Seasonally and spatially varied controls of climatic factors on net primary productivity in alpine grasslands on the Tibetan Plateau

Abstract

Vegetation net primary productivity (NPP) is a core parameter regulating carbon cycles of terrestrial ecosystem, which also has close relations with climates. The alpine ecosystems on the Tibetan Plateau (TP) are highly sensitive to climate changes. However, systematic analyses on the seasonal and annual responses of NPP to climatic factors in different grassland types on the TP are still lacking. In this study, the spatial and temporal patterns of NPP and their responses to temperature, precipitation and solar radiation during 2001–2015 at seasonal and annual temporal scales were investigated based on outputs of an improved Carnegie–Ames–Stanford Approach (CASA) model. The improved CASA model showed solid performances in simulating NPP in reference to field observations (R2 = 0.79, P < 0.001), resulting in mean error (ME) of −16.68, root mean square error (RMSE) of 87.59 g C·m−2·yr−1, and mean relative error (MRE) of −4.29%, respectively. The annual NPP displayed different altitude dependences between the regions below and above 3500 m, which could be attributed to the altitude associated precipitation variations. The temporal trends of the seasonal and annual NPP exhibited high spatial heterogeneity. For the entire alpine grasslands, solar radiation exerted stronger influences on annual NPP than temperature and precipitation did. The responses of NPP to climatic factors also varied among grassland types and seasons. For alpine meadow, solar radiation and temperature were the dominant climatic factors in controlling the NPP variability in spring and summer, respectively, while the effect of precipitation was weak in all seasons. On the contrary, precipitation played a more crucial role in influencing NPP than temperature and solar radiation in both summer and autumn for alpine steppe. Our results shed further lights on the mechanism underlying the responses of alpine ecosystem to climate changes. The improved understanding can provide guidelines for alpine grassland management.