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Bamboos are arborescent monocotyledons that have no secondary growth, but can continually produce conduits with diameters appropriate to the current size of the plant. Here, we studied bamboo hydraulic architecture to address the mechanisms involved in compensating for the increase in hydraulic resistance during ontogeny. We measured the hydraulic weighted vessel diameters (Dh ) at different distances from the apex along the stem of Bambusa textilis. The hydraulic resistance of different components and individuals of different heights were quantified using the high-pressure flowmeter method. The Dh showed tip-to-base widening with a scaling exponent in the range of those reported for trees. Although theoretical hydraulic conductivity decreased from base-to-tip, leaf-specific conductivity did not change. Leaves contributed the most to the whole-shoot hydraulic resistance, followed by the leaf-bearing branches. Roots contributed c. 13% to whole-plant resistance. Interestingly, taller individuals showed lower whole-shoot resistance owing to an increased number of resistances in parallel (side-branches), while leaf-specific resistance was independent of plant size. Tip-to-base vessel widening and height-independent constant leaf-specific conductance could be mechanisms for hydraulic optimization in B. textilis. Similar patterns have also been found in woody plants with secondary growth, but this bamboo exhibits them without secondary growth.

Vulnerability curves (VCs) describe the loss of hydraulic conductance against increasing xylem tension, providing valuable insights about the response of plant water transport to water stress. Techniques to construct VCs have been developed and modified continuously, but controversies continue. We compared VCs constructed using the bench-top dehydration (BD), air-injection-flow (AI), pneumatic-air-discharge (PAD), optical (OP) and X-ray-computed microtomography (MicroCT) methods for tropical trees and lianas with contrasting vessel lengths. The PAD method generated highly vulnerable VCs, the AI method intermediate VCs, whereas the BD, OP and MicroCT methods produced comparable and more resistant VCs. Vessel-length and diameter accounted for the overestimation ratio of vulnerability estimated using the AI but not the PAD method. Compared with directly measured midday embolism levels, the PAD and AI methods substantially overestimated embolism, whereas the BD, MicroCT and OP methods provided more reasonable estimations. Cut-open vessels, uncertainties in maximum air volume estimations, sample-length effects, tissue cracks and shrinkage together may impede the reliability of the PAD method. In conclusion, we validate the BD, OP and MicroCT methods for tropical plants, whereas the PAD and AI need further mechanistic testing. Therefore, applications of VCs in estimating plant responses to drought need to be cautious.

Tree trunks not only provide physical support for canopy leaves but also supply and store water for transpiration. However, the relationships between trunk hydraulic properties and canopy leaf physiology in tropical trees are not well-understood. In this study we concurrently measured morning and midday canopy leaf photosynthesis (A), stomatal conductance (gs), and leaf water potentials (ΨL) in 40 tropical trees representing 14 species at the beginning of the rainy season in Xishuangbanna, Southwest China. We also measured trunk sapwood capacitance (C), wood density, and sap flux density to assess their association with canopy leaf physiology. Among the 14 studied species, only three and four species did not show a significant midday reduction in A and gs respectively. The diurnally maximum A and gs were significantly positively related to sapwood hydraulic capacitance, maximum sap flux density (midday), and sap flux density at 11:00. Those species with lower wood density and higher C showed a lower reduction in ΨL at midday, whereas, species with high C, and large values of maximum sap flux density also showed high carbon assimilation at midday. Our results provide new insights into the close coordination between canopy physiology and trunk sapwood hydraulic properties in tropical trees.

Water management and irrigation practices are persistent challenges for many agricultural systems, exacerbated by changing seasonal and weather patterns. The wild blueberry industry is at heightened susceptibility due to its unique growing conditions and uncultivated nature. Stress detection in agricultural fields can prompt management responses to mitigate detrimental conditions, including drought and disease. We assessed airborne spectral data accompanied by ground sampled water potential over three developmental stages of wild blueberries collected throughout the 2019 summer on two adjacent fields, one irrigated and one non-irrigated. Ground sampled leaves were collected in tandem to the hyperspectral image collection with an unoccupied aerial vehicle (UAV) and then measured for leaf water potential. Using methods in machine learning and statistical analysis, we developed models to determine irrigation status and water potential. Seven models were assessed in this study, with four used to process six hyperspectral cube images for analysis. These images were classified as irrigated or non-irrigated and estimated for water potential levels, resulting in an R2 of 0.62 and verified with a validation dataset. Further investigation relating imaging spectroscopy and water potential will be beneficial in understanding the dynamics between the two for future studies.

