*Heat-Induced Tree Mortality as A Global Phenomenon*

In previous posts, I have pointed out the climatically-induced water deficit is the primary cause of tree mortality in the Mediterranean Climate Zone of Sierra Nevada. In this post, I will exam the prevalence of this phenomenon on global scale. I will also discuss the mechanism of excessive heat in making trees more vulnerable and what type of human influence can deteriorate the environment of forest systems.


Is heat-induced tree death a global phenomenon? Yes, according to researches that attempted to review the causes of tree death in different regions of the world. There are problem of heat-induced tree death in virtually every continents, suggests global temperature rising is alternating the earth's forest structure. The map below indicated the primary factors limit the net productivity of forest biomes in different regions. From the map, we can see most of the temperate forest is limited by temperature and water stress in North America and Asia. For Europe, the primary limiting factor is sunlight due to the fact that Europe is more humid than those two continent. However, since the most productive and  temperate forests are in American and Asian continent, it can be conclude that the excessive heat should be rank as the most important challenge for the future health of temperate forests.

Under a warmer environment, the growing season is lengthened and the decomposing rate will be speeded up. The downside is the seasonal draught may become more frequent and the competition between trees for resource might become more intense. Under excessive heats, trees may choose to reduce its rate of transpiration to prevent further water lost (isohydry). This would led to a reduction in photosynthesis rate to produce carbohydrate for trees to consume, and if the draught is longer than the trees used to adapt, the carbohydrate store inside those tree will run out as respiration still have energy cost, this situation is call carbon starvation. This is one way that excessive heats can cause tree mortality.


Another scenario is described as anisohydry strategy, which means trees to continue transpiration at about normal rate to store carbon inside tree bodies. Such mechanism is ate risk of putting plants into cavitation, which the air disolved in liquid releases and damage the xylem of trees. Under lengthen draught, the water availability decrease rapidly and the tree have difficulties to inject solutes into damaged cell to repair it.


Apart from carbon starvation and cavitation, the hydraulic deficit can limit the metabolic rate of trees, make them less able to produce resins and other chemicals that could be use to defend against biotic attacks, especially insects. A warmer climate can create favourable conditions for insects to boost its population and eventually turn into pests. Pest problem is regarded as the most serious challenge for temperate forests because pest can have larger active area than water deficits, and it is more mobile. A warmer climate may also create favourable conditions for the introduction of exotic species that alters the native composition of genetic diversity and promote competition for water, nutrients and space. In areas that winters usually cold, the raise in temperature would promote tree respirations and cause the deficits of stored carbon for overcome summer draught.




Reference


Allen C. D. et al., (2010), "A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests", Forest Ecology and Management, Vol. 259 pp. 660–84.
http://www.mesc.usgs.gov/Products/Publications/22509a/22509a.pdf


Wilson K. et al., (2005), "A vulnerability analysis of the temperate forests of south central Chile", Biological Conservation, Vol. 122 pp. 9–21.
http://www.sciencedirect.com/science/article/pii/S0006320704002654

Videos

An example of the negative effects of commercial logging.

http://www.dailymotion.com/video/xeqzgh_effects-of-commercial-logging-in-ch_tech#rel-page-10


Clearcutting's threats.

http://www.youtube.com/watch?v=rgu99-lFTIA


Beautiful Temperate Forest Biome.

http://www.youtube.com/watch?v=nVe8jF9jEeE

*Introduction to Temperate Forest*

What is temperate forest?

Temperate forest is forest that in temperate climate zone, it is also the home of some of the world’s largest living organisms, noticeably giant trees that could reach over 100 meters in height. Tree species in temperate forests can either be coniferous (needle-leaf) or deciduous (drop leaves every year). In some parts of the world, it is common to find mixed temperate forest that has both broadleaf and needle-leaf species. The word “temperate” suggests the climate condition of where this type of forest grown is not extreme (when compare with the most extreme climate). However, in some of the temperate forests, especially for those in the high latitude areas, winter can last over 5 months and barely have more than 180 days that are frost-free.

The Temperature-Precipitation Graph of a City (43°52′N 126°33′E) in Temperate Forest Zone

The deciduous species prevail in areas with relatively mild winters. The coniferous species are more common in areas with more extreme winters. The most humid period of the year for most of the world's temperate forest are summers, and the precipitation ranges between 650mm to over 3000mm. For forests that receive annual rain fall over 1400mm, we describe them as "temperate rainforest".

Where are they?

Temperate forests exist between 30°and 55°latitude. It covers much of the areas of Eastern Asian, North American and European continents of the latitude zone. The majority of the temperature forest biomes now lies between 40°and 50°latitude. There are much more temperate forest in the northern hemisphere than the south. 

Temperate Forest (Green) in the World.

How does it different from other forest type ?


When compare with tropical forest, temperate forest is easily distinguished by it four-seasons character. Tropical forest usually just have rainy season and dry season. However, temperate forest in some ways are very similar to its close neighbour - boreal forest. They both can experience severe winters with heavy snows. The boundaries between boreal and temperate forests are always subject to debate. In general, boreal forest is charactered by the dominance of needle-leaf trees, and its usually shorter in heights. Unlike temperate forests that are distributed around the world, there are only two major boreal forest biomes that existed on Eurasia and North America continents.

The distribution of Boreal Forests.

Reference

Alaback P.B. (1991), "Comparative ecology of temperate rainforests of the Americas along analogous climatic gradients.: Rev. Chil. Hist. Nat. 64: 399–412.
http://www.mendeley.com/research/comparative-ecology-of-temperate-rainforests-of-the-america-along-analogous-climatic-gradients/

Molles M. C. Jr., (2010), "Ecology: Concepts and Applications", New York: McGraw-Hill, Chap. 2.

*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