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Context: Forest–grassland ecotones, the transitional zones between forests and grasslands, often harbor high levels of biodiversity. According to the portfolio theory, communities with such high biodiversity should exhibit high stability in ecosystem functioning. On the contrary, the theory of 'bi-stability' or 'multistability' predicts that ecosystems exhibiting multiple states or regimes (e.g. forest and grassland) are less stable. Understanding the stability of forest–grassland ecotone has broad implications for forest management and restoration. Objectives: In this study, by quantifying landscape heterogeneity and temporal invariability of regional climatic and vegetational metrics, we aimed to reveal how landscape diversity influences the stability of ecosystem functioning in the forest–grassland ecotone. Methods: We focused on the forest–grassland ecotone in northern China (i.e. from the forest belt in the Greater Khingan Mountains to the steppe grassland in Inner Mongolia) and used remote sensing data of land cover dynamics from 1992 to 2015 to analyze the landscape diversity and ecosystem dynamics at different grain sizes. To measure the temporal stability of climatic variables and vegetational dynamics, we calculated the interannual variation of temperature (T), wind speed (WS), precipitation (P), and the net primary productivity (NPP) on the grided sampling cells at different spatial scale over the same period. Results: Our results showed that landscape diversity across this forest–grassland ecotone was positively related to the stability of T and NPP, but negatively related to that of WS and P. These opposite patterns may be explained by the differential nature of ecosystem variables, namely whether they involve transporting substance across space (WS and P) or not (T and NPP). We also found that as the spatial scale increases, the diversity-stability relationships strengthened for all three climatic variables (i.e., T, P, WS). However, the stabilizing effects of landscape diversity on NPP first increased but then decreased with increasing grain size, peaking at ~ 30 km. Conclusions: Our empirical analyses demonstrate strong, but context-dependent diversity-stability relationships, shedding light on the paradox in the stability of ecosystem functioning in the ecotone. These findings also have implications for the management of landscape resources to maintain the sustainability of forest–grassland ecotones. [ABSTRACT FROM AUTHOR]

Introduction: Continuous nitrogen deposition increases the nitrogen content of terrestrial ecosystem and affects the geochemical cycle of soil nitrogen. Forestgrassland ecotone is the interface area of forest and grassland and is sensitive to global climate change. However, the structure composition and diversity of soil microbial communities and their relationship with soil environmental factors at increasing nitrogen deposition have not been sufficiently studied in forestgrassland ecotone. Methods: In this study, experiments were carried out with four nitrogen addition treatments (0 kgN·hm-2·a-1,10 kgN·hm-2·a-1,20 kgN·hm-2·a-1 and 40 kgN·hm-2·a-1) to simulate nitrogen deposition in a forest-grassland ecotone in northwest Liaoning Province, China. High-throughput sequencing and qPCR technologies were used to analyze the composition, structure, and diversity characteristics of the soil microbial communities under different levels of nitrogen addition. Results and discussion: The results showed that soil pH decreased significantly at increasing nitrogen concentrations, and the total nitrogen and ammonium nitrogen contents first increased and then decreased, which were significantly higher in the N10 treatment than in other treatments (N:0.32 ~ 0.48 g/kg; NH4 +-N:11.54 ~ 13 mg/kg). With the increase in nitrogen concentration, the net nitrogen mineralization, nitrification, and ammoniation rates decreased. The addition of nitrogen had no significant effect on the diversity and structure of the fungal community, while the diversity of the bacterial community decreased significantly at increasing nitrogen concentrations. Ascomycetes and Actinomycetes were the dominant fungal and bacterial phyla, respectively. The relative abundance of Ascomycetes was negatively correlated with total nitrogen content, while that of Actinomycetes was positively correlated with soil pH. The fungal community diversity was significantly negatively correlated with nitrate nitrogen, while the diversity of the bacterial community was significantly positively correlated with soil pH. No significant differences in the abundance of functional genes related to soil nitrogen transformations under the different treatments were observed. Overall, the distribution pattern and driving factors were different in soil microbial communities in a forest-grassland ecotone in northwest Liaoning. Our study enriches research content related to factors that affect the forest-grassland ecotone. [ABSTRACT FROM AUTHOR]

Context: Montane grasslands and forest-grassland ecotones are unique and dynamic components of many landscapes, but the processes that regulate their dynamics are difficult to observe over ecologically relevant time spans. Objectives: We aimed to demonstrate the efficacy of using grassland-forest ecotone trees to reconstruct spatial and temporal properties of the historical fire regime in a complex landscape of montane forests and adjacent grasslands. Methods: We sampled and crossdated fire-scarred trees along ecotones and compared variations in historical fire occurrence within and among nine adjoining valle basins in a 10,158 ha landscape. We analyzed fire year extensiveness, climate regulation, and the occurrence of consecutive fire years. Results: The resulting tree-ring record covers 1240–2005 AD, with 296 trees recording 125 replicated fire years during the analysis period 1601–1902 AD. Mean fire intervals for all events recorded on two or more trees ranged from 4.7 to 13.6 years in individual valles, and a mean of 2.4 ± 1.7 (SD) years at the landscape scale. Between 1660 and 1902, extensive fires occurring in six or more valles occurred 15 times, on average at ~ 17-year intervals; 29 moderately widespread fires (3–5 valles) occurred during this period, at 8.7 year intervals on average. Widespread events occurred in years with a significantly lower Palmer Drought Severity Index (PDSI) preceded by years of significantly positive PDSI, indicating conditions favorable for fine fuel production. Spatial reconstruction of fire extent revealed multiple occurrences of consecutive-year fires burning non-overlapping areas, associated with persistent low PDSI anomalies preceded by positive conditions in antecedent years. Conclusions: A landscape spatiotemporal approach to reconstructing fire regimes of montane forest-grassland complexes provides a valuable baseline for guiding prescribed and natural fire management at large spatial scales. [ABSTRACT FROM AUTHOR]

