J. Stephen Brewer

Effects of Fire Season and Herbivory on Reproductive Success in a Clonal Forb, Pityopsis Graminifolia

1 In 1990 and 1992, we experimentally manipulated fire season in a longleaf pine sandhill community in north Florida, USA, and studied floral induction, bud herbivory, clone fecundity, and seedling dynamics of a perennial forb, Pityopsis graminifolia. In addition, some proximate cues of fire-related floral induction were examined using a factorial arrangement of clipping season and fertilization treatments in 1992. 2 Flowering was induced by fire. Results from the clipping/fertilization experiment showed that removal of canopy and litter is an important inductive cue associated with fire. Floral induction of clipped shoots was further influenced by soil fertility and the season during which clipping occurred. 3 Fire season influenced the proportion of shoots that bolted, the size of bolting shoots, and bud herbivory. Floral induction was greater following May and August (growing-season) fires than following January fires in 1990 and 1992. Bud herbivory by white-tailed deer was lower following growing-season fires than following January fires in 1990 and 1992. As a result, in 1992, fecundity of growing-season-burned clones was greater than that of January-burned clones, with greater shoot size leading to greater fecundity in May-burned clones than in August-burned clones. 4 Seedling emergence in early 1991, following 1990 fires, was highest in May-burned plots, intermediate in August-burned plots, and lowest in January-burned plots. Seedling survivorship through December 1992 was low in all fire season plots, but still varied in response to fire season (6% in May-burned plots, 2% in August-burned plots, and 0% in January-burned plots). Successful reproduction of 1991 cohorts following 1992 fires only occurred in May-burned plots. 5 Results from the current study suggest that reproductive success in P. graminifolia is greatest in May, intermediate in August and lowest in January. These rankings are the same as the relative likelihoods that longleaf pine savannahs will burn at these different times of the year.

Nutrient effects on the composition of salt marsh plant communities along the southern atlantic and Gulf coasts of the United States

Nutrient availability is known to mediate plant community structure in many systems, but relatively few studies of nutrient effects have been done in systems where strong gradients in physical stress might constrain the effects of nutrients. Recent studies in New England, United States, salt marshes indicate that nutrients may strongly mediate plant community composition by increasing the competitive ability of stress-tolerant species that are normally displaced by competition to recently-disturbed or low-intertidal habitats. It is unknown whether these results can be generalized to salt marshes in other geographic regions that experience different climates, tidal regimes, and edaphic conditions. To address the generality of these results from New England, we fertilized seven different mixtures of salt marsh plants at study sites on the southeast and Gulf coasts of the U.S. Two of these mixtures were studied in both geographic regions. Consistent with results from New England, fertilization always increased the biomass of the low-marsh dominantSpartina alterniflora and usually led to it increasing in dominance at the expense of high-marsh species. Fertilization also led to increased community dominance byDistichlis, but only in a mixture where it was already common. Fertilization led to changes in plant dominance patterns in four of the seven types of mixtures that we studied. Results were not a function of edaphic conditions, at least within the range included in our study, and were consistent between the southeastern and Gulf coasts, which experience markedly different tidal regimes. The broad similarity of these results suggests that changes in nutrient input may lead to predictable changes in the composition of similar salt marsh plant communities across large geographic areas despite site to site variation in the abiotic environment.

Current and presettlement tree species composition of some upland forests in northern Mississippi1

compared present-day tree species composition of the two old-growth forests and the two younger forests with the composition of bearing trees (i.e., trees identified by land surveyors in the 1830s) in upland and lowland areas within townships encompassing the four sites. Bearing tree samples were similar in composition to an explorers account of old-growth woodlands that dominated the region during sparse subsistence settlement by Chickasaw Indians. There were striking differences between present-day and bearing-tree samples. One of the old-growth forests (Bailey Woods) was more similar in species composition to the mid-successional secondgrowth forests than to bearing-tree samples. The shade-intolerant, but fire-tolerant, Quercus marilandica Muenchh. was nearly absent from present-day upland forests (< 1% of all stems 2 10 cm dbh; range 0-3%) but was the most common species among presettlement upland trees (42% of all bearing trees in the township containing the field sites). Liquidambar styraciflua L., often regarded as both a pioneer species and a common constituent of alluvial floodplain and mesophytic terrace forests, was the single most abundant tree species in the understory and midcanopy of one of the old-growth forests (Bailey Woods) and in all size classes of trees of both younger forests. It was completely absent from presettlement records of upland trees, but was an abundant bearing tree in nearby sections containing lowland alluvial forests. I hypothesize that open oak woodlands dominated the upland landscape of Lafayette County, Mississippi before extensive settlement. These woodlands were neither mid-successional nor late-successional forests but were a unique fire-dependent community type that is absent from north Mississippi today.

