The canary in the coal mine: Sprouts as a rapid indicator of browse impact in managed forests

Forest managers are frequently confronted with sustaining vegetation diversity and structure in landscapes experiencing high ungulate browsing pressure. Often, managers monitor browse damage and risk to plant communities using vegetation as indicators (i.e., phytoindicators). Although useful, the efficacy of traditional phytoindicators is sometimes hampered by limited distribution and abundance, variable browse susceptibility, and lagged responses. In contrast, sprouts possess traits which make them readily available and attractive to browsers, yet fairly resilient to tissue loss. Here, we experimentally evaluate whether hardwood tree stump sprouts are effective and sensitive phytoindicators of deer browse pressure. We measured sprout abundance and height in fenced and unfenced plots at 17 shelterwood harvested sites scattered across a 6500 km² region where deer densities varied by nearly an order of magnitude. We found browsing did not alter the proportion of stumps sprouting and sprout density; however, browse pressure varied among the four most abundant species. Acer rubrum and Acer saccharum were heavily browsed, although browse pressure on A. saccharum decreased in areas with greater canopy openness. Fagus grandifolia and Prunus serotina were less preferred. Differences in palatability altered size hierarchies. Averaged across all species, browsing reduced sprout height by 39%, relative to protected sprouts. Under ambient browsing, P. serotina was 60-100% taller than other species and significantly taller than A. saccharum and F. grandifolia. However, within fences A. saccharum and A. rubrum doubled in size, relative to browsed individuals, and were as tall as P. serotina. Deer impact on sprout height within unfenced forest stands was negatively correlated with estimated deer densities (R² = 0.46). Thus, we suggest sprout surveys can provide a measure of impact across much larger areas. Our results demonstrate that sprouts, particularly those of Acer species, offer an abundant, easily measured, and reliable indicator of browse pressure. Moreover, browse impacts on sprouts emerged before impacts were detected on seedling abundance, height, or biomass. We argue sprouts can warn of imminent browse risk to seedlings (and perhaps non-woody vegetation) and thereby allow managers to take actions to mitigate or avert losses to the regenerating seedling cohort.