Leaf habit determines the hydraulic and resource-use strategies in tree saplings from the Sonoran Desert.

The drought susceptibility of woody saplings may explain their low survival in arid environments. Therefore, it is critical to determine which morphological and physiological traits are more responsive to drought among young plants. This study tested whether plant responses to experimental drought differ between two plant functional groups: the deciduous and evergreen species. We predicted that deciduous species would present a tighter stomatal control under drought, coupled with fast carbon fixation under no stress, tending toward isohydry and faster growth rates than the evergreen species. Using 1-year-old saplings from three evergreen and four deciduous Sonoran Desert tree species, we evaluated their hydraulic and gas exchange traits under three experimental irrigation conditions: high, intermediate and low water availability. We measured CO2 assimilation rates (A), stomatal conductance (gs), the level of iso-anisohydry (as the plant's ability to maintain constant their water potential) and seven morphological and growth-related traits throughout 2 months. Under high water availability, saplings reached their maximum values of A and gs, which were significantly higher for deciduous than evergreen species. Correlations among hydroscape area (HA) and leaf traits positioned species along the iso/anisohydric continuum. Deciduous species presented isohydric characteristics, including low HA, high gs, A and Huber values (HVs), and traits indicative of a faster use of resources, such as low stem-specific density (SSD) and low leaf mass per area (LMA). By contrast, evergreen species showed traits that indicate slow resource use and anisohydric behavior, such as high HA, SSD and LMA, and low gs, A and HVs. Deciduous species drastically reduced gas exchange rates in response to drought, while evergreen maintained low rates independently of drought intensity. Overall, desert saplings showed strategies concordant with the iso-anisohydric continuum and the fast-slow use of resources.

Patterns of leaf trait variation underlie ecological differences among sympatric tree species of Damburneya in a tropical rainforest.

PREMISE: Although ecological differentiation driven by altitude and soil is hypothesized to promote coexistence of sympatric tree species of Damburneya (Lauraceae), the mechanistic role of leaf functional variation on ecological differentiation among co-occurring species remains unexplored. We aimed to determine whether the patterns of leaf trait variation reflect ecological differences among sympatric Damburneya species. We tested whether trait correlations underlying functional strategies and average species traits vary in response to local soil heterogeneity along an altitudinal gradient, potentially affecting species distributions. METHODS: At two contrasting altitudes (100, 1100 m a.s.l.) in a Mexican tropical rainforest, we characterized soil chemical and physical properties and sampled four Damburneya species to quantify five leaf functional traits. We used linear models to analyze paired and multivariate trait correlations, spatial and interspecific effects on trait variation, and trait response to local soil heterogeneity. Relative contributions of intra- and interspecific variation to local trait variability were quantified with an ANOVA. RESULTS: Soil nutrient availability was higher at low altitude, but all species had a high leaf N:P ratio across altitudes suggesting a limited P supply for plants. Species distribution differed altitudinally, with some species constrained to low or high altitude, potentially reflecting soil nutrient availability. Leaf traits responded to altitude and local soil properties, suggesting interspecific differences in functional strategies according to the leaf economics spectrum (conservative vs. acquisitive). CONCLUSIONS: The interspecific divergence in functional strategies in response to local environmental conditions suggests that trait variation could underlie ecological differentiation among Damburneya sympatric species.

