毛乌素沙地南缘沙丘生物结皮中微生物分布特征

为探明半干旱沙区生物结皮中微生物分布特征,对毛乌素沙地南缘沙丘生物结皮中微生物数量进行了测定。结果表明:微生物总数从丘顶到丘间地呈递增趋势,除丘顶与迎风坡、迎风坡与背风坡结皮层微生物总数差异不显著外,其他各地貌部位结皮层微生物数量之间差异显著。同一地貌部位结皮层、0～5和5～10cm土层微生物垂直分布有变化,其变化规律为:除迎风坡放线菌数量呈先增加后递减、迎风坡微生物总数、细菌、真菌和丘顶真菌数量随剖面的加深呈递减外,其他各地貌部位微生物数量均呈先降低,后增加的趋势。微生物类群的组成表现为细菌最多,放线菌次之,真菌最少。在丘间地细菌所占微生物总数的比例与丘顶相比有所增加,而放线菌和真菌的比例有所减少。结皮下0～5和5～10cm土层微生物分布与土壤含水量的变化同步,说明土壤水分可能是影响微生物垂直分布的重要因子。     

绿洲―荒漠交错带土壤水分变化特征初步研究

 以甘肃省的民勤绿洲为例,选择土壤水分最为亏缺的7月,对白刺(Nitraria tangutorum)沙包不同迎风面,以及绿洲和荒漠交错地带的土壤水分进行了取样分析。结果表明,对于白刺沙包而言,其上层和深层土壤水分是迎风坡大于背风坡,中间层次（40～80 cm）则刚好相反。在绿洲与荒漠的交错带上,灌丛沙包与沙丘间低地水分变化有很大的不同,总体而言,丘间地水分含量大于灌丛沙包,但在距离绿洲406 m处,二者均出现一个土壤水分的最低值。在丘间地表层,水分变化是距离绿洲愈近,土壤含水量愈高;而深层土壤水分的变化趋     

典型沙生植物烛台虫实根系形态特征对沙丘微生境的响应

烛台虫实(Corispermum candelabrum)是一种广泛分布于科尔沁沙地的典型沙生植物,通过研究根系对生境的响应机制有助于理解沙生植物对于环境胁迫的适应性。本实验对生长在沙丘不同微生境(丘间地与沙丘迎风坡相连的过渡带、迎风坡底部、迎风坡中部、坡顶和背风坡)的烛台虫实采用挖掘法分层获取地下根系,进行烛台虫实根系形态特征的比较研究。结果表明:烛台虫实的根系主要集中于0~10 cm层且主要由直径为0~1 mm的根系组成。沿着过渡带到沙丘顶部的方向,烛台虫实地上、地下生物量、根长密度、表面积密度和组织密度均逐渐减少,但背风坡的这几项指标有所增加;比根长、比表面积的变化则呈现相反趋势。根系的各项指标与沙丘不同微生境土壤含水量具有显著相关性。因此,在过渡带风沙干扰较低、水分状况较好的环境,虫实可以通过根系的大范围生长进行养分和水分的吸收;当处于沙丘顶部风蚀沙埋强烈、水分匮乏的环境时,虫实通过较高的资源吸收利用效率获取足够的养分,维持生存。烛台虫实可以通过根系的合理配置适应不同类型的沙地微生境。     

绿洲―荒漠交错带土壤水分变化特征初步研究

以甘肃省的民勤绿洲为例,选择土壤水分最为亏缺的7月,对白刺(Nitraria tangutorum)沙包不同迎风面,以及绿洲和荒漠交错地带的土壤水分进行了取样分析。结果表明,对于白刺沙包而言,其上层和深层土壤水分是迎风坡大于背风坡,中间层次（40～80 cm）则刚好相反。在绿洲与荒漠的交错带上,灌丛沙包与沙丘间低地水分变化有很大的不同,总体而言,丘间地水分含量大于灌丛沙包,但在距离绿洲406 m处,二者均出现一个土壤水分的最低值。在丘间地表层,水分变化是距离绿洲愈近,土壤含水量愈高;而深层土壤水分的变化趋     

