K. Ruthrof

Advances in restoration ecology: rising to the challenges of the coming decades

Simultaneous environmental changes challenge biodiversity persistence and human wellbeing. The science and practice of restoration ecology, in collaboration with other disciplines, can contribute to overcoming these challenges. This endeavor requires a solid conceptual foundation based in empirical research which confronts, tests and influences theoretical developments. We review conceptual developments in restoration ecology over the last 30 years. We frame our review in the context of changing restoration goals which reflect increased societal awareness of the scale of environmental degradation and the recognition that inter-disciplinary approaches are needed to tackle environmental problems. Restoration ecology now encompasses facilitative interactions and network dynamics, trophic cascades, and above- and below ground linkages. It operates in a non-equilibrium, alternative states framework, at the landscape scale, and in response to changing environmental, economic and social conditions. Progress has been marked by conceptual advances in the fields of trait-environment relationships, community assembly, and understanding the links between biodiversity and ecosystem functioning. Conceptual and practical advances have been enhanced by applying evolving technologies, including treatments to increase seed germination and overcome recruitment bottlenecks, high throughput DNA sequencing to elucidate soil community structure and function, and advances in satellite technology and GPS tracking to monitor habitat use. The synthesis of these technologies with systematic reviews of context dependencies in restoration success, model based analyses and consideration of complex socio-ecological systems will allow generalizations to inform evidence based interventions. Ongoing challenges include setting realistic, socially acceptable goals for restoration under changing environmental conditions, and prioritizing actions in an increasingly space-competitive world. Ethical questions also surround the use of genetically modified material, translocations, taxon substitutions, and de-extinction, in restoration ecology. Addressing these issues, as the Ecological Society of America looks to its next century, will require current and future generations of researchers and practitioners, including economists, engineers, philosophers, landscape architects, social scientists and restoration ecologists, to work together with communities and governments to rise to the environmental challenges of the coming decades.

Landscape-scale assessment of tree crown dieback following extreme drought and heat in a Mediterranean eucalypt forest ecosystem

Mediterranean regions are under increasing pressure from global climate changes. Many have experienced more frequent extreme weather events such as droughts and heatwaves, which have severe implications for the persistence of forest ecosystems. This study reports on a landscape-scale assessment investigating potential associated factors of crown dieback in dominant tree species following an extreme dry and hot year/summer of 2010/11 in the Northern Jarrah Forest of Western Australia. Analyses focussed on the influence of (i) geology, (ii) topography, (iii) climate, and (iv) fire history. The results showed that trees on specific soils were more likely to show canopy dieback. Generally, trees on rocky soils with low water holding capacity were found to be affected more frequently. Other explanatory factors identified that dieback occurred (i) on sites that were close to rock outcrops, (ii) in areas that received a slightly higher amount of annual rainfall compared to the surrounding landscape, (iii) on sites at high elevations and (vi) on steep slopes, and (v) in areas that were generally slightly warmer than their surroundings. These results expand our understanding of how landscape-scale factors contribute to the effects of an extreme drought and heating event in Mediterranean forest ecosystems, and give indications of where changes are likely to occur within the landscape in the future. The analogues with other Mediterranean climate regions make the results of this study transferable and a starting point for further investigations.

An ecoclimatic framework for evaluating the resilience of vegetation to water deficit

The surge in global efforts to understand the causes and consequences of drought on forest ecosystems has tended to focus on specific impacts such as mortality. We propose an ecoclimatic framework that takes a broader view of the ecological relevance of water deficits, linking elements of exposure and resilience to cumulative impacts on a range of ecosystem processes. This ecoclimatic framework is underpinned by two hypotheses: (i) exposure to water deficit can be represented probabilistically and used to estimate exposure thresholds across different vegetation types or ecosystems; and (ii) the cumulative impact of a series of water deficit events is defined by attributes governing the resistance and recovery of the affected processes. We present case studies comprising Pinus edulis and Eucalyptus globulus, tree species with contrasting ecological strategies, which demonstrate how links between exposure and resilience can be examined within our proposed framework. These examples reveal how climatic thresholds can be defined along a continuum of vegetation functional responses to water deficit regimes. The strength of this framework lies in identifying climatic thresholds on vegetation function in the absence of more complete mechanistic understanding, thereby guiding the formulation, application and benchmarking of more detailed modelling.

Restoration treatments improve seedling establishment in a degraded Mediterranean-type Eucalyptus ecosystem.

