Climate differentiates forest structure across a residential macrosystem

Perspectives on socio-ecological studies in the Northern and Southern Hemispheres

Socio-ecology studies the relationships between human activities and natural systems and their importance in management and public policy. Our objective was to analyse how published papers in countries with a high Human Development Index (HDI) perform socio-ecological studies and compare them between the Northern and Southern Hemispheres. To do this, we used the Scopus platform as a source for searching and obtaining scientific papers about socio-ecological studies conducted in countries from the Northern and Southern Hemispheres. We calculated the number (n) of papers published per year and classified them using the main subject areas of the SCImago Journal & Country Rank database. Then, we analysed whether papers included specific recommendations for natural system management, nature conservation, policies or governance structures, or science in general. Besides, we studied whether the papers addressed socio-ecological studies related to flora and fauna and from what specific group of organisms or systems. Data were compared using the chi-square (χ ²) test (Pearson p < 0.005). A total of 467 papers were analysed, where 34% were from the Southern Hemisphere (mainly Argentina, Australia, Chile, and South Africa) and 66% from the Northern Hemisphere (mainly the USA, Canada, and Spain). The Northern Hemisphere (mainly North America and Europe) played a major role in the socio-ecological knowledge exchange than the Southern Hemisphere (South America and Africa). The results showed socio-ecological studies focused mainly on generating management recommendations in social and environmental science fields. The number of studies coming from the Northern Hemisphere was significantly higher than those from the Southern Hemisphere. Most of them were conducted at a local level (e.g., watersheds or human settlements) in three different systems (i) terrestrial (e.g., forests or grasslands), (ii) freshwater (e.g., rivers or streams) and (iii) marine (e.g., coastlines or seas). Most of the studies (70%) were conducted in production systems, where the majority included livestock (mainly bovine) and aquatic fisheries (e.g., salmon, artisanal coastal fishing, or trout). Most vegetation papers (65%) were on native forests. Papers on wildlife made up 30% of all animal-related studies, with mammals, birds, and marine invertebrates (such as collars) being the most extensively researched species. This work highlighted the socio-ecological approach that was used in the analysed countries with greater HDI to develop management options for natural systems.

Valuing the Role of Time in Urban Ecology

Cities and towns are complex ecosystems with features that can vary dramatically in space and time. Our knowledge of the spatial structure of urban land and ecological systems is expanding. These systems have been investigated across spatial scales, urban to rural gradients, networks of urban macrosystems, and global megalopolises. However, the temporal dimensions of urban ecosystems – such as those related to ecological cycles and historical legacies – are far less understood and investigated. Here, we outline the main dimensions of time that can shape how events in urban ecosystems unfold, which we categorize as: (i) time flows and duration, (ii) synchrony, lags, and delays, (iii) trends and transitions, (iv) cycles and hysteresis, (v) legacies and priming, (vi) temporal hotspots and hot moments, and (vii) stochastic vs. deterministic processes affecting our ability to forecast the future of cities and the species that live in them. First, we demonstrate the roles of these understudied dimensions by discussing exemplary studies. We then propose key future research directions for investigating how processes over time may regulate the structure and functioning of urban land and biodiversity, as well as its effects on and implications for urban ecology. Our analysis and conceptual framework highlights that several temporal dimensions of urban ecosystems – like those related to temporal hotspots/moments and stochastic vs. deterministic processes – are understudied. This offers important research opportunities to further urban ecology and a comprehensive research agenda valuing the “Urban Chronos” – the change of urban ecosystems through time.

Tree Transpiration and Urban Temperatures: Current Understanding, Implications, and Future Research Directions

The expansion of an urban tree canopy is a commonly proposed nature-based solution to combat excess urban heat. The influence trees have on urban climates via shading is driven by the morphological characteristics of trees, whereas tree transpiration is predominantly a physiological process dependent on environmental conditions and the built environment. The heterogeneous nature of urban landscapes, unique tree species assemblages, and land management decisions make it difficult to predict the magnitude and direction of cooling by transpiration. In the present article, we synthesize the emerging literature on the mechanistic controls on urban tree transpiration. We present a case study that illustrates the relationship between transpiration (using sap flow data) and urban temperatures. We examine the potential feedbacks among urban canopy, the built environment, and climate with a focus on extreme heat events. Finally, we present modeled data demonstrating the influence of transpiration on temperatures with shifts in canopy extent and irrigation during a heat wave.

