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
没有评论:
发表评论