哀牢山山地森林不同附生地衣功能群的水分关系和光合生理特征

附生地衣是热带和亚热带山地森林生态系统中重要的结构性组分, 在生物多样性保护、环境监测、养分循环中发挥着重要作用。附生地衣按共生藻、生活型和繁殖策略的不同可划分为不同的功能群, 不同附生地衣功能群的分布格局存在较大的差异, 然而其生理生态机制仍不清楚。该研究以我国西南地区哀牢山亚热带山地森林中的附生地衣优势类群为研究对象, 对该地区蓝藻地衣、阔叶地衣、狭叶地衣及枝状地衣4种功能群的8种附生地衣的水分关系、光合生理特征等进行了测定分析, 结果显示: 不同功能群附生地衣的持水力和失水速率均存在差异, 其中蓝藻地衣具有较高的最大水分含量, 而枝状地衣的失水速率较快; 过高和过低的水分含量都会抑制附生地衣的光合作用, 但抑制程度有所差异; 蓝藻地衣的光合作用最适水分含量比较高, 表明它们的光合生理活动对水分条件要求较高, 所以它们偏好潮湿的生境, 同时蓝藻地衣的光补偿点比较低但光饱和点却不低, 反映出它们具有较宽的光强适应范围, 所以蓝藻地衣能够同时分布于强光和弱光生境中; 枝状地衣的光合最适水分含量较低, 表明它们的光合生理活动对水分条件要求不是很高, 能够适应较为干旱的环境, 同时枝状地衣的光补偿点和光饱和点都很高, 说明它们的光合生理活动对光照条件要求比较高, 所以它们广泛分布于强光生境中; 阔叶和狭叶地衣的光补偿点比较高, 说明它们更适应有充足光照条件的生境。     

云南哀牢山原生林及次生林群落附生地衣物种多样性与分布

附生地衣是森林附生植物的重要类群之一, 在维护森林生态系统的物种多样性以及水分和养分循环等方面发挥着重要作用。作者于2005年12月至2006年5月利用树干取样法调查了云南哀牢山徐家坝地区原生山地常绿阔叶林及其次生群落栎类萌生林、滇山杨(Populus bonatii)林和花椒(Zanthoxylum bungeanum)人工林中525株不同种类和径级树木距地面 0–2.0 m处附生地衣的组成和分布, 并收集了各个群落地面上凋落的地衣, 分析了林冠层附生地衣的物种组成。研究结果表明, 该区森林中附生地衣物种比较丰富。共收集到附生地衣61种, 分属17科29属, 其中原生林、栎类萌生林、滇山杨林和花椒人工林分别有51、53、46和23种。在树干距地面 0–2.0 m位置, 各群落中的附生地衣组成明显不同;但在林冠层中, 各群落内的附生地衣基本相似。原生林中附生地衣种类较多, 但分布不均匀。树干附生地衣的Shannon-Wiener和Simpson多样性指数以栎类萌生林最高, 分别为2.71和0.89;花椒林和滇山杨林次之, 分别为2.43–2.45和0.88–0.89;原生林最低, 为1.25和0.67。树干方位、宿主种类和宿主径级等都对附生地衣的物种组成和多样性有着重要影响, 附生地衣更多地出现于树干南向方位, 云南越桔(Vaccinium duclouxii)的附生地衣最为丰富, 胸径5.0–25.0 cm的树木上附生地衣较多。哀牢山山地森林群落中丰富的附生地衣种类及物种多样性在维系本区山地森林生态系统生物多样性格局方面具有重要的作用。     

Responses of the distribution of four epiphytic cyanolichens to habitat changes in subtropical forests