Mangroves growing in tropical and subtropical intertidal zones face challenges from warming and altered rainfall patterns associated with global climate change. Intraspecific variation in hydraulic traits may allow a mangrove species to acclimate to novel climatic conditions, yet little is known regarding the potential for adaptive plasticity in these traits. In this study, we aimed to quantify the variation in plant hydraulic traits of two widespread mangrove species growing across a latitudinal gradient. We investigated the xylem hydraulic structure and function of Avicennia marina and Aegiceras corniculatum, across three sites spanning a latitudinal gradient of 17.45° in eastern Australia. We found that both species were highly resistant to xylem embolism and that there was significant intraspecific variation in hydraulic traits between sites. The highest embolism resistance and sapwood-specific hydraulic conductivity (KS) were found at the lowest latitude site that had the highest mean annual temperature and precipitation. A. marina showed no differences in vessel size and density among sites. It has other special features such as successive cambia enhancing its ability to adapt to a large environmental gradient. In contrast, A. corniculatum showed higher vessel densities at lower latitudes. There was a significant and positive correlation (R2 = 0.72, P &lt, 0.05) between KS and embolism resistance across species and sites, suggesting the absence of a tradeoff between hydraulic efficiency and safety. Both embolism resistance and KS were negatively correlated with wood density but positively with vessel wall reinforcement. This study reveals that these two widespread mangrove species were adapted to warmer climates by enhancing both hydraulic efficiency and safety.

Identifying differences in ecophysiology between simple and compound leaves can help understand the adaptive significance of the compound leaf form and its response to climate change. However, we still know surprisingly little about differences in water and nutrient use, and photosynthetic capacity between co-occurring compound-leaved and simple-leaved tree species, especially in savanna ecosystems with dry-hot climate conditions. From July to September in 2015, we investigated 16 functional traits associated with water use, nutrients, and photosynthesis of six deciduous tree species (three simple-leaved and three compound-leaved species) coexisting in a valley-savanna in Southwest China. Our major objective was to test the variation in these functional traits between these two leaf forms. Overall, overlapping leaf mass per area (LMA), photosynthesis, as well as leaf nitrogen and phosphorus concentrations were found between these coexisting valley-savanna simple- and compound-leaved tree species. We didn&rsquo, t find significant differences in water and photosynthetic nitrogen or phosphorus use efficiency between simple and compound leaves. Across these simple- and compound-leaved tree species, photosynthetic phosphorus use efficiencies were positively related to LMA and negatively correlated with phosphorus concentration per mass or area. Water use efficiency (intrinsic water use efficiency or stable carbon isotopic composition) was independent of all leaf traits. Similar ecophysiology strategies among these coexisting valley-savanna simple- and compound-leaved species suggested a convergence in ecological adaptation to the hot and dry environment. The overlap in traits related to water use, carbon assimilation, and stress tolerance (e.g., LMA) also suggests a similar response of these two leaf forms to a hotter and drier future due to the climate change.

Characterizing differences in key functional traits between simple-leaved (SL) and compound-leaved (CL) tree species can contribute to a better understanding of the adaptive significance of compound leaf form. In particular, this information may provide a mechanistic explanation to the long-proposed fast-growth hypothesis of CL tree species. Here, using five SL and five CL tree species co-occurring in a typical temperate forest of Northeast China, we tested whether higher hydraulic efficiency underlies potentially high photosynthetic capacity in CL species. We found that the CL species had significantly higher hydraulic conductance at the whole-branch level than the SL species (0.52 ± 0.13 vs 0.15 ± 0.04 × 10-4 kg m-2 s-1 Pa-1, P = 0.029). No significant difference in net photosynthetic rate (14.7 ± 2.43 vs 12.5 ± 2.05 μmol m-2 s-1, P = 0.511) was detected between these two groups, but this was largely due to the existence of one outlier species in each of the two functional groups. Scrutinization of the intragroup variations in functional traits revealed that distinctions of the two outlier species in wood type (ring- vs diffuse-porous) from their respective functional groups have likely contributed to their aberrant physiological performances. The potentially high photosynthetic capacity of CL species seems to require ring-porous wood to achieve high hydraulic efficiency. Due to its limitation on leaf photosynthetic capacity, diffuse-porous wood with lower hydraulic conductivity largely precludes its combination with the 'throw-away' strategy (i.e., annually replacing the stem-like rachises) of compound-leaved tree species, which intrinsically requires high carbon assimilation rate to compensate for their extra carbon losses. Our results for the first time show clear differentiation in hydraulic architecture and CO2 assimilation between sympatric SL and CL species, which contributes to the probing of the underlying mechanism responsible for the potential fast growth of trees with compound leaves.

