Abstract
Understory plants are an important element of forests, having a considerable influence on ecosystem functioning and canopy-tree development following disturbance. Recent bark beetle outbreaks across western North American forests have caused extensive canopy mortality, creating new growing conditions that provide the opportunity for changes within the intact understory. Over a five-year period following peak mountain pine beetle (MPB) activity across lodgepole pine-dominated forests in Rocky Mountain National Park, Colorado, we measured the changes in plant diversity, cover, and dominance by lifeform and quantified tree regeneration rates. Average species richness and diversity increased, but overall plant cover did not change. Graminoids appeared to benefit the most, increasing in average cover, richness, and relative dominance. The rise in graminoid dominance was largely at the expense of shrubs, which showed little ability to benefit from overstory mortality within the first years following attack. Most plant responses were positively related to the total tree basal area lost since the peak of the outbreak, suggesting that increased resource availability following tree death may benefit understory plants. However, a negative relationship between several understory variables and tree sapling density provides evidence that understory plants compete with saplings for the newly available resources. Tree seedling density nearly doubled over the duration of the study, indicating a strong regeneration pulse. Among species, lodgepole pine displayed the greatest tree seedling establishment. This is one of the first studies to use repeated measurements to describe this often-overlooked component of forest change associated with MPB disturbance.
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Acknowledgements
We are grateful to Matt Diskin for conceiving the understory study, assisting with data collection in 2008, and contributing to development of the 2013 resampling scheme. We thank Kellen Nelson for his statistical assistance and discussions contributing to this manuscript and Paige Parry for providing helpful comments on a previous version of this manuscript. We also thank Phil Georgakakos, Rachel Ray, and Chris Deaderick for their help with field data collection and Rocky Mountain National Park staff for their logistical support.
Funding
This research was funded by the Rocky Mountain Conservancy and a McIntire–Stennis award to Colorado State University.
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Pappas, G.S., Tinker, D.B. & Rocca, M.E. Understory vegetation response to mountain pine beetle disturbance in northern Colorado lodgepole pine forests. Plant Ecol 221, 1293–1308 (2020). https://doi.org/10.1007/s11258-020-01082-5
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DOI: https://doi.org/10.1007/s11258-020-01082-5