Climate warming and precipitation redistribution modify tree–grass interactions and tree species establishment in a warm‐temperate savanna

Savanna tree–grass interactions may be particularly sensitive to climate change. Establishment of two tree canopy dominants, post oak (Quercus stellata) and eastern redcedar (Juniperus virginiana), grown with the dominant C₄ perennial grass (Schizachyrium scoparium) in southern oak savanna of the United States were evaluated under four climatic scenarios for 6 years. Tree–grass interactions were examined with and without warming (+1.5 °C) in combination with a long‐term mean rainfall treatment and a modified rainfall regime that redistributed 40% of summer rainfall to spring and fall, intensifying summer drought. The aim was to determine: (1) the relative growth response of these species, (2) potential shifts in the balance of tree–grass interactions, and (3) the trajectory of juniper encroachment into savannas, under these anticipated climatic conditions. Precipitation redistribution reduced relative growth rate (RGR) of trees grown with grass. Warming increased growth of J. virginiana and strongly reduced Q. stellata survival. Tiller numbers of S. scoparium plants were unaffected by warming, but the number of reproductive tillers was increasingly suppressed by intensified drought each year. Growth rates of J. virginiana and Q. stellata were suppressed by grass presence early, but in subsequent years were higher when grown with grass. Quercus stellata had overall reduced RGR, but enhanced survival when grown with grass, while survival of J. virginiana remained near 100% in all treatments. Once trees surpassed a threshold height of 1.1 m, both tiller number and survival of S. scoparium plants were drastically reduced by the presence of J. virginiana, but not Q. stellata. Juniperus virginiana was the only savanna dominant in which neither survival nor final aboveground mass were adversely affected by the climate scenario of warming and intensified summer drought. These responses indicate that climate warming and altered precipitation patterns will further accelerate juniper encroachment and woody thickening in a warm‐temperate oak savanna.     

祁连山典型灌丛降雨截留特征

基于2010年6月至10月的野外试验数据,研究了祁连山金露梅、高山柳、沙棘和鬼箭锦鸡儿灌丛降雨截留特征,分析了降雨量和雨强对灌丛降雨截留过程的影响。结果表明:试验期间共降雨298.6 mm,在降雨量<2.1 mm时,降雨被全部截留,实际发生穿透和茎流的降雨为283.1 mm;金露梅灌丛穿透雨量、茎流量和截留量分别为175.8 mm(62.0%)、9.5 mm(3.4%)和62.0 mm(34.6%),高山柳为179.8 mm(63.5%)、9.1 mm(3.2%)和63.5 mm(33.3%),沙棘分别为148.1mm(52.3%)、22.5mm(8.0%)和52.3 mm(39.7%),鬼箭锦鸡儿分别为170.4 mm(60.2%)、11.8 mm(4.2%)和60.2 mm(35.6%);灌丛穿透雨量、茎流量和截留量均与降雨量呈显著线性正相关(P<0.001);穿透率、茎流率和截留率与降雨量呈指数函数关系(P<0.05);平均雨强与截留率关系以指数函数拟合最好(P<0.05)。在降雨性质相同的情况下,植被形态特征是影响灌丛降雨截留的重要因素。     

内蒙古典型草原小叶锦鸡儿灌丛化对水分再分配和利用的影响

灌丛化是全球草原地区存在的主要环境问题。通过对内蒙古典型草原区小叶锦鸡儿灌丛和草地斑块冠层降雨再分配、地表径流、土壤含水量的对比观测,研究了小叶锦鸡儿灌丛化对该区水分再分配和利用的影响。结果表明,灌丛和草地斑块的冠层截留量分别占降雨量的20.86%和7.88%,灌丛和草地斑块的平均地表径流系数分别为5.95%和17.19%。土壤含水量观测结果显示,0—60 cm土层中,降雨事件过程中,灌丛斑块较草地斑块能捕获更多水分,灌丛斑块植被冠层下方土壤含水量高于草地斑块;而在雨后无有效降水补充土壤水分的前提下,0—60 cm土层中,灌丛斑块土壤水分蒸散发量高于草地斑块,其中0—10cm土层中灌丛斑块土壤水分蒸散发速率低于草地斑块,10—60 cm土层中灌丛斑块土壤水分蒸散发速率高于草地斑块。研究认为,在水分为关键性限制因子的干旱半干旱区,小叶锦鸡儿灌丛化过程增加草原生态系统中水分分布的空间异质性,灌丛斑块能捕获、利用更多水分以维持更多的生物量。     

