Most global biomes are shaped by disturbances, such as fire or herbivory, that damage or kill the aboveground biomass of plants. In many of these biomes, however, the damaged plants do not die; rather, they persist through sprouting. In disturbance-prone environments, resprouting from meristems stabilizes plant populations where disturbance may cause demographic bottlenecks. The advantage of resprouting is that it confers persistence under disturbance; however, sprouting has disadvantages such as potentially reducing sexual reproduction and limiting gene flow (Bond & Midgley, 2001; Lamont & Wiens, 2003). Understanding resprouting is critically important for understanding long-term vegetation dynamics, extinction risks, carbon balance and woody plant management. A Vegetation Function Working Group (Working Group 67; http://www.vegfunction.net/wg/67/67_Sprouting.htm) was established in 2009 to identify challenges encountered in developing coherent models of the functional role of resprouting in fire-prone environments, with an emphasis on savanna and Mediterranean biomes. Fire is the most pervasive disturbance and is integral to the evolutionary ecology of these biomes. This broad disciplinary group met at the International Ecological Conference (INTECOL) in Brisbane in September 2009 and reconvened in July 2010 at The University of New England, Armidale, Australia, to review progress. In this report we highlight major challenges encountered in developing unifying models of the functional role of resprouting.

1. In fire-prone ecosystems, bark protects the stem bud bank from fire. Absolute bark thickness is a good indicator of this protective function, but it depends on stem size as well as inherent differences between species. Relative bark thickness (i.e. relative to stem diameter) takes the latter into account. We argue that relative bark thickness is an important functional trait offering insights to the evolution of species persistence in fire-prone habitats. 2. During growth ontogeny different species can acquire absolutely thick bark through having: (i) relatively thick bark (i.e. an early commitment to thick bark) or (ii) relatively thin bark but fast stem diameter growth rates. We test the hypothesis that the most effective way of protecting tree stems from frequent fire is by having relatively thick-barked small stems. We predict that species with higher relative bark thickness are more common in fire-prone habitats. In habitats with long fire-free intervals such as rainforest, delayed investment in bark thickness results in thin bark. 3. We examined the relative bark thickness of woody congeners from Australian non-fire-prone forest and fire-prone savanna and in other tree-dominated systems world-wide. We determined the relative cost of acquiring absolute bark thickness of 0.5 cm for different rates of bark allocation. The insulating benefits of bark were considered a linear function of bark thickness. 4. Synthesis. We suggest that relatively thick bark minimizes the costs of acquiring absolutely thick bark, and it confers greater protection to smaller stems. The cost of acquiring thick bark prevents small trees from merely accumulating bark as a consequence of fast height or stem diameter growth. Accordingly, our field survey indicated that forest species had relatively thin bark and acquired thick bark only as a consequence of very large size, while fire-prone savanna species had relatively thick-barked small stems. Based on this, relative bark thickness appears to be a good predictor of local fire regimes and is a useful plant functional trait.