Hydraulics play an important role in causing low growth rate and dieback of aging Pinus sylvestris var. mongolica trees in plantations of Northeast China

Coupled hydraulics and carbon economy underlie age-related growth decline and revitalisation of sand-fixing shrubs after crown removal

Crown removal revitalises sand-fixing shrubs that show declining vigour with age in drought-prone environments; however, the underlying mechanisms are poorly understood. Here, we addressed this knowledge gap by comparing the growth performance, xylem hydraulics and plant carbon economy across different plant ages (10, 21 and 33 years) and treatments (control and crown removal) using a representative sand-fixing shrub (Caragana microphylla Lam.) in northern China. We found that growth decline with plant age was accompanied by simultaneous decreases in soil moisture, plant hydraulic efficiency and photosynthetic capacity, suggesting that these interconnected changes in plant water relations and carbon economy were responsible for this decline. Following crown removal, quick resprouting, involving remobilisation of root nonstructural carbohydrate reserves, contributed to the reconstruction of an efficient hydraulic system and improved plant carbon status, but this became less effective in older shrubs. These age-dependent effects of carbon economy and hydraulics on plant growth vigour provide a mechanistic explanation for the age-related decline and revitalisation of sand-fixing shrubs. This understanding is crucial for the development of suitable management strategies for shrub plantations constructed with species having the resprouting ability and contributes to the sustainability of ecological restoration projects in water-limited sandy lands.

A trade-off between leaf hydraulic efficiency and safety across three xerophytic species in response to increased rock fragment content

Limited information is available on the variation of plant leaf hydraulic traits in relation to soil rock fragment content (RFC), particularly for xerophytes native to rocky mountain areas. In this study, we conducted a field experiment with four gradients of RFC (0, 25, 50 and 75% ν ν-1) on three different xerophytic species (Sophora davidii, Cotinus szechuanensis and Bauhinia brachycarpa). We measured predawn and midday leaf water potential (Ψleaf), leaf hydraulic conductance (Kleaf), Ψleaf induced 50% loss of Kleaf (P50), pressure-volume curve traits and leaf structure. A consistent response of hydraulic traits to increased RFC was observed in three species. Kleaf showed a decrease, whereas P50 and turgor loss point (Ψtlp) became increasingly negative with increasing RFC. Thus, a clear trade-off between hydraulic efficiency and safety was observed in the xerophytic species. In all three species, the reduction in Kleaf was associated with an increase in leaf mass per area. In S. davidii, alterations in Kleaf and P50 were driven by leaf vein density (VLA) and Ψtlp. In C. szechuanensis, Ψtlp and VLA drove the changes in Kleaf and P50, respectively. In B. brachycarpa, changes in P50 were driven by VLA, whereas changes in both Kleaf and P50 were simultaneously influenced by Ψtlp. Our findings suggest that adaptation to increased rockiness necessarily implies a trade-off between leaf hydraulic efficiency and safety in xerophytic species. Additionally, the trade-off between leaf hydraulic efficiency and safety among xerophytic species is likely to result from processes occurring in the xylem and the outside-xylem hydraulic pathways. These findings contribute to a better understanding of the survival strategies and mechanisms of xerophytes in rocky soils, and provide a theoretical basis for the persistence of xerophytic species in areas with stony substrates.

Contrasting growth responses to drought in three tree species widely distributed in northern China

Tree species-specific responses to drought are urgently needed for assessing the impacts of current climate change on forest ecosystems. Here, we characterized the resistance, recovery, resilience, and growth recovery periods in response to different drought events based on tree-ring width index data (>30 years) for three tree species widely distributed in northern China, among which larch (Larix principis-rupprechtii Mayr) and Mongolian pine (Pinus sylvestris L. var. mongolica Litv.) are two major species used for afforestation, and birch (Betula platyphylla Suk.) is one of the most common natural tree species. Despite no significant effects of mild drought on tree growth, severe drought events significantly reduced the growth of all species, with contrasting species-specific responses. Larch trees had the lowest resistance and resilience among the three species, and Mongolian pine trees were more resistant but had a longer recovery period than birch trees. The drought responses varied with tree size. Large Mongolian pine and birch trees were more resistant but large larch trees were much more vulnerable than small trees during severe droughts. Smaller birch trees had higher resilience to severe droughts. Our study shows species-specific differences in drought responses and suggests that drought responses are tree-size dependent and drought-intensity associated, which further provides a guidance for selecting optimal cultivated tree species and designing forest managements in this region.

