Differential predation on mangrove propagules in open and closed canopy forest habitats

Amounts of seed predation by grapsid crabs (Brachyura: Grapsidae) on two species of mangroves (Aegiceras corniculatum and Avicennia marina) were compared among different habitats in an Australian mangrove forest. For Avicennia, comparisons were between canopy gaps and the adjacent forest understory for six, mid intertidal, gaps of different sizes. For Aegiceras the comparisons were among canopy gaps in the high intertidal; open, accreting mud/sand banks where mangroves were colonizing in the low intertidal; and in the forest understory in both the high and low intertidal zones. These were repeated in the high salinity (35\%) downstream portion and the low salinity (0–5\%) upstream portion of a tidal river.Predation on Avicennia was significantly higher in the understory than in adjacent canopy gaps. Within a canopy opening, predation was greatest in the smallest gaps and lowest in the largest gaps. Predation on Aegiceras was greater in the high intertidal compared to the low intertidal, but no differences were found between river mouth and upstream locations. In the high intertidal zone of the forest, there were no differences in predation between canopy gap or forest understory sites for Aegiceras. In the low intertidal zone, however, significant differences in amount of predation were found between habitats. More Aegiceras propagules were consumed in the understory than on adjacent accreting sandbanks.Frequency of tidal inundation, which in turn affects the amount of time available to forage, is hypothesized to account for differences in predation between low and high intertidal forests and between small and large canopy gaps. Our results also suggest that shade intolerance in these two species may actually reflect an escape from predators, successful when the seeds are dispersed into open areas such as canopy gaps or mud banks.     

Measurement of the diffusion coefficient for salt in salt flat and mangrove soils

Excluded salt accumulated at mangrove roots must be transported away from the root zone by diffusive processes, due to the low permeability of most mangrove soils. The diffusion coefficient for salt in mangrove soils determines the rate of this diffusive transport but has not been determined experimentally before. In this work we used a 12-month long-time series of salt concentration profiles measured in a sediment core over which fresh water was continuously circulated, to determine the diffusion coefficient for salt in the soil. Salt concentrations were measured using an electrical conductivity probe that was developed for use in hypersaline (salt concentration up to and in excess of 120g/l) conditions. A modified formula was experimentally determined to relate electrical conductivity to salt concentration and temperature, applicable up to a salt concentration of 200g/l. This was done because standard formulae relating these variables do not apply in the hypersaline conditions often encountered in salt flat sediments. The salt concentration profiles were used in a simple mathematical model to determine a sediment diffusion coefficient for salt in a salt flat sediment. This value of D=(4.6±0.2) ×10^–5m²/day was approximately half that calculated theoretically.     

Comparative physiology of salt and water stress

Plant responses to salt and water stress have much in common. Salinity reduces the ability of plants to take up water, and this quickly causes reductions in growth rate, along with a suite of metabolic changes identical to those caused by water stress. The initial reduction in shoot growth is probably due to hormonal signals generated by the roots. There may be salt-specific effects that later have an impact on growth; if excessive amounts of salt enter the plant, salt will eventually rise to toxic levels in the older transpiring leaves, causing premature senescence, and reduce the photosynthetic leaf area of the plant to a level that cannot sustain growth. These effects take time to develop. Salt-tolerant plants differ from salt-sensitive ones in having a low rate of Na+ and Cl-- transport to leaves, and the ability to compartmentalize these ions in vacuoles to prevent their build-up in cytoplasm or cell walls and thus avoid salt toxicity. In order to understand the processes that give rise to tolerance of salt, as distinct from tolerance of osmotic stress, and to identify genes that control the transport of salt across membranes, it is important to avoid treatments that induce cell plasmolysis, and to design experiments that distinguish between tolerance of salt and tolerance of water stress.     

盐度对稀释平板法研究红树林区土壤微生物数量的影响

在使用稀释平板法分离潮间带红树林及其对照光滩土壤微生物以及计数时,多数情况下使用陈海水制作培养基和稀释水,很少考虑培养基和稀释水的盐度对最终计数结果的影响.使用稀释平板法研究了盐度对福建九龙江口红树林区与深圳福田红树林保护区土壤微生物平板计数的影响,结果表明培养基与稀释水盐度对微生物数量有明显的影响.统计分析显示细菌的海水稀释效果优于淡水,而放线菌与真菌则刚好相反(P<0.05,一个例外).海水不适合配制红树林区土壤微生物平板计数的培养基,从0～35,高盐度的平板培养基会降低微生物的数量,尤其是放线菌的数量,尽管培养基的盐度对真菌影响无规律,但细菌数量在低盐度时比在高盐度和不加氯化钠时要多.根据盐度效应,提出了稀释平板技术应用于潮间带的红树林及其相应光滩时的优化方法,认为细菌应该用海水作无菌稀释水,而放线菌和真菌则应用淡水作稀释水;包括光滩在内的红树林区土壤微生物分离与计数的培养基宜控制较低盐度范围.     