Marginal water-use efficiency plays a critical role in plant carbon–water coupling relationships. We investigated the ecosystem marginal water-use efficiency (λ) of a tropical seasonal evergreen forest to (1) determine the general pattern of λ across time, (2) compare different models for calculating λ, and (3) address how λ varies with soil water content during different seasons. There was a U-shaped diurnal pattern in λ, which was higher in the early morning and late afternoon. At other times of the day, λ was lower and remained constant. Ecosystem λ was higher in the wet season than in the dry season. All three models successfully captured the diurnal and seasonal patterns of λ but differed in the calculated absolute values. The idea that λ is constant on a subdaily scale was partly supported by our study, while a constant λ was only true when data from the early morning and late afternoon were not included. The λ increases with soil water content on a seasonal scale, possibly because early morning λ remained low in dry conditions when the soil water content was low.

The differences in crown conductance sensitivity (m) to vapor pressure deficit (VPD) and water consumption between tropical evergreen and deciduous tree are not well-understood despite the importance of such information for the development of plantation and water-resource management strategies in regions that experience seasonal drought and increasing water deficits. To this end, during the wet season we measured sap flow in 60 individual trees representing 21 tropical angiosperm broadleaf timber species (11 evergreen and 10 deciduous) growing in plantations in a marginal tropical area of southwestern China. The results showed that m was strongly correlated with reference crown conductance (Gcref) at 1 kPa VPD across all species, with a slope value of 0.46, which was significantly different from the proposed theoretical value of 0.6, suggesting the study species as anisohydric. The m was positively correlated with tree diameter at the height of 1.3 m (diameter at breast height, DBH) but for a given DBH, m was significantly higher in evergreens than in deciduous trees. Further, whole-tree mean daily water consumption was strongly and positively correlated with DBH, but at a given DBH, evergreen species exhibited larger water consumption than deciduous species. Vessel diameter (related to xylem hydraulic conductance) and sapwood area were also positively correlated with DBH, and evergreen species had significantly larger sapwood area than deciduous species at a given DBH. In conclusion, we found that 1) crown conductance of evergreen trees is more sensitive to VPD compared to deciduous species and 2) evergreen trees consume more water than deciduous trees, partly because of having higher peak transpiration rate and, larger sapwood area (i.e., larger xylem area for water transport and storage). Therefore, we suggest that in tropical regions associated with seasonal drought timber plantations with more deciduous species could potentially be considered as a management possibility for achieving a balance between timber production and water conservation, even in the wet season, which needs to be tested at the plantation level.

The time scale of stomatal closure and xylem cavitation during plant dehydration, as well as the fate of embolized organs, are under debate, largely due to methodological limitations in the evaluation of embolism. While some argue that complete stomatal closure precedes the occurrence of embolism, others believe that the two are contemporaneous processes that are accompanied by daily xylem refilling. Here, we utilize an optical light transmission method to continuously monitor xylem cavitation in leaves of dehydrating grapevine (Vitis vinifera) in concert with stomatal conductance and stem and petiole hydraulic measurements. Magnetic resonance imaging was used to continuously monitor xylem cavitation and flow rates in the stem of an intact vine during 10 d of dehydration. The results showed that complete stomatal closure preceded the appearance of embolism in the leaves and the stem by several days. Basal leaves were more vulnerable to xylem embolism than apical leaves and, once embolized, were shed, thereby preventing further water loss and protecting the hydraulic integrity of younger leaves and the stem. As a result, embolism in the stem was minimal even when drought led to complete leaf shedding. These findings suggest that grapevine avoids xylem embolism rather than tolerates it.