Analysis of soil water use by exotic and native vegetation in a semi-arid area and their associated interspecific competition

The exotic vegetation used in dryland vegetation restoration projects is characterized by its fast-growing and deep-rooted system, which enables it to expedite the restoration of ecosystem functions and enhance biodiversity. However, the interspecific relationship between exotic and native vegetation and soil water uptake in these restored ecosystems remains unclear, limiting our ability to evaluate the succession process and sustainability of restored ecosystems. In this study, stable isotope techniques and a proportional similarity index were used to investigate soil water use strategies and interspecific relationships between exotic and native vegetation. The results showed significant differences between the soil water use strategies of both exotic and native vegetation between seasons and species, where the proportions of deep soil water (30-100 cm) used by exotic shrubs (Caragana korshinskii) and exotic grass (Medicago sativa) were significantly higher than those used by the co-occurring native grass (Stipa bungeana) (p < 0.05). As soil water storage declined, exotic vegetation increased its utilization of deep soil water, whereas native grasses relied more on surface water (0-10 cm). This suggests that deep-rooted exotic vegetation has greater adaptability and access to water resources than shallow-rooted native vegetation. However, a prolonged decline in soil water storage led to increased competition for surface soil water (0-30 cm) between the exotic and native vegetation. This may increase the risk of degradation of exotic vegetation, particularly in situations with lower soil water content in the deep layers. Overall, this study highlights the variation in water-use strategies and interspecies relationships between exotic and native vegetation and their implications for ecosystem succession, which provides valuable insights for developing future vegetation restoration strategies and managing restored ecosystems.

Climate-driven sapwood-specific hydraulic conductivity and the Huber value but not leaf-specific hydraulic conductivity on a global scale

Efficient water transport is crucial for plant growth and survival. Plant hydraulic conductivity varies between functional groups and biomes and is strongly influenced by changing environmental conditions. However, correlations of conductivity-related hydraulic traits with climatic variables are not fully understood, preventing clarification of plant form and function under climate change scenarios. By compiling leaf-specific hydraulic conductivity (K_L), sapwood-specific hydraulic conductivity (K_s), and Huber values (H_v, sapwood area to leaf area ratio) along with climatic variables including mean annual temperature (MAT), mean annual precipitation (MAP) and aridity index (AI) for 428 species across a wide range of plant functional types (PFTs) and biomes at a global scale, we found greater variability of K_L within PFTs and biomes than across PFTs and biomes. Interaction effects between PFTs and biomes on K_L and K_s were found. The interaction between MAT and MAP played a significant role in K_s and H_v (t = 3.89, P < 0.001 for K_s and t = -5.77, P < 0.001 for H_v). With increasing AI, K_s increased and H_v decreased. K_L was not influenced by the investigated climatic variables. Our study provides a better understanding of the dynamics of hydraulic structure and function across functional groups and biomes and of the abiotic drivers of their large-scale variations.

Xylem hydraulics strongly influence the niche differentiation of tree species along the slope of a river valley in a water-limited area

Xylem hydraulic characteristics govern plant water transport, affecting both drought resistance and photosynthetic gas exchange. Therefore, they play critical roles in determining the adaptation of different species to environments with various water regimes. Here, we tested the hypothesis that variation in xylem traits associated with a trade-off between hydraulic efficiency and safety against drought-induced embolism contributes to niche differentiation of tree species along a sharp water availability gradient on the slope of a unique river valley located in a semi-humid area. We found that tree species showed clear niche differentiation with decreasing water availability from the bottom towards the top of the valley. Tree species occupying different positions, in terms of vertical distribution distance from the bottom of the valley, showed a strong trade-off between xylem water transport efficiency and safety, as evidenced by variations in xylem structural traits at both the tissue and pit levels. This optimized their xylem hydraulics in their respective water regimes. Thus, the trade-off between hydraulic efficiency and safety contributes to clear niche differentiation and, thereby, to the coexistence of tree species in the valley with heterogeneous water availability.