新疆准噶尔盆地不同径级梭梭和白梭梭的水分来源

在干旱区,水是植物生长发育的主要限制性因子。运用稳定氧同位素技术探究了准东地区生长在相邻地段不同径级的梭梭(Haloxylon ammodendron)和白梭梭(Haloxylon persicum)的水分来源。通过测定不同径级梭梭和白梭梭的小枝木质部水、不同土层的土壤水以及地下水的δ~(18)O同位素值,运用MixSIAR模型、平均吸水深度模型和直观图法分析不同径级梭梭和白梭梭对各潜在水源的利用比例和主要吸水层位。结果表明:生长在丘间低地的四个径级梭梭主要水源是土壤水,随着径级的增长,梭梭水分利用方式更加灵活,趋向于利用稍浅层的土壤水。生长在沙丘顶部的四个径级白梭梭主要水源是土壤水,随着径级的增长,白梭梭更趋向于利用深层土壤水。梭梭和白梭梭通过不同的水分利用策略合理的利用干旱区有限的水源。     

科尔沁沙地流动沙丘掘穴蚁蚁丘分布及影响因素

以科尔沁沙地典型流动沙丘作为研究对象,通过样方法调查分析了不同地形上掘穴蚁的蚁丘密度、直径、盖度及其与土壤环境的关系,讨论了不同地形和土壤环境对掘穴蚁筑丘活动的影响,并探讨了蚁丘的空间分布格局.结果表明：受到地形变化影响的土壤环境和地形共同影响掘蚁穴的筑巢活动,蚁丘密度表现为丘顶>背风坡>迎风坡;蚁丘直径和盖度均表现为丘顶>迎风坡>背风坡,并且亦均受到蚁丘密度的制约.蚁丘的空间格局表现为随机分布型.地形及土壤环境的共同作用决定了掘穴蚁的筑巢行为.     

海岸不同坡向滨麦光合特性与风速异质环境的关系

在自然条件下,测定分析了夏季和秋季静风天及秋季大风天,海岸沙丘不同坡向环境因子及滨麦株高、叶片叶绿素含量、土壤含水量和光合日变化特性,以期明晰滨麦响应不同风速环境的生理调控机理。结果表明,不同坡向环境异质性明显,滨麦形态可塑性强。海岸迎风坡风大、温度低、湿度大,滨麦植株低矮、叶绿素含量较高;背风坡温度高、土壤干旱、空气流动差,滨麦植株高大,叶绿素含量低。在夏秋季静风天,迎风坡和背风坡滨麦叶片Pn出现光合"午休"现象,但迎风坡滨麦日均Gs、Tr、Pn均显著高于背风坡;而在秋季大风天,迎风坡和背风坡滨麦叶片Pn"午休"现象消失,并且背风坡滨麦日均Pn、Tr、Gs均显著高于迎风坡滨麦。同坡向相比,秋季大风天迎风坡滨麦日均Pn、Tr、Gs明显较秋季静风天低,而背风坡滨麦日均叶片Pn、Tr、Gs却较静风天分别增高126%、66.3%、134%。强海风吹袭引发迎风坡温度降低、滨麦叶片摇摆、气孔导度降低导致Pn下降,而强海风使背风坡空气流动加快、温度降低、气孔导度增大、"午休"消失使Pn增高。不同坡向滨麦对不同海风风速的适应表现出明显的光合生理可塑性,它在滨麦适应不同风力、提高其光合速率和增加物质积累上具有重要作用。而滨麦的形态和光合生理可塑性可能是其在不同海风强度下生存、生长、实现种群扩张的重要生理调控机理,这一特性在未来作物、牧草和树木抗风、抗盐育种中具有重要应用价值。     

