We often hear claims about 200, 300-year old trees being removed or preserved in Arlington County, and, while we do have a handful of those, most trees are not that old. When aging or dating a tree, you have to take to take three major things into account: History, Site characteristics, and Species.

History

Urban foresters do informal tree ring counts on removed trees, and have employed outside support on comparing these ring counts to climate and other events. Rings will tell you the most information on the age of the tree, and you can often get within a decade of the tree’s birth.

That being said, doing some digging in the library can often get you a better idea of when a tree started growing. Here are some major aspects of Arlington County and events that strongly influenced trees in the area:

Civil War

While disturbance before the Civil war definitely happened, finding information on this is difficult. The war itself has sparse information, but one thing is common knowledge in aging trees. Sight lines for artillery and resource needs cause the removal of almost all trees in Arlington County. This means most of our trees are not likely to be older than 150 years. Where trees were preserved for shade for soldier, there is a chance for older-aged trees, and one of the two remaining old growth forests, on Arlington Cemetery, behind the Lee House, was preserved. This forest appears to have been unlogged since before the late 1700s, but is a striking exception to the rule.

Farming

Much of Arlington county was farmed, in the 1800s, and we have records of dairy farms in the county until the 1990s. These areas typically had very little trees, except as hedgerows or trees near the homes.  Checking old maps, like the one on the right, from 1934, can tell you whether the land was used for forest or not. If the land use was farmland, you can exclude most ages beyond that date. We are blessed with an abundance of historical data, being in the DC region, and this can aide in this research.

Many digital scans of these maps are available on our website, as well, in our GIS Gallery: http://gis.arlingtonva.us/gallery/index.html

Post-World War 2 development

The rush to build housing for new federal employees, and the new Pentagon brought with it large subdivisions in Arlington County. The Fairlington community was built during the war, to house thousands of new employees. If you drive through the community now, there are sizeable willow oaks and Darlington oaks gracing the streets. These trees were mere sprigs when this development was completed, but can only be 70-80 years old by now, despite their current size (see images on the left).

Recent disturbance

Home redevelopment, street projects, and other disturbance happens all the time in this busy county. Finding out from locals about projects can tell you whether a tree was removed and replaced. Neighborhood conservation projects and county tree planting programs have not always been around, and this information can often be gathered from neighbors around the tree.

 

Site characteristics

The site in which a tree grows up greatly influences the speed of its growth. Poor sun exposure, scarce access to water and oxygen, nutrients, and other limiting factors can slow down the growth of a tree drastically. While a tree of the same species in the middle of a forest may only grow a conservative 0.2 inches a year in diameter, in an open field it may shoot up at 1 inch annually. This means street trees, which often do have restrictions, but more access to light, can often be much younger at the same size as forest-grown trees.

Species

One of the oldest broadleaf trees in the Eastern United States is a Black Tupelo (Nyssa sylvatica) in New England (News link) It’s almost 600 years old, but it’s only 30 inches in diameter. This species grows so slowly that one could not guess its age by looking at it. On the other side of the spectrum, the elm tree on the right, which was removed last year, at 40 inches in diameter, only has rings for 40 years of age.

Some species will not get very large. Finding a dogwood with a diameter larger than 15 inches is a rarity, and can indicate good growing conditions and old age. Knowing your species, and their biological characteristics can help greatly in providing estimates.

Tools for dating trees

All of this information will give you an idea of the age of the tree. If you are doing in-depth research, you will need more information, so ring counts (the field of dendrochronology, for those Scrabble fans out there) are often the best approach. If removing the tree is not an option (which it often isn’t), coring a tree, or drilling a small hole to extract a rod, can be a great way to get an idea of its age. This technique extracts a sample of the tree rings, usually not harming the tree, which can then be analyzed. Many trees are hollow, however, and estimates are typically the only approach, along with the other tools above.

More information:

About Tree Rings, University of Arizona: http://ltrr.arizona.edu/about/treerings

What tree rings tell us about our environment: http://www.environmentalscience.org/dendrochronology-tree-rings-tell-us

Morton Arboretum worksheet to age a tree (PDF): https://www.mortonarb.org/files/Find%20the%20Age%20of%20a%20Tree%20-%20middle%20school.pdf

Components of an Eastern Deciduous Forest

The best designer of the eastern deciduous forest in which we live is nature. Given enough time and lack of human disturbance, almost any spot in Northern Virginia with some soil, will turn into this ecosystem. Our climate and environment encourage the type of growth that brings with it tall trees and dense woods. If you looked at all the layers of a healthy forest, what would go into designing a forest from the ground up?

Soil

Without proper soil, you don’t get much growth. There are plants that can handle very poor, thin soils, but that will not get you a full forest very quickly. Because our forest tends to be dominated by species that prefer acidic soils, such as oaks, beech, and tuliptree, you would start with an acidic soil. Almost just as important is the soil composition. Gardeners will attest to the prevalence of clay in their soil, and the difficulty of digging in this clay, but we do also have sand and silt in our soil. A good mix of the three, with a focus on the water-retaining clay, gets us a good soil to start with.

If we end up with an acidic, well-composed, uncompacted soil, that’s step 1 of our forest. Our useable soil typically goes about 1-2 feet down, so that’s a target to hold. Where space is restricted (planters, tree pits), we often ask for more to compensate, but in an actual forest, this is plenty. Bringing in soil from other areas can be costly, so a great method of providing this layer of our forest is to mix organic material with clean existing soil on site, to provide the basis of our forest.

 

Groundcover and herbaceous plants

A forest is not just made out of trees. Ephemerals take advantage of low canopy cover in the earlier part of the year to catch energy from the sun, and then disappear. Grasses and other groundcovers hold the soil, and provide food for earthbound wildlife.

