Post-fire tree stress and growth following smoldering duff fires

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 8C) 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 8C for over an hour following ignition, with lethal temperatures of shorter duration recorded 20 cm 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 8C 10 cm 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 (2–5 mm diameter) 120 days following burning were more strongly correlated with the duration of lethal heating 5 cm 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