Host tree utilization by epiphytic orchids in different land-use intensities in Kathmandu Valley, Nepal

We studied the influence of site conditions on epiphytic orchids under a subtropical climate in the Kathmandu Valley, Nepal. We analysed 96 systematically distributed grid points situated in Kathmandu Valley across a land-use intensity gradient (national park to urbanised city area). Geographical Information System (GIS) and remote sensing were used for classification of land-use types. We identified 23 species of epiphytic orchids, within 13 genera, from 42 different host tree species. Host preference is obvious for some orchid species (e.g., Dendrobium nobile), with certain tree species (e.g., Schima wallichii, Ficus religiosa) hosting more orchid species than others. The orchid Rhynchostylis retusa was the most common species found on many different host tree species across the land-use intensity gradient. Host species and host bark characteristics (e.g., rugosity, pH and exposure to wind) played a vital role for orchid distribution, with lower abundance in areas of higher impact. Under strong human impact (urban city area), F. religiosa was the dominant host tree, with large individual trees (mean diameter in breast height, dbh?=?1.3?m) providing the habitat for considerable populations of R. retusa individuals. In general, epiphytic orchids were found on larger host trees in urban areas than in areas of lower human impact. We found that some hosts are more likely to harbour orchid species, especially native host species. Older larger trees with rougher bark, low pH, exposed to wind and reduced human impact provided better habitats for orchids. We suggest these characteristics should be considered in urban planning to reduce human impact on the associated orchid epiphytic community.     

Deciduousness in tropical trees and its potential as indicator of climate change: A review

In tropical forests, deciduousness is an outcome of integrated effect of drought, tree characteristics and soil moisture conditions and thus it is a reliable indicator of seasonal drought experienced by different tree species. Variations in the deciduousness are associated with several ecophysiological characteristics, such as varying allocation pattern of metabolic products, resource capture and conservation, water relations and stem water storages, annual carbon sequestration, timing of reproductive event initiation, extent of separation of vegetative and reproductive events and leaf strategies, and it helps in maintenance of water balance and protection of tree organs during the seasonal drought. Tropical forests support mosaics of tree functional types showing marked differences in the duration of deciduousness (from leaf exchanging to >8 months deciduous), as a result of varying degree of water stress experienced by physiognomy, distribution and wood anatomy of tropical trees. Wide variations in deciduousness in the same species growing at different sites suggest the high sensitivity of tropical trees to small changes in growing habitat. In the present review we have explored the ecological significance of deciduousness in tropical trees with emphasis on: (a) inter- and intraspecies plasticity in deciduousness, (b) various capacity adaptations related with the duration of deciduousness, (c) relationship between tree stem water status and deciduousness, and (d) probable effect of impending climate change on tropical trees. An attempt has also been made to establish deciduousness as climate change indicator in the dry tropics. There is need to develop capabilities to detect and predict the impact of climate change on deciduousness through long-term phenological network in tropics. Remote sensing techniques can generate valuable ecological information such as leaf level drought response and phenological patterns. Deciduousness has the potential to emerge as an important focus for ecological research to address critical questions in global modeling, monitoring, and climate change.     

Variation in woody plant mortality and dieback from severe drought among soils, plant groups, and species within a northern Arizona ecotone

Vegetation change from drought-induced mortality can alter ecosystem community structure, biodiversity, and services. Although drought-induced mortality of woody plants has increased globally with recent warming, influences of soil type, tree and shrub groups, and species are poorly understood. Following the severe 2002 drought in northern Arizona, we surveyed woody plant mortality and canopy dieback of live trees and shrubs at the forest–woodland ecotone on soils derived from three soil parent materials (cinder, flow basalt, sedimentary) that differed in texture and rockiness. Our first of three major findings was that soil parent material had little effect on mortality of both trees and shrubs, yet canopy dieback of trees was influenced by parent material; dieback was highest on the cinder for pinyon pine (Pinus edulis) and one-seed juniper (Juniperus monosperma). Ponderosa pine (Pinus ponderosa) dieback was not sensitive to parent material. Second, shrubs had similar mortality, but greater canopy dieback, than trees. Third, pinyon and ponderosa pines had greater mortality than juniper, yet juniper had greater dieback, reflecting different hydraulic characteristics among these tree species. Our results show that impacts of severe drought on woody plants differed among tree species and tree and shrub groups, and such impacts were widespread over different soils in the southwestern U.S. Increasing frequency of severe drought with climate warming will likely cause similar mortality to trees and shrubs over major soil types at the forest–woodland ecotone in this region, but due to greater mortality of other tree species, tree cover will shift from a mixture of species to dominance by junipers and shrubs. Surviving junipers and shrubs will also likely have diminished leaf area due to canopy dieback.     

