Contrasting Hydraulic Efficiency and Photosynthesis Strategy in Differential Successional Stages of a Subtropical Forest in a Karst Region

Understanding the successional process from a disturbed forest to a mature forest is essential for species recovery and conservation initiatives. The resource acquisition and drought tolerance of plants can be instructive to predictions of species abundance and distribution for different forests. However, they have not been adequately tested at different successional stages in karst regions. Here, we selected seven dominant species in an early-succession forest and 17 species in a late-succession forest in a karst region of southwestern China. Resource acquisition-related traits such as hydraulic conductivity and photosynthetic rate, and drought tolerance-related traits, including turgor loss point and wood density, were measured. We found that species in the early-succession forest had a higher hydraulic conductance and photosynthetic rate than those in the late-succession forest, while leaf water potential at turgor loss point and wood density showed nonsignificant differences between the two forests. In addition, we observed a significant negative relationship between photosynthetic rate and drought tolerance in the early-succession forest, which was not identified in late-succession forests. Our study indicates that resource acquisition rather than drought tolerance was the key factor explaining plant distributions in forests at different successional stages in karst regions. We also suggest that the resource acquisition and drought tolerance trade-off hypothesis is not always supported for karst region species. Our study could inform about the design of species replacements in successional forests and provide forest management and restoration guidelines for karst regions.

Vulnerability to cavitation is linked to home climate precipitation across eight eucalypt species

Vulnerability to cavitation in leaves is the result of highly adaptive anatomical and physiological traits that can be linked to water availability in a species' climate of origin. Despite similar gross leaf morphology, eucalypt species are often confined to specific climate envelopes across the variable rainfall environments of Australia. In this study, we investigate how the progression of cavitation differs among eucalypts and whether this is related to other hydraulic and physical leaf traits. We used the Optical Visualisation technique to capture cavitation progression across the leaves of eight eucalypt species (Angophora crassifolia, Corymbia tessellaris, Eucalyptus atrata, Eucalyptus grandis, Eucalyptus laevopinea, Eucalyptus longifolia, Eucalyptus macrandra, Eucalyptus tereticornis) from a wide range of climates and grown in a common garden setting. Vulnerability to cavitation, represented by the leaf water potential required for 50% cavitation of leaf vessels, varied significantly among species (-3.48 MPa to -8.25 MPa) and correlated linearly with home climate precipitation and leaf SLA (R2 of 0.64 and 0.75, respectively). P12-P88, the range of water potentials between which 12% to 88% of cavitation occurs, was decoupled from P50 but also correlated with leaf SLA (R2 of 0.72). We suggest the magnitude of P12-P88 may be representative of a species' drought strategy - a large P12-P88 signifying leaves that exhibit drought tolerance (retention of leaves under drought conditions) and a small P12-P88 signifying drought avoidance (leaf shedding after a threshold of drought is reached). Our results agree with other studies that highlight these cavitation metrics as genetically fixed traits. Turgor loss point, on the other hand, may be more plastic, as evidenced by the low variability of this trait across these eucalypt species grown in a common garden environment. Further study will help to establish the SLA-related anatomical traits that impart cavitation resistance and to extend these conclusions to a greater number of species and home climates.

Mediterranean trees vulnerability to climate change will not be minimized through hydraulic safety traits adjustments.

