A study site of an ever-wet tropical rainforest. Photos courtesy of Chris Smith-Martin.
Earth’s climate is entering a new era, with increasingly severe temperature extremes, floods, droughts and more. Can living things survive in the face of such dramatic change?
Here’s some good news: Research led by Chris Smith-Martin, assistant professor of plant and microbial biology at the University of Minnesota, suggests that tropical tree species may have more ability to thrive in the face of drought than previously thought.
The research, published August 12 in Nature, looked at physiological traits related to drought resistance in 18 tree species across the island of Puerto Rico, from the dry southern coast to lush inland forests. It found that water-storing and damage-minimizing traits vary within species to a greater degree than was anticipated based on research in other climates.
“In the wetter forest, species tended to have more drought avoidance traits, such as storing water in tissues, specifically in stems and in leaves, whereas in the drier side species tended to be more drought tolerant with high resistance to air blockages forming in their xylem,” Smith-Martin says. “The most exciting finding from this study is that within the same species, individuals in the drier sites tended to be overall more drought resistant than individuals of the same species in the wetter sites. In the face of climate change, this is potentially good news, because they can adjust their drought resistance across the rainfall gradient.”
It’s well known that different tree species differ in drought resistance. And previous studies in temperate and Mediterranean ecosystems have found only a narrow range in drought resistance within species. But until now, we knew relatively little about intraspecific (within species) variation in tropical trees’ drought-surviving traits — particularly important because of the enormous role tropical forests play in storing carbon and so mitigating climate change.
To extend that knowledge, Smith-Martin and colleagues looked at how drought resistance traits of each of the 18 species studied varied across the island’s wide rainfall gradient. All told, the researchers collected data on 11 water-related physiological traits from 290 individual trees. Specific characteristics studied included how readily trees can resist the formation of air blockages, or emboli, in their xylem conduits which block the movement of water from roots to leaves, and how much water they can store in their tissues to help them from drying out.
As expected, some species were more drought resistant than others. The surprise came in assessing drought resistance traits — both drought tolerance and drought avoidance — within species. The researchers found that this intraspecific variation was much higher than they had expected.
“For a species with more intraspecific variation, the expectation is that they will probably do better under future climate scenarios,” Smith-Martin says. “Also, they may be able to maintain their distribution ranges better, whereas species with less of this variation, if there are changes in rainfall patterns, they may start reducing their distribution range because they just don't have this flexibility in drought resistance across gradients.”
In addition to potentially providing reassurance about the ability of tropical forests to adapt to changing conditions, the research offers valuable insights into how forests might respond under various scenarios.
“This is definitely important for informing ecosystem demography modeling under future climate scenarios,” Smith-Martin says. –Mary Hoff