PREMISA: Aunque se ha propuesto que la diferenciación ecológica impulsada por la variación del suelo y la altitud ha promovido la coexistencia de especies arbóreas simpátricas de Damburneya (Lauraceae), el papel de la variación de funcional foliar como mecanismo para explicar las diferencias ecológicas entre especies que coocurren sigue sin explorarse. Nuestro objetivo fue determinar si los patrones de variación de los rasgos funcionales foliares reflejan diferencias ecológicas entre especies simpátricas de Damburneya. Hipotetizamos que, tanto las correlaciones entre los rasgos que determinan las estrategias funcionales, como los valores promedio de los rasgos de las especies, varían en respuesta a la heterogeneidad local del suelo a lo largo de un gradiente altitudinal, afectando potencialmente la distribución de las especies. MÉTODO: En dos altitudes contrastantes (100, 1000 m s.n.m) en una selva húmeda tropical mexicana caracterizamos propiedades físicas y químicas del suelo y muestreamos cuatro especies de Damburneya para cuantificar cinco rasgos funcionales foliares. Usamos modelos lineales para analizar las correlaciones pareadas y multivariadas entre rasgos, los efectos espaciales e interespecíficos en la variación de rasgos, y la respuesta de los rasgos a la heterogeneidad local del suelo. Además, cuantificamos la contribución relativa de la variación intra- e interespecífica en la varianza local de los rasgos con un análisis de varianza. RESULTADOS: La disponibilidad de nutrientes en el suelo fue mayor en la zona baja, pero todas las especies tuvieron valores altos de N:P foliar en ambas altitudes, lo que sugiere un suministro limitado de P para las plantas. La distribución de las especies difirió altitudinalmente y algunas de ellas se restringieron a zonas altas o bajas, reflejando potencialmente la disponibilidad de nutrientes en el suelo. Los rasgos foliares respondieron a la altitud y a las propiedades edáficas locales, sugiriendo diferencias interespecíficas en las estrategias funcionales según el espectro de economía foliar (conservativa vs. adquisitiva). CONCLUSIONES: La divergencia interespecífica en las estrategias funcionales en respuesta a las condiciones ambientales locales sugiere que la variación de rasgos podría explicar algunas diferencias ecológicas entre las especies simpátricas de Damburneya.

Above- and below-ground trait coordination in tree seedlings depend on the most limiting resource: a test comparing a wet and a dry tropical forest in Mexico.

The study of above- and below-ground organ plant coordination is crucial for understanding the biophysical constraints and trade-offs involved in species' performance under different environmental conditions. Environmental stress is expected to increase constraints on species trait combinations, resulting in stronger coordination among the organs involved in the acquisition and processing of the most limiting resource. To test this hypothesis, we compared the coordination of trait combinations in 94 tree seedling species from two tropical forest systems in Mexico: dry and moist. In general, we expected that the water limitation experienced by dry forest species would result in stronger leaf-stem-root coordination than light limitation experienced by moist forest species. Using multiple correlations analyses and tools derived from network theory, we found similar functional trait coordination between forests. However, the most important traits differed between the forest types. While in the dry forest the most central traits were all related to water storage (leaf and stem water content and root thickness), in the moist forest they were related to the capacity to store water in leaves (leaf water content), root efficiency to capture resources (specific root length), and stem toughness (wood density). Our findings indicate that there is a shift in the relative importance of mechanisms to face the most limiting resource in contrasting tropical forests.

Functional recovery of secondary tropical forests.

One-third of all Neotropical forests are secondary forests that regrow naturally after agricultural use through secondary succession. We need to understand better how and why succession varies across environmental gradients and broad geographic scales. Here, we analyze functional recovery using community data on seven plant characteristics (traits) of 1,016 forest plots from 30 chronosequence sites across the Neotropics. By analyzing communities in terms of their traits, we enhance understanding of the mechanisms of succession, assess ecosystem recovery, and use these insights to propose successful forest restoration strategies. Wet and dry forests diverged markedly for several traits that increase growth rate in wet forests but come at the expense of reduced drought tolerance, delay, or avoidance, which is important in seasonally dry forests. Dry and wet forests showed different successional pathways for several traits. In dry forests, species turnover is driven by drought tolerance traits that are important early in succession and in wet forests by shade tolerance traits that are important later in succession. In both forests, deciduous and compound-leaved trees decreased with forest age, probably because microclimatic conditions became less hot and dry. Our results suggest that climatic water availability drives functional recovery by influencing the start and trajectory of succession, resulting in a convergence of community trait values with forest age when vegetation cover builds up. Within plots, the range in functional trait values increased with age. Based on the observed successional trait changes, we indicate the consequences for carbon and nutrient cycling and propose an ecologically sound strategy to improve forest restoration success.

Leaf water relations reflect canopy phenology rather than leaf life span in Sonoran Desert trees.

