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With lengthening growing seasons but increased temperature variability under climate change, frost damage to plants may remain a risk and could be exacerbated by poleward planting of warm-adapted seed sources. Here, we study cold adaptation of tree populations in a wide-ranging coniferous species in western North America to inform limits to seed transfer. Using tree-ring signatures of cold damage from common garden trials designed to study genetic population differentiation, we find opposing geographic clines for spring frost and fall frost damage. Provenances from northern regions are sensitive to spring frosts, while the more productive provenances from central and southern regions are more susceptible to fall frosts. Transferring the southern, warm-adapted genotypes northward causes a significant loss of growth and a permanent rank change after a spring frost event. We conclude that cold adaptation should remain an important consideration when implementing seed transfers designed to mitigate harmful effects of climate change. Assisted migration has been proposed to aid trees in altering their ranges under climate change. Here, Montwé et al. use common garden experiments to show that lodgepole pine populations vary in their cold susceptibility, suggesting seed transfer may increase the risk of frost damage.

Northern forests at the leading edge of their distributions may not show increased primary productivity under climate warming, being limited by climatic extremes such as drought. Looking beyond tree growth to underlying physiological mechanisms is fundamental for accurate predictions of forest responses to climate warming and drought stress. Within a 32-year genetic field trial, we analyze relative contributions of xylem plasticity and inferred stomatal response to drought tolerance in regional populations of a widespread conifer. Genetic adaptation leads to varying responses under drought. Trailing-edge tree populations produce fewer tracheids with thicker cell walls, characteristic of drought-tolerance. Stomatal response explains the moderate drought tolerance of tree populations in central areas of the species range. Growth loss of the northern population is linked to low stomatal responsiveness combined with the production of tracheids with thinner cell walls. Forests of the western boreal may therefore lack physiological adaptations necessary to tolerate drier conditions. Northern tree populations may not benefit under climate change, with implications for assisted migration and range expansion. Here, Isaac-Renton et al. show that leading-edge lodgepole pine populations have fewer characteristics of drought-tolerance, so may not adapt to tolerate drier conditions.

The increasing frequency of severe drought and heat events under climate change is a major cause for concern for forest productivity and survival. One potential solution to improve forest resilience to drought may involve tree removal to reduce competition. To quantify potential benefits of partial cutting systems under drought, we use tree-ring data from a long-term partial cutting experiment in a dry ecosystem in western Canada, composed primarily of mature interior Douglas-fir ( Pseudotsuga menziesii var. glauca ). We ask: (1) How does tree removal change growth responses to drought and (2) how persistent are effects across multiple drought events? We compare growth responses in 83 trees up to 400 years in age from eight treatment units in a randomized experimental design representing a logging treatment with 25% basal area retention and a control (100% basal area retention). Retention harvesting was conducted in winter of 2002/2003 after a moderate drought in the summer of 2002. The site experienced a more severe drought in 2009 and a drought-heat event in 2015. In 2020, we obtained tree height and diameter data as well as tree increment cores from canopy dominant Douglas-fir, which were used to derive basal area increments. Based on these growth increments, we calculated four indices of resilience to drought. Growth increments of remaining individuals substantially increased after tree removal. Relative to the control, the low retention treatment expressed significantly higher post-drought growth increments compared to growth increments after the drought years of 2002 and 2009, i.e., higher drought recovery. Growth increments in the low retention treatment also had a significantly higher ability to regain pre-drought growth levels after the 2002 and 2009 droughts, i.e., higher drought resilience. However, the treatment did not show higher resilience under a heat-drought event in 2015. Although the treatment effects on drought response indices diminished over time, the absolute growth increments of residual trees remained higher than the control 17 years after harvest, despite repeated droughts. These results suggest that partial cutting on moisture-limited sites provides immediate and medium term benefits for remaining trees in terms of drought resilience, which may help forests adapt to climate change.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.An amendment to this paper has been published and can be accessed via a link at the top of the paper.

As northern latitudes experience rapid winter warming, there is an urgent need to assess the effect of varying winter conditions on tree growth and forest carbon sequestration potential. We examined tree growth responses to variability in cold-season (November–April) frequency of freeze days (FFD) over 1951 to 2018 using tree-ring data from 35,217 trees and 57 species at 4,375 sites distributed across Canada. We found that annual radial growth responses to FFD varied by species, with some commonalities across genera and clades. The growth of gymnosperms with late spring leaf-out strategies was negatively related to FFD; years with high FFD were most detrimental to the annual growth of Pinus banksiana, Pinus contorta, Larix lyalli, Abies amabilis, and Abies lasiocarpa. In contrast, the growth of angiosperms with early leaf-out strategies, namely, Populus tremuloides and Betula papyrifera, was better in the coldest years, and gymnosperms with intermediate leaf-out timing, such as widespread Picea mariana and Picea glauca, had no consistent relationship to FFD. Tree growth responses to FFD were further modulated by tree size, tree age, regional climate (i.e., mean cold-season temperature), and local site conditions. Overall, our results suggest that moderately warming winters may temporarily improve the growth of widespread pines and some high-elevation conifers in western Canada, whereas warming winters may be detrimental to the growth of widespread boreal angiosperms. Our findings also highlight the value of using species-specific climate-growth relationships to refine predictions of forest carbon dynamics.

The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author.

The presentation of the authors' names and (or) special characters in the title of the pdf file of the accepted manuscript may differ slightly from what is displayed on the item page. The information in the pdf file of the accepted manuscript reflects the original submission by the author.

Recent experiments suggest that introduced, non-migratory Canada geese (Branta canadensis) may be facilitating the spread of exotic grasses and decline of native plant species abundance on small islets in the Georgia Basin, British Columbia, which otherwise harbour outstanding examples of threatened maritime meadow ecosystems. We examined this idea by testing if the presence of geese predicted the abundance of exotic grasses and native competitors at 2 spatial scales on 39 islands distributed throughout the Southern Gulf and San Juan Islands of Canada and the United States, respectively. At the plot level, we found significant positive relationships between the percent cover of goose feces and exotic annual grasses. However, this trend was absent at the scale of whole islands. Because rapid population expansion of introduced geese in the region only began in the 1980s, our results are consistent with the hypothesis that the deleterious effects of geese on the cover of exotic annual grasses have yet to proceed beyond the local scale, and that a window of opportunity now exists in which to implement management strategies to curtail this emerging threat to native ecosystems. Research is now needed to test if the removal of geese results in the decline of exotic annual grasses.