Indices of tree adaptive capacity were created for five strategies.

1) Tree phenotypic plasticity: The ability of individual trees to adapt their phenotype to better cope with drought and/or warming may be especially important under climate change. If phenotypic plasticity is high, a tree may be able to alter its morphology rapidly (e.g., leaf characteristics) in response to a given stressor. These changes may allow individuals to withstand a stressor and persist. The index presented here was developed using data on leaf morphology. However, any ecophysiological characteristic that underlies a tree’s ability to respond to climatic stress could be linked with phenotypic plasticity.

2) Population phenotypic diversity: Over time, climate change is expected to exceed tree phenotypic plasticity. Because genotype and phenotype are linked, there is a higher probability that a population might contain a better suited phenotype when population-level genetic diversity is high. The potential phenotypic diversity index used here compares the level of expected heterozygosity in a population.

3) Genetic exchange within populations: Genetic exchange is expected to help shape overall response of a population to climate change over the longer term. In order for population-level adaptation in response to novel environmental conditions to occur, better suited phenotypes and its associated genetic material need to be disseminated between parents, recombined and passed on to offspring. Genetic recombination that results in adaptation happens through sexual reproduction (i.e. through seeds). For trees, high gene mixing within a population tend to be associated with species that produce large quantities of seed based on genetic material from unrelated parents and that are effective at seed dispersal. Therefore, the index for this strategy considers the number of viable seeds that are genetically distinct.

4) Genetic exchange between populations: Not only can species exchange genetic material within a population, but also between populations. Gene flow through pollen or seeds transfers better suited genes between populations, contributing to their overall adaptation. The index for this strategy considers the level of genetic differentiation among populations as an indicator of the potential for natural or artificial gene flow and population connectivity.

5) Genetic exchange between species: Genetic exchange can also occur through hybridization where species’ ranges overlap or between species without physiological barriers to reproduction. Hybridization could lead to the genes from one species’ gene pool being incorporated into the gene pool of another (i.e., introgression). When hybridization occurs frequently, there is a higher likelihood of forming new combinations of genes and phenotypes that could tolerate novel climate conditions. This index uses the number of hybrids a species is known to form.

Indices were developed for the five strategies: Individual adaptation, Population phenotypic diversity, Genetic exchange within populations, Genetic exchange between populations and Genetic exchange between species. An index value was attributed to each species based on its relative ability to adapt to changes in climate. Five classes were defined of increasing adaptive capacity (from dark to light red): low, medium-low, intermediate, medium-high and high. Missing data values are classified as Data not available.

Data confidence is assessed depending on the strategy: levels for Individual adaptation, Population phenotypic diversity and Genetic exchange between populations are based on the level of variability in the values for a given species. For Genetic exchange within populations, confidence level is based on the availability of trait data related to reproductive capacity and dispersal. For Genetic exchange between species, confidence was assessed if observed hybridization is supported by a literature reference. This information was ranked into five confidence classes: low, medium-low, intermediate, medium-high, high. Missing data values are attributed Data not available.

For a given species, each coloured tile represents a corresponding index value (reds) and confidence value (blues).

For more information on index development and how adaptive capacity classes were defined, please see the publication “Finding common ground: Toward comparable indicators of adaptive capacity of tree species to a changing climate” (Available online: https://doi.org/10.1002/ece3.8024).

Another set of strategies that characterize species’ response to a climate stressor – are presented on the Sensitivity page.

Glossary - Glossary of terms used in vulnerability assessments