Plants from arid environments display covarying traits to survive or resist drought. Plant drought resistance and ability to survive long periods of low soil water availability should involve leaf phenology coordination with leaf and stem functional traits related to water status. This study tested correlations between phenology and functional traits involved in plant water status regulation in 10 Sonoran Desert tree species with contrasting phenology. Species seasonal variation in plant water status was defined by calculating their relative positions along the iso/anisohydric regulation continuum based on their hydroscape areas (HA)-a metric derived from the relationship between predawn and midday water potentials-and stomatal and hydraulic traits. Additionally, functional traits associated with plant water status regulation, including lamina vessel hydraulic diameter (DHL), stem-specific density (SSD) and leaf mass per area (LMA) were quantified per species. To characterize leaf phenology, leaf longevity (LL) and canopy foliage duration (FD) were determined. Hydroscape area was strongly correlated with FD but not with leaf longevity (LL); HA was significantly associated with SSD and leaf hydraulic traits (DHL, LMA) but not with stem hydraulic traits (vulnerability index, relative conductivity); and FD was strongly correlated with LMA and SSD. Leaf physiological characteristics affected leaf phenology when it was described as canopy FD better than when described as LL. Stem and leaf structure and hydraulic functions were not only relevant for categorizing species along the iso/anisohydric continuum but also allowed identifying different strategies of desert trees within the 'fast-slow' plant economics spectrum. The results in this study pinpoint the set of evolutionary pressures that shape the Sonoran Desert Scrub physiognomy.

Distribution and conservation of species is misestimated if biotic interactions are ignored: the case of the orchid Laelia speciosa.

The geographic distribution of species depends on their relationships with climate and on the biotic interactions of the species. Ecological Niche Models (ENMs) mainly consider climatic variables only and may tend to overestimate these distributions, especially for species strongly restricted by biotic interactions. We identified the preference of Laelia speciosa for different host tree species and include this information in an ENM. The effect of habitat loss and climate change on the distribution of these species was also estimated. Although L. speciosa was recorded as epiphyte at six tree species, 96% of the individuals were registered at one single species (Quercus deserticola), which indicated a strong biotic interaction. We included the distribution of this host tree as a biotic variable in the ENM of L. speciosa. The contemporary distribution of L. speciosa is 52,892 km2, which represent 4% of Mexican territory and only 0.6% of the distribution falls within protected areas. Habitat loss rate for L. speciosa during the study period was 0.6% per year. Projections for 2050 and 2070 under optimistic and pessimistic climate change scenarios indicated a severe reduction in its distribution. Climaticaly suitable areas will also shift upwards (200-400 m higher). When estimating the distribution of a species, including its interactions can improve the performance of the ENMs, allowing for more  accurate estimates of the actual distribution of the species, which in turn allows for better conservation strategies.

Differentiation in the water-use strategies among oak species from central Mexico.

Oak species (Fagaceae: Quercus) differ in their distribution at the landscape scale, specializing to a certain portion of environmental gradients. This suggests that functional differentiation favors habitat partitioning among closely related species. To elucidate the mechanisms of species coexistence in oak forests, we explored patterns of interspecific variation in functional traits involved in water-use strategies. We tested the hypothesis that oak species segregate along key trade-offs between xylem hydraulic efficiency and safety, and between hydraulic safety and drought avoidance capacity, leading to species niche partitioning across a gradient of aridity. To do so, we quantified biophysical and physiological traits in four red and five white oak species (sections Lobatae and Quercus, respectively) across an aridity gradient in central Mexico. We also explored the trade-offs guiding species differentiation, particularly between the drought tolerance versus water acquisition capacity, and determined whether the water-use strategy was associated with the portion of the environmental gradient that the species occupy. In a trait-by-trait analysis, we detected differences between white and red oak species. However, a larger part of the variation was explained at the species rather than at the section level. We detected two primary axes of trait covariation. The first exhibited differences between species with dense tissues and species with soft tissues (the tissue construction cost axis); however, the oak sections did not constitute separate groups, while the second suggested a trade-off between xylem resistance to cavitation and tree deciduousness. As expected, the water-use strategies of the species were related to the environment; oak species from arid areas had more deciduousness and a higher instantaneous water-use efficiency. In contrast, their humid counterparts had less deciduousness and had a xylem that was more resistant to embolisms. Altogether, these results suggest that aridity filters closely related species, resulting in habitat partitioning and niche divergence.

