Abstract Exotic plant species may facilitate their invasion into native communities through the modification of ecosystem disturbances such as fire regimes. Where frequent fires are common, invasive plants that suppress fire may induce a positive feedback which further suppresses fire and promotes their continued invasion. In the pine rockland savanna ecosystem of south Florida, the frequent understory fire regime may be altered by the fire-resistant invasive shrub Brazilian pepper (Schinus terebinthifolius). In this thesis, I document the interaction of Brazilian pepper and fire in these savannas. I show that fire causes significant (30%-50%) mortality among low-density populations of Brazilian pepper. However, Brazilian pepper exhibits rapid growth and reproduces quickly following fire, and in the absence of fire it has a low mortality rate. Furthermore, Brazilian pepper can cause a reduction in fire temperature from 47° C at low densities, to almost 200° C at high densities, where it can completely impede fire spread. This creates the potential for Brazilian pepper to initiate a fire-suppressing feedback if it can reach a density threshold during extended fire-free intervals. At a landscape scale, I analyzed digital aerial photographs to show that fire frequency correlates with the extent of Brazilian pepper invasion into pine savanna fragments in southern Florida. In savannas where fire is frequent, Brazilian pepper does not heavily invade, but savannas that are heavily invaded tend to be unburned for more than 20 years. This supports both the regulation of low-density Brazilian pepper populations by fire, and the potential for high-density Brazilian pepper populations to suppress fire and facilitate further invasion.

No description available.

Burning and grazing are key processes in the natural disturbance regime of the Fescue Prairie. Burning, grazing and their interacting effects on plant species diversity (H¡¯), species richness, and heterogeneity in species composition were studied at two spatial scales for two years in a remnant Fescue Prairie near Saskatoon, Saskatchewan. Cattle distribution in relation to plant communities was also studied. At the plot scale (100 m2), burning increased H¡¯ (P<0.01) (x=1.75) compared to unburned treatments (x=1.54) (S.E.¡À0.058). Burning, grazing, and burning + grazing had no significant effect (P>0.10) on species richness; richness varied between years (P=0.04), averaging 14.2 species m-2 in year one versus 15.8 species m-2 in year two (S.E.¡À 0.65). Spatial heterogeneity (P>0.25; x=46%; S.E.¡À3.0) and temporal heterogeneity in species composition (P>0.21; x=42%; S.E.¡À3.8) were not affected by burning, grazing, or their interaction. Burning + grazing increased tiller densities in <i>Elymus lanceolatus</i> (68%) and those of <i>Festuca hallii</i> (11%) (P<0.001) compared to the control. Burning decreased total aboveground net primary production (ANPP) (P<0.001) (x=305 g m-2) compared to unburned treatments (x=500 g m-2; S.E.¡À30.8). Grazing and burning + grazing had no effect on total ANPP or graminoid ANPP (P¡Ý0.36). At the scale of Kernen Prairie (130 ha), H¡¯ increased between 1996 (P<0.05) (x=1.10) and 2005 (x=1.40; S.E.¡À0.094). Species richness increased from 5.2 species 0.25 m-2 in 1996, to 6.8 species 0.25 m-2 in 2005 (S.E.¡À0.505). Heterogeneity in plant species composition tended to increase after prescribed burning was started in 1986 and after grazing began in 2006. Cattle preferred <i>Bromus inermis-</i> and <i>Poa pratensis-</i>dominated plant communities, areas with intermediate amounts of total aboveground standing crop of plants, and areas in which shrub densities exceeded 16 stems 0.25 m-2. In the short term, burning and environmental conditions had greater effects on species diversity, richness, and heterogeneity in species composition than grazing or the interaction of burning and grazing. Different responses may be expected with different combinations of timing, frequency, and intensity of burning and grazing at different sites under ever changing environmental conditions.

