Hydrologic refugia,plants, and climate change

Climate, physical landscapes, and biota interact to generate heterogeneous hydrologic conditions in space and over time, which are reflected in spatial patterns of species distributions. As these species distributions respond to rapid climate change, microrefugia may support local species persistence in the face of deteriorating climatic suitability. Recent focus on temperature as a determinant of microrefugia insufficiently accounts for the importance of hydrologic processes and changing water availability with changing climate. Where water scarcity is a major limitation now or under future climates, hydrologic microrefugia are likely to prove essential for species persistence, particularly for sessile species and plants. Zones of high relative water availability – mesic microenvironments – are generated by a wide array of hydrologic processes, and may be loosely coupled to climatic processes and therefore buffered from climate change. Here, we review the mechanisms that generate mesic microenvironments and their likely robustness in the face of climate change. We argue that mesic microenvironments will act as species‐specific refugia only if the nature and space/time variability in water availability are compatible with the ecological requirements of a target species. We illustrate this argument with case studies drawn from California oak woodland ecosystems. We posit that identification of hydrologic refugia could form a cornerstone of climate‐cognizant conservation strategies, but that this would require improved understanding of climate change effects on key hydrologic processes, including frequently cryptic processes such as groundwater flow.     

Hydraulic architecture and the evolution of shoot allometry in contrasting climates

We used pairs of congeneric shrub species from contrasting habitats to test for repeated evolutionary divergence in leaf-stem allometry and shoot hydraulic architecture in response to water availability. Allometric relationships and mean ratios between leaf size (individual and total area and mass per shoot) and stem cross-sectional area were compared between habitats using six species pairs representing three genera (Arctostaphylos, Baccharis, Ceanothus). We measured correlations among evolutionary changes in allometric, morphological, and physiological traits using phylogenetic independent contrasts. Allometric analysis revealed habitat differences: slopes were homogeneous among species (=1.46), but the more mesic-adapted species generally supported more leaf area at a common stem cross-sectional area. Reducing bivariate allometry to a ratio obscured this pattern because ratios varied with stem size, which was unrelated to habitat. Mean individual leaf size also was not correlated with either water availability or leaf-stem allometry. Stem hydraulic conductivity was generally lower in the xeric-adapted species of each pair, and its evolution mirrored changes in shoot allometry. This study provides evidence for repeated evolutionary divergence in shoot allometry and hydraulic architecture associated with water availability and demonstrates the importance of shoot allometry to water relations, independent of leaf size.     

Adaptation, niche conservatism, and convergence: comparative studies of leaf evolution in the California chaparral

Small leaves and low specific leaf area (SLA) have long been viewed as adaptations to Mediterranean-type climates in many species of evergreen woody plants. However, paleobotanical and floristic evidence suggests that in many cases these traits originated prior to the advent of the summer-drought climate regime. In this study, molecular phylogenies and ancestral state reconstructions were used to test the hypothesis of adaptive leaf evolution in 12 lineages of evergreen shrubs in the California chaparral. Across all lineages there was a small but significant shift toward lower SLA, but there were no trends in leaf size evolution. For individual lineages, adaptive changes were detected in only three cases for SLA and in one case for leaf size. Three of these cases of evolutionary change were observed in taxa derived from cool temperate ancestors (e.g., Heteromeles). In contrast, most lineages originating from subtropical ancestors exhibited relative stasis in leaf trait evolution (e.g., Ceanothus). The absence of change suggests that ancestors of chaparral taxa had already acquired appropriate traits that contributed to their success under Mediterranean-type climates. These results illustrate how biogeographic history may influence patterns of trait evolution and adaptation and highlight the contribution of ecological sorting processes to the assembly and functional ecology of regional biotas.     

A broader model for C₄ photosynthesis evolution in plants inferred from the goosefoot family (Chenopodiaceae s.s.)

C(4) photosynthesis is a fascinating example of parallel evolution of a complex trait involving multiple genetic, biochemical and anatomical changes. It is seen as an adaptation to deleteriously high levels of photorespiration. The current scenario for C(4) evolution inferred from grasses is that it originated subsequent to the Oligocene decline in CO(2) levels, is promoted in open habitats, acts as a pre-adaptation to drought resistance, and, once gained, is not subsequently lost. We test the generality of these hypotheses using a dated phylogeny of Amaranthaceae s.l. (including Chenopodiaceae), which includes the largest number of C(4) lineages in eudicots. The oldest chenopod C(4) lineage dates back to the Eocene/Oligocene boundary, representing one of the first origins of C(4) in plants, but still corresponding with the Oligocene decline of atmospheric CO(2). In contrast to grasses, the rate of transitions from C(3) to C(4) is highest in ancestrally drought resistant (salt-tolerant and succulent) lineages, implying that adaptation to dry or saline habitats promoted the evolution of C(4); and possible reversions from C(4) to C(3) are apparent. We conclude that the paradigm established in grasses must be regarded as just one aspect of a more complex system of C(4) evolution in plants in general.     