Exploiting water versus tolerating drought: water-use strategies of trees in a secondary successional tropical dry forest.

In seasonal plant communities where water availability changes dramatically both between and within seasons, understanding the mechanisms that enable plants to exploit water pulses and to survive drought periods is crucial. By measuring rates of physiological processes, we examined the trade-off between water exploitation and drought tolerance among seedlings of trees of a tropical dry forest, and identified biophysical traits most closely associated with plant water-use strategies. We also explored whether early and late secondary successional species occupy different portions of trade-off axes. As predicted, species that maintained carbon capture, hydraulic function and leaf area at higher plant water deficits during drought had low photosynthetic rates, xylem hydraulic conductivity and growth rate under non-limiting water supply. Drought tolerance was associated with more dense leaf, stem and root tissues, whereas rapid resource acquisition was associated with greater stem water storage, larger vessel diameter and larger leaf area per mass invested. We offer evidence that the water exploitation versus drought tolerance trade-off drives species differentiation in the ability of tropical dry forest trees to deal with alternating water-drought pulses. However, we detected no evidence of strong functional differentiation between early and late successional species along the proposed trade-off axes, suggesting that the environmental gradient of water availability across secondary successional habitats in the dry tropics does not filter out physiological strategies of water use among species, at least at the seedling stage.

Root depth and morphology in response to soil drought: comparing ecological groups along the secondary succession in a tropical dry forest.

Root growth and morphology may play a core role in species-niche partitioning in highly diverse communities, especially along gradients of drought risk, such as that created along the secondary succession of tropical dry forests. We experimentally tested whether root foraging capacity, especially at depth, decreases from early successional species to old-growth forest species. We also tested for a trade-off between two mechanisms for delaying desiccation, the capacity to forage deeper in the soil and the capacity to store water in tissues, and explored whether successional groups separate along such a trade-off. We examined the growth and morphology of roots in response to a controlled-vertical gradient of soil water, among seedlings of 23 woody species dominant along the secondary succession in a tropical dry forest of Mexico. As predicted, successional species developed deeper and longer root systems than old-growth forest species in response to soil drought. In addition, shallow root systems were associated with high plant water storage and high water content per unit of tissue in stems and roots, while deep roots exhibited the opposite traits, suggesting a trade-off between the capacities for vertical foraging and water storage. Our results suggest that an increased capacity of roots to forage deeper for water is a trait that enables successional species to establish under the warm-dry conditions of the secondary succession, while shallow roots, associated with a higher water storage capacity, are restricted to the old-growth forest. Overall, we found evidence that the root depth-water storage trade-off may constrain tree species distribution along secondary succession.

Successional changes in functional composition contrast for dry and wet tropical forest.

We tested whether and how functional composition changes with succession in dry deciduous and wet evergreen forests of Mexico. We hypothesized that compositional changes during succession in dry forest were mainly determined by increasing water availability leading to community functional changes from conservative to acquisitive strategies, and in wet forest by decreasing light availability leading to changes from acquisitive to conservative strategies. Research was carried out in 15 dry secondary forest plots (5-63 years after abandonment) and 17 wet secondary forest plots (< 1-25 years after abandonment). Community-level functional traits were represented by community-weighted means based on 11 functional traits measured on 132 species. Successional changes in functional composition are more marked in dry forest than in wet forest and largely characterized by different traits. During dry forest succession, conservative traits related to drought tolerance and drought avoidance decreased, as predicted. Unexpectedly acquisitive leaf traits also decreased, whereas seed size and dependence on biotic dispersal increased. In wet forest succession, functional composition changed from acquisitive to conservative leaf traits, suggesting light availability as the main driver of changes. Distinct suites of traits shape functional composition changes in dry and wet forest succession, responding to different environmental filters.

