长期生态研究中的若干重要问题及趋势

长期生态研究（LTER）是对生态过程进行长期的监测,研究各种生态因子的相互作用及生态过程,从而揭示出生态系统和环境的长期变化。为生态系统评价管理提供科学依据。本文探讨了长期生态研究的核心问题及其时空尺度,揭示出长期生态所注重的问题及其对策,并以国际长期生态研究计划为例论述了生态研究所关注的内容并提出了研究的发展趋势,这对中国的长期生态研究及陆地生态系统评价有着积极的指导作用。     

Don’t miss the forest for the trees! Evidence for vertical differences in the response of plant diversity to disturbance in a tropical rain forest

Ecological studies in tropical rain forests traditionally focus on trees above a threshold diameter at breast height (dbh), since ignoring plant species of the other structural compartments is believed to be an acceptable tradeoff between exhaustiveness and effectiveness. However, the consequences of missing species below a threshold dbh value have been largely neglected so far. We evaluated whether the response of species diversity of ≥10-cm dbh trees was similar to the response of other structural ensembles (namely treelets, saplings, and terricolous herbs) in a lowland tropical rain forest, to three disturbance regimes: natural gap dynamics (control), and selective logging with and without additional thinning. We studied forest vegetation composition and diversity in a 20-yr replicated field experiment comprising nine 1 ha permanent plots established in a semi-deciduous rain forest of the Congo Basin and equally distributed among the three treatments. Once corrected by stem density, species richness was similar between logged (20 years since logging) and untouched old-growth forest stands with respect of trees, but higher with respect of treelets. As disturbance intensity increased, species richness increased within sapling layers but decreased within herb layers, while species spatial turnover (beta diversity) increased in both cases. Regarding the parameters of the partitioned rarefaction curves and relative abundance distribution curves, no correlation was found between trees and any of the other structural compartments. Whilst tree and treelet species composition was similar among treatments, the understories still reflected past disturbance intensity, with a strong response of the sapling and herb layers. These results show that ecological studies based solely on tree layers (dbh ≥ 10 cm) are misleading because their response to disturbance cannot be used as a surrogate for the response of other structural ensembles. Long-lasting effects of anthropogenic disturbance on the sapling bank and the herb layer may durably influence the long-term forest dynamics. Since overstory but not understory plant communities have recovered from human disturbances 20 years after silvicultural operations, African tropical rain forest ecosystems may not be as resilient to selective logging as previously thought.     

Global environmental changes in terrestrial ecosystems. International issues and strategic solutions: introduction

Human activities are having major impact on biogeochemical cycles and ecosystems worldwide. Rapid urbanization and changes in rural populations are affecting ecosystems in often-drastic ways. Ecologists are using long term monitoring and experimental studies to understand and to help mitigate the effects of these changes. The collection of 15 papers in this special volume “global changes in terrestrial ecosystems” describes some of these studies, particularly concentrating on effects on terrestrial ecosystems and landscapes, and demonstrating the importance of understanding the problems at both the multi-scale and international levels.     

The UK Environmental Change Network after twenty years of integrated ecosystem assessment: Key findings and future perspectives

The UK Environmental Change Network (ECN), the UK's Long-Term Ecosystem Research (LTER) network, has now been operating for over twenty years. It was established in 1992 as a set of terrestrial sites at which sustained observations relevant to a range of ecological indicators and environmental parameters could be made. An additional ECN freshwater network was launched in 1994. In this paper we provide a brief history of the network, and describe its current structure and role within a complementary wider range of UK environmental monitoring and observation programmes that are either more focussed on specific parameters or habitats, or operate at different temporal and spatial scales. We then provide a review of the other papers within this Special Issue, which exemplifies the broad range of environmental concerns that ECN data and sites are helping to address. These include network-wide summaries of environmental and biological trends over the first two decades of monitoring, more site-specific assessment of the ecological impacts of local pressures resulting from changes in management, biological and ecosystem service indicator development, and the testing of new monitoring technologies. We go on to consider: (i) future directions of network development and adaptation in light of recently emerging environmental concerns, dwindling financial resources and the consequent need for greater efficiency; (ii) the desire for tighter integration with other monitoring and observation programmes both nationally internationally; (iii) opportunities raised by recent technological developments; and (iv) the need to process and make available data more rapidly to increase the capacity of ECN sites as early warning systems. In its first two decades of operation the ECN has accumulated a robust set of baseline data that describe environmental and biological variability across a range of habitats in unprecedented detail. With appropriate, informed development, these should prove invaluable in discerning the causes and consequences of environmental change for decades to come.     

Are forest‐shrubland mosaics of the Cape Floristic Region an example of alternate stable states?

