Shrub-mediated effects on soil nitrogen determines shrub-herbaceous interactions in drylands of the Tibetan Plateau

Abstract

IntroductionShrub promotes the survival, growth and reproduction of understory species by buffering the environmental extremes and improving limited resources (i.e., facilitation effect) in arid and semiarid regions. However, the importance of soil water and nutrient availability on shrub facilitation, and its trend along a drought gradient have been relatively less addressed in water-limited systems.MethodsWe investigated species richness, plant size, soil total nitrogen and dominant grass leaf δ13C within and outside the dominant leguminous cushion-like shrub Caragana versicolor along a water deficit gradient in drylands of Tibetan Plateau.ResultsWe found that C. versicolor increased grass species richness but had a negative effect on annual and perennial forbs. Along the water deficit gradient, plant interaction assessed by species richness (RIIspecies) showed a unimodal pattern with shift from increase to decrease, while plant interaction assessed by plant size (RIIsize) did not vary significantly. The effect of C. versicolor on soil nitrogen, rather than water availability, determined its overall effect on understory species richness. Neither the effect of C. versicolor on soil nitrogen nor water availability affected plant size.DiscussionOur study suggests that the drying tendency in association with the recent warming trends observed in drylands of Tibetan Plateau, will likely hinder the facilitation effect of nurse leguminous shrub on understories if moisture availability crosses a critical minimum threshold.

Warming affects leaf light use efficiency and functional traits in alpine plants: evidence from a 4-year in-situ field experiment

“…Plant communities exhibit both positive and negative interactions between different plant species (Callaway and Walker, 1997). Cui et al (2023) confirmed that grasses exhibited a higher competitive ability compared to other functional groups, primarily attributed to their increased investment in roots and enhanced capacity for resource uptake.…”

Density‐dependent species interactions modulate alpine treeline shifts

“…It showed that facilitation and competition occur simultaneously, regardless of vegetation density. Facilitation processes are acting through the accumulation of soil nutrients (Cui et al, 2023;Grau et al, 2012) by maintaining suitable microclimatic conditions (Chen et al, 2020;Lyu et al, 2016) and by ameliorating physical conditions (Renard et al, 2016). Generally, any species found in a community experiencing stress could be facilitated (Liancourt & Dolezal, 2021).…”

Divergent responses of above- and belowground ecosystem functioning to shrub encroachment in the Tibetan semi-arid alpine steppes

“…Alpine ecosystem on the Tibetan Plateau has undergone rapid climate changes and intensity anthropogenic activities in recent decades (Kuang et al 2016), which are also associated with substantial structural and functional variations of alpine ecosystems, particularly the prevalently ecological process known as shrub encroachment (Wu et al, 2021;Zhao et al, 2023). In general, compared with herbaceous species, shrub species with greater aboveground and root biomass can provide more organic matter input soils (Zhou et al 2017;Cui et al, 2023) and enhance the accumulations of soil carbon and nitrogen (Li et al, 2019;Zhao et al, 2023). Shrub encroached into alpine grassland ecosystems can increase biodiversity, soil fertility, soil infiltration processes, ultimately attributing to strengthen ecosystem functioning (Butterfield et al, 2016;Cai et al, 2020).…”

“…Nevertheless, the degree by which shrub encroachment caused variations in the ecosystem functioning could vary with the traits of encroaching shrubs (e.g., Leguminosae or Non-Leguminosae species). In semi-arid regions, Leguminosae shrubs can overcome drought stress and nutrients constraints to promote species richness, vegetation productivity, and carbon accumulation (Saixiyala et al, 2017;Ale et al, 2023). However, the encroachment of Leguminosae shrubs was differently associated with the ecosystem functioning than that of none-leguminous shrubs (Li et al, 2016;Zhao et al, 2023).…”

“…Much less attention has been given to recent widespread encroachments of shrubs in semi-arid alpine ecosystems, such as leguminous shrub (Caragana spinifera ) and none-leguminous shrub (Dasiphora fruticosa ). These encroaching shrubs was expected to provide favorable habitat for herbaceous plants due to increased shrubs abundance and sprawling canopy that provide increased ultraviolet radiation protection and buffering of extreme soil temperatures and drought stress, as well as an increase in soil nutrients availability (Cui et al, 2023;Zhao et al, 2023), which are associate with strengthening ecosystem functioning. These dwarf shrubs may be associated with different above-and belowground effects, including increased vegetation productivity and increased soil nutrient contents.…”