title: Natural regeneration of floodplain vegetation in semi-arid agricultural landscapes
Creators: name: Zivec, Peta L
Publisher: name: Griffith University
Year: 2022
Subjects: subject: Revegetation
subject: restoration
subject: secondary forest
subject: agricultural landscapes
subject: animal scat
subject: leaf litter
subject: propagule bank
subject: soil seed banks
subject: assisted restoration
subject: establishment

description: Few areas on earth remain untouched by human activity. Agricultural land has become the largest terrestrial biome globally with ~ 40% of earth’s surface occupied by pastures and croplands. This has resulted in widespread biodiversity loss and ecosystem alteration, which is being further exacerbated by climate change. Urgent and widespread revegetation of agricultural landscapes is required to slow and remediate these impacts. Simultaneously, the extent of abandoned agricultural land, often referred to as old-fields, is increasing worldwide due to a range of socioeconomic factors, providing significant opportunities for revegetation. Natural regeneration, whereby an area is left to revegetate on its own, with minimal human intervention, can be a cost-effective and large-scale restoration approach well suited to some, but not all, agricultural landscapes. While there are numerous success stories regarding natural regeneration from around the world, natural regeneration in other areas is significantly constrained by low propagule supply and/or high levels of land degradation. Considerable knowledge gaps regarding patterns and drivers of natural regeneration remain and improved understanding of these processes is critical for informing improved decisions regarding effective revegetation strategies for degraded landscapes. This thesis examines natural regeneration of vegetation within agricultural landscapes, specifically of old-fields, with a focus on floodplains of the northern Murray-Darling Basin, Australia. More specifically, the research presented here sought to: 1) understand global trends of natural regeneration of woody vegetation in agricultural landscapes and its key drivers,; 2) describe patterns and drivers of natural regeneration within floodplain old-fields of the northern Murray-Darling Basin; 3) understand the regenerative capacity from major propagule sources within agricultural floodplain landscapes of the northern Murray-Darling Basin; and 4) investigate how different hydrological events influence the regeneration of riparian vegetation in semi-arid agricultural landscapes. Chapter 1 introduces the thesis, presenting background information on revegetation within agricultural landscapes, restoration approaches, a conceptualisation of states and trajectories of natural regeneration, and a description of the study area. In Chapter 2, I explore the global literature on natural regeneration within agricultural landscapes through a systematic quantitative literature review designed to provide a deeper understanding of the compositional and structural characteristics of naturally regenerating woody vegetation, key factors driving these patterns, and the global state of knowledge in this field. To conduct this review, I surveyed an initial 947 research papers from across the globe and selected a shortlist of 84 primary empirical studies based on a set of inclusion and exclusion criteria. Among the shortlisted studies, it was observed that regenerating vegetation does not return completely to that of remnant reference conditions in relation to composition and structural characteristics, and most commonly exhibits novel attributes in relation to both. Land-use history, propagule supply, species interactions (i.e., facilitation and competition), and soil properties emerged as the four broad factors driving patterns in naturally regenerating woody vegetation in oldfields globally. Overall, the review found that despite vegetation regrowing with a novel compositional and structural character, this approach does provide a viable form of restoration in a wide range of conditions. In Chapters 3, 4, and 5, the results are then presented of several field and glasshouse studies designed to better understand patterns, processes, and drivers of natural regeneration in floodplain old-fields across the northern Murray-Darling Basin. Chapter 3 explores patterns and drivers of extant woody vegetation regrowth on old-fields distributed amongst four major regions representing a broad aridity gradient across the northern Murray-Darling Basin. The aim of this chapter is to identify key compositional and structural characteristics of naturally regenerating woody vegetation, and drivers of these, across the study area. This study found that after approximately 30–40 years, woody vegetation in these old-fields begins to regenerate in greater density to that of remnant reference sites. Nevertheless, regenerating old-fields did support woody vegetation of similar species composition to remnant reference sites. Mean annual rainfall, time since abandonment, and presence of remnant paddock trees were identified as major drivers of natural regeneration in this study. Overall, this chapter demonstrates that natural regeneration can be a viable form of restoration within these landscapes, provided adequate time for recovery is allowed. For successful natural regeneration to occur, there must be adequate plant propagules within a site and/or being supplied to the area. Chapter 4 explores how various major seed banks (i.e., soil, animal scat and leaf litter) contribute to the overall regenerative capacity of old-fields in the northern Murray-Darling Basin. The year-long seedling emergence experiment studied seed banks from the 12 old-fields as well as adjacent remnant vegetation patches investigated in Chapter 3, to understand how land-use history influences regenerative capacity. This study found that each seed bank type played an important role in contributing to the overall regenerative success, although, there was little difference between seed banks of different land-use histories, indicating high levels of seed bank resilience to agricultural land-use disturbances. The results suggest that soil seed banks are vital for storing high herb species richness. Additionally, animal scats support relatively high abundances of emerging seedlings, indicating that zoochory is likely to be a vital seed dispersal mode within these landscapes. Finally, leaf litter, while producing very low abundances of emerging seedlings overall, contained the highest abundance of germinable seeds of woody species in these oldfields. Very low numbers of woody tree seedlings emerged from the experiment overall, however, indicating that trees are unlikely to store seeds within these seed banks in these habitats and likely rely on more immediate supply from remnant paddock trees or vegetation patches. This research indicated that seed banks are important contributors to natural regeneration of these old-fields, but mainly with respect to understory and midstory species. Within semi-arid ecosystems, water regimes are critical in driving plant establishment and vegetation regeneration and previous research has shown the importance of flooding in shaping plant population and vegetation dynamics within arid and semi-arid floodplain habitats. Knowledge gaps remain, however, regarding the importance of flooding versus rainfall events. In this chapter, I present the results of a field experiment examining the establishment of Eucalyptus camaldulensis seedlings and understory vegetation in response to differing hydrological events (flooding, rainfall and drought) in two different habitat types (remnant riparian zone and adjacent floodplain old-field). Imposed flooding, rainfall and drought treatments were conducted on 180 Eucalyptus camaldulensis and in situ extant vegetation within 18 grazing enclosure plots over a 9-month experiment. Overall, there was very high seedling mortality, with ~3% of Eucalyptus seedlings surviving the entire experiment. Flooding was critical, however, in driving this survival of seedlings and supporting the resilience of understory cover and diversity over this period. The survival of seedlings up until 10 weeks was higher in riparian plots than in floodplain old-field plots, although habitat type had less of an effect within flooded plots. The results of this study demonstrates that rainfall alone is unlikely to be sufficient to enable adequate vegetation regeneration in these habitats. Overall, the combined results of this project clearly demonstrate that floodplain old-fields of the northern Murray-Darling Basin exhibit significant resilience to previous agricultural landuse at landscape scales. Furthermore, this thesis demonstrates that natural regeneration is a viable approach to revegetating old-field floodplain habitats in this region, adding to the broad range of diverse case studies reporting effective natural regeneration in agricultural landscapes globally. The final chapter (Chapter 6) of this thesis synthesises the key findings and management implications of the knowledge generated by this thesis, as well as identifying future research needs which can better support effective regeneration strategies of such landscapes.