Rocky Substrate and the Lower Treeline Ecotone of Yellowstone's Northern Range

Lower treeline ecotones are influenced by several processes, including fire, grazing, and climatic variables, but rocky substrate is an under-recognized factor affecting their patterns and dynamics. On the Northern Range of Yellowstone National Park, we hypothesized that north-facing slopes and riparian zones would explain the distribution of most forested areas, but that rocky substrate would explain a substantial additional portion of the lower treeline, and that advancement of conifers into grassland has occurred along rocky substrate. We used GIS software and change detection analysis to assess the spatial distribution and change of the forest with respect to north-facing slopes, riparian areas, and rocky substrate. As in other lower treeline studies, we found that north-facing slopes and riparian zones contained the majority of forested land (63%). However, areas with rocky substrate accounted for more forested area than north-facing slopes and riparian zones combined (67.4%), including 28% of forest that was not explained by the first two variables. Change detection analysis between 1954 and 2006 revealed that conifer advancement was insignificant but the number of forested patches increased and the mean size of patches decreased. These results suggest a more fragmented lower treeline environment that may indicate forest advancement in small patches. Field surveys revealed that rocky substrate did not provide conifers with protection from browsing ungulates or fire, yet conifers preferentially established on rocky areas. Our landscape analysis suggests that weathered rock increases soil coarseness in a landscape dominated by fine glacial till, which improves conditions for seedling establishment by reducing competition with grasses and shrubs sufficiently to overcome the negative influence of dry, nutrient-poor microsites in a moisture-stressed environment.     

高山树线交错带的景观格局与生态过程

由于对气候变化的敏感性, 近年来高山树线交错带成为全球变化研究中的热点问题。高 山树线交错带所指示的是一个复杂生态系统的特征, 反映出了由低处植被向高山植被的转化, 其 景观格局是综合了种子定居, 树木生长以及树木繁殖等多个树木生态过程和特定高山环境下的 地理特征格局的适应结果。一方面, 树木通过自身生理生态学上的调整对高山极端环境进行适 应。一方面, 高山特定地理环境特征对树线的景观格局进行影响, 因而高山树线交错带成为研究 景观格局- 生态过程相互关系的一个重要载体。通过运用3S 技术, 可以将高山树线交错带研究中 的树线景观结构和生理生态过程结合在一起, 并验证在其景观格局形成过程中地理特征和人为 因子的相对重要性。     

Geomorphic Controls of Spatial Pattern and Process at Alpine Treeline*

This study examines the role of surface geomorphic features in tree establishment at the alpine treeline in Glacier National Park, Montana, through the presentation of a multiscale, conceptual model of biogeomorphic relationships at alpine treeline. Empirical observations gathered through a multiscale field methodology over three summers serve as a base for the model. The model highlights the importance of surface geomorphic features, specifically boulders and terrace risers, in creating favorable local site conditions, largely by protecting seedlings from wind. The sheltering effect of surface features enables initial seedling establishment, and in some cases survival, above current treeline locations, thereby initiating a positive feedback effect that encourages subsequent tree establishment. Geomorphic features are therefore important in linking scales of pattern and process at the alpine treeline ecotone.     

Feedback-Driven Response to Multidecadal Climatic Variability at an Alpine Treeline

The Pacific Decadal Oscillation (PDO) has significant climatological and ecological effects in northwestern North America. Its possible effects and their modification by feedbacks are examined in the forest-tundra ecotone in Glacier National Park, Montana, USA. Tree ring samples were collected to estimate establishment dates in 10 quadrats. Age-diameter regressions were used to estimate the ages of uncored trees. The temporal pattern of establishment and survival was compared to the pattern of the PDO. A wave of establishment began in the mid-1940s, rose to a peak rate in the mid-1970s, and dropped precipitously beginning ca. 1980 to near zero for the 1990s. The period of establishment primarily coincided with the negative phase of the PDO, but the establishment and survival pattern is not correlated with the PDO index. The pattern indicates a period during which establishment was possible and was augmented by positive feedback from surviving trees. Snow may be the most important factor in the feedback, but studies indicate that its effects vary locally. Spatially differentiated analyses of decadal or longer periodicity may elucidate responses to climatic variation.     

Landform Influences on Treeline Patchiness and Dynamics in a Changing Climate

Local landforms and microtopography control site conditions, spatial patterns, and dynamics in treeline landscapes. Several topography-related treeline types are presented and their responses to a warming climate discussed. On rugged mountain terrain, pronounced changes in vertical range and variety of treeline landscapes will not take place as long as debris slides and avalanches occur regularly. On intensively eroded steep terrain, trees will mainly colonize convexities. On trough shoulders and similar gentle topography with irregular mosaics of convex and concave landforms, tree establishment is most likely to take place on convex topography. On gentle slopes and rolling uplands, where the proportion of wind-swept terrain is comparatively large, microsite facilitation appears to be a precondition for tree establishment. At higher elevations, the relative importance of shelter-providing landforms will increase due to windier conditions. At the beginning of treeline rise, seedling establishment, growth, and survival are closely related to local landforms and microtopography and their effects on site conditions. Later, the feedback from trees on their environment may overrule the effects of landforms.     

