Climatic Control of the High-Latitude Vegetation Greening Trend and Pinatubo Effect

Abstract

A biogeochemical model of vegetation using observed climate data predicts the high northern latitude greening trend over the past two decades observed by satellites and a marked setback in this trend after the Mount Pinatubo volcano eruption in 1991. The observed trend toward earlier spring budburst and increased maximum leaf area is produced by the model as a consequence of biogeochemical vegetation responses mainly to changes in temperature. The post-Pinatubo decline in vegetation in 1992-1993 is apparent as the effect of temporary cooling caused by the eruption. High-latitude CO 2 uptake during these years is predicted as a consequence of the differential response of heterotrophic respiration and net primary production.Satellite observations over the past two decades indicate a trend toward longer growing seasons and greater annual net primary production (NPP) in high latitudes (1-3). Using a dynamic vegetation model that predicts leaf area index (LAI), primary production, and net ecosystem carbon exchange from first principles (4 -7), we show that the trend and variability in the satellite observations are consistent quantitatively with independent climate data and qualitatively with net ecosystem carbon exchange, which was independently calculated from atmospheric CO 2 concentration measurements (8).We analyzed data for a 1982-1998 interval for which we had access to both climate and satellite data. A previous analysis of 10 years of data from the Advanced Very High Resolution Radiometer (AVHRR) indicated a progressive greening of the boreal zone. A steady increase in annual maximum LAI during 1981-1991 was associated with a slight advance of spring budburst and delay of autumn abscission (1). Subsequent work spanning 1981-1999 has confirmed these findings (2, 9), but doubts about the validity of the trend have persisted because of the need for data corrections for instrumental and navigational drift, intercalibration of successive instruments, and consideration of aerosol effects (10). Such doubts could be dispelled if the interannual variations in greenness and growing season length were shown to be quantitatively consistent with independent expectations on the basis of climate variability and/or with independent reconstructions of changes in regional CO 2 balance. This analysis uses a climate-driven terrestrial carbon cycle model capable of simulating the interannual variability of LAI and the components of the terrestrial carbon balance.We compare monthly LAI anomalies for the boreal zone, derived from a recent version of the AVHRR data, with monthly LAI anomalies independently predicted by the LPJ Dynamic Global Vegetation Model (LPJ-DGVM), a biogeochemical process model driven by monthly climate observations and by the global mean CO 2 concentration increase (see Materials and Methods in supporting online material). Both simulated and observed LAI anomalies show an overall increasing trend, but periods lasting several years with positive or negative deviations from the trend can be discerned in the ob...

Modelling Tundra Vegetation Response to Recent Arctic Warming

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