Genetic engineering of forest woody plants

The present state of genetic engineering (GE) of forest woody plants is considered with special reference to the materials of the International Conference "Wood, Breeding, Biotechnology and Industrial Expectations" held in France in June, 2001. Main tree species subjected to GE are listed, aims of constructing transgenic plants discussed, and methods described. Major achievements in the field are considered along with the problems associated with the employment of GE in the breeding of forest woody plants.     

Genetic Engineering of Forest Woody Plants

The present state of genetic engineering (GE) of forest woody plants is considered with special reference to the materials of the International Conference Wood, Breeding, Biotechnology and Industrial Expectations held in France in June, 2001. Main tree species subjected to GE are listed, aims of constructing transgenic plants discussed, and methods described. Major achievements in the field are considered along with the problems associated with the employment of GE in the breeding of forest woody plants.     

The diversity of drought adaptation in the wide

Life on the earth is highly dependent on the properties and functions of water. In front of water limitation, herbaceous, woody and epiphyte plants have developed a wide diversity of drought tolerance mechanisms at the molecular, metabolic and physiological levels. The strategies of adaptation to drought have been listed in regard of the level of organization: molecules, cells, whole plant. Root development and water uptake, transpiration and micro- and macromorphological adaptations, and water status and osmotic adjustment have important consequences on drought adaptation. The relationship between these characters and mechanisms and the productivity of cultivated plants are the basis of the breeding for drought tolerance.     

Stand dynamics modulate water cycling and mortality risk in droughted tropical forest

Transpiration from the Amazon rainforest generates an essential water source at a global and local scale. However, changes in rainforest function with climate change can disrupt this process, causing significant reductions in precipitation across Amazonia, and potentially at a global scale. We report the only study of forest transpiration following a long‐term (>10 year) experimental drought treatment in Amazonian forest. After 15 years of receiving half the normal rainfall, drought‐related tree mortality caused total forest transpiration to decrease by 30%. However, the surviving droughted trees maintained or increased transpiration because of reduced competition for water and increased light availability, which is consistent with increased growth rates. Consequently, the amount of water supplied as rainfall reaching the soil and directly recycled as transpiration increased to 100%. This value was 25% greater than for adjacent nondroughted forest. If these drought conditions were accompanied by a modest increase in temperature (e.g., 1.5°C), water demand would exceed supply, making the forest more prone to increased tree mortality.     

四川都江堰亚热带针阔混交林中主要晚生型外生菌根真菌类群的空间分布

晚生型外生菌根真菌通常出现在森林演替的后期,是成熟林中的优势外生菌根真菌类群.对四川都江堰一片亚热带针阔混交林中的菌根真菌地上群落进行调查,并应用二元逻辑回归分析对晚生型外生菌根真菌的主要类群,即鹅膏菌科、牛肝菌科和红菇科,与周围(5 m×5 m样方)树种组成的关系进行研究.还应用次级变量分析方法对主要外生菌根真菌类群的空间格局进行了分析.结果表明,非外生菌根树种及某些外生菌根树种对特定类群菌根真菌子实体的出现有抑制作用,而不同类群外生菌根真菌在克隆生长上的差异并不是子实体空间分布的决定因素.我们认为,当研究自然林中外生菌根子实体的空间分布时,除了宿主植物的分布,也应考虑非宿主植物的分布以及菌根真菌相互作用的影响.     

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.     

四川都江堰亚热带针阔混交林中主要晚生型外生菌根真菌类群的空间分布

晚生型外生菌根真菌通常出现在森林演替的后期,是成熟林中的优势外生菌根真菌类群。对四川都江堰一片亚热带针阔混交林中的菌根真菌地上群落进行调查,并应用二元逻辑回归分析对晚生型外生菌根真菌的主要类群,即鹅膏菌科、牛肝菌科和红菇科,与周围(5m×5m样方)树种组成的关系进行研究。还应用次级变量分析方法对主要外生菌根真菌类群的空间格局进行了分析。结果表明,非外生菌根树种及某些外生菌根树种对特定类群菌根真菌子实体的出现有抑制作用,而不同类群外生菌根真菌在克隆生长上的差异并不是子实体空间分布的决定因素。我们认为,当研究自然林中外生菌根子实体的空间分布时,除了宿主植物的分布,也应考虑非宿主植物的分布以及菌根真菌相互作用的影响。     