The Effects of Drought and Shade on the Performance,Morphology and Physiology of Ghanaian Tree Species

In tropical forests light and water availability are the most important factors for seedling growth and survival but an increasing frequency of drought may affect tree regeneration. One central question is whether drought and shade have interactive effects on seedling growth and survival. Here, we present results of a greenhouse experiment, in which seedlings of 10 Ghanaian tree species were exposed to combinations of strong seasonal drought (continuous watering versus withholding water for nine weeks) and shade (5% irradiance versus 20% irradiance). We evaluated the effects of drought and shade on seedling survival and growth and plasticity of 11 underlying traits related to biomass allocation, morphology and physiology. Seedling survival under dry conditions was higher in shade than in high light, thus providing support for the “facilitation hypothesis” that shade enhances plant performance through improved microclimatic conditions, and rejecting the trade-off hypothesis that drought should have stronger impact in shade because of reduced root investment. Shaded plants had low biomass fraction in roots, in line with the trade-off hypothesis, but they compensated for this with a higher specific root length (i.e., root length per unit root mass), resulting in a similar root length per plant mass and, hence, similar water uptake capacity as high-light plants. The majority (60%) of traits studied responded independently to drought and shade, indicating that within species shade- and drought tolerances are not in trade-off, but largely uncoupled. When individual species responses were analysed, then for most of the traits only one to three species showed significant interactive effects between drought and shade. The uncoupled response of most species to drought and shade should provide ample opportunity for niche differentiation and species coexistence under a range of water and light conditions. Overall our greenhouse results suggest that, in the absence of root competition shaded tropical forest tree seedlings may be able to survive prolonged drought.     

Depletion of regenerative bud resources during cyclic drought: What are the implications for fire management?

Summary   Resprouting is a common regenerative strategy in plant taxa that occurs in fire and drought-prone environments. When plants are forced to use bud resources in quick succession as a result of repeated disturbances, recovery vigour may be diminished. The loss of bud resources through the combined effect of successive fire and drought is likely to be more damaging for plant survival and persistence than one or other disturbance on its own. In this study, we examine the resprouting response of seven trees and two shrubs after fire and drought in woodland communities in the New England and Bioregion of New South Wales. We also investigate whether there is a cumulative impact on plant vitality as a result of the combined disturbances of fire followed by drought. Preliminary results suggest that resprouting after drought occurs from buds located on the same morphological parts of tree and shrub species as after fire, although the response reflects the intensity of impact. Mortality in nine species affected by drought was similar to that in plants affected by both fire and drought. Since a drought between successive fires has the potential to deplete bud resources and debilitate plants, drought should be taken into account when determining fire regimes, and a severe drought between two fires should be considered in a similar way to an unplanned burn. Failure to do so may lead to reduced vigour and excessive mortality in resprouting species after planned fire.     

A commensal network of epiphytic orchids and host trees in an Atlantic Forest remnant: A case study revealing the important role of large trees in the network structure

Application of metrics derived from network theory could elucidate the structural organization of orchid assemblages, and help identify the host tree species on which they depend, as well as predicting the impacts of removing host tree species. In this study, we used nestedness, modularity, connectance and robustness, to identify the factors that predict the structure of a quantitative orchid–host tree network in a remnant of Atlantic Forest in Brazil. The network exhibited low nestedness (NODF = 14.07; P = 0.03; WNODF = 5.3; P = 0.02) and no modularity. It was highly robust to the random elimination of host tree species, but showed low robustness when host trees with more interactions started to be eliminated. The nested pattern found was attributed to a combination of two main host tree traits, height and diameter (given by DBH measure). Thus, our analyses reflect the importance of tall and large host tree species, which seem to play an important role in the network structure, providing a substrate for orchid species with different habitat associations and representing a decisive factor in both nested and robust patterns found.     

