In a dramatic response to global warming, high altitude forests in five continents of the world have been "browning" since the 1990s. They have been steadily losing foliage and showing lesser photosynthetic activity.
When satellites measure sunlight that has bounced off the surface of the earth, different surfaces have different reflection patterns. Areas with lush foliage where photosynthetic rates are high appear green, and areas with less foliage look brown. There is a formula to express the "greenness" (indicating high photosynthetic rate) or "brownness" (indicating decreased photosynthetic rate) of an area, called Normalised Difference Vegetation Index (NDVI).
Jagdish Krishnaswamy from the Ashoka Trust for Research in Ecology and the Environment, Bangalore; Robert John from the Indian Institute of Science Education and Research, Kolkata, and Shijo Joseph from Center for International Forestry Research at Indonesia, used NDVI to study changes in tropical mountain vegetation across 25 years, from 1982 to 20061. The researchers concentrated on protected areas found between 1000 and 6000 metres above sea level, to minimise effects of human activity on photosynthesis patterns. They came up with 47 such areas across five continents, covering more than 50,000 square kilometres — all located in the world's biodiversity hotspots.
They analysed how photosynthetic rate responded to two aspects of climate change — rainfall and temperature. Greening, or an increase in photosynthetic rate, is caused by increase in temperature and/or increase in precipitation. Browning is caused by increased temperature, but decreased precipitation, that leads to a drop in photosynthetic rate.
A global shift in the rate of photosynthesis
Until the mid 1990s, the NDVI increased, indicating a "greening" across areas. Then came an abrupt shift from "greening" to "browing" across all 47 protected areas. "The browning was with respect to the maximum greenness attained in each year — so it was a decline in the maximum photosynthetically active leaf biomass attained in the entire year," Krishnaswamy, the first author of the paper, told Nature India . "Something was affecting the ability of tropical mountain vegetation to sustain the same canopy biomass in the early 1990s".
While there was an increase in temperature in these areas, there were no clear trends in rainfall. The authors say this causes "moisture stress" — an increase in temperature not accompanied by a concomitant increase in moisture. This usually causes a drop in photosynthetic rate, or "browning".
The exact timing of browning varied between regions, but was within a 7-year period in the 1990s for all of them. Anping Chen from the University of Princeton, who was not involved with the study, is not surprised by the synchronous shift. "Continuously rising temperature is usually regarded as the main reason for such a trend shift. In early years, warming enhanced vegetation growth. Yet, continuous warming without an accompanied increase in precipitation in late years makes water a major limiting factor for vegetation growth, and thus the greening trend was stalled or reversed."
Across all regions, the trends in NDVI tracked elevation — there was lesser browning at higher elevations. The authors hypothesise that as temperatures increased, moisture was probably not a limiting factor in most of these high elevation areas. This removed restrictions placed on photosynthesis by low temperatures, and the degree of browning was lesser. "Even though we did not see a clear increase in greening with elevation, we are seeing a decrease in browning, which fits our hypothesis", said Krishnaswamy.
From the study, it also became clear that not just the two variables —temperature and rainfall — were affecting patterns in NDVI. There is a clear indication that other climatic or nonclimatic drivers of vegetation change in tropical mountains. "The role of processes such as reactive nitrogen deposition, CO2 fertilization, and forest regrowth are known from several studies, even from lowland tropics, but their importance in tropical mountains has not been evaluated," they write.
More research is necessary to understand these complex processes, but studies have shown that satellite data is valuable in getting long-term data and testing hypotheses. With temperature being a crucial factor affecting climate change, understanding its effects on photosynthesis is of importance.
