The Stem Exclusion Bottleneck: Identifying the Key Drivers of Plant Diversity in Temperate Forests using a Long-Term Experimental Approach

by

Alejandro A. Royo, Michael J. Chips, Tim Nuttle, Mary Beth Adams, Walter P. Carson

Research Ecologist (AAR), USDA Forest Service, Northern Research Station, Irvine, PA 16329; Graduate Student (MJC), Department of Biology, 154A Crawford Hall, University of Pittsburgh, Pittsburgh, PA 15260; Senior Ecologist/Principal (TN), Civil and Environmental Consultants, Inc., Ecological Services Division, 333 Baldwin Road, Pittsburgh, PA 15205; Research Soil Scientist (MBA), USDA Forest Service, Northern Research Station, 180 Canfield Street, Morgantown, WV 26505; Associate Professor (WPC), Department of Biology, 154A Crawford Hall, University of Pittsburgh, Pittsburgh, PA 15260. AAR is corresponding author. To contact, call 814-563-1040 or email aroyo@fs.fed.us

Abstract – Eastern forests are experiencing dramatic species composition shifts and plant diversity declines. Fire suppression, smaller canopy gaps, and overabundant deer have been shown to drive many of these changes. After restoring fire and gaps to the landscape, herbaceous and woody diversity often increases, but only when deer are at moderate densities. However, we do not understand how disturbances and herbivory mediate plant communities as the forest transitions into the stem exclusion stage, which for herbs may erase any of the initial increase in light availability created by large canopy gaps. We tested the hypothesis that effects of disturbances and herbivory would persist for the woody plants and disappear for herbs as the forest transitions into the stem exclusion phase. To test this hypothesis using a fully factorial experiment, where we restored fire, cut canopy gaps, and excluded deer using fences. We then re-assessed response of woody and herbaceous plants after 12 years. We found deer browsing reduced sapling richness and diversity, especially under open canopies, where sapling diversity doubled when browsing was excluded. In contrast, gap effects did not persist for herbs but browsing increased herbaceous diversity by approximately 40% in burned plots because browsing removed a dominant, fast-growing shrub (Rubus allegheniensis). We are the first to demonstrate that disturbances and herbivory cause contrasting responses in herbs and saplings over the long term (12 years), likely driven by high stem density of the sapling layer that reduces both above- and belowground resource availability to the herbaceous layer.