Conclusion

This series of blog is on the topic of temperate forests under climate change. So far, I have made the following points:

1. The clearing of forests and projects designed to extract water for human use account for much of the problem related to water stress in temperate forests;

2. Temperate forests could be benefit from a warmer climate provide they enjoy sufficient water supply;

3. Canopy density and the saturation pressure of air have been identified as key issues affects the water loss in temperate forests;

4. Forests in Mediterranean Climate Zone are forecasted to face more severe water deficits in summers;

5. Climate induced tree mortality by water deficits have been reported across the world’s temperate forests;

6. Biotic agents especially insects are also the important factors contributing to tree death, and climate warming would favour insects pest developments.

In conclusion, temperate forest under climate warming will undergo structure changes, it opens the possibility of challenges as well as opportunities. As the only ecosystem that have expended in size in the 20th century due to human plantings, the strategies to maintain a productive forestry will need to be reviewed an rethought in order to meet the environmental change that could alternate one of the world's most productive biome.

*Insect Pests Dynamic Under Global Warming of Temperate Forests*

For the increase in global tree mortality under climate warming, biotic agents are important causes. Having mentioned the topic in the previous blog, now I will give more details in this post. Biotic agents that can cause tree death include insect pests and fungi pathogens. Insect pests usually can cause more damage than the fungi pathogens because insects are more mobile and more resistant to external environment. They both have high fecundity thus can evolve very fast but insects are especially good at the "trail and error" type of evolutionary adaptation (Hill, 2011). Foresters have already identified insect pests cause as mush as five times the economic impact than forest fire (Logan 2003), then to understand the dynamics of insect pests in temperate forests under global warming is essential for any study on forestry.


The most obvious (and also the most important) change of climate warming on the dynamic of insect pests is the disruption of phonological synchrony. Phenological synchrony is the match in phase of population development of insects, their predators and their nutrients. The nature has set these in a correct timing to ensure both the insects and their predator can enjoy sufficient amount of food to keep their population at carrying level, and without the danger of spoil their long-term prospects. If such pattern is disrupted by a warming climate, what might happened is the insects start to develop earlier that their predators and nutrients. Insects then would consume more nutrients earlier and avoid the risk of predation. This can make the pressure on plants very high (too much being eaten and too slow regenerations).


Another change equally important change is the extended range of activity of insect pests under a warmer climate. Evidences from North American temperate forests suggest that has already became a reality (Parmesan, 2006). Pests attack new land is not a new story. Over the past millenniums there are uncountable alternations of the biomes on the earth's terrains. However, if global warming can intensify the activities of native pests, as well as provoke invasions of new pests, it would make the whole pest dynamics more complex for the forest systems to cope. The contribution factor may also comes form evolutionary changes on the insects’ genetics. Warmer climate creates more favourable conditions for the breeding of insects, this in turn will lead to intensified genetic recombination of insect populations, and eventually could led to greater genetic fitness and make insects pest adapt into their environment more comfortably.


Having discussed the fecundity of insect pests, I will now review the changes on mortality under global warming. Over the past, chilly winter an important mechanism to reduce the pathogens in temperate forests. Severe temperature usually well below the freezing point of water in many forests can kill pests very effectively. Foresters identify this mechanism as “cold-limited mortality”. Global warming might change this, for example, the outbreaks of southern pine beetle have had their range extended northward for about 200km when the minimum temperature have increased by 3.3 degrees Celsius in the USA (Tran, 2007). The prediction for new pests population under warmer climate is the invasion of southern species towards north because of the reduction on cold-limited mortality (the reverse in southern hemisphere of course).


Finally, global warming might “normalize” insect pest outbreak. If the outbreak of insect pests intensified under a warmer climate and become a common trend, then we need to rethink our opinion on whether this should be classified as pest outbreak or not. However, there are self-destructive factors too in intensified insect activities, because an more active pest dynamic will rise the likelihood of forests fires as dead trees can serve as fuel wood. Forest fires of natural causes are good for the health of trees and they can kill pathogens (Parker,2006).

