Combustion in coniferous forests dependent on frequent fire plays two important roles. First, fire levels the competitive playing field by suppressing fire sensitive species that would otherwise outcompete fire adapted species. Second, the spatial variability of fire intensity creates patches of mortality that then drive plant community assembly through neutral processes. In the absence of fire, both processes disappear with consequences for plant community assembly. Here we examine how assembly rules change from neutral dominated to those driven by competitive advantage conferred by some life history traits and the implications for fire management in frequently burned forests. The role of spatial heterogeneity in fire energy release was investigated in experimental burns in longleaf pine (Pinus palustris) sandhills and flatwoods at Eglin Air Force Base in Florida, USA. To test for the link between mortality and fire intensity we measured fire radiative power and energy in 14 experimental burns using infrared thermography and monitored pre- and post-fire plant demography in 48 plots including 7 plots that were unburned. In 9 plots, we experimentally manipulated coarse fuels loads to test their influence on plant mortality.
The spatial variability in fire energy release was driven by the difference in combustion characteristics between fine fuels (pine needles, grasses, forbs) and coarser fuels (pine cones, twigs, branches). While there was a wide range of energy release linked to variable fine fuel loads driven by overstory pine density, coarser fuels on average released ~7 MJ more energy than fine fuels (mean of 0.64 MJ). Plants recieving more than 4 MJ of fire radiative energy were almost 3 times more likely to die than plants burned at the lower intensity in fine fuels (logistic regression, odds ratio 2.78, p =0.023). Recruitment into the mortality patches was mostly explained by a simple neutral model. In the fire excluded plots, mortality occurred in the absence of fire and plants with certain suites of life history traits (e.g. large stature, clonal, woody) had a higher probability of recruitment than others (sessile, herbaceous, small stature). The effect of the lack of fire moves the system from being driven from neutral, primarily stochastic dynamics towards a more deterministic competition driven assembly. While a continuum from neutral to niche based assembly rules can exist spatially, our results suggest the continuum can also occur through time within the same space.