 蓝藻地衣是附生植物类群的重要组成部分, 在森林生态系统的环境监测和养分循环中发挥着重要作用。该研究在云南哀牢山亚热带森林系统的2种原生和6种次生森林群落中, 以粉缘绵毛衣(Leioderma sorediatum)、天蓝猫耳衣(Leptogium azureum)、网肺衣(Lobaria retigera)和双缘牛皮叶(Sticta duplolimbata) 4种常见蓝藻地衣为对象, 共设立120个样地, 调查了它们在3 600株树木0–2 m树干上的分布, 探讨其分布特征及与森林类群、宿主种类以及林龄等生境因子的关系。研究发现4种蓝藻地衣在森林群落间的分布模式明显不同。除双缘牛皮叶的盖度和频度在原生苔藓矮林中最高外, 其他3种蓝藻地衣的最高值均出现于次生林如厚皮香(Ternstroemia gymnanthera)林和滇山杨(Populus bonatii)林中; 而哀牢山地区广布的原生木果柯(Lithocarpus xylocarpus)林中, 4种蓝藻地衣极为少见。4种地衣都能生长于10多个树种上, 但明显表现出对厚皮香、滇山杨和硬壳柯(Lithocarpus hancei)等树种的偏好性, 以及对小花山茶(Camellia forrestii)等的排斥性。森林群落的林龄、胸径、最大胸径、林冠开阔度、基面积、树木密度和树种多样性等因子的变化均对4种附生蓝藻地衣的分布产生重要影响, 但在景观尺度上影响程度相对较小, 在不同森林群落内部却有各自的重要作用。其中, 林龄、林冠开阔度和宿主胸径是影响蓝藻地衣分布的最重要的生境因子。     

亚热带森林系统4种附生蓝藻地衣的分布对生境变化的响应

蓝藻地衣是附生植物类群的重要组成部分, 在森林生态系统的环境监测和养分循环中发挥着重要作用。该研究在云南哀牢山亚热带森林系统的2种原生和6种次生森林群落中, 以粉缘绵毛衣(Leioderma sorediatum)、天蓝猫耳衣(Leptogium azureum)、网肺衣(Lobaria retigera)和双缘牛皮叶(Sticta duplolimbata) 4种常见蓝藻地衣为对象, 共设立120个样地, 调查了它们在3600株树木0-2 m树干上的分布, 探讨其分布特征及与森林类群、宿主种类以及林龄等生境因子的关系。研究发现4种蓝藻地衣在森林群落间的分布模式明显不同。除双缘牛皮叶的盖度和频度在原生苔藓矮林中最高外, 其他3种蓝藻地衣的最高值均出现于次生林如厚皮香(Ternstroemia gymnanthera)林和滇山杨(Populus bonatii)林中; 而哀牢山地区广布的原生木果柯(Lithocarpus xylocarpus)林中, 4种蓝藻地衣极为少见。4种地衣都能生长于10多个树种上, 但明显表现出对厚皮香、滇山杨和硬壳柯(Lithocarpus hancei)等树种的偏好性, 以及对小花山茶(Camellia forrestii)等的排斥性。森林群落的林龄、胸径、最大胸径、林冠开阔度、基面积、树木密度和树种多样性等因子的变化均对4种附生蓝藻地衣的分布产生重要影响, 但在景观尺度上影响程度相对较小, 在不同森林群落内部却有各自的重要作用。其中, 林龄、林冠开阔度和宿主胸径是影响蓝藻地衣分布的最重要的生境因子。     

亚热带森林附生地衣压力-体积曲线分析及其适用性

附生地衣是森林生态系统中的重要组成部分, 在维护森林物种多样性以及水分和养分循环等方面发挥着重要作用。地衣类群的水势特征以及压力-体积(PV)曲线及相关参数是否适用于探索地衣等变水性生物类群应对水分胁迫机制, 仍有待阐明。该研究以中国西南地区云南哀牢山亚热带山地森林中5个类群15种常见附生地衣为对象, 探讨了其PV曲线及水势参数的功能群和物种水平的变化。结果显示, 蓝藻型地衣的内部含水量(WC_internal)和共质体水含量(Rs)显著高于绿藻型地衣, 是其1倍以上, 但其他参数表现出较高的一致性。功能群水平上, 地衣仅WC_internal、Rs和膨压损失点的相对含水量(RWC_TLP)具有显著性差异; 但物种间差异均非常显著。结合主成分分析进一步发现, PV曲线及相关参数在评估地衣整体应对水分胁迫以及揭示生境水分条件选择策略时具有较大的限制性, 受到共生藻和生长型的显著影响; 但内部最大持水力可以解释蓝藻型地衣的生境适应策略, 而部分参数如饱和渗透势(Ψsat)和RWC_TLP可以分别用于解释狭叶地衣、阔叶地衣和枝状地衣对生境水分条件的适应。研究表明PV曲线及水势参数不适用于地衣类群整体的抗旱性评价, 和其他生物类群进行抗旱性比较时更要慎用。     