Drought resistance in early and late secondary successional species from a tropical dry forest: the interplay between xylem resistance to embolism, sapwood water storage and leaf shedding.

The mechanisms of drought resistance that allow plants to successfully establish at different stages of secondary succession in tropical dry forests are not well understood. We characterized mechanisms of drought resistance in early and late-successional species and tested whether risk of drought differs across sites at different successional stages, and whether early and late-successional species differ in resistance to experimentally imposed soil drought. The microenvironment in early successional sites was warmer and drier than in mature forest. Nevertheless, successional groups did not differ in resistance to soil drought. Late-successional species resisted drought through two independent mechanisms: high resistance of xylem to embolism, or reliance on high stem water storage capacity. High sapwood water reserves delayed the effects of soil drying by transiently decoupling plant and soil water status. Resistance to soil drought resulted from the interplay between variations in xylem vulnerability to embolism, reliance on sapwood water reserves and leaf area reduction, leading to a tradeoff of avoidance against tolerance of soil drought, along which successional groups were not differentiated. Overall, our data suggest that ranking species' performance under soil drought based solely on xylem resistance to embolism may be misleading, especially for species with high sapwood water storage capacity.

Coordinated evolution of leaf and stem economics in tropical dry forest trees.

With data from 15 species in eight families of tropical dry forest trees, we provide evidence of coordination between the stem and leaf economic spectra. Species with low-density, flexible, breakable, hydraulically efficient but cavitationally vulnerable wood shed their leaves rapidly in response to drought and had low leaf mass per area and dry mass content. In contrast, species with the opposite xylem syndrome shed their costlier but more drought-resistant leaves late in the dry season. Our results explain variation in the timing of leaf shedding in tropical dry forests: selection eliminates combinations such as low-productivity leaves atop highly vulnerable xylem or water-greedy leaves supplied by xylem of low conductive efficiency. Across biomes, rather than a fundamental trade-off underlying a single axis of trait covariation, the relationship between leaf and stem economics is likely to occupy a wide space in which multiple combinations are possible.

Morphological and physiological differentiation of seedlings between dry and wet habitats in a tropical dry forest.

A common observation in tropical dry forests is the habitat preference of tree species along spatial soil water gradients. This pattern of habitat partitioning might be a result of species differentiation in their strategy for using water, along with competing functions such as maximizing water exploitation and tolerating soil water stress. We tested whether species from drier soil conditions exhibited a tolerance strategy compared with that of wet-habitat species. In a comparison of 12 morphophysiological traits in seedlings of 10 closely related dry and wet-habitat species pairs, we explored what trade-offs guide differentiation between habitats and species. Contrary to our expectations, dry-habitat species showed mostly traits associated with an exploitation strategy (higher carbon assimilation capacity, specific leaf area and leaf-specific conductivity and lower water-use efficiency). Strikingly, dry-habitat species tended to retain their leaves longer during drought. Additionally, we detected multiple strategies to live within each habitat, in part due to variation of strategies among lineages, as well as functional differentiation along the water storage capacity-stem density (xylem safety) trade-off. Our results suggest that fundamental trade-offs guide functional niche differentiation among tree species expressed both within and between soil water habitats in a tropical dry forest.

Hydraulics and life history of tropical dry forest tree species: coordination of species' drought and shade tolerance.

Plant hydraulic architecture has been studied extensively, yet we know little about how hydraulic properties relate to species' life history strategies, such as drought and shade tolerance. The prevailing theories seem contradictory. We measured the sapwood (K(s) ) and leaf (K(l) ) hydraulic conductivities of 40 coexisting tree species in a Bolivian dry forest, and examined associations with functional stem and leaf traits and indices of species' drought (dry-season leaf water potential) and shade (juvenile crown exposure) tolerance. Hydraulic properties varied across species and between life-history groups (pioneers vs shade-tolerant, and deciduous vs evergreen species). In addition to the expected negative correlation of K(l) with drought tolerance, we found a strong, negative correlation between K(l) and species' shade tolerance. Across species, K(s) and K(l) were negatively correlated with wood density and positively with maximum vessel length. Consequently, drought and shade tolerance scaled similarly with hydraulic properties, wood density and leaf dry matter content. We found that deciduous species also had traits conferring efficient water transport relative to evergreen species. Hydraulic properties varied across species, corresponding to the classical trade-off between hydraulic efficiency and safety, which for these dry forest trees resulted in coordinated drought and shade tolerance across species rather than the frequently hypothesized trade-off.