古尔班通古特沙漠沙丘微地形变化对草本植物多样性影响

为探究微地形对草本植物物种多样性的影响,选择古尔班通古特沙漠垄状沙丘为研究区,从4—9月连续4次进行地上植被调查,研究了沙丘坡向和坡位对草本植物多样性时空变化的影响。结果表明:沙丘草本植物种以菊科为主,生活型多为短命植物;迎风坡植物密度显著少于背风坡,且随调查期的后延,两坡向植物密度变化均呈"V"字型,共有种以背风坡相对较多;α多样性指数在迎风坡及背风坡均为坡上部最大,坡底最小且有显著差异;从4—9月,各坡位α多样性指数呈减少和"V"字型2种变化趋势;随坡位升高,迎风坡和背风坡物种替代性指数降低,相似性指数增加,两者在背风坡均表现为显著变化;随调查时间后延,两坡向上物种替代指数增加,相似性指数降低,但替代性指数均无显著差异,沙丘坡向和坡位对草本植物α多样性和β多样性时空格局均产生显著影响。因此,保护和恢复沙漠植物多样性,应考虑沙丘坡向和坡位生境异质性,并结合植物多样性在时间上的变化特征以制定科学的沙漠管理措施。     

沙地生境沙拐枣(Calligonum mongolicum)种群特征及其扩张

沙拐枣(Calligonum mongolicum)是分布于我国北方荒漠、半荒漠的一种主要固沙植物,研究沙拐枣种群在沙地生境中的扩张对沙漠-绿洲过渡带的生态恢复和重建管理具有重要的实践意义.以巴丹吉林沙漠延伸带与河西走廊中部张临高绿洲过渡带为研究区域,采用踏查与样方调查相结合的方法对4种不同沙地生境(固定沙丘、半流动沙丘迎风坡、背风坡、丘间低地)中沙拐枣种群特征、繁殖规律、种群的扩张域和扩张速率进行了研究.结果表明:(1) 沙地生境中沙拐枣以有性繁殖为主.固定沙丘、半流动沙丘丘间低地、迎风坡和背风坡萌蘖苗密度占总密度的比值分别为35%、45%、2%、9%;实生苗密度比值分别为65%、55%、98%、91%.(2)不同生境中沙拐枣种群都为增长型种群;(3) 固定沙丘、半流动沙丘迎风坡、背风坡、丘间低地中,沙拐枣种群无性繁殖的扩张域和平均扩张速率分别为1～5m和0.67m/a、1～8m和0.64m/a、0～2m和0.15m/a、1～4m和0.41m/a.同时,3种生境中沙拐枣种群密度及其扩张速率都随着年龄的增长而下降.(4)沙拐枣种群在侵入新的生境过程中,开始于有性繁殖,利用无性繁殖拓展生存空间.     

张北地区退化杨树防护林的水分利用特征

在张北地区,以杨树为主的防护林出现不同程度的退化现象,水分是干旱区植物生存的主要限制因子,为揭示水分与防护林退化的关系,本文基于稳定氢氧同位素技术,通过对比小叶杨枝条水与潜在水源的同位素值,探究不同退化程度下小叶杨的水分来源及其对各水源的利用比率.结果表明:不同退化程度的杨树林水分来源不同,随退化程度的加深,小叶杨的水分来源从深层逐渐向表层转移.无退化的小叶杨主要利用320~400 cm的土壤水,利用率为25.1%;轻度退化的小叶杨主要利用120~180、180~240和240~320 cm的土壤水,对这3层的利用率总和将近50.0%,而对其他土壤水利用较少;中度退化的小叶杨主要利用20~40、40~60和60~80 cm的土壤水,对这3层土壤水每层利用率在17.5%~20.9%范围,对120 cm及其以下的土壤水利用率均低于10.0%;而重度退化的小叶杨主要利用0~20 cm的表层土壤水分,利用率为30.4%,明显高于其他水源.杨树防护林在衰退过程中的水分来源逐渐变浅,而林地浅层土壤较低的土壤含水量无法满足杨树的正常水分需求,加速了杨树林的退化和死亡.     