Restoration of degraded Mediterranean-type ecosystems (MTEs) with long, hot and dry summers is challenging. To develop management guidelines, we evaluated techniques that could improve seedling establishment in two degraded Eucalyptus gomphocephala DC (tuart) woodlands, given weed and herbivore control. These techniques aimed to mimic favourable conditions for species that primarily recruit following disturbance events (e.g. fire). Trial 1 investigated the response of 5-month-old seedlings and broadcast seed in plots that contained a created ashbed, were ripped, or were ripped and contained an ashbed. Trial 2 examined the response of 5-month-old seedlings to treatments providing a nutrient or moisture source (slow-release fertiliser tablet, chelating agent, slow-release fertiliser tablet plus chelating agent, zeolite, hydrated hydrophilic co-polymers and dry hydrophilic co-polymers). Results indicated that created ashbeds enhance establishment for a range of species and reduce weed cover, with or without ripping. Broadcast seeding was not successful in returning species to site. Higher growth rates were recorded in seedlings treated with a nutrient source. The present study has shown that it is possible to re-establish local plant species in degraded woodlands through several techniques that mimic disturbance (e.g. fire). Strong early growth may be the vital start seedlings need in MTEs in the face of reinvading weed species, herbivory and a drying climate.

Research frontiers for improving our understanding of drought-induced tree and forest mortality

Accumulating evidence highlights increased mortality risks for trees during severe drought, particularly under warmer temperatures and increasing vapour pressure deficit (VPD). Resulting forest die-off events have severe consequences for ecosystem services, biophysical and biogeochemical land-atmosphere processes. Despite advances in monitoring, modelling and experimental studies of the causes and consequences of tree death from individual tree to ecosystem and global scale, a general mechanistic understanding and realistic predictions of drought mortality under future climate conditions are still lacking. We update a global tree mortality map and present a roadmap to a more holistic understanding of forest mortality across scales. We highlight priority research frontiers that promote: (1) new avenues for research on key tree ecophysiological responses to drought; (2) scaling from the tree/plot level to the ecosystem and region; (3) improvements of mortality risk predictions based on both empirical and mechanistic insights; and (4) a global monitoring network of forest mortality. In light of recent and anticipated large forest die-off events such a research agenda is timely and needed to achieve scientific understanding for realistic predictions of drought-induced tree mortality. The implementation of a sustainable network will require support by stakeholders and political authorities at the international level.

Linking restoration outcomes with mechanism: the role of site preparation, fertilisation and revegetation timing relative to soil density and water content

Global land use and ongoing climate change highlight the importance of ecological restoration as an emerging discipline and underscore the need for successful revegetation techniques. To link mechanistic drivers of seedling establishment with techniques to increase revegetation success, we undertook field-based experiments in degraded peri-urban woodlands in Mediterranean southwestern Australia using two iconic tree species. Over the course of an entire growing season, our objectives were to: (1) characterise soil moisture profiles in relation to site preparation techniques (ripping and created ashbeds) and (2) determine whether early seedling establishment can be increased through site preparation techniques (ripping and ashbeds), plant treatments (various fertilisers and biochar) and early planting during the wet season. Ripping significantly reduced soil compaction and was associated with a significant soil moisture stratification; moisture penetrated to greater depths in soils that were ripped or treated with ashbeds. Particular site preparation techniques (ripping) and early planting significantly increased early establishment, health and growth of tree seedlings. Fertilisation effects varied by species with generally neutral effects on seedlings. Finally, seedlings planted in ripped soils had significantly longer, deeper root systems accessing portions of the soil profile with higher summer moisture. Techniques such as ripping, even in deep sandy soils, reduce soil compaction and alter moisture availability within the soil profile, promoting deeper root growth and thus increasing revegetation success in these degraded Mediterranean ecosystems. Linkage of revegetation outcomes with plant response and physical soil properties associated with particular treatments provides critical knowledge for both restoration scientists and land managers.

A Critical Evaluation of Interventions to Progress Transdisciplinary Research

Transdisciplinary research is widely being promoted for its potential to effectively address complex issues, such as ecosystem management in a changing climate. Working across disciplines and with broader society can benefit greatly from continuous evaluation to improve transdisciplinary practices. However, methods for such continuous self-reflection are scarce, with little evidence of the application of social science concepts, theory, or methods. This article presents a case study of how researchers from different disciplinary backgrounds have familiarized themselves with the key social science concepts of “structure” and “agency” to reflect on the integrative research efforts of a research center in southwestern Australia. They identified influential “structures” as the geographical separation of the centers research groups, contrasting research cultures, and little previous engagement with the social sciences. Evidence of “agency” comprised various interventions to promote collaboration. Intriguingly, these interventions rendered some challenging paradoxes.

