Savannas are mixed tree-grass systems maintained by complex interactions and feedbacks between bottom-up (resources) and top-down (e.g., fire and herbivory) forces. Much research in savannas has focused on understanding what enables tree-grass coexistence. In contrast, tree-tree interactions have received limited attention despite being equally important for determining savanna structure. Tree-tree interactions can be facilitative, or competitive, with differential implications for woody cover and savanna structure. Further, these interactions can change across life stages of woody plants. In this study, we assessed patterns of tree spatial distributions to infer the nature of tree-tree interactions (whether facilitative or competitive) across two life stages, saplings and adults, in an Indian savanna.
Mesic savanna in southern India.
We assessed spatial patterns of trees using point pattern analysis and inferred the nature of tree-tree interactions by examining neighbourhood effects, both conspecific and heterospecific, on plant survival and growth across the two life stages.
We found that saplings were strongly aggregated while adult trees were randomly distributed. Despite being strongly aggregated, sapling growth and survival over 4 years were unaffected by neighbours (adults and saplings). Instead, initial size (basal area at first census) was the single best predictor of both sapling growth and survival. In contrast, adult neighbours negatively affected adult tree growth with effects being more pronounced for conspecific neighbours than heterospecifics. Other factors such as drought and fire, although important for sapling and adult performance, did not occur during our study period.
Our results suggest that tree-tree interactions in this mesic savanna vary from neutral at the sapling stage to negative at the adult stage. Neighbourhood density influenced adult tree growth but not survival. Suppressive effects of neighbours on adult tree growth, although weak, suggest a potential role for competitive tree-tree interactions in constraining tree cover in Indian mesic savannas.
Understanding how nutrient availability and soil conditions influence ecosystem functionality is crucial for predicting changes in the plant community and informing ecological restoration programs. Here, we aimed to characterize the ecological strategies of the most abundant species in an ironstone ecosystem, to understand how edaphic parameters drive these strategies, and to investigate how species taxonomic, functional, and phylogenetic diversities differ among habitats within the ecosystem.
The study was conducted in three municipalities in the southern portion of the Espinhaço mountain range in Minas Gerais, Brazil. We sampled plots in three habitat types with distinct soil characteristics.
We employed the CSR framework to characterize the ecological strategies of the most common species across 87,100-m2 plots. We also measured 19 edaphic parameters in these plots to evaluate their influence on ecological strategies.
Most of the 46 sampled species displayed a high proportion of stress-tolerant strategies due to their small leaves with high dry matter content. This pattern reflects the preponderance of S-selection at the plot level. Conversely, plots with finer sand and relatively fertile conditions had more competitive strategies, and the species within showed larger leaves with less biomass investment.
This study highlights the role of abiotic filters in shaping CSR strategies in ironstone campo rupestre, emphasizing the trade-off between growth and resource conservation. The low growth rates of species reveal the ecosystem's vulnerability to increased soil removal and disturbances. Additionally, small-scale edaphic variation relates mainly to physical properties, selected species composition, and functionality.
Climate change and forest management have profoundly altered forest ecosystem dynamics, with impacts on canopy composition and understory vegetation. Forest type affects bryophyte ecosystem processes, particularly related to carbon (C) and nitrogen (N) cycling. Within the boreal-temperate ecotone, the Wabanaki-Acadian Forest harbors a diverse canopy composition with many species at their distributional limit, making it sensitive to climate change and human alterations, with unknown impacts on bryophytes and their functions.
We aimed to quantify how canopy composition and moisture affect forest-floor bryophyte biomass, and C and N stocks.
We sampled in 49 stands representing six forest types of the Wabanaki-Acadian Forest of New Brunswick (Canada): mesic coniferous, wet coniferous, mesic deciduous, wet deciduous, mixed, and cedar forests.
First, we developed an allometric model to predict bryophyte bulk density. We then estimated bryophyte biomass and C and N contents for five functional groups at each site using the ground layer indicator method and measured forest and soil characteristics at each site (e.g., forest composition, soil pH, moisture class). Linear models; multivariate, similarity percentages; and indicator species analyses were used for data analysis.
The highest bryophyte biomass and C and N stocks were found in conifer-dominated forests, especially where Sphagnum was abundant, whereas they were low in mixed and deciduous forests which also had a different functional composition. There was a strong negative nonlinear relationship between bryophyte biomass and broadleaf litter.
We provide the first reports of bryophyte biomass and C and N stocks in the Wabanaki-Acadian Forest, which were consistent with its latitudinal location at the southern edge of the boreal forest. Our findings highlight the connections between forest composition and bryophytes in the Acadian Forest and reveal the potential contribution of forest-floor bryophytes to C and N stocks.
Do multiple environmental conditions filter plant communities in non-native plantations similarly to the natural forests? Do upper forest layers act as additional filters on understory plant diversity? How do multiple environmental conditions and forest structure affect rewilding?
Trentino region (Italy).
Using a stratified random sampling design approach encompassing different elevations and aspects in non-native old-established black pine plantations of the eastern European Alps, we sampled 55 plots over 6200 km2 in which we collected data on abiotic conditions and species cover for each forest layer (tree, shrub, herb), and forest regeneration (i.e., woody juvenile species). We adopted structural equation models (SEMs) to investigate the causal relationships between multiple environmental conditions and forest layer interactions on alpha and beta plant diversity. Furthermore, we ran redundancy analysis (RDA) to test for the effect of multiple environmental conditions and forest structural parameters on the ecology of the herbaceous communities.