西北干旱区泡泡刺灌丛的降雨再分配特征及影响因素分析

定量分析植被冠层对降雨再分配过程的影响,是认识陆地生态系统水文循环的重要环节。然而,由于干旱区天然植被分布稀疏、形态结构特殊,其降雨再分配过程的测算较为困难,相关研究较少,特别是关于荒漠低矮灌丛的降雨再分配研究鲜有报道。本文以河西走廊中段临泽绿洲—荒漠过渡带的天然建群种泡泡刺灌丛(Nitraria sphaerocarpa)为研究对象,基于3年逐个单次降雨事件的观测数据分析了生长季泡泡刺灌丛的降雨再分配特征及主要影响因素,量化了泡泡刺灌丛覆盖下实际进入土壤的有效降雨量及其空间分布特征。结果表明:(1)生长季泡泡刺灌丛的平均穿透率、树干茎流率和冠层截留损失率分别为87.89%、1.61%和10.50%;(2)降雨量是影响泡泡刺灌丛降雨再分配特征的关键气象因素,其与穿透雨量、树干茎流量、冠层截留损失量之间具有显著的统计关系(P<0.001);(3)与干旱区其他稀疏植被相比,泡泡刺灌丛的穿透率和集流率较高,冠层截留损失率较低,与其特殊的植被形态特征有关,相关分析的结果表明,泡泡刺灌丛的穿透雨量与植被面积指数和株高呈显著的负相关关系(P<0.001),树干茎流量与树干倾角呈显著的正...     

苏南丘陵区毛竹林冠截留降雨分布格局

降雨穿过林冠层时,由于林冠的拦截作用,改变了降雨分布格局。林冠截留是一个复杂的水文过程,受降水特征及林分特性的影响较大。本文以北亚热带苏南丘陵地区人工毛竹林(Phyllostachys edulis)为研究对象,利用2007年度各场次降雨观测数据,分析了降雨量和降雨强度与林冠截留降雨的关系,研究了林冠截留过程的特点。结果表明：（1） 研究期间共观测到102次降水事件,降水总量为1110.8mm,单次最大降雨量为110.0㎜,最小为0.55㎜,事件平均降水量为10.89mm,且绝大部分降雨为低雨强、中雨量级的降雨事件。（2） 研究期间林冠截留总量为171.72mm,占同期降雨总量的15.46%。单次林冠截留量变幅为0.21—4.55mm,截留率变幅为1.3%—100%,且随林外降雨量的递增,林冠截留率呈现递减的变化趋势,二者的关系用幂函数（I0=117.34P-0.9106）拟合效果较好。（3）在林外次降雨量小于5㎜的条件下,事件降雨量占年降雨总量的5.0%,相应的降雨事件频率为9.8%,此时林冠截留量随降雨量的增大而增加,其变幅为0.55—1.9㎜,截留量与降雨量的关系用对数函数（I=0.4931Ln(P) 0.9493）进行拟合效果较好;在单场降雨量大于5㎜时,林冠截留量随各场次降雨量的增加,其变动幅度和频率均大大增强,变化范围在0.21—4.55㎜之间,二者相关性较差。（4）降雨在7—10mm雨量级范围内时,林外降雨量和林冠截留量分别为245.14mm和47.9㎜,占其全年总量的比例均为最大,分别为22.07%和27.9%;各雨量级林冠平均截留量与平均降雨量的关系表现为对数函数关系（R2=0.7287）,而截留率与平均降雨量的关系表现为极显著的幂函数关系（R2=0.9817）,且林冠对降雨的截留作用在雨量级较小时,表现十分显著。（5）全年单场降雨强度小于0.06mm/min时,降雨事件频数为73.53%,降雨量占其总量的57.93%,林冠截留量占其总量的89.34%,降雨事件平均截留率（23.84%）远高于雨强大于0.06mm/min时的降雨事件平均截留率（3.92%）。本研究结果为长江中下游丘陵山区水土保持林和水源涵养林体系建设提供了理论依据。     

