Abstract
Niche differentiation arising in functional trait diversity is expected to increase the potential for species coexistence, but empirical evidence for these relationships is sparse. We test whether grazing increases the functional diversity of leaf traits and niche differentiation in phosphorus limited Tibetan alpine meadows. We measured five traits in the leaf economic spectrum (LES; LC, leaf carbon concentration; LN, leaf nitrogen concentration; LP, leaf phosphorus concentration; SLA, specific leaf area; and LDMC, leaf dry matter content) for all species occurring in grazed and ungrazed plots at each of five sites. By comparing indicators of the fundamental and realized niches of co‐occurring plants in both grazed and ungrazed plots, we quantified a grazing‐mediated competitive effect on trait divergence and convergence. This trait response reflects the relative importance of niche differentiation and competitive exclusion in response to grazing. We found that while grazing induced LP divergence, both LC and LN tended to converge under grazing. Grazing had no effect on either SLA or LDMC diversity. When all five traits are considered together as a functionally integrated suite (LES hypervolume), there is no evidence for either divergence or convergence in response to grazing. Although grazing promotes functionally relevant diversity in LP that enables niche differentiation in competition for scarce soil available P, these results suggest that coordinated shifts in other LES traits sustain effective overall foliar function despite shifts in LP.
Highlights
The mechanisms affecting species coexistence and community assembly necessarily involve a balance between trait convergence and divergence among coexisting species
This shift of grazed communities toward trait divergence is associated with increasing elevation of the abundMPDSES-p/aMPDSES regression without change in slope (Fig. 1)
When niche differentiation is greater than competitive exclusion, species can form a stable community with both high trait divergence and high biodiversity
Summary
The mechanisms affecting species coexistence and community assembly necessarily involve a balance between trait convergence and divergence among coexisting species. Empirical evidence for niche differentiation promoting species coexistence and community diversity, is sparse (Siepielski and McPeek 2010, Thompson et al 2010, Freschet et al 2011) This paucity of empirical support can be attributed either to potential trait divergence being offset by counterbalancing abiotic or biotic effects that are simultaneously causing convergence (Swenson and Enquist 2009, Gotzenberger et al 2012) or to methodological constraints in separating the effects of abiotic and biotic process on trait convergence and divergence (Mayfield and Levine 2010, de Bello et al 2012; Kraft et al, in press). We report the results of an exclosure experiment in Tibetan alpine meadows that was designed to isolate the biotic effects on trait divergence and convergence in grazed versus ungrazed plant communities
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