When building from the ground up, a complete forest also has:

– Ephemerals, such as spring beauty (Claytonia virginica), Virginia bluebell (Mertensia virginica), and Dutchman’s breeches (Dicentra cucullaria)
– Grasses and grass-like plants, such as eastern bottlebrush grass (Elymus hystrix) and Pennsylvania sedge (Carex pensylvanica)
– Ferns, such as Christmas fern (Polystichum acrostichoides), and wood ferns (Dryopteris spp.)
– Other Herbaceous plants, such as Jack-in-the-pulpit (Arisaema triphyllum), and mayapple (Podophyllum peltatum)
– Vines, such as Virginia creeper (Parthenocissus quinquefolia) and grape (Vitis spp.)

This layer can be difficult to emulate perfectly, as some ephemerals require unusual soil compositions, and species such as orchids and trilliums often don’t survive transplanting. However, there are many grasses, ferns, and vines are fairly tolerant of transplanting,  which can be used to prepare the site for other seeds to come in naturally. Keeping the soil in place and alive is the best way to have this forest provide the habitat for the rarer plants to come in.

Shrubs

The next level up is the shrub layer. There’s no real physiological difference between most shrubs and trees, but the general definition relates to mature size. Most shrubs don’t reach above 20 feet, when fully grown. This shrub layer provides much of the shelter and cover for forest animals. In forests with an unhealthy population of deer, this layer is often suppressed, along with the lower plants. Healthy native shrub growth is a great indicator of a healthy forest, and usually indicates the other aspects of the forest are in place, in a fairly balanced way. Unfortunately, many invasive shrubs, such as Honeysuckle (Lonicera spp.) and Autumn olive (Elaeagnus umbellata) have out-competed our native shrubs, providing little value to our ecosystem, except for some shelter and berries with little nutritional value.

Common shrubs in our forest include spicebush (Lindera benzoin), witch hazel (Hamamelis virginiana), Arrowwood (Viburnum dentatum), and blueberry (Vaccinium spp.).

Creating the shrub layer can be a great way to improve the forest, but deer pressure can make it a frustrating process. Deer exclosures and individual deer fencing can be a great way to address this issue.

Understory and midstory Canopy

The forest is an ever-changing fight for access to light. The groundcovers and shrubs have typically adapted to lower light conditions, but there are always new trees coming through, trying to get to the top. All of our tree species can exist in this middle level of canopy at one point in their lives, but it typically takes the opening of the canopy to let through the really intolerant species. For example, Tuliptree (Liriodendron tulipifera), will live as a sapling, building up energy, sometimes for up to several decades, to wait for an opening in the canopy. All of the accumulated energy in its roots will then allow for exponential growth into the canopy, to prevent others from taking this access to light.

Understory trees that stay under 30-40 feet are present in a healthy forest, and rise above the shrub layer. These are most commonly dogwoods (Cornus spp), redbuds (Cercis), serviceberry (Amelanchier spp.), hornbeam (Carpinus caroliniana), Pawpaw (Asimina triloba), American holly (Ilex opaca), and Sassafras (Sassafras albidum).

The mid-story is typically composed of all the trees looking to reach the next level, and can be composed of any of our hundreds of native species. This layer can be emulated through transplanting of landscape-sized trees, but care has to be taken with placement. If a plant has no access to light, it will not thrive, and not likely survive.

Overstory Canopy

Ultimately, what is essential to the forest is the overstory canopy. Trees larger than 40 fe

et, providing high cover for animals, intercepting the majority of the rainwater and sunlight. The trees in this layer are typically older, and have accumulated massive reserves of energy through their access to light and lack of competition. Some overstory trees even provide their own habitat for other plants, like vines and epiphytes, plants growing in small soil pockets along branches.

In our region, our overstory is dominated largely by oaks, beech, tuliptree, and hickory. Some unique ecosystems with very high moisture contents also have dominant red maples, willow, and sweetgum canopy.

Creating this layer of the forest takes extreme patience. Transplanting large canopy trees is financially burdensome, and establishment is difficult. The best approach is to plant for the future, with all of the aforementioned layers in place, and, after establishing the plants, letting nature take its course.

The Complete Forest

Arlington is working to incorporate all the elements of the forest into our ecosystem, understanding the stresses of urban influence. While letting nature do its job is an essential component, invasive species, deer pressure,  pollutants, and non-native insects can cause human-made havoc on our environment, and we must stay vigilant in identifying and addressing these problems in time.

Of course soil and plants are not the only aspect of forests that make it a healthy ecosystem. Water, in the form of groundwater, streams, lakes, and wetlands, provides the lifeblood for the forest, animals spread and germinate seed, and fungi make nutrients available to our plants, and break down dead material. The forest does not work without these aspects, and if you are looking for a permanently self-sustaining forest, measuring the presence of key organisms and soil fungus health can be a great indicator of success.

The best way to learn about these layers is to explore all of our region’s forests, and to see all the different growth habits of every habitat. Learn to identify habitat types, and why certain plants are in different situations. This way you can learn to best incorporate these elements into your urban forest.

For more information on forest structure in Virginia, check out this publication by the Virginia State Extension and Virginia Tech: https://pubs.ext.vt.edu/465/465-315/465-315_pdf.pdf

Urban foresters often get questions about private tree removal, including everything from whether it’s legal to remove trees, to what to look for when choosing a tree removal company. This post aims to outline some of the restrictions to tree removal, and the questions property owns should ask when moving forward on tree removal.

Does my tree need to be removed?

Deciding whether to remove a tree can be a big decision. Make sure you are certain that a tree is a hazard before you choose to remove it. When in doubt, we recommend seeking the help of an independent International Society for Arboriculture (ISA)-certified arborist to assess the health of a tree. Certified arborists can assess hazard, the need to remove a tree, and how to address specific issues. Sometimes trees that don’t appear to be a hazard have hidden defects, only detectable by trained professionals and equipment, and conversely, sometimes apparently hazardous trees may be preserved. One resource to find certified arborists can be found at the following site: Good Tree Care

Is it legal to remove my tree?