N limitation increases along a temperate forest succession: evidences from leaf stoichiometry and nutrient resorption

温带森林演替加剧了氮限制：来自叶片化学计量和养分重吸收的证据 森林生产力和碳汇功能在很大程度上取决于土壤氮和磷的有效性。然而,迄今为止,养分限制随森林演替的时间变化仍存在争议。叶片化学计量和养分重吸收是预测植物生长养分限制的重要指标。基于此,本研究测定了温带森林4个演替阶段所有木本植物叶片和凋落叶中氮和磷的含量,并分析了演替过程中非生物因子和生物因子如何影响叶片化学计量和养分重吸收。研究结果表明,在个体尺度上,叶片氮磷含量在演替末期显著增加,而叶片氮磷比无显著变化;氮的重吸收效率随演替显著增加,然而磷的重吸收效率先增加后减少;氮重吸收效率与磷重吸收效率的比值仅在演替末期显著增加。此外,植物氮素循环对土壤养分的响应比磷素循环更弱。在群落尺度上,叶片氮磷含量随森林演替呈现先降低后升高的趋势,主要受香农-维纳多样性指数和物种丰富度的影响;叶片氮磷比随演替而显著变化,主要由胸径的群落加权平均值决定;氮的重吸收效率增加,主要受物种丰富度和胸径的影响,而磷的重吸收效率相对稳定。因此,氮重吸收效率与磷重吸收效率的比值显著增加,表明随着温带森林演替,氮限制加剧。这些结果可能反映了较高生物多样性群落中物种间对有限资源的激烈竞争,强调了生物因子在驱动森林生态系统养分循环中的重要性,为中国温带和北方森林可持续经营的施肥管理提供了参考。     

叶附生苔植物物种多样性分布格局及生态成因

叶附生苔植物(以下简称叶附生苔)是一类附生在维管植物叶片表面的、一般只出现在热带雨林和常绿阔叶林中的苔类植物。它们具有重要的生态功能, 如影响碳、氮、水循环, 对气候变化和森林破碎化反应敏感, 可用作环境变化的指示植物。该文对叶附生苔的形态特性、物种多样性的研究历史和进展、地理分布格局, 以及对环境的要求(附主和生境的特性)等进行了综述, 探讨了叶附生苔多样性分布格局形成的可能原因, 即环境(空气湿度高、林冠层发育好、干扰少)和适应特征(无性繁殖、形体微小)的选择。根据全球叶附生苔研究进展状况及所存在的问题提出未来可开展的研究方向, 如叶附生苔的形成原因、与附主间的物质交换和能量流动机制、在森林生态系统中的生态功能、气候变化指示作用研究等, 大尺度上的研究也值得关注。     

不同水分条件下三种附生地衣的光合作用特性

附生地衣是哀牢山湿性常绿阔叶林生态系统中重要的结构性组分。通过对该区域山地森林中3种典型附生地衣平滑牛皮叶 (Sticta nylanderiana)、网肺衣 (Lobaria retigera) 和橄榄斑叶 (Cetrelia olivetorum)在不同水分条件下的光合光响应及荧光参数的测定分析,结果显示,附生地衣光补偿点 (LCP)、光饱和点 (LSP)较高,对强光适应能力较强。在水分胁迫 (含水量5%~10%) 条件下,3种附生地衣的最大净光合速率 (Pmax) 仅为17~50nmol·g-1·s-1。随着含水量的增加,地衣的最大净光合速率 (Pmax) 与暗呼吸速率 (Rday) 逐渐增大,LCP降低,而LSP随之提高,这表明3种附生地衣具备“阳生植物”的某些特性,从而能够在一定程度上适应野外光照较强的灌丛、向阳林冠等生境。地衣叶绿素光反应中心初始荧光参数 (F0) 和最大光化学效率 (Fv/Fm) 随含水量下降而显著降低,暗示其光反应中心对水分有很强的敏感性。水分条件的改善有助于附生地衣的光反应中心进入到较高的生理活性状态。     

