Human altered disturbance patterns and forest succession: impacts of competition and ungulate herbivory
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Human activities are altering patterns of ungulate herbivory and wildfire regimes globally with large potential impacts on plant community succession and ecosystem resilience. Aspen (Populus tremuloides) is a keystone species which co-exists with conifer species across temperate forests in North America. Aspen sucker regeneration which is the foundation of aspen–conifer forests succession is often a targeted food source by multiple ungulate species. Using a region-wide exclosure network across a broad gradient of aspen–conifer overstory abundance, we empirically tested the effects of ungulate herbivory and conifer competition (that increases with fire suppression), on the regeneration and recruitment of aspen forests over a 4-year period. The study results indicate that ungulate herbivory and increasing abundance of overstory conifers dramatically reduced aspen regeneration and recruitment success. The average height of aspen suckers exposed to ungulate herbivory was 72% shorter than aspen suckers in fenced plots and resulted in 24% less recruitment. There was a 9% decrease in aspen recruitment and 12% decrease in average aspen height with every 20% increase in overstory conifer density. Aspen suckers were most vulnerable to herbivory at 70 cm height, with the probability of herbivory decreasing under 50 cm or above 90 cm. Steep slope angles and higher winter precipitation increased aspen regeneration and recruitment success. Reduction in aspen recruitment in response to ungulate herbivory and competition by conifers may result in loss of biodiversity, altered forest function and loss of key ecosystem services because of the important role that aspen plays in facilitating forest succession and biodiversity.
KeywordsAspen Deer Elk Fire Herbivory Populus tremuloides Ungulates
We acknowledge the important contributions of Justin Taylor, Rebecca Lee, and Ho Yi Wan for assistance in data collection. We are grateful for the assistance of the Fishlake, Dixie and Manti-Lasal National Forests for their assistance in establishing the exclosure networks.
Author contribution statement
SS conceived the ideas and SS and AR set up the experimental design, AR and JM collected the data, JM analyzed the data, JM led the writing of the manuscript with the assistance of both SS and AR.
Funding for this study was provided by the Utah Division of Natural Resources (2012).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflicts of interest.
This article does not contain any studies with human participants or animals performed by any of the authors.
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