These are some of the questions you should ask yourself before choosing to move forward on tree removal or work. This is not an exhaustive list of restrictions, but addresses the majority of issues we face.

Private v. public ownership

The first step to take is to ascertain the ownership of the tree. Trees in the right-of-way, along streets, in parks, or other public properties, are under the care of the agency controlling that land. Removing a tree on public land without permission can have legal and financial ramifications. Common local owners of public trees are Arlington County, the Virginia Department of Transportation, and the National Park Service. Contact the agency before deciding to move forward on tree removal. Rough outlines of property lines can be found on this map: Real estate map.

Jointly-owned trees

Trees don’t always respect property boundaries, and often grow large enough to have a trunk that straddles a property line. If any work is to be performed on this tree, seek written permission from the other property owner(s). The same counts with trees co-owned with public entities, as described above.

Resource Protection Areas (RPA)

Resource protection areas in Arlington County include sreams, wetlands, and buffers of riparian habitat adjacent to these waters. These buffers extend at least 100 feet from streams and wetlands, but can be wider where there are “critical slopes” (slopes steeper than 25%, or 45 degrees). Protecting trees in these areas helps keep our streams healthier and prevent major erosion on the stream banks.  If the tree is within an RPA, you need permission from the Department Of Environmental Services.  Contact Christin Jolicoeur prior to proposed tree removal if an RPA is present on your property. Find a map of RPAs here: Watershed & Resource Protection Areas

Locally-designated historic properties

Trees larger than 15 inches diameter at breast height  (DBH(at 4.5 feet height)) on locally-designated historic properties require a certificate of appropriateness, reviewed by the Historic Affairs and Landmark Review Board (HALRB), and get issued by Arlington County, after approval. County Urban foresters  will make an assessment of the tree, and provide a report for the HALRB to consider. Contact the Historic Preservation department at 703-228-3831 or Email. For a map of locally-designated historic properties, consult the following map: Historic Sites and Districts

Specimen, heritage, and memorial trees

Arlington county code allows for a tree on private property to be designated a specimen, heritage, or memorial tree. This type of designation requires county board approval, and legally protects the designated tree on the property by adding a covenant to the deed. This designation protects the tree with a potential civil penalty of up to $2,500 if removed without permission. To find out more about specimen, heritage, and memorial trees, follow this link: Specimen trees

Site plan restrictions

Large projects, such as big office buildings, residential condominiums, and other projects in the urban corridors of Arlington, often go through a rezoning, or “special exception” process, which binds the property to site plan conditions. These conditions often include language on trees on the project, and may restrict removal of trees or major changes to a landscape. Most single-family homes and small buildings do not fall under these restrictions, but when in doubt, ask the building engineer or owner for the history on the project for these restrictions. Find out more about site plans here: https://projects.arlingtonva.us/private-development/

Development

Development disturbing 2,500 Square feet or more of soil requires a landscape conservation plan. This landscape conservation plan requires 20% of tree canopy after development, met either through preservation and/or planting of trees. Projects performed by-right (according to their zoning designation), without the restrictions to tree removal mentioned above, do not have a requirement to preserve trees, but Arlington County has made preservation more attractive through additional credit given for properly-preserved trees. Find out more about land disturbance applications here: Chesapeake Bay Preservation Ordinance

What to look for in a tree care company

While Arlington County cannot make specific recommendations, these are some guidelines to look through before picking a company to perform work on your tree.

Pruning and specialized work

If the decision for tree care involves detailed pruning, specialized care, or other work, it’s recommended to pick a company with certified arborists on staff to perform or supervise the work. Improper pruning or tree care can lead to more damage than benefit, and following proper arboricultural practices is key to preserving healthy trees.

Removal

Detailed arboricultural knowledge may not be necessary for tree removal, but it is recommended to seek a company with proper proof of insurance. Tree removal can involve significant hazards, heavy limbs, and other potential damage to property or people. Proper insurance protects you from accidents and improper work practices. There is no license required to perform tree work in Virginia, so always ask if the company performing work on your property is properly protected.

Is it legal for my neighbor to remove a tree on their property?

Aside from the above-mentioned restrictions, removing trees is legal on private property in Virginia, without local government permission. Recognizing the value of trees and communicating this value to other residents is one of the most powerful tools to prevent unnecessary removal of trees, and Arlington County works closely with their partners to spread the word on the value of trees.

Links

Tree removal in Arlington County: https://environment.arlingtonva.us/trees/care-for-trees/tree-removal/

The value of trees: http://www.na.fs.fed.us/spfo/pubs/uf/techguide/values.htm

Maps of Arlington County: http://gis.arlingtonva.us/gallery/index.html

Soil is one of the most critical aspects of urban street tree survival. When given the chance to live and grow in the appropriate quantity of healthy soil, trees can handle a lot of negative environmental factors.

A big movement in urban forestry in recent years has been to expand the accessible soil volume for trees, to improve tree health, and consequently, the value it provides to us. These techniques were developed to address limitations in Arlington County’s urban corridors and they serve as our best efforts in this less-than-ideal environment for trees.

If space is not an issue, then tree plantings should be explored from a forest perspective where soil connects their roots and provides access to the water table. This is where the bulk of our tree canopy will come from, but providing a healthy street tree inventory is not precluded through these technologies.

This article utilizes technical language. For any clarification, contact the author at vverweij@arlingtonva.us.

How much soil do trees need?

A truly healthy tree requires access to enough uncompacted soil to receive the necessary water, mineral nutrients, and oxygen it needs to thrive.

