Climate and Plant Production

In 1998, Gallina Churkina, then in Steve Running’s research group at the university of Montana, published an important paper that documented the spatial variability of the role of climate as a terrestrial productivity. Net primary production (NPP) cannot be measured directly since it is a carbon flux (photosynthesis minus respiration) and model simulations often provide the closest estimate. Churkina and Running used the model BIOME-BGC to simulate global NPP and defined the importance of temperature, water, and radiation as limiting factors using three different curves. To each grid cell of the world map they produced were associated 3 values based on the degree of limitation by annual mean temperature, water balance coefficient, and the percentage of sunshine hours per year. We have reproduced in Data Basin the three layers that were used to produce the final results (click here). Each layer corresponds to the degree of importance that temperature, water or radiation exerts on production. Results showed that climate controls NPP in most vegetation biomes but also that in certain areas something else than climate e.g. nutrient availability, salinity, … was the limiting factor.

 

In terms of climate change, the authors remarked that new conditions would likely alter the degree of importance of the environmental controls of NPP. For example, warmer temperatures would enhance evaporative demand and decrease water availability. Water limitation could then become a new limiting factor in regions where it was not limiting before. Elevated nitrogen deposition could also alleviate nitrogen limitations and stimulate production that would increase water demand and exacerbate water limitation.

 

More recently, Running’s group published another paper using satellite imagery that showed a general increase in global productivity of terrestrial systems that was interpreted as the result of a warming trend (Nemani et al. 2003), a relaxing of the temperature limitation. In 2010, at the end of the warmest decade on record (“since instrumental measurements of temperatures began in the 1880s”), and Running are now showing a decrease in global productivity (click here) that they attribute to severe regional droughts particularly in the southern hemisphere. One is reminded of the “green up – brown down” hypothesis formulated years ago, whereby an environmental threshold of temperature and water availability is passed below which plants can green up and store carbon in a warm and moist world but above which hot and dry conditions cause carbon losses through reduced growth and increased mortality associated with fires and pest outbreaks causing diebacks and “brown” forests. Comparing Churkina’s original map of water limitation with Zhao’s map of reduction in NPP for the last decade, one can identify areas where water limitation was already important in the 20th century but also others (Indonesia) where it was not. We can then focus on those areas and try to understand what has caused the decline in production. Drought conditions, pollution, land use change, overuse of resources? Our ability to elucidate the causes of change will help us better prepare for the challenges ahead of us.

 

It is important to recognize that our world is constantly changing and that we have an amazing opportunity to document the changes. Comparing maps produced to synthesize current understanding of ecological processes from different periods enables us to discuss change. This is what we have tried to do here. We can also compare observed biological changes with projections of climate that general circulation models have been producing for the last 30 years and discuss the trends in climate that were projected and the hypotheses that had been formulated then.

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