中国森林生态系统林冠层降雨截留特征

森林生态系统作为陆地生态系统的主体,其发达的林冠层通过调节降水量、改变降水强度等深刻影响着流域全过程水文通量及水分输出。以中国广泛开展的典型森林降雨再分配过程的年尺度监测数据为基础,揭示中国不同类型森林生态系统的降雨再分配及林冠层降雨截留特征,阐明森林生态系统林冠层截留特征与降雨、植被要素的关系。结果表明:我国不同森林生态系统年穿透雨量处于141.4-2450.0 mm之间,年穿透雨率为36.3%-92.3%。5种典型森林生态系统多年平均穿透雨量((445.3±252.9)-(1230.6±479.6) mm)占同期多年平均降雨量的(72.6±9.2)%-(77.4±8.9)%。不同森林生态系统年树干茎流量介于0-508.2 mm之间,占同期年降雨量的0-25.8%。5种典型森林生态系统树干茎流量多年平均值((9.8±17.3)-(87.8±81.6) mm)占同期多年平均降雨量的(1.4±1.9)%-(5.4±4.6)%。不同森林生态系统林冠层年降雨截留范围在25.7-812.9 mm之间,占年降雨量的4.2%-55.6%。5种典型森林生态系统多年平均林冠截留量((154.2±81.6)-(392.2±203.5) mm)占同期年平均降雨量的(18.7±7.4)%-(25.9±8.3)%。进一步分析表明,我国森林生态系统穿透雨量、树干茎流量和林冠层截留量随观测区年降雨量的增加而呈显著增大(P<0.05),年穿透雨率、年树干茎流率随年降雨量的增加呈显著线性上升趋势(P<0.05),而年林冠截留率与年降雨量呈显著的负相关关系(P<0.01),降雨量、叶面积指数是深刻影响森林生态系统林冠层降雨截留率等特征的重要因素。整体上,不同类型森林生态系统林冠截留降雨能力存在明显差异,林冠层截留率突出表现为:落叶林大于常绿林、针叶林大于阔叶林。     

太岳山不同郁闭度油松人工林降水分配特征

利用2011年5-9月生长季观测的30场降雨数据,分析了山西太岳山不同郁闭度下油松人工林林冠截留、穿透雨以及树干茎流与降雨量的关系,以及林冠截留过程的特点.结果表明:(1)实验观测期间,该地区降雨总量为634.79mm,单次平均降雨量为21.16mm,单次最大降雨量为58.15mm,单次最小降雨量为0.54mm.其中,8月份的降雨总量最大,为190.77mm,6月份的降雨总量最小,为41.81mm.(2)郁闭度为0.8的油松人工林林冠截留量与降雨量呈一元线性关系,郁闭度为0.7、0.6和0.5均呈幂函数关系;对于各郁闭度的油松人工林,其林冠截留率与降雨量均呈对数函数关系;穿透雨量、树干茎流量与降雨量均呈明显的一元线性关系,穿透雨量和树干茎流量都随着降雨量的增加而增加.(3)不同郁闭度油松人工林之间林冠截留、穿透雨和树干茎流不同,总的趋势为随着郁闭度的减小,林冠截留量减小,穿透雨量增大,树干茎流量增大.林冠截留量与郁闭度表现出正相关关系,而穿透雨量、树干茎流量都与郁闭度表现出负相关关系.(4)各郁闭度林冠截留量、穿透雨量和树干茎流量的月动态变化与总降水量的月变化基本一致.     

A preliminary assessment of rain throughfall beneath Portulacaria afra canopy in subtropical thicket and its implications for soil carbon stocks

Subtropical thicket dominated by the leaf- and stem-succulent tree Potulacaria afra (spekboom) accumulates extraordinarily high amounts of soil organic carbon for a semi-arid ecosystem. This has been attributed to high leaf litter production of canopy trees — especially spekboom — and the relatively cool beneath-canopy temperatures, which reduces mineralization of organic matter. High rainfall interception by the dense thicket canopy may also contribute to reduced mineralization of soil organic carbon stocks via a reduced Birch effect (wet-dry cycles) and overall reduced moisture of organic matter. Here we provide preliminary data on one easily measurable component of canopy interception, namely throughfall, which is the amount of rain falling on the canopy that is not intercepted by the canopy or diverted as stemflow. For 23 rainfall events > 1 mm in the period Jan-Jun 2006, we measured 275 ± 21 mm gross rainfall at three spekboom thicket sites. Mean throughfall was 56.4% across all events and ≤ 20% for small (≤ 5 mm) events. These values are the lowest recorded in the literature. Throughfall in tropical and temperate forests ranges from 70 to 90%, Mediterranean woodland trees 70-80%, and Savanna trees 75-84%. The low rates of throughfall recorded in this study support the hypothesis that the extreme accumulation of soil organic carbon in thicket soils is partly due to interception of rainfall and concomitant constraints on soil microbial activity. The extraordinary high interception of rainfall by the spekboom thicket canopy warrants further research at the ecosystem level.     