Exotic plant communities shift water-use timing in a shrub-steppe ecosystem

Semiarid areas in the US have realized extensive and persistent exotic plant invasions. Exotics may succeed in arid regions by extracting soil water at different times or from different depths than native plants, but little data is available to test this hypothesis. Using estimates of root mass, gravimetric soil water, soil-water potential, and stable isotope ratios in soil and plant tissues, we determined water-use patterns of exotic and native plant species in exotic- and native-dominated communities in Washington State, USA. Exotic and native communities both extracted 12 ± 2 cm of water from the top 120 cm of soil during the growing season. Exotic communities, however, shifted the timing of water use by extracting surface (0–15 cm) soil water early in the growing season (i.e., April to May) before native plants were active, and by extracting deep (0–120 cm) soil water late in the growing season (i.e., June to July) after natives had undergone seasonal senescence. We found that δ ¹⁸O values of water in exotic annuals (e.g., −11.8 ± 0.4 ‰ for Bromus tectorum L.) were similar to δ ¹⁸O values of surface soil water (e.g., −13.3 ± 1.4 ‰ at −15 cm) suggesting that transpiration by these species explained early season, surface water use in exotic communities. We also found that δ ¹⁸O values of water in taprooted exotics (e.g., −17.4 ± 0.3 ‰ for Centaurea diffusa Lam.) were similar to δ ¹⁸O values of deep soil water (e.g., −18.4 ± 0.1 ‰ at −120 cm) suggesting that transpiration by these species explained late season, deep water use. The combination of early-season, shallow water-use by exotic winter-actives and late-season, deep water-use by taprooted perennials potentially explains how exotic communities resist establishment of native species that largely extracted soil water only in the middle of the growing season (i.e., May to June). Early season irrigation or the planting of natives with established root systems may allow native plant restoration.     

Chlorophytes of biological soil crusts in Gurbantunggut Desert, Xinjiang Autonomous Region, China

In this paper, chlorophytes collected from 253 biological soil crust samples in Gurbantunggut Desert in Xinjiang Autonomous Region, China were studied by field investigation and microscopical observation in lab. The flora composition, ecological distribution of chlorophytes in the desert and dynamic changes of species composition of chlorophytes in different developing stages of biological soil crusts are preliminarily analyzed. Results showed that there were 26 species belonging to 14 genera and 10 families, in which unicellular chlorophytes were dominant. There existed some differences in distribution of varied sand dune positions. The taxa of chlorophytes in leeward of sand dunes are most abundant, but the taxa in windward, interdune and the top of sand dunes reduced gradually. Chlorophytes were mainly distributed within the crust and the taxa of chlorophytes decrease obviously under the crust. In the developing stages of the biological soil crust, species diversity of chlorophytes changed a little, but species composition presented some differences. Chlorococcum humicola, Chlorella vulgaris, Chlamydomonas ovalis and Chlamydomonas sp. nearly existed in all developing stages of biological crusts. In several former stages of the biological soil crust there were spherical chlorophytes and filamentous ones. When moss crust formed, filamentous chlorophytes disappeared, such as Microspora and Ulothrix. __________ Translated from Arid Zone Research, 2006, 23(2): 189–193 [译自: 干旱区研究]     

Habitat degradation,topography and rainfall variability interact to determine seed distribution and recruitment in a sand dune grassland