Trait evolution, community assembly, and the phylogenetic structure of ecological communities

Taxa co-occurring in communities often represent a nonrandom sample, in phenotypic or phylogenetic terms, of the regional species pool. While heuristic arguments have identified processes that create community phylogenetic patterns, further progress hinges on a more comprehensive understanding of the interactions between underlying ecological and evolutionary processes. We created a simulation framework to model trait evolution, assemble communities (via competition, habitat filtering, or neutral assembly), and test the phylogenetic pattern of the resulting communities. We found that phylogenetic community structure is greatest when traits are highly conserved and when multiple traits influence species membership in communities. Habitat filtering produces stronger phylogenetic structure when taxa with derived (as opposed to ancestral) traits are favored in the community. Nearest-relative tests have greater power to detect patterns due to competition, while total community relatedness tests perform better with habitat filtering. The size of the local community relative to the regional pool strongly influences statistical power; in general, power increases with larger pool sizes for communities created by filtering but decreases for communities created by competition. Our results deepen our understanding of processes that contribute to phylogenetic community structure and provide guidance for the design and interpretation of empirical research.     

Leaf position,light levels,and nitrogen allocation in five species of rain forest pioneer trees

We used path analysis to ask whether leaf position or leaf light level was a better predictor of within-plant variation in leaf nitrogen concentration in five species of rain forest pioneer trees (Cecropia obtusifolia, Ficus insipida, Heliocarpus appendiculatus, Piper auritum, and Urera caracasana) from the Los Tuxtlas Biological Station, Veracruz, Mexico. Three hundred seventy-five leaves on 28 plants of the five species were analyzed for leaf nitrogen concentration, leaf mass per area, and leaf light interception at different positions (= nodes) along a shoot. Mean values of leaf nitrogen concentration ranged from 0.697 to 0.993 g/m² in the five species, and varied by as much as 2.24 g/m² among leaves on individual plants. Leaf position on the shoot explained significantly more of the within-plant variation in leaf nitrogen concentration than did leaf light level in four of the five species: Cecropia obtusifolia, Heliocarpus appendiculatus, Piper auritum (branch leaves only), and Urera caracasana. However, individual species differed considerably in the patterns of nitrogen allocation and leaf mass per area among leaves on a shoot. These results suggest that leaf nitrogen deployment in these plants is, in part, developmentally constrained and related to the predictability of canopy light distribution associated with plant growth form.     

Variation in nuclear DNA content across environmental gradients: a quantile regression analysis

The nuclear DNA content of angiosperms varies by several orders of magnitude. Previous studies suggest that variation in 2C DNA content (i.e. the amount of DNA in G1 phase nuclei, also referred to as the 2C-value) is correlated with environmental factors, but there are conflicting reports in the literature concerning the nature of these relationships. We examined variation in 2C DNA content for 401 species in the ecologically diverse California flora in relation to the mean July maximum temperature, January minimum temperature, and annual precipitation within the geographical ranges of these species. Species with small 2C-values predominate in all environments. Species with large 2C-values occur at intermediate July maximum temperatures, and decline in frequency at both extremes of the July temperature gradient, and with decreasing annual precipitation. Our analysis demonstrates the utility of quantile regression for statistical inference of complex distributions such as these. The method supports our observation that relationships between nuclear DNA content and environmental factors are stronger for species with large 2C-values.     

Self-shading, carbon gain and leaf dynamics: a test of alternative optimality models

A simple model of shoot-level carbon gain is presented addressing the optimal number and life span of leaves in relation to alternative optimality criteria: (1) maximizing carbon export from the shoot, or (2) maximizing the rate of leaf production at the shoot tip. Additionally, the processes that cause declining assimilation with leaf age are considered in relation to (1) leaf position on the shoot (e.g., self-shading) versus (2) leaf age per se. Using these alternative scenarios, only a model based on position-dependent assimilation and maximization of leaf production rates resulted in quantitative predictions for all aspects of leaf dynamics on the shoot (i.e., leaf number, life span, and birth rate), while other approaches predicted that one or more parameters would be infinite. This formulation of the model also predicted that leaves should be maintained on the shoot until the diurnal carbon balance declines to zero, in contrast with other scenarios which predict that leaves should be shed while maintaining a positive carbon balance. Predictions of the model were supported by the results of a field study of carbon gain and leaf dynamics in saplings of three species of tropical pioneer trees (Carica papaya, Cecropia obtusifolia, and Hampea nutricia) which differ in the number of leaves per shoot. The results illustrate that in these fast-growing plants, leaf production and height growth may be more appropriate measures of performance than net carbon export from the shoot, and suggest that leaf senescence is primarily a function of the position of a leaf within the canopy, rather than its chronological age. Received: 13 October 1998 / Accepted: 27 January 1999     

Flammability and serotiny as strategies: correlated evolution in pines

Fire may act as a selective force on plants both through its direct effects by killing or wounding susceptible individuals and through its effect on the environment: the post-fire environment may select specific physiological traits or life histories. We used phylogenetic independent contrasts to test the hypothesis that fire has selected for correlated evolution among alternative suites of traits in pines: a survival/avoidance suite characterized by thick bark, height, and self-pruning of dead branches; and a fire-embracing strategy in which plants invest little into survival, exhibit traits which enhance flammability, and use fire as a means to cue seedling establishment to the post-fire environment through serotinous cones. We created a set of alternative 'supertree' phylogenies for the genus Pinus from published sources. Using these alternative phylogenies, published ecological data for 38 pine species, and newly collected morphological data, we demonstrate that much variation in trait evolution occurs along a fire-surviving/fire-embracing axis. Pines vary in their susceptibility to ignition since a tree that retains dead branches is more likely to carry a fire into the canopy than a tree that self-prunes. The evolution of increased flammability may have altered evolutionary trajectories prompting an evolutionary switch from a fire-surviving to a fire-embracing life history. Alternatively, the fire-embracing strategy may in fact select for increased flammability to ensure canopy ignition and the realization of serotinous seed-release.