川西亚高山典型森林生态系统截留水文效应

截留是水文循环的一个重要过程,水文功能是森林生态系统功能的重要方面,林冠和枯落物截留实现对大气降水的二次分配过程.为深入认识生态系统截留的水文效应,采用野外观测和人工降雨模拟试验相结合的方法,研究了2008年和2009年5-10月贡嘎山亚高山峨眉冷杉中龄林、峨眉冷杉成熟林和针阔混交林的冠层枯落物截留能力.结果表明,峨眉冷杉中龄林2008年林冠截留率为20.9％,针阔混交林2008年和2009年林冠截留率分别为23.0％和23.6％,林冠截留率的年际间变化不大,林冠截留主要受到降雨特征影响.3种林型枯落物饱和持水能力分别为5.1、5.1和5.7 mm,显著高于林冠的饱和持水能力,但由于冠层的截留蒸发速率较高,林冠截留蒸发仍是生态系统截留蒸发的主要组成部分.     

秦岭天然次生油松林冠层降雨再分配特征及延滞效应

为了研究秦岭典型地带性植物油松林冠层降雨再分配特征及延滞效应,选择陕西宁陕县秦岭森林生态系统国家野外科学观测研究站55龄天然次生油松林,从2006-2008年(5-10月份)对林外降水、穿透降雨和树干茎流进行定位观测。利用其中100次实测数据进行分析研究,结果表明:总降雨量为1576.4 mm,穿透降雨量为982.9 mm,树干茎流量为69.5 mm,冠层截留量为524. 0mm,分别占总降雨量的62.4%、4.4%和33.2%。降雨分配与降雨量级密切相关,降雨量级增大,穿透降雨率和茎流率呈增大趋势,截留率呈降低趋势,变化幅度分别为46.6%-68.9%、0.8%-9.2%、53.4%-22.0%。穿透降雨量、树干茎流量和林冠截留量与林外降雨量之间的关系分别为:TF=0.6548P-0.4937,R²＝0.9596;SF=-0.2796+0.0452P+0.0005P²,R²＝0.8179;I=0.5958P^0.8175,R²＝0.8064。降雨事件发生后,穿透降雨和树干茎流出现的时间与降雨发生的时间并不同步,均表现出一定的延滞性,随着降雨量级增大,滞后时间表现出逐渐缩短的趋势((78.5±8.8)-(16.0±0.0) min,(111.0±33.0)-(41.2±0.0) min)。降雨终止时,特别是当降雨量>10.0 mm,穿透降雨终止时间也存在一定的延滞性((3.2±2.6)-(12.0±0.0) min)。但树干茎流终止时间先于降雨终止时间,降雨量级越小,树干茎流终止时间愈早((-58.3±21.5)-(-9.8±0.0) min)。     

两种温带落叶阔叶林降雨再分配格局及其影响因子

林冠对降雨的再分配是森林生态系统的重要生态水文过程,影响着生态系统生物地球化学循环。于2012年5—10月连续测定帽儿山森林生态站的两种温带天然次生林型——蒙古栎林和杂木林的林外降雨、穿透雨及树干径流,旨在量化其降雨再分配过程及其影响因素。结果表明:蒙古栎林的平均穿透雨、树干径流、林冠截留分别占同期林外降雨的76%、7%和17%;杂木林分别占85%、5%和10%。根据模型估算,当降雨量分别超过1.0mm和0.7mm时蒙古栎和杂木林开始出现穿透雨;当降雨量超过3.0mm开始出现树干径流。当降雨量超过5.6mm,树干径流体积会随着树木胸径的增加而显著增加;而当降雨量低于5.6mm则出现相反趋势。穿透雨、树干径流及林冠截留的绝对量均随降雨量的增大而显著增加,但其占降雨量的比例却表现出不同的变化趋势。穿透雨量的空间变异随降雨量和降雨强度的增大显著减小。两种林型的降雨再分配格局因受降雨量、降雨强度等降雨特征和林冠结构的影响而产生差异。     