Within-crown plasticity of hydraulic properties influence branch dieback patterns of two woody plants under experimental drought conditions

In recent year, widespread declines of Populus bolleana Lauche trees (P. bolleana, which dieback from the top down) and Haloxylon ammodendron shrubs (H. ammodendron, which dieback starting from their outer canopy) have occurred. To investigate how both intra-canopy hydraulic changes and plasticity in hydraulic properties create differences in vulnerability between these two species, we conducted a drought simulation field experiment. We analyzed branch hydraulic vulnerability, leaf water potential (Ψ), photosynthesis (A), stomatal conductance (gs), non-structural carbohydrate (NSCs) contents and morphological traits of the plants as the plants underwent a partial canopy dieback. Our results showed that: (1) the hydraulic architecture was very different between the two life forms; (2) H. ammodendron exhibited a drought tolerance response with weak stomatal control, and thus a sharp decline in Ψ while P. bolleana showed a drought avoidance response with tighter stomatal control that maintained a relatively stable Ψ; (3) the Ψ of H. ammodendron showed relative consistent symptoms of drought stress with increasing plant stature, but the Ψ of P. bolleana showed greater drought stress in higher portions of the crown; (4) prolonged drought caused P. bolleana to consume and H. ammodendron to accumulate NSCs in the branches of their upper canopy. Thus, the prolonged drought caused the shoots of the upper canopy of P. bolleana to experience greater vulnerability leading to dieback of the upper branches first, while all the twigs of the outer canopy of H. ammodendron experienced nearly identical degrees of vulnerability, and thus dieback occurred uniformly. Our results indicate that intra-canopy hydraulic change and their plasticity under drought was the main cause of the observed canopy dieback patterns in both species. However, more work is needed to further establish that hydraulic limitation as a function of plant stature was the sole mechanism for causing the divergent canopy dieback patterns.

Interactive effect between tree ageing and trunk-boring pest reduces hydraulics and carbon metabolism in Hippophae rhamnoides

Sea-buckthorn (Hippophae rhamnoides) is widely distributed across the Eurasian continent. Recently sea-buckthorn has shown premature ageing and decline when confronted with water deficiency and Holcocerus hippophaecolus damage in northwest China and the Loess Plateau region. However, the physiological process of sea-buckthorn senescence in response to drought and pest damage is still unknown. In this study, 4-year-old (4y), 15-year-old normal growth (15yN) and 15-year-old seriously moth-damaged sea-buckthorn plants (15yH) were used as the research objects. The growth of branches and roots, branch water potential and percentage loss of conductivity (PLC), branch vulnerability to embolism (quantified by P₅₀, xylem water potential at 50 % of PLC), branch xylem parenchyma cell viability, photosynthesis and the non-structural carbohydrate (NSC) content in branches and roots in dry and wet seasons were measured. The results showed that the length, basal diameter of 1-year-old branches and the leaf area of 4y trees were significantly larger than that of 15yN and 15yH trees, and the fine root density of 15yH trees was significantly lower than that of 15yN trees in all measured areas. The branch-specific hydraulic conductivity of 15yN and 15yH trees was only 50.2 % and 12.3 % of that of 4y trees, and the P₅₀ of 4y, 15yH and 15yN trees was -3.69 MPa, -2.71 MPa and -1.15 MPa, respectively. The midday water potential and photosynthetic rate were highest in 4y trees, followed by 15yN and then 15yH trees in both the dry season and wet seasons, while branch PLC declined in the opposite direction (15yH trees highest, 4y trees lowest). The degree of PLC repair within a day was highest in 4y trees, followed by 15yN and then 15yH trees, and the viability of xylem cells was consistent with this pattern. The branch xylem starch and NSC content of 4y and 15yN trees were significantly higher than that of 15yH trees in the dry season, and the root starch and NSC content of 4y trees were significantly higher than that of 15yH trees in the two seasons. The above results suggest that the hydraulic properties of the normal elderly and seriously pest-damaged sea-buckthorn were significantly worse than in juvenile plants. Narrower early wood width and vessel density, high embolism vulnerability and weak embolism repair capacity led to the decline in water-conducting ability, and similarly further affected photosynthesis and the root NSC content. The decline in xylem parenchyma cell viability was the main reason for the limited embolism repair in the branches.