The Role of Thresholds and Fine-Scale Processes in Driving Upper Treeline Dynamics in the Bighorn Mountains,Wyoming

This research investigates whether abrupt changes indicative of a threshold response occurred in the spatial and temporal patterns of tree establishment within upper treeline ecotones in the Bighorn Mountains of north-central Wyoming, with a particular focus on assessing whether treeline advance occurred. Dendroecological techniques were used to reconstruct the spatiotemporal patterns of tree establishment at multiple spatial scales (local and landscape). Increases in the elevational extent of treeline and tree density above timberline were reconstructed for each decade of the 20^th century. Regime-shift analysis was used to detect threshold changes in tree establishment. Marked increases in tree establishment and density occurred at local and landscape scales during the 20th century, particularly during the post-1970 period and in areas above timberline. Varying degrees of treeline advance occurred during this time, yet establishment upslope from timberline was often contingent on the availability of sheltered microsites in the lee of boulders. This highlights the importance of both broad-scale climate inputs and fine-scale site conditions in governing the rate and pattern of tree establishment in high-elevation treeline environments. Overall, the abrupt changes in tree establishment over time and space since 1970 suggest that bioclimatic thresholds can strongly influence ecotonal dynamics at upper treeline.     

Precipitation and Temperature Estimation Error at Alpine Treeline Ecotones Using the Mountain Climate Simulator Model (MT-CLIM)

In order to improve modeling of alpine treeline responses to climate change, estimations of snowfall at treeline sites are needed. The MT-CLIM climate model was evaluated for this purpose by extrapolating precipitation and temperature from standard weather stations at lower elevations to 30 alpine SNOTEL study sites across the western United States. Quantification of the topography between the base stations and the SNOTEL sites was used in inverse distance weighting and compared to straight-line weighting. The predicted temperature and precipitation under different weighting methods were compared to observed data over three months during the winter of 2006-2007. The errors were mapped and their spatial pattern analyzed. Error patterns indicate strong gradients, particularly in the Pacific Northwest, that are suggestive of areas where additional characteristics of atmosphere-land interactions and boundary layer climatology need to be considered in modeling applications.     

高寒草甸氮素循环研究进展

高寒草甸氮素循环过程中的物理过程、化学过程和生物过程与土壤、植物的发生、发育紧密联系，因此对物理、化学、生物三个不同过程高寒草甸氮素循环的研究现状进行了分析。     

Mapping Ecotone Movements: Holocene Dynamics of the Forest Tension Zone in Central Lower Michigan,Usa

This paper presents a novel method for detecting and mapping movements of ecotone boundaries, over both time and space, using existing fossil pollen data. The method combines existing statistical techniques, including discriminant analysis and spatial interpolation, to generate statistically robust maps of ecotone boundaries over time. This method was applied to provide a comprehensive vegetative history of the forest tension zone in the Lower Peninsula of Michigan during the last 10,000 years. Pollen data for nine lakes in the Lower Peninsula were analyzed with discriminant analysis and spatially interpolated with inverse distance weighted to track ecotone movements of the forest tension zone. The forest tension zone has been in existence over the last 10,000 years and has been highly variable over space and time, with at least three major latitudinal shifts, spanning 320 km and 6° of latitude, as well as several less pronounced shifts. These shifts occurred primarily in response to millennial-scale oscillations in climate such as the warmer and dryer conditions associated with the mid-Holocene. A slighter shift was evident in association with the cooler and drier conditions of the Little Ice Age (600-100 cal yr BP). This research demonstrates the sensitivity of Great Lakes forest communities to Holocene climate change and suggests a similar sensitivity of future ecotone dynamics in response to modern global warming.     

大兴安岭北部大白山高山林线动态与气候变化的关系

高山林线植被对气候变化十分敏感,已成为全球变化研究的热点.研究了大兴安岭北部大白山高山林线的树木生长和群落更新动态及其与气候变化、火干扰等因素的关系.结果表明,林线树木的生长对气候变化十分敏感,但其敏感性随着海拔的降低而减弱;在高海拔,林线树木的径向生长与上年生长季后期(8月)降水负相关,而与上年初秋(9月)温度正相关,这限制作用随着海拔的降低而逐渐消失;与此相反,低海拔树木生长与当年冬末春初(3月)的温度负相关,但随着海拔上升这种限制作用消失.分析结果还表明,本地区的群落更新主要受火干扰驱动,而与气候变化没有显著关系.不同树种在火灾后更新的时间存在差异,这种差异又因海拔的不同而异,反映出不同树种的更新策略及对环境变化适应能力的差异.大兴安岭北部的高山林线在树木生长对气候变化的敏感性、与气候因子的关系及群落更新动态等方面均与干旱区林线有明显的差异,这些差异与气候条件、树种、更新驱动力等方面的不同有密切的关系.     