Calibrating the radial-growth averaging method for detecting releases in old-growth forests of coastal British Columbia,Canada

Dendroecological methods that use growth releases to reconstruct the history of canopy disturbances are most useful when calibrated for specific species in specific forest types. In this study, we calibrate the radial-growth averaging method to detect growth releases of western redcedar (Thuja plicata Donn ex D. Don), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and Pacific silver fir (Abies amabilis Dougl. ex J. Forbes) following fine-scale canopy disturbances in old-growth forests of coastal British Columbia, Canada. Our goal is to establish a version of the method that will capture the full range of growth increases that occur for the study species following natural, fine-scale canopy gaps, yet will account for the effects of climatic variability on radial growth and growth increases. We used traditional dendroclimatological techniques and the radial-growth averaging method to examine how climatic and regional-scale factors contributed to radial growth and growth increases. In addition, we did a sensitivity analysis, using both ring widths and basal area increments, to explore how varying the values of three parameters of the radial-growth averaging method (threshold, moving average, and window) influenced the proportion of trees that showed a release pattern following the formation of canopy gaps of known timing of origin. Although radial growth of western redcedar and western hemlock was significantly associated with climate, percent-growth change derived from residual chronologies rarely exceeded 25%, which defined our minimum threshold for a release. For the sensitivity analysis, two general trends were common to all three species: (1) as threshold increased, the proportion of trees that showed a release pattern decreased, particularly for western redcedar and (2) a higher proportion of trees showed a release pattern using a 10-year versus a 5-year window, particularly for thresholds <100%. The greatest proportion of trees showed a release pattern using a 25% threshold, 5-year moving average, and 10-year window for both ring widths and basal area increments. Overall, a higher proportion of trees showed a release pattern using basal area increments as opposed to ring widths. Therefore, basal area increments are better suited to assess releases in these old-growth stands that have large inter- and intra-species variability in tree size. By establishing these empirically-based criteria, we have achieved the first step towards quantifying attributes of growth releases of trees in the study stands, allowing future studies to capture the variability of past disturbance events and predict changes in forest structure and composition over time.     

模拟降雨条件下玉米植株对降雨再分配过程的影响

为系统测定玉米(Zea mays)不同生长阶段的穿透雨、茎秆流和冠层截留,采用室内模拟降雨法测定了不同降雨强度、不同叶面积指数玉米冠下穿透雨和茎秆流,采用喷雾法测定了玉米不同生长阶段的冠层截留。对其进行了量化分析,并探讨了三者与玉米叶面积指数和降雨强度的关系,阐明了玉米冠下穿透雨的空间分布特征。结果表明:玉米冠下穿透雨量占冠上总降雨量比例为30.97%—94.02%,平均为63.92%;茎秆流量占降雨量比例的变化范围为5.68%—75.70%,平均为35.28%;冠层截留量在其全生育期内变化范围为0.02—0.43 mm,平均为0.16 mm,所占总降雨量比例最大仅为1%。随玉米生长,穿透雨量逐渐降低,茎秆流量和冠层截留量逐渐增加。穿透雨与茎秆流呈现此消彼长的关系,其中穿透雨率平均由93.55%降至36.23%;茎秆流率平均由5.98%增加至70.42%。降雨强度与穿透雨量和茎秆流量呈正相关关系,但是二者占总降雨量的比例与降雨强度关系不显著(P0.05)。随着玉米生长,穿透雨冠下空间分布由均匀逐渐趋向于不均匀,使降雨经过冠层后趋于向行中汇集,但在玉米生长后期,集中于行中的穿透雨量也因叶片衰败而随之降低。揭示了玉米对降雨的再分配作用特征,可为农田水分有效利用、农田生态水文过程机理和坡耕地土壤侵蚀防治提供理论依据。     

