Size-dependent enhancement of water relations during post-fire resprouting

In resprouting species, fire-induced topkill causes a reduction in height and leaf area without a comparable reduction in the size of the root system, which should lead to an increase in the efficiency of water transport after fire. However, large plants undergo a greater relative reduction in size, compared with small plants, so we hypothesized that this enhancement in hydraulic efficiency would be greatest among large growth forms. In the ecotone between long-leaf pine (Pinus palustris Mill.) savannas and wetlands, we measured stomatal conductance (g s), mid-day leaf water potential (Ψ leaf), leaf-specific whole-plant hydraulic conductance (K L.p), leaf area and height of 10 species covering a range of growth forms in burned and unburned sites. As predicted, K L.p was higher in post-fire resprouts than in unburned plants, and the post-fire increase in K L.p was positively related to plant size. Specifically, large-statured species tended to undergo the greatest relative reductions in leaf area and height, and correspondingly experienced the greatest increases in K L.p. The post-fire increase in K L.p was smaller than expected, however, due to a decrease in absolute root hydraulic conductance (i.e., not scaled to leaf area). The higher K L.p in burned sites was manifested as an increase in g s rather than an increase in Ψ leaf. Post-fire increases in g s should promote high rates of photosynthesis for recovery of carbohydrate reserves and aboveground biomass, which is particularly important for large-statured species that require more time to recover their pre-fire size.