Tree carbon allocation explains forest drought‐kill and recovery patterns

The mechanisms governing tree drought mortality and recovery remain a subject of inquiry and active debate given their role in the terrestrial carbon cycle and their concomitant impact on climate change. Counter‐intuitively, many trees do not die during the drought itself. Indeed, observations globally have documented that trees often grow for several years after drought before mortality. A combination of meta‐analysis and tree physiological models demonstrate that optimal carbon allocation after drought explains observed patterns of delayed tree mortality and provides a predictive recovery framework. Specifically, post‐drought, trees attempt to repair water transport tissue and achieve positive carbon balance through regrowing drought‐damaged xylem. Furthermore, the number of years of xylem regrowth required to recover function increases with tree size, explaining why drought mortality increases with size. These results indicate that tree resilience to drought‐kill may increase in the future, provided that CO₂ fertilisation facilitates more rapid xylem regrowth.     

Changes in DNA methylation fingerprint of Quercus ilex trees in response to experimental field drought simulating projected climate change

Rapid genetic changes in plants have been reported in response to current climate change. We assessed the capacity of trees in a natural forest to produce rapid acclimation responses based on epigenetic modifications. We analysed natural populations of Quercus ilex, the dominant tree species of Mediterranean forests, using the methylation‐sensitive amplified polymorphism (MSAP) technique to assess patterns and levels of methylation in individuals from unstressed forest plots and from plots experimentally exposed to drought for 12 years at levels projected for the coming decades. The percentage of hypermethylated loci increased, and the percentage of fully methylated loci clearly decreased in plants exposed to drought. Multivariate analyses exploring the status of methylation at MSAP loci also showed clear differentiation depending on stress. The PCA scores for the MSAP profiles clearly separated the genetic from the epigenetic structure, and also significantly separated the samples within each group in response to drought. Changes in DNA methylation highlight the large capacity of plants to rapidly acclimate to changing environmental conditions, including trees with long life spans, and our results demonstrate those changes. These changes, although unable to prevent the decreased growth and higher mortality associated with this experimental drought, occurred together with a dampening in such decreases as the long‐term treatment progressed.     

Quercus robur survival at the rear edge in steppe: Dendrochronological evidence

Climate change is altering forest ecosystems worldwide, particularly in steppe landscapes, where the rare tree communities are challenged with steadily increasing droughts. In the steppe of Eastern Europe, amid dry conditions, Quercus robur occupies mostly riverine habitats and ravines. Here we study the climate sensitivity and drought vulnerability of a Q. robur population located at the rear edge of the species range, in the steppe of Ukraine. The population occupies two adjacent but clearly contrasting in their microclimatic conditions sites: a river floodplain and a steep-sloping river bank. We develop tree earlywood, latewood, and total ring width site-level chronologies and evaluate their relationship with regional climate variables and the local river's water level using response function analysis. We find that trees growing in the floodplain and at the steep river bank have exhibited slightly different growth patterns. The trees at the flooded site have benefited from water proximity, which facilitated their earlywood growth. These trees have responded positively to the current May and previous September precipitation and previous and current May temperatures. At the non-flooded site, the trees have experienced warm temperatures and the lack of precipitation in June. The extreme drought episodes have triggered a decrease in the latewood and total ring width in trees from both sites. We established that oak growth in the floodplain had been depressed by an unknown stressor around 1900, therefore limiting our ability to identify the more beneficial steppe habitat for Q. robur conclusively. Nevertheless, our results provide a dendrochronological evidence of Q. robur survival in a dry steppe environment and lend new insights into local microclimatic factors contributing to it.     

Patterns of drought tolerance in major European temperate forest trees: climatic drivers and levels of variability

The future performance of native tree species under climate change conditions is frequently discussed, since increasingly severe and more frequent drought events are expected to become a major risk for forest ecosystems. To improve our understanding of the drought tolerance of the three common European temperate forest tree species Norway spruce, silver fir and common beech, we tested the influence of climate and tree‐specific traits on the inter and intrasite variability in drought responses of these species. Basal area increment data from a large tree‐ring network in Southern Germany and Alpine Austria along a climatic cline from warm‐dry to cool‐wet conditions were used to calculate indices of tolerance to drought events and their variability at the level of individual trees and populations. General patterns of tolerance indicated a high vulnerability of Norway spruce in comparison to fir and beech and a strong influence of bioclimatic conditions on drought response for all species. On the level of individual trees, low‐growth rates prior to drought events, high competitive status and low age favored resilience in growth response to drought. Consequently, drought events led to heterogeneous and variable response patterns in forests stands. These findings may support the idea of deliberately using spontaneous selection and adaption effects as a passive strategy of forest management under climate change conditions, especially a strong directional selection for more tolerant individuals when frequency and intensity of summer droughts will increase in the course of global climate change.     

