Strategies of tree species to adapt to drought from leaf stomatal regulation and stem embolism resistance to root properties

Physiological mechanisms and drought resistance assessment of four dominant species on the Loess Plateau under drought stress

The escalating frequency and severity of droughts have caused growth decline and increased mortality risk for plantations on the Loess Plateau. The main aim of this study was to explore the physiological mechanisms of four native dominant species during drought-induced mortality and evaluate their drought resistance capabilities. Drought was induced by withholding water from potted seedings, and we compared patterns in pit membrane damage, hydraulic function, and non-structural carbohydrates (NSC) dynamics across four tree species with distinct anatomical features. Our findings reveal species-specific vulnerability thresholds: Pinus tabulaeformis (-2.86 Mpa), Quercus liaotungensis (-1.92 Mpa), Robinia pseudoacacia (-0.109 Mpa), and Syringa reticulata (-0.93 Mpa). Additionally, drought stress was found to compromise pit membrane integrity, with water potential thresholds identified as R.pseudoacacia (-1.37 Mpa), S. reticulata (-2.20 Mpa), Q. liaotungensis (-2.39 Mpa), and P. tabulaeformis (-1.85 Mpa). The study concludes that R. pseudoacacia and S. reticulata exhibit greater susceptibility to hydraulic failure under severe drought conditions, leading to increased mortality risks. In contrast, Q. liaotungensis and P. tabulaeformis demonstrate enhanced drought tolerance and survival capacity. Our research elucidates the physiological mechanisms of drought-induced mortality, emphasizing the critical role of pit membrane damage in this process. These findings not only provide valuable insights into the drought resistance of native dominant species but also establish a scientific foundation for future artificial forest transformation initiatives on the Loess Plateau.

Soil CO2 and CH4 response to experimental warming under various tree species compositions in a temperate harwood forest

Under climate change, some forest ecosystems appear to be transitioning into net source of carbon dioxide (CO2), raising questions about the future role of soil respiration rate (Rs), which depends on hydroclimatic conditions. Conversely, well-drained forest soils could become more significant sinks of methane (CH4) under warming. The main objective of this study was to assess the effects of artificial soil warming on Rs and CH4 fluxes in a sugar maple forest at the northern limit of Quebec temperate deciduous forests in eastern Canada, and to evaluate the effect of species composition on soil response to warming. We measured Rs and CH4 fluxes during the snow-free period of 2021 and 2022 in 32 plots distributed across three forest types, half of which were artificially heated by approximately 2 °C with heating cables. Forest soils were a very consistent sink for CH4 and it did not respond to artificial soil warming nor was it sensitive to variations in soil moisture, ionic activity in soil solution and forest types. However, we observed an increase in Rs in response to warming in the heated plots, but only up to a threshold of about 15 °C, beyond which Rs started to slow down in respect to the control plots. We also observed a weakening of the exponential relationship between Rs and soil temperature beyond this threshold. This trend varied across the forest types, with hardwood-beech stands being more sensitive to warming than mixedwoods and other hardwoods. This greater response of hardwood-beech stands to warming resulted in a more significant downshift of Rs, starting from a colder temperature threshold, around 10-12 °C. This study highlights a potential plateauing of Rs despite rising soil temperature, at least in eastern Canada's temperate deciduous forest, but this trend could vary from one forest type to another.

Supplementary Information

The online version contains supplementary material available at 10.1007/s44378-025-00045-4.

Response mechanism of water status and photosynthetic characteristics of Cotoneaster multiflorus under drought stress and rehydrated conditions

Introduction

Plant physiology response and adaptation to drought stress has become a hotspot in plant ecology and evolution. Cotoneaster multiflorus possesses high ecological, ornamental and economic benefits. It has large root system and tolerance to cold, drought and poor soil. Therefore, C. multiflorus is considered as one of the most important tree species for ecological restoration in arid and semi-arid areas. However, little is known about the physiological mechanisms, molecular mechanisms and drought strategies of how C. multiflorus responds to drought stress. Therefore, exploring the physiological response mechanisms, molecular mechanisms and adaptive strategies of C. multiflorus in response to drought is important for its growth in arid and semi-arid regions.

