For the first time, researchers have studied the effects of water stress on forests over a 20-year period. A team of biologists from the University of Florida has conducted systemic forest inventories of trees in the eastern United States from the 1980s to the 2000s. They looked specifically at forest biomass, tree species composition, and climate variability. The researchers found that decades of changes in water deficit have had a direct effect on forest biomass, causing an influx of trees that are more tolerant to drought but slower-growing. This shift results in significant changes in forest species composition with their accompanying ecological effects and, moreover, affects the capacity of the forest biomass (the mass of living trees) for carbon sequestration. Healthy forests play a key role in global ecosystems as they contain much of the terrestrial biodiversity on the planet and act as a net sink for capturing atmospheric carbon.
Heat-induced water stress, an effect of hydrological drought, is caused by rising temperatures that, in turn, cause precipitation to fall as rain rather than snow in colder regions, as well as increased evaporation and transpiration. The researchers measured moisture levels in the soil through the Palmer drought severity index to examine average water availability and loss over each season of the study period. “Although climate change has been less dramatic in the eastern U.S. compared to some other regions, such as Alaska and the southwestern U.S., we were interested to see if there were signals in forest inventory data collected by the U.S. Forest Service that might indicate climate-induced changes in eastern U.S. forests over the last few decades,” says Jeremy Lichstein, senior author and UF assistant professor of biology.
“We compared forests in the 1980s of a given age to forests of the same age in the 2000s,” says Lichstein. “In areas where the climate got wetter, our analysis showed increases in biomass over the two decades, whereas in the areas that got drier, there were decreases in biomass. When we look at the eastern U.S. as a whole, there was an overall trend towards a drier climate from the 1980s to the 2000s, and therefore the overall effect of climate over the two decades was to reduce forest biomass.”
Overall, the study shows that forest biomass and tree species composition and their combined impact on carbon storage are affected by climatic variability on a sensitive and short timeline — just a few decades. “It is premature to say whether or not the amplification effect that we documented is a widespread phenomenon,” says Lichstein. “We hope that our findings will stimulate further research into relationships between species composition, ecosystem function, and climate variability.”