In order to determine when an urban tree will reach its peak maturity and provide the maximum amount of benefits to its environment, we have to examine up to 1,000 cubic feet of soil volume for every large canopy tree (oaks, maples, lindens). The soil volume of one tree could be shared with the soil volume of another tree; therefore, the individual soil volume per tree may be lower than what is examined.

With a depth of three feet being the typical depth of tree roots in this region, this would require a 333 Square ft. area of soil. At a spacing of 25 ft. for trees (with 25 feet on both sides equaling 50 feet), this would require about about a 7 ft. open planting strip.

In many urban areas it is restrictive to accommodate such dimensions due to competing interests; therefore, planners and urban foresters work to create accessible soil space underneath sidewalks. There are many techniques that companies and universities are continuing to develop solutions for. Over time we will observe these solutions’ success rates. but several of these have already been in place for more than 15 years, and show positive levels of success.

Current techniques in expanding soil volume

The simplest and usually cheapest approach is to provide a simple open planting strip for trees. The advantages are many-fold, including reduced impervious cover, and full access to soil for the tree, to water and nutrients. This is appropriate for residential areas, but is typically not preferred in heavy commercial areas, where benches, fire hydrants, sidewalks, street lights, utilities, and access to parking are both desired and required. The following techniques provide some solutions to work with these conflicts.

Sidewalk support technologies

One of the major factors of healthy soil is the level of compaction. Uncompacted soil is soil with enough space in between the particles to allow water and air to occupy those spaces. Many current construction techniques require heavy compaction of their soil, to support sidewalks. This is one reason older projects have healthier trees, as requirements for compaction were significantly more lax, allowing for root intrusion. Poor compaction may create a great environment for tree roots, but unfortunately, also destabilizes sidewalks, so this is not a solution, unless we can alleviate the stability issue through some of these techniques.

Continuous soil panels under sidewalks

This technique is fairly simple. It’s essentially a reinforced sidewalk in between tree pits, which allows uncompacted soil to live in harmony with a sidewalk reinforced by steel rods, which are placed before pouring concrete. This technique has been used in most large projects in Arlington county, and no issues have been found with sidewalk cracking, from the few older projects we have investigated.

What limits this technique is the width of the area you can bridge with these steel rods. Once you get past 6 feet in width, the steel rods lose the capacity to support the sidewalk and whatever may be on top of it. If you look the calculation from above, 6 feet gets you almost to our goal of 1,000 cubic feet, but not quite there. This is why we also explore other techniques below.

Structural cells

One of the more recently developed technologies are structural cells. These products look like large open plastic crates, with the capacity to support the sidewalk and anything on those sidewalks, while also containing soil accessible to nearby trees. This technique is fairly advanced, and requires expert installation, but it has been used in several projects in Arlington County. One of the larger projects is a county Park, Penrose Square. All the trees in this urban park are in structural cells, allowing them to grow larger and faster, while also supporting the harder gravel surface above the soil. This was installed in 2013, and even with minimal watering, all the trees are doing very well.

Limitations to this technology do exist, with concerns about cost, the nature of adding more plastic to our environment, and maintenance when working with under-pavement utilities.

Structural soils

Structural soils are a different technology, which focuses more on the texture of the soil to support the sidewalk and trees. Because sidewalks and other hardscape need a certain level of compaction, changing the composition of the soil to create large aggregates, mixed in with healthy top soil, is one option to attempt to relieve the issues of urban soil space. You can see a diagram on the left to illustrate how this might work.

There is currently significant debate about the effectiveness of this technology, and we will continue to monitor research on this. With that in mind, this soil has been used in several projects, and is generally fairly easy to work with, since it acts like regular soil.

Arlington county requirements and future

Above its standard requirement of a 5 X 12 X 3 Ft. (W X L X D)  tree pit, which provides 180 cubic feet of soil, Arlington county does not require an explicit soil volume amount on all of its projects. However, several sector plans have included soil volume recommendations. Rosslyn’s new sector plan, for example includes significant goals for soil volume for street trees. Staff, when working with private projects, advocate for expanded soil volumes beyond the required 5 X 12 tree pit.  Continuous soil panels (bringing the soil volume up to 750 cubic ft) have become standard, and we are exploring options on requiring specific soil volumes, where appropriate.

On county projects, soil volumes have become more of a target, and several projects with expanded soil volume are underway. Mosaic Park has structural cells planned for its plaza, and other transportation projects have started to include more significant open planting strips and alternative techniques. Administrative regulation 4.3, the regulation prescribing tree planting and soil for Arlington County projects, is being rewritten with proposed ideal soil volume targets.

The issue of providing a healthy environment for our street trees goes beyond soil, but this research and advancements in planning documents and practice helps us address some major issues facing urban trees. Improved environmental conditions can help us look at a wider palette of species, and help integrate more appropriate trees into our ecosystem. Healthier, more long-lived trees help us get the return on investment from these trees, in environmental and social benefits in the long term.

Links

• Arlington County design standards (tree planting and tree pit details)
• Arlington County Landscape Standards
• Casey Trees tree space design study
• Deeproot recommendations on soil volume
• Soil Profile rebuilding for compacted soil
• American Society for Landscape Architecture information on Structural Soils and suspended pavements:
  • http://thefield.asla.org/2014/01/30/structural-soil-part-1/
  • http://thefield.asla.org/2014/02/19/structural-soil-part-2/
  • http://thefield.asla.org/2014/04/24/planting-trees-in-suspended-pavement/

From the Natural Arlington Blog:

What is Urban Tree Canopy?

One of the assessments Arlington County uses to measure the success of our urban forestry program is through measuring tree canopy percentage, or Urban Tree Canopy (UTC) throughout the County. Virginia Tech defines Urban Tree Canopy as the layer of leaves, branches, and stems of trees that cover the ground when viewed from above. Urban tree canopy provides many benefits to communities including improving water quality, conserving energy, lowering city temperatures, reducing air pollution, enhancing property values, providing wildlife habitat, facilitating social and educational opportunities, and providing aesthetic benefits.

