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Decrease in global average productivity driven by temperature-induced moisture stress

High-altitude forests indicates the amount of carbon that trees and plants absorb is not rising due to climate change.
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What effect is climate change truly having on forests?
What effect is climate change truly having on forests?

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On a recent walk through the forest, my thoughts wandered to the stories we can read from the trees. I gazed upon trees that appeared as though they had lost their vitality — as though they had no blood in their veins — and suddenly my thoughts turned to a recent analysis on forest monitoring using satellite data. What effect is climate change truly having on forests — and could my research have something to say about it?

Together with two colleagues from India, Jagdish Krishnaswamy of the Ashoka Trust for Research in Ecology and the Environment, and Robert John of the Indian Institute of Science Education and Research, I have been working to assess the impact of climate change on global primary production — that is, the rate of fixation of carbon through the process of photosynthesis. Existing science says that global average productivity is on the rise due to climate change. However, most of these studies were carried out at the global scale without separating out drivers of local land-use change, such as expansion of agricultural areas. Therefore, an increase in global primary production attributed solely to climate change was not logical to us. So the first question we faced was how to separate out the impacts of local land-use drivers from the global environmental-change drivers.

Figure 1.

Figure 1. Trends in NDVI greenness (a proxy of primary productivity) of mountain vegetation over the period 1982–2006 for five continental regions located in pantropical latitudes.

We realized that Protected Areas (PAs) are the only the places where the biotic pressure is kept low and so represented natural experimental sites to test the impact of climate change. Within PAs, we selected only National Parks with the highest protection status. We also decided to bring additional criteria such as higher elevation — National Parks at least 3,000 meters in elevation (where temperature is a constraint in photosynthetic activity) — and to omit areas within National Parks less than 1,000 meters in elevation (to minimize effects of biotic pressure and fire).

As the experiment progressed, we were surprised to see that primary productivity curves went down after the mid-1990s in all five regions we studied (South America, Central America, Africa, South Asia and Southeast Asia) (Fig 1). The results contradicted earlier findings that productivity is increasing due to climate change.

Figure 2.

Figure 2. The greening along elevation gradient over time in five continental regions in the pantropical latitudes.

What might be happening? We decided to study the trajectories of the temperature and precipitation trends during the same time period. We observed a consistent increase in temperature with no clear trend in precipitation. The increase in temperature — unaccompanied by an increase in moisture — might have caused the reduction in the rate of photosynthesis.

If ‘temperature-induced moisture stress’ is a limiting factor in photosynthesis, it should not have an impact on moisture abundance at higher elevations. We tested this hypothesis by measuring the rate of photosynthesis along the elevation gradient. We found that the rate of photosynthesis is increasing along the elevation gradient over time (i.e., greening along the elevation gradient), indicating that a warming climate removes temperature constraints in the photosynthetic process in moisture-abundant higher-elevation areas (Fig. 2).

We continued our research to extract the residual trends after accounting for the effects of temperature and precipitation changes. We found a significant residual trend indicating the influence of other climatic and non-climatic factors such as nutrient deposition, CO2 fertilization and spread of aerosols on global primary production.

This is big news! As a colleague told me: “There is so much ecological plant literature based on small-scale and lab manipulations predicting increases in productivity: This may well be the case in controlled experiments, but your results show that they fail to capture the complexity of real-world responses to climate change.”

Tony Joseph, a researcher based at the University of Canterbury, wrote to me about a recent study of his that found that the temperature effect on carbon exchange is regulated by soil water content. “Soil water deficit induces an inhibition in the photosynthetic capacity of the leaves due to stomatal limitation, and reduction in carboxylation efficiency and RuBP regeneration capacity,” he wrote.

PC Abhilash, a professor at Banaras Hindu University, wrote: “Although it has been perceived that a warming climate — particularly increasing carbon dioxide — will enhance the productivity up to a certain extent, the current study proved that there are also other tipping elements which would be considered while predicting the productivity of forest systems under changing global climate.”

In the course of my work, I think about how the changes we have caused to climate feedback mechanisms will affect our existence on this planet. How long can we continue on this path?

For more information about this topic, please contact Shijo Joseph at s.joseph@cgiar.org.

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