Estimating fire interval bounds using vital attributes: implications of uncertainty and among-population variability

Identifying the range of appropriate fire return intervals is crucial for ecosystem management in fire-prone environments. Plant vital attributes and changes in their associated trait values with time since fire are important indicators of suitable fire interval bounds to conserve biodiversity. However, using vital attributes to derive prescriptions for acceptable fire intervals remains challenging due to (1) uncertainty regarding how traits are best measured, (2) uncertainty in the acceptable ranges of trait values to avoid local extinctions, and (3) potential for variability among populations in the time taken postfire to reach trait threshold values. Using a time-since-fire gradient in contrasting mallee and mallee-heath vegetation types of southwestern Australia, we calculate, compare, and aggregate fire interval bound predictions from nine serotinous non-resprouters and seven serotinous resprouters across these three sources of uncertainty or variation. Relationships between time since fire and both trait measures reflecting minimum fire interval (mean number of closed fruit per plant or proportion of plants with closed fruit) were typically significant, had reasonable goodness of fit, and showed similar patterns of change with time since fire. Significant relationships reflecting maximum fire interval were less frequent but were more commonly detected using direct measures of mortality than using evidence for decline in reproductive potential. Of the two sources of uncertainty, trait value threshold selection caused more substantial differences in estimated interval bounds than trait measurement method. Variation between populations increased with greater estimated minimum interval length and, in some species, rendered interval estimates of limited practical value. On balance, we conclude that measures of vital attribute traits offer a transparent approach for estimating fire interval bounds at the plant community level, but selection of trait value thresholds is in need of stronger biological justification in their application. Further, variation between populations should be explicitly sampled if fire interval estimates are to be applied across the landscape.