*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