| Soil moisture-forced increase in mountain forest NPP over the past 40 years in central Tianshan Mountains |
| Paper ID : 1067-ADA2013 |
| Authors: |
|
Hongyan Liu * Yiheyuan Lu 5 |
| Abstract: |
| Located in the desert region in Central Asia, the vertically distributed mountain forests are very sensitive to climate change. To test the alternative hypotheses of soil moisture driven or CO2 driven NPP dynamics in the upper and lower timberline in this region, we systematically collect tree rings from 24 plots with size of 25 m × 25 m at different elevations on the Tianshan Mountains. Trees with d.b.h.>5 cm were all cored in each plot to reconstruct NPP dynamics in it. Spatial and temporal variations of temperature and precipitation along an elevational gradient on the Tianshan Mountains were comprehensively discussed based on nine vertically distributed meteorological stations. We use the interpolated climatic data to correlate with tree ring width indices and NPP in the study area, and further discuss the spatiotemporal patterns and mechanisms of the response of tree growth and forest net primary productivity to climatic changes in the study area. We found a very significant increasing trend in annual NPP of the Schrenk spruce forest from 1961 to 2003 (R2=0.51; P<0.01). Tree-ring estimated annual NPP (abbreviated as TR-NPP) was compared with averaged annual NPP of LPJ-DGVM (abbreviated as LPJ-NPP) and NDVI. It is found that annual TR-NPP, LPJ-NPP and NDVI have the similar increasing trend during the recent 20 years. It is confirmed that the forest vegetation activities have been enhancing significantly in recent decades. This study shows stronger increase trend in NPP in temperate dark coniferous forest in the study area than in the boreal zone due to a stronger increasing trend of soil moisture in central Tianshan Mountains than surrounding regions. Simulation by a Dynamic Global Vegetation Model (DGVM), LPJ, also yielded very consistent interannual changes. The forest NPP increase on the Tianshan Mountains is strongly associated with soil moisture represented by the Palmer Drought Severity Index (PDSI), as shown by a high correlation coefficient between them (R>0.5 for all the months; P<0.01). Significant increase in PDSI from 1961 to 2003 was found for all the months, which might contribute to the significant increase of annual NPP during this period. A linear decrease in fraction of the tree-ring estimated aboveground NPP to the LPJ simulated total NPP with Mean Annual Precipitation (MAP) supports the hypothesis that more NPP is allocated to roots to acquire water and nutrient in lower precipitation. Doubling CO2 concentration during the last 40 years contributes to a maximum of 13.2% of NPP growth, as simulated by the LPJ-DGVM. Our study implies a synergetic effect of available soil water and CO2 concentration on carbon fixation for the temperate arid mountainous forests. The effect of CO2 fertilization is associated with the growing season soil moisture (R2=0.1; P<0.05), weakening under dry climate and vice versa. High CO2 concentration improves water use efficiency and they both contribute to carbon uptake by forest, which was simply ascribed to CO2 fertilization. |
| Keywords: |
| Net primary productivity (NPP); Tree ring width; LPJ-DGVM; Soil moisture; CO2 fertilization; Tianshan Mountains |
| Status : Abstract Accepted |
© 2010 - 2024 All Rights Reserved. Powered by sinaweb.