Wed, Aug 04, 2021:On Demand
Background/Question/Methods
A century of browsing by overabundant deer has caused depauperate understory and forest floor communities throughout the eastern deciduous forest. Herbivory may also have cryptic impacts on plant phenology, with unknown consequences. Recent empirical research demonstrates that tree leafout now occurs earlier in the year and spring wildflowers are unable to advance their phenology quickly enough to maintain their ephemeral window before canopy closure. We hypothesize that browsing may directly and indirectly prevent wildflowers from advancing the timing of their emergence, leafout, flowering, and fruiting. Browsing can directly delay phenology by stressing plants or removing flowers, while the dense browse-tolerant understory layer could cause indirect delays by reducing light and resource availability. We tested these hypotheses over 12 weeks from April to July, using a fully factorial long-term experiment consisting of six deer exclosures and paired control plots established in 1996. We removed the recalcitrant cohort of beech and sugar maple saplings from half of each exclosure and control in 2012. We also measured temperature and moisture differences and canopy closure within and outside of exclosures to control for the effects of the exclosures themselves, not just the removal of herbivores, on phenology.
Results/Conclusions Browsing halved the number of species that flowered and reduced the species richness by 25%. The removal of the recalcitrant understory layer increased species richness by 10% but had no effect on phenology. Yet, herbivory delayed the timing of both flowering and fruiting by ~1 week. Browsing did not alter mean date of plant or leaf emergence. By contrast, Heberling et al. (2019) showed that trees have advanced leafout by almost 2 weeks, while spring wildflowers have only advanced their leafout by ~6 days. Surprisingly, the removal of the recalcitrant understory layer also delayed flowering by more than a week, regardless of browsing. Air and soil temperature were significantly lower when the recalcitrant layer was removed early in the growing season, potentially explaining the delayed flowering in the removal sites. Long-term over-browsing by deer significantly delays plant reproductive phenology in forest understories, but environmental variability does not explain these differences. The phenological delays caused by herbivory are a potential contributor to forest wildflowers failing to keep up with phenological advances by canopy trees in response to climate change.
Results/Conclusions Browsing halved the number of species that flowered and reduced the species richness by 25%. The removal of the recalcitrant understory layer increased species richness by 10% but had no effect on phenology. Yet, herbivory delayed the timing of both flowering and fruiting by ~1 week. Browsing did not alter mean date of plant or leaf emergence. By contrast, Heberling et al. (2019) showed that trees have advanced leafout by almost 2 weeks, while spring wildflowers have only advanced their leafout by ~6 days. Surprisingly, the removal of the recalcitrant understory layer also delayed flowering by more than a week, regardless of browsing. Air and soil temperature were significantly lower when the recalcitrant layer was removed early in the growing season, potentially explaining the delayed flowering in the removal sites. Long-term over-browsing by deer significantly delays plant reproductive phenology in forest understories, but environmental variability does not explain these differences. The phenological delays caused by herbivory are a potential contributor to forest wildflowers failing to keep up with phenological advances by canopy trees in response to climate change.