Tree canopy studies are usually performed using high-resolution satellite photo, and involves significant analysis by Geographic Information Systems technicians. Simpler methods of estimating tree canopy percentages do exist, such as iTree Canopy (http://www.itreetools.org/canopy/), but for comparable, scientifically-sound studies, UTC’s more accurate assessments are often used.

There are some limitations to urban tree canopy data, because this top-down data does not speak to what species of trees are in the community, the health of the trees, whether we have a vertically complete forest (trees, shrubs, grasses), or is an over-browsed unsustainable forest. Consistency between different methods of UTC analysis has also made comparison an issue, especially when used for scientific purposes, but abstractions can be used to create comparable data.

Studies performed

Arlington’s first tree canopy study was performed in 2008 by Virginia Tech. It showed us where our densest tree canopy was, and what areas needed extra focus. This study gave us a tree canopy percentage of 43 percent. With a target of 40 percent recommended by American Forests in areas like Arlington, we exceeded that goal. It also set a baseline to compare future studies. This study is available here:

2008 Tree Canopy report

Since 2008, another study was performed for the region, by Casey Trees and the University of Vermont. This study used the same resolution and similar methods to the 2008 study, and showed a significant decrease in tree canopy in Arlington County. Using this data, Arlington County shows about 40 percent tree canopy. You can find a map of the analysis here:

2011 Tree Canopy report

To truly identify trends in Arlington County on tree canopy, we will need to see future studies performed on the County. Recent studies have started for the entirity of the Chesapeake Bay (of which Arlington County is part), and this will include state-wide tree canopy assessments. It’s likely new data will be gleaned from this study to develop stronger trends. More information can be found here:

Chesapeake Bay Tree Canopy Goals

The trend in tree canopy

A reduction from 43 percent to 40 percent is a significant decrease for such a short period of time. From closer analysis, it is clear that no one source of impact (development, invasive plants, deer, disease) is at fault. It appears that all factors are involved in the reduction of canopy. Cross-analysis was performed with intensity of development, parks prone to deer browse and invasive plants, and areas with disease-prone trees, and it seemed that all issues were the cause here.

Some civic associations were hit particularly hard between the two analyses, such as Alcova heights, Arlington Ridge and Lyon Village. Some increases in neighborhoods in the Northwest were noted, but they were largely offset by surrounding losses. A map of changes in tree canopy can be found here:

Change in tree canopy 2008-2011

How is Urban Tree Canopy data used?

Data is only useful if it helps us communicate an issue and plan for the best outcome in projects. Our tree canopy data has been used with:

Planning for protection: Communicating where contiguous, high value forest patches are in the County can help drive park planning, road construction, and other facility needs. Using UTC data in planning was used successfully to educate about the losses incurred by individual projects, and to drive conversations of prevention. Showing which civic associations show below average tree canopy can help communicate the need for tree protection for both public and private projects.

Outreach: Some neighborhoods with low tree canopy have been targeted by outreach from our tree canopy fund partners to help residents become aware of programs for planting trees on their projects. This has led to strong ties with communities otherwise not typically involved in natural resources management, such as the affordable housing community.

Potential tree canopy and intelligent planting: A more positive way to look at tree canopy data is to identify possible planting spaces in the County. This allows the Parks department and other organizations to find areas where the most impact can be had for planting. Large reforestation projects tend to be easier to manage than individual street trees or plantings, and this data can help us identify potential locations for this.

Other analysis

UTC analysis gives us a great idea of the general quantity of tree cover, and can help show trends in tree canopy in the County. However, its limitations don’t give us a full understanding of the function and value of our urban forest. Some future projects are being considered right now, which will help us get a more comprehensive idea of the makeup of our natural world:

iTree Eco: Staff hopes to perform a 200-plot statistical survey of trees throughout the County, which will give us an idea of the species makeup and sizes of trees in the County. More information on this program is available here: http://www.itreetools.org/eco/.

Preliminary study results from Washington, DC’s iTree Eco study can be found here, to show what kind of data this study produces: Casey Trees iTree Study

Finding new spaces to plant and preserve: Many natural areas can be expanded in parks, along rights-of-way, without impeding existing use, and improving our environment. Using Tree canopy data can help us identify where these areas may be. Reducing mowed areas can help reduce maintenance cost and carbon emissions, and make our parks more sustainable.

Bringing in social factors into the analysis: The City of Baltimore, as well as other pilot studies in the country, have analyzed how tree canopy correlates to wealth, social capital, and other social factors, such as age, family size and ethnic background. Using these kinds of analyses, these communities have improved their outreach in areas with low tree canopy, and significantly improved support for improving the natural side of their cities. Detailed census data exists for Arlington County, and this may be a part of the Urban Forest Masterplan update.

More at the following link:

Comparing socio-economic factors and tree canopy

Resources

Arlington County Forest Health: Arlington’s tree canopy

United States Forest Service document on tree canopy: http://www.nrs.fs.fed.us/urban/utc/

Urban tree canopy studies in Virginia: http://gep.frec.vt.edu/va_utc.html

iTree Canopy: http://www.itreetools.org/canopy/

iTree Eco: http://www.itreetools.org/eco/

Over the last few years, Arlington has compiled resources and gathered input from experts and the public on what trees work best in Arlington to provide our community with the greatest benefit now and for the future. Urban foresters have put together several lists of recommended trees for different situations. These lists can be found here, and this article will speak to some of the aspects we look for in trees to plant in our community. All issues are weighed when selecting species for these lists, and no tree is perfect. A tree that may work in one place may not work somewhere else, so always do your research before choosing a tree that may be with you for decades.

