Friday, February 1, 2013

Forests and Climate Change

Forests are critical to the planet; they store carbon, cycle nutrients and water, increase biodiversity, and protect topsoil. Those are just a few of the myriad of eco-services that forests provide to the planet, and when preserved, increase forest resilience. The idea of resilience, an ill-defined, non-scientific term, is incredibly important to forests, as they are under attack from two major anthropogenic factors: deforestation from land-use conversion and climate change. This post will focus on the effects forests may face from a changing climate.

I am not going to debate whether or not humans are altering the climate on the planet; we are. I will offer a concise perspective on atmospheric CO2. We have great data on atmospheric CO2 for the last 400,000 years from glacial ice cores. This data shows that we experienced fluctuations in atmospheric CO2 every 50,000-100,000 years with peaks of 290ppm, and troughs of 190ppm. About 20,000 years ago the planet began warming again. The previous data shows that we could expect a peak CO2 ~290-300ppm, however NASA shows current CO2 at 394ppm, an anomalously high peak. A previously reliable trend is now significantly altered.

How does this affect the forest? In a very complicated way. Climate change will affect plant's productivity as they rely on CO2 for photosynthesis. So, given enough water and nutrition, increased CO2 could increase the productivity of forests on fertile soils. This assertion has been observed in some scientific experiments using CO2-enriched growth chambers. However this is much more difficult to observe in the field because of many complex and poorly understood environmental interactions. One of these interactions that we do understand is that atmospheric ozone (O3) decreases productivity, canceling out the gains of added CO2.

This uncertainty is confounded when indirect effects of climate change are considered. If we experience more punctuated weather patterns, we will see more flooding, and more droughts, on an increasingly unpredictable schedule. The water stress of drought and water logging from flooding will decrease productivity, and increase the occurrence of large disturbance events. Natural disturbance (disturbance rates that ecosystems evolved with over the last 400,000 years) events are good for forests, they begin ecosystem succession, and restore nutrients to the soil. Succession is the process that a piece of land goes through when converting to a forest. But if droughts, floods, hurricanes, wild fires, insect outbreaks and other events all increase in occurrence (as predicted), forest carbon stores will decrease by constantly restarting succession. Much in the way a timber forest plantation will never store as much carbon as a natural forest if it is constantly harvested (disturbance event) and replanted (natural succession).

Another important impact of climate change that is already being observed is the effect of increased temperatures. Vegetation zones are most generally described by minimum temperatures of a given area. When the minimum temperature starts increasing (even slightly) plants from warmer regions begin moving into that area, and outcompete the native vegetation. This “migration” of plants, moving north or south based on temperature minimum means occurs naturally. However under anthropogenic-induced climate change, rapid increases in temperature (compared to natural rates of increase) may move vegetative zones more quickly than long-lived trees can migrate. This can cause the extirpation or even extinction of a species if it gets “jumped” by a more short-lived species, or one with a more advantageous reproductive strategy.

When discussing climate change, it is important to note the uncertainties. Scientists agree the climate is changing, and that humans have a significant role in it, but to exactly what extent, and to what end is still uncertain. There are many feedbacks that have the potential for sudden extreme changes; such as an altering of ocean circulation, precipitation patterns, and changes in carbon cycling. If the warming oceans result in a new circulation pattern, that cools European waters instead of warming them, dramatic ecological changes would ensue. There are numerous theories and ideas about the feedback loops and how they may change, and they highlight the uncertainty of predicting climate change.

It is important to increase public awareness about climate, how it impacts our lives, and what we may see in the future. If we could begin valuing an adaptable, resilient forest, we would be taking one positive step toward mitigating climate change. We need forests for natural resources, and eco-services; so we should start managing them in a way that promotes both, not just the former.

Sources and Further Reading
-J., King, Pregitzer K., Zak D., Sober J., Isebrands J., Dickson R., Hendrey G., and Karnosky D. "Fine-root Biomass and Fluxes of Soil Carbon in Young Stands of Paper Birch and Trembling Aspen as Affected by Elevated Atmospheric CO 2 and Tropospheric O 3."Oecologia 128.2 (2001): 237-50. Print.
-Special thanks to NASA's new comprehensive climate site

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