The idea of alternate stable states (ASS) has been used to explain the juxtaposition of distinct vegetation types within the same climate regime. ASS may explain the co‐existence of relatively inflammable closed‐canopy Afrotemperate forest patches (‘Forest’) within fire‐prone open‐canopy Fynbos in the Cape Floristic Region (CFR) on sandstone‐derived soils. We evaluated the hypothesis that although fire and local topography and hydrology likely determined the paleogeographic boundaries of Forest, present‐day boundaries are additionally imposed by emergent edaphic properties and disturbance histories. We studied vegetation and edaphic properties of Forest‐Transition‐Fynbos vegetation at two sites within the CFR on sandstone‐derived soils and tracked historical change using aerial photography. Whereas Forest and Fynbos have changed little in extent or density since 1945, transition vegetation increased into areas formerly occupied by Fynbos. Forest soils were ubiquitously more nutrient‐rich than Fynbos soils, with transition soils being intermediate. These edaphic differences are not due to geological differences, but instead appear to have emerged as a consequence of different nutrient cycling within the different ecosystems. Soil nutrients are now so different that a switch from Fynbos to Forest is unlikely, in the short term (i.e. decades). Floristically and nutritionally, transitional vegetation is more similar to Fynbos than Forest and may be less resilient to changes in exogenous drivers (e.g. fire). Our findings are consistent with the idea that geologically Forest and Fynbos are largely fire‐derived long‐term ASS, with the stability of each state reinforced by marked soil nutrient differences. In contrast, the intermediate transitional vegetation that might switch states is unlikely to be stable.     

Integrating Social Science into the Long-Term Ecological Research (LTER) Network: Social Dimensions of Ecological Change and Ecological Dimensions of Social Change

The integration of the social sciences into long-term ecological research is an urgent priority. To address this need, a group of social, earth, and life scientists associated with the National Science Foundations (NSF) Long-Term Ecological Research (LTER) Network have articulated a conceptual framework for understanding the human dimensions of ecological change for the LTER Network. This framework explicitly advocates that what is often divided into natural and human systems be considered a single, complex social-ecological system (SES). In this paper, we propose a list of core social science research areas, concepts, and questions; identify the need for multiscale investigatory frameworks crucial for implementing integrated research; and suggest practical approaches for integration. In sum, this paper is a general outline for empirical and cross-site research projects where investigators agree that bringing together social, biological, and earth scientists can lead to synthetic approaches and a unified understanding of the mechanisms regulating SES. Although the motivation for this goal is specific to the LTER Network and similar projects, we believe that the issues and ideas presented here are widely applicable to other interdisciplinary SES studies.     

Relationship between grass species richness and ecosystem stability in Kruger National Park, South Africa

The relationship between grass species richness and ecosystem stability was investigated in the Kruger National Park. A total of 135 489 individual grasses were identified from 189 sites spread across 19 485 km² of savanna biome, making this one of the largest studies of its kind. At each site, grass percentage abundance and standing crop were measured at one year intervals to provide an index of ecosystem function. A severe drought that affected the region between 1991 and 1993 provided a convenient perturbation. At the height of the drought, mean species richness declined by 12.7% while standing crop declined by 38.1%, from 3199 to 1979 kg ha^?1. Percentage grass abundance declined to 87.5% of its pre‐drought value. After the drought had passed species richness, standing crop and percentage abundance recovered to 92.1%, 113.8% and 92.8% of their pre‐perturbation values, respectively. Statistical analysis of these responses revealed that grass assemblages of low species richness were more resistant to drought than assemblages of high species richness. Species‐poor sites also showed better recovery from perturbation after the drought had passed. These findings suggest that ecosystem stability may be negatively related to grass species richness in South African savanna grasslands.     

Development, potentials, and challenges of the International Long-Term Ecological Research (ILTER) Network

In order to review the development, the potentials, and the challenges of the International Long-Term Ecological Research (ILTER) Network, the necessity for the development of the ILTER Network, the development of ILTER activities in the world, and the body structures of ILTER Network are described. As an example for showing the potentials of ILTER activities, exemplary results of tree growth studies carried out by the author utilizing long-term daily weather data at the Hubbard Brook Experimental Forest, New Hampshire, USA are presented and future potentials are discussed. At this time, as the ILTER Network is situated at a critical point, challenges for the ILTER Network are discussed with regard to its further development. In this review, the author tries to maintain that as the long-term ecological research, whose results can secure comparability in the dimension of time and space, can provide us with a fundamental basis for sound understanding and management of ecosystems, we can publicize the activities and findings from the ILTER Network to the public, on from local to international levels, to continue further efforts around the world for our sustainable future.     