Question: Understanding the mechanisms underlying how habitat degradation, topography and rainfall variability interactively affect seed distribution and seedling recruitment is crucial for explaining plant community patterns and dynamics. Interactions between these major factors were studied together in a semiarid sand dune grassland. Location: Eastern Inner Mongolia, China. Methods: The study system used four sites of fixed, semifixed, semishifting and shifting sand dune grasslands, representing a gradient of habitat degradation. We investigated the density of germinable seeds deposited in the top 5 cm of soil and in situ seedling emergence (number of seedlings emerging early in the growing season) and establishment (number of plants recruited at the end of the growing season) at three topographic positions (dune top, windward and leeward sides) within each site over 2 years that differed in rainfall. Habitat characteristics (i.e. vegetation cover, plant species composition and diversity, soil moisture and nutrient availability and soil erodibility) of the four sites were also measured. Results: Habitat degradation (i.e. decreased vegetation cover and enhanced wind erosion rate) significantly reduced the size of the germinable soil seed bank. On average, germinable seed number from the high‐vegetation cover fixed dune was 36‐fold larger than the low‐vegetation cover shifting dune, and eight‐ and two‐fold larger, respectively, than the semishifting and semifixed dunes with intermediate vegetation cover. We observed within‐habitat variability in seed distribution, but among‐topographic position variation differed among habitats. Seedling recruitment showed large between‐year, and among‐ and within‐habitat variability, but these variations varied significantly depending on the response variables evaluated (i.e. initial seedling density, final plant density, emergence rate and recruitment rate). Path analysis revealed complex density‐dependent positive and negative, direct and indirect effects of germinable seed density and initial seedling density on recruitment, but the relative importance of these density‐dependent effects varied depending on habitat type and rainfall availability. Conclusion: Our results suggest that habitat degradation, microtopography and rainfall availability interact in shaping sand dune seed bank and plant community recruitment patterns and dynamics. Their effects were mainly mediated through changes in both the biotic and abiotic environment during the process of habitat deterioration.     

Vertical distribution, biomass, production and turnover of fine roots along a topographical gradient in a sandy shrubland

In an artificial Salix gordejevii Chang et Skv. plantation of the Horqin sandy land, we investigated vertical distribution (in 0–100 cm depth), biomass (FRD), fine root production (FRP), fine root length density (FRLD) and turnover of fine roots (<2 mm diameter) at three sites (dune top, midslope and bottom of dune) along leeward slopes. Meanwhile, the correlation between FRP and soil available resources was analyzed. Our results indicate that more than 65% of total fine root biomass is distributed in 0–40 cm depth, and the patterns are different at three sites. The mean monthly FRD ranges from 227 to 324 g·m^?2, and they follows the order: dune top > midslope > bottom of dune. Ingrowth cores were harvested after 2, 3, 4, 5, 6 and 8 months of installation. At the first five sampling times, FRP and FRLD (0–40 cm) follows the same order with FRD along the topographical gradient, while FRP harvested after 8 months does not follow the same tendency, they are 348, 402 and 356 g·cm^?2 in dune top, midslope and bottom of dune, respectively. Fine root turnover ranges from 1.04–1.92 year^?1, and fine root turnover (20–40 cm) increases from dune top to bottom of dune along the topographical gradient. Correlation analysis between FRP and soil available resources indicates that only mean soil volumetric water content significantly correlates with annual FRP, which suggests that soil water content might be more crucial for shrub growth than fertility along the topographical gradient.     

浑善达克沙地不同微地形的土壤物理性质和草本群落分布及其相关性分析

采用样方调查和室内检测相结合法对内蒙古浑善达克沙地迎风坡、坡顶、背风坡和丘间地不同土层(0~10、10~20和20~40 cm)的土壤物理性质(包括含水量、田间持水量、容重、总孔隙度和毛管孔隙度)及草本群落的生产力(包括盖度和地上部生物量)和物种多样性(包括Margalef丰富度指数、Simpson多样性指数、Shannon-Wiener多样性指数和Pielou均匀度指数)进行比较分析;在此基础上,采用Pearson相关性分析法对不同微地形的土壤物理性质与草本群落各指标间以及草本群落生产力与物种多样性各指标间的相关性进行分析.结果表明:4种微地形的土壤物理性质总体上差异显著,但不同土层的土壤物理性质总体上无明显差异;从丘间地、背风坡、坡顶到迎风坡,土壤的含水量、田间持水量、总孔隙度和毛管孔隙度依次递减,而土壤容重则依次递增,说明迎风坡的土壤结构较差且水分散失较多,而丘间地的土壤结构和水分状况均相对较好.不同微地形间草本群落生产力和物种多样性总体上也存在明显差异,从背风坡、丘间地、坡顶到迎风坡,草本群落的生产力和物种多样性依次递减,仅Margalef丰富度指数表现为在丘间地最高、在迎风坡最低,说明迎风坡的草本群落物种多样性较低,群落稳定性差,而背风坡和丘间地的草本群落物种多样性和生产力水平均较高,群落稳定性较好.相关性分析结果表明:该区域的草本群落生产力和物种多样性与土壤容重呈负相关,与土壤的其他物理性状呈正相关,但仅部分指标间有显著或极显著相关性;草本群落生产力与物种多样性各指标间呈正相关,但多数指标间的相关性并不显著.研究结果显示:微地形能够显著影响浑善达克沙地的土壤物理性质和草本群落的分布特征,气候和人为干扰使该沙地的土壤物理性质受到严重破坏,导致草本群落生产力降低并处于不稳定状态.     