Structural stability and health issues

Because of our urban nature, we have to be very aware of the impact trees can have on surrounding structures, roads, utilities, and of course people. Historically, many trees were picked to plant because of their fast growth (such as Silver maples or Bradford pears), but these trees were not always the most structurally sound trees. While some of the less structurally-sound native species still have a place in less damage-prone areas such as natural areas or floodplain restorations, these species are generally kept off the lists.

Another issue to consider when picking a species is susceptibility to pathogens like insects, fungus, bacteria, or fungi. Foresters have learned hard lessons over time, seeing entire streets planted with American Elms or Green Ash die from their respective pathogens, Dutch Elm Disease, and the Emerald Ash Borer. We take this into account when recommending trees, and recommend resistant varieties (as with the Elm), or restrict planting of certain species (as with the Ash).

Aesthetics and form

A tree’s appearance is often one of the first things taken into account when selecting species. Trees are still primarily seen as a landscape feature, even with growing appreciation of their other essential values. This is why certain species that may have low environmental value, but high cultural value, such as ornamental cherries and European hornbeams remain on our recommended tree lists. They are non-invasive, attractive trees, that add to our community with their color and beauty. Luckily, we also have many trees that are both beautiful and have a high value. Almost nothing beats a mature oak in its majesty, while also providing some of the greatest wildlife value. We seek to keep this balance equal, and there is a place for every tree on our lists.

Form and size is part of the aesthetic of a tree, but often is looked at to fit the space in which the trees are grown. Some trees are selected to be particularly narrow, short, broad, vase-shaped, pyramidal, the list goes on. This is often related to how people want their site to look, but can also be done to reduce interference with utility lines, provide sight lines for signs and roads, or overhead clearance for paths underneath.

Resistance to urban conditions

In an urban environment, we also have to worry about the stresses that come with roads, an increased pollution load. Some pollutants are obvious, such as exhaust and road salt, but other pollutants may

be less clear, such as pet waste, lead from lead paint, or pH-altering chemicals from surrounding concrete. Trees have to be able to adapt to these stressors, and not all trees can handle this. Unfortunately, many trees that adapt readily to short-term stress are opportunistic, invasive species, and the damage caused by these species to our natural areas is too great to risk additional invasion. However, there remain species native and non-invasive on our list, that have adapted to urban environments, or naturally occur in environments of high stress, such as floodplains or mountainous environments.

Oaks and maples, London planetrees, tupelo, and other species on our lists are noted for their pollution tolerance, and are prioritized for street tree planting. It does give us a much more restrictive list to pick from, so ultimately, much of our tree canopy’s diversity will have to come from trees in natural areas, or more protected landscaped areas on private and public land.

Benefit to our environment and natural landscape

One of the major aspects of supporting trees in our county is for the environmental and natural benefit trees provide to our community and the Chesapeake Bay. Many of our regulations and tree canopy requirements were originally created for the benefit to stormwater runoff reduction and pollution reduction. Large canopy trees provide the most interception of stormwater, capture the most carbon, and, in the right place, live the longest. The Urban Forest Masterplan outlines many of the benefits we seek from planting and maintaining trees in the county.

A high premium is also placed on trees native to our region, as native trees fare better in the long term, and provide the greatest benefit to our community. The Natural Resource Management plan lays

out how we coordinate with all agencies and the public to promote native species, and these species are prioritized when planting trees. There exist occasions where native trees may not be appropriate, due to space, cultural or aesthetic restrictions, which is why non-invasive non-native trees are still on our lists, but we work to find and foster the right places for our highest value native trees.

All of these considerations have to be balanced with a need for diversity. Having learned over time that monocultures, or large groves of the same species, can lead to serious issues, Arlington county tries to balance the species planted, so we have as diverse palette of trees as we can. This sometimes conflicts with a desire for uniformity, and compromises can be difficult to be reached. Willow oaks and red maples, for example, are great urban trees, but are widely overplanted throughout the county. Where possible, we restrict the use of these trees, but there remains a high demand for them from developers, as they tend to be very reliable.

Tree selection

Every tree reacts differently to a site, and care should be taken when picking your species. There is no tree that meets all of the restrictions discussed in this article, and there is no “golden bullet” tree. Consulting with arborists, our extension service, and landscape architects, along with digging around the internet for research are all tools to find the tree that fits your site best. Days of research went into compiling the lists on our website, and they are still evolving, as we learn more about each tree, and get input from the community. Never hesitate to reach out to staff with suggestions or new information, as these recommendations are part of a living document, and mature with the trees we plant.

Resources

• Recommended Trees for Arlington County: https://environment.arlingtonva.us/trees/plant-trees/recommended-trees/
• Right Tree, Right Place: http://www.arborday.org/trees/righttreeandplace/
• Native Plant resources: http://www.plantnovanatives.org/ and  http://www.nps.gov/plants/pubs/chesapeake/pdf/chesapeakenatives.pdf
• Comprehensive atlas of plants in Virginia: vaplantatlas.org
• Where to find native trees: http://vnps.org/conservation/plant-nurseries/
• List of all nurseries in Virginia: http://www.nurserytrees.com/States/state%20Virginia.htm

Inspecting trees at one of our historic forts, Fort Ethan Allen, foresters noticed a high level of death in the canopy. There had been a killer on the loose (See figure 1). Emerald Ash borer (EAB) had killed over 80% of the ash trees in the fort area. This pest has been a major killer of our ash trees for the last few years, and it struck again this year, at several parks (Ft. Ethan Allen, Quincy Park, and Doctor’s Run were particularly hard hit) While the impact is much larger in the Midwest, where the tree species diversity is much smaller, it has had a major impact on some of our most beloved parks and street trees.