The importance of associations with saprotrophic non-Rhizoctonia fungi among fully mycoheterotrophic orchids is currently under-estimated: novel evidence from sub-tropical Asia

Background and Aims Most fully mycoheterotrophic (MH) orchids investigated to date are mycorrhizal with fungi that simultaneously form ectomycorrhizas with forest trees. Only a few MH orchids are currently known to be mycorrhizal with saprotrophic, mostly wood-decomposing, fungi instead of ectomycorrhizal fungi. This study provides evidence that the importance of associations between MH orchids and saprotrophic non-Rhizoctonia fungi is currently under-estimated.Methods Using microscopic techniques and molecular approaches, mycorrhizal fungi were localized and identified for seven MH orchid species from four genera and two subfamilies, Vanilloideae and Epidendroideae, growing in four humid and warm sub-tropical forests in Taiwan. Carbon and nitrogen stable isotope natural abundances of MH orchids and autotrophic reference plants were used in order to elucidate the nutritional resources utilized by the orchids.Key Results Six out of the seven MH orchid species were mycorrhizal with either wood- or litter-decaying saprotrophic fungi. Only one orchid species was associated with ectomycorrhizal fungi. Stable isotope abundance patterns showed significant distinctions between orchids mycorrhizal with the three groups of fungal hosts.Conclusions Mycoheterotrophic orchids utilizing saprotrophic non-Rhizoctonia fungi as a carbon and nutrient source are clearly more frequent than hitherto assumed. On the basis of this kind of nutrition, orchids can thrive in deeply shaded, light-limiting forest understoreys even without support from ectomycorrhizal fungi. Sub-tropical East Asia appears to be a hotspot for orchids mycorrhizal with saprotrophic non-Rhizoctonia fungi.     

Drought-induced tree growth decline in the desert margins of Northwestern China

Water deficiency is the primary limiting factor for tree growth in arid and semi-arid areas. Droughts associated with rising temperatures have increased in severity and frequency globally over the past few decades, making the trees in the drought-prone sites first be affected by water shortages. However, our understanding of tree growth status in these areas, and of their response to drought, is currently insufficient; especially in the context of global warming. Here, we studied 94 Chinese pine (Pinus tabulaeformis) and 86 spruce (Picea crassifolia) trees from different altitudes [2,100–2400 m above sea level (a.s.l.)] distributed at the desert margins of Northwestern China to explore tree growth and drought response from multiple perspectives using dendroecological approaches. Significant growth decline, across all tree species and altitudes, was detected in response to an interdecadal trend towards a drier climate. Moreover, the extent of tree growth decline, the proportion of affected trees, and the degree of moisture dependence have all tended to increase in each sample site, most likely due to enhanced drought severity and duration in recent decades. The more sensitive and susceptible trees were found at lower elevations (drier sites) and may signify a higher vulnerability to heating-induced drought stress. Tree resistance to drought showed strong negative correlation with drought severity across all sample sites. However, the connection between post-drought tree resilience and drought intensity is weak, perhaps because the samples were all collected from living trees, while those that had died were not sampled. The priority for future work should be to combine surviving and dead trees simultaneously, thus achieving a more representative view of tree resilience to drought; this will improve our knowledge of forest dynamics and even ecosystem succession in these vulnerable and sensitive environments.     

Response of native Hawaiian woody species to lava-ignited wildfires in tropical forests and shrublands

Wildfires are rare in the disturbance history of Hawaiian forests but may increase in prevalence due to invasive species and global climate change. We documented survival rates and adaptations facilitating persistence of native woody species following 2002–2003 wildfires in Hawaii Volcanoes National Park, Hawaii. Fires occurred during an El Niño drought and were ignited by lava flows. They burned across an environmental gradient occupied by two drier shrub-dominated communities and three mesic/wet Metrosideros forest communities. All the 19 native tree, shrub, and tree fern species demonstrated some capacity of postfire persistence. While greater than 95% of the dominant Metrosideros trees were top-killed, more than half survived fires via basal sprouting. Metrosideros trees with diameters >20 cm sprouted in lower percentages than smaller trees. At least 17 of 29 native woody species colonized the postfire environment via seedling establishment. Although the native biota possess adaptations facilitating persistence following wildfire, the presence of highly competitive invasive plants and ungulates will likely alter postfire succession.     