北京西山侧柏林冠层不同高度处叶片水分利用效率

以北京西山广泛分布的侧柏林为研究对象,综合考虑冠层不同高度处气象因子、大气CO₂浓度以及大气CO₂中碳同位素组成的差异,对其冠层不同高度处叶片的瞬时水分利用效率和短期水分利用效率分别进行了测定,以期为区域森林生态系统固碳与耗水研究提供理论依据,为区域森林生态系统经营与维护提供技术支撑.结果表明: 侧柏林冠层不同高度处叶片的瞬时水分利用效率随冠层高度的变化规律表现为上层>中层>下层,多种气象因子协同影响气孔运动,使瞬时水分利用效率受气孔限制;侧柏林冠层不同高度处的环境因子、大气CO₂浓度以及大气CO₂的δ13C均存在一定差异,导致了林冠各层叶片的短期水分利用效率的变化.林冠上层叶片通过提高水分利用效率适应环境.     

Forest canopy structure analyzed by using aerial photographs

A method was developed using aerial photographs to analyze forest canopy structure. Digital elevation models of both the land and canopy surface in a mesh of 5 m intervals were made from aerial photographs taken in winter (without tree leaves) and summer (with leaves), respectively, in a 60 ha area of temperate deciduous forest. The difference between the two elevation values at each point was regarded as the canopy height, and a canopy height profile was constructed. The estimated canopy structure was compared with that obtained by ground observations in a 6 ha part of the study area. Large gaps (>100 m²) were adequately detected by the method, and the gap size distribution obtained was similar to the one observed on the ground. The method was found to be effective in analyzing the forest canopy structure of large areas, but it is not suitable for the detection of small gaps.     

Australian tropical forest canopy crane: New tools for new frontiers

Abstract An industrial crane was installed in the Daintree lowland rainforest in 1998 to provide a new means of accessing the canopy. Approximately 0.95 ha of forest, including 680 trees of 82 species with a diameter at breast height of greater than 10 cm, are accessible using the crane. The site was hit by a Category 3 Cyclone in 1999 and has shown a remarkable rate of recovery. The crane has been used for a very wide range of research including tree physiology and ecology, interactions with vertebrate and invertebrate biodiversity and studies of carbon and water fluxes. Results from studies on this crane and 11 other cranes around the world are changing views of the importance of the rainforest canopy.     

Role of root hydrophobic barriers in salt exclusion of a mangrove plant Avicennia officinalis

Salt exclusion at the roots and salt secretion in the leaves were examined in a mangrove, Avicennia officinalis. The non‐secretor mangrove Bruguiera cylindrica was used for comparative study of hydrophobic barrier formation in the roots. Bypass flow was reduced when seedlings were previously treated with high salt concentration. A biseriate exodermis was detected in the salt‐treated roots, along with an enhanced deposition of hydrophobic barriers in the endodermis. These barriers reduced Na⁺ loading into the xylem, accounting for a 90–95% salt exclusion in A. officinalis. Prominent barriers were found in the roots of B. cylindrica even in the absence of salt treatment. A cytochrome P450 gene that may regulate suberin biosynthesis was up‐regulated within hours of salt treatment in A. officinalis roots and leaves, corresponding with increased suberin deposition. X‐ray microanalysis showed preferential deposition of Na⁺ and Cl^? in the root cortex compared with the stele, suggesting that the endodermis is the primary site of salt exclusion. Enhanced salt secretion and increased suberin deposition surrounding the salt glands were seen in the leaves with salt treatment. Overall, these data show that the deposition of apoplastic barriers increases resistance to bypass flow leading to efficient salt exclusion at the roots in mangroves.     

Predicting the limits to tree height using statistical regressions of leaf traits

Leaf morphology and physiological functioning demonstrate considerable plasticity within tree crowns, with various leaf traits often exhibiting pronounced vertical gradients in very tall trees. It has been proposed that the trajectory of these gradients, as determined by regression methods, could be used in conjunction with theoretical biophysical limits to estimate the maximum height to which trees can grow. Here, we examined this approach using published and new experimental data from tall conifer and angiosperm species. We showed that height predictions were sensitive to tree-to-tree variation in the shape of the regression and to the biophysical endpoints selected. We examined the suitability of proposed end-points and their theoretical validity. We also noted that site and environment influenced height predictions considerably. Use of leaf mass per unit area or leaf water potential coupled with vulnerability of twigs to cavitation poses a number of difficulties for predicting tree height. Photosynthetic rate and carbon isotope discrimination show more promise, but in the second case, the complex relationship between light, water availability, photosynthetic capacity and internal conductance to CO(2) must first be characterized.     

