Coordination between leaf biomechanical resistance and hydraulic safety across 30 subtropical woody species

Background and Aims Leaf biomechanical resistance protects leaves from biotic and abiotic damage. Previous studies have revealed that enhancing leaf biomechanical resistance is costly for plant species and leads to an increase in leaf drought tolerance. We thus predicted that there is a functional correlation between leaf hydraulic safety and biomechanical characteristics. Methods We measured leaf morphological and anatomical traits, pressure–volume parameters, maximum leaf hydraulic conductance (Kleaf-max), leaf water potential at 50% loss of hydraulic conductance (P50leaf), leaf hydraulic safety margin (SMleaf), and leaf force to tear (Ft) and punch (Fp) of 30 co-occurring woody species in a subtropical evergreen broadleaved forest. Linear regression analysis was performed to examine the relationships between biomechanical resistance and other leaf hydraulic traits. Key Results We found that higher Ft and Fp values were significantly associated with a lower (more negative) P50leaf and a larger SMleaf, thereby confirming the correlation between leaf biomechanical resistance and hydraulic safety. However, leaf biomechanical resistance showed no correlation with Kleaf-max, although it was significantly and negatively correlated with leaf outside-xylem hydraulic conductance. In addition, we also found that there was a significant correlation between biomechanical resistance and the modulus of elasticity by excluding an outlier. Conclusions The findings of this study reveal leaf biomechanical-hydraulic safety correlation in subtropical woody species.

Large scale assessment of post-drought climate sensitivity of tree-growth

<p>Hotter droughts will have an increasingly influential role in shaping forest ecosystems in the future. Risks include decreases in species richness, altered species distributions, forest dieback and changed function as carbon sink. A common method to study the impacts of droughts on forests is the quantification of reductions in biomass productivity via secondary growth – approximated by ring-width measurements –, including duration until growth rates return to pre-drought levels, so-called legacy periods. However, while these metrics are practical and relatively easy to measure, the underlying governing mechanisms are not, and thus poorly understood. Consequently, it is uncertain if drought-induced reductions in secondary growth are due to corresponding decreases in total physiological function or high plasticity, and if recovery times are due to lasting damage or adaptation with more carbon allocated to drought-mitigating structures.</p><p>The principle of the most limiting factor for tree-growth can be used to track temporal variations in climate-growth relationships. Similarly, the considerable strain hotter drought constitutes for tree-growth, and the need to repair damaged structures or alter carbon allocation, may imply temporary climate sensitivity deviations during legacy periods. Identifying their existence and quantifying subsequent differences in these deviations can help to shed light on strategies used by trees to respond to droughts.</p><p>Here, we detect and quantify deviations in climate-growth relationships during hotter drought legacy periods and assess how they differ according to clade (angiosperms – gymnosperms), site aridity and hydraulic safety margin. We do this by applying a linear mixed model on all ring-width indices (RWI) in the global-scale International Tree-Ring Data Bank (ITRDB) which exhibit a positive correlation with Standardized Precipitation-Evapotranspiration Index (SPEI). We apply a combined climatological and ecological definition for drought events and use site-dependent SPEI time-scales to allow for specific climate dependencies.</p><p>Results show heterogeneous post-drought climate sensitivity deviations, which are broadly categorized in three groups: 1) angiosperms growing in arid sites become increasingly sensitive to climate for 2 – 4 years; 2) angiosperms in mesic sites and or with high hydraulic safety margin show abrupt and complete disruption of the climate-growth relationship for the first year after droughts, which turn into a decrease in climate sensitivity for an additional 1 – 3 years; 3) gymnosperms in arid sites become less sensitive to climate for 2 – 4 years, although without the abrupt disruption seen in group 2. We discuss these results and their implications in an ecophysiological context, including future research avenues.</p><p>In conclusion, the results clearly show a functional legacy effect that is not detected through measurements of reductions in biomass accumulation alone, hinting at differential strategies employed by trees to cope with hotter droughts. This is a first step towards a better understanding of the mechanisms underlying hotter drought legacies which may help to improve ecosystem models and better predict how trees will respond to drought in a warming future climate.</p>

Leaf hydraulic safety margin and safety–efficiency trade-off across angiosperm woody species

Leaf hydraulic conductance and the vulnerability to water deficits have profound effects on plant distribution and mortality. In this study, we compiled a leaf hydraulic trait dataset with 311 species-at-site combinations from biomes worldwide. These traits included maximum leaf hydraulic conductance ( K leaf ), water potential at 50% loss of K leaf (P50 leaf ), and minimum leaf water potential ( Ψ min ). Leaf hydraulic safety margin (HSM leaf ) was calculated as the difference between Ψ min and P50 leaf . Our results indicated that 70% of the studied species had a narrow HSM leaf (less than 1 MPa), which was consistent with the global pattern of stem hydraulic safety margin. There was a positive relationship between HSM leaf and aridity index (the ratio of mean annual precipitation to potential evapotranspiration), as species from humid sites tended to have larger HSM leaf . We found a significant relationship between K leaf and P50 leaf across global angiosperm woody species and within each of the different plant groups. This global analysis of leaf hydraulic traits improves our understanding of plant hydraulic response to environmental change.

The sequence and thresholds of leaf hydraulic traits underlying grapevine varietal differences in drought tolerance

Adapting agriculture to climate change is driving the need for the selection and breeding of drought-tolerant crops. The aim of this study was to identify key drought tolerance traits and determine the sequence of their water potential thresholds across three grapevine cultivars with contrasting water use behaviors, Grenache, Syrah, and Semillon. We quantified differences in water use between cultivars and combined this with the determination of other leaf-level traits (e.g. leaf turgor loss point, π TLP), leaf vulnerability to embolism (P50), and the hydraulic safety margin (HSM P50). Semillon exhibited the highest maximum transpiration (Emax), and lowest sensitivity of canopy stomatal conductance (Gc) to vapor pressure deficit (VPD), followed by Syrah and Grenache. Increasing Emax was correlated with more negative water potential at which stomata close (Pgs90), π TLP, and P50, suggesting that increasing water use is associated with hydraulic traits allowing gas exchange under more negative water potentials. Nevertheless, all the cultivars closed their stomata prior to leaf embolism formation. Modeling simulations demonstrated that despite a narrower HSM, Grenache takes longer to reach thresholds of hydraulic failure due to its conservative water use. This study demonstrates that the relationships between leaf hydraulic traits are complex and interactive, stressing the importance of integrating multiple traits in characterizing drought tolerance.