云南哀牢山地区森林附生维管植物多样性及区系特征

附生植物作为山地森林生态系统中重要的结构性成分,在维持森林生态系统生物多样性格局、水分和养分循环等方面发挥着重要作用。本文通过野外调查、标本查阅并结合相关文献,对云南哀牢山地区附生维管植物物种组成及分布进行了系统研究。结果显示,哀牢山地区附生维管植物共有23科83属218种,其中附生蕨类和兰科植物最丰富。附生蕨类有34属93种,以附生-石生蕨类生活型占优势,其中水龙骨科17属62种,占附生蕨类的66.67%,瓦韦属(Lepisorus)和石韦属(Pyrrosia)分别有13种和10种。附生兰科植物有26属65种,其中石斛属(Dendrobium)和石豆兰属(Bulbophyllum)分别有12种和8种。该地区附生维管植物属的分布具有明显的热带性质并以热带亚洲分布居多。附生植物生长于生境因子变化剧烈、资源有限的林冠,对环境变化敏感,极易遭受破坏且破坏后难以恢复,不少附生植物具有很高的药用、观赏等价值。因此,应加强对附生维管植物这一特殊类群的保护。     

矿质养分输入对森林生物固氮的影响

生物固氮是森林生态系统重要的氮素来源,并且在全球氮循环中占有重要的地位。近代以来,因人类活动加剧而导致氮沉降的增加以及其它矿质养分元素(如磷、钼、铁等)输入的改变已成为影响森林生态系统生物固氮的重要因素之一,并引起了学术界的普遍关注。综述了国内外关于森林生物固氮对矿质养分输入的响应及机理。主要内容包括:(1)森林生物固氮的概念及主要的测定方法;(2)矿质养分输入对森林生物固氮的影响。整体上讲,氮素输入抑制了森林生物固氮,磷和其他营养元素输入则表现为促进作用。氮和磷、磷和微量元素同时添加均提高了森林的固氮量;(3)矿质养分改变森林生物固氮的机理。包括生物作用机制(如改变地表层固氮菌的数量或群落丰度、改变结瘤植物的根瘤生物量和附生植物的丰度或盖度)和环境作用机制(如引起土壤酸化、改变碳源物质的含量);(4)探讨了矿质养分输入对森林生物固氮影响研究中所存在的问题,并对未来该领域的研究提出建议。     

Phyllosphere nitrogen relations: reciprocal transfer of nitrogen between epiphyllous liverworts and host plants in the understorey of a lowland tropical wet forest in Costa Rica

Epiphyllous bryophytes on tropical rainforest plants acquire nutrients from throughfall and free-living N2-fixing organisms, but may also depend directly on host leaf leachates. By contrast, after drying events bryophytes lose significant quantities of nutrients through leaching that can be taken up by host leaves. To assess a potential nutritional interdependency, nitrogen fluxes between epiphyllous liverworts and their host leaves (Carludovica drudei, Costus laevis, Dieffenbachia concinna, Pentagonia wendlandii) were quantified by in situ15N-labelling techniques in a lowland rainforest, Piedras Blancas National Park, Costa Rica. Depending on host species, epiphyllous bryophytes met between 1 and 57% of their N demand from host leaf leachates. Externally supplied 15N was taken up both by epiphylls and host leaves, but N from epiphyll leachates accounted for < 2.5% of host leaf N after 14 d. Long-term observations (180 d) demonstrated the highly dynamic nature of phyllosphere N of the investigated tropical rainforest understorey and an intermittent sink capacity of epiphyllous bryophytes.     