The effects of tree characteristics on rainfall interception in urban areas

Trees in urban areas have significant effects on the urban ecosystem. They can be used to improve the water cycle in urban areas by increasing evaporation and reducing runoff through rainfall interception. Street trees placed in planters on impervious areas reduce runoff by intercepting rainfall and by temporarily storing raindrops on leaves. Therefore, understanding tree canopy geometry and the effect of rainfall interception is important in urban hydrology. In this study, we assessed the effect of tree canopy morphology on rainfall interception using four major street tree species, Sophora japonica L., Ginkgo biloba L., Zelkova serrata (Thunb.) Makino, and Aesculus turbinata Blume, in Seoul, South Korea. We measured throughfall for each tree and also derived three-dimensional data of tree canopy morphology with a terrestrial laser scanner. Tree height, canopy crown width, leaf area index (LAI), leaf area density, mean leaf area, and mean leaf angle were used to determine canopy morphology. The interception rate was mostly affected by the LAI; a higher LAI tended to result in a higher interception rate. Leaf area affected the rainfall interception rate when trees had similar LAIs. These findings on individual tree canopy rainfall interception can help us to understand the importance of rainfall interception in hydrology and for ecological restoration when planning urban green spaces.

     

Productivity responses to altered rainfall patterns in a C<Subscript>4</Subscript>-dominated grassland

Rainfall variability is a key driver of ecosystem structure and function in grasslands worldwide. Changes in rainfall patterns predicted by global climate models for the central United States are expected to cause lower and increasingly variable soil water availability, which may impact net primary production and plant species composition in native Great Plains grasslands. We experimentally altered the timing and quantity of growing season rainfall inputs by lengthening inter-rainfall dry intervals by 50%, reducing rainfall quantities by 30%, or both, compared to the ambient rainfall regime in a native tallgrass prairie ecosystem in northeastern Kansas. Over three growing seasons, increased rainfall variability caused by altered rainfall timing with no change in total rainfall quantity led to lower and more variable soil water content (0–30 cm depth), a ~10% reduction in aboveground net primary productivity (ANPP), increased root to shoot ratios, and greater canopy photon flux density at 30 cm above the soil surface. Lower total ANPP primarily resulted from reduced growth, biomass and flowering of subdominant warm-season C₄ grasses while productivity of the dominant C₄ grass Andropogon gerardii was relatively unresponsive. In general, vegetation responses to increased soil water content variability were at least equal to those caused by imposing a 30% reduction in rainfall quantity without altering the timing of rainfall inputs. Reduced ANPP most likely resulted from direct effects of soil moisture deficits on root activity, plant water status, and photosynthesis. Altered rainfall regimes are likely to be an important element of climate change scenarios in this grassland, and the nature of interactions with other climate change elements remains a significant challenge for predicting ecosystem responses to climate change.     

两种不同林分截留能力的比较研究

根据林冠对大气降雨截留的实测资料,采取同雨量下取最大值的方法近似确定了两种不同林分的截留能力与雨量的关系并进行了比较。结果表明,林冠枝叶空间分布均匀的人工林分,截留能力随雨量增加较快,不仅对〈３０ｍｍ的中小雨量表现出了较强的截留作用,而且容易满足林冠的饱和截留容量,尽管天然林对〈３０ｍｍ的雨量截留能力相对较弱,但对超过３０ｍｍ的降雨其截留能力大于人工林。林冠饱和截留容量与冠层枝叶量有关,天然林大于     

An Ecohydrological Cellular Automata Model Investigation of Juniper Tree Encroachment in a Western North American Landscape

Woody plant encroachment over the past 140 years has substantially changed grasslands in western North American. We studied encroachment of western juniper (Juniperus occidentalis var. occidentalis) into a previously mixed shrub–grassland site in central Oregon (USA) using a modified version of Cellular Automata Tree–Grass–Shrub Simulator (CATGraSS) ecohydrological model. We developed simple algorithms to simulate three encroachment factors (grazing, fire frequency reduction, and seed dispersal by herbivores) in CATGraSS. Local ecohydrological dynamics represented by the model were first evaluated using satellite-derived leaf area index and measured evapotranspiration data. Reconstructed pre-encroachment vegetation cover percentages and the National Land Cover Database (NLCD 2006) vegetation map were used to estimate parameters for encroachment factors to represent juniper encroachment in CATGraSS. Model sensitivity experiments examined the influence of each encroachment factor and their combinations on trajectories of modeled percent cover of each plant functional type and emergent spatial vegetation patterns in the modeled domain. Simulation results identified grazing as the key factor leading to juniper encroachment, by reducing shrub and grass cover and promoting the formation of juniper tree clusters. Reduced fire frequency and increased seed dispersal by grazers further amplified juniper encroachment into grassland patches between clusters of juniper trees. Each encroachment factor showed different consequences on modeled vegetation patterns. Time series of modeled plant cover and spatial patterns of plant functional types were found to be consistent with an existing conceptual model described in the literature. The proposed model provides a tool that can be used to improve our understanding of the drivers and processes of woody plant encroachment and vegetation response to global change.     