Drought-induced tree death in savanna

Increasing densities of woody plants in savannas has been attributed to both elevated atmospheric CO₂ and reduced burning with grazing management, such that the biome could represent a substantial carbon sink. However, we show that extreme droughts (less than two-thirds expected rainfall over 3 years) occur in the drier half of the savanna biome and can cause substantial tree death. An Australian case study reveals that a net increase in tree cover over five decades of above-average rainfall was offset by sudden tree death during drought. The relationship between woody cover change and rainfall is moderated by competition with growth being facilitated by low woody cover and drought-induced death more likely as the woody component of savanna increases. The results are not supportive of a sustained increase in the woody component of xeric savannas resulting from CO₂ fertilization or land management. Extensive tree death in savanna regions will become a stark consequence of climate change if predictions of increasing severity and frequency of drought are realized.     

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。     

岩溶木本植物对干旱的生理生态适应

受全球气候变暖和季风气候影响,西南岩溶区年降水量及其在季节间的分配发生明显变化,无雨期频率和持续时间增加,且基岩风化严重,基质储水能力差,致使岩溶木本植物面临的季节性和地质性干旱加剧。该文通过参考相关文献分析结构性状和生理调节探讨岩溶木本植物如何适应地质性和季节性干旱。结果表明岩溶木本植物应对干旱的策略与其他干旱、半干旱区的植物大体一致,主要有抗旱和避旱两种策略:抗旱性植物一般具有比叶面积小、叶肉多汁、储水组织发达、细胞液浓度高等适应干旱的特征,可通过增加木材密度、增强木质部导管的抗栓塞性和提高水分利用效率以适应干旱; 避旱植物则可通过小而密的气孔和叶脉、发达的表皮毛、栅栏组织和维管束鞘等结构特征减少水分丧失,并可通过落叶、深根吸收深层水源和脱落酸(ABA)介导提早关闭气孔以适应干旱。虽然关于岩溶植物形态结构和生理调节对干旱适应机制的研究取得了一定进展,但仍然存在一些亟待解决的问题,例如:深入研究岩溶地区基岩水分状况及其对植物的贡献; 加强岩溶木本植物根系结构和生物量分配、树木构型及根际微生物与木本植物干旱适应的协同关系研究; 同时探索如何将岩溶植物生态适应研究成果应用于生产实践中,科学指导石漠化治理与生态修复。     

林木耗水调控机理研究进展

林木的蒸腾耗水量是造林设计与环境水分研究的重要参数。本文就林木耗水的气孔与非气孔调节机制、木质部空穴和栓塞的发生和恢复机理、树体组织水容等方面进行了综述,对它们在树木水分传输过程中的调控作用和意义开展了探讨。目前在蒸腾气孔调节方面,包括,蒸腾午休、夜间蒸腾、气孔振荡和补偿现象等气孔行为的研究工作有待深入。栓塞木质部和空穴化导管恢复的临界条件与重新充注对植物水分运输的重要生理作用要进一步加强。树体组织水容对树木水分传输和耗水的调控机制问题应加以重视。     

Diversity of Tropical Tree Seedling Responses to Drought

Drought is an important seedling mortality agent in dry and moist tropical forests, and more severe and frequent droughts are predicted in the future. The effect of drought on leaf gas exchange and seedling survival was tested in a dry-down experiment with four tree species from dry and moist forests in Bolivia. Seedlings were droughted and wilting stage and gas exchange were monitored. Drought led to a gradual reduction of photosynthesis and stomatal conductance over time, whereas respiration and photosynthetic water-use efficiency initially increased with drought and then declined. Seedlings gradually went through the different wilting stages, until they eventually died, but the trajectory differed for the four species. The strong relationship between wilting stage and photosynthesis means that simple field observations can provide valuable information on plant physiological performance. Three different drought strategies were identified. Dry forest species Ceiba samauma shed its leaves and survived. The moist forest species Cariniana ianeirensis postponed drought stress by having low rates of transpiration and high water-use efficiency. Dry forest Astronium urundeuva and moist forest Triplaris americana followed an opportunistic strategy; they are early successional species that can quickly grow to maturity but periodic drought can be lethal. Strikingly, dry and moist forest species did not differ clearly in their drought tolerance strategies.     