Overcoming restoration thresholds and increasing revegetation success for a range of canopy species in a degraded urban Mediterranean-type woodland ecosystem

Widespread decline of Mediterranean-type ecosystem (MTE) woodlands can result in a loss of soil- and canopy-stored seed banks. This can drive woodlands across a biotic threshold, where natural regeneration cannot occur. Without management intervention, these woodlands will suffer local extinction. Using a Mediterranean-type, degraded woodland as a case study, we undertook field trials over 3 years, with the aim of increasing revegetation success by (1) introducing propagules of key canopy species to overcome this biotic threshold and (2) applying commonly used revegetation treatments (abiotic treatments such as the addition of nutrient and water resources, two types of tree guards, and combinations of these). We found that (1) control plants had low establishment success, confirming the crossing of a biotic threshold and the practical irreversibility of the degraded state without intervention, (2) plant establishment was often significantly higher for treated than for control seedlings and (3) supplementation of nutrient and water resources seems to be critical in terms of increasing early seedling establishment for some species. We suggest that in declining woodlands that have crossed biotic thresholds, merely adding propagules does not ensure successful revegetation. The present study has practical implications for restoration activities in degraded MTE communities where biotic thresholds may have already been crossed.

Invasion by Eucalyptus megacornuta of an Urban Bushland in Southwestern Australia

Abstract Eucalypts have been planted extensively beyond their natural range but few of these plantings have become invasive. The Kings Park bushland in Western Australia is an example of a native bushland in which a few nonlocal eucalypt species (Warted Yate, sugar gum, and lemon-scented gum) were introduced as ornamentals and have since spread into the bushland. Although the population expansion was obvious, little was known about the factors influencing this pattern, making management options unclear. The study objective was to investigate the factors facilitating the invasion of Kings Park by Warted Yate. The study has shown that ecological characteristics that may be facilitating the Warted Yate invasion in Kings Park include mass recruitment of seedlings subsequent to fire, high seedling survival rates, high levels of serotinous seed storage, and a high germination rate. It seems that this species has a high resilience in a fire-prone environment. However, it appears that Warted Yate is not as invasive as sugar gum. The Kings Park situation highlights the risks associated with planting introduced eucalypts at the interface between native and managed systems. Nomenclature: Corymbia citriodora; Eucalyptus cladocalyx; Warted Yate, Eucalyptus megacornuta. Additional index words: Ecological study of invasive eucalypts, restricted natural distribution.

Topsoil Stockpiling in Restoration: Impact of Storage Time on Plant Growth and Symbiotic Soil Biota

Addressing plant-soil relationships within restoration science may improve success and reduce costs. Here we assess the question of topsoil storage time: how does stockpile age impact plant biomass and soil microbial activity, particularly root symbionts such as rhizobia and arbuscular mycorrhizal fungi (AMF)? Working in Western Australia and in sandy soils, we grew a legume species Acacia saligna (Fabaceae) in one-, two-, three-, five- and ten-year-old stockpile soils under controlled glasshouse conditions. We assessed whether plant biomass, specific root length, and root diameter decreased with stockpile age. Further, we investigated how stockpile age affected the distribution and the number of effective nodules, nodule biomass and AMF colonization in roots. These above and belowground traits were chosen because they reflect the A. saligna response to growing in soils stockpiled for 1–10 years. We hypothesised that in older stockpiles there would be fewer rhizobial and AMF propagules which would constrain A. saligna growth, whereas in younger stockpiles there would be more rhizobial and AMF propagules, and A. saligna growth would be facilitated via these root symbionts. Using generalized linear mixed models, we found that total plant biomass was the lowest but AMF percent colonization was the highest when A. saligna was grown in ten-year-old soils, suggesting that AMF spores and hyphae are present in old stockpiles. Our results demonstrate that AMF communities may be initially disrupted by soil disturbance and storage, and then begin to re-establish between 5–10 years after stockpiling. However, other soil microbial communities, such as fungal pathogens that were not assessed in this study may have been responsible for decreased A. saligna biomass in older soils. Further research, particularly on other soil microbial communities, is needed to understand restoration success using stockpiled soil older than ten years.