History

Emerald ash borer is an insect that was introduced through the import of non-native ash trees. It was first introduced in the mid-1990s, and had major impact on the Midwest, starting in Michigan, causing billions of dollars in damage. It was introduced to our area through a careless landscape company introducing infected ash trees.
The loss has recently been linked not only to the cost of removal and treatment, but also the indirect health effects of the loss of nature (click here for more information).

Identification

The EAB is a shiny green insect that kills ash trees by eating the living part of the tree, right under the bark. The insect is easily identifiable, and foresters found dead specimens throughout Arlington, after finding the trees they had infested (see Figure 2). The hole they make to enter the tree is fairly easily-identified, too, having a distinct D-shape (Figure 3) However, when you find the insect, it’s often too late, and keeping a close eye on the health of your ash tree is the best way to identify and manage for this pest.
The most obvious signs of stress and a likely infestation from EAB are when the tips of your ash tree’s branches show significant dieback (Figure 4). When the tree’s leaves are mostly gone from the very tips of your tree, it’s time to call an arborist to inspect your tree. When you catch it at this stage, there is usually still a chance to treat the tree or make an educated management decision on whether to remove the tree.

Impact to Arlington and Treatment

Ash trees are at their southernmost range in Arlington, which means you will not likely find many ash trees South of Arlington. This means that they are only a small part of our canopy, and the impact is relatively small. However, we are home to some of Virginia’s largest ash trees, and they are still an essential part of our ecosystem.

Arlington county takes the threat from EAB very seriously, and has focused resources on treating champion ash trees and large (>30 inch) street trees for the pest. Both Maryland and Virginia state are working on releasing predatory wasps, which will hopefully help as a biological control for the borer. We will continue to treat our highest value trees as long as we can, and hope to have these trees outlive the greatest impact to the species.

If you have an ash tree on your property, or suspect you do, feel free to send pictures for identification to the Virginia Tech Extension, who can help identify trees and insects to see if you need to think about treatment. The Extension agent can be reached here: http://offices.ext.vt.edu/arlington/. More resources are available below.

Additional Resources

• Comprehensive site on emerald ash borer: http://www.emeraldashborer.info/
• Decision guide for homeowners to manage for EAB: http://extension.entm.purdue.edu/EAB/pdf/NABB_DecisionGuide.pdf
• Information on predatory wasps: http://www.nrs.fs.fed.us/disturbance/invasive_species

Last month, in the week of June 15, we celebrated National Pollinator Week. There has been significant discussion about the challenges pollinators (especially bees and monarch butterflies) face in our world. Pollinators, both native (native bees, flies, wasps, hummingbirds, etc.) and non-native (honeybees) provide an essential service to our lives and ecosystems (bats also pollinate plants, but we do not have any fruit-eating bat species in Arlington). Not only do they provide us with one third of the food we eat through the reproduction of flowering plants and their seeds, but they also keep our flowering plants going in our natural world.

Alonso Abugattas, Arlington’s Natural Resource Manager, recently spoke about different pollinators like beetles and bees, and touched on some of the limitations of focusing only on milkweed for pollinator value, so it’s time to talk about the importance of trees in a pollinator’s life cycle.

Leaves: Baby Food for Many Pollinators

While this blog will also address the value of flower nectar, the real meat and potatoes for many pollinators, especially butterflies and moths, lie in the leaves. Where monarch butterflies eat primarily milkweed leaves, many other species are dependent on tree leaves for their early stages of development. For example, the spicebush swallowtail caterpillar (Papilio troilus, Pictured above right), only feeds on spicebush (Lindera benzoin) and sassafras (Sassafras albidum), both native understory trees.  The zebra swallowtail (Eurytides Marcellus), only feeds on young pawpaw (Asimina triloba) leaves.

Many species are not as picky as the aforementioned species, and feed on a wide variety of trees. Our state insect, the tiger swallowtail butterfly (Papilio glaucus, pictured below right), finds a host on tuliptree (Liriodendron tulipifera), black cherry (Prunus serotina), sweetbay magnolia (Magnolia virginiana), and many other native host plants.

Douglas Tallamy, in his research on lepidoptera (moths and butterflies) identified a list of plant species with the highest value to this group of insects.As mentioned in an earlier blog post, one of our most common tree species, the oaks (Quercus species), are the host of the most lepidoptera in the region. This makes oaks some of the most valuable pollinator plants to plant around. With all their other values, they really are an extremely valuable species to plant and preserve around here. Closely followed by the oaks are native cherries, like black cherry (Prunus serotina), willows (Salix), birches (Betula) and poplars (Populus). Keep in mind that even though there are non-native species in these plant groups, the native version provide the greatest benefit to our pollinators.

Flowers: Nature’s Source of Nectar

What’s more popularly known as food for pollinators is the nectar from flowers. Nectar provides immediate energy for all species in what we know of as pollinators, from bees turning it into honey, to hummingbirds feeding from buckeye flowers mid-air (pictured left). Even here, not all flowers are created equal in pollinator value, but research has highlighted some high-value trees here, as well.

Some of the species with great nectar benefit include some already-popular landscape trees, such as Red Maple (Acer rubrum), Eastern Redbud (Cercis Canadensis), American linden (Tilia americana) and Blackgum (Nyssa sylvatica). Also noted in some of the research are smaller trees with human food value, as well, such as American Plum (Prunus americana), Persimmon (Diospyros virginiana), and Black Cherry (Prunus serotina). All of the above species, when given the right space, work well in Arlington.

Let’s help make Arlington a home for our pollinators. Plant and protect our native plants, and work to provide a great habitat for all.

