Interactive effects of flooding and deer (Odocoileus virginianus) browsing on floodplain vegetation and nitrogen dynamics

Background/Question/Methods

Periodic flooding promotes rapid vegetation turnover and nitrogen cycling along floodplains. However, the intensity and selectivity of ungulate herbivory has the potential to modify plant community responses to flooding, with associated effects on the soil microclimate and rates of nitrogen cycling. Such changes may feedback to further influence floodplain plant communities and influence rates of nitrogen transfer across aquatic-terrestrial transition zones. To study the interactive effects of flooding and herbivory on floodplain vegetation and nitrogen dynamics, 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. 

Results/Conclusions

Pre-flood browsing by Odocoileus viginianus (white-tailed deer) reduced annual tree height growth rates, thereby extending the duration that saplings were completely submerged during the flood events. Tree mortality rates ranged from approximately 40% in high elevation plots that experienced low levels of deer browsing (<30% of stems) to as high as 98% in low elevation plots that experienced high levels of deer browsing (>80% of stems). Phalaris arundinacea (reed canarygrass, an exotic herbaceous species) colonized some portions of all plots, except for those situated at high elevations and protected by exclosures.

Potential nitrification and mineralization rates depended on floodplain elevation and time since the flood pulse, but were generally highest in plots dominated by Phalaris. Relative to other vegetation types, Phalaris maintained a dense litter layer and had soils with higher organic matter which maintained higher soil moisture following the flood pulse. In contrast, heavily browsed plots had the most open canopies with the warmest, driest, and heaviest soils. Net dissolved inorganic nitrogen accumulation (ammonium and nitrate) following the flood pulse averaged 0.46 (+/-0.13) µg/g/d in Phalaris plots, 0.09 (+/-0.11) µg/g/d in heavily browsed plots, and (0.16 (+/-0.05) µg/g/d in unbrowsed plots.

Hence, herbivory can interact with the local flooding regime of rivers to delay forest recruitment, leave young forests vulnerable to invasion by exotic herbaceous species, and in turn modify rates of nitrogen cycling.