Intensive research activity alters short-term seedling dynamics in a tropical forest

Researchers can have unintentional, yet significant effects on their study systems. We tested for the effects of an intensive tree census on seedling dynamics in a 50-ha permanent forest plot on Barro Colorado Island, Panama. At the community level, and for different shade-tolerance guilds, we found no significant differences in seedling recruitment or survival inside compared to controls outside the plot. However, among growth forms, canopy trees and lianas exhibited significantly lower seedling survival inside the plot. Results suggest that intense researcher activity impacts short-term vegetation dynamics, but effects do not accumulate over time.     

Phytometers are underutilised for evaluating ecological restoration

Ecological restoration increases, but evaluation of restoration efforts is inadequate because reliable performance indicators are lacking. As plants are important actors in ecological restoration, we suggest that they be used as metres, i.e. phytometers, of restoration success. Phytometer plants are transplanted to different conditions to integrate measures of the prevailing conditions. We analysed 109 studies for the use of phytometers and especially their applicability to evaluate ecological restoration. Most studies employed single species and life-stages and focused on habitat conditions and environmental impacts. Most experiments were conducted on grasslands in moist temperate regions. Growth was the dominant response variable, in long-term studies often combined with reproductive output and plant survival. Only five studies specifically evaluated ecological restoration, implying that its potential is not yet realised. We found phytometers promising in evaluating restoration outcomes given that they are easy to measure, can provide rapid results, and serve as integrative indicators of environmental conditions with the ability of covering many aspects of plant life and ecosystem processes. To evaluate restoration success with high resolution and generality, we suggest a combination of different phytometer species, life-forms and life-stages, and experimental periods >1 year to reduce effects of transplantation and between-year variation and to account for time lags in ecological processes and changes after restoration.     

基于生态系统服务的生态安全格局构建与生态韧性评估——以长株潭城市群为例

构建生态安全格局对于保障区域生态安全、优化国土生态空间具有重要意义,科学评估生态韧性是城市高质量发展的重要议题之一。以长株潭城市群为研究对象,基于“源地识别-阻力面构建-廊道提取”的生态安全格局构建模式,定量评估固碳释氧、土壤保持和水源涵养三项生态系统服务功能以识别源地,结合自然和人为因素构建阻力面,使用Linkage Mapper工具基于最小成本路径理论识别生态廊道,提出了一套基于生态安全格局构建过程的生态韧性评估框架,并结合Fragstats和Conefor Sensinode软件对其进行综合评价。研究结果显示：（1）长株潭城市群共55块生态源地,面积为7747.75km²,占城市群总面积的27.58%,东部和南部源地较为集中,西部较分散,林地是生态源地的主要组成部分;生态廊道共113条,廊道总长度为1458.97km,平均长度为12.91km,主要分布于研究区西部和北部。（2）结合生态源地和生态廊道构建了长株潭城市群“一心一环”生态安全格局,可有效缓解长株潭城市群中部生态用地“空心化”的格局。（3）研究区55块生态源地的韧性指数介于0.1265—0.4682之间,空间上呈现出东高西低,南高北低的特征;113条生态廊道的韧性指数介于0.0027—0.8002之间,韧性指数较高的廊道主要分布在研究区东部和南部,韧性指数较低的廊道分布在西部、中部和北部。西部和中部或将成为影响长株潭城市群生态安全的关键区域,在城市规划过程中应重点关注。研究结果对优化城市空间布局、维护区域生态安全和促进区域一体化发展具有重要意义,并为区域生态韧性的评估提供了新的思路。     

Characterization of Households and its Implications for the Vegetation of Urban Ecosystems

Our understanding of the dynamics of urban ecosystems can be enhanced by examining the multidimensional social characteristics of households. To this end, we investigated the relative significance of three social theories of household structure—population, lifestyle behavior, and social stratification—to the distribution of vegetation cover in Baltimore, Maryland, USA. Our ability to assess the relative significance of these theories depended on fine-scale social and biophysical data. We distinguished among vegetation in three areas hypothesized to be differentially linked to these social theories: riparian areas, private lands, and public rights-of-way (PROWs). Using a multimodel inferential approach, we found that variation of vegetation cover in riparian areas was not explained by any of the three theories and that lifestyle behavior was the best predictor of vegetation cover on private lands. Surprisingly, lifestyle behavior was also the best predictor of vegetation cover in PROWs. The inclusion of a quadratic term for housing age significantly improved the models. Based on these research results, we question the exclusive use of income and education as the standard variables to explain variations in vegetation cover in urban ecological systems. We further suggest that the management of urban vegetation can be improved by developing environmental marketing strategies that address the underlying household motivations for and participation in local land management.