Cold environments, such as polar systems, are highly vulnerable to global change drivers such as climate change and invasive species. Therefore, it is essential to assess what drives the diversity of native and invasive species in these systems. We investigated what drives native and alien plant species richness on sub-Antarctic Marion Island and determined the scale-dependence of these drivers. Native and alien plant species richness was surveyed at “small” (1 m2) and “large” (9 m2) sampling grains. Difference in richness between the two sampling grains was calculated to assess how local turnover contributes to species assemblage. The factors driving richness at both grains, and the differences in richness between the two grains, were analysed using simultaneous auto-regressive models. Drivers related to energy and environmental heterogeneity were correlated with native richness, whilst drivers related to productivity were related to alien richness. Biotic interactions with a cushion plant facilitated native richness, but restricted alien richness at low elevations. Further, some drivers of richness depended on spatial grain. Native richness was positively related to northness at large, but not small grain size, suggesting that higher northness increases local turnover at a grain size > 1 m2. On the other hand, topographical wetness index (TWI) boosted native richness at small but not large grains, implying that competition for water limits coexistence at low TWI (i.e., low moisture availability) only at small grain. Differences in native species richness between large and small grain sizes were more pronounced at low elevations, suggesting higher compositional heterogeneity at low altitudes. Our study highlights that drivers of plant species richness in a polar ecosystem differ between native and alien plant species. Additionally, the effects of some drivers on richness differ between sampling grains, and considering these differences provides insight into drivers of local patterns of species assemblage.

Environmental and biotic factors drive species richness patterns, but the nature of this relationship can vary with sampling grain. Understanding the scale-dependent effects of these factors is crucial for interpreting species richness patterns in ecosystems experiencing rapid environmental change. We investigated the effects of local environmental drivers on plant species richness at small (1 x 1 m) and large (3 x 3 m) sampling grains, and the factors correlated with differences in richness between the two grains, on a sub-Antarctic island. Broadly, richness was higher in warmer (i.e., lower altitude, north-facing) and wetter (i.e., higher topographic wetness index, lower distance from drainage line) sites, and in more topographically heterogenous (i.e., steeper slopes) sites. Additionally, there was some evidence of competition with a keystone plant limiting species richness, though this was only evident at low elevations. However, the effects of several drivers on richness depended on spatial grain. Differences in species richness between large and small grain sizes were more pronounced at low elevations, indicating that there is more compositional heterogeneity at low altitudes at both grains. Richness was positively related to northness at large grain size but not at small grain size, suggesting that higher northness increases local turnover at a grain size > 1 m2. On the other hand, TWI boosted richness at small but not large grains, implying that competition limited coexistence at low TWI, and that higher TWI only resulted in more species coexisting at a grain of 1 m2, while having no effect on richness at large grains. Our study therefore highlights that drivers of plant species richness can vary with sampling grain, suggesting that environmental effects on local species turnover affect richness patterns at different grains. Assessing how the influence of such drivers differ with grain size provides insight into local patterns of species assemblage.

Patterns and drivers of succession provide insight into the mechanisms that govern community assembly and are indicators of community resilience and stability but are still poorly understood in microbial communities. We assessed whether the successional trends of woody vegetation are mirrored by foliar fungal endophyte communities of three tree species that are abundant across the woody successional gradient using a total amplicon sequencing approach. Additionally, we test the relative contribution of host identity, abiotic predictors, biotic factors, and spatial distance between sites in predicting community composition and species richness of endophyte communities. Unlike the woody community, endophyte communities showed no consistent evidence of deterministic successional trends. Host identity was the most important factor structuring fungal endophyte community composition. Spatial distance played some role in explaining differences in community composition, but the effects of this and other environmental variables were small and not consistent between different host species. Much of the variation in endophyte composition remained unexplained. Host identity was most important in predicting endophyte richness. Although endophyte communities showed no deterministic succession, community assembly was most strongly influenced by host identity and spatial distance.

1. Fire and frost represent two major hurdles for the persistence of trees in open grassy biomes and have both been proposed as drivers of grassland-forest boundaries in Africa. 2. We assess the response of young tree seedlings, which represent a vulnerable stage in tree recruitment, to traumatic fire and frost disturbances. 3. In a greenhouse experiment, we investigated how seedling traits predicted survival and resprouting ability in response to fire vs frost; we characterised survival strategies of seedlings in response to the two disturbances, and we documented how the architecture of surviving seedlings is affected by fire vs frost injury. 4. Survival rates were similar under both treatments. However, different species displayed different levels of sensitivity to fire and frost. Seedling survival was higher for older seedlings and seedlings with more basal leaves. Survivors of a fire event lost more biomass than the survivors of a frost event. However, the architecture of recovered fire and frost treated seedlings were mostly similar. Seedlings that recovered from fire and frost treatments were often shorter than those that had not been exposed to any disturbance, with multiple thin branches, which may increase vulnerability to the next frost or fire event. 5. Synthesis. Fire caused more severe aboveground damage compared to frost, suggesting that trees in these open grassland systems may be subjected to a seedling release bottleneck maintained by fire. However, the woody species composition will almost certainly be influenced by phenomena that affect the timing and frequency of seedling exposure to damage, as mortality was found to be dependent on seedling age. Therefore, changes in fire regime and climate (esp. changes that bring about less frost and reduced fire intensity and frequency) are likely to result in changes in the composition and the structure of the woody components of these systems.