Effects of fire and drought in a tropical eucalypt savanna colonized by rain forest

Aim This study documents the effects of multiple fires and drought on the woody structure of a north Australian savanna never grazed by domestic stock. Location The study was conducted in a 500 ha pocket of Eucalyptus‐dominated savanna surrounded by a late Quaternary lava flow. The flow is known as the Great Basalt Wall, located c. 50 km northeast of Charters Towers in semi‐arid north‐eastern Australia. This region was exposed to the largest 5‐year rainfall deficit on record between 1992 and 1996. Methods All individual woody plants were tagged within a 1.56 ha plot. Species were segregated into their habitat affinities (rain forest, ecotone, savanna) and regeneration strategy (resprouter, seeder). The survivorship of plants within these categories was analysed in relation to fire intensity from the first fire, and to each of four fires lit between 1996 and 2001. Results Before the first fire, the plot contained thirty‐one tree species including twenty‐one typical of the surrounding dry rain forest. These rain forest species were represented by small individuals and constituted <1% of the total basal area of woody plants. The basal area of savanna trees was 7.5 m² ha^?1 at the commencement of monitoring, although 31% had recently died and others had major crown damage. Further death of the drought debilitated savanna trees was substantial during the first year of monitoring and the basal area of live savanna trees declined to 1.1 m² ha^?1 after 5 years. Most species from both rain forest and savanna were classified as resprouters and are capable of regenerating from underground organs after fire. Species without this ability (rain forest seeders and ecotone seeders) were mostly eliminated after the first two consecutive fires. Among resprouters, survivorship declined as fire intensity increased and this was more pronounced for rain forest than for savanna species. Repeated burning produced a cumulative effect of decreasing survivorship for rain forest resprouters relative to savanna resprouters. Main conclusions The study provides evidence that savanna and rain forest trees differ in fire susceptibility and that recurrent fire can explain the restricted distribution of rain forest in the seasonally arid Australian tropics. The time of death of the savanna trees is consistent with the regional pattern after severe drought, and highlights the importance of medium term climate cycles for the population dynamics of savanna tree species and structure of Australian savannas.     

基于无人机和决策树算法的榆树疏林草原植被类型划分和覆盖度生长季动态估计

植被覆盖度是评估生态环境质量与植被生长的重要指标,也是全球众多陆面过程模型和生态系统模型中表达植被动态的重要参数。卫星遥感和地面测量是估算植被覆盖度的常见方法。然而,如何精确估计榆树疏林草原上木本、草本不同类型植被的覆盖度仍然具有挑战性。无人机飞行系统有效的补充了区域尺度低空间分辨率的卫星遥感影像与样地尺度实地调查之间的缺口,为精确的监测、评估疏林草原的植被动态提供了新途径。利用无人机监测平台和决策树算法构建了一套快速、准确、自动获取景观尺度植被类型和估算植被覆盖度的自动化工具,以浑善达克沙地榆树疏林草原为对象,应用无人机监测平台对榆树疏林草原长期定位监测大样地2017年生长季植被状况进行7次监测。结果表明:1)无人机植被监测平台数据飞行高度100 m,获取的样地数字正射影像空间分辨率为2.67 cm/像元,远高于高分卫星影像,利用决策树算法基于数字正射影像可以实现自动划分榆树疏林草原木本和草本植被类型和估算植被覆盖度; 2)生长季内榆树疏林草原木本植被覆盖度为(19±2)%,草本植被覆盖度为(50±8)%,植被总覆盖度为(69±9)%,相对于木本植被,草本植被生长季内盖度变幅较大; 3)在整个生长季中,木本植被和草本植被对植被总覆盖度的平均贡献率分别为27%和73%,草本植被对植被总盖度的贡献远大于木本植被,榆树疏林草原植被的盖度主要受草本植被的影响。本研究证明无人机监测平台是一种高效、准确的植被监测工具,结合机器学习算法,实现了景观尺度植被类型的自动划分和不同类型植被覆盖度快速获取;在浑善达克沙地榆树疏林草原地区首次获取了木本植被和草本植被覆盖度的生长季动态。该平台未来可应用于各种类型生态系统植被类型划分、监测和评估。     

