Although it may be difficult for us to observe in our short lifetimes, the composition of trees in a forest can be a very variable thing. When a forest is clear-cut or thinned by fire, the first trees to rebound are often the fastest growing—those that can sprout quicker than their competitors. However, over time, these speedy forerunners often get supplanted by sturdier, slower species that eventually grow taller and cast their predecessors into shadow and eventual obscurity.
As the composition of the forest changes, its influence on the climate at large also shifts: Some trees produce more oxygen, some consume more carbon dioxide, and others release more water into the air. To investigate how these dynamics change as the forest ages, researchers at the University of Michigan Biological Station artificially accelerated the natural succession of a deciduous forest in northern Michigan by killing off a large number of aspen and birch trees to make way for the up-and-coming red maples, red oaks, and white pines that typically characterize a more mature forest. Over the course of a 3-year study, Matheny et al. measured differences in gas exchange, transpiration rates, and other variables between the altered forest and plots that were left alone.
Overall, transpiration—the process that carries moisture from roots to leaves and eventually into the air—was found to be reduced by approximately 15% in the altered plots. However, several individual tree species, including red oaks and white pines, showed an increase in the amount of water they released into the atmosphere.
The total transpiration was also discovered to be influenced by a species-specific relationship between sap flux and how saturated the atmosphere is with water. This relationship changes as the day progresses, with transpiration occurring most in the morning and tapering off as the day goes on.
The researchers concluded that understanding how the rates of nutrient exchange between trees and the atmosphere change as forests age may help modelers understand how the climate as a whole will be affected in the future. It may also shed light on how the management of forests may influence future climate. (Journal of Geophysical Research: Biogeosciences, doi:10.1002/2014JG002804, 2014)