Simulation of the climatic productivity of forest vegetation in Three Gorges Reservoir Area

Abstract

Forest biological production plays an important role in forest ecosystems and is significantly related to the terrestrial carbon cycle and the ecological changes occurring worldwide, so it must be considered during the evaluation of ecosystem functions. The Three Gorges Reservoir Area continues to be a hotspot for research because of its complex topography, rich species diversity and wide variety community types. The construction of the reservoir threatens the ecostability of the surrounding and downstream areas. Estimating and simulating forest climatic productivity has become necessary and will also provide evidence of vegetation restoration and land use in the Three Gorges Reservoir Area and even in the entire Yangtze River Basin. We developed a method designed to link together data from forest inventories and ecological research sites, including data from 104 permanent sample plots and 118 temporary sample plots, and combined that plot data with meteorological, topographical, leaf area index, live-biomass and net primary productivity (NPP) data collected from across the Three Gorges Reservoir Area using a GIS system. Using this large database and statistical models, the actual and potential productivities of different forest types were estimated and the annual mean temperature/ precipitation, latitude, longitude and altitude were documented. Then the distribution patterns of biological production of major forest types in Three Gorges Reservoir Area were analyzed. To acquire actual measurement data of mature forest productivity is critical to predicting and simulating forest climatic productivity. Data based on natural or near natural conditions were selected from a database of forest production and used to estimate the productivity of major forest types. The criteria for selecting these data included: (1) Forest age: mature or near mature forests were selected for this part of the analysis. Based on the age group division standards of the State Forestry Administration, forests more than 60 years old were treated as mature or near mature for evergreen broad-leaved forest, deciduous broad-leaved forest, coniferous and broadleaved mixed forest and cypress forest. Masson pine forest, coniferous forest and temperate forest were selected if more than 50 years and Chinese fir forest was selected if more than 26 years. (2) Selected forests had a soil which was greater than 60cm thick. (3) Stand density is between 1,000-2,000 trees/hm2. (4) Midslope forests with slopes less than 35° were chosen for analysis. The NPP data within 0.1° (Latitude) × 0.1° (Longtitude) × 100 m (Height) were chosen for use in the analysis of forest climatic productivity. If one grid had multiple sets of data, we took the average of the three largest datasets. The amounts of annual evapotranspiration in an area significantly influenced NPP of different forest types in the Three Gorges Reservoir Area. NPP of evergreen broad-leaved forests is related to average annual evapotranspiration in the form of the exponential curve when graphed. For coniferous forests and deciduous broad-leaved forests, the relationship between the NPP and evapotranspiration is in the form of a parabolic surface. NPP of coniferous-deciduous mixed forests is related to annual actual evapotranspiration in the form of an exponential curve when graphed. The potential productivity of the Three Gorges Reservoir Area is 11.26 t • hm-2-a-1, which is 1.48 times higher than actual productivity. Forest productivity responded differently in five different modeled climate scenarios. The productivity of evergreen broad-leaved forests, deciduous broad - leaved forests and coniferous-deciduous mixed forests are positively correlated with modeled increases in temperature and precipitation; with a temperature increase of 2°C and a 20% increase in precipitation, their NPP increased by 24.34%, 22.5% and 15.98%, respectively, but NPP of evergreen coniferous forests decreased by 5. 55%. Total potential productivity changed slightly (-0.53% to +5.51%) with the changing of the climate as discussed above, but theoretical NPP increased 29.51% with the modeled rise in temperature and precipitation described above.