Canopy Transpiration and Stomatal Conductance Dynamics of Ulmus pumila L. and Caragana korshinskii Kom. Plantations on the Bashang Plateau, China

Constructing protective forests to control water and soil erosion is an effective measure to address land degradation in the Bashang Plateau of North China, but forest dieback has occurred frequently due to severe water deficits in recent decades. However, transpiration dynamics and their biophysical control factors under various soil water contents for different forest functional types are still unknown. Here, canopy transpiration and stomatal conductance of a 38-year-old Ulmus pumila L. and a 20-year-old Caragana korshinskii Kom. were quantified using the sap flow method, while simultaneously monitoring the meteorological and soil water content. The results showed that canopy transpiration averaged 0.55 ± 0.34 mm d−1 and 0.66 ± 0.32 mm d−1 for U. pumila, and was 0.74 ± 0.26 mm d−1 and 0.77 ± 0.24 mm d−1 for C. korshinskii in 2020 and 2021, respectively. The sensitivity of canopy transpiration to vapor pressure deficit (VPD) decreased as soil water stress increased for both species, indicating that the transpiration process is significantly affected by soil drought. Additionally, canopy stomatal conductance averaged 1.03 ± 0.91 mm s−1 and 1.34 ± 1.22 mm s−1 for U. pumila, and was 1.46 ± 0.90 mm s−1 and 1.51 ± 1.06 mm s−1 for C. korshinskii in 2020 and 2021, respectively. The low values of the decoupling coefficient (Ω) showed that canopy and atmosphere were well coupled for both species. Stomatal sensitivity to VPD decreased with decreasing soil water content, indicating that both U. pumila and C. korshinskii maintained a water-saving strategy under the stressed water conditions. Our results enable better understanding of transpiration dynamics and water-use strategies of different forest functional types in the Bashang Plateau, which will provide important insights for planted forests management and ecosystem stability under future climate changes.

Key Strategies Underlying the Adaptation of Mongolian Scots Pine (Pinussylvestris var. mongolica) in Sandy Land under Climate Change: A Review

Forest degradation and mortality have been widely reported in the context of increasingly significant global climate change. As the country with the largest total tree plantation area globally, China has a great responsibility in forestry management to cope with climate change effectively. Mongolian Scots pine (Pinus sylvestris var. mongolica) was widely introduced from its natural sites in China into several other sandy land areas for establishing shelterbelt in the Three-North Shelter Forest Program, scoring outstanding achievements in terms of wind-breaking and sand-fixing. Mongolian Scots pine plantations in China cover a total area of ~800,000 hectares, with the eldest trees having >60 years. However, plantation trees have been affected by premature senescence in their middle-age stages (i.e., dieback, growth decline, and death) since the 1990s. This phenomenon has raised concerns about the suitability of Mongolian Scots pine to sandy habitats and the rationality for further afforestation, especially under the global climate change scenario. Fortunately, dieback has occurred only sporadically at specific sites and in certain years and has not spread to other regions in northern China; nevertheless, global climate change has become increasingly significant in that region. These observations reflect the strong drought resistance and adaptability of Mongolian Scots pines. In this review, we summarized the most recent findings on the ecohydrological attributes of Mongolian Scots pine during its adaptation to both fragile habitats and climate change. Five main species-specific strategies (i.e., opportunistic water absorb strategy, hydraulic failure risk avoidance strategy, water conservation strategy, functional traits adjustment strategy, rapid regeneration strategy) were summarized, providing deep insights into the tree–water relationship. Overall, the findings of this study can be applied to improve plantation management and better cope with climate-change-related drought stress.