Reference

Dukes J. S. et al., (2009), "Responses of insect pests, pathogens, and invasive plant species to climate change in the forests of northeastern North America: What can we predict?", Canadian Journal of Forest Research, Vol. 39, pp. 231-48.

http://ddr.nal.usda.gov/bitstream/10113/26723/1/IND44171952.pdf

Hill J. K. et al., (2011), "Climate Change and Evolutionary Adaptations at Species' Range Margins", Annual Review of Entomology, Vol. 56, pp. 143-59.

http://www.annualreviews.org/doi/pdf/10.1146/annurev-ento-120709-144746

Logan J. A. et al., (2003), "Assessing the impacts of global warming on forest pest dynamics", Frontiers in Ecology and the Environment, Vol. 1:3, pp. 130-37.

http://www.esajournals.org/doi/abs/10.1890/1540-9295%282003%29001%5B0130%3AATIOGW%5D2.0.CO%3B2?journalCode=fron

Parmesan C., (2006), "Ecological and evolutionary responses to recent climate change.", Annu. Rev. Ecol. Syst., Vol. 37, pp. 637–69.

http://www.annualreviews.org/doi/abs/10.1146/annurev.ecolsys.37.091305.110100

Parker T. J. et al., (2006), "Interactions among fire, insects and pathogens in coniferous forests of the interior western United States and Canada", Agriculture and Forest Entomology, Vol. 8, Iss. 3, pp. 167-89.

http://www.sustainablenorthwest.org/bmfp/parker__clancy__mathiassen.pdf

Tran J. K. et al., (2007), "Impact of minimum winter temperature on the population dynamics of Dendroctonus frontalis", Ecol. Appl., Vol. 17, pp. 882-99.

http://www.esajournals.org/doi/abs/10.1890/06-0512

*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

*The Important Issues of Water Stress*

As my previous blog suggested, the net productivity of temperate forests may increase due to global warming, if there is enough water to facilitate forests growth. I had also addressed the abundance of water would be the limiting factor, in this blog, I will address some important issues of water stress in temperate forests.

There are mainly tow types of water inputs in temperate forests biomes. One is snowfall in winters, which will melt when the weather gets warmer in spring. The other is the rainfall, which mostly occur over summers for the majority of temperate forests. For both types of inputs, the height and the density of forest canopy can have a strong role to play in keeping such moisture contents.

For the winter snowfalls, denser canopies can reduce the energy transmitted by the sun on the snow accumulated on the ground. Therefore the snow will melt slowly and provide steady moisture for soil instead to become surface runoff. In summers, denser canopies with several layers can effectively reduce water loss by making diurnal evaporation. Also, the difference between canopies can produce water cycles and this in turn will affect the patterns of rainfall (Gielen, 2010).

In general, natural old grown forests that have multiple layers of canopy have higher ability to lock up moisture than planted forests by men. The biomass density is therefore linked with the issue of water stress. When consider the biomass density together with the total biomass, we can get a good idea about the capability of a forest to lock it water.

Apart from the microclimate influenced by forest canopies, the global climate shift is the other factor that affects the issue of water stress. On one hand, warmer air could hold more moisture and in theory this could less to less but heavier rains. On the other hand, a study shows because the stomata of plants opens less widely under doubled concentration of CO2, there are also predictions on increase efficiency of water use by plants (Betts, 2007).

On the human side, the over-exploration of forest resource and the fragmentation of forest are among the worst things that men can do to make forest ecosystems to loss its ability to contain its water.

Reference


Betts R. et al., (2007), "Projected increase in continental runoff due to plant responses to increasing carbon dioxide", Nature, Vol. 448, pp. 1037–41.
http://www.nature.com/nature/journal/v448/n7157/full/nature06045.html


Gielen B. et al., (2010), "Decadal water balance of a temperate Scots pine forest (Pinus sylvestris L.) based on measurements and modelling", Biogeosciences, Vol. 7, pp. 1247–61.
http://www.biogeosciences.net/7/1247/2010/bg-7-1247-2010.pdf