The physiological mechanisms underlying drought responses are poorly documented in mangroves, which experience nearly constant exposure to saline water. We measured gas exchange, foliar abscisic acid (ABA) concentration, and vulnerability to embolism in a soil water-withholding experiment of two co-occurring mangroves, Avicennia marina (Forsskål) Vierhapper (Verbenaceae) and Bruguiera gymnorrhiza (L.) Savigny (Rhizophoraceae). A. marina showed higher photosynthesis and transpiration than B. gymnorrhiza under well-watered conditions. Cavitation resistance differed significantly between species, with 50% cavitation occurring at a water potential (P50) of −8.30 MPa for A. marina and −2.83 MPa for B. gymnorrhiza. This large difference in cavitation resistance was associated with differences in stomatal closure and leaf wilting. The rapid stomatal closure of B. gymnorrhiza was correlated with ABA accumulation as water potential declined. Meanwhile, stomatal closure and declining water potentials in A. marina were not associated with ABA accumulation. The safety margins, calculated as the difference between stomatal closure and embolism spread, differed between these two species (1.59 MPa for A. marina vs. 0.52 MPa for B. gymnorrhiza). Therefore, A. marina adopts a drought tolerance strategy with high cavitation resistance, while B. gymnorrhiza uses a drought avoidance-like strategy with ABA-related sensitive stomatal control to protect its vulnerable xylem.

In recent years, plants in sandy soils have been impacted by increased climate variability due to weak water holding and temperature buffering capacities of the parent material. The projected impact spreads all over the world, including New England, USA. Many regions of the world may experience an increase in frequency and severity of drought, which can be attributed to an increased variability in precipitation and enhanced water loss due to warming. The overall benefits of biochar in environmental management have been extensively investigated. This review aims to discuss the water holding capacity of biochar from the points of view of fluid mechanics and propose several prioritized future research topics. To understand the impacts of biochar on sandy soils in-depth, sandy soil properties (surface area, pore size, water properties, and characteristics) and how biochar could improve the soil quality as well as plant growth, development, and yield are reviewed. Incorporating biochar into sandy soils could result in a net increase in the surface area, a stronger hydrophobicity at a lower temperature, and an increase in the micropores to maximize gap spaces. The capability of biochar in reducing fertilizer drainage through increasing water retention can improve crop productivity and reduce the nutrient leaching rate in agricultural practices. To advance research in biochar products and address the impacts of increasing climate variability, future research may focus on the role of biochar in enhancing soil water retention, plant water use efficiency, crop resistance to drought, and crop productivity.

We report a novel form of xylem dysfunction in angiosperms: reversible collapse of the xylem conduits of the smallest vein orders that demarcate and intrusively irrigate the areoles of red oak (Quercus rubra) leaves. Cryo-scanning electron microscopy revealed gradual increases in collapse from approximately −2 MPa down to −3 MPa, saturating thereafter (to −4 MPa). Over this range, cavitation remained negligible in these veins. Imaging of rehydration experiments showed spatially variable recovery from collapse within 20 s and complete recovery after 2 min. More broadly, the patterns of deformation induced by desiccation in both mesophyll and xylem suggest that cell wall collapse is unlikely to depend solely on individual wall properties, as mechanical constraints imposed by neighbors appear to be important. From the perspective of equilibrium leaf water potentials, petioles, whose vessels extend into the major veins, showed a vulnerability to cavitation that overlapped in the water potential domain with both minor vein collapse and buckling (turgor loss) of the living cells. However, models of transpiration transients showed that minor vein collapse and mesophyll capacitance could effectively buffer major veins from cavitation over time scales relevant to the rectification of stomatal wrong-way responses. We suggest that, for angiosperms, whose subsidiary cells give up large volumes to allow large stomatal apertures at the cost of potentially large wrong-way responses, vein collapse could make an important contribution to these plants’ ability to transpire near the brink of cavitation-inducing water potentials.

