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Researchers study effects of water stress on forests over 20 year period

Published: 22 March 2018
Forest trees

A team of biologists from the University of Florida, working with colleagues from the University of Southampton, Princeton University and the Estonian University of Life Sciences have analysed forest inventories of trees in the eastern United States from the 1980s to 2000s. The team looked specifically at forest biomass, tree species composition, and climate variability.

Writing in the journal Nature, the researchers reveal that in some parts of the eastern United States, water deficits have led to a decline in forest biomass, causing an influx of trees that are more tolerant to drought but slower-growing.

This shift in forest species composition affects the capacity of forest biomass (the mass of living trees) to store carbon. Healthy forests play a key role in global ecosystems as they contain much of the terrestrial biodiversity on the planet and sequester atmospheric carbon. Just as climate change affects the forests, so do the forests affect climate.

Water stress can be caused by rising temperatures (which lead to increased evaporation and plant water-demand), decreases in rainfall, or a combination of the two. To study changes in soil moisture, the researchers used the Palmer drought severity index to examine average water availability and loss over the study period.

Justin Sheffield, Professor of Hydrology and Remote Sensing in Geography and Environment at the University of Southampton, and contributor to the research, said: “Increased drought severity is one of the expected outcomes of climate change in many regions of the world, but how this will impact on forests is not well understood. This study provides observational evidence of large-scale impacts and more importantly shows that these impacts are more complex than previously thought.”

“Although climate change has been less dramatic in the eastern US compared to some other regions, such as Alaska and the south-western US, we were interested to see if there were signals in forest inventory data that might indicate climate-induced changes in eastern US forests over the last few decades,” said Jeremy Lichstein, senior author and University of Florida assistant professor of biology. “The changes we documented are in some sense subtle, which is probably why no one had previously noticed them. Without a long-term dataset comprised of millions of data points, we probably could not have detected these changes.”

Forests are affected by other human activities such as farming or logging, and many are in a stage of ecological succession with lower biomass than mature forests. This history of disturbance made the researchers’ analysis challenging. To solve this, researchers compared forests on the basis of their age.

“We compared forests in the 1980s of a given age (for example, an 80-year-old forest) to forests of the same age in the 2000s,” said 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 US 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.”

Drought-tolerant tree species tend to allocate more carbon to fine roots and less to their leaves and woody parts that would sequester more carbon. Lichstein said that although they expected an increase in drought-tolerant species to prevent biomass losses triggered by water deficits, the opposite appears to be true.

“Functional shifts amplified the effects of climate by making forest biomass more responsive to drying or wetting,” he said. “In hindsight, this makes sense, because drought-tolerant species tend to be slow growing. So, if drought causes a shift towards more drought-tolerant species, biomass will decline compared to forests dominated by fast-growing, drought-intolerant species.”

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,” said Lichstein. “We hope that our findings will stimulate further research into relationships between species composition, ecosystem function, and climate variability.”

 

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