Species faced with rapidly shifting environments must be able to move, adapt, or acclimate in order to survive. One mechanism to meet this challenge is phenotypic plasticity: altering phenotype in response to environmental change. Here, we investigated the magnitude, direction, and consequences of changes in two key phenology traits (fall bud set and spring bud flush) in a widespread riparian tree species, Populus fremontii. Using replicated genotypes from 16 populations from throughout the species’ thermal range, and reciprocal common gardens at hot, warm, and cool sites, we identified four major findings: (a) There are significant genetic (G), environmental (E), and GxE components of variation for both traits across three common gardens; (b) The magnitude of phenotypic plasticity is correlated with provenance climate, where trees from hotter, southern populations exhibited up to four times greater plasticity compared to the northern, frost‐adapted populations; (c) Phenological mismatches are correlated with higher mortality as the transfer distances between provenance and garden increase; and (d) The relationship between plasticity and survival depends not only on the magnitude and direction of environmental transfer, but also on the type of environmental stress (i.e., heat or freezing), and how particular traits have evolved in response to that stress. Trees transferred to warmer climates generally showed small to moderate shifts in an adaptive direction, a hopeful result for climate change. Trees experiencing cooler climates exhibited large, non‐adaptive changes, suggesting smaller transfer distances for assisted migration. This study is especially important as it deconstructs trait responses to environmental cues that are rapidly changing (e.g., temperature and spring onset) and those that are fixed (photoperiod), and that vary across the species’ range. Understanding the magnitude and adaptive nature of phenotypic plasticity of multiple traits responding to multiple environmental cues is key to guiding restoration management decisions as climate continues to change.

Evolution has been viewed as occurring primarily through selection among individuals. We present a framework based on multilevel selection for evaluating evolutionary change from individuals to communities, with supporting empirical evidence. Essential to this evaluation is the role that interspecific indirect genetic effects play in shaping community organization, in generating variation among community phenotypes, and in creating community heritability. If communities vary in phenotype, and those phenotypes are heritable and subject to selection at multiple levels, then a community view of evolution must be merged with mainstream evolutionary theory. Rapid environmental change during the Anthropocene will require a better understanding of these evolutionary processes, especially selection acting at the community level, which has the potential to eliminate whole communities while favoring others. Expected final online publication date for the Annual Review of Ecology, Evolution, and Systematics, Volume 51 is November 2, 2020. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Aim  Edaphic heterogeneity may be an important driver of population differentiation in the Amazon but remains to be investigated in trees. We compared the phylogeographic structure across the geographic distribution of two Protium (Burseraceae) species with different degrees of edaphic specialization: Protium alvarezianum, an edaphic specialist of white‐sand habitat islands; and Protium subserratum, an edaphic generalist found in white sand as well as in more widespread soil types. We predicted that in the edaphic specialist, geographic distance would structure populations more strongly than in the edaphic generalist, and that soil type would not structure populations in the edaphic generalist unless habitat acts as a barrier promoting population differentiation. Location  Tropical rain forests of the Peruvian and Brazilian Amazon, Guyana and French Guiana. Methods  We sequenced 1209–1211 bp of non‐coding nuclear ribosomal DNA (internal transcribed spacer and external transcribed spacer) and a neutral low‐copy nuclear gene (phytochrome C) from P. subserratum (n = 65, 10 populations) and P. alvarezianum (n = 19, three populations). We conducted a Bayesian phylogenetic analysis, constructed maximum parsimony haplotype networks and assessed population differentiation among groups (soil type or geographic locality) using analysis of molecular variance and spatial analysis of molecular variance. Results  The edaphic specialist exhibited considerable genetic differentiation among geographically distant populations. The edaphic generalist showed significant genetic differentiation between the Guianan and Amazon Basin populations. Within Peru, soil type and not geographic distance explained most of the variation among populations. Non‐white‐sand populations in Peru exhibited lower haplotype/nucleotide diversity than white‐sand populations, were each other’s close relatives, and formed an unresolved clade derived from within the white‐sand populations. Main conclusions  Geographic distance is a stronger driver of population differentiation in the edaphic specialist than in the generalist. However, this difference did not appear to be related to edaphic generalism per se as adjacent populations from both soil types in the edaphic generalist did not share many haplotypes. Populations of the edaphic generalist in white‐sand habitats exhibited high haplotype diversity and shared haplotypes with distant white‐sand habitat islands, indicating that they have either efficient long‐distance dispersal and/or larger ancestral effective population sizes and thus retain ancestral polymorphisms. These results highlight the importance of edaphic heterogeneity in promoting population differentiation in tropical trees.

Populus fremontii (Fremont cottonwood) is recognized as one of the most important foundation tree species in the southwestern USA and northern Mexico because of its ability to structure communities across multiple trophic levels, drive ecosystem processes and influence biodiversity via genetic-based functional trait variation. However, the areal extent of P. fremontii cover has declined dramatically over the last century due to the effects of surface water diversions, non-native species invasions and more recently climate change. Consequently, P. fremontii gallery forests are considered amongst the most threatened forest types in North America. In this paper, we unify four conceptual areas of genes to ecosystems research related to P. fremontii’s capacity to survive or even thrive under current and future environmental conditions: (i) hydraulic function related to canopy thermal regulation during heat waves; (ii) mycorrhizal mutualists in relation to resiliency to climate change and invasion by the non-native tree/shrub, Tamarix; (iii) phenotypic plasticity as a mechanism for coping with rapid changes in climate; and (iv) hybridization between P. fremontii and other closely related Populus species where enhanced vigour of hybrids may preserve the foundational capacity of Populus in the face of environmental change. We also discuss opportunities to scale these conceptual areas from genes to the ecosystem level via remote sensing. We anticipate that the exploration of these conceptual areas of research will facilitate solutions to climate change with a foundation species that is recognized as being critically important for biodiversity conservation and could serve as a model for adaptive management of arid regions in the southwestern USA and around the world.