Increasing gross primary production (GPP) in the urbanizing landscapes of southeastern Michigan

In order to understand the impact of urbanizing landscapes on regional gross primary production (GPP), we analyzed changes in land-cover and annual GPP over an urban-rural gradient in ten Southeastern Michigan counties between 1991 and 1999. Landsat and AVHRR remote sensing data and biophysical parameters corresponding to three major landcover types (i.e., built-up, tree, and crop/grass) were used to estimate the annual GPP synthesized during the growing season of 1991 and 1999. According to the numbers of households reported by the U.S. Census in 1990 and 2000, the area settled at urban (>1 housing unit acre(-1)), suburban (0.1 to 1 housing units acre(-1)), and exurban (0.025 to 0.1 housing units acre(-1)) densities expanded, while the area settled at rural (<0.025 housing units acre(-1)) densities reduced. GPP in this urbanizing area, however, was found to increase from 1991 to 1999. Increasing annual GPP was attributed mainly to a region-wide increase in tree cover in 1999. In addition, the estimated annual GPP and its changes between 1991 and 1999 were found to be spatially heterogeneous. The exurban category (including constantly exurban and exurban converted from rural) was associated with the highest annual GPP as well as an intensified increase in GPP. Our study indicates that low-density exurban development, characterized by large proportions of vegetation, can be more productive in the form of GPP than the agricultural land it replaces. Therefore, low-density development of agricultural areas in U.S. Midwest, comprising significant fractions of highly productive tree and grass species, may not degrade, but enhance, the regional CO, uptake from the atmosphere.