Individual heterogeneous growth in longleaf pines: The effect of fire
Within a population, individuals frequently differ in the rate at which they grow, and this rate can be impacted by both biotic factors (including competitors, pathogens, and genetic variation) and abiotic factors (including microsite differences). We investigated the effect of fire frequency, year, and tree age on variation in radial growth heterogeneity in longleaf pine (Pinus palustris P. Mill.) individuals in a southwest Florida sandhill community. Tree-ring data were collected from pines in experimental plots, each with one of four levels of prescribed fire-return interval. The fire-return intervals approximated 1-, 2-, 5-, and 7-year fire frequencies, or the plot was left unburned. Prescribed burns have been ongoing since 1976. Tree age and basal area increment were calculated from radial tree-ring growth. We examined the periods before and after 1976 (when experimental burn treatments began) in separate analyses using linear mixed effects models. We treated burn frequency and tree age as fixed effects and individual tree and year as random effects.
In the period after prescribed burns began, estimated among-individual variance in growth was five times larger than estimated between-year variance. Comparatively, the variances of these two factors are much more similar in the time period during which fire was excluded from the area. This indicates that longleaf pines exhibit some individual-level, persistent heterogeneous growth when fire is incorporated into the plots, and less heterogeneous growth when fire is excluded. This suggests the hypothesis that some fire regimes may increase between-individual tree growth variance. Due to the heterogeneous nature of fire, each tree often experiences it differently, and growth may change accordingly. Also, fire commonly reduces competition, which could make pine growth heterogeneity more distinct by underscoring the importance of other factors, like genetics or microhabitats.