Forest and woodland replacement patterns following drought-related mortality.

Forest vulnerability to drought is expected to increase under anthropogenic climate change, and drought-induced mortality and community dynamics following drought have major ecological and societal impacts. Here, we show that tree mortality concomitant with drought has led to short-term (mean 5 y, range 1 to 23 y after mortality) vegetation-type conversion in multiple biomes across the world (131 sites). Self-replacement of the dominant tree species was only prevalent in 21% of the examined cases and forests and woodlands shifted to nonwoody vegetation in 10% of them. The ultimate temporal persistence of such changes remains unknown but, given the key role of biological legacies in long-term ecological succession, this emerging picture of postdrought ecological trajectories highlights the potential for major ecosystem reorganization in the coming decades. Community changes were less pronounced under wetter postmortality conditions. Replacement was also influenced by management intensity, and postdrought shrub dominance was higher when pathogens acted as codrivers of tree mortality. Early change in community composition indicates that forests dominated by mesic species generally shifted toward more xeric communities, with replacing tree and shrub species exhibiting drier bioclimatic optima and distribution ranges. However, shifts toward more mesic communities also occurred and multiple pathways of forest replacement were observed for some species. Drought characteristics, species-specific environmental preferences, plant traits, and ecosystem legacies govern postdrought species turnover and subsequent ecological trajectories, with potential far-reaching implications for forest biodiversity and ecosystem services.

Increasing and declining native species in urban remnant grasslands respond differently to nitrogen addition and disturbance.

Background and Aims: Atmospheric nitrogen deposition and natural fire regime suppression are key drivers of vegetation change in urbanizing grasslands. Some species thrive under these conditions, while others face local extinction. In the natural grasslands that surround Melbourne, Australia, biotic homogenization has occurred with intensifying urbanization. Some native species have become rarer (decreaser species) across the landscape, while others have become more widespread (increaser species). This study experimentally examined the response of increaser and decreaser plant species to nitrogen addition/depletion, and examined the presence/absence of annual disturbance to the vegetation. Methods: Decreaser and increaser species were planted into 60 field plots established in an urban Melbourne grassland and examined over 2 years. Annual removal of above-ground biomass occurred in half the plots to simulate biomass removal via fire, with the remaining plots undisturbed. Soil nitrogen was depleted in one-third of plots, one-third received no nitrogen treatment and one-third were fertilized with nitrogen. Increaser plant species were predicted to persist in the absence of disturbance, and thrive when fertilized. In contrast, high mortality was predicted for decreaser species in the absence of disturbance, with fertilization providing no advantage. Key Results: Seedling mortality for increaser and decreaser species was unrelated to the treatments. The mortality of decreaser species was high (69 %), and the mortality of increaser species low (20 %). However, seedling growth was related to the treatments. The total biomass of decreaser species was highest in annually disturbed plots, with growth suppressed in undisturbed plots. In contrast, the total biomass of increaser species was unrelated to the disturbance regime, but responded positively to nitrogen enrichment. Conclusions: The results provide evidence that by affecting plant growth, declines in biomass removal and atmospheric nitrogen deposition could be key drivers of biotic homogenization in urban grasslands.

Fire regime, not time-since-fire, affects soil fungal community diversity and composition in temperate grasslands.

Frequent burning is commonly undertaken to maintain diversity in temperate grasslands of southern Australia. How burning affects below-ground fungal community diversity remains unknown. We show, using a fungal rDNA metabarcoding approach (Illumina MiSeq), that the fungal community composition was influenced by fire regime (frequency) but not time-since-fire. Fungal community composition was resilient to direct fire effects, most likely because grassland fires transfer little heat to the soil. Differences in the fungal community composition due to fire regime was likely due to associated changes that occur in vegetation with recurrent fire, via the break up of obligate symbiotic relationships. However, fire history only partially explains the observed dissimilarity in composition among the soil samples, suggesting a distinctiveness in composition in each grassland site. The importance of considering changes in soil microbe communities when managing vegetation with fire is highlighted.