荒漠草原中间锦鸡儿冠层截留特征

以荒漠草原人工中间锦鸡儿（Caragana intermedia）为研究对象,利用2016-2017年监测获取的26次降雨事件,对比分析了两组灌丛（自然组和人工组）的冠层截留特征。结果表明：（1）试验期间共观测到有效降雨33次,总降雨量为251 mm,次平均降雨量和降雨强度分别为7.6 mm和1.14 mm/h,以雨量<2 mm,雨强<1 mm/h和降雨历时2-5 h的降雨出现次数最多;（2）自然组和人工组中间锦鸡儿平均截留量分别为1.11 mm和0.72 mm,平均截留率分别为24.81%和15.95%,两组灌丛截留存在极显著差异（P < 0.01）;（3）在雨量级>15 mm时,自然组（4.57%,CV=73.38%）和人工组（5.25%,CV=51.96%）平均截留率变异性相差最大;（4）自然组和人工组截留量与降雨量,降雨历时和降雨强度之间的关系可以用幂函数描述,截留率与三者的关系均用指数函数描述较好。在降雨特征相同的情况下,灌丛形态特征是影响中间锦鸡儿冠层截留的关键因素。     

应用修正的Gash解析模型对岷江上游亚高山川滇 高山栎林林冠截留的模拟

修正的Gash解析模型具有较好的物理基础,是目前估算和预测林冠截留的有效工具。基于2007年6月到9月的降雨数据、气象和林分结构资料,采用修正的Gash解析模型对岷江上游川滇高山栎林冠截留进行了模拟。结果表明,林冠持水能力为0.23mm,树干持水能力为0.041mm,树干茎流系数为0.013,使林冠饱和的降雨为0.3mm;研究期间实测的林冠截留量为80.2mm,占总降雨量的16.5%,应用模型模拟的林冠截留总量为81.8mm,高于实测值1.6mm(高于实测值2.0%),林冠截留和树干茎流周累积量模拟值均与实测值有很好的一致性,敏感性分析结果显示,应用修正的Gash解析模型进行岷江上游川滇高山栎林冠截留模拟研究中,模型最易受林冠持水能力影响,其次为郁闭度、树干持水能力,影响最小的林分参数为树干茎流系数。     

中亚热带红壤区油桐(Vernicia fordii)林冠水文效应特征

于从2004年7月到2005年9月,对南方红壤区油桐人工林的穿透雨、树干流和林冠截留的水文特征进行了监测,并对其影响因素进行初步分析,结果表明：在整个测定期间,油桐林穿透雨占总降雨量75.6%±8.6%,树干流占3.6%±1.1%,而截留量占20.8%±9.1%。油桐林冠对降雨的再分配受到降雨量和降雨强度的影响,随着雨量的增加,穿透雨、树干流和截留量相应地提高,并且树干流和截留量在高的雨量下逐渐趋于稳定;随着降雨强度的增加,穿透雨率逐渐升高,而树干流率和截留率降低。在不同雨量级间,油桐穿透雨具有显著性的差异,但树干流的差异不显著。油桐林下水分输入存在明显的空间异质性,穿透雨在不同观测点间具有显著性差异,靠近树干的林冠内部穿透雨低于林冠边缘,而且随着降雨量或降雨强度的增加,穿透雨的空间异质性(穿透雨率的变异系数CV)降低;树干流对降雨也具有明显的汇集作用,在树干周围输入的雨量是林外降雨量20~70倍,并且随着降雨量的增加,这种汇集效应（漏斗比率）先提高后降低。同时油桐单株树干流(cm³•mm^-1)与胸径、树高和冠层面积均呈显著正相关(p<0.05),但是与枝下高的相关性不显著(p>0.05)。