*Climatically Induced Tree Mortality*

In the previous blog, I talked about the important issue of water stress in the world’s temperate forests, and I have pointed the predictions of double CO2 concentration would raise the net productivity and the decomposing rate if there are sufficient water. While this is true, the climate change in some case would also cause draught, especially in Mediterranean climate zone. The climate change caused draughts would increase the level of competition of resource, in particular water, and in such situations, tree mortality is anticipated to rise.

Mediterranean climate zone can be found in southern Europe and the California State and parts of Australia. The weather of the zone is charactered by hot and dry summers and wet winters. Most of the precipitation of temperate forests in the Mediterranean zone is in the form of snowfall during winters, and in summer forests receive less rain when comparing with most of the other temperate forests. Because trees stop growing during winters, they therefore cannot benefit from the most water-abundant time of the year. In summers, wind that blow through the region comes from tropic continent, usually of higher temperature and have less water contents (Barry, 2010).

Summers are the most robust season for tree growth, hot and dry wind that promotes evapotranspiration would take water out from soils and cause water stress for trees.

Medium scale researches shows forest have greater evapotranspiration rate than crop fields (Bona, 2008) in the Eastern part of USA.  Because forests have lower albedo and this reflect less solar energy into the sky. Therefore forested areas should have higher temperature than crop fields. A study over 22 years period on an old grown temperate forest in Sierra Nevada in California have yield some insightful results about the future of temperate forests under global warming, especially in regions similar to Mediterranean Zone.


First, the increase in tree mortality is positively related to temperature driven water deficit. Tree death in dryer years is more common than wet years because of stronger evaporative stress. Therefore, higher tree mortality is predicted under a warmer climate that could lower regional summer precipitation. The current increase in mortality is estimated about 3% annually. Secondly, the increase in mortality is a common trend across all species, including both shade-tolerant and shade-intolerant trees. This suggests climate change in this region does not favour a particular type of tree, and it can only explained by a stress factor is of fundamental importance to all vegetation. Thirdly, the increase in mortality has predominately in small trees. Small trees have less developed root systems, their ability to extract soil moisture is lower than large trees and are more vulnerable to water stress.

The research also shower the recruitment rate over the 22 years period increased little, and the trees in higher altitude enjoy lower mortality than trees in lower altitude. The colder temperature that slowdown snowmelt in summers could reduced the water stress of forest on mountain slopes with higher altitudes.

Reference


Barry R. G. & Chorley R. J., (2010), "Atmosphere, Weather and Climate", London: Routledge, pp.287-90.


Bonan G. B., (2008), "Forests and Climate Change: Forcings, Feedbacks, and the Climate Benefits of Forests", Science, Vol. 320.
http://www.sciencemag.org/content/320/5882/1444.full


van Mantgem P. J. & Stephenson N. L., (2007), "Apparent climatically induced increase of tree mortality rates in a temperate forest", Ecology Letters, Vol. 10, pp.909–16.
http://onlinelibrary.wiley.com/doi/10.1111/j.1461-0248.2007.01080.x/full