Regeneration in old climatically-buffered infertile landscapes

Abstract

Old, climatically-buffered, infertile landscapes (OCBILs) occur in at least 12 out of 35 terrestrial biodiversity hotspots. Despite having a scattered global distribution, plant communities in OCBILs show clear similarities in structural and functional aspects. Soil infertility, especially severe phosphorus (P) deficiency, is a strong environmental filter that leads to a clear predominance of nutrient-conserving, slow-growing strategies among OCBIL plant species. Although associations between P-impoverished habitats and traits of adult individuals are well known, the particularities of regenerative phases, such as seedlings, juveniles and resprouts, are rarely investigated, limiting our understanding of community assembly and species distribution. In this thesis, I studied the ecophysiological strategies of regeneration in two OCBILs, the campo rupestre in Brazil and the kwongan in Southwest Australia, from a community perspective. In the first study (Chapter 2), I investigated how ontogenetic shifts in plant ecological strategies in the campo rupestre compare with those of a productive nearby habitat. I measured key leaf functional traits and calculated scores for competitor (C), stress-tolerator (S), ruderal (R) strategies of the CSR system for juveniles of conspecific adults of species naturally occurring either in unproductive (campo rupestre) or more productive (forest) habitats. Juveniles exhibited a more R-selected strategy in both habitats when compared with adults, but were not necessarily “R” compared to the whole flora spectrum. Strategies of campo rupestre juveniles were more S-selected and less R-selected than forest juveniles, suggesting that a functional habitat-dependent specialisation is the main driver of selection for ecological strategy in juveniles, despite the remarkable ontogenetic shifts in ecological strategies. In order to assess shifts in P-use strategies in post-fire regeneration forms, i.e. resprouts (burnt adults) and seedlings, I studied 10 species of three dominant families, Fabaceae, Myrtaceae and Proteaceae, from burnt and unburnt kwongan sites (Chapter 3). I assessed differences in leaf total P, inorganic P (Pi) and organic P (Po) concentrations among regeneration forms, and their association with soil P availability, seed P content and other plant ecophysiological traits. Seedlings had greater leaf total phosphorus concentration, [P], than conspecific unburnt adults, mostly due to a greater leaf [Pi]. Proteaceae seedlings had remarkably high concentrations of leaf total P, Pi and organic P fractions, associated with a higher seed P content. Burnt adults exhibited a short-term increase in leaf total [P], but burnt and unburnt adults had similar leaf total [P] one year after a fire, when soil [Pi] was still increased at the burnt site. This study revealed the spectrum of variation in P-use and -allocation strategies within the community, and demonstrated the singular strategies of Proteaceae seedlings from Southwest Australia. In the third study (Chapter 4), I aimed to characterise main attributes associated with environmental heterogeneity in ferruginous campo rupestre (i.e. established in ironstone outcrops) and to test whether seedling survival, growth and abundance, and adult species richness were influenced by specific environmental conditions. I surveyed microhabitats with the presence of each of two dominant shrubs, Mimosa calodendron (Fabaceae) and Lychnophora pinaster (Asteraceae), and microhabitats lacking both species. Plots associated with the two shrubs exhibited more developed substrates than those lacking these plants, and had greater seedling abundance and adult richness. Larger crown area of the dominant shrubs and understorey plant cover were negatively associated with species richness and seedling abundance, respectively. Growth, but not seedling survival, was predicted by some of the measured attributes. This study showed that regeneration from seed in ferruginous campo rupestre is marked by complex spatial heterogeneity, driven by abiotic and biotic factors, which helps to understand the community structure. Finally, I discussed particularities of OCBILs that preclude landscape-scale seed-based restoration of degraded areas. Issues related to seed sourcing, seed quality/availability, and dormancy-breaking comprise relevant hurdles that limit the use of native seeds for restoration (Chapter 5). I also presented some alternatives for overcoming these challenges, and highlighted the need for investments in restoring those exceptionally-biodiverse landscapes. Overall, my studies revealed that OCBILs are marked by limited and slow regeneration from seeds, due to restricted seed availability, lack of suitable sites for germination and seedling establishment, and inherently slow growth of seedlings. Although I unveiled general patterns for OCBILs, spatial heterogeneity and different species strategies within communities play an important role in their structuring, contributing to species coexistence and the remarkable biodiversity in these systems. I surmise that the slow regeneration was selected in communities mainly as a response to P-impoverished soils, combined with long-term climatic and geological stability.