Variation in Nickel Content in the Nickel-Hyperaccumulating Shrub Psychotria douarrei (Rubiaceae) from New Caledonia1

Plants that hyperaccumulate Ni contain > 1000 ppm (dry wt.) in their tissues. Variation of Ni content within hyperaccumulating plant species is poorly explored. Using the Ni-hyperaccumulating shrub Psychotria douarrei, we documented variation of leaf Ni levels within individual shrubs, and variation with respect to plant size and leaf age. Plant size did not correlate significantly with leaf Ni content, and leaf Ni content did not correlate significantly with soil Ni content. Older leaves contained twice as much Ni as younger leaves. Older leaves also contained greater concentrations of Ca, Fe, and Cr but less K, P, and Cu. Five elements (Zn, Pb, Co, Mn, Mg) showed no significant variation due to leaf age. We also examined the effect of leaf age on epiphyll cover, finding increased epiphyll cover on the upper surface of older leaves. The dominant leafy liverwort epiphyll had a relatively high Ni content (400 ppm), suggesting that epiphylls of Ni hyperaccumulators obtain some Ni from host leaves. Individual shrubs differed in mean leaf Ni content almost two-fold (14,900-27,700 ppm). Variation among branches within individuals also ranged widely; however, this intraplant variability was not strongly correlated with the mean leaf Ni content of an individual shrub. We concluded that Ni contents in leaves of P. douarrei vary considerably due to leaf age, among individual shrubs, and among branches within a shrub.     

Ectopic expression of Kxhkn5 in the viviparous species Kalanchoe?��?Houghtonii induces a novel pattern of epiphyll development

KxhKN5 (class 1 KNOX gene) was cloned from Kalanchoe × houghtonii with strong tendency to form epiphylls on leaves. KxhKN5 appear to be homologue of BP of A. thaliana on the basis of phylogeny, expression and phenotype analysis. Beside the modification of several plant and leaf traits, the appearance of epiphylls was drastically reduced by both the silencing and the over-expression of KxhKN5 in most of the generated clones. In silenced clones, epiphyll production followed the morphogenetic pathway of the WT plants: somatic embryos outbreak in the centre of each leaf-pedestal, grown in the notch between leaf indentations and were supported by a suspensor. The connection between the epiphyll and the mother plant did not include any vasculature and as a result, the epiphylls dropped easily from the mother plant. The most represented category of over expressor clones, disclosed a novel pattern of epiphyll development: the leaf-pedestals were absent and single bud outbreaks in each leaf notch. Buds developed into shoots which remained attached to the maternal plant by a strong vascular connection. The leaves supporting shoots, produced a thickened midrib and veins, and their lamina ceased expanding. Finally, the leaf/shoot structure resembles a lateral branch. The ectopic expression of KxhKN5 in K. × houghtonii induces a process comparable to the alternation of leaf and shoot formation in other species and support the idea, that it is the variation in shared molecular and developmental processes which produces the growth of specific structures.     

Determinants of Lichen Diversity in a Rain Forest Understory

Change in lichen diversity is often used as a bioindicator to estimate effects of atmospheric pollution, but natural variation in lichen cover and species richness can be very high. We examined the top-down effects of spore-consuming ants and the bottom-up effects of nutrient and light availability on lichen diversity associated with the leaf surface of the rain forest understory plant, Piper cenocladum. Plots containing P. cenocladum were randomly assigned to treatments in factorial experiments that included high and low light levels, nutrient enrichment, and presence and absence of the ant mutualist, Pheidole bicornis . At the conclusion of the experiments, plants were harvested and size of leaves, secondary metabolite content (amides), epiphyll cover, and the species richness of the lichens (which comprised 85% of the epiphyll community) were quantified. Epiphyll cover (mosses, liverworts, and lichens) was greater on plants that had ant-mutualists and balanced resources. Lichen species richness was greater for plants with balanced resources, particularly for those with high light availability. Relationships between toxins and lichen cover and richness were weak and unclear. In this system, natural sources of variation were reliable determinants of lichen diversity and both biotic and abiotic influences were important.     