Historically, where forest habitats are deemed as the pristine landscape state, anthropogenic habitats such as managed grasslands or open spaces are often perceived to be antagonistic and of secondary conservation priority. Traditionally, studies on biodiversity responses to ecological variation, i.e. edge effect, have mostly focused on forest habitats. Yet recently there has been increased attention on communities beyond the forest edge in an effort to better understand how interactions between forests and adjacent habitats may potentially affect regional biodiversity. However, in Europe and the Mediterranean basin (a biodiversity hotspot), areas with high landscape heterogeneity and high edge density, there is a paucity of studies analysing the community responses across forest and 'beyond edge' habitats across ecotones. In a protected area of central Italy, we investigated the responses of ground-dwelling arthropods [Araneae (spiders), Chilopoda (centipedes) and Carabidae (ground beetles)], which were differentiated into habitat-specific guilds (forest, edge and grassland species) across a forest-grassland ecotone. We investigated the extent to which a habitat edge influenced communities of arthropods associated with either the forest or grassland, and how far from the edge this effect penetrated into each habitat. Twelve 150 m-transects perpendicular to a forest-grassland edge were established and arthropods were sampled at nine progressive distances across the ecotone. An indicator species analysis was used to detect species significantly associated with forest, edge-belt or grassland habitats, which were assumed representative of the respective communities. Logistic models of indicator species richness and abundances were used to describe responses of grassland and forest communities across the ecological boundaries. We found that grassland and edge habitats had habitat specialists and higher species richness compared to the forest habitat. Moreover, the occurrence of grassland-specific species was influenced by the presence of an edge up to 15 m from the habitat border. In contrast forest-associated indicator species were not affected by proximity to the habitat edge, rather individuals typical of forest habitats tended to 'spill over' into grassland habitats. These findings support the hypothesis that in a forest-grassland mosaic, forest species are less sensitive to an edge and influence the community beyond the forest edge and into the grassland more than the reverse, i.e. the effect was asymmetric. From these data, we estimated that a minimum grassland habitat width of 600 m is necessary for grassland species to maintain a core area that is relatively unaffected by the spillover of species from adjacent forest habitats. Incorporating the directional influences of adjacent communities on each other allows for an empirical assessment of habitat vulnerability that doesn't a priori value the conservation of one habitat over another. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

• Pixels with greater subpixel percent tree cover generally had earlier greenup dates (GUDs). • A large area of significantly earlier GUD was observed across the Hulunbuir forest-grassland ecotone. • Increases in spring temperature and percent tree cover contributed to the earlier trends in GUD. Changes in climate and land cover are potential drivers of land surface phenology changes. Here, we investigate whether subpixel percent tree cover (PTC) change is an important driver of trends in satellite derived vegetation spring greenup date (GUD) across the Hulunbuir temperate forest-grassland ecotone in northeastern China. GUD was estimated using the MODIS-derived enhanced vegetation index time series during 2001–2020 with a spatial resolution of 500 m. To understand the influential mechanisms of PTC on GUD, we examined relationships between the spatial variations in GUD and PTC at multiple spatio-temporal extents. Forested pixels with greater PTC were found to have generally earlier GUDs for all forest types. The GUD of forests was also generally earlier than that of grassland. On the other hand, we observed approximately 23.7% and 1.2% significantly earlier and later trends in GUD across the region, respectively. Meanwhile, widespread increases in preseason land surface temperature (LST) and PTC were detected. Both increases in LST and PTC contributed to the earlier GUD in the forested region. Specifically, we found negative correlations (Spearman correlation coefficient -0.17 to -0.55) between the change slopes of GUD and PTC in every forest and grassland type. The results highlight the important impacts of subpixel PTC on GUD variations, and improve the understanding of ecosystem changes under the effects of climate and human activities (e.g., afforestation) over the Hulunbuir temperate forest-grassland ecotone. [ABSTRACT FROM AUTHOR]

Questions: What is the structure of the anthropogenic upper forest-grassland ecotone and are there differences in the spatial relationships between the tree species involved? Location: Valfurva Valley, Italian central Alps. Methods: We conducted a spatial distribution and structure analysis in three 1-ha permanent plots along an altitudinal gradient, from the treeline to the sub-alpine forest. We reconstructed the age structure from cores from each individual with diameter > 4 cm at 50 cm height. Results: All tree species and age classes examined had a clumped structure. The cluster tendency was more evident at the treeline where the environmental conditions are more severe. In the sub-alpine forest there was a repulsion between Pinus cembra and Pious mugo but at the treeline P. cembra was frequently found downslope from P. mugo. Conclusions: Although human influence has been the main driving force in shaping the present forest structure, in the last few decades natural dynamics have become the predominant force acting on forest structure and processes, showing a higher magnitude as altitude increases. Our results emphasize the existence of facilitating and interfering mechanisms between different species. P. cembra seems to be favoured compared to the other tree species. [ABSTRACT FROM AUTHOR]