Seedling Response to Simulated Browsing and Reduced Water Availability: Insights for Assisted Migration Plantations

To facilitate forest transition to future climate conditions, managers can use adaptive silvicultural tools, for example the assisted translocation of tree species and genotypes to areas with suitable future climate conditions (i.e., assisted migration). Like traditional plantations, however, assisted migration plantations are at risk of failure because of browsing by ungulate herbivores. The ability of seedlings to tolerate browsing could also be hampered by low water availability, as is expected under climate change. Using a greenhouse experiment with five eastern North American tree species, we evaluated the effects of simulated winter browsing and reduced water availability on the growth (total biomass, shoot:root ratio), survival, and chemical composition (nitrogen, total phenolics, flavonoids) of seedlings. We compared seedlings from three geographic provenances representing three climate analogues, i.e., locations with a current climate similar to the climate predicted at the plantation site at a specific time (here: current, mid-century and end of the century). We hypothesized that seedlings would allocate resources to the system (shoots or roots) affected by the most limiting treatment (simulated browsing or reduced water availability). Additionally, we evaluated whether the combination of treatments would have an additive or non-additive effect on the growth, survival and chemical composition of the seedlings. Quercus rubra seedlings reacted only to the water reduction treatment (changes in biomass and N concentration, dependent on geographic provenance) while Pinus strobus reacted only to the simulated browsing treatment (biomass and chemical composition). We also observed non-additive effects of reduced water availability and simulated browsing on Prunus serotina, Acer saccharum and Thuja occidentalis. In general, shoot:root ratio and investment in chemical defense did not vary in response to treatments. The regrowth response observed in Q. rubra and A. saccharum suggests that these species could tolerate periodic browsing events, even when water availability is reduced. More information is required to understand their long-term tolerance to repeated browsing events and to harsher and more frequent water stress. We highlight the importance of species-specific growth and allocation responses that vary with geographic provenance, which should be considered by managers when planning climate-adapted strategies, such as assisted migration.

Planting stock type and seasonality of simulated browsing affect regeneration establishment of Quercus rubra

Animal herbivory is a major limiting factor to successful oak (Quercus spp.) regeneration. Although bare-root seedlings are the most commonly used nursery stock type for oak plantings in the eastern United States, container seedlings may better resist planting stress and help enable seedlings to overcome browsing pressure. Four stock types (1 + 0 bare-root seedlings and seedlings in 164, 336, and 520 mL containers) of northern red oak (Quercus rubra L.) were planted on two reforestation sites in Indiana, USA, which were fenced to exclude white-tailed deer (Odocoileus virginianus Zimmerman). Seedlings were then subjected to three simulated browsing treatments (control unclipped, dormant clipped, and summer clipped). Container seedlings exhibited higher relative growth rates on both sites; for example, at one site, control seedlings in 336 mL containers had relative height growth of 558% compared with 79% for bare-root control seedlings. On both sites, summer-browsed seedlings of all stock types had negligible height growth, and summer browsing reduced survival at one site by 23% for all stock types compared with control seedlings. Browsing of seedlings during the dormant period did not affect growth for any stock type. Container seedlings may help facilitate rapid establishment of planted oak seedlings, but browse protection is necessary to ensure oak regeneration success in areas of large populations of deer.