Sprouting by semi-arid plants: testing a dichotomy and predictive traits

A widely-used description of vegetation response to fire is that species can be clearly classified as sprouters or non-sprouters. We aimed to assess: (1) how well this dichotomous classification (sprouter/non-sprouter) described the responses of a semiarid flora to experimental disturbance; (2) how similar were sprouting responses to treatments mimicking intense herbivory and fire; (3) how well easily-measured traits could predict sprouting. Sprouting was assessed for 45 species from a range of growth forms (grasses, forbs, sub-shrubs, woody shrubs and trees) from semi-arid southeastern Australia. We used two treatments: clipping at stem base, and clipping followed by burning with a blowtorch. A dichotomy accounted for /60% of deviance explained by species identity. Models with three or four groups were not substantially better. The dichotomy was not between 0% and 100% sprouting, rather between ‘weak’ and ‘strong’ sprouters. Probabilities of sprouting for weak sprouters were 23% after clipping and 6% after burning, while strong sprouters had sprouting probabilities of 90% after clipping and 79% after burning. While sprouting varied in space and time, the dichotomy was robust to this variation. Sprouting ability increased with size in most of the species with variable sprouting. Sprouting was partially related to growth form; grasses sprouted strongly, chenopods weakly, and forb and woody species covered the range of sprouting. Strong sprouters were likely to have more stems per plant, greater basal area, shorter potential height and deeper buds than weak sprouters. A hierarchical model that used growth form and then stems-per-plant provided a simple, robust predictor of sprouting. Four-fifths of species responded consistently to clipping and burning while one-fifth of species were strong sprouters after clipping but weak sprouters after burning. Burning reduced sprouting most in intermediate sprouters. Differences between sprouting after clipping and burning reflected increased intensity and were related to the depth of buds below ground.