Nutrient enrichment diminishes plant diversity and density, and alters long-term ecological trajectories, in a biodiverse forest restoration

Abstract

Nutrient enrichment can negatively affect natural plant communities and result in the loss of species diversity and productivity. Despite this, fertiliser (especially phosphorus) is typically applied to restore highly biodiverse communities. Long-term effects of nutrient addition to restored plant communities, particularly those adapted to inherently low nutrient soils, have received little attention. We report results of a large-scale 20-year field experiment established in West Australian jarrah forest restored after bauxite mining Three P-application rates were applied (0, 80 and 120 kg ha−1) once at the beginning of the experiment, and plant communities monitored after 1, 6, 13 and 20 years. One year after the onset of restoration, native plant species richness and plant density was highest at 80 and 120 kg P ha−1. Subsequently, native species richness, plant density, and the richness and density of seeder and slow-growing resprouter species were highest without fertilisation, establishing the negative impact of P enrichment on plant community and ecosystem development in P impoverished soils. Total plant cover was similar for all P treatments across the chronosequence which, when combined with higher stem densities at zero P, suggests zero P favoured smaller, slower growing species. Applied-P initially favoured weeds and ephemerals and, while these species declined over time, other species were lost from these plots. The similarity of the restored communities to unmined reference jarrah forest increased over time and was consistently highest at in the absence of P fertiliser. Jarrah forest restoration is assumed to follow the initial floristic model of plant succession. However, we question this assumption and instead suggest that successional outcomes are contingent on P fertilisation rather than initial floristics per se. Applied P retarded recruitment of resprouter species that were present at zero P, debunking the assumption under IFM that these species do not disperse to restored areas. Consequently, based on the most comprehensive long-term study of P-fertilisation in the context of restoration of P-impoverished ecosystems yet reported, we propose that P limitation is important for the recreation of species diversity in inherently P impoverished forests. These results highlight the necessity of long-term experiments for understanding forest successional dynamics and implications for restoration practices.