Inconsistent changes in NPP and LAI determined from the parabolic LAI versus NPP relationship

Abstract

Rapidly altering vegetation dynamics, is an aspect of global environmental change which has consequences for ecosystem structure, function, and the provision of ecosystem services. However, the responses of vegetation greenness and vegetation productivity to environmental changes may be inconsistent. Currently, we do not know how these inconsistent trends of greenness and productivity vary across the globe, nor the main causes of that variability. We used leaf area index (LAI) and net primary productivity (NPP) to represent vegetation greenness and productivity, respectively. The results showed that about 42% of the global vegetated area showed inconsistent trends of LAI and NPP from 2000 to 2015. These inconsistent trends were mainly found in humid areas. A parabolic relationship was found between the NPP and LAI of global vegetation, with a turning point in LAI of global vegetation of 4.5 ± 0.4 m2 m−2. The ratio of NPP to LAI (NPP/LAI), net primary production per unit leaf area, decreases as the LAI increases. The variability of NPP/LAI is significantly correlated with the consistency of NPP and LAI changes in different land cover types. NPP/LAI is lower in tree-dominated biomes with higher canopies (such as forests), and higher in areas dominated by short vegetation (such as shrubland and grassland). Drylands have higher NPP/LAI than humid regions which are perhaps due to high rates of net photosynthesis and narrow leaves. The results of this study not only clearly portray the relationship between vegetation greenness and productivity but also help us to understand the impact of vegetation dynamics on global carbon cycling in a changing environment.