Spatiotemporal Analysis of the Nonlinear Negative Relationship between Urbanization and Habitat Quality in Metropolitan Areas

Urbanization intensity (UI) affects habitat quality (HQ) by changing land patterns, nutrient conditions, management, etc. Therefore, there is a need for studies on the relationship between UI and HQ and quantification of separate urbanization impacts on HQ. In this study, the relationship between HQ and UI and the direct and indirect impacts of urbanization on HQ were analyzed for the Yangtze River Delta Urban Agglomeration (YRDUA) from 1995 to 2010. The results indicated that the regional relationship between HQ and UI was nonlinear and negative, with inflection points where urbanization reached 20% and 80%. Furthermore, depending on different urbanization impacts, the relationship types generally changed from a steady decrease to stable in different cities. Negative indirect impacts accelerate habitat degradation, while positive impacts partially offset habitat degradation caused by land conversion. The average offset extent was approximately 28.23%, 17.41%, 22.94%, and 16.18% in 1995, 2000, 2005, and 2010, respectively. Moreover, the dependency of urbanization impacts on human demand in different urbanization stages was also demonstrated. The increasing demand for urban land has exacerbated the threat to ecological areas, but awareness about the need to protect ecological conditions began to strengthen after the antagonistic stage of urbanization.

Effect of planting alien Robinia pseudoacacia trees on homogenization of Central European forest vegetation

Biological homogenization is a process of biodiversity loss driven by the introduction and invasion of widespread species and the extinction of specialized, endemic species. This process has accelerated in recent years due to intensive human activities. We focused our study on large areas of forest vegetation that have not yet been intensively studied. Forest management, especially the planting of alien trees, could play a key role in the homogenization process because alien trees can act as habitat 'transformers' influencing vegetation through creating different environmental conditions. Several types of native forests (hardwood floodplain forests, oak forests, and oak-hornbeam forests) have in many regions been replaced by Robinia pseudoacacia plantations. The huge diversity of native broadleaved deciduous forests in the Pannonian and Carpathian regions, with many local differences and considerable geographical variability, could be exposed to the homogenization process due to the planting of Robinia pseudoacacia. We used 282 paired plots of Robinia pseudoacacia-dominated forests and native forests with a distance of 50-250 m among them under the same environmental conditions to avoid the influence of the variability of local environmental conditions on the forest undergrowth. We found out that the replacement of native forests by plantations of Robinia pseudoacacia plays a crucial role in the homogenization process in forest vegetation by unifying microenvironmental conditions of stands and removing the geographically specified variability of plant communities from previous four classes to single one. The replacement reduced total species pool from 422 to 372 species and supported the occurrence of widespread, generalist plant species in the undergrowth.

Substantial declines in urban tree habitat predicted under climate change

Globally, local governments are increasing investment in urban greening projects. However, there is little consideration of whether the species being planted will be resilient to climate change. We assessed the distribution of climatically suitable habitat, now and in the future, for 176 tree species native to Australia, commonly planted across Australia's Significant Urban Areas (SUAs) and currently grown by commercial nurseries. Species' occurrence records were obtained from inventories and herbaria, globally and across Australia, and combined with baseline climate data (WorldClim, 1960-1990) and six climate scenarios for 2030 and 2070 using climatic suitability models (CSMs). CSMs for each species were calibrated and projected onto baseline and future scenarios. We calculated changes in the size of climatically suitable habitat for each species across each SUA, and identified urban areas that are likely to have suitable climate for either fewer or more of our study species under future climate. By 2070, climatically suitable habitat in SUAs is predicted to decline for 73% of species assessed. For 18% of these species, climatically suitable area is predicted to be more than halved, relative to their baseline extent. Generally, for urban areas in cooler regions, climatically suitable habitat is predicted to increase. By contrast, for urban areas in warmer regions, a greater proportion of tree species may lose climatically suitable habitat. Our results highlight changing patterns of urban climatic space for commonly planted species, suggesting that local governments and the horticultural industry should take a proactive approach to identify new climate-ready species for urban plantings.

