Interactive effects of flooding and deer (Odocoileus virginianus) browsing on floodplain forest recruitment

Floodplain forests have historically been resilient to the effects of flooding because the tree species that inhabit these ecosystems regenerate and grow quickly following disturbances. However, the intensity and selectivity of ungulate herbivory in floodplains has the potential to modify the community-level effects of flooding by delaying forest recruitment and leaving sites vulnerable to invasive species. We established a series of exclosures along an elevation gradient in an actively recruiting floodplain forest along the Upper Mississippi River prior to three large-magnitude flood events. Pre-flood browsing by Odocoileus virginianus (white-tailed deer) ranged from 20% to 85% of all available stems, and reduced subsequent annual tree height growth from 60 cm/yr to approximately 35 cm/yr, regardless of elevation. Tree mortality, in contrast, was positively correlated with both pre-flood browsing rates and the duration of the growing season that the ground elevation of plots was flooded. Mortality rates ranged from approximately 40% in plots that experienced low levels of deer browsing (<30% of stems) and short flood durations (<40 days) to as high as 98% in plots that experienced high levels of deer browsing (>80% of stems) and long flood durations (>50 days). Longer flood durations led to larger shifts in tree community composition, away from heavily browsed and less flood tolerant Acer saccharinum L. (silver maple) and Populus deltoides (cottonwood) and toward species that were more flood tolerant and not preferred by deer. Phalaris arundinacea (reed canarygrass) colonized some portions of all plots, except for those situated at high elevations and protected by exclosures.Hence, herbivory can interact with the local flooding regime of rivers to delay recruitment of some tree species, resulting in shifts in successional trajectories, and leaving young forests vulnerable to invasion by exotic herbaceous species.

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► Effects of herbivory and flooding were studied in a floodplain forest.
► Herbivory reduced tree height growth from 60 cm/yr to 35 cm/yr.
► Tree mortality rates increased from 40% to 98% with herbivory and flood duration.
► Flooding promoted shifts away from heavily browsed and less flood tolerant species.
► Herbivory and flooding interacted to promote invasion by Phalaris arundinacea.