Land use can profoundly alter material cycling in forest ecosystems by modifying the species composition and structure of forest communities. Changes in the cycling of water through transpiration have important bioclimatic consequences, including impacts on ecosystem services through changes in local cooling. However, the long-term effects of land use on transpiration are not well known. In the Great Lakes Region, forest regrowth has been ongoing for a century or more, but second growth forests remain more deciduous than historic forests and consist of younger and smaller trees. These persistent differences in composition and structure may modify transpiration and local cooling. We investigated the effect of historic land use on transpiration by monitoring sap flux in an old-growth (400+ y.o.) and mature second growth (80 y.o.) forest in the Great Lakes Region. Sap flux measurements were supplemented by meteorology and forest surveys to facilitate stand-level interpretation, and paired with canopy surface temperature measurements to investigate the relationship between transpiration and surface cooling. We test the hypothesis that the greater tree age and relative abundance of evergreen species in old growth forest will result in reduced transpiration and reduced surface cooling relative to second growth forest.
Results/Conclusions
Old growth forest had lower tree density and greater basal area than second growth forest. Evergreen trees represented >10% of old growth basal area but were absent from second growth forest. When sap flow was scaled to the stand level, total transpiration was lesser in the old growth forest despite greater potential evapotranspiration and basal area. These differences were stronger for transpiration per unit sapwood area, indicating that older and larger old growth trees may utilize their sapwood more conservatively than second growth trees. Surface temperatures were cooler relative to air temperature for the second growth forest, suggesting that increased transpiration may play a role in surface cooling. Our findings indicate that species composition and age- or size-related sapwood characteristics contribute to reduced transpiration in old growth forest, and that those reductions in transpiration may result in warmer canopy surfaces. If Great Lakes Region forests maintain their current structure and composition, high transpiration rates will continue to promote regional surface cooling in the future.