Abstract
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.
Highlights
Rising temperatures and increasing frequency of natural disturbances due to climate change will affect the future viability of forest ecosystems [1]
Most differences among the geographic provenances were negligible, suggesting that different seed sources could have similar levels of tolerance to water stress and browsing. This result should be considered with care because local adaptation to water stress and herbivory pressure is possible, this is more often documented for resistance to herbivore abilities [50,51,52]
We compared a limited number of seed sources on a relatively small latitudinal gradient from 65◦ N to 72◦ N, and from only one geographic provenance for each climate analogue
Summary
Rising temperatures and increasing frequency of natural disturbances due to climate change will affect the future viability of forest ecosystems [1]. 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) [4,5,6]. In the Northern hemisphere, assisted migration usually means moving seedlings from southern to more northern locations, with managers aiming to match genotypes and plantations sites using the climatic information at the source site, such as temperature and mean precipitation e.g., [7]. Especially ungulates, are increasingly recognized as a major barrier to artificial and natural tree regeneration [9,10], but they are rarely considered in current species or genotype selection e.g., [7,11,12]
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