Greater hydraulic safety contributes to higher growth resilience to drought across seven pine species in a semi-arid environment

Quantifying inter-specific variations of tree resilience to drought and revealing the underlying mechanisms are of great importance to the understanding of forest functionality, particularly in water-limited regions. So far, comprehensive studies incorporating investigations in inter-specific variations of long-term growth patterns of trees and the underlying physiological mechanisms are very limited. Here, in a semi-arid site of northern China, tree radial growth rate, inter-annual tree-ring growth responses to climate variability, as well as physiological characteristics pertinent to xylem hydraulics, carbon assimilation and drought tolerance were analyzed in seven pine species growing in a common environment. Considerable inter-specific variations in radial growth rate, growth response to drought and physiological characteristics were observed among the studied species. Differently, the studied species exhibited similar degrees of resistance to drought-induced branch xylem embolism, with water potential corresponding to 50% loss hydraulic conductivity ranging from -2.31 to -2.96 MPa. We found that higher branch hydraulic efficiency is related to greater leaf photosynthetic capacity, smaller hydraulic safety margin and lower woody density (P < 0.05, linear regressions), but not related to higher tree radial growth rate (P > 0.05). Rather, species with higher hydraulic conductivity and photosynthetic capacity were more sensitive to drought stress and tended to show weaker growth resistance to extreme drought events as quantified by tree-ring analyses, which is at least partially due to a trade-off between hydraulic efficiency and safety across species. This study thus demonstrates the importance of drought resilience rather than instantaneous water and carbon flux capacity in determining tree growth in water-limited environments.

Unlocking Drought-Induced Tree Mortality: Physiological Mechanisms to Modeling

Drought-related tree mortality has become a major concern worldwide due to its pronounced negative impacts on the functioning and sustainability of forest ecosystems. However, our ability to identify the species that are most vulnerable to drought, and to pinpoint the spatial and temporal patterns of mortality events, is still limited. Model is useful tools to capture the dynamics of vegetation at spatiotemporal scales, yet contemporary land surface models (LSMs) are often incapable of predicting the response of vegetation to environmental perturbations with sufficient accuracy, especially under stressful conditions such as drought. Significant progress has been made regarding the physiological mechanisms underpinning plant drought response in the past decade, and plant hydraulic dysfunction has emerged as a key determinant for tree death due to water shortage. The identification of pivotal physiological events and relevant plant traits may facilitate forecasting tree mortality through a mechanistic approach, with improved precision. In this review, we (1) summarize current understanding of physiological mechanisms leading to tree death, (2) describe the functionality of key hydraulic traits that are involved in the process of hydraulic dysfunction, and (3) outline their roles in improving the representation of hydraulic function in LSMs. We urge potential future research on detailed hydraulic processes under drought, pinpointing corresponding functional traits, as well as understanding traits variation across and within species, for a better representation of drought-induced tree mortality in models.

A clear trade-off between leaf hydraulic efficiency and safety in an aridland shrub during regrowth

It has been suggested that a trade-off between hydraulic efficiency and safety is related to drought adaptation across species. However, whether leaf hydraulic efficiency is sacrificed for safety during woody resprout regrowth after crown removal is not well understood. We measured leaf water potential (ψ_leaf ) at predawn (ψ_pd ) and midday (ψ_mid ), leaf maximum hydraulic conductance (K_leaf-max ), ψ_leaf at induction 50% loss of K_leaf-max (K_leaf P₅₀ ), leaf area-specific whole-plant hydraulic conductance (LSC), leaf vein structure and turgor loss point (π_tlp ) in 1- to 13-year-old resprouts of the aridland shrub (Caragana korshinskii). ψ_pd was similar, ψ_mid and K_leaf P₅₀ became more negative, and K_leaf-max decreased in resprouts with the increasing age; thus, leaf hydraulic efficiency clearly traded off against safety. The difference between ψ_mid and K_leaf P₅₀ , leaf hydraulic safety margin, increased gradually with increasing resprout age. More negative ψ_mid and K_leaf P₅₀ were closely related to decreasing LSC and more negative π_tlp , respectively, and the decreasing K_leaf-max arose from the lower minor vein density and the narrower midrib xylem vessels. Our results showed that a clear trade-off between leaf hydraulic efficiency and safety helps C. korshinskii resprouts adapt to increasing water stress as they approach final size.