Changing partners in the dark: isotopic and molecular evidence of ectomycorrhizal liaisons between forest orchids and trees

In the mycorrhizal symbiosis, plants exchange photosynthates for mineral nutrients acquired by fungi from the soil. This mutualistic arrangement has been subverted by hundreds of mycorrhizal plant species that lack the ability to photosynthesize. The most numerous examples of this behaviour are found in the largest plant family, the Orchidaceae. Although these non-photosynthetic orchid species are known to be highly specialized exploiters of the ectomycorrhizal symbiosis, photosynthetic orchids are thought to use free-living saprophytic, or pathogenic, fungal lineages. However, we present evidence that putatively photosynthetic orchids from five species which grow in the understorey of forests: (i) form mycorrhizas with ectomycorrhizal fungi of forest trees; and (ii) have stable isotope signatures indicating distinctive pathways for nitrogen and carbon acquisition approaching those of non-photosynthetic orchids that associate with ectomycorrhizal fungi of forest trees. These findings represent a major shift in our understanding of both orchid ecology and evolution because they explain how orchids can thrive in low-irradiance niches and they show that a shift to exploiting ectomycorrhizal fungi precedes viable losses of photosynthetic ability in orchid lineages.     

Mark-mark scatterplots improve pattern analysis in spatial plant ecology

Point process statistics provides valuable tools for many ecological studies, where ‘points’ are commonly determined to represent the locations of plants or animals and ‘marks’ are additional items such as species or size. In the statistical analysis of marked point patterns, various correlation functions are used such as the mark variogram or the mark correlation function. Often the interpretation of these functions is not easy and the non-spatial ecologist is in need of support. In order to make the analysis of spatial point patterns more accessible to ecologists, we introduced and tested a new graphical method, the mark-mark scatterplot. This plot visualises the marks of point pairs of inter-point distances r smaller than some small distance r_max. We tested the application of the mark-mark scatterplot by reconsidering three quite different tree patterns: a pattern of longleaf pine trees from the southern US which was strongly influenced by fires, a tropical tree pattern of the species Shorea congestiflora from Sri Lanka and a Scots pine pattern from Siberia (Russia). The new method yielded previously undetected cause-effect information on mark behaviour at short inter-point distances and thus improved the analysis with mark correlation functions as well as complemented the information they provided. We discovered important new correlations in clusters of trees at close proximity. The application of the mark-mark scatterplot will facilitate the interpretation of point process summary statistics and will make point process analysis more accessible to ecologists not specialized in point process statistics.     

Genetic Variation of Drought Tolerance in Pinus pinaster at Three Hierarchical Levels: A Comparison of Induced Osmotic Stress and Field Testing

Understanding the survival capacity of forest trees to periods of severe water stress could improve knowledge of the adaptive potential of different species under future climatic scenarios. In long lived organisms, like forest trees, the combination of induced osmotic stress treatments and field testing can elucidate the role of drought tolerance during the early stages of establishment, the most critical in the life of the species. We performed a Polyethylene glycol-osmotic induced stress experiment and evaluated two common garden experiments (xeric and mesic sites) to test for survival and growth of a wide range clonal collection of Maritime pine. This study demonstrates the importance of additive vs non additive effects for drought tolerance traits in Pinus pinaster, and shows differences in parameters determining the adaptive trajectories of populations and family and clones within populations. The results show that osmotic adjustment plays an important role in population variation, while biomass allocation and hydric content greatly influence survival at population level. Survival in the induced osmotic stress experiment presented significant correlations with survival in the xeric site, and height growth at the mesic site, at population level, indicating constraints of adaptation for those traits, while at the within population level no significant correlation existed. These results demonstrate that population differentiation and within population genetic variation for drought tolerance follow different patterns.     

Plastic bimodal growth in a Mediterranean mixed-forest of Quercus ilex and Pinus halepensis

Mediterranean tree species have evolved to face seasonal water shortages, but may fail to cope with future increases in drought frequency and intensity. We investigated stem radial increment dynamics in two typical Mediterranean tree species, Aleppo pine (Pinus halepensis), a drought-avoiding species, and holm oak (Quercus ilex), a drought-tolerant species, in a mixed forest and on contrasting slope aspects (south- and north-facing). Intra- and inter-annual growth patterns were modelled using the VS-Lite2 model for each tree species and slope-aspect. Both species showed a bimodal growth pattern, with peaks coinciding with favourable conditions in spring and autumn. A bimodal growth pattern is always observed in P. halepensis, while in Q. ilex is facultative, which suggests different strategies adopted by these species to cope with summer drought. More specifically, trees on south-facing slope showed a more evident bimodal pattern and more intra-annual density fluctuations. In recent decades, the intensity of both growth peaks has diminished and drifted away due to the increased summer drought. The VS-Lite2 model reveals a niche partitioning between both species. Differences in growing season’s length and timings of growth peaks in both species are relevant for their coexistence and should be considered for estimating mixed-forest responses under climate change scenarios.     