High- and low-elevation sites had reduced alpha diversity likely because of frost and drought climatic constraints. Shaded conditions due to tree canopy cover directly reduced alpha shrub diversity, and indirectly the herb diversity. Beta diversity of each layer was mainly influenced by slope and elevation. The composition of trees set off a cascading effect, shaping shrub communities and, in turn, the herbaceous and juvenile layers below.
Non-native old-established plantations close to the native range of black pine showed high diversity coupled with scarce regeneration. Our insights are crucial for understanding the rewilding of old-established plantation forests. Future plantation programs should consider both the multiple spatial scales of the environmental conditions of the sites and the interaction among vegetation layers to achieve effective rewilding.
Climate has long been recognized as an important factor shaping plant phenology. Seasonal climatic variables can exert a strong influence on flowering and fruiting timing. Further, if phenological responses to environmental gradients are phylogenetically conserved in plant species, evolutionary history might be a strong determinant of phenological periods. In this study, we evaluated whether closely related species are phenologically similar, whether there is an association between the composition of flowering or fruiting species and environmental gradients, and to what extent such associations are mediated by the phylogenetic relationships among species.
Araucaria forest at São Francisco de Paula, South Brazil.
We assessed the phylogenetic conservatism of reproductive phenology in a climbing plant community (78 spp.), an understudied life form that highly contributes to ecosystem dynamics. For this, we calculated the phylogenetic signal of species' flowering and fruiting peaks and conducted a phylobetadiversity analysis and model selection considering temporal autocorrelation.
We found a weak phylogenetic signal in flowering and no signal for fruit phenology at the species scale. However, we found a strong influence of shared evolutionary history on the association between phenological patterns and environmental gradients at the assemblage scale, with different phylogenetic lineages influenced by distinct variables. The daylength, a variable that exhibits subtle changes and consistent patterns across years, explained most of both flowering and fruiting of larger clades (rosids vs asterids), while for fruiting lower level clades (families) were influenced by the canopy openness of tree species.
The results highlight the fingerprint of phylogenetic relatedness in the association of plant phenology and environmental gradients, enabling the identification of at which phylogenetic levels these gradients are acting. Our approach and results bring a new perspective, showing that even in the absence of a phylogenetic signal in species phenological peaks, there is a phylogenetic conservatism between plant phenology and environmental gradients.
Biological invasions significantly impact plant communities, affecting and threatening biodiversity at regional and local scales worldwide. The invasion process may be influenced by climate change, particularly by the increasing frequency of extreme droughts. Here we evaluate (1) the level of exotic plant invasion along an aridity gradient and its relationship with environmental variables, and (2) the cumulative effect of prolonged droughts on native plant communities and the invasion of exotic species.
Rangelands located along an aridity gradient from the arid Patagonian steppe to the humid Pampa region in Argentina, South America. The study sites (11) encompass a wide range of mean annual precipitation (170–950 mm year−1) and diverse vegetation physiognomic characteristics.
We conducted floristic surveys to assess plant community composition, diversity, and the cover of native and invasive exotic species in (1) 11 sites located along an aridity gradient, and (2) a 4-year drought simulation experiment established in nine sites. At each experimental site, rain-out shelters intercepted 50%–60% of incoming rainfall to simulate drought conditions. We then used linear mixed models to analyze the effects of drought on the cover, richness, and dominance of the plant community.
Our findings reveal (1) a positive relationship between plant invasion and rangeland humidity at the regional scale. In the humid extreme, invasive exotic species dominated the plant community, whereas in the arid extreme native species were more abundant than invasive exotic species. (2) Experimental droughts reduced native plant cover in arid and semi-arid rangelands and promoted the cover, dominance, and richness of invasive exotic species in humid counterparts, reducing native species richness.
This study suggests a potential intensification of biological invasions in response to increased occurrences of droughts predicted by climate change. Management strategies for invaded humid rangelands are imperative, and early warnings are crucial during dry years in arid and semi-arid rangelands, where droughts could enhance the abundance of invasive exotic species.
What is the position of the two selected invasive alien species in the regional trait space? Does the probability of occurrence of a species depend on the interaction of its similarity to and the cover of the selected invasive species, Asclepias syriaca and Conyza canadensis?
Hungary, Kiskunság, sandy grasslands.
We used data from a stratified random grassland survey (plot size: 25 m2, n = 161). Our analysis focused on the two most common invasive species of studied habitats: clonal perennial A. syriaca and annual C. canadensis. Trait similarity between species was measured by weighted Gower distance using three sets of traits: (1) 12 traits covering all aspects of life history, (2) traits related to reproduction and (3) traits related to persistence. Calculated similarities were used to describe the position of the two focal invasive species in the trait space generated by all species. We fitted generalised mixed-effects models with the presence/absence of all species except A. syriaca and C. canadensis as dependent variables and trait similarity to, and cover of invasive species as predictors.
We found that both invasive species were in the trait range of the regional species pool. C. canadensis was closer to the centre of regional trait space than A. syriaca. Similarity to A. syriaca and its cover interacted to influence the occurrence of the rest of the species: Where the cover of A. syriaca was higher, similar species occurred with a higher probability. We found no such effect of similarity in the case of C. canadensis.
These results suggest that for perennial invasive species A. syriaca, trait similarity may influence the composition of co-occurring species. In contrast, the results of annual C. canadensis suggest a more random assembly.
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