Methods

We investigated the response and coupling mechanisms of water status, photosynthetic properties and chloroplast fluorescence parameters in C. multiflorus in response to drought and rehydrated after drought, especially the importance of nocturnal sap flow and nocturnal water refilling to maintain its own water balance in response to drought stress. In addition, we studied the stress response of C. multiflorus transcriptome factors, and we also discussed drought adaptation strategies of C. multiflorus.

Results

C. multiflorus adapted to drought stress by a series of structural and physiological mechanisms, such as promoting closing stomata, increasing nocturnal sap flow. When rehydrated after undergoing severe drought stress, its physiological activities such as photosynthesis, water status, chlorophyll fluorescence parameters and other physiological activities have rapidly resumed. This showed C. multiflorus had strong tolerance to drought. In addition, water status, photosynthetic characteristics, and chloroplast fluorescence parameters of C. multiflorus were highly coupled. Nocturnal sap flow and nocturnal water refilling were very important for C. multiflorus to maintain its own water balance in response to drought stress. Finally, C. multiflorus will strengthen the drought defense mechanism by gene regulation of various metabolisms, such as promoting stomatal closure, reducing transpiration water loss, and vigorously regulating water balance. C. multiflorus responded to drought stress by avoiding or reducing water deficit in plant organs and tissues. Therefore, the shrub C. multiflorus is a drought-tolerant plant.

Discussion

We explored the response mechanisms of water status, photosynthetic characteristics, and chloroplast fluorescence parameters of C. multiflorus in drought and rehydrated after drought stress, especially the response mechanisms of nocturnal sap flow and nocturnal water refilling in response to drought stress, and identified the physiological coupling mechanisms, molecular mechanisms and drought types of C. multiflorus in response to drought.

Heartwood/Sapwood Characteristics of Populus euphratica Oliv. Trunks and Their Relationship with Soil Physicochemical Properties in the Lower Tarim River, Northwest China

The characteristics of heartwood and sapwood not only reflect tree growth and site quality but also provide insights into habitat changes. This study examines the natural Populus euphratica Oliv. forest in the Arghan section of the lower Tarim River, comparing the heartwood and sapwood characteristics of P. euphratica at different distances from the river, as well as at varying trunk heights and diameters at breast height (DBH). The objective was to examine the correlation between these characteristics and the physicochemical properties of the soil to better understand the ecological response strategies of P. euphratica in arid environments. Results indicated that heartwood radius, sapwood width, sapwood area, and heartwood moisture content decreased with increasing trunk height, following the pattern: 0.3 m > 0.8 m > 1.3 m. In contrast, heartwood density increased as trunk height increased. Most of the heartwood and sapwood indicators increased with larger tree diameters. In the case of P. euphratica with a DBH of less than 45 cm, the difference in moisture content between heartwood and sapwood was not significant (p > 0.05) at heights of 0.3 m and 0.8 m. However, at a height of 1.3 m, the difference was significant (p < 0.05). Soil analysis revealed that factors such as total nitrogen, available potassium, and water content significantly influenced the physical characteristics of P. euphratica heartwood and sapwood across different sites. Redundancy analysis (RDA) further demonstrated that total nitrogen, available phosphorus, and soil moisture were significantly correlated with the physical properties of P. euphratica heartwood and sapwood, further validating the critical role of soil nutrients in shaping the wood characteristics of P. euphratica. These findings highlighted the specific adaptations of P. euphratica in the lower Tarim River to the arid desert environment, reflected in the observed relationships between soil conditions and the physical characteristics of heartwood and sapwood.