Short-term effects of fire and competition on growth and plasticity of the yellow pitcher plant, Sarracenia alata (Sarraceniaceae)

Although recurrent fires are widely assumed to reduce competitive interference of plants of pine savannas, rarely has this assumption been tested explicitly. This 2-yr study reports on the interactive effects of fire and neighbors on short-term growth responses and plasticity in allocation patterns of a carnivorous plant, the yellow pitcher plant, Sarracenia alata. This species relies upon pitfall traps (pitchers) to attract and capture insects to obtain nutrients. Neighbors reduced the growth rate of individual ramet transplants (phytometers) in one but not both years of the study. The effect of neighbors on total (i.e., both above- and belowground) productivity of phytometers was not reduced by a winter fire. Neighbors had a greater effect on large plants than on small plants. Although fire did not affect the growth rate of phytometers in the short term, allocation patterns were greatly altered by both neighbors and fire. Allocation to pitchers increased at the expense of belowground organs following fire and in the absence of neighbors at the unburned site. Results of the current study suggest that adult pitcher plants may tolerate competition from neighboring vegetation by reducing allocation to costly pitchers during years without fire.

Phosphorus addition reduces invasion of a longleaf pine savanna (Southeastern USA) by a non-indigenous grass (Imperata cylindrica)

Imperata cylindrica is an invasive C4 grass, native to Asia and increasing in frequency throughout the tropics, subtropics, and southeastern USA. Such increases are associated with reduced biodiversity, altered fire regimes, and a more intense competitive environment for commercially important species. We measured rates of clonal spread by I. cylindrica from a roadside edge into the interior of two longleaf pine savannas. In addition, we measured the effects of fertilization with nitrogen and phosphorus on clonal invasion of one of these sites. Clonal invasion occurred at both sites and at similar rates. Older portions of an I. cylindrica sward contained fewer species of native pine-savanna plants. Clonal growth rates and aboveground mass of I. cylindrica were reduced by the addition of phosphorus relative to controls by the second growing season at one site. As a group, native species were not affected much by P-addition, although the height of legumes was increased by P addition, and the percent cover of legumes relative to native non-legumes decreased with increasing expected P limitation (i.e., going from P-fertilized to controls to N-fertilized treatments). Clonal invasion was negatively correlated with the relative abundance of legumes in control plots but not in P-fertilized plots. Species richness and percent cover of native plants (both legumes and non-legumes) were dramatically lower in N-fertilized plots than in controls or P-fertilized plots. Species richness of native plants was negatively correlated with final aboveground mass of I. cylindrica in control and P-fertilized plots, but not in N-fertilized plots. The results suggest that I. cylindrica is a better competitor for phosphorus than are native pine-savanna plants, especially legumes, and that short-lived, high-level pulses of phosphorus addition reduce this competitive advantage without negatively affecting native plant diversity. Ratios of soil P to N or native legume to non-legume plant species may provide indicators of the resistance of pristine pine savannas to clonal invasion by I. cylindrica.

Effects of competition, litter, and disturbance on an annual carnivorous plant (Utricularia juncea)

The effects of removal of live plants and litter (using herbicide, clipping, and raking) on seedling establishment were examined in Utricularia juncea (bladderwort), an annual carnivorous plant of low frequency in wet, nutrient-poor pine savannas of the southeastern United States. In addition, the spatial distribution of this species in relation to crayfish disturbances was determined. The creation of competition-free gaps in the groundcover canopy in May 1996 (using herbicide) promoted establishment of this species at two sites by September 1997. Standing dead and litter left in herbicide-treated plots inhibited establishment. Density was near zero in undisturbed plots. Natural occurrences of this species were associated with crayfish mounds, which bury plant litter as they erode and increase in area. These results suggest that Utricularia juncea is a fugitive species that depends on disturbances or litter-free microsites to become established in wet, nutrient-poor seepage savannas in southern Mississippi. It is hypothesized that the production of carnivorous traps combined with relatively high allocation to reproductive structures (>90%) and the production of a persistent seed bank make it well-adapted to nutrient-poor and disturbed habitats.