The relationship between leaf water repellency and leaf traits in three distinct biogeographical regions

Leaf water repellency (LWR) is the degree to which water droplets repel from a leaf surface. LWR has been suggested as an adaptation to maximize gas exchange during photosynthesis and as a mechanism to increase hydrological inputs beneath the canopy. This article examined the relationships between LWR and leaf traits in 33 species across three distinct biogeographical regions (tropical montane cloud forests in Guatemala; tropical dry forests in Guatemala; and foothills-grassland vegetation in the U.S.). The objectives of this study were to determine if leaf area, leaf thickness, leaf biomass, specific leaf area, leaf toughness, stomatal density, and the presence of epiphylls explained variation in LWR in the 33 species. LWR was greatest in the dry foothills-grassland ecotone in Colorado and lowest in humid cloud forest of the Sierra de las Minas. Most possible pairs of species were significantly different from each other at each study area. Significant variation in all leaf traits was found among species and sites. LWR was not explained by leaf area, leaf thickness, leaf biomass, and specific leaf area. The presence of epiphylls influenced LWR on the abaxial leaf surfaces of species of the Sierra de las Minas. The article concludes that leaf traits related to size and mass do not influence LWR; however, the variation in LWR in species between distinct biogeographical regions remains an important research area in ecohydrology.     

Leaf Water Repellency as an Adaptation to Tropical Montane Cloud Forest Environments

Adaptations that reduce water retention on leaf surfaces may increase photosynthetic capacity of cloud forests because carbon dioxide diffuses slower in water than air. Leaf water repellency was examined in three distinct ecosystems to test the hypothesis that tropical montane cloud forest species have a higher degree of leaf water repellency than species from tropical dry forests and species from temperate foothills-grassland vegetation. Leaf water repellency was measured by calculating the contact angle of the leaf surface and the line tangent to a water droplet through the point of contact on the adaxial and the abaxial surface. Leaf water repellency was significantly different between the three study areas. The hypothesis that leaf water repellency is higher in cloud forest species than tropical dry forests and temperate foothills-grassland vegetation was not confirmed in this study. Leaf water repellency was lower for cloud forest species (adaxial surface = 50.8°; abaxial surface = 82.9°) than tropical dry forest species (adaxial surface = 74.5°; abaxial surface = 87.3°) and temperate foothills-grassland species (adaxial surface = 77.6°; abaxial surface = 95.8°). The low values of leaf water repellency in cloud forest species may be influenced by presence of epiphylls and loss of epicuticular wax on the leaf surfaces.     

N2 Fixation (C2H2 reduction) by epiphylls on coffee,Coffea arabica

Nitrogen fixation (C₂H₂ reduction) by epiphylls on coffee,Coffea arabica, grown in sites with different degrees of shade, was determined. Coffee leaves with nitrogen-fixing epiphylls were found in all sites in approximately equal numbers. Rates of C₂H₂ reduction were similar for all sites and throughout the year, averaging 3.21 nmoles C₂H₂ reduced leaf with epiphylls^–1 day^–1. Apparently, neither rates of activity nor abundance of leaves with nitrogen-fixing epiphylls is related to the degree of shade in a site. No correlation was found between percent epiphyll cover and the presence or magnitude of nitrogen-fixing activity. Calculated annual fixation by epiphylls on coffee was low, ranging from 0.7 g N₂ ha^–1 year^–1 for the shadeless site to 1.4 g N₂ ha^–1 year^–1 for the site withIngajinicuil shade trees. These results suggest that epiphyll fixation is not an important source of nitrogen for the coffee ecosystem studied.     

Abundance and Leaf Damage Caused by Gall‐Inducing Insects in a Mexican Tropical Dry Forest1

We evaluated the temporal and spatial patterns of abundance and the amount of damage caused by gall‐inducing insects (GII) in deciduous and riparian habitats in a seasonal tropical dry forest in Mexico. Plants occurring in these habitats differ in their phenology and moisture availability. Deciduous habitats are seasonal and xeric, while riparian habitats are aseasonal and mesic. We found 37 GII species and each one was associated with a specific plant species. In total, 19 species (51.3%) were present in deciduous habitats, 13 species (35.2%) in riparian habitats, and only 5 species (13.5%) occurred in both. Abundance and leaf damage by GII were greater in deciduous than in riparian habitats during the wet season. For each GII species that occurred in both habitats, host plant species supported greater abundance and leaf damage by GII in deciduous habitats during the wet season. These results indicate a greater association of GII species with host plants in deciduous than in riparian habitats during the wet season. In riparian habitats, 11 plant species (61.1%) had greater density of GII in the dry than in the wet season. Similarly, leaf damage by GII was significantly greater in the dry than in the wet season in riparian habitats for 12 plant species (66.7%). Dry forest plants of riparian habitats presented two peaks of leaf‐flushing: GII colonized leaves produced in the first peak at the beginning of the wet season, and accumulated or recolonized leaves in the second peak at the beginning of the dry season. The levels of leaf damage by GII detected in this study in the rainy season were considerably higher than those obtained for folivorous insects in other neotropical forests, suggesting that this GII guild might have an important impact on their host plant species in this tropical community.     