Xylem vulnerability to cavitation and hydraulic efficiency are directly linked to fine-scale bordered pit features in water-conducting cells of vascular plants. However, it is unclear how pit characteristics influence water transport and carbon economy in tropical species. The primary aim of this study was to evaluate functional implications of changes in pit characteristics for water relations and photosynthetic traits in tropical Ficus species with different growth forms (i.e. hemiepiphytic and non-hemiepiphytic) grown under common conditions. Intervessel pit characteristics were measured using scanning electron microscopy in five hemiepiphytic and five non-hemiepiphytic Ficus species to determine whether these traits were related to hydraulics, leaf photosynthesis, stomatal conductance and wood density. Ficus species varied greatly in intervessel pit structure, hydraulic conductivity and leaf physiology, and clear differences were observed between the two growth forms. The area and diameter of pit aperture were negatively correlated with sapwood-specific hydraulic conductivity, mass-based net assimilation rate, stomatal conductance (gs), intercellular CO2 concentration (Ci) and the petiole vessel lumen diameters (Dv), but positively correlated with wood density. Pit morphology was only negatively correlated with sapwood- and leaf-specific hydraulic con- ductivity and Dv. Pit density was positively correlated with gs, Ci and Dv, but negatively with intrinsic leaf water-use efficiency. Pit and pit aperture shape were not significantly correlated with any of the physiological traits. These findings indicate a significant role of pit characteristics in xylem water transport, carbon assimilation and ecophysiological adaptation of Ficus species in tropical rain forests.

Because the xylem in leaves is thought to be at the greatest risk of cavitation, reliable and efficient methods to characterize leaf xylem vulnerability are of interest. We report a method to generate leaf xylem vulnerability curves (VCs) by gas injection. Using optical light transmission, we visualized embolism propagation in grapevine (Vitis vinifera) and red oak (Quercus rubra) leaves injected with positive gas pressure. This resulted in a rapid, stepwise reduction of transmitted light, identical to that observed during leaf dehydration, confirming that the optical method detects gas bubbles and provides insights into the air-seeding hypothesis. In red oak, xylem VCs generated using gas injection were similar to those generated using bench dehydration, but indicated 50% loss of conductivity at lower tension (∼0.4 MPa) in grapevine. In determining VC, this method eliminates the need to ascertain xylem tension, thus avoiding potential errors in water potential estimations. It is also much faster (1 h per VC). However, severing the petiole and applying high-pressure gas could affect air-seeding and the generated VC. We discuss potential artifacts arising from gas injection and recommend comparison of this method with a more standard procedure before it is assumed to be suitable for a given species.

We tested for a tradeoff across species between plant maximum photosynthetic rate and the ability to maintain photosynthesis under adverse conditions in the unfavorable season. Such a trade-off would be consistent with the observed trade-off between maximum speed and endurance in athletes and some animals that has been explained by cost-benefit theory. This trend would have importance for the general understanding of leaf design, and would simplify models of annual leaf carbon relations. We tested for such a trade-off using a database analysis across vascular plants and using an experimental approach for 29 cycad species, representing an ancient plant lineage with diversified evergreen leaves. In both tests, a higher photosynthetic rate per mass or per area in the favorable season was associated with a stronger absolute or percent decline in the unfavorable season. We resolved a possible mechanism based on biomechanics and nitrogen allocation; cycads with high leaf toughness (leaf mass per area) and higher investment in leaf construction than in physiological function (C/N ratio) tended to have lower warm season photosynthesis but less depression in the cool season. We propose that this trade-off, consistent with cost-benefit theory, represents a significant physio-phenological constraint on the diversity and seasonal dynamics of photosynthetic rate.

Tropical lianas deploy most of their leaves towards the top of the forest canopy, whereas trees exhibit a more stratified crown. Forest canopies are often exposed to hot and windy conditions, and how lianas cope with the extremely high transpirational demands under these environments remains unknown. We investigated stem hydraulic properties, leaf drought tolerance, diurnal changes in leaf and stem water potentials (Ψleaf and Ψstem), stomatal conductance (gs), photosynthetic rate, sap flow and stem native percentage loss of conductivity (PLC) for four liana species in a tropical forest in southwest China. Five co-occurring tree species were also selected for comparison. Lianas reached maximal transpiration at a relatively lower vapour pressure deficit (

