Fire may have different effects on the relative availability of nitrogen (N) and phosphorus (P) because N volatilization occurs at lower temperatures than P volatilization, and fire-mediated changes in soil nutrient availability may affect foliar nutrient concentrations. We assessed the short-term effects of fire on soil and plant nutrients and 15 N isotopic signatures in a palmetto flatwoods ecosystem in central Florida. Fire caused a short-term increase in extractable ammonium (NH 4 + ) and phosphate (PO 4 3-). The increase in PO 4 3-was greater than the increase in NH 4 + , resulting in a decrease in the soil extractable N: P ratio shortly after fire. Similarly, foliar %P of the palmetto Serenoa repens (W. Bartram) Small increased more than foliar %N, resulting in a decrease in foliar N:P ratios shortly after fire. Soil δ 15 N and the difference between foliar and soil δ 15 N did not vary with time since fire; however, foliar δ 15 N of S. repens decreased after fire. Foliar %N of Quercus geminata Small and ericaceous shrubs was positively correlated with soil extractable inorganic N, while foliar %P of S. repens was positively correlated with soil extractable PO 4 3-. Variation in foliar δ 15 N after fire and the positive relationship between soil and foliar nutrients suggest that both increased soil nutrient availability and reallocation of nutrients from below-to aboveground can be important for plant nutrient status after fire in palmetto flatwoods.

The probability of stem survival after fire is strongly influenced by energy allocation to bark because bark thickness affects heat transfer during fire. Greater relative investment in inner bark versus outer bark should also enhance survival because of greater moisture content of inner bark. We measured stem diameter, bark thickness, and habitat preference of five species typical of long-leaf pine savannas, and six species characteristic of adjacent wetlands (pocosins), and calculated relative bark thickness, the inner bark proportion, radial growth, and bark accumulation of each species. We hypothesized that savanna species have thicker bark and greater relative investment in inner bark than pocosin species, because fires occur more frequently in savannas than pocosins. As hypothesized, savanna species have relatively thicker bark than pocosin species. Relative bark thickness and the rate of bark accumulation were correlated with the mean location of a species along the pocosin-to-savanna gradient. However, the inner bark proportion did not differ between savanna and pocosin species. Our results indicate that relative bark thickness is likely the primary bark trait affecting fire-induced topkill and influencing the distribution of species along the pocosin-to-savanna gradient.

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In frequently burned ecosystems, many plants persist by repeated resprouting from basal or belowground buds. This strategy requires that plants reach a balance between biomass loss and recovery, which depends on the shape of the relationship between pre- and post-fire size. Previous analyses of this relationship, however, have focused on the size of the largest stem, which ignores the importance of the multi-stem growth habit that is common in pyrogenic ecosystems. We hypothesized that the presence of multiple stems causes a substantial shift in the relationship between pre- and post-fire size and in the relationship between pre-fire size and size recovery. We measured the height and basal diameter, then calculated volume and biomass, of all stems of six tree species before and nine months after complete removal of aboveground biomass via coppicing. The number of resprouts was correlated with the original number of stems for four species. For all species, the relationship between pre-coppicing and resprout size fit a positive curvilinear function, and the shape of this curve did not differ for maximum and total stem size. Smaller individuals recovered a larger proportion of their pre-coppicing size than larger individuals, but the shape of the size recovery curves were the same regardless of whether the analysis was performed with all stems or only the largest stem. Our results indicate that measuring only the largest stem of multi-stemmed individuals is sufficient to assess the ability of individuals to recover after complete loss of aboveground biomass and persist under frequent burning.

Soil nutrients and herbaceous plant growth are patchily distributed in desert ecosystems, often restricted to "fertility islands" created by perennial shrubs. Although fire has been historically uncommon in southwestern American hot deserts (e.g., the Mojave), these regions have experienced more severe fires due to recent invasions of exotic species that increased proneness to fire. Nevertheless, the effects of fire on soil nutrients in SW deserts, including via the removal of shrubs by fire, remain unclear. We assessed the spatio-temporal impacts of fire on soil nutrient availability in burned and unburned areas of the Mojave Desert. The study was conducted in shrublands dominated by Larrea tridentata. We investigated both the short (seven months after fire) and long (seven years after fire) term effects of fire on soil nutrient availability within a microhabitat gradient spanning from under the shrub canopy to open inter-shrub areas. We found that nitrogen (N) and potassium (K) were higher under the canopy of burned L. tridentata seven months after fire. In contrast, seven years after fire, N and K availability were lower around shrubs that were killed by fire. Over the short-term, fire had a positive effect on soil nutrients. However, over the long-term, the fertility island effect diminished after removal of shrubs by fire, and the differential availability of nutrients such as N and K became more similar under shrubs and in open inter-shrub areas. This reinforces the key role of L. tridentata in influencing the distribution of soil nutrients and provides support for the hypothesis that post-fire herbaceous plant growth will be less restricted to areas under shrubs.

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Background: Resprouting is an effective strategy for persistence of perennial plants after disturbances such as fire. However, can disturbances be so frequent that they limit resprouting? We examined the effects of fire and mowing frequency on eight species of resprouting shrubs in Florida scrub, USA, using a factorial field experiment. We burned or mowed plots at four disturbance return intervals (DRI): either annually, biennially, every three years, or once in six years (with all plots being treated in the sixth year to control for time since disturbance). We analyzed plant growth responses (height, aboveground biomass, number of stems) based on sampling pre treatment, and six months, one year, two years, and four years post-treatment. We also measured non-structural carbohydrates (NSC) and soil properties to evaluate these factors as potential drivers of resprouting responses. Results: Fire temperatures were hot (mean maxima 414 to 698°C among burn days), typical of larger fires in Florida scrub. Plant biomass and heights were affected by DRI (being suppressed by frequent disturbance, especially initially) and varied among species with palmettos recovering biomass faster, and species within the same genus generally showing similar responses. Biomass recovery in mown versus burned treatments showed comparable effects of DRI and similar trajectories over time. Numbers of stems were affected by DRI, disturbance type, and species, and increased after disturbances, especially with less frequent disturbances and mowing, and subsequently declined over time. NSC concentrations varied among species and over time and were positively related to biomass. One year post disturbance, soil moisture and organic matter content were higher in mown plots, while pH was higher in burned plots. Given the slightly lower elevation of the mown plots, we interpreted these differences as site effects. Soil properties were not affected by DRI and did not affect biomass responses. Conclusions: Although very frequent disturbances reduced shrub growth responses, the magnitude of plant responses was modest and the effects temporary. Because resprouting shrubs in Florida scrub appear resilient to a range of disturbance return intervals, frequent fire or mowing can be used effectively in restorations.