Requirements for the spatial storage effect are weakly evident for common species in natural annual plant assemblages

Coexistence in spatially varying environments is theorised to be promoted by a variety of mechanisms including the spatial storage effect. The spatial storage effect promotes coexistence when: (i) species have unique vital rate responses to their spatial environment and, when abundant, (ii) experience stronger competition in the environmental patches where they perform better. In a naturally occurring southwest Western Australian annual plant system we conducted a neighbour removal experiment involving eleven focal species growing in high-abundance populations. Specifically, we measured species' fecundity across a variety of environmental gradients in both the presence and absence of neighbours. For the environmental variables that we measured, there was only limited evidence for species-specific responses to the environment, with a composite variable describing overstory cover and leaf litter cover being the best predictor of fecundity for a subset of focal species. In addition, although we found strong evidence for intra-specific competition, positive environment-competition covariance was only detected for one species. Thus, positive environment-competition covariance may not be as common as expected in populations of species growing at high abundance, at least when tested in natural assemblages. Our findings highlight the inherent limitations of using natural assemblages to study spatial coexistence mechanisms, and we urge empirical ecologists to take these limitations into account when designing future experiments

Regional climate and local-scale biotic acceptance explain native–exotic richness relationships in Australian annual plant communities

Native and exotic species richness is expected to be negatively related at small spatial scales where individuals interact, and positive at larger spatial scales as a greater variety of habitats are sampled. However, a range of native-exotic richness relationships (NERRs) have been reported, including positive at small scales and negative at larger scales. We present a hierarchical metacommunity framework to explain how contrasting NERRs may emerge across scales and study systems, and then apply this framework to NERRs in an invaded winter annual plant system in southwest Western Australia. We analysed NERRs at increasing spatial scales from neighbourhoods (0.09 m) to communities (225 m) to metacommunities (greater than 10 ha) within a multilevel structural equation model. In contrast to many previous studies, native and exotic richness were positively related at the neighbourhood scale and were not significantly associated at larger scales. Heterogeneity in soil surface properties was weakly, but positively, associated with native and exotic richness at the community scale. Metacommunity exotic richness increased strongly with regional temperature and moisture availability, but relationships for native richness were negative and much weaker. Thus, we show that neutral NERRs can emerge at larger scales owing to differential climatic filtering of native and exotic species pools

Restoration thinning permits stems to capitalize on high‐rainfall years in a regenerating endangered forest ecosystem

Abstract 1. Passively regenerating native vegetation presents a cost‐effective opportunity to sequester carbon and reinstate habitat in heavily cleared agricultural landscapes. 2. However, in some cases a few woody species recolonize in dense, low‐diversity stands that are slow to self‐thin. 3. Restoration thinning of over‐dominant species has been proposed to accelerate ecosystem recovery, but its longer term efficacy remains uncertain, and is likely to depend strongly on rainfall. 4. This study focuses on a restoration thinning experiment established in 2007 in dense brigalow (Acacia harpophylla) regrowth in Queensland, Australia. Using variation in rainfall between 2007 and 2017, we examined the interactive effects of neighbourhood density and moisture availability on the growth and survival of A. harpophylla. 5. We also compared the strength of A. harpophylla density effects on itself to its effects on a sparsely distributed co‐occurring tree species (Casuarina cristata) that was codominant in the original forest. 6. Our results provide clear evidence that thinning permits A. harpophylla to grow rapidly during periods of high rainfall, and that interspecific competition betweenA. harpophylla and C. cristata is relatively weak. As such, thinning of dense A. harpophylla could be combined with seeding or planting of co‐occurring tree species with complementary niches to further accelerate forest recovery in this extensive regrowth ecosystem

Data from: Regional climate and local-scale biotic acceptance explain native-exotic diversity relationships in Australian annual plant communities

Native and exotic species richness is expected to be negatively related at small spatial scales where individuals interact, and positive at larger spatial scales as a greater variety of habitats are sampled. However, a range of native-exotic richness relationships (NERRs) have been reported, including positive at small scales and negative at larger scales. We present a hierarchical metacommunity framework to explain how contrasting NERRs may emerge across scales and study systems, and then apply this framework to NERRs in an invaded winter annual plant system in south-west Western Australia. We analysed NERRs at increasing spatial scales from neighbourhoods (0.09 m2) to communities (225 m2) to metacommunities (>10 ha) within a multi-level structural equation model. In contrast to many previous studies, native and exotic richness were positively related at the neighbourhood scale and were not significantly associated at larger scales. Heterogeneity in soil surface properties was weakly, but positively, associated with native and exotic richness at the community-scale. Metacommunity exotic richness increased strongly with regional temperature and moisture availability, but relationships for native richness were negative and much weaker. Thus, we show that neutral NERRs can emerge at larger scales due to differential climatic filtering of native and exotic species pools