Effects of topography,soil moisture,wind and grazing on Festuca seedlings in a Patagonian grassland

Abstract. The effects of topography, soil moisture, wind and grazing on the emergence and survival of seedlings of Festuca spp. were examined in the steppe zone of Patagonia, Argentina. Ungrazed and grazed field treatment plots were established on a plain and a north-facing slope at the Media Luna Ranch (43° 36′S, 71° 25′W). On the leeward and windward sides of each of 15 Festuca plants, 0.1 m × 0.4 m quadrats were censused bimonthly for seedling emergence and survival over three growing seasons. Three categories were distinguished: recently germinated and up to the first leaf, two to four leaves, and from five leaves up to one tiller. Soil moisture content, litter cover and frost heaving effects were also determined for each treatment at each sampling date. Festuca spp. showed two emergence peaks, one in late fall and the other in early to mid-spring. Seedling emergence was significantly correlated with soil moisture content in the 0–5 cm of the soil during the three growing seasons. Seedlings that emerged in the fall had higher survivorship than those that emerged in spring. Seedling emergence and survival was significantly (p < 0.01) lower on slopes, in the grazing treatment, and on windward sides of adult plants. In this grassland, an increase in the availability of safe sites for seedling emergence and survival might be achieved by protecting vegetation from grazing, particularly on north-facing slopes.     

Whitebark pine facilitation at treeline: potential interactions for disruption by an invasive pathogen

In stressful environments, facilitation often aids plant establishment, but invasive plant pathogens may potentially disrupt these interactions. In many treeline communities in the northern Rocky Mountains of the U.S. and Canada, Pinus albicaulis, a stress‐tolerant pine, initiates tree islands at higher frequencies than other conifers – that is, leads to leeward tree establishment more frequently. The facilitation provided by a solitary (isolated) P. albicaulis leading to tree island initiation may be important for different life‐history stages for leeward conifers, but it is not known which life‐history stages are influenced and protection provided. However, P. albicaulis mortality from the non‐native pathogen Cronartium ribicola potentially disrupts these facilitative interactions, reducing tree island initiation. In two Rocky Mountain eastern slope study areas, we experimentally examined fundamental plant–plant interactions which might facilitate tree island formation: the protection offered by P. albicaulis to leeward seed and seedling life‐history stages, and to leeward krummholz conifers. In the latter case, we simulated mortality from C. ribicola for windward P. albicaulis to determine whether loss of P. albicaulis from C. ribicola impacts leeward conifers. Relative to other common solitary conifers at treeline, solitary P. albicaulis had higher abundance. More seeds germinated in leeward rock microsites than in conifer or exposed microsites, but the odds of cotyledon seedling survival during the growing season were highest in P. albicaulis microsites. Planted seedling survival was low among all microsites examined. Simulating death of windward P. albicaulis by C. ribicola reduced shoot growth of leeward trees. Loss of P. albicaulis to exotic disease may limit facilitation interactions and conifer community development at treeline and potentially impede upward movement as climate warms.     

