Forests are a critical cog in the global water cycle: Trees pull water from the ground and release it into the atmosphere as vapor through pores in their leaves in a process called transpiration, which can drive temperatures and rainfall across the globe. Forests are also dynamic ecosystems, with both natural events, such as pest infestations and droughts, and anthropogenic activities like logging potentially causing dramatic changes in forest structure. Despite the important roles forests play, the relationship between forest structure and the global water cycle is not well understood.
To help fill gaps in our understanding of this relationship, Aron et al. compared two forest sites in Michigan to find out how disturbances in forest structure can influence water transport from the land surface to the atmosphere.
The team selected two adjacent field sites at the University of Michigan Biological Station in Northern Lower Michigan: an undisturbed, control site dominated by bigtooth aspen and paper birch and a site where researchers in 2008 purposefully killed aspen and birches, giving this disturbed site a much more open canopy than the control. The arrangement of trees within a forest influences the amount of light and heat that reaches the ground, affecting not just transpiration but also other processes like evaporation and entrainment—the process by which air above the canopy is mixed into the canopy—which also contribute to the amount of water vapor that reaches the atmosphere.
Taking advantage of the fact that each of these processes results in distinct isotopic signals in water vapor, the researchers measured stable water isotopes at six heights in the two forest sites during the spring, summer, and fall of 2016.
The results revealed that the disturbed canopy was both drier and warmer than the undisturbed control site. The control site also exhibited a more stratified isotopic profile, suggesting less vertical mixing of the air in the forests, whereas the more open canopy appeared to encourage more mixing. The differences between the two sites were most prominent in the summer and spring.
The study demonstrates that forest canopy can regulate the rate at which moisture and energy are returned to the atmosphere at a local scale, which can in turn influence water retention and the makeup of forest ecosystems. The results provide important context for researchers interested in modeling how both forest ecology and water cycles will evolve as climate change progresses. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2019JG005118, 2019)
—Kate Wheeling, Freelance Writer