Habitat differentiation between polyploid and diploid plants are frequently observed, with polyploids usually occupying more stressed environments. In woody plants, polyploidization can greatly affect wood characteristics but knowledge of its influences on xylem hydraulics is scarce. The four Betula species in NE China, representing two diploids and two polyploids with obvious habitat differentiation, provide an exceptional study system for investigating the impact of polyploidization on environmental adaptation of trees from the point view of xylem hydraulics. To test the hypothesis that changes in hydraulic architecture play an important role in determining their niche differentiation, we measured wood structural traits at both the tissue and pit levels and quantified xylem water transport efficiency and safety in these species. The two polyploids had significantly larger hydraulic weighted mean vessel diameters than the two diploids (45.1 and 45.5 vs 25.9 and 24.5 μm) although the polyploids are occupying more stressed environments. As indicated by more negative water potentials corresponding to 50% loss of stem hydraulic conductivities, the two polyploids exhibited significantly higher resistance to drought-induced embolism than the two diploids (-5.23 and -5.05 vs -3.86 and -3.13 MPa) despite their larger vessel diameters. This seeming discrepancy is reconciled by distinct characteristics favoring greater embolism resistance at the pit level in the two polyploid species. Our results showed clearly that the two polyploid species have remarkably different pit-level anatomical traits favoring greater hydraulic safety than their congeneric diploid species, which have likely contributed to the abundance of polyploid birches in more stressed habitats; however, less porous inter-conduit pits together with a reduced leaf to sapwood area may have compromised their competitiveness under more favorable conditions. Contrasts in hydraulic architecture between diploid and polyploid Betula species suggest an important functional basis for their clear habitat differentiation along environmental gradients in Changbai Mountain of NE China.

Freezing resistance through avoidance or tolerance of extracellular ice nucleation is important for plant survival in habitats with frequent subzero temperatures. However, the role of cell walls in leaf freezing resistance and the coordination between leaf and stem physiological processes under subzero temperatures are not well understood. We studied leaf and stem responses to freezing temperatures, leaf and stem supercooling, leaf bulk elastic modulus and stem xylem vessel size of six Patagonian shrub species from two sites (plateau and low elevation sites) with different elevation and minimum temperatures. Ice seeding was initiated in the stem and quickly spread to leaves, but two species from the plateau site had barriers against rapid spread of ice. Shrubs with xylem vessels smaller in diameter had greater stem supercooling capacity, i.e., ice nucleated at lower subzero temperatures. Only one species with the lowest ice nucleation temperature among all species studied exhibited freezing avoidance by substantial supercooling, while the rest were able to tolerate extracellular freezing from −11.3 to −20 °C. Leaves of species with more rigid cell walls (higher bulk elastic modulus) could survive freezing to lower subzero temperatures, suggesting that rigid cell walls potentially reduce the degree of physical injury to cell membranes during the extracellular freezing and/or thaw processes. In conclusion, our results reveal the temporal–spatial ice spreading pattern (from stem to leaves) in Patagonian shrubs, and indicate the role of xylem vessel size in determining supercooling capacity and the role of cell wall elasticity in determining leaf tolerance of extracellular ice formation. Fil: Zhang, Yong Jiang. Chinese Academy of Sciences; República de China. Harvard University; Estados Unidos Fil: Bucci, Sandra Janet. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de la Patagonia ; Argentina Fil: Arias, Nadia Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de la Patagonia ; Argentina Fil: Scholz, Fabian Gustavo. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de la Patagonia ; Argentina Fil: Hao, Guang You. Chinese Academy of Sciences; República de China Fil: Cao, Kun Fang. Guangxi University; China Fil: Goldstein, Guillermo Hernan. University of Miami; Estados Unidos. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Ecología, Genética y Evolución; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina

Despite lianas being fundamental components of tropical and subtropical forest ecosystems throughout the world, the physiological characteristics of this growth form are not well known. Different behaviors at the seedling stage were until recently largely unnoticed. In one extreme of a continuum of adaptive traits, freestanding liana seedlings invest a large proportion of biomass in self-support tissue while on the other extreme support-seeker seedlings invest more resources in rapid elongation of slender stems with an efficient hydraulic conductive system. Adult lianas often have lower wood density and higher specific leaf area than trees and have most of their leaves deployed at the top of the canopy, experiencing high irradiance and transpirational demands, which requires effective regulation of water loss to avoid desiccation. Recent studies show that lianas have faster stomatal responses to increasing vapor pressure deficit (VPD) and exhibit stronger partial stomatal closure compared to trees. Strong stomatal control and efficient water transport help lianas maintain leaf water potential (Ψleaf) within a safe hydraulic range to avoid xylem dysfunction despite their low stem water storage capacity, which is achieved at a minimum cost in terms of carbon assimilation. Liana colonization of tree crowns can significantly reduce tree growth and transpiration with consequences for carbon and water economy at individual tree and ecosystem levels.