Burning and grazing are key processes in the natural disturbance regime of the Fescue Prairie. Burning, grazing and their interacting effects on plant species diversity (H¡¯), species richness, and heterogeneity in species composition were studied at two spatial scales for two years in a remnant Fescue Prairie near Saskatoon, Saskatchewan. Cattle distribution in relation to plant communities was also studied. At the plot scale (100 m2), burning increased H¡¯ (P<0.01) (x=1.75) compared to unburned treatments (x=1.54) (S.E.¡À0.058). Burning, grazing, and burning + grazing had no significant effect (P>0.10) on species richness; richness varied between years (P=0.04), averaging 14.2 species m-2 in year one versus 15.8 species m-2 in year two (S.E.¡À 0.65). Spatial heterogeneity (P>0.25; x=46%; S.E.¡À3.0) and temporal heterogeneity in species composition (P>0.21; x=42%; S.E.¡À3.8) were not affected by burning, grazing, or their interaction. Burning + grazing increased tiller densities in <i>Elymus lanceolatus</i> (68%) and those of <i>Festuca hallii</i> (11%) (P<0.001) compared to the control. Burning decreased total aboveground net primary production (ANPP) (P<0.001) (x=305 g m-2) compared to unburned treatments (x=500 g m-2; S.E.¡À30.8). Grazing and burning + grazing had no effect on total ANPP or graminoid ANPP (P¡Ý0.36). At the scale of Kernen Prairie (130 ha), H¡¯ increased between 1996 (P<0.05) (x=1.10) and 2005 (x=1.40; S.E.¡À0.094). Species richness increased from 5.2 species 0.25 m-2 in 1996, to 6.8 species 0.25 m-2 in 2005 (S.E.¡À0.505). Heterogeneity in plant species composition tended to increase after prescribed burning was started in 1986 and after grazing began in 2006. Cattle preferred <i>Bromus inermis-</i> and <i>Poa pratensis-</i>dominated plant communities, areas with intermediate amounts of total aboveground standing crop of plants, and areas in which shrub densities exceeded 16 stems 0.25 m-2. In the short term, burning and environmental conditions had greater effects on species diversity, richness, and heterogeneity in species composition than grazing or the interaction of burning and grazing. Different responses may be expected with different combinations of timing, frequency, and intensity of burning and grazing at different sites under ever changing environmental conditions.

Conserving structural and compositional diversity in Fescue Prairie requires reintroducing natural disturbances according to their historic regime. Fire is an important natural process that may be a source of spatial heterogeneity in Fescue Prairies. The effects of burning in all months of the year except January and February were evaluated in a Fescue Prairie in central Saskatchewan for 6 years following burning on 2 sites that had not been previously burned and 2 sites that had been burned 5 years earlier. Except for burning in March, burning reduced cover of litter (P<0.01) and <i>Festuca hallii </i> (Vasey) Piper (P=0.01) while increasing bare soil (P<0.01) for 1 to 5 years. Cover of <i>Elymus lanceolatus </i>(Scribn. & J.G. Sm.) Gould (P<0.01), graminoids (P=0.02), and species evenness (P=0.01) increased with burning frequency. Burning in late-summer reduced cover of graminoids (P=0.03), plants other than the dominant grasses (P=0.03), and total plant cover (P=0.02). Burning increased the spatial variance (s2) in litter cover (P<0.01) and bare soil (P<0.01) for 1 to 3 years. Aside from burning in early spring, burning reduced s2 in total standing crop (P=0.02) and <i>F. hallii</i> (P=0.01). Variability in the cover of <i>E. lanceolatus </i>(P<0.01) and graminoids (P=0.04) increased with burning frequency. Canonical correspondence analysis (CCA) indicated that pre-burn history had a dominant effect on plant community composition, explaining 13% of the variation (P<0.01). The cumulative effects of repeated burning, annual variability in weather, and exposure to temperature extremes may have caused a shift in the composition of the plant community. The first 4 ordination axes explained 22% of the variation in plant community composition after burning, indicating that many other environmental or site variables controlled community composition. A range of burning dates and frequencies should be reintroduced or maintained in Fescue Prairie to create a mosaic of plant communities in various stages of recovery after burning. A mosaic will increase the structural and compositional diversity in remnant Fescue Prairies.

Conserving structural and compositional diversity in Fescue Prairie requires reintroducing natural disturbances according to their historic regime. Fire is an important natural process that may be a source of spatial heterogeneity in Fescue Prairies. The effects of burning in all months of the year except January and February were evaluated in a Fescue Prairie in central Saskatchewan for 6 years following burning on 2 sites that had not been previously burned and 2 sites that had been burned 5 years earlier. Except for burning in March, burning reduced cover of litter (P<0.01) and <i>Festuca hallii </i> (Vasey) Piper (P=0.01) while increasing bare soil (P<0.01) for 1 to 5 years. Cover of <i>Elymus lanceolatus </i>(Scribn. & J.G. Sm.) Gould (P<0.01), graminoids (P=0.02), and species evenness (P=0.01) increased with burning frequency. Burning in late-summer reduced cover of graminoids (P=0.03), plants other than the dominant grasses (P=0.03), and total plant cover (P=0.02). Burning increased the spatial variance (s2) in litter cover (P<0.01) and bare soil (P<0.01) for 1 to 3 years. Aside from burning in early spring, burning reduced s2 in total standing crop (P=0.02) and <i>F. hallii</i> (P=0.01). Variability in the cover of <i>E. lanceolatus </i>(P<0.01) and graminoids (P=0.04) increased with burning frequency. Canonical correspondence analysis (CCA) indicated that pre-burn history had a dominant effect on plant community composition, explaining 13% of the variation (P<0.01). The cumulative effects of repeated burning, annual variability in weather, and exposure to temperature extremes may have caused a shift in the composition of the plant community. The first 4 ordination axes explained 22% of the variation in plant community composition after burning, indicating that many other environmental or site variables controlled community composition. A range of burning dates and frequencies should be reintroduced or maintained in Fescue Prairie to create a mosaic of plant communities in various stages of recovery after burning. A mosaic will increase the structural and compositional diversity in remnant Fescue Prairies.

