Deer browsing overwhelms extended leaf phenology benefits: A test case with Rubus allegheniensis and a recalcitrant hay-scented fern layer
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Forest Ecology and Management
Plant species coexistence can be promoted by multiple tradeoffs including temporal niche separation via differences in phenology. Namely, if phenological differences afford longer leaf life-spans that confer species the opportunity to exploit light and fix carbon during periods relatively free of other competitors, then coexistence, or even competitive superiority, may arise. Phenological niche separation explanations, including the Extended Leaf Phenology (ELP) hypothesis, have garnered considerable support as a mechanistic explanation for both exotic invasive species success and maintenance of native plant species. The benefits conferred by a phenological advantage, however, can be nullified if tissue losses from browsing are coincident with this phenological window of opportunity. This study experimentally tests the role of phenological advantage and white-tailed deer (Odocoileus virginianus) browsing, alone and in concert, in structuring coexistence between the native forest shrub Rubus allegheniensis, which possesses an extended phenological window, and the native invasive fern Dennstaedtia punctilobula. Browsing treatments (ambient versus excluded) were factorially crossed with shading treatments (none versus shading timed to eliminate phenological advantage). The results demonstrate the phenological advantage is an essential growth mechanism for Rubus as seedlings deprived of the ELP advantage were, on average, 50% shorter than those not treated with shade. However, this benefit was largely dependent on herbivory pressure, as deer browsing largely nullified the growth benefits provided by the phenological advantage. This experiment is the first experimental study documenting how browsing impacts affect a plant species by curtailing a phenological coexistence advantage. The results suggest elucidating the interacting roles of the ELP and browsing is critical, particularly as the phenological niche advantage does not necessarily constitute a fitness advantage where browsing pressure is high. Results from this work also provide broader insight into how exotic plant species invasions that are assumed to be aided by a phenological (ELP) advantage, may be modulated through their browse susceptibility. Overall, these results suggest understanding the co-occurring roles of phenology and browsing may guide management decisions to maintain or promote understory plant diversity.
Royo, Alejandro A.; Stanovick, John S. 2019. Deer browsing overwhelms extended leaf phenology benefits: A test case with Rubus allegheniensis and a recalcitrant hay-scented fern layer. Forest Ecology and Management. 448: 294-299. https://doi.org/10.1016/j.foreco.2019.06.019.