Patterns of plant species richness in a wet slash-pine (Pinus elliottii) savanna'

nificantly more common adjacent to trees than away from trees. Densities of seedlings of this species were also greater near trees than away from trees. The results of this study show that 1) local species richness and the occurrence of most herbaceous species are negatively associated with the occurrence of woody species (pines and shrubs), and 2) the distribution, local abundance, and richness of shrubs is positively associated with the occurrence of trees, the latter of which may serve as safe perch sites for animal dispersal-vectors and facilitate establishment of shrubs. Although experiments must be done to be certain, results of this descriptive study suggest that factors influencing patterns of species richness and composition within wet savannas may be regulated by a balance between competitive and facilitative effects of pines on herbs and shrubs, respectively.

Inferring relationships between native plant diversity and Lonicera japonica in upland forests in north Mississippi, USA

ABSTRACT Question: Do anthropogenic disturbances interact with local environmental factors to increase the abundance and frequency of invasive species, which in turn exerts a negative effect on native biodiversity? Location: Mature Quercus-Carya and Quercus-Carya-Pinus (oak-hickory-pine) forests in north Mississippi, USA. Methods: We used partial correlation and factor analysis to investigate relationships between native ground cover plant species richness and composition, percent cover of Lonicera japonica, and local and landscape-level environmental variables and disturbance patterns in mature upland forests. We directly measured vegetation and environmental variables within 34 sampling subplots and quantified the amount of tree cover surrounding our plots using digital color aerial photography. Results: Simple bivariate correlations revealed that high species richness and a high proportion of herbs were associated with low Lonicera japonica cover, moist and sandy uncompacted soils, low disturbance in the surrounding landscape, and periodic prescribed burning. Partial correlations and factor analysis showed that once we accounted for the environmental factors, L. japonica cover was the least important predictor of composition and among the least important predictors of species richness. Hence, much of the negative correlation between native species diversity and this invasive species was explained by soil texture and local and landscape-level land-use practices. Conclusions: We conclude that negative correlations between the abundance of invasive species and native plant diversity can occur in landscapes with a gradient of human disturbance, regardless of whether there is any negative effect of invasive species on native species. Nomenclature: Radford et al. (1968).

WHY DON'T CARNIVOROUS PITCHER PLANTS COMPETE WITH NON‐CARNIVOROUS PLANTS FOR NUTRIENTS?

Most field studies suggest that competition for nutrients is important in nutrient-limited systems. Nevertheless, the mechanisms by which species avoid competition for nutrients in nutrient-poor but species-rich communities remain poorly understood. I used a model system, the carnivorous pitcher plant Sarracenia alata, to measure competition for nutrients. In a field competition experiment, I increased the potential for nutrient-niche overlap between established juvenile pitcher plants and their non-carnivorous neighbors by denying a unique source of nutrients (prey) to pitcher plants. Contrary to predictions based on nutrient-niche complementarity, prey exclusion did not reduce the performance of pitcher plants competing with non-carnivorous neighbors. Hence, differences in nutrient-source specialization did not explain the coexistence of Sarracenia alata and its non-carnivorous neighbors in the field. Instead, prey exclusion reduced the positive growth response to increased light following the removal o...

Current and historical composition and size structure of upland forests across a soil gradient in north Mississippi

Abstract Comparisons of current and historical tree species composition and size structure along natural productivity gradients are useful for inferring effects of disturbance regimes and productivity on patterns of succession. We tabulated occurrences and estimated diameters of 3483 General Land Office bearing trees across 19 survey townships along an upland soil texture and organic matter gradient in north Mississippi. We then contrasted this presettlement composition and structure with that of 2998 trees in sampling plots within present-day mature (>100 years old) upland forests contained within the survey townships. Presettlement upland communities appeared to consist of non-successional communities, in which the most abundant trees were shade-intolerant, fire-tolerant trees (e.g., Quercus marilandica [blackjack oak]) in both large and small size classes across the entire soil gradient. These fire-prone pre-settlement assemblages differed greatly from present-day mature uplands, which were transitional assemblages of upland and floodplain trees, with mesophytic floodplain species (both early and late-successional) dominating the smaller size classes.