Title:

Assisted Migration and Drought in the Northeast U.S.

Forestry assisted migration, the planting of tree species and populations to match forests’ future climate conditions, has been proposed as a tool to mitigate the negative impacts of climate change on forest health and productivity in the Northeast U.S. Successful implementation of assisted migration requires guidelines for selecting species and ecotypes adapted to anticipated future climate conditions, including increases in drought frequency and intensity; however, this information is lacking for the Northeast. To address this knowledge gap, we conducted a two-year common garden experiment in a semi-open greenhouse (Durham, NH) to test how seven species from three source locations respond to drought: black cherry (Prunus serotina), black walnut (Juglans nigra), eastern white pine (Pinus strobus), sugar maple (Acer saccharum), southern red oak (Quercus falcata), northern red oak (Q. rubra), and white oak (Q. alba) from nurseries in New Hampshire, Michigan, and Virginia (n=450). Height and diameter were measured monthly during the growing season and biomass was measured at the end of the experiment. Physiological measurements including foliar gas exchange and water potential were measured to elucidate the mechanisms underlying growth results. Black cherry drought and control plants exhibited the highest relative growth rates compared to other species and treatments while eastern white pine showed consistently low height growth rates across all treatments and the lowest treatment effect of any species. Ecotype effects were generally weaker than species effects but were notable in cases including black cherry height and red oak diameter growth. Stronger interspecific responses to the drought treatment relative to ecotype differences imply that species selection may be particularly important when considering reforestation for drought resiliency. However, inconsistencies in the growth responses of different ecotypes underscores the need to conduct common garden experiments that encompass a wider range of ecotypes and sufficient sample size to separate out effects of species and ecotypes on drought response.

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