Cette étude repose sur une analyse comparative des distributions d’éléments traces (ET) dans les podzols de deux forêts adjacentes qui se distinguent principalement par leur régime de perturbation. La forêt du site de l’Île a été protégée des perturbations majeures pendant plusieurs siècles, tandis que le site du Versant a subi un feu majeur il y a 100 ans. Bien qu’ils aient évolué à partir du même matériel parental et sous les mêmes conditions climatiques et topographiques, les sols de ces deux sites présentent des différences marquées au niveau de leur morphologie et de leurs propriétés chimiques. Notamment, le sol de l’Île, qui est dominée par des conifères, a un pH significativement plus faible dans l’entièreté du profil, un horizon B podzolique qui contient significativement plus d’oxydes de fer cristallin et des horizons organiques quatre fois plus épais en comparaison au sol du Versant, un peuplement de feuillus. L’objectif de ce projet de recherche était alors de démontrer comment les différentes conditions biogéochimiques affectent la rétention et la mobilisation des ET et plus spécifiquement, comment l’âge des forêts affecte le cycle des ET à travers son influence sur la pédogénèse. Dans chacun des deux sites, quatre pédons ont été creusés puis tous leurs horizons de sol ont été échantillonnés (total de 79 échantillons) afin de mesurer leurs concentrations extraites à l’acides et solubles dans l’eau en As, Cd, Cu, Pb, V et Zn. Les différences majeures entre les deux sites se trouvaient surtout dans les horizons organiques, où les concentrations totales de tous les ET étaient significativement plus élevées sur le Versant. Un cyclage biologique accru sur le Versant, par la présence de feuillus, dont Betula alleghaniensis, explique en partie les concentrations élevées dans la couche d’humus, particulièrement pour le Cd, le Cu et le Zn, dont les concentrations étaient jusqu’à 5 fois plus élevées dans l’horizon F du Versant. Cependant, le pourcentage d’ET en solution était significativement plus élevé sur l’Île pour tous les ET, à l’exception de l’As. Le très faible pH de l’Île a contribué à la mobilisation de ces ET. La rétention de Cd et de Zn était plus sensible à la variation de pH que les autres ET. Une quantité significative de Zn a été lessivée en dehors du profil de la forêt non-perturbée, car le stock total du profil de sol en Zn (mg/m2 par cm d’épaisseur) y est 4.5 fois plus faible que sur le Versant. Cette étude soulève l’impact direct et indirect de la dynamique de la végétation sur la pédogénèse et sur le cycle des ET dans les podzols. / This study is based on a comparative analysis of the distribution of trace elements (ET) in the podzolic soils of two adjacent forested sites that distinguish themselves based on their disturbance regime. The Island site was sheltered from major disturbances for several centuries, whereas a stand-replacing wildfire burned the area 100 years ago on the Hillslope site. Even though the soils of both sites developed from the same parent material and under the same climatic and topographic conditions, they display major morphological and chemical differences. The Island is a coniferous stand that has a significantly lower soil pH through the entire profile, a podzolic B horizon that contains significantly more crystalline iron oxide and organic horizons that are four times thicker than at the Hillslope, a deciduous stand. The objective of this research is to demonstrate how different biogeochemical conditions affect the retention and mobilisation of TE and more specifically how stand age influences the cycling of TE in soils through its impact on pedogenesis. In each site, four soil profiles were dug and all their horizons were sampled (total of 79 samples) in order to measure their acid-extracted and water-soluble concentrations of As, Cd, Cu, Pb, V and Zn. Major differences between the two sites are mainly found in organic horizons, where acid-extracted TE concentrations were significantly greater on the Hillslope. The enhanced biocycling caused by deciduous trees including Betula alleghaniensis contributed to the high concentrations in the humus layer of the Hillslope, especially of Cd, Cu and Zn, which were 5 times higher in the F horizon compared to the Island. Also, a higher percentage of TE was soluble on the Island for all TE, except for As. The solubilisation of TE was associated to the low soil pH of the older stand. Cd and Zn retention were more sensible to pH variation than other TE. A significant quantity of Zn was leached outside of the soil profile on the Island since the summed Zn content of the entire profile (mg Zn /m2 surface /cm thickness) was 4.5 times lower than on the Hillslope. This study highlights the direct and indirect impacts of vegetation on soil genesis and TE cycling in podzols.

No description available.

No description available.