Ecological differentiation in xylem cavitation resistance is associated with stem and leaf structural traits.

Cavitation resistance is a critical determinant of drought tolerance in tropical tree species, but little is known of its association with life history strategies, particularly for seasonal dry forests, a system critically driven by variation in water availability. We analysed vulnerability curves for saplings of 13 tropical dry forest tree species differing in life history and leaf phenology. We examined how vulnerability to cavitation (P50) related to dry season leaf water potentials and stem and leaf traits. P50-values ranged from -0.8 to -6.2 MPa, with pioneers on average 38% more vulnerable to cavitation than shade-tolerants. Vulnerability to cavitation was related to structural traits conferring tissue stress vulnerability, being negatively correlated with wood density, and surprisingly maximum vessel length. Vulnerability to cavitation was negatively related to the Huber-value and leaf dry matter content, and positively with leaf size. It was not related to SLA. We found a strong trade-off between cavitation resistance and hydraulic efficiency. Most species in the field were operating at leaf water potentials well above their P50, but pioneers and deciduous species had smaller hydraulic safety margins than shade-tolerants and evergreens. A trade-off between hydraulic safety and efficiency underlies ecological differentiation across these tropical dry forest tree species.

Stability of pollen-ovule ratios in pollinator-dependent versus autogamous Clarkia sister taxa: testing evolutionary predictions.

It has been proposed that natural selection should favor distinct temporal patterns of sex allocation in selfing vs pollinator-dependent taxa. In autogamous selfers in which pollen receipt is highly reliable, selection should favor genotypes that maintain low and stable pollen to ovule (P : O) ratios throughout flowering. By contrast, in outcrossers the optimum P : O ratio of an individual's flowers will depend on pollinator abundances and mating opportunities, both of which may vary over time. In this case, selection may favor temporal variation among flowers in the P : O ratio. An opposing prediction is that selfing taxa will be developmentally more unstable than outcrossers because of lower homeostasis caused by high homozygosity. We compared temporal changes in the P : O ratio in two pairs of sister taxa in the genus Clarkia. We examined hundreds of glasshouse-raised maternal families representing three wild populations each of the outcrossing, insect-pollinated Clarkia unguiculata, the facultatively autogamous Clarkia exilis and the outcrossing and selfing subspecies of Clarkia xantiana: ssp. xantiana and parviflora, respectively. Temporal change in the P : O ratio was significantly greater in both outcrossers than in their selfing sister taxa, although the proportional changes in the P : O ratio (relative to the first bud produced) did not differ significantly between sister taxa (0.07 < P < 0.10). Our results provide partial support for the hypothesis that the P : O ratio is more stable in selfing than in outcrossing taxa and reject the hypothesis that selfers are less stable.

Evolution of mating system and the genetic covariance between male and female investment in Clarkia (onagraceae): selfing opposes the evolution of trade-offs.