A Critical Appraisal of the Global Warming Debate

Major changes are occurring in the global warming debate. Popular alarmist views are giving way to more balanced assessments of the situation. There is now greater emphasis on the lack of consensus among climatologists on fundamental scientific issues related to climate change.     

绿洲边缘人工固沙植被自组织过程

在对植被自组织过程研究进展评述的基础上,研究了河西走廊荒漠绿洲边缘人工固沙植被格局40年来自组织及种群自疏过程,发现在干旱区,特别是降水量200mm以下的荒漠区,绿洲边缘雨养或者降水和地下水共同维系的沙丘人工植被也会发生自组织过程,原来基本均匀栽植的植被出现斑块化,最大斑块面积、斑块密度、斑块聚集度在20~30a出现,但种群自疏过程并未遵循随植被发育年龄变化的规律,而与生境与绿洲的距离及生境地下水埋深显著相关。未来应加强在气候变化和人类干扰条件下植被格局发生突变的阈值范围的研究,也应加强干旱区人工植被自组织过程及其变化等的研究。     

农牧交错区土地利用动态与生态效应变化--以榆阳区为例

以榆阳区1988-2003年土地数据为依据,在分析研究区土地利用的数量和结构变化特点的基础上,运用国内外学者在生态价值估算的方法与模式,结合该区的具体情况对估算方法进行修正,然后对研究区研究时段内各年度的生态系统服务功能经济价值进行了估算,并以1988年为基准测算了土地利用类型的数量和结构变化所引起的生态系统服务功能经济价值变化情况,分析了其变化的时段特点及变化原因,定量地揭示了在生态脆弱区环境与经济发展的关系。研究表明：1988-2003年,榆阳区土地利用变化所产生的环境响应总体米说是积极的,生态系统服务功能经济价值年度总量在波动中提高;营造防护林、退耕还林及农用地内部结构调整政策的实施实现了经济和环境双赢的预期效果。     

长白山北坡林线长白落叶松与岳桦对环境适应的对比分析

以长白山北坡林线长白落叶松（Larix olgensis）和岳桦（Betula ermanii）为研究对象,通过野外样方调查、树轮技术分析,采用趋势线模型拟和、单因素方差等统计方法从各种参数分析长白落叶松和岳桦径向树高生长的对比关系并揭示2个种群动态对环境适应性的对比差异。结果显示：垫状生长形的长白落叶松海拔最高（均值：2 017 m）,其次为旗形树（2 005 m）,正常形态的海拔最低（1 992 m）,所在生境差异显著（p < 0.05）。正常形态的长白落叶松生长速度大于岳桦（树高方程斜率：长白落叶松0.750 9>岳桦0.253 6）,随着旗形树的出现,速度相似（斜率：长白落叶松0.375 4>岳桦0.253 6）,变成垫状后,岳桦占优。长白落叶松受到压制种群扩张速度不如岳桦,长白山林线上侵表现为以岳桦种群为主力,长白落叶松仍是岳桦主要的伴生树种。     

未来气候变暖情形下青藏高原多年冻土分布初探

基于未来温室气体中等排放情景下气候模式给出的气候预测结果的高分辨率降尺度分析结果,运用两种方法(年均温法和高程模型法)模拟了1980-1999,2030-2049和2080-2099年3个时段青藏高原多年冻土分布.结果表明,以年均地温-1℃作为多年冻土划分依据的年均温法模拟的目前(1980-1999年)高原多年冻土面积为127.99万km2,与世界数据中心给出的青藏高原现代多年冻土面积为129.12万km2的估算接近(误差率仅为0.86%);到本世纪中期(2030-2049年),高原多年冻土面积减少为87.26万km2,退化率达到31.82%;而到本世纪末(2080-2099年),高原多年冻土面积只有69.25万km2,较目前将退化45.89%.不同高度带的对比分析还发现,与高原及其邻近地区年均气温的升高一般随海拔高度而增加的趋势相反,未来高原多年冻土的退化率将随着海拔高度增加而降低.在全球变暖过程中的冻土退化,特别是高原东南部冻土向西北部的逐步退缩,对高原冻土区工程稳定性的影响应引起我们的足够重视.     

