How does Alpine Plant Productivity Respond to the Environmental Changes via Canopy Structure and Physiology on the Tibetan Plateau?

Ecosystems absorb CO₂ from the atmosphere through photosynthesis (i.e., Gross Primary Productivity, GPP) which is controlled by vegetation canopy structure and physiology. Canopy structure is usually represented by vegetation greenness, and determines the fraction of absorbed solar radiation during photosynthesis (f_PAR, Forkel et al., 2016), while canopy physiology indicates the potential maximum assimilation productivity by absorbing full photosynthetically active radiation (A₀, Hu et al., 2018). As one of the most sensitive regions to climate change, the Tibetan Plateau (TP) has, after 1998, experienced a slowdown of what was the previously an increasing trend in vegetation greenness. However, it is unclear whether the GPP trend showed a similar shift to vegetation greenness around 1998 over the TP and how alpine GPP responded to environmental changes through canopy structure and physiology before and after 1998.

With the help from Dr. Wang and Prof. Yang, we used a validated productivity model (Pmodel, Wang et al., 2017) to simulate the GPP changes on the TP from 1982 to 2015 and to decompose the overall environmental impacts on GPP trends in relation to both canopy structure (f_PAR) and physiology (A₀) effects. We found that from 1982, the regional mean GPP of the TP showed a vigorously increasing trend, while after the late 1990s the rate of increase slowed down. The results indicate that change in canopy greenness contributed much more (about 3/4) to the reduction of the GPP’s upward trend than canopy physiology effects after 1998 (Figure 1). From 1982 to 1998, climate warming effect and atmospheric CO₂ fertilization played dominant roles in GPP increase on the TP. After 1998, reduced benefits of warming and the stress due to atmospheric aridity overwhelmed the positive effects of CO₂ fertilization and the slight radiation enhancement, thus diminishing the persistent greening, and leading to the slowdown of increasing GPP.  Our results reveal a strong coupling between alpine GPP and canopy greenness and provides further insight into the climate-carbon feedback on the TP.

This work is going to be submitted. We will provide the DOI when it is accepted.

References:

Forkel, M., N. Carvalhais, C. Rödenbeck, R. Keeling, M. Heimann, K. Thonicke, et al. (2016). Enhanced seasonal CO2 exchange caused by amplified plant productivity in northern ecosystems. Science, 351(6274), 696-699. 10.1126/science.aac4971

Hu, Z., H. Shi, K. Cheng, Y.-P. Wang, S. Piao, Y. Li, et al. (2018). Joint structural and physiological control on the interannual variation in productivity in a temperate grassland: A data-model comparison. Global Change Biology, 24(7), 2965-2979. https://doi.org/10.1111/gcb.14274

Wang, H., I. C. Prentice, T. F. Keenan, T. W. Davis, I. J. Wright, W. K. Cornwell, et al. (2017). Towards a universal model for carbon dioxide uptake by plants. Nature Plants, 3(9), 734-741. 10.1038/s41477-017-0006-8