Contrasting effects of wildfire and climate on radial growth of Pinus canariensis on windward and leeward slopes on Tenerife, Canary Islands

Little is known concerning the effects of wildfires on tree radial growth and their climatic response under contrasting regimes of fog water inputs on oceanic islands. On Tenerife, Canary Islands, windward slopes are humid with high-fog frequency due to influence of wet trade winds, while climate on leeward slopes is more arid. We used tree-ring records of Pinus canariensis Sweet ex Spreng. to quantify the effects of a fire of known date on radial growth and determine the main limiting climatic factors for growth. Radial growth patterns and their responsiveness to fire severity and climatic variation differed between windward and leeward slopes. Surface fire did not significantly impact growth, while crown fire caused short-term growth reduction, and even cessation, more pronounced on the windward slope. Growth rates, tree-ring common signal, and climate sensitivity were smaller on the windward slope, with cold winters, and summer water stress limiting growth. On the leeward slope, climate explained a greater amount of growth variation mainly due to negative effects of high October–December sea-level pressures causing dry winter conditions. Contrasting growth dynamics on both slopes may result from diverging physiological effects of water inputs and reduced radiation caused by fog drip. Our findings suggest that dating growth suppressions and absent rings are useful to date past high-severity crown fires in P. canariensis forests, in addition to ordinary fire scars dating indicative of low-severity surface fires.     

福建平潭岛海岸不同演化阶段草丛沙堆表面老鼠艻叶水势日变化特征

以福建省平潭岛海岸不同演化阶段草丛沙堆表面典型沙生植物老鼠艻为对象,研究其叶水势日变化特征及其影响因素.结果表明: 老鼠艻叶水势日变化以“W”型为主,午间出现短暂升高现象.发育阶段沙堆表面老鼠艻叶水势日均值较稳定阶段高,且夜间水分恢复能力更强;老鼠艻叶水势与大气水势和相对湿度呈显著正相关,与气温呈显著负相关,但与不同深度土壤水势无显著相关性;发育阶段沙堆丘间地老鼠艻叶水势在10:00—16:00皆高于迎风坡和背风坡,迎风坡除10:00和12:00外皆低于背风坡.不同坡位叶水势日变异程度表现为背风坡>迎风坡>丘间地,但无显著差异.沙堆迎风坡老鼠艻叶水势与大气水势和相对湿度呈显著正相关,与气温呈显著负相关;沙堆背风坡和丘间地老鼠艻叶水势与气象因子有一定相关性,但不显著.     

Water source partitioning among trees growing on shallow karst soils in a seasonally dry tropical climate

The sources of water used by woody vegetation growing on karst soils in seasonally dry tropical regions are little known. In northern Yucatan (Mexico), trees withstand 4–6 months of annual drought in spite of the small water storage capacity of the shallow karst soil. We hypothesized that adult evergreen trees in Yucatan tap the aquifer for a reliable supply of water during the prolonged dry season. The naturally occurring concentration gradients in oxygen and hydrogen stable isotopes in soil, bedrock, groundwater and plant stem water were used to determine the sources of water used by native evergreen and drought-deciduous tree species. While the trees studied grew over a permanent water table (9–20 m depth), pit excavation showed that roots were largely restricted to the upper 2 m of the soil/bedrock profile. At the peak of the dry season, the δ¹⁸O signatures of potential water sources for the vegetation ranged from 4.1 ± 1.1‰ in topsoil to −4.3 ± 0.1‰ in groundwater. The δ¹⁸O values of tree stem water ranged from −2.8 ± 0.3‰ in Talisia olivaeformis to 0.8 ± 1‰ in Ficus cotinifolia, demonstrating vertical partitioning of soil/bedrock water among tree species. Stem water δ¹⁸O values were significantly different from that of groundwater for all the tree species investigated. Stem water samples plotted to the right of the meteoric water line, indicating utilization of water sources subject to evaporative isotopic enrichment. Foliar δ¹³C in adult trees varied widely among species, ranging from −25.3 ± 0.3‰ in Enterolobium cyclocarpum to −28.7 ± 0.4‰ in T. olivaeformis. Contrary to initial expectations, data indicate that native trees growing on shallow karst soils in northern Yucatan use little or no groundwater and depend mostly on water stored within the upper 2–3 m of the soil/bedrock profile. Water storage in subsurface soil-filled cavities and in the porous limestone bedrock is apparently sufficient to sustain adult evergreen trees throughout the pronounced dry season.