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.
Figure 1: The water cycle. Source: Salinity Management Guide, USGS
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.
Figure 2: Sycamores struggling under drought conditions.
Source: Wikipedia
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.
Figure 3: Sugar Maple, a tree not likely to survive in the long term in Arlington. Source: Wikipedia
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
Figure 4: Dieback caused by mountain pine beetle. Source: Simon Fraser University
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/