*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

*Could Temperate Forest Benefits From Climate Change? Yes, If There Is Enough Water.*

Temperate forests are the Biomes with heaviest settlements during the entire life history of humans, and human activities are the main cause of the transformation on the earth’s surface. The current trends of Global Warming will affect nearly all aspect of global biomes, including the productivities of forests. It has long been suggested that a warmer climate can made forests more productive, and since temperate forest is the only biome that expended during the 20^th century, many believe the net productivity of temperate forests will be increased. In this blog, I will review this idea. Because it is difficult to draw conclusion on forestry over a short term, since forest grow relatively slowly, so I will review the predictions made in the past on “today’s situation”, if there are modern literature agree with those points made in the past, then we would know those models are valid and could use it to suggest future forest dynamics.

John Pastor and Wilfred Post argued that if the temperature would rise 2-4 degree Celsius above normal temperature (1988 prediction), the temperate forests might benefit from such are rise and gain an increase in net productivities. This is primarily due to the rise in temperature is due to enhanced CO2 concentration which would also increase the stomaltal activity of leafs. The rate of photosynthesis and general water use efficiency will increase as a result of that. In some species, the rate of water use efficiency would double. The woody plants then could absorb more carbon and growing at a faster rate. However, if species that are low in carbon storage becomes more common in northern hemisphere, such as oaks and pine, the net carbon storage would be lower than a model based on current species distribution. Enhanced CO2 concentration would also mean a faster decomposing rate and faster nutrient cycle.

The limiting factor is water. If the water balance is in the right phase, then the positive effects will be amplified and negatives are constrained. Up to date researches shows the predictions made in the past so far largely valid. The continue monitoring of Amazon rain forest showed that although the total area decreased due to human activity, the ability of trees to absorb carbon have increased (the Economist, 2010). This has made the process of deforestation of Amazon rain forest have not net effect on the contribution to greenhouse gases. Furthermore, other temperate forest biomes have also observed increases in productivity, such as northeastern Chinese temperate forests.

However, the increase in temperature will be much more than what was predicted in 1980s. The 4 degree Celsius target is deemed to be missed. Warmer air means it would be less likely to rain, and there are huge uncertainties about the possibility of future hydrological cycles under climate change. It is possible the massive draught caused by climate cannot compensate the improvement in water efficiency of plants.

To sum up, climate change will benefit temperate and boreal forests in areas where soil moisture is stable, but we still need to address the issue of water stress that some major forest biomes could face.

Reference:

Pastor J. & Post W. M., (1988), "Response of northern forests to CO2-induced climate change", Nature, Vol. 334.
http://www.nature.com/nature/journal/v334/n6177/abs/334055a0.html

The economist:  http://www.economist.com/node/17093495

*Water Security and Temperature Biomes*

Temperate forest climate zones are the hosts of the Eurasian Civilizations, which are the most prevalent human life history stories on Earth. The Studies of temperate forest and its current dynamic under climate change is important as it can reflect the reactions of the world most relied ecosystems. For a healthy temperate forest, water is essential. If a temperate forest ecosystem is experiencing water scarcity, then the question of water security should be examined to assess the healthiness of the ecosystem. According to popular definition, water security is the capacity of a population to have access to potable water (wikipedia http://en.wikipedia.org/wiki/Water_security). This definition not only apply to human populations, but also applicable to other species too. When considering the issue of water security, we not only need to pay attention to the flowing water in liquid form, but also we need to consider visual water such as food, plant and other water containing substance that we are utilizing from the ecosystems.

On its own, agriculture is the largest consumer of water, it is estimated to make up about 69% of global water consumptions (WBCSD). It is also the dominate cause of deforestation of temperate forest. Over the past centuries, most of the temperate forest biomes in Europe, Asia, and America have experienced land clearing for framing. While the clearing of forests clearly reduced the biodiversity and total biomass, it also provides challenges for the water security for both humans and ecosystems. Agriculture contributes staple crops, and it then can be turn into other forms of proteins, none of these processes are not water intensive. The concern of conservationists on the transformation of temperate forests into mono-cropping fields is primarily associated with water security for humans and biomes (Vorosmatry, 2010). The reduction of temperature forest biomes will make the crisis of water even more severe.

For humans, technological improvement in water purification methods and better pipe network would alleviate some pressure, however, this does not generate any water for ecosystems. Furthermore, water projects like dams and reservoir are the recently just became major cause of deforestation, the long term effects of major engineering projects are being questioned by scholars. In some case badly designed and managed artificial water storage structures have very strong links to the deterioration of water quality and degradation of forest biomes. Pollutions from agricultural fertilizer and chemicals have no less influence when comparing with projects listed above.

Reference


WBCSD, (2009), Facts and Trends: Water, Vision 2.
http://www.unwater.org/downloads/Water_facts_and_trends.pdf


Vorosmatry et. al., (2010), "Global threats to human water security and river biodiversity", Nature, Vol 467.
http://www.nature.com/nature/journal/v467/n7315/full/nature09440.html

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