Post-fire resprouting capacity of seasonally dry forest species - Two quantitative indices

Qualitative measures of resprouting capacity often fail to capture inter- and intra-species variation, whereas available quantitative methods can be complex and time-consuming, hindering broad-scale comparative studies. Here, we propose two quantitative indices that can be applied in a standard way in different regions. We sampled 1046 plants of 20 dominant species (6 shrubs, 7 trees and 7 tree/shrubs) from the seasonally dry forests of the arid Chaco, central-western Argentina. Sampling was conducted in burned field sites one growing season after fire. For each sampled plant we measured the diameter of the main burned trunk (MTD) and main resprout (MRD), and the number of burned trunks (TN) and resprouts (RN); we then built estimated composite diameters for trunks and resprouts (ECTD and ECRD) and we calculated two alternative Resprouting Capacity Indices: RCI 1 (ECRD/ECTD); and RCI 2 (MRD/MTD). The indices were validated against a measure of Resprouting Vigour (RV) that included detailed measurement of all trunks and resprouts for a subset of sampled plants. In all cases, variables indicative of fire severity were measured and included in the analyses. The RCIs and RV were highly related, both at the species and growth form levels. Fire severity had no significant effect on these relationships, but growth form affected RCI 2. All species were capable of resprouting, showing considerable inter-species variation for the two proposed RCIs. Species rank differed considerably between RCIs and survival-only estimations. RCI 1 was higher in tree/shrubs (i.e. species regarded as trees or shrubs) and trees than in shrubs. All species showed decreasing resprouting capacity with increasing ECTD. Our results support the use of the proposed RCIs as a robust tool to assess resprouting capacity, providing more details than survival-based assessments. Choosing one or the other implies a trade-off between accuracy and simplicity, and may depend on the scale and objective of the study, and resprouting patterns of studied species. Species, growth form and individual plant size are relevant in explaining post-fire resprouting capacity and survival.