Leaf-branch vulnerability segmentation occurs all year round for three temperate evergreen tree species

Seasonal changes in hydraulic functions of eight temperate tree species: divergent responses to freeze-thaw cycles in spring and autumn

Freeze-thaw cycles (FTCs) are the major seasonal environment stress in the temperate and boreal forests, inducing hydraulic dysfunction and limiting tree growth and distribution. There are two types of FTCs in the field: FTCs with increasing temperature from winter to spring (spring FTCs); and FTCs with decreasing temperature from autumn to winter (autumn FTCs). While previous studies have evaluated the hydraulic function during the growing season, its seasonal changes and how it adapts to different types of FTCs remain unverified. To fill this knowledge gap, the eight tree species from three wood types (ring- and diffuse-porous, tracheid) were selected in a temperate forest undergoing seasonal FTCs. We measured the branch hydraulic traits in spring, summer, autumn, and early, middle and late winter. Ring-porous trees always showed low native hydraulic conductance (Kbranch), and high percentage loss of maximum Kbranch (PLCB) and water potential that loss of 50% maximum Kbranch (P50B) in non-growing seasons (except summer). Kbranch decreased, and PLCB and P50B increased in diffuse-porous trees after several spring FTCs. In tracheid trees, Kbranch decreased after spring FTCs while the P50B did not change. All sampled trees gradually recovered their hydraulic functions from spring to summer. Kbranch, PLCB and P50B of diffuse-porous and tracheid trees were relatively constant after autumn FTCs, indicating almost no effect of autumn FTCs on hydraulic functions. These results suggested that hydraulic functions of temperate trees showed significant seasonal changes, and spring FTCs induced more hydraulic damage (except ring-porous trees) than autumn FTCs, which should be determined by the number of FTCs and trees' vitality before FTCs. These findings advance our understanding of seasonal changes in hydraulic functions and how they cope with different types of FTC in temperate forests.

Seasonality in embolism resistance and hydraulic capacitance jointly mediate hydraulic safety in branches and leaves of oriental cork oak (Quercus variabilis Bl.)

Seasonality in temperate regions is prominent during the era of increased climatic variability. A hydraulic trait that can adjust to seasonally changing climatic conditions is crucial for tree safety. However, little attention has been paid to the intraspecific seasonality of drought-related traits and hydraulic safety of keystone forest trees. We examined seasonal variations in the key morphological and physiological traits as well as multiple hydraulic safety margins (SMs) at the branch and leaf levels in oriental cork oak (Quercus variabilis Bl.), which is predominant in Chinese temperate forests. Pneumatic measurements indicated that, as seasons progressed, the water potential at which 50% of branch embolisms occur (P50_branch) decreased from -3.34 to -4.23 MPa, with a coefficient of variation (CV) of 9.08%. Sapwood capacitance ranged from 48.19 to 248.08 kg m-3 MPa-1, peaking in autumn and reaching minimum in winter (CV 60.58%). Rehydration kinetics confirmed higher leaf embolism vulnerability (P50_leaf) in spring and autumn than those in summer, with values ranging from -1.06 to -3.02 MPa (CV 39.85%). All leaf pressure-volume (PV) traits shifted with growth, with CVs ranging from 6.95% to 46.69%. Sapwood density had significant negative correlations with P50_branch and hydraulic capacitance for elastic water storage, whereas leaf mass per area was linearly associated with PV traits but not with P50_leaf. Furthermore, the branch typical SMs (difference between branch midday water potential and P50_branch) were consistently >1.84 MPa, and vulnerability segmentation was prevalent throughout, implying a plausible hydraulic foundation for the dominance of Q. variabilis. Diverse hydraulic response patterns existed across seasons, leading to positive SMs mediated by the aforementioned physiological traits. Although Q. variabilis exhibits a high level of hydraulic safety, its susceptibility to sudden summer droughts may increase due to global climate change.

Adaptive strategies to freeze-thaw cycles in branch hydraulics of tree species coexisting in a temperate forest

Freeze-thaw cycles (FTCs) limit the distribution and survival of temperate tree species. Tree species with different wood types coexist in temperate forests and are subjected to the same FTCs. It is essential to understand how these trees differentially cope with xylem hydraulic failure induced by FTCs in the field. The branch hydraulic traits and nonstructural carbohydrate concentration of six coexisting tree species in a temperate forest were measured from mid-winter to early spring when the FTCs occurred from January to April. The percentage loss of hydraulic conductivity (PLC) was lower, and the water potential inducing a 50% loss of hydraulic conductivity (P50) was more negative in tracheid trees than in ring- and diffuse-porous trees, suggesting tracheid trees with narrow tracheid diameters showed less vulnerable to embolism and provided a lower degree of hydraulic failure during FTCs (stronger resistance). Ring-porous trees always showed lower hydraulic conductivity and higher PLC and P50, and these traits did not change during FTCs, suggesting that they might lose the hydraulic functions in winter and abandon the last year xylem. The P50 in diffuse-porous increased after several FTCs (frost fatigue), but that in tracheid species continued to increase (or even decrease) until the end of FTCs (69 cycles), suggesting that tracheid trees were less sensitive to frost fatigue than diffuse-porous trees. Soluble sugar concentration in deciduous trees negatively correlated with PLC at the end of FTCs, indicating that the effect of soluble sugar on refilling embolism occurred in early spring. While the soluble sugar concentration of deciduous trees decreased, that of two evergreen tracheid trees gradually increased, possibly due to the winter photosynthesis of evergreen leaves. Our results suggest temperate trees adopt different strategies to cope with the same FTCs. These findings enrich the understanding of plant hydraulics and carbon physiology in winter and provide insights into the response of different species coexisting in temperate forests under climate change.

Different hydraulic and photosynthetic responses to summer drought between newly sprouted and established Moso bamboo culms

The subtropical regions in China are prone to recurrent summer droughts induced by the Western Pacific Subtropical High-Pressure, which has induced the death of tens of millions of culms of Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau), a widely distributed giant bamboo with high economic and ecological values. In the future, the intensity and frequency of the summer drought are projected to increase in these areas due to global climate change, which may lead to significant age-specific mortality of Moso bamboo. So far, it is still unclear about the age-specific response mechanisms of hydraulic traits and carbon balance of Moso bamboo when it is suffering to an ongoing summer drought. This study aimed to investigate the hydraulic and photosynthetic responses of newly sprouted (1 year old) and established (2-5 years old) culms of Moso bamboo to summer drought, which was manipulated by throughfall reduction in Lin'an of Zhejiang. The results showed that both newly sprouted and established culms had a gradually weakening hydraulic conductivity and photosynthesis during the whole drought process. In the early stage of the manipulated drought, the established culms had more loss of hydraulic conductivity than the newly sprouted culms. However, the newly sprouted culms had significant more loss of hydraulic conductivity and lower photosynthetic rates and stomatal conductance in the middle and late stages of the manipulated drought. The results suggest that the newly sprouted culms were more susceptible to summer drought than established culms due to the combined effects of hydraulic damage and photosynthetic restriction, explaining why the newly sprouted culms have higher mortality than elder culms when subjected to extreme drought. These findings provided insights into the mechanisms of Moso bamboo's age-specific drought-induced mortality, which will help for the anti-drought management of bamboo.