Short-Lived Tree Species and Their Role as Indicators for Plant Diversity in the Restoration of Natural Forests

Anthropogenic forests, particularly conifer monocultures, today constitute a large proportion of Central European woodland. Conversion of such forest stands into abundantly structured mixed‐species woodland is within the focus of ecosystem restoration and is considered to affect forest biodiversity. Short‐lived tree species play an important role in such conversion processes and may serve as focal species. However, not much is known about their relationship with forest biodiversity. In this study, the short‐lived tree species, European mountain ash (Sorbus aucuparia L.), European white birch (Betula pendula Roth), Downy birch (B. pubescens Ehrh.), and Glossy buckthorn (Frangula alnus P. Mill.), commonly occurring throughout Central Europe, are investigated with regard to their relationship with plant diversity. The focus is on their occurrences in Scots pine (Pinus sylvestris L.)–dominated forests in the Northeast German lowlands. A significant increase in vascular plant diversity is revealed in stands with the selected species’ presence, in comparison to stands without them. Increase in plant species numbers is highest where the respective species occurs in the tree and/or shrub layer, compared with their presence only in the herb layer. For bryophyte species, there is a less strong inverse relationship. An analysis of different species groups, such as threatened, woody, and typical forest species of higher plants, reveals no decrease in species numbers in these groups if short‐lived tree species are present. It is concluded that short‐lived tree species can be indicators for plant diversity assessment within forest restoration processes. As to causal explanations, effects of differing site conditions, assessed by use of Ellenberg indicator values, are discussed as well as possible active effects of the tree species changing their environment.     

Seedling Traits Determine Drought Tolerance of Tropical Tree Species

Water availability is the most important factor determining tree species distribution in the tropics, but the underlying mechanisms are still not clear. In this study, we compared functional traits of 38 tropical tree species from dry and moist forest, and quantified their ability to survive drought in a dry‐down experiment in which wilting and survival were monitored. We evaluated how seedling traits affect drought survival, and how drought survival determines species distribution along the rainfall gradient. Dry forest species tended to have compound leaves, high stem dry matter content (stem dry mass/fresh mass), and low leaf area ratio, suggesting that reduction of transpiration and avoidance of xylem cavitation are important for their success. Three functional groups were identified based on the seedling traits: (1) drought avoiders with a deciduous leaf habitat and taproots; (2) drought resisters with tough tissues (i.e., a high dry matter content); and (3) light‐demanding moist forest species with a large belowground foraging capacity. Dry forest species had a longer drought survival time (62 d) than moist forest species (25 d). Deciduousness explained 69 percent of interspecific variation in drought survival. Among evergreen species, stem density explained 20 percent of the drought survival. Drought survival was not related to species distribution along the rainfall gradient, because it was mainly determined by deciduousness, and species with deciduous seedlings are found in both dry and moist forests. Among evergreen species, drought survival explained 28 percent of the variation in species position along the rainfall gradient. This suggests that, apart from drought tolerance, other factors such as history, dispersal limitation, shade tolerance, and fire shape species distribution patterns along the rainfall gradient.     