Quantifying Key Points of Hydraulic Vulnerability Curves From Drought-Rewatering Experiment Using Differential Method

Precise and accurate estimation of key hydraulic points of plants is conducive to mastering the hydraulic status of plants under drought stress. This is crucial to grasping the hydraulic status before the dieback period to predict and prevent forest mortality. We tested three key points and compared the experimental results to the calculated results by applying two methods. Saplings (n = 180) of Robinia pseudoacacia L. were separated into nine treatments according to the duration of the drought and rewatering. We established the hydraulic vulnerability curve and measured the stem water potential and loss of conductivity to determine the key points. We then compared the differences between the calculated [differential method (DM) and traditional method (TM)] and experimental results to identify the validity of the calculation method. From the drought-rewatering experiment, the calculated results from the DM can be an accurate estimation of the experimental results, whereas the TM overestimated them. Our results defined the hydraulic status of each period of plants. By combining the experimental and calculated results, we divided the hydraulic vulnerability curve into four parts. This will generate more comprehensive and accurate methods for future research.

Influence of Warmer and Drier Environmental Conditions on Species-Specific Stem Circumference Dynamics and Water Status of Conifers in Submontane Zone of Central Slovakia

The frequency and intensity of droughts and heatwaves in Europe with notable impact on forest growth are expected to increase due to climate change. Coniferous stands planted outside the natural habitats of species belong to the most threatened forests. In this study, we assess stem circumference response of coniferous species (Larix decidua and Abies alba) to environmental conditions during the years 2015–2019. The study was performed in Arboretum in Zvolen (ca. 300 m a.s.l., Central Slovakia) characterised by a warmer and drier climate when compared to their natural habitats (located above 900 m a.s.l.), where they originated from. Seasonal radial variation, tree water deficit (ΔW), and maximum daily shrinkage (MDS) were derived from the records obtained from band dendrometers installed on five mature trees per species. Monitored species exhibited remarkably different growth patterns under highly above normal temperatures and uneven precipitation distribution. The magnitudes of reversible circumference changes (ΔW, MDS) were species-specific and strongly correlated with environmental factors. The wavelet analysis identified species-specific vulnerability to drought indicated by pronounced diurnal stem variation periodicity in rainless periods. L. decidua exhibited more strained stem water status and higher sensitivity to environmental conditions than A. alba. Tree water deficit and maximum daily shrinkage were found appropriate characteristics to compare water status of different tree species.

Carrying capacity for vegetation across northern China drylands

Revegetation and afforestation across drylands for establishing sustainable ecosystems requires a comprehensive understanding of the carrying capacity for vegetation (CCV) at the regional scale. To determine the CCV across drylands in northern China, we developed a technical framework based on two measures of leaf area index (LAI): maximum LAI (Max-LAI) and safe LAI (Safe-LAI), and their thresholds, CCV_max and CCV_safe, for six drylands (Horqin, Hulun Buir, Otindag, Mu Us, Tengger, and Junggar) using remote sensing datasets from 2000 to 2014. We also predicted dynamics of CCV of the drylands over the next decade (2015-2024) by establishing optimal prediction models based on environmental factors (temperature, precipitation, potential evapotranspiration, and elevation). According to these models, the Max-LAI threshold (range: 0.36-1.03 m²/m²) and Safe-LAI threshold (0.29-0.70 m²/m²) declined from east to west with decreases in aridity index. Under current climatic variability and anthropogenic disturbances, the CCV in most drylands would have positive increments (approximately 15%), except in the Horqin (approximately -15%) and Tengger (slight changes), during the following decade. This indicates that there is scope for improving vegetation coverage in most drylands, except in the Horqin and Tengger. Our results suggest that revegetation and ecosystem management to prevent ongoing desertification should be carried out at the regional scale. Although it does not account for biocrusts, artificially introduced vegetation, underground water, and other vegetation attributes (e.g., density and biomass), our technical framework and results might nonetheless be valuable in evaluating regional ecological security and guiding vegetation restoration of drylands across northern China.

