The impact of trees on temporal variability in urban carbon and water budgets.

Abstract

Urbanization is responsible for some of the fastest rates of land-use change around the world, with important consequences for local, regional, and global climate. Vegetation can represent a significant proportion of many urban and suburban landscapes and modifies climate by altering local exchanges of heat, water vapor, and CO2. To assess the contribution of plant functional types to urban ecosystem processes of water loss and carbon gain in a suburban neighborhood of Minneapolis-Saint Paul, Minnesota, USA, I investigated 1) the microclimate effects of different forest types over time, 2) the relative importance of environmental and biological controls on urban tree transpiration and canopy photosynthesis, and 3) the relative importance of trees and turfgrass on the spatial and seasonal variation in suburban evapotranspiration. Regardless of plant functional type, I found that seasonal patterns of soil and surface temperature were controlled by differences in stand-level leaf area index, and that sites with high leaf area index had soil and surface temperatures 7°C and 6°C lower, respectively, than sites with low leaf area index. Plant functional type differences in canopy structure and growing season length largely explained why evergreen needleleaf trees had significantly higher annual transpiration (307 kg H2O m-2 yr-1) and canopy photosynthesis (1.02 kg C m-2 yr-1) rates per unit canopy area than deciduous broadleaf trees (153 kg H2O m-2 yr-1 and 0.38 kg C m-2 yr-1, respectively), offering an approach to scale up the tree component of urban water and carbon budgets. Turfgrass represented the largest contribution to annual evapotranspiration in recreational and residential land-use types (87% and 64%, respectively), due to a higher fractional cover and greater daily water use than trees. Component-based estimates of suburban evapotranspiration underestimated measured water vapor fluxes by 3%, providing a useful approach to predict the seasonal patterns of evapotranspiration in cities. These finding have implications for the management of urban ecosystem services related to climate, carbon sequestration, and hydrology, and for predicting the impacts of climate change on urban ecosystems.