Pattern and dynamics of the ground vegetation in south Swedish Carpinus betulus forests: importance of soil chemistry and management

The vegetation and environmental conditions of south Swedish hornbeam Carpinus betulus forests are described with data from 35 permanent sample plots The main floristic gradient of the ground vegetation is closely related to acid-base properties of the top soil base saturation, pH and organic matter content Other floristic differences are related to tree canopy cover and the distance of the sample plots to the Baltic coast Species richness of herbaceous plants typical of forests increases with soil pH, The number of other herbaceous species, occurring in both forests and open habitats, and of woody species is not related to pH Comparisons of vegetation data from 1983 and 1993 show relatively small compositional differences of the herbaceous forest flora The number of other herbaceous species increased considerably m those plots where canopy trees had been cut after 1983 The number of new species in managed plots increases with soil pH Species losses and gains of the herbaceous forest flora between 1983 and 1993 are generally lower as compared with other herbaceous species and woody species However, the ground cover of herbaceous forest species, especially of Oxalis acetosella and Lamium galeobdolon , was considerably lower in 1993 as compared to 1983 in both unmanaged and managed plots Possible explanations for this decrease are current soil acidification and drought during the growing season     

Water source utilization and foliar nutrient status differs between upland and flooded plant communities in wetland tree islands

Tree islands in the Everglades wetlands are centers of biodiversity and targets of restoration, yet little is known about the pattern of water source utilization by the constituent woody plant communities: upland hammocks and flooded swamp forests. Two potential water sources exist: (1) entrapped rainwater in the vadose zone of the organic soil (referred to as upland soil water), that becomes enriched in phosphorus, and (2) phosphorus-poor groundwater/surface water (referred to as regional water). Using natural stable isotope abundance as a tracer, we observed that hammock plants used upland soil water in the wet season and shifted to regional water uptake in the dry season, while swamp forest plants used regional water throughout the year. Consistent with the previously observed phosphorus concentrations of the two water sources, hammock plants had a greater annual mean foliar phosphorus concentration over swamp forest plants, thereby supporting the idea that tree island hammocks are islands of high phosphorus concentrations in the oligotrophic Everglades. Foliar nitrogen levels in swamp forest plants were higher than those of hammock plants. Linking water sources with foliar nutrient concentrations can indicate nutrient sources and periods of nutrient uptake, thereby linking hydrology with the nutrient regimes of different plant communities in wetland ecosystems. Our results are consistent with the hypotheses that (1) over long periods, upland tree island communities incrementally increase their nutrient concentration by incorporating marsh nutrients through transpiration seasonally, and (2) small differences in micro-topography in a wetland ecosystem can lead to large differences in water and nutrient cycles.     

Some effects of animal grazing and browsing on vegetation

Summary

This is a review of some problems posed by research on tree biology. First, phase change and in vitro culture are discussed as they affect vegetative propagation of woody plants. Then the breeding of timber trees, with an evident and important trend to clonal propagation is considered. Tropical timber tree breeding has enormous potential but has only just started (on very few species). The trend to making tropical woodlands into planted pure stands, of converting production forestry into, ‘tree farming’, an aspect of agriculture, so to speak, is apparent. ‘Ideotypes’ have fallen out of favour but some important ideas as to biomass and its allocation remain and well-defined economic objectives must become paramount. A broad review of the tropical agricultural context with regard to trees, suggests an acute need for far better understanding of them and for much introduction and breeding to generate new woody food-crops.     

Influence of Seasonal Variations in Soil Water Availability on Gas Exchange of Tropical Canopy Trees

Seasonal variations in environmental conditions influence the functioning of the whole ecosystem of tropical rain forests, but as yet little is known about how such variations directly influence the leaf gas exchange and transpiration of individual canopy tree species. We examined the influence of seasonal variations in relative extractable water in the upper soil layers on predawn leaf water potential, saturated net photosynthesis, leaf dark respiration, stomatal conductance, and tree transpiration of 13 tropical rain forest canopy trees (eight species) over 2 yr in French Guiana. The canopies were accessed by climbing ropes attached to the trees and to a tower. Our results indicate that a small proportion of the studied trees were unaffected by soil water depletion during seasonal dry periods, probably thanks to efficient deep root systems. The trees showing decreased tree water status (i.e., predawn leaf water potential) displayed a wide range of leaf gas exchange responses. Some trees strongly regulated photosynthesis and transpiration when relative extractable water decreased drastically. In contrast, other trees showed little variation, thus indicating good adaptation to soil drought conditions. These results have important applications to modeling approaches: indeed, precise evaluation and grouping of these response patterns are required before any tree‐based functional models can efficiently describe the response of tropical rain forest ecosystems to future changes in environmental conditions.