Compared to tropical and temperate forests, subtropical forests have received little attention in physiological and ecological studies until now, and the contribution of this ecosystem type to the global carbon cycle has not been fully assessed. In this chapter we discuss results on the carbon balance of subtropical forests at different spatial and temporal scales, analyze the potential limitation of seasonal low temperatures and water deficits on physiological processes of subtropical trees, and characterize the uniqueness of subtropical forest ecosystems in terms of carbon economy. Results from multiple techniques and scales were included in the carbon balance assessment. The largest two regions with subtropical forests are located in Asia and South America. The net ecosystem carbon gain of subtropical forests in these two regions, which have annual precipitations larger than 800 mm, is probably neither strongly limited by soil water availability nor by seasonal low temperatures. Relatively low evapotranspiration in the winter/dry season and high soil water-holding capacity help maintain good water availability for trees in most subtropical forests. High solar radiation, light penetration and low ecosystem respiration in winter may compensate for the negative effects of low temperatures on gross photosynthesis . Therefore, subtropical forests in many areas can assimilate carbon in excess of respiration throughout the year and they are, probably, among the largest terrestrial carbon sinks across terrestrial ecosystems worldwide. In addition, because leaf and ecosystem respiration respond to temperature changes to a larger extent compared to ecosystem carbon assimilation, a negative relationship between net ecosystem carbon gain and mean annual temperature was found in Asian subtropical and tropical forests. This relationship suggests that global warming may weaken the carbon sink strength of these forest ecosystems. These results indicate the important contribution of subtropical forests to the global carbon cycle and the potentially negative response of these forests to global warming. We hope this information will promote additional physiological and ecological research and conservation in subtropical forests.

Light is critical for plant establishment, growth, and survival in wet tropical forests. The objective of this chapter is to analyze paradigms of photosynthetic performance and life history traits of tropical forest trees to contrasting light environments across the forest floor, gaps and upper canopy. Physiological and morphological plasticity as well as genetically fixed adaptive traits are analyzed, including leaf optical properties and photoprotection from high irradiance. Photosynthetic adaptations to contrasting light environments of closely related species are discussed. This approach has the advantage among comparative studies of adaptations across species in that genetic relationships among species are known. Species-specific variations in maximum photosynthetic rates, which reflect the degree of adaptation to growth irradiance, are shown to be gradual, suggesting that classification into two distinct functional groups in terms of light requirements is somewhat arbitrary. Trees growing in gaps or in the upper canopy rely strongly on biochemical mechanisms to dissipate excess energy and to avoid damage to the light reaction centers and photosystems. Consistent with their high photosynthetic capacity, light demanding species are capable of plastic changes in hydraulic architecture , such as increases in hydraulic conductivity under high irradiance, which makes them more competitive in open habitats.

Forests play an important role in global mercury (Hg) cycling. To explain the high Hg accumulation in subtropical forest ecosystems, we studied temporal dynamics of Hg, carbon (C), nitrogen (N), and sulfur (S) in forest soil profiles, as well as litterfall flux and precipitation, in an old-growth moist evergreen broadleaf (EB) forest and a mossy coppice (MC) forest from South China over seven years. The mean soil Hg concentration was 257 ± 14 ng g −1 in the O-horizon and 248 ± 15 ng g −1 in the A-horizon for the EB forest, and 94 ± 27 ng g −1 in the O-horizon and 70 ± 11 ng g −1 in the A-horizon for the MC forest. Annual variations in Hg concentration were suggested to be associated with variations in precipitation and litterfall biomass. Significant vertical Hg transport was only observed in the MC forest, which was attributed to its lower organic matter content. Correlation and stoichiometry analyses further suggested that the dynamics in Hg concentration in the forest floor was also closely linked to the variation in S concentration. Additionally, the difference in the soil Hg pool between these two forests was attributed to different litterfall biomass fluxes.