Press-Pulse Odocoileus Virginianus Herbivory in Relict Tsuga Canadensis Stands in the Western Upper Peninsula of Michigan, USA

Grace L Parikh,Christopher R Webster

doi:10.3390/f10060496

Abstract

Ungulate herbivory occurring within a forest plant community’s natural range of variation may help maintain species diversity. However, acute or chronically elevated levels of herbivory can produce dramatic changes in forest communities. For example, chronically high levels of herbivory by white-tailed deer (Odocoileus virginianus Zimmerman) in regions of historically low abundance at northern latitudes have dramatically altered forest community composition. In eastern hemlock (Tsuga canadensis L. Carrière) stands where deer aggregate during winter, high deer use has been associated with a shift towards deciduous species (i.e., maples [Acer spp.]) dominating the regeneration layer. Especially harsh winters can lead to deer population declines, which could facilitate regeneration of species that have been suppressed by browsing, such as hemlock. To enhance our understanding of how fluctuations in herbivory influence regeneration dynamics, we surveyed regeneration and deer use in 15 relict hemlock stands in the western Upper Peninsula of Michigan in 2007 and again in 2015. With the exception of small seedlings (0.04–0.24 m height), primarily maples whose abundance increased significantly (p < 0.05), we observed widespread significant declines (p < 0.05) in the abundance of medium (0.25 ≤ 1.4 m height) and large regeneration (>1.4 m tall ≤ 4 cm diameter at breast height) over the study period. Midway through our study period, the region experienced a high severity winter (i.e., “polar vortex”) which resulted in a substantial decline in the white-tailed deer population. Given the dominance of maples and dearth of hemlock in the seedling layer, the decline in the deer population may fail to forestall or possibly hasten the trend towards maple dominance of the regeneration layer as these stands recover from pulses of acute herbivory associated with high-severity winters and the press of chronically high herbivory that precedes them.

Highlights

The consequences of ungulate herbivory for individual plants and plant populations are not uniform among species, often resulting in clear “winners” and “losers” [1]
Given the dominance of maples and dearth of hemlock in the seedling layer, the decline in the deer population may fail to forestall or possibly hasten the trend towards maple dominance of the regeneration layer as these stands recover from pulses of acute herbivory associated with high-severity winters and the press of chronically high herbivory that precedes them
Stands ranged in area from 0.4 ha to 46.7 ha, with overstories ranging in basal area from 26.4 m2 ha−1 to 59.6 m2 ha−1 of which the proportion of hemlock ranged from 35.6% to 83.8% (Table 1)

Summary

Introduction

The consequences of ungulate herbivory for individual plants and plant populations are not uniform among species, often resulting in clear “winners” and “losers” [1]. Forests 2019, 10, 496 variation of the baseline may foster plant species diversity, for example, by reducing the abundance of palatable but highly competitive species [14] This chronic-acute framework conceptually is similar to the press-pulse framework used to describe the influence of climate trends and extreme weather events on biological systems [15]. This framework recognizes that most organisms are adapted to short- and long-term climate variability, but as the underlying trend changes the threshold between survivable events and extinction extremes is crossed more frequently [15]. An advantage of this framework is that it recognizes the coupling of slow and fast drivers of change

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