Search

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]

This study aimed to explore the ecological stoichiometric characteristics of soil carbon ( C ), nitrogen ( N), and phosphorus( P) in a northern forest-grass ecotone and evaluate its response to influencing factors to reveal the limiting factors of soil nutrients and the driving mechanism of ecosystem evolution in the region. Sampling points were set up at an interval of 25 km from north to south in the study area for field investigation and soil sample collection. The soil ecological stoichiometric characteristics and their relationship with environmental factors were analyzed using statistics and redundancy analysis( RDA). The results showed that: the average concentration of soil organic carbon( SOC), total nitrogen( TN), and total phosphorus( TP) in the 0-20 cm soil layer was 36.76, 2.65, and 0.60 g • kg-1, respectively, and the C N P ratio was 61:4:1. In the 20-30 cm soil layer, the contents of SOC, TN, and TP decreased, they were 27.95, 2.30, and 0.55 g•kg-1, respectively, and the C: N: P ratio was 51:4:1. The region was characterized by N deficiency. For different vegetation types, the contents of SOC, TN, and TP were in the following order: forest>forest and sandy grassland>sandy grassland, and there were significant differences among the three vegetation areas( P<0.05). The contents of SOC, TN, and TP increased with an increase in precipitation, altitude, and temperature. In the 0-20 cm soil layer, the C: N, C: P, and N: P ratios increased with an increase in precipitation and altitude, but decreased with an increase in temperature. In the 20-30 cm soil layer, C: P and N: P ratios increased with an increase in precipitation and altitude but did not change significantly with temperature. C: N ratio increased with an increase altitude but did not change significantly with precipitation and temperature. RDA indicated that the main driving factor of soil ecological stoichiometry in the area was vegetation, followed by soil, terrain, and climate. The results revealed the response mechanism of soil ecological stoichiometry to influencing factors, and this is an important guide to ecological restoration and soil quality health development in forest-grassland ecotones in northern China. [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]

Distinct guidelines have been proposed in Romania and Switzerland for the management of pasture-woodlands that either focused on the regulation of grazing pressure (Romanian production perspective) or overall tree cover (Swiss conservation perspective). However, the landscape structural diversity and the cover of forest-grassland ecotones, which are both crucial for nature conservation value, were not explicitly considered. We aimed to compare the country-specific management guidelines regarding their efficiency for the conservation of the structurally diverse forest-grassland mosaics in the light of recent land-use and climate change. In strategic simulation experiments using the process-based model of pasture-woodland ecosystems WoodPaM, we analyzed the relationships among drivers for the formation of mosaic patterns (grazing intensity, climate change) and the resulting landscape properties (tree cover, forest-grassland ecotones, mosaic structure) during the past until today (2000 AD). The results showed that tree canopy densification following recent climate warming is likely to trigger landscape structural shifts. Medium grazing pressure promoted the development of the full range of pasture-woodland habitats and is therefore confirmed as a management strategy that balances agronomic demands and nature conservation value. Tree cover is rejected as a criteria to monitor pasture-woodland conservation status, because its relationship to landscape structural diversity and to the cover of forest-grassland ecotones did not hold for changing climate. Our results suggest “experimental-retrospective” analysis as a useful tool to test conclusions from expert knowledge. [ABSTRACT FROM AUTHOR]

Resprouting of trees and shrubs in forest-grassland ecotones is a key process to understand the dynamics of these systems under different disturbance regimes. This study integrates resprouting of grassland shrubs and pioneer forest trees ('gullivers'), burned in subtropical lowland grassland and cut in temperate highland grassland of Rio Grande do Sul, Brazil. Per grassland site, 20 individuals each of 1-2 grassland shrub species (Asteraceae) and two forest tree species (Myrsinaceae, Myrtaceae) were tagged, and post-disturbance survival and growth monitored for 1 year at 2-4 month intervals. Differences in resprouting vigour (summed-up basal area of resprouted shoots per pre-disturbance summed-up area of basal stems), and in density and allometry of resprouted shoots (allocation mode) were compared between tree and shrub species by linear mixed effects modelling and multiple comparisons, using the Tukey test. All grassland shrub individuals resprouted and regained 73-142 % (species average) of pre-disturbance basal area within one year, as opposed to 14-24 % in trees. All Myrtaceae 'gullivers' resprouted, but up to two-thirds of Myrsine individuals did not survive disturbance. Tree species tended to produce either many slender or few stout shoots, while shrub species were intermediate between these extremes. Forest trees regained 22-46 % of pre-disturbance height, independent of allocation mode, and grassland shrubs up to 73 %. This suggests that grassland fires allow grassland shrubs but not forest trees to persist and to grow to reproductive size. Differing sprout allocation modes may reflect allometric constraints rather than strategies to outgrow the fire-prone grass matrix. [ABSTRACT FROM AUTHOR]

We conducted a study on small-mammal composition, abundance and diversity across altitudinal and vegetational gradients in the Urucum Mountains (from 150 to 1000 m asl) in western Brazil, a complex biogeographic region. Small mammals were collected in 31 sampling units distributed along altitudinal and vegetational gradients (forest and grassland), totalling 18 112 trap-nights for pitfall and 3500 trap-nights for live-trap. Community variation among sampling units was assessed by randomization tests, setting altitude, vegetation, locality and time as factors, and using presence/absence data. Correlation and logistic regression analyses were run for species and diversity along gradients of altitude and vegetation, according to abundance and presence/absence data. Nineteen species (seven marsupial and 12 rodent) and 355 individuals were recorded. The species composition of small mammals differed according to altitude, vegetation type, locality and time. Species diversity varied significantly according to altitude. Species were influenced solely by altitude or vegetation, or by both vegetation and altitude concomitantly. The small-mammal community was divided in two groups according to biogeographic affinities. The grassland group is capable of invading forest habitats. The results are discussed in light of current hypotheses that attempt to explain community variation along altitudinal gradients around the world. [ABSTRACT FROM PUBLISHER]

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]

We assessed landscape-scale invasions of openings in mountain forests by native tree species since EuroAmerican settlement (ca. 1870-1899). We reconstructed historical openings across a 250,240 ha area in the Medicine Bow Mountains, Wyoming, using notes from the original General Land Office (GLO) surveys, and compared historical openings to modern openings interpreted from digital aerial photography. We constructed logistic regression models to describe and predict tree invasion, based on a set of environmental and land use predictors. Openings have decreased by about 7.3% in the last ca. 110 years. Invasion was more likely to occur on moister sites, indicated by high values for steady-state wetness, low values for evaporative demand, proximity to streams, and topographic settings in basins or channels. More invasion also occurred on unprotected public land, in openings surrounded by lodgepole pine and aspen, and on mesic soils. The relatively slow rates of invasion in the study area may be driven by climate and land use. [ABSTRACT FROM AUTHOR]