高寒金露梅灌丛草甸植被反射率特征

根据祁连山海北高寒金露梅灌丛草甸2004年观测的太阳总辐射和反射辐射资料,分析了高寒金露梅灌丛草甸植被反射率（A）的变化特征。结果表明,金露梅灌丛草甸A在日间或一年间均表现出＂U＂型变化规律,晴天、昙天和阴云天气状况下的早晚略有差异,中午前后的A值三者无明显差异。季节不同下垫面性质不同的影响,A的分布不同。海北高寒金露梅灌丛草甸A的月平均在植物生长季内5～9月为0.15,植物非生长季的10月～翌年4月为0.20,年平均A为0.18。     

沙漠绿洲过渡带典型下垫面风速脉动特征——以塔克拉玛干沙漠南缘策勒绿洲为例

以塔克拉玛干沙漠南缘策勒绿洲西部沙漠-绿洲过渡带为研究区,选择植被盖度 < 5%裸沙样地、植被盖度10%～15%柽柳样地和植被盖度20%～25%骆驼刺样地,利用可移动梯度风测量系统,获取了系统性天气过程中距离地表20 cm、40 cm、60 cm、100 cm和200 cm高度的风速及200 cm高度的风向系列数据,对3个样地平均风速、风速脉动和风向脉动进行分析和研究。结果表明:(1)植被的存在大幅度消弱了平均风速,随着植物密度的增加平均风速逐渐降低,风速廓线显示平均风速发生明显降低的转折高度与植被冠层的平均高度相对应。(2)3个样地脉动风速均近似服从高斯分布,可见植被的存在并未改变风速脉动分布规律。各样地风速脉动强度均随指示风速的增大而增大,随距离地表高度的降低而下降。风速脉动相对值总体上呈现随植物密度增大而增大的趋势,植物群落结构的差异增加了相对风速脉动垂直分布的复杂性。(3)风向脉动幅度以裸沙样地最小、柽柳样地次之、骆驼刺样地最大,即随植物密度增加而增大。同一样地内风向脉动值不随指示风速的变化而发生明显变化。     

Landscape Analysis of the Forest-Tundra Ecotone in Rocky Mountain National Park,Colorado*

Landscapes in the ecotone between forest and tundra contain a mosaic of patches of trees, meadows, lakes, disturbed areas, and other features. The structure of this mosaic affects species habitat and potential ecotone response to global change. However, the alpine forest-tundra ecotone may be insensitive to climatic change if it is a climatic relict or is frequently disturbed. We used GIS and multivariate statistics to (1) analyze landscape structure in transects across the ecotone in Rocky Mountain National Park, (2) identify the major variants of forest-tundra ecotone, and (3) identify the influence of the environment and natural disturbances on variation in the landscape structure of the ecotone. There are six major types of ecotone varying in the amount of natural disturbances, permanent features (e.g., lakes), closed forest, patch forest, and krummholz. Variation is primarily related to slope, elevation, aspect, and geology associated with the morphology of the mountains and the disturbances they produce. The ecotone is not strongly structured by natural disturbances; thus, it may be more strongly controlled by and sensitive to climatic change than in areas where disturbance is more prevalent. Monitoring of potential ecotone response to global change is feasible, if tailored to the types of ecotone and their expected response.     

滇西北生态脆弱区土地利用变化及其生态效应 ——以云南省永胜县为例

土地利用/土地覆被变化及其生态环境效应研究是当前地理学全球变化研究的核心内容,而生态系统服务功能则是生态学、生态经济学研究的热点问题。作者利用滇西北生态脆弱区--永胜县1988年、1994年和1999年的TM遥感影像,采用景观生态学的空间格局指数分析研究区近10年来的土地利用动态变化特征,并以生态系统服务功能衡量相应土地利用类型的相对生态价值,评价区域土地利用/土地覆被变化的生态效应,尝试将二者有机结合起来。研究表明,全县景观总体构成表现为景观破碎度较高,斑块数目较多;林地和未利用地一直占很大比重,林地的不断增加与未利用地的迅速减少是研究时段内土地利用变化的突出特征;研究时段内全县土地利用变化带来了较好的生态效应,土地利用生态效益持续提高,并表现出了较大的空间差异。     

全球暖化问题的反思

阐述了近年来出现的破纪录高温现象,列举了全球暖化的实例证据;指出了影响全球气温变化的重要事件;从温室效应方面讨论地表温度变化和全球暖化问题;从南太平洋海平面的变化来证明全球暖化是否存在的问题;从热岛效应出发论证了全球暖化与土地利用间的关系;用实例和历史数据讨论有关全球暖化的6个威胁问题.提出了几个值得深思的问题,认为很难令人相信全球暖化有愈来愈烈的趋势.全球暖化是否存在?目前的资料并不充分.全球暖化与热岛效应的影响有着密切的关系,气候变迁的灾难说法尚有许多疑义.