Variations and climate constraints of terrestrial net primary productivity over Mongolia

The ecosystems of Mongolia are among the most sensitive to global climate change because they are located in the circumpolar boreal zone, with its arid and semiarid climate. This study used the Carnegie-Ames-Stanford Approach (CASA) model parameterized by meteorological and remote-sensing data to quantify vegetation net primary productivity (NPP) for Mongolia for 1982–2011 and further constructed a map of the relative contributions of climatic constraints to NPP. The results showed that 30-year average NPP in Mongolia varied spatially from 18.1 to 662.8 g C/m², with an average of 228.2 g C/m². From the map of climatic constraints on NPP, it was estimated that temperature constrains NPP variation over about 3.9% of Mongolia's land area, whereas precipitation constraints are dominant over about 77.5%. NPP in temperature-constrained regions has continuously increased from 1982 to 2011, with an annual increment of 2.18 g C/m² (R = 0.61, p < 0.01) with continuous climate warming (0.07 °C/year, R = 0.73, p < 0.01). In contrast, the NPP trend in precipitation-constrained regions has changed from increasing (R = 0.68, p = 0.01) to decreasing (R = -0.38, p = 0.12) since 1994, which can be attributed to a reversal of precipitation trends in 1994. At national scale, NPP appeared stable during 1982–2011, due primarily to the high proportion of precipitation-constrained area in Mongolia, which obscured NPP trends in the temperature-constrained regions. Given that NPP trends clearly varied with different climatic constraints, the importance of differentiating climatic constraints on NPP for exploring NPP trends under different climatic drivers should be emphasized. The seasonal-scale analysis further showed that summer climate and associated vegetation productivity is an important factor regulating NPP over the entire growing season, due primarily to the high proportion of summer NPP and precipitation for the entire growing season, and warmer temperatures in summer relative to those in spring and autumn. In addition to temperature and precipitation, radiation is an important factor responsible for the NPP variations, and the shape of the correlations between radiation and NPP with changing precipitation gradient suggests that the impact process of radiation on NPP with changing water stress is similar to that of temperature, but its impact is weaker than that of temperature.