Learn More

• Pollinator Partnership: http://www.pollinator.org/pollinators.htm
• Native plants of Northern Virginia: http://www.plantnovanatives.org/
• What to plant for Butterfly and moth benefit: http://www.bringingnaturehome.net/what-to-plant.html
• West Virginia handbook for pollinators (page 82 has a list of woody species for pollinators): http://www.xerces.org/wp-content/uploads/2009/11/WVPH-SEC.pdf
• What species attract what pollinators: http://www.ree.usda.gov/ree/news/Attractiveness_of_Agriculture_crops_to_pollinating_bees_Report-FINAL.pdf

 

This post was edited on 7/17/2015 to reflect corrections to species pictured.

Every year, I’m impressed by how trees handle standing outside in 90 to over 100 degree weather, without being able to move and, while providing shade, often not getting much shade themselves. Of course trees have evolved in very different ways from humans. This article will talk about how trees deal with regular summer heat and the long-term aspects of how climate change will affect trees.

The Days of Summer

Just like humans, trees sweat out water. The process of evapotranspiration is how trees transport water through their roots, trunks, branches and eventually out of their leaves. This process brings nutrients and water to all parts of the tree, and essentially allows the tree to breathe. A large oak tree can transpire up to 40,000 gallons of water per year. This process is extremely useful in stormwater reduction, as it returns the moisture back to the atmosphere, instead of having it run off into our streets and streams.

While this process is very beneficial, it does have its limits. If the ground is very dry, the tree struggles to keep up demands from the leaves for water and nutrients. The first line of defense against high temperatures and moisture loss is for the leaves to close their gas exchange holes, known as stomata. This prevents transpiration in the most exposed leaves, but also prevents growth, hindering the tree in the long term. The second, more drastic way of dealing with drought is to drop leaves, reducing demand. This can be a very costly solution for a tree, because it will also cut off the ability to gain nutrients from those leaves. It rarely happens in Arlington, but all trees, even evergreen trees, have this capacity.

The Catch 22 of Climate Change

There is a great irony in the predictions for climate change. Trees play a large part in sequestering carbon in their roots, soil, and organic leaf litter, but also have to deal with the consequences of climate change.

Trees as Climate Damping

Carbon dioxide in our atmosphere is one of the major drivers of climate change on the earth, and trees and their supporting soil play a large role in abating and slowing down its effects. Through photosynthesis, growing their trunks with the resulting stored energy, and storing carbon in the soil, trees are one of the best solutions to this issue. Along with that, trees shade pavement; reducing evaporation of asphalt, and cool housing; reducing energy costs for heating and cooling. With trees being able to store up to a ton of carbon over a 40 year period, and their associated energy cost reductions, they are one of the greatest tools in the fight against climate change.

Responding to Weather Demand on Trees from Climate Change

Temperature Changes

Unfortunately, trees will also have to live with significantly wilder changes in temperature. The old term of Global warming really confused the issue, as our area has experiences some of the harshest winters in the last two years. This, along with years of record heat in the summer, has really taxed some of our trees. In Arlington, we are in a particularly special place, with trees typically found only in the North and only in the South converging in one place. We have native Eastern white pine (Pinus strobus), Sugar Maple (Acer saccharum) and American linden (Tilia americana) from the North, along with native Sweetbay magnolia (Magnolia virginiana), River Birch (Betula nigra), and Southern Red Oak (Quercus falcata) from the South. The Northern species are adapted to the cold, but will have to deal with higher overall temperatures over time. While some of the latter Southern species will probably be able to adapt to higher temperatures, the extremes in winter temperatures have taken a toll on these species.

We have had to, by recommendation from the Virginia Department of Forestry, stop recommending the planting of one of my favorite trees, the Sugar Maple. While it is not illegal to plant this species, it is not likely to be able to handle the long-term changes in our climate. To replace these trees in our plant palette, we have started to look southward for options. While non-native species may be an option, Arlington County prefers to plant native species, and expanding our definition of what is native to this region may be the best solution to the predicted pressures to our trees.

Increased Severity of Storms

Along with temperature extreme changes, it is also predicted that our storms will increase in frequency and severity. While we have not had major storms in the last two years, this is more of an exception than a rule. Hurricanes, ice storms, and other wind events cause major damage to our trees, and we need to be cognizant of this in our planning. Planting trees in groves, to distribute energy from storms, and ensuring trees are not planted in inappropriate locations, near critical facilities, are some of the preventive measures we can take.  Additionally, providing preventive pruning and hazard removal can be used, but this is more of an extreme solution to a potential problem, and may expose other trees to additional threats. Dealing with storms is a difficult topic to plan for, as it is not a predictable budget item, but we need to be aware of the impact they will have on our trees.

Insect Pests

Finally, changes in climate have caused some serious issues throughout the country. The most glaring example of climate change causing extensive damage from pests is the Mountain Pine Beetle in the Rocky Mountains. This is a native pest to the rocky mountains, which was previously restricted in range, but with climate change, its populations exploded beyond control. This pest left behind a swath of destruction throughout the Rocky Mountains, and is now threatening to invade our area through Canada. Pine is an essential part of much of our urban forest, and this pest, along with other invading pests (such as Hemlock woolly adelgid), could become a major threat.

 

Trees are Working for the Future

Temperature, storms, loss of tree species, and dealing with invasive pests are tough issues to address, but while we may despair at what affects our trees, we also know we are working with the solution. Take care of your trees so they live to an old age, and plant appropriate, climate change-tolerant trees where space allows, and do our part in slowing down climate change.

Read more:

Find out if a species will live with climate change at the Climate Change Tree Atlas: http://www.nrs.fs.fed.us/atlas/tree/
Virginia’s climate vulnerability assessment: http://www.bewildvirginia.org/climate-change/virginias-climate-vulnerability-assessment.pdf
Changing climate, Shifting forests: https://www.americanforests.org/magazine/article/changing-climate-shifting-forests/
Deer; the other threat aggravating climate change: http://blog.nature.org/science/2013/08/22/too-many-deer/