Modeling browse impacts on sapling and tree recruitment across forests in the northern United States

Understanding the patterns of tree recruitment is essential to quantifying the future health and productivity of forest ecosystems. Using national forest inventory information, we incorporated browse impact measurements into models of sapling (2.5-12.7 cm diameter at breast height (DBH)) and overstory tree (≥12.7 cm DBH) ingrowth across the northern United States. Ingrowth was modeled with standard and zero-inflated techniques using discrete Poisson and negative binomial distributions. Zero-inflated models using stand attributes and browse impacts provided the best fit statistics for modeling the occurrence and frequency of ingrowth over a 5-year time period. Results indicate that stands with very high browse impact would contain 50.0% fewer ingrowth saplings compared with stands with no browse impact. Greater browse impacts similarly yielded a negative effect on overstory tree ingrowth, but to a lesser degree than saplings. Despite the stochastic nature of ingrowth observations, incorporating browse impacts may be essential in determining accurate levels of ingrowth in forests where herbivory constrains forest regeneration objectives.