Silviculture at establishment of hardwood plantations is relatively ineffective in the presence of deer browsing

Abstract

Ungulate browsing limits forest regeneration on many reforestation and restoration sites. Silviculture can be used to mitigate the effects of ungulate damage by promoting rapid early growth of planted seedlings, but benefits may depend upon site characteristics and ungulate browse pressure. We studied the interactions among browsing by white-tailed deer (Odocoileus virginianus), use of genetically select seed sources, controlled-release fertilization (CRF) at planting, and site type (harvest openings and plantations) in a nine-year hardwood forest regeneration study. The experiment consisted of paired deer exclosure and control plots, with fertilization and seed source, established at two reforested clearcut sites and three afforested agricultural field sites in Indiana, USA. Our objectives were to examine treatment effects on growth (height and diameter), survival, and stem quality of four temperate deciduous hardwood species (northern red oak, white oak, black walnut, and black cherry). For all species, fencing had the greatest significant positive influence on survival (non-fenced: 50–72%, fenced: 71–75% by year 8) and growth (81–178% greater height and 90–167% greater diameter by year 8), as well as stem quality ratings. Fencing also increased (by 50–78%) the probability that black cherry and black walnut at afforested sites (as well as northern red oak at both site types) would reach free-to-grow status by year 5. We observed gains in height and diameter from CRF only during the first three years for fenced black cherry (11% greater height and 14% greater diameter in year 3), and for white oak regardless of fencing (13% greater height and 10% greater diameter in year 3). Genetically select seed sources had the greatest and most consistent growth benefit for black walnut (81% greater height and 50% greater diameter by year 8). Early growth was improved in genetically select P. serotina vs. non-select sources (11% greater height in year 3) but differences faded by the fifth growing season, while superior growth of genetically select Q. rubra began to manifest only after year 5 (16% greater height 21% greater diameter by year 8). In addition, select northern red oaks had an 8% greater probability of reaching free-to-grow status by year 5 and black walnuts at afforested sites had a 13% greater probability of reaching free-to-grow status. Without protection from herbivory, genetically improved sources did not realize their full potential for enhanced growth. Our results from this nine-year-long hardwood plantation experiment confirm that without browse protection, additional silvicultural treatments are unlikely to improve regeneration performance.