Post-fire tree stress and growth following smoldering duff fires

Abstract

Understanding the proximate causes of post-fire conifer mortality due to smoldering duff fires is essential to the restoration and management of coniferous forests throughout North America. To better understand duff fire-caused mortality, we investigated tree stress and radial growth following experimental fires in a long-unburned forest on deep sands in northern Florida, USA. We burned basal fuels surrounding 80 mature Pinus palustris Mill. in a randomized experiment comparing the effects of basal burning treatments on stem vascular meristems; surficial roots; root and stem combinations; and a non-smoldering control. We examined the effects of duration of lethal temperatures (>60°C) on subsequent pine radial growth and root non-structural carbohydrates (starch and sugar). Duff and mineral soil temperatures in the experimental fires consistently exceeded 60°C for over an hour following ignition, with lethal temperatures of shorter duration recorded 20cm below the mineral soil surface. Duff heating was best explained by day-of-burn Oe horizon moisture (P =0.01), although little variation was explained (R ² =0.24). Post-fire changes in latewood radial increment in the year following fires was related to duration of temperatures >60°C 10cm deep in the mineral soil (P =0.07), but explained little variability in post-fire growth (R ² =0.17). In contrast, changes in non-structural carbohydrate content in coarse roots (2-5mm diameter) 120 days following burning were more strongly correlated with the duration of lethal heating 5cm below the mineral soil surface (P =0.02; R ² =0.53). Results from this study implicate the role of mineral soil heating in the post-fire decline of mature longleaf pine following restoration fires in sandy soils.