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Tradeoffs between the energetic benefits and costs of traits can shape species and trait distributions along environmental gradients. Here we test predictions based on such tradeoffs using survival, growth, and 50 photosynthetic, hydraulic, and allocational traits of tenEucalyptusspecies grown in four common gardens along an 8-fold gradient in precipitation/pan evaporation (P/E_p) in Victoria, Australia. Phylogenetically structured tests show that most trait-environment relationships accord qualitatively with theory. Most traits appear adaptive across species within gardens (indicating fixed genetic differences) and within species across gardens (indicating plasticity). However, species from moister climates have lower stomatal conductance than others grown under the same conditions. Responses in stomatal conductance and five related traits appear to reflect greater mesophyll photosynthetic sensitivity of mesic species to lower leaf water potential. Our data support adaptive cross-over, with realized height growth of most species exceeding that of others in climates they dominate. Our findings show that pervasive physiological, hydraulic, and allocational adaptations shape the distributions of dominantEucalyptusspecies along a subcontinental climatic moisture gradient, driven by rapid divergence in speciesP/E_pand associated adaptations.

 

Despite the recognized importance of hydraulic capacitance as a mechanism used by plants to maintain hydraulic functioning during high transpiration, characterizing the dynamics of capacitance remains a challenge.

We used a novel ‘two-balance method’ to investigate relationships between stem rehydration kinetics and other hydraulic traits in multiple tree species, and we developed a model to explore stem rehydration kinetics further.

We found that: (1) rehydration time constants and the amount of water uptake occurring during rehydration differed significantly across species; (2) time constants did not change with declining water potential (Ψ), while water uptake increased at lower Ψ in some species; (3) longer time constants were associated with lower wood density, higher capacitance and less negative stem pressures causing 50 % loss of hydraulic conductivity (P50); (4) greater water uptake occurred in stems with lower wood density and less negative P50 values; and (5) the model could estimate the total hydraulic resistance of the rehydration path, which cannot be measured directly.

Overall, the two-balance method can be used to examine rehydration dynamics quickly and thoroughly in detached woody stems. This method has the potential to improve our understanding of how capacitance functions across tree species, which is an often-overlooked component of whole-plant hydraulics.

 

Abstract Winter annuals comprise a large fraction of warm-desert plant species, but the drivers of their diversity are little understood. One factor that has generally been overlooked is the lack of obvious means of long-distance seed dispersal in many desert-annual lineages, which could lead to genetic differentiation at small spatial scales and, ultimately, to speciation and narrow endemism. If our gene-flow hypothesis is correct, individual winter-annual species should have populations with genetic spatial structures implying short distances of gene flow. To test this idea, we sampled six populations of Eschscholzia parishii (Papaveraceae) in three pairs of watersheds within a 28-km radius in southern California. We quantified genetic diversity and structure and inferred the distance of gene flow in these populations using single nucleotide polymorphisms derived from genotyping-by-sequencing. Estimated distances of gene flow were quite small (σ = 10.4–14.9 m), with strong genetic structure observed within and between populations. Kinship declined steeply with ln distance (r2 = 0.85). Petal size and shape differed significantly between the northernmost and southernmost populations. These findings support the hypothesis that the high diversity of warm-desert winter annuals might result, in part, from genetic differentiation within species at small spatial scales driven by poor seed dispersal. 

Photosynthetic sensitivity to drought is a fundamental constraint on land‐plant evolution and ecosystem function. However, little is known about how the sensitivity of photosynthesis to nonstomatal limitations varies among species in the context of phylogenetic relationships.

Using saplings of 10Eucalyptusspecies, we measured maximum CO₂‐saturated photosynthesis usingA–c_icurves at several different leaf water potentials (ψ_leaf) to quantify mesophyll photosynthetic sensitivity to ψ_leaf(MPS), a measure of how rapidly nonstomatal limitations to carbon uptake increase with declining ψ_leaf. MPS was compared to the macroclimatic moisture availability of the species’ native habitats, while accounting for phylogenetic relationships.

We found that species native to mesic habitats have greater MPS but higher maximum photosynthetic rates during non‐water‐stressed conditions, revealing a trade‐off between maximum photosynthesis and drought sensitivity. Species with lower turgor loss points have lower MPS, indicating coordination among photosynthetic and water‐relations traits.

By accounting for phylogenetic relationships among closely related species, we provide the first compelling evidence that MPS inEucalyptusevolved in an adaptive fashion with climatically determined moisture availability, opening the way for further study of this poorly explored dimension of plant adaptation to drought.

 

Pressure–volume curves are a widely used analytical framework to derive several key physiological traits related to plant–water relations, including a species’ turgor loss point, osmotic potential at full turgor, and the elasticity of cell walls. We developed a novel protocol, including the preparation and treatment of fern gametophytes, to generate data for pressure–volume curve analyses using thermocouple psychrometry.

Gametophytes of the fern speciesPolystichum lemmoniiwere grown from spore, harvested, and subjected to a series of drying intervals. We constructed pressure–volume curves using thermocouple psychrometers to calculate gametophyte water potential and a balance to measure relative water loss.

We present the first protocol for fern gametophyte pressure–volume curves that can accurately determine key physiological traits in fern gametophytes such as the turgor loss point and osmotic potential at full turgor.

 

In the stems of terrestrial vascular plants studied to date, the diameter of xylem water‐conducting conduitsDwidens predictably with distance from the stem tipLapproximatingD ∝ L^b, withb ≈ 0.2. Because conduit diameter is central for conductance, it is essential to understand the cause of this remarkably pervasive pattern. We give reason to suspect that tip‐to‐base conduit widening is an adaptation, favored by natural selection because widening helps minimize the increase in hydraulic resistance that would otherwise occur as an individual stem grows longer and conductive path length increases. Evidence consistent with adaptation includes optimality models that predict the 0.2 exponent. The fact that this prediction can be made with a simple model of a single capillary, omitting much biological detail, itself makes numerous important predictions, e.g. that pit resistance must scale isometrically with conduit resistance. The idea that tip‐to‐base conduit widening has a nonadaptive cause, with temperature, drought, or turgor limiting the conduit diameters that plants are able to produce, is less consistent with the data than an adaptive explanation. We identify empirical priorities for testing the cause of tip‐to‐base conduit widening and underscore the need to study plant hydraulic systems leaf to root as integrated wholes.

 

The vast majority of measurements in the field of plant hydraulics have been on small‐diameter branches from woody species. These measurements have provided considerable insight into plant functioning, but our understanding of plant physiology and ecology would benefit from a broader view, because branch hydraulic properties are influenced by many factors. Here, we discuss the influence that other components of the hydraulic network have on branch vulnerability to embolism propagation. We also modelled the impact of changes in the ratio of root‐to‐leaf areas and soil texture on vulnerability to hydraulic failure along the soil‐to‐leaf continuum and showed that hydraulic function is better maintained through changes in root vulnerability and root‐to‐leaf area ratio than in branch vulnerability. Differences among species in the stringency with which they regulate leaf water potential and in reliance on stored water to buffer changes in water potential also affect the need to construct embolism resistant branches. Many approaches, such as measurements on fine roots, small individuals, combining sap flow and psychrometry techniques, and modelling efforts, could vastly improve our understanding of whole‐plant hydraulic functioning. A better understanding of how traits are coordinated across the whole plant will improve predictions for plant function under future climate conditions.