Shifting range limits are predicted for many species as the climate warms. However, the rapid pace of climate change will challenge the natural dispersal capacity of long-lived, sessile organisms such as forest trees. Adaptive responses of populations will, therefore, depend on levels of genetic variation and plasticity for climate-responsive traits, which likely vary across the range due to expansion history and current patterns of selection. Here, we study levels of genetic and plastic variation for phenology and growth traits in populations of red spruce ( Picea rubens ), from the range core to the highly fragmented trailing edge. We measured more than 5000 offspring sampled from three genetically distinct regions (core, margin and edge) grown in three common gardens replicated along a latitudinal gradient. Genetic variation in phenology and growth showed low to moderate heritability and differentiation among regions, suggesting some potential to respond to selection. Phenology traits were highly plastic, but this plasticity was generally neutral or maladaptive in the effect on growth, revealing a potential liability under warmer climates. These results suggest future climate adaptation will depend on the regional availability of genetic variation in red spruce and provide a resource for the design and management of assisted gene flow. This article is part of the theme issue ‘Species’ ranges in the face of changing environments (Part II)’.

Background and Aims Grasses in subfamily Pooideae live in some of the world’s harshest terrestrial environments, from frigid boreal zones to the arid wind-swept steppe. It is hypothesized that the climate distribution of species within this group is driven by differences in climatic tolerance, and that tolerance can be partially explained by variation in stomatal traits. Methods We determined aridity index (AI) and minimum temperature of the coldest month (MTCM) for 22 diverse Pooideae accessions and one outgroup, and used comparative methods to assess predicted relationships for climate traits versus fitness traits, stomatal diffusive conductance to water (gw), and speed of stomatal closure following drought and/or cold. Key Results Results demonstrate that AI and MTCM predict variation in survival/regreening following drought/cold, and gw under drought/cold is positively correlated with ẟ 13C-measured water use efficiency (WUE). However, the relationship between climate traits and fitness under drought/cold was not explained by gw or speed of stomatal closure. Conclusions These findings suggest that Pooideae distributions are at least partly determined by tolerance to aridity and above freezing cold, but that variation in tolerance is not uniformly explained by variation in stomatal traits.

Local adaptation is increasingly being integrated into macroecological models, offering an evolutionary perspective that has been largely missing from climate change biogeography. Genomic offsets, which quantify the disruption of existing genotype-environment associations under environmental change, are an informative landscape genomic tool that allows for the incorporation of intra-specific adaptive differentiation into forecasts of climate suitability and thus management planning. Gradient forest (GF), a method originally developed to model community turnover along environmental gradients, is now the most commonly used approach in genomic offset estimation. However, major hurdles in the application of GF-derived offsets are (1) an inability to interpret the absolute magnitude of genomic offsets in a biologically meaningful way and (2) uncertainty in how genomic offsets compare to established species-level approaches like Ecological Niche Models (ENMs). We used both ENMs and novel, genomic offset metrics derived from GF modeling of genomic variation along climatic gradients to assess the climate change vulnerability of red spruce (Picea rubens), a cool-temperate tree species endemic to eastern North America. We show how genomic offsets can be standardized relative to contemporary genomic variation across the landscape to better represent their biological significance and facilitate comparisons among studies. In three common gardens, we found a significant negative relationship between standardized genomic offsets and red spruce growth and higher explanatory power for standardized offsets than (raw) climate transfer distances. We also derived new threshold-based metrics that we term Donor and Recipient Importance and which quantify transferability of propagules between donor and recipient localities while minimizing disruption of genotype-environment associations. This approach leverages landscape genomic information to account for local adaptation when predicting climate suitability. ENMs and our novel genomic offset metrics largely agreed in forecasting drastic northward range shifts. Combining several offset-based metrics, we show that the projected northward shift of suitable climate mainly applies to populations located in the center and northern parts of the current range, whereas southern populations might be able to persist in situ owing to specific local climate adaptations. The novel metrics thus yield refined, region-specific prognoses for local persistence and show how management could be improved by considering assisted migration.