Tree-Ring Analysis Reveals Density-Dependent Vulnerability to Drought in Planted Mongolian Pines

Population density influences tree responses to environmental stresses, such as drought and high temperature. Prolonged drought negatively affects the health of Mongolian pines in forests planted by the Three-North Shelter Forest Program in North China. To understand the relationship between stand density and drought-induced forest decline, and to generate information regarding the development of future management strategies, we analyzed the vulnerability to drought of planted Mongolian pines at three stand densities. A tree-ring width index for trees from each density was established from tree-ring data covering the period 1988–2018 and was compared for differences in radial growth. Resistance (Rt), recovery (Rc), resilience (Rs), and relative resilience (RRs) in response to drought events were calculated from the smoothed basal area increment (BAI) curves. The high-density (HDT) group showed a consistently lower tree-ring width than the border trees (BT) and low-density (LDT) groups. The BAI curve of the HDT group started to decrease five years earlier than the LDT and BT groups. Pearson correlation analysis revealed that the radial growth of all of the groups was related to precipitation, relative humidity (RH), potential evapotranspiration (ET0), and standardized precipitation evapotranspiration index (SPEI) in the previous October and the most recent July, indicating that Mongolian pine trees of different densities had similar growth–climate relationships. Over the three decades, the trees experienced three severe drought events, each causing reduced tree-ring width and BAI. All of the groups showed similar Rc to each drought event, but the HDT group exhibited significantly lower Rt, Rs, and RRs than the BT group, suggesting that the HDT trees were more vulnerable to repeated drought stress. The RRs of the HDT group decreased progressively after each drought event and attained <0 after the third event. All of the groups showed similar trends regarding water consumption under varying weather conditions, but the HDT group showed significantly reduced whole-tree hydraulic capability compared with the other two groups. From these results, HDT trees exhibit ecophysiological memory effects from successive droughts, including sap flux dysfunction and higher competition index, which may prevent recovery of pre-drought growth rates. HDT trees may be at greater risk of mortality under future drought disturbance.

Different ways to die in a changing world: Consequences of climate change for tree species performance and survival through an ecophysiological perspective

Anthropogenic activities such as uncontrolled deforestation and increasing greenhouse gas emissions are responsible for triggering a series of environmental imbalances that affect the Earth's complex climate dynamics. As a consequence of these changes, several climate models forecast an intensification of extreme weather events over the upcoming decades, including heat waves and increasingly severe drought and flood episodes. The occurrence of such extreme weather will prompt profound changes in several plant communities, resulting in massive forest dieback events that can trigger a massive loss of biodiversity in several biomes worldwide. Despite the gravity of the situation, our knowledge regarding how extreme weather events can undermine the performance, survival, and distribution of forest species remains very fragmented. Therefore, the present review aimed to provide a broad and integrated perspective of the main biochemical, physiological, and morpho-anatomical disorders that may compromise the performance and survival of forest species exposed to climate change factors, particularly drought, flooding, and global warming. In addition, we also discuss the controversial effects of high CO2 concentrations in enhancing plant growth and reducing the deleterious effects of some extreme climatic events. We conclude with a discussion about the possible effects that the factors associated with the climate change might have on species distribution and forest composition.

Deepening Rooting Depths Improve Plant Water and Carbon Status of a Xeric Tree during Summer Drought

