Increased soil nutrition and decreased light intensity drive species loss after eight years grassland enclosures

Jingpeng Li,Nianxi Zhao,Yubao Gao,Hongtao Xie,Zhirong Zheng

doi:10.1038/srep44525

Abstract

Enclosures (fenced, grazing or clipping) within a certain period of years are the most common tools for restoration of degraded grasslands in temperate regions. Short-term enclosures can improve biodiversity and productivity by effectively relieving grazing pressure, while long-term enclosures can reduce species diversity. We therefore carried out a field experiment to investigate the specific causes of the reduced species diversity in Hulunbeier grassland of northern China. After eight years of enclosure, the significantly increased soil available nitrogen (AN) and available phosphorus (AvP) in enclosure community reduced nitrogen (N) limitation but most vegetation was still N limited. Many environmental factors led to decreased species richness, but increased soil AN and decreased light intensity at the community bottom were the most significant ones. Community density decreased independently of soil nutrition but significantly related to decreased species richness. Density of dominant canopy species increased, while dominant understory species decreased during assemblage-level thinning; therefore, the random-loss hypothesis was not supported. The dominant understory species responded to lower light availability by increasing their height, leaf area, and chlorophyll content. Moreover, our results were expected to provide some specific guidance for the restoration mode selection of degraded grasslands in northern China.

Highlights

Enclosures within a certain period of years are the most common tools for restoration of degraded grasslands in temperate regions
The assemblage-level thinning hypothesis predicts that N-enrichment reduces the N limitation on grassland productivity, thereby increasing community biomass and individual plant size[11,16]. This in turn leads to an increased overall competitive intensity in the community, which causes community-level thinning[17,18] and decreased density, as small individuals of all species randomly die for weaker competitiveness[16]
The interspecific competitive exclusion hypothesis assumes that mortality is not equal among species, and predicts that subdominant species are suppressed by dominant species, and the competitive exclusion of subdominant species reduces richness[17,19]

Summary

Introduction

Enclosures (fenced, grazing or clipping) within a certain period of years are the most common tools for restoration of degraded grasslands in temperate regions. Two hypotheses were proposed: (1) in contrast to unenclosed plots, soil N and P (phosphorous) content should increase significantly in enclosure plots, and nutrient limitation effects on enclosure plots should weaken while light competition effects on species diversity increase; and (2) the growth of understory individuals, which were primarily annuals and biennials should be suppressed by vertical light attenuation as community structure changes over eight years.

Results

Conclusion