<p><span>Climate change in the Mediterranean region leads to an intensification of summer droughts. These episodes of extreme water stress threaten the survival of tree species and, by the same token, would affect the structure and ecosystem services of woodlands. Indeed, in conditions of prolonged and intense drought, one of the major risks for trees is the hydraulic failure due to high embolism level. Xylem embolism risk depends essentially on various leaf and hydraulic traits including (i) the vulnerability of their xylem to cavitation, (ii) the turgor loss point (a surrogate for stomatal control) and (iii) their cuticular transpiration (gmin). The two former traits can be used to compute hydraulic safety margins (HSM). </span></p><p><span>In order to assess whether trees will survive future climatic conditions, it is necessary to quantify and assess the plasticity of these traits to intensified drought. In this study, we used three rainfall exclusion experiments established in mature forests in south-eastern France (Font-blanche, Puéchabon and O3HP experimental sites) to measure these traits and evaluate their ability to adjust to aggravated drought conditions for three Mediterranean widespread species: </span><span><em>Quercus ilex</em></span><span>, </span><span><em>Quercus Pubescens</em></span><span>, and </span><span><em>Pinus halepensis</em></span><span>. We performed pressure-volume curves of trees from rainfall exclusion and control plots to see if adjustments of gmin and leaf hydraulic traits involved in stomatal regulation occurred in these three species. Using the optical method and cavitron, we also quantified the plasticity of xylem vulnerability to cavitation by comparing the values of water potential leading to a 50% reduction in plant hydraulic conductance (P50). </span></p><p><span>Our results show that </span><span><em>Quercus pubescens</em></span><span> has the lowest HSM while </span><span><em>Quercus ilex</em></span><span> has the highest. In addition, gmin is higher for </span><span><em>Quercus pubescens</em></span><span> than for the other two species. All together these results suggest that </span><span><em>Quercus pubescens</em></span><span> is the most vulnerable to drought among the three studied. Globally, for most traits and species no significant difference was found between treatments. The only exception was for </span><span><em>Quercus ilex</em></span><span> that exhibited lower turgor loss point (Ψtlp) in the dry treatment. Drought acclimation for these species may rather depend on other traits, such as leaf area reduction or rooting depth. To integrate the role of these traits to estimate the historic and future mortality risk for these species, the use of hydraulic based models will be of interest. </span></p>

Intraspecific drought tolerance of Betula pendula genotypes: an evaluation using leaf turgor loss in a botanical collection

Key message The results showed a significant relationship between the potential evapotranspiration of the provenance collection site and the leaf turgor loss point and significant positive differences in drought tolerance between provenances and subspecies of B. pendula. Abstract The ecosystem services provided by urban trees make substantial contributions to the quality of urban living and securing resilience towards the challenges posed by a changing climate. Water deficits are a major abiotic stress for trees in urban environments and, in many regions, this stress is likely to be amplified under future climate scenarios. Although wide variation in drought tolerance exists at the species level, many species also show substantial intraspecific variation in drought tolerance. The aim of this study is to evaluate how drought tolerance, inferred from the water potential at leaf turgor loss point (ΨP0), varies in Betula pendula from different geographical origins and determine if the observed drought tolerance can be related to the local climate and seasonal water balance from the provenance of origin, despite the trees now being established in similar soil and climatic conditions within a single botanical collection. Six subsp. betula, five subsp. mandshurica and two subsp. szechuanica were evaluated, giving a total of 12 different provenances. The results showed a significant relationship between the potential evapotranspiration of the provenance collection site and the leaf turgor loss point and significant positive differences in drought tolerance between provenances and subspecies of B. pendula. By directing efforts towards identifying more drought-tolerant genotypes, it will be possible to diversify the palette of trees that could confidently be integrated by urban tree planners and landscape architects into the urban landscape. The results of this study on different ecotypes of B. pendula clearly show that it is possible to find more drought-tolerant plant material.

Leaf turgor loss point at full hydration for 41 native and introduced tree and shrub species from Central Europe

The last years, Central European forests have suffered from drought as a direct consequence of climate change. All these forests have a long management history and it lies in the landowner’s responsibility to replant damaged forests. Hence, landowners and the government are searching currently for species suitable to replant in areas affected by tree die-offs. It is a matter of fact that good knowledge of drought resistance of species is a critical measure for the current replanting efforts. We determined a widely recognized trait for leaf drought tolerance (leaf water potential at turgor loss point at full hydration, πtlp) in 41 woody species native or introduced in Central Europe. The osmometric rapid assessment method was used to measure the leaf osmotic potential at full hydration (πosm) of sun-exposed leaves and converted to πtlp. Mean πtlp of the native species was −2.33 ± 0.33 MPa. The less negative πtlp was found in the introduced species Aesculus hypocastania and was at −1.70 ± 0.11 MPa. The most negative πtlp, and thus the potentially highest drought tolerance, were found in the introduced species Pseudotsuga menzesii and was at −3.02 ± 0.14 MPa. High or less negative πtlp is associated with lower drought tolerance, whereas low or more negative πtlp stands for higher resistance to drought stress. For example, the two native species Illex aquifolium and Alnus glustinosa are species naturally associated with moist habitats and are characterized by the least negative πtlp of −1.75 ± 0.02 and −1.76 ± 0.03 MPa, respectively.