Effects of time after fire and location along a savanna-to-wetland gradient on photosynthesis, water relations, and nutrient status of two resprouting shrubs

Background/Question/Methods

Fire-adapted ecosystems often occur adjacent to ecosystems that burn less frequently, and transitions between ecosystems with different fire regimes can be gradual. Along these dynamic ecotones, microhabitat factors such as soil moisture, nutrient availability, and light availability may vary spatially and temporally. Plant species traits and physiological function across microhabitats may affect the ability of species to persist or spread into adjacent ecosystems under changing fire regimes. In the southeastern United States, lowland stream-head pocosins (i.e., wetlands) frequently occur within a matrix of fire-adapted longleaf pine (Pinus palustris) savanna. Our objective was to assess the effects of time after fire and location along the savanna-to-pocosin gradient on plant physiological traits. Specifically, we measured photosynthesis, water relations, nutrient status, allocation (e.g., the ratio of leaf area to sapwood area), and leaf traits (e.g., leaf mass per area) of Clethra alnifolia, a deciduous shrub, and Ilex glabra, an evergreen shrub, in sites 2 and 5 years after prescribed fire. We sampled individuals of both species along transects that spanned from upland savannas to lowland stream-head pocosins. 

Results/Conclusions

Time after fire and gradient location did not have an effect on net CO₂ assimilation, water use efficiency (WUE), or photosynthetic nitrogen use efficiency (PNUE). There was, however, a significant difference between species; Ilex had higher net CO₂ assimilation and WUE, while Clethra had higher PNUE. Clethra had a lower leaf mass per area than Ilex, and thus, lower foliar N than Ilex on an area basis. Mid-day leaf water potential increased along the savanna-to-pocosin gradient. For both species, the ratios of leaf area:sapwood area, leaf biomass:stem biomass, and leaf area:stem biomass were higher 2 years after fire, but were not affected by gradient location. Clethra had higher ratios of leaf area:sapwood area, leaf biomass:stem biomass, and leaf area:stem biomass than Ilex. We found that species identity tended to have a greater effect on physiological traits than time after fire or gradient location. Thus, our results suggest that different physiological strategies allow resprouting shrubs to persist along the savanna-to-pocosin ecotone.