Effects of plant-soil feedback on tree seedling growth under arid conditions

Aims Plants are able to influence their growing environment by changing biotic and abiotic soil conditions. These soil conditions in turn can influence plant growth conditions, which is called plant–soil feedback. Plant–soil feedback is known to be operative in a wide variety of ecosystems ranging from temperate grasslands to tropical rain forests. However, little is known about how it operates in arid environments. We examined the role of plant–soil feedbacks on tree seedling growth in relation to water availability as occurring in arid ecosystems along the west coast of South America.Methods In a two-phased greenhouse experiment, we compared plant–soil feedback effects under three water levels (no water, 10% gravimetric moisture and 15% gravimetric moisture). We used sterilized soil inoculated with soil collected from northwest Peru (Prosopis pallida forests) and from two sites in north-central Chile (Prosopis chilensis forest and scrublands without P. chilensis).Important findings Plant–soil feedbacks differed between plant species and soil origins, but water availability did not influence the feedback effects. Plant–soil feedbacks differed in direction and strength in the three soil origins studied. Plant–soil feedbacks of plants grown in Peruvian forest soil were negative for leaf biomass and positive for root length. In contrast, feedbacks were neutral for plants growing in Chilean scrubland soil and positive for leaf biomass for those growing in Chilean forest soil. Our results show that under arid conditions, effects of plant–soil feedback depend upon context. Moreover, the results suggest that plant–soil feedback can influence trade-offs between root growth and leaf biomass investment and as such that feedback interactions between plants and soil biota can make plants either more tolerant or vulnerable to droughts. Based on dissecting plant–soil feedbacks into aboveground and belowground tissue responses, we conclude that plant–soil feedback can enhance plant colonization in some arid ecosystems by promoting root growth.     

Foliar demand and resource economy of nutrients in dry tropical forest species

Important phenological activities in seasonally dry tropical forest species occur within the hot‐dry period when soil water is limiting, while the subsequent wet period is utilized for carbon accumulation. Leaf emergence and leaf area expansion in most of these tree species precedes the rainy season when the weather is very dry and hot and the soil cannot support nutrient uptake by the plants. The nutrient requirement for leaf expansion during the dry summer period, however, is substantial in these species. We tested the hypothesis that the nutrients withdrawn from the senescing leaves support the emergence and expansion of leaves in dry tropical woody species to a significant extent. We examined the leaf traits (with parameters such as leaf life span, leaf nutrient content and retranslocation of nutrients during senescence) in eight selected tree species in northern India. The concentrations of N, P and K declined in the senescing foliage while those of Na and Ca increased. Time series observations on foliar nutrients indicated a substantial amount of nutrient resorption before senescence and a ‘tight nutrient budgeting’. The resorbed N‐mass could potentially support 50 to 100% and 46 to 80% of the leaf growth in terms of area and weight, respectively, across the eight species studied. Corresponding values for P were 29 to 100% and 20 to 91%, for K 29 to 100% and 20 to 57%, for Na 3 to 100% and 1 to 54%, and for Ca 0 to 32% and 0 to 30%. The species differed significantly with respect to their efficiency in nutrient resorption. Such interspecific differences in leaf nutrient economy enhance the conservative utilization of soil nutrients by the dry forest community. This reflects an adaptational strategy of the species growing on seasonally dry, nutrient‐poor soils as they tend to depend more or less on efficient internal cycling and, thus, utilize the retranslocated nutrients for the production of new foliage biomass in summer when the availability of soil moisture and nutrients is severely limited.