Ungulate browsers promote herbaceous layer diversity in logged temperate forests.

Edward K Faison,Glenn Motzkin,Joshua M Rapp,Stephen Destefano,David R Foster

doi:10.1002/ece3.2223

Abstract

Ungulates are leading drivers of plant communities worldwide, with impacts linked to animal density, disturbance and vegetation structure, and site productivity. Many ecosystems have more than one ungulate species; however, few studies have specifically examined the combined effects of two or more species on plant communities. We examined the extent to which two ungulate browsers (moose [Alces americanus]) and white‐tailed deer [Odocoileus virginianus]) have additive (compounding) or compensatory (opposing) effects on herbaceous layer composition and diversity, 5–6 years after timber harvest in Massachusetts, USA. We established three combinations of ungulates using two types of fenced exclosures – none (full exclosure), deer (partial exclosure), and deer + moose (control) in six replicated blocks. Species composition diverged among browser treatments, and changes were generally additive. Plant assemblages characteristic of closed canopy forests were less abundant and assemblages characteristic of open/disturbed habitats were more abundant in deer + moose plots compared with ungulate excluded areas. Browsing by deer + moose resulted in greater herbaceous species richness at the plot scale (169 m2) and greater woody species richness at the subplot scale (1 m2) than ungulate exclusion and deer alone. Browsing by deer + moose resulted in strong changes to the composition, structure, and diversity of forest herbaceous layers, relative to areas free of ungulates and areas browed by white‐tailed deer alone. Our results provide evidence that moderate browsing in forest openings can promote both herbaceous and woody plant diversity. These results are consistent with the classic grazing‐species richness curve, but have rarely been documented in forests.

Highlights

Large herbivores are leading drivers of terrestrial plant composition and dynamics and important determinants of biodiversity and a host of ecosystem services (Diaz et al 2007; Hegland et al 2013; Borer et al 2014)
Two ungulate browsers had strong additive effects on the structure and composition of temperate forest herbaceous layers 5–6 years after complete canopy removal from logging. This experiment is the first to examine the individual effects of one ungulate browser vs the combined effects of two browsers on herbaceous layer vegetation in temperate forests of North America and perhaps temperate forests anywhere
Our goals were to examine how the recolonization of low densities of moose to a system browsed by low densities of deer altered the plant community, and how moderate intensity browsing by deer + moose altered the community differently from low intensity browsing by deer alone

Summary

Introduction

Large herbivores are leading drivers of terrestrial plant composition and dynamics and important determinants of biodiversity and a host of ecosystem services (Diaz et al 2007; Hegland et al 2013; Borer et al 2014). In addition to site conditions (e.g., disturbance, vegetation structure, and soil productivity), herbivore density is an important determinant of ungulate impacts on plant communities. The humped-back grazing curve has been documented in grassland ecosystems (Mwendera et al 1997; Olff and Ritchie 1998; Suominen et al 2003), it has rarely been reported in forests, in part because (1) forest ungulates are generally not stocked at extremely high levels (Hegland et al 2013), (2) few experiments have manipulated multiple levels of ungulate densities in forests (Wisdom et al 2006), and (3) because ungulate-forest experiments have predominantly occurred in areas of moderate-to-high deer densities (>8.5 kmÀ2; Russell et al 2001).

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