Frequent fire or mowing inhibits resprouting vigor in dominant shrubs of Florida scrub

Background/Question/Methods

Frequent fire may constrain fire regimes supporting resprouting plants because carbohydrate stores may be exhausted after multiple resprouting episodes. We explored whether frequent fires or mowing (often used as a fire surrogate) would affect vigor and carbohydrate levels in eight species of resprouting shrubs dominant in Florida scrub, a pyrogenic shrubland. The experimental design provided equal times-since-disturbance for all plots, but used prescribed fire and mowing to subject plots to different disturbance return intervals (DRI: 1, 2, 3, or 6 times in six years), with all plots (N = 32, 4 replicates for each treatment) disturbed during the sixth year. We measured fire intensities with type-K thermocouples and dataloggers during most burns. For each study species in each plot, we measured stem densities, heights, and crown lengths before and six months after the last disturbances. We also estimated non-structural carbohydrate (NSC) concentrations before and shortly after treatments with the anthrone method.

Results/Conclusions

Post-disturbance recovery was inhibited by frequent fire or mowing. Across all species, 6-month stem heights varied significantly among species and were significantly lower in frequently disturbed plots. Plots with taller stems at the start of the experiment produced greater stem heights post-disturbance. The source of disturbance (burn, mow) did not affect stem heights, nor were interactions among factors significant. Analyses of individual species responses varied. For example, in the dominant oak Quercus inopina, heights increased with DRI and were higher in mown (vs. burned) plots. Overall, three species had post-disturbance heights significantly related to pre-burn heights, three species had heights related to DRI, and one species had heights related to disturbance type. Although individual species patterns varied in significance, most had the greatest height growth with infrequent fire and the least height growth with annual fire. Responses appear unrelated to measured fire temperatures. However, insoluble and soluble carbohydrate concentrations each varied significantly by species, and by DRI, but did not differ between burning and mowing. Because very frequent disturbances have the potential to affect carbohydrate reserves and inhibit resprouting, and because responses vary among species, very frequent fires or mowing could alter the composition and structure of Florida scrub.