Exploring the effects of drought on trees of different sizes is an important research topic because the size-dependent mortality pattern of the major dominant species significantly affects the structure and function of plant communities. Here we studied the physiological performance and non-structural carbohydrates (NSCs) dynamics of a small xeric tree species, Haloxylon ammodendron (C.A.Mey.) of different tree size with varying rooting depth, during summer drought. We measured predawn (Ψpd) and midday (Ψm) leaf water potential, osmotic potential at saturated turgor (π100), and turgor lost point (Ψtlp), stomatal conductance (gs) at noon, maximum photochemical efficiency of photosystem II (Fv/Fm) in the morning, and NSCs concentration, from June–September. Our results demonstrated that the summer drought reduces the overall performance of physiological traits of the small young trees more than the larger adult trees. Ψpd, gs and Fv/Fm dropped larger in the small-diameter groups than the larger diameter groups. Substantial osmotic adjustments were observed in small size individuals (with lower π100 and Ψtlp) to cope with summer drought. Furthermore, mean concentration of NSCs for the leaf and shoot were higher in September than in July in every basal stem diameter classes suggested the leaf and shoot acted as reserve for NSC. However the root NSCs concentrations within each basal stem diameter class exhibited less increase in September than in the July. At the same time, the small young tress had lower root NSCs concentrations than the larger adult tree in both July and September. The contrasting root NSC concentrations across the basal stem diameter classes indicated that the roots of smaller trees may be more vulnerable to carbon starvation under non-lethal summer drought. The significant positive relationship between rooting depth and physiological traits & root NSCs concentration emphasize the importance of rooting depth in determining the seasonal variation of water status, gas exchange and NSCs.

Drought tolerance traits do not vary across sites differing in water availability in Banksia serrata (Proteaceae)

Interspecific variation in plant hydraulic traits plays a major role in shaping species distributions across climates, yet variation within species is poorly understood. Here we report on intraspecific variation of hydraulic traits in Banksia serrata (L.f.) sampled from three sites characterised by contrasting climates (warm-wet, warm-dry and cool-wet). Hydraulic characteristics including vulnerability to embolism, hydraulic conductance, pressure-volume traits and key morphological traits were measured. Vulnerability to embolism in leaf and stem, defined by the water potential inducing 50 and 88% loss of hydraulic conductivity (P50 and P88 respectively), did not differ across sites. However, plants from the warm-dry environment exhibited higher stem conductivity (Ks) than the cool-wet environment. Leaf turgor loss point (TLP) did not vary among sites, but warm-dry site plants showed lower leaf capacitance (C*FT) and higher modulus of elasticity (ε) than the other two sites. Plants from the cool-wet site had lower specific leaf area (SLA) and plants from the warm-dry site had lower sapwood density (WD). Overall, key hydraulic traits were generally conserved across populations despite differences in mean site water availability, and the safety-efficiency trade-off was absent in this species. These results suggest that B. serrata has limited ability to adjust hydraulic architecture in response to environmental change and thus may be susceptible to climate change-type drought stress.

Biomass Accumulation and Carbon Sequestration in an Age-Sequence of Mongolian Pine Plantations in Horqin Sandy Land, China

The Mongolian pine (Pinus sylvestris L. var. mongolica Litv.) was first introduced to the southeastern Horqin sandy land in the mid-1950s. Since then, it has been widely planted and has become the most important conifer species in Northern China, providing significant ecological, economic and social benefits. However, its function in sequestering carbon at different developmental stages has been little studied. In this study, twenty plots inventory and destructive sampling of eight trees were conducted in 12-, 19-, 34-, 48- and 58-year-old Mongolian pine stands of China. Allometric biomass equations (ABEs) for tree components were established and used to determine the magnitude and distribution of tree biomass and carbon density. The carbon density of the understory, forest floor and soil was also determined. The ABEs with age as the second variable could simply and accurately determine the biomass of plantation tree branches, foliage and fruit, which were considerably influenced by age. With increasing stand age, the proportion of stem biomass to total tree biomass increased from 22.2% in the 12-year-old stand to 54.2% in the 58-year-old stand, and the proportion of understory biomass to total ecosystem biomass decreased, with values of 7.5%, 4.6%, 4.4%, 4.1% and 3.0% in the five stands. The biomass of the forest floor was 0.00, 1.12, 2.04, 6.69 and 3.65 Mg ha−1 in the five stands. The ecosystem carbon density was 40.2, 73.4, 92.9, 89.9 and 87.3 Mg ha−1 in the 12-, 19-, 34-, 48-, and 58-year-old stands, in which soil carbon density accounted for the largest proportion, with values of 67.4%, 76.8%, 73.2%, 63.4%, and 57.7% respectively. The Mongolian pine had the potential for carbon sequestration during its development, especially in the early stages, however, in the later growth stage, the ecosystem carbon density decreased slightly.