Responses of stem growth and canopy greenness of temperate conifers to dry spells

Dry spells strongly influence biomass production in forest ecosystems. Their effects may last several years following a drought event, prolonging growth reduction and therefore restricting carbon sequestration. Yet, our understanding of the impact of dry spells on the vitality of trees' above-ground biomass components (e.g., stems and leaves) at a landscape level remains limited. We analyzed the responses of Pinus sylvestris and Picea abies to the four most severe drought years in topographically complex sites. To represent stem growth and canopy greenness, we used chronologies of tree-ring width and time series of the Normalized Difference Vegetation Index (NDVI). We analyzed the responses of radial tree growth and NDVI to dry spells using superposed epoch analysis and further explored this relationship using mixed-effect models. Our results show a stronger and more persistent response of radial growth to dry spells and faster recovery of canopy greenness. Canopy greenness started to recover the year after the dry spell, whereas radial tree growth remained reduced for the two subsequent years and did not recover the pre-drought level until the fourth year after the event. Stem growth and canopy greenness were influenced by climatic conditions during and after drought events, while the effect of topography was marginal. The opposite responses of stem growth and canopy greenness following drought events suggest a different impact of dry spells on trees´ sink and source compartments. These results underscore the crucial importance of understanding the complexities of tree growth as a major sink of atmospheric carbon.

Leaf water relations determine the trade-off between ozone resistance and stomatal functionality in urban tree species

Urban trees possess different capacities to mitigate ozone (O3) pollution through stomatal uptake. Stomatal closure protects trees from oxidative damage but limits their growth. To date, it is unclear how plant hydraulic function affect stomatal behaviour and determine O3 resistance. We assessed gas exchange and hydraulic traits in three subtropical urban tree species, Celtis sinensis, Quercus acutissima, and Q. nuttallii, under nonfiltered ambient air (NF) and elevated O3 (NF60). NF60 decreased photosynthetic rate (An) and stomatal conductance (gs) only in Q. acutissima and Q. nuttallii. Maintained An in C. sinensis suggested high O3 resistance and was attributed to higher leaf capacitance at the full turgor. However, this species exhibited a reduced stomatal sensitivity to vapour pressure deficit and an increased minimal gs under NF60. Such stomatal dysfunction did not decrease intrinsic water use efficiency (WUE) due to a tight coupling of An and gs. Conversely, Q. acutissima and Q. nuttallii showed maintained stomatal sensitivity and increased WUE, primarily correlated with gs and leaf water relations, including relative water content and osmotic potential at turgor loss point. Our findings highlight a trade-off between O3 resistance and stomatal functionality, with efficient stomatal control reducing the risk of hydraulic failure under combined stresses.

Ectopic expression of TTP gene from human in poplar promotes xylem differentiation and confers plant drought tolerance

The CCCH zinc finger proteins play critical roles in a wide variety of growth, development, and stress responses. Currently, limited reports are available about the roles of animal CCCH proteins in plants. In this study, we report the identification of human TTP (hTTP) with functional similarity to PdC3H17 in a hybrid poplar. hTTP and PdC3H17 shared highly similar tandem CCCH zinc-finger RNA-binding domains. The fragments excluding the CCCH domain of both hTTP and PdC3H17 possessed transcriptional activation activities in yeast cells. Compared to the controls, ectopic expression of hTTP in poplar caused dwarfism, and resulted in significant increases in stem xylem vessel number and photosynthetic and ROS-scavenging abilities, thereby enhancing plant tolerance to drought stress. Our results suggest that hTTP may perform a function in poplar through the PdC3H17-mediated system, and provide an example for the application of animal genes in plants.

