Simulated effects of temperature and precipitation change in several forest ecosystems

The Nutrient Cycling Model (NuCM) was used to investigate the effects of in creased temperature (+4 degrees C) and changing precipitation (increased an d decreased) on biogeochemical cycling at six forest sites in the United St ates: a Picea rubens forest at Nolan Divide in the Great Smoky Mountains, N orth Carolina; mixed deciduous forests at Walker Branch, Tennessee and Cowe eta, North Carolina; a Pinus tacda forest at Duke, North Carolina; a P. eli ottii forest at Bradford, Florida; and a P. conrota/P, jeffreyii forest at Little Valley, Nevada. Simulations of increased temperature indicated incre ased evapotranspiration and reduced water Aux, Simulations of changes in pr ecipitation indicated disproportionately large variations in soil water flu x because of the relative stability of evapotranspiration with changes in p recipitation. Increased temperature caused N release from forest floors at all sites. Ar the N-saturated Nolan Divide site, this resulted in no change in N uptake or growth but increased soil solution Al and NO; and increased N leaching losses. At the N-limited sites, the release of N from the fores t floor caused increased growth, and, in some cases, increased NO; leaching as well, indicating that N released from the forest floor was not efficien tly taken up by the vegetation. Increased precipitation caused increased gr owth, and decreased precipitation caused reduced growth in the N-limited si tes because of changes in wet N deposition. Changes in precipitation had no effect on growth in the N-saturated Nolan Divide site, but did cause large changes in soil solution mineral acid anion and Al concentrations, Increas ed precipitation caused long-term decreases in soil exchangeable base catio ns in most cases because of the disproportionately large effects on soil wa ter flux; however, increased precipitation caused decreases in exchangeable base cations in cases when atmospheric deposition was a major source of ba se cations for the system. The simulation results illustrate the extreme complexity of the possible re sponses of nutrient cycling processes to climate change. By virtue of the f act that the NuCM model does not contain physiological algorithms, these si mulations demonstrate that changes in temperature and precipitation can pro duce widely varying ecosystem-level responses through their effects on biog eochemical cycling processes alone and that generalizations about the relat ive importance of temperature versus precipitation changes are hazardous. ( C) 2000 Elsevier Science B.V. All rights reserved.