Tree invasions into treeless areas: mechanisms and ecosystem processes

Non-native tree invasions occur not only in woodland or forest vegetation, but also into areas with little or no native tree presence. Limiting factors for tree establishment and survival include seasonal or annual drought, low nutrient availability, cold temperature extremes, fire, and other abiotic conditions to which trees are poorly adapted as well as biotic conditions such as herbivory and lack of soil mutualist inoculum. Tree invasions of grasslands and semi-arid riparian areas in particular are now widespread and frequently result in the rapid conversion of these habitats to woodlands or forests. In some cases, these invasions are the result of a change in extrinsic conditions such as climate, fire, and/or grazing that remove what have been previous barriers to tree establishment. However, in other cases, tree species with particular life-history and dispersal traits fill open niches or outcompete native species. Significant examples of tree invasion into treeless areas can be seen with invasions of Pinus species into temperate grasslands and fynbos shrublands, Melaleuca quinquenervia and Triadica sebifera into grassy wetlands, Prosopis and Tamarix species into semi-arid riparian zones, and Acacia and Morella invasions into nutrient-poor shrublands and barrens. The establishment of trees into treeless areas may have strong impacts on ecosystem processes, influencing biogeochemical cycling, carbon sequestration and cycling, and ecohydrology, as well possible edaphic legacies that persist even if trees are removed.     

Digital image analysis of different crown shape of Platycladus orientalis

Platycladus orientalis (L.) Franco is a beautifully crown-shaped evergreen tree species used for ornamental cultivation. These trees are also important afforestation plants at hill sites containing calcareous parent rocks and exhibit high tolerance to drought and barren sites. However, Platycladus orientalis trees with abnormal crowns, such as fusiform and torch-form, have been identified at sites with extreme drought and barren hills in Shandong, China, although the abnormal crowns does not reduce the ornamental value of these trees. In the present study, we used the RGB imaging and geographical statistical analyses through the construction of meteorological indices. The results indicate that variations of abnormal Platycladus orientalis crowns are associated with both the internal metabolism of these trees and the external environment. Crown shapes are strongly affected by the local dry, hot and windy meteorological environment, particularly individuals planted at poor hill site conditions. In response to extreme events of drought, high temperature and strong winds, the twigs and scale leaves of Platycladus orientalis typically wilt from the lower part to the upper part of the crown. The fusiform and torch-form crowns are formed through the wilting of partial twigs and scale leaves to avoid the entire wilt of the trees, thereby saving the life of the tree at the expense of partial twigs and scale leaves.     

木本植物水力系统对干旱胁迫的响应机制

干旱导致树木死亡对生态系统功能和碳平衡有重大影响。植物水分运输系统失调是引发树木死亡的主要机制。然而, 树木对干旱胁迫响应的多维性和复杂性, 使人们对植物水分运输系统在极端干旱条件下的响应以及植物死亡机理的认识还不清楚。该文首先评述衡量植物抗旱性的指标, 着重介绍可以综合评价植物干旱抗性特征的新参数——气孔安全阈值(SSM)。SSM越高, 表明气孔和水力性状之间的协调性越强, 木质部栓塞的可能性越低, 水力策略越保守。然后, 阐述木本植物应对干旱胁迫的一般响应过程。之后, 分别综述植物不同器官(叶、茎和根)对干旱胁迫的响应机制。植物达到死亡临界阈值的概率和时间, 取决于相关生理和形态学特征的相互作用。最后, 介绍木本植物水力恢复机制, 并提出3个亟待开展的研究问题: (1)改进叶片水分运输(木质部和木质部外水力导度)的测量方法, 量化4种不同途径的叶肉水分运输的相对贡献; (2)量化叶片表皮通透性变化, 以便更好地理解植物水分利用策略; (3)深入研究树木水碳耦合机制, 将个体结构和生理特征与群落/景观格局和过程相关联, 以便更好地评估和监测干旱诱导树木死亡的风险。