Sex allocation theory has assumed that hermaphroditic species exhibit strong genetically based trade-offs between investment in male and female function. The potential effects of mating system on the evolution of this genetic covariance, however, have not been explored. We have challenged the assumption of a ubiquitous trade-off between male and female investment by arguing that in highly self-fertilizing species, stabilizing natural selection should favor highly efficient ratios of male to female gametes. In flowering plants, the result of such selection would be similar pollen:ovule (P:O) ratios across selfing genotypes, precluding a negative genetic correlation (r(g)) between pollen and ovule production per flower. Moreover, if selfing genotypes with similar P:O ratios differ in total gametic investment per flower, a positive r(g) between pollen and ovule production would be observed. In outcrossers, by contrast, male- and female-biased flowers and genotypes may have equal fitness and coexist at evolutionary equilibrium. In the absence of strong stabilizing selection on the P:O ratio, selection on this trait will be relaxed, resulting in independence or resource-based trade-offs between male and female investment. To test this prediction, we conducted artificial selection on pollen and ovule production per flower in two sister species with contrasting mating systems. The predominantly self-fertilizing species (Clarkia exilis) consistently exhibited a significant positive r(g) between pollen and ovule production while the outcrossing species (C. unguiculata) exhibited either a trade-off or independence between these traits. Clarkia exilis also exhibited much more highly canalized gender expression than C. unguiculata. Selection on pollen and ovule production resulted in little correlated change in the P:O ratio in the selfing exilis, while dramatic changes in the P:O ratio were observed in unguiculata. To test the common prediction that floral attractiveness should be positively genetically correlated with investment in male function, we examined the response of petal area to selection on pollen and ovule production and found that petal area was not consistently genetically correlated with gender expression in either species. Our results suggest that the joint evolutionary trajectory of primary sexual traits in hermaphroditic species will be affected by their mating systems; this should be taken into account in future theoretical and comparative empirical investigations.

Relationships among ecologically important dimensions of plant trait variation in seven neotropical forests.

BACKGROUND AND AIMS: When ecologically important plant traits are correlated they may be said to constitute an ecological 'strategy' dimension. Through identifying these dimensions and understanding their inter-relationships we gain insight into why particular trait combinations are favoured over others and into the implications of trait differences among species. Here we investigated relationships among several traits, and thus the strategy dimensions they represented, across 2134 woody species from seven Neotropical forests. METHODS: Six traits were studied: specific leaf area (SLA), the average size of leaves, seed and fruit, typical maximum plant height, and wood density (WD). Trait relationships were quantified across species at each individual forest as well as across the dataset as a whole. 'Phylogenetic' analyses were used to test for correlations among evolutionary trait-divergences and to ascertain whether interspecific relationships were biased by strong taxonomic patterning in the traits. KEY RESULTS: The interspecific and phylogenetic analyses yielded congruent results. Seed and fruit size were expected, and confirmed, to be tightly related. As expected, plant height was correlated with each of seed and fruit size, albeit weakly. Weak support was found for an expected positive relationship between leaf and fruit size. The prediction that SLA and WD would be negatively correlated was not supported. Otherwise the traits were predicted to be largely unrelated, being representatives of putatively independent strategy dimensions. This was indeed the case, although WD was consistently, negatively related to leaf size. CONCLUSIONS: The dimensions represented by SLA, seed/fruit size and leaf size were essentially independent and thus conveyed largely independent information about plant strategies. To a lesser extent the same was true for plant height and WD. Our tentative explanation for negative WD-leaf size relationships, now also known from other habitats, is that the traits are indirectly linked via plant hydraulics.

Life history, floral development, and mating system in Clarkia xantiana (Onagraceae): do floral and whole-plant rates of development evolve independently?

Autogamously self-fertilizing taxa have evolved from outcrossing progenitors at least 12 times in the annual wildflower genus, Clarkia (Onagraceae). In C. xantiana, individuals of the selfing subspecies (ssp. parviflora) flower at an earlier age, produce successive flowers more rapidly, and produce flowers that complete their development more rapidly than their outcrossing counterparts (ssp. xantiana). Two hypotheses have been proposed to explain the joint evolution of these whole-plant and individual floral traits. The accelerated life cycle hypothesis proposes that selection favoring a short life cycle in environments with short growing seasons (such as those typically occupied by parviflora) has independently favored genotypes with early reproduction, synchronous flower production, and rapidly developing, self-fertilizing flowers. The correlated response to selection hypothesis similarly proposes that selection in environments with short growing seasons favors early reproduction, but that rapid floral development and increased selfing evolve as correlated responses to selection due to genetic linkage (or pleiotropy) affecting both whole-plant and floral development. We conducted a greenhouse experiment using maternal families from two field populations of each subspecies to examine covariation between floral and whole-plant traits within and among populations to seek support for either of these hypotheses. Our results are consistent with the accelerated life cycle hypothesis but not with the correlated response to selection hypothesis.