Relationship of water transport to anatomical features in the mangrove Laguncularia racemosa grown under contrasting salinities

The purpose of this study was to investigate the xylem anatomy and hydraulic characteristics of the mangrove Laguncularia racemosa grown under contrasting salinities. The study addressed the hypothesis that, at high salinity, water transport capacity may decrease in association with higher water use efficiency. Plants were grown in media to which 0, 15 and 30 NaCl was added. Vessel density and diameter were determined in transverse sections of stem and midrib leaves in terminal shoots, and hydraulic parameters were measured. In stems, the vessel density increased with salinity, while the anatomical diameter (d(a)) and hydraulic diameter (d(h)) declined; in leaves, these parameters remained unchanged with salinity. Huber value and hydraulic and specific conductivities decreased with salinity. Leaf blade resistance increased with salinity and represented the largest fraction of twig resistance. Xylem anatomy and leaf tissue of L. racemosa appeared to be modulated by salinity, which led to a coordinated decline in hydraulic properties as salinity increased. Therefore, these structural changes would reflect functional water use characteristics of leaves under salinity.     

Water availability predicts forest canopy height at the global scale

The tendency of trees to grow taller with increasing water availability is common knowledge. Yet a robust, universal relationship between the spatial distribution of water availability and forest canopy height (H) is lacking. Here, we created a global water availability map by calculating an annual budget as the difference between precipitation (P) and potential evapotranspiration (PET) at a 1‐km spatial resolution, and in turn correlated it with a global H map of the same resolution. Across forested areas over the globe, H_mean increased with P‐PET, roughly: H_mean (m) = 19.3 + 0.077*(P‐PET). Maximum forest canopy height also increased gradually from ~ 5 to ~ 50 m, saturating at ~ 45 m for P‐PET > 500 mm. Forests were far from their maximum height potential in cold, boreal regions and in disturbed areas. The strong association between forest height and P‐PET provides a useful tool when studying future forest dynamics under climate change, and in quantifying anthropogenic forest disturbance.     

田间小麦群体内叶片气孔阻力垂直差异研究

根据田间实测资料, 本文给出了田间小麦群体中叶片不同部位和冠层不同层次叶面气孔阻力的差异, 及其与环境因素特别是土壤水分含量关系的分析结果。结果表明, 小麦冠层各不同层次和叶片不同部位的气孔阻力有明显差异。就冠层上部单个叶片而言, 由叶尖到叶基气孔阻力逐渐增大;叶片正面的气孔阻力小于叶片反面对应部位的气孔阻力。对于水汽和CO₂传导的贡献70%是由冠层上部的两片叶子给出的;群体内不同层次叶片相应部位的气孔阻力是由冠层上部向下逐渐增大, 冠层不同层次叶片和叶片不同部位的气孔对环境因素变化的反空不同, 如当土壤水分不足时, 群体下部叶片和叶片反面的气孔张度首先变小, 且变小的程度较大。     

Photosynthesis and related physiological processes in two mangrove species,Rhizophora mucronata and Ceriops tagal,at Gazi Bay,Kenya

Measurements were carried out of the gas exchange properties (namely, photosynthesis, stomatal conductance and transpiration rates), water use efficiency and water relations of two mangrove species, Rhizophora mucronata and Ceriops tagal at Gazi Bay, Kenya. Rhizophora mucronata had significantly higher photosynthetic rates than C. tagal. Internal CO₂ concentrations were higher during the wet season than the dry season in both species. Gas exchange properties were correlated positively with photon flux density in both species. Leaf water potentials were highest during the morning and lowest at midday and were also highest in the lower canopy leaves in both species. The two mangrove species had conservative water use. Management potential for the East African mangroves based on the results of this study is suggested.     

基于林冠塔吊的附生植物生态学研究进展

林冠是生物圈中物种最丰富却最鲜为人知的生境之一。它在森林与大气的物质、能量交换过程中发挥着至关重要的作用。但因林冠调查技术的限制, 林冠及生存在其中的附生植物在生态系统中的功能尚未得到足够的重视。塔吊在三维空间中作业具有“全方位、高精度、非破坏、可重复”的特征。林冠塔吊已成为当前林冠学研究的标志, 并为林冠附生植物研究提供了契机。国际上, 欧美国家利用塔吊技术对林冠层附生植物多样性与空间分布等进行了大量的研究, 取得了丰硕的成果。该文介绍了塔吊的构造、林冠塔吊建设历史和站点分布及国际林冠研究组织等概况, 并对依托塔吊开展的附生植物研究进展进行了评述。此外, 还简要介绍了我国塔吊建设与林冠生态学发展情况。在系统分析国内外附生植物研究现状基础上, 从附生植物多样性、附生植物空间格局与维持机制、生态适应性、与林冠动物的关系以及附生植物对气候变化的响应等5个方面对今后基于林冠塔吊开展附生植物研究进行了展望。