Live fast, die young: Accelerated growth, mortality, and turnover in street trees

Municipalities are embracing greening initiatives as a key strategy for improving urban sustainability and combatting the environmental impacts of expansive urbanization. Many greening initiatives include goals to increase urban canopy cover through tree planting, however, our understanding of street tree ecosystem dynamics is limited and our understanding of vegetation structure and function based on intact, rural forests does not apply well to urban ecosystems. In this study, we estimate size-specific growth, mortality, and planting rates in trees under municipal control, use a box model to forecast short-term changes in street tree aboveground carbon pools under several planting and management scenarios, and compare our findings to rural, forested systems. We find accelerated rates of carbon cycling in street trees with mean diameter growth rates nearly four times faster in Boston, MA, USA (0.78 ± 0.02 cm yr-1) than in rural forest stands of MA (0.21 ± 0.02 cm yr-1) and mean mortality rates more than double rural forested rates (3.06 ± 0.25% yr-1 in street trees; 1.41 ± 0.04% yr-1 in rural trees). Despite the enhanced growth of urban trees, high mortality losses result in a net loss of street tree carbon storage over time (-0.15 ± 0.09 Mg C ha-1 yr-1). Planting initiatives alone may not be sufficient to maintain or enhance canopy cover and biomass due to the unique demographics of urban ecosystems. Initiatives to aid in the establishment and preservation of tree health are central for increasing street tree canopy cover and maintaining/increasing carbon storage in vegetation. Strategic combinations of planting and maintenance will maximize the viability of greening initiatives as an effective climate mitigation tool.

Temperature Variability Differs in Urban Agroecosystems across Two Metropolitan Regions

Climatically similar regions may experience different temperature extremes and weather patterns that warrant global comparisons of local microclimates. Urban agroecosystems are interesting sites to examine the multidimensional impacts of climate changes because they rely heavily on human intervention to maintain crop production under different and changing climate conditions. Here, we used urban community gardens across the California Central Coast metropolitan region, USA, and the Melbourne metropolitan region, Australia, to investigate how habitat-scale temperatures differ across climatically similar regions, and how people may be adapting their gardening behaviors to not only regional temperatures, but also to the local weather patterns around them. We show that, while annual means are very similar, there are strong interregional differences in temperature variability likely due to differences in the scale and scope of the temperature measurements, and regional topography. However, the plants growing within these systems are largely the same. The similarities may be due to gardeners’ capacities to adapt their gardening behaviors to reduce the adverse effects of local temperature variability on the productivity of their plot. Thus, gardens can serve as sites where people build their knowledge of local weather patterns and adaptive capacity to climate change and urban heat. Climate-focused studies in urban landscapes should consider how habitat-scale temperature variability is a background for interesting and meaningful social-ecological interactions.

Impervious surface area is a key predictor for urban plant diversity in a city undergone rapid urbanization

Urban biodiversity has increasingly been recognized by the scientific community and environmental policymakers as a part of conservation efforts worldwide. However, most studies on urban biodiversity focus on cities in developed countries. An information gap exists for urban biodiversity of cities in developing countries. Here we focused on variability in plant diversity, a major component of biodiversity, in a Chinese city that has undergone rapid urbanization in recent time. The influence of urbanization was determined by comparing plant diversity and proportion of exotic/endemic plant species with the intensity of urbanization across the study area. We used percentage of total impervious surface area (PTIA) as an indicator of urbanization intensity, ranging from 5% to 95% across the study area. In the study area, a total of 321 plant species was recorded, totaling 83 trees, 113 shrubs and 125 herbs. Plant diversity, measured by number of plant taxa and other indices, was driven by PTIA; an increase in PTIA reduced plant diversity. In addition, the ratio of exotic to endemic plant species increased as PTIA increased. Among the exotic species, most of the tree and shrub species were purposely introduced. Above 40% PTIA, plant diversity decreased sharply and the proportion of exotic species rose. As a valuable predictor of urban biodiversity, PTIA can thus be used as a key criterion for urban planning to ameliorate urban biodiversity. Further, our findings extend current understanding of urban biodiversity for cities in developing countries.

The Provision of Urban Ecosystem Services Throughout the Private-Social-Public Domain: A Conceptual Framework

As cities are largely private systems, recent investigations have assessed the provision of ecosystem services from the private realm. However, these assessments are largely based on the concept of ownership and fail to capture the complexity of service provision mediated by interactions between people and ecological structures. In fact, people interact with ecological structures in their role of land tenants and stewards, further modulating the provision of ecosystem services. We devise a theoretical framework based on the concepts of ownership, tenancy, and stewardship, in which people, as mediators of ecosystem services, regulate the provision of services throughout the private-social-public domain. We survey relevant literature describing these dimensions and propose a comprehensive framework focused on the private-social-public domain. Our framework can advance ecosystem service research and enhance the provision of ecosystems services. The inclusion of people's individual, social and public roles in the mediation of ecosystem services could improve how benefits are planned for, prioritized, and optimized across cities.