Modeling Grassland Net Primary Productivity and Water-use Efficiency along an Elevational Gradient of the Northern Tianshan Mountains

Mountain ecosystems are a vital component of the global biogeosphere and are widely recognized as being highly diverse. The investigations of mountain ecosystems may provide critical insights into understanding climate variability and its impacts. Among all the indicators of ecosystem function, terrestrial net primary productivity (NPP) is considered a major one because it represents the greatest annual carbon flux from the atmosphere to the biosphere.

Water-use efficiency (WUE) defines how efficiently individual plants use water to produce biomass and it provides a useful index for understanding the metabolism of terrestrial ecosystems as well as for evaluating the degradation of grasslands. Clearly examining the variations in WUE across the elevational gradient is useful for predicting the effects of climate change on vegetation productivity.

Northern Tianshan Mountains-southern Junggar Basin (TMJB), located in the center of the Central Eurasia continent, has representative grasslands, and as a result, is very meaningful for global carbon cycling research to estimate the ecosystem function of grasslands. Therefore, this result explored NPP and WUE in grasslands along an elevational gradient ranging from 400 to 3,400 m asl in the TMJB using the Biome-BGC model.

The results showed that the NPP increased by 0.05 g C/(m²·a) with every increase of 1-m elevation, reached the maximum at the mid-high elevation (1,600 m asl), and then decreased by 0.06 g C/(m²·a) per 1-m increase in elevation. The grassland NPP was positively correlated with temperature in alpine meadow (2,700–3,500 m asl), mid-mountain forest meadow (1,650–2,700 m asl) and low-mountain dry grassland (650–1,650 m asl), while positive correlations were found between NPP and annual precipitation in plain desert grassland (lower than 650 m asl). An increase (from 0.08 to 1.09 g C/(m²·a)) in mean NPP for the grassland in TMJB under a real climate change scenario was observed from 1959 to 2009.

Remarkable differences in WUE were found among different elevations. In general, WUE increased with decreasing elevation, because water availability is lower at lower elevations; however, at elevations lower than 540 m asl, they did observe a decreasing trend of WUE with decreasing elevation, which may be due to the sharp changes in canopy cover over this gradient.

The research suggests that the NPP simulated by Biome-BGC is consistent with field data, and the modeling provides an opportunity to further evaluate interactions between environmental factors and ecosystem productivity. The result was published in Journal of Arid Land on September 2013, 5(3): 354-365.