The influence of fire on forest birds at multiple scales

© 2014 Dr. Holly SittersImproved understanding of the impact of fire on fauna is required because the frequency and severity of fire are predicted to increase under climate change, and the implications for biodiversity are largely unknown. To better understand the characteristics of fire regimes that sustain avian diversity, my thesis tests two overarching hypotheses: (i) that bird diversity increases with fire-mediated landscape heterogeneity; and (ii) that bird diversity increases with fine-scale heterogeneity in vegetation structure and plant species diversity. To test my first hypothesis, I examined bird responses to inter-patch variation in fire age class and vegetation type using landscape sampling units at a large spatial scale (60,000 ha). At a smaller scale (400 ha), I used a before-after control-impact experiment to investigate the effects of intra-patch variation in fire severity on bird diversity and the occurrence of individual species. To test my second hypothesis, I used measurements of vegetation structure and plant diversity to explain patterns in taxonomic diversity, functional diversity and species’ occurrence. Birds were surveyed across a 70-year chronosequence spanning four broad vegetation types, from heathland to wet forest. Results provided some support for both hypotheses. First, bird diversity was positively associated with landscape heterogeneity at the inter- and intra-patch levels. Second, bird functional evenness was positively related to fine-scale structural heterogeneity, and 13 of 15 modelled species responded to elements of habitat structure measured at fine scales. Only four of the 13 species responded to time since fire, indicating that time is unlikely to be a useful surrogate for bird occurrence in systems characterised by variable rates of post-fire structural development. Although I identified positive relationships between bird diversity and fire-mediated heterogeneity at multiple scales, results indicate that older vegetation is of disproportionate importance to the region’s birds, and that the preservation of old vegetation is paramount. Management strategies that use controlled application of patchy, low-severity fire to break up large areas of mature vegetation are likely to enhance avian diversity, ecosystem function and resilience, while conserving species reliant on older vegetation

Case Study: The Ecology of Mixed-Severity Fire in Mountain Ash Forests

The eucalypt forests of southeast Australia are among the most flammable ecosystems worldwide. Most of the forest in the region is dominated by a single overstory eucalypt species. Here we describe stands of mountain ash (Eucalyptus regnans), which is the tallest flowering plant species in the world, at heights approaching 100 m

Above-ground and soil-seedbank species diversity in the juvenile growth-stage

We used fire-history maps to stratify the landscape into four growth stages: 0-3 years since fire (juvenile); 4–10 years (young); 11–34 years (mature); and >34 years (old). We established 71 sites across the range of growth stages within each region. At each site, we counted all above-ground shrub, sub-shrub, and herbaceous species in six 3 × 3 m quadrats, two each at ridge, mid-slope (midway between ridge and gully) and lower-slope/gully locations. The soil seedbank was sampled by extracting soil cores (6 cm diameter, 5 cm depth) and treating, germinating, and identifying the seeds within

Soil-seedbank species diversity in the juvenile growth-stage

We used fire-history maps to stratify the landscape into four growth stages: 0-3 years since fire (juvenile); 4–10 years (young); 11–34 years (mature); and >34 years (old). We established 71 sites across the range of growth stages within each region. The soil seedbank was sampled by extracting soil cores (6 cm diameter, 5 cm depth) and treating, germinating, and identifying the seeds within

R script for growth-stage optimisation, sensitivity analysis, and abundance change

R script for growth-stage optimisation, sensitivity analysis, and abundance chang

Data from: Survey design for precise fire management conservation targets

Common goals of ecological fire management are to sustain biodiversity and minimize extinction risk. A novel approach to achieving these goals determines the relative proportions of vegetation growth stages (equivalent to successional stages, which are categorical representations of time since fire) that maximize a biodiversity index. The method combines data describing species abundances in each growth stage with numerical optimization to define an optimal growth-stage structure which provides a conservation-based operational target for managers. However, conservation targets derived from growth-stage optimization are likely to depend critically on choices regarding input data. There is growing interest in use of growth-stage optimization as a basis for fire management, thus understanding of how input data influence the outputs is crucial. Simulated datasets provide a flexible platform for systematically varying aspects of survey design and species inclusions. We used artificial data with known properties, and a case-study dataset from southeastern Australia, to examine the influence of (i) survey design (total number of sites, and their distribution among growth stages) and (ii) species inclusions (total number of species and their level of specialization) on the precision of conservation targets. Based on our findings, we recommend that survey designs for precise estimates would ideally involve at least 80 sites, and include at least 80 species. Greater numbers of sites and species will yield increasingly reliable results, but fewer might be sufficient in some circumstances. An even distribution of sites among growth stages was less important than the total number of sites, and omission of species is unlikely to have a major influence on results as long as several species specialize on each growth stage. We highlight the importance of examining the responses of individual species to growth stage before feeding survey data into the growth-stage optimization black box, and advocate use of a resampling procedure to determine the precision of results. Collectively, our findings form a reproducible guide to designing ecological surveys that yield precise conservation targets through growth-stage optimization, and ultimately help sustain biodiversity in fire-prone systems

Above-ground species diversity in the mature growth-stage

We used fire-history maps to stratify the landscape into four growth stages: 0-3 years since fire (juvenile); 4–10 years (young); 11–34 years (mature); and >34 years (old). We established 71 sites across the range of growth stages within each region. At each site, we counted all above-ground shrub, sub-shrub, and herbaceous species in six 3 × 3 m quadrats, two each at ridge, mid-slope (midway between ridge and gully) and lower-slope/gully locations

Soil-seedbank species diversity in the mature growth-stage

We used fire-history maps to stratify the landscape into four growth stages: 0-3 years since fire (juvenile); 4–10 years (young); 11–34 years (mature); and >34 years (old). We established 71 sites across the range of growth stages within each region. The soil seedbank was sampled by extracting soil cores (6 cm diameter, 5 cm depth) and treating, germinating, and identifying the seeds within

Soil-seedbank species diversity in the young growth-stage

We used fire-history maps to stratify the landscape into four growth stages: 0-3 years since fire (juvenile); 4–10 years (young); 11–34 years (mature); and >34 years (old). We established 71 sites across the range of growth stages within each region. The soil seedbank was sampled by extracting soil cores (6 cm diameter, 5 cm depth) and treating, germinating, and identifying the seeds within

Above-ground species diversity in the young growth-stage

We used fire-history maps to stratify the landscape into four growth stages: 0-3 years since fire (juvenile); 4–10 years (young); 11–34 years (mature); and >34 years (old). We established 71 sites across the range of growth stages within each region. At each site, we counted all above-ground shrub, sub-shrub, and herbaceous species in six 3 × 3 m quadrats, two each at ridge, mid-slope (midway between ridge and gully) and lower-slope/gully locations