Finding balance: Tree-ring isotopes differentiate between acclimation and stress-induced imbalance in a long-term irrigation experiment

Scots pine (Pinus sylvestris L.) is a common European tree species, and understanding its acclimation to the rapidly changing climate through physiological, biochemical or structural adjustments is vital for predicting future growth. We investigated a long-term irrigation experiment at a naturally dry forest in Switzerland, comparing Scots pine trees that have been continuously irrigated for 17 years (irrigated) with those for which irrigation was interrupted after 10 years (stop) and non-irrigated trees (control), using tree growth, xylogenesis, wood anatomy, and carbon, oxygen and hydrogen stable isotope measurements in the water, sugars and cellulose of plant tissues. The dendrochronological analyses highlighted three distinct acclimation phases to the treatments: irrigated trees experienced (i) a significant growth increase in the first 4 years of treatment, (ii) high growth rates but with a declining trend in the following 8 years and finally (iii) a regression to pre-irrigation growth rates, suggesting the development of a new growth limitation (i.e. acclimation). The introduction of the stop treatment resulted in further growth reductions to below-control levels during the third phase. Irrigated trees showed longer growth periods and lower tree-ring δ13 C values, reflecting lower stomatal restrictions than control trees. Their strong tree-ring δ18 O and δ2 H (O-H) relationship reflected the hydrological signature similarly to the control. On the contrary, the stop trees had lower growth rates, conservative wood anatomical traits, and a weak O-H relationship, indicating a physiological imbalance. Tree vitality (identified by crown transparency) significantly modulated growth, wood anatomical traits and tree-ring δ13 C, with low-vitality trees of all treatments performing similarly regardless of water availability. We thus provide quantitative indicators for assessing physiological imbalance and tree acclimation after environmental stresses. We also show that tree vitality is crucial in shaping such responses. These findings are fundamental for the early assessment of ecosystem imbalances and decline under climate change.

Drought survival in conifer species is related to the time required to cross the stomatal safety margin

The regulation of water loss and the spread of xylem embolism have mostly been considered separately. The development of an integrated approach taking into account the temporal dynamics and relative contributions of these mechanisms to plant drought responses is urgently needed. Do conifer species native to mesic and xeric environments display different hydraulic strategies and temporal sequences under drought? A dry-down experiment was performed on seedlings of four conifer species differing in embolism resistance, from drought-sensitive to extremely drought-resistant species. A set of traits related to drought survival was measured, including turgor loss point, stomatal closure, minimum leaf conductance, and xylem embolism resistance. All species reached full stomatal closure before the onset of embolism, with all but the most drought-sensitive species presenting large stomatal safety margins, demonstrating that highly drought-resistant species do not keep their stomata open under drought conditions. Plant dry-down time to death was significantly influenced by the xylem embolism threshold, stomatal safety margin, and minimum leaf conductance, and was best explained by the newly introduced stomatal margin retention index (SMRIΨ50) which reflects the time required to cross the stomatal safety margin. The SMRIΨ50 may become a key tool for the characterization of interspecific drought survival variability in trees.

PtrVCS2 Regulates Drought Resistance by Changing Vessel Morphology and Stomatal Closure in Populus trichocarpa

Drought has severe effects on plant growth, forest productivity, and survival throughout the world. Understanding the molecular regulation of drought resistance in forest trees can enable effective strategic engineering of novel drought-resistant genotypes of tree species. In this study, we identified a gene, PtrVCS2, encoding a zinc finger (ZF) protein of the ZF-homeodomain transcription factor in Populus trichocarpa (Black Cottonwood) Torr. & A. Gray. ex Hook. Overexpression of PtrVCS2 (OE-PtrVCS2) in P. trichocarpa resulted in reduced growth, a higher proportion of smaller stem vessels, and strong drought-resistance phenotypes. Stomatal movement experiments revealed that the OE-PtrVCS2 transgenics showed lower stomata apertures than wild-type plants under drought conditions. RNA-seq analysis of the OE-PtrVCS2 transgenics showed that PtrVCS2 regulates the expression of multiple genes involved in regulation of stomatal opening and closing, particularly the PtrSULTR3;1-1 gene, and several genes related to cell wall biosynthesis, such as PtrFLA11-12 and PtrPR3-3. Moreover, we found that the water use efficiency of the OE-PtrVCS2 transgenic plants was consistently higher than that of wild type plants when subjected to chronic drought stress. Taken together, our results suggest that PtrVCS2 plays a positive role in improving drought adaptability and resistance in P. trichocarpa.