Autogenic Succession in a Subtropical Savanna: Conversion of Grassland to Thorn Woodland

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecological Monographs.Abstract. Dense thorn woodlands occupy what are thought to have been grasslands and savannas prior to settlement of the Rio Grande Plains of Texas. However, the tenet that grasslands have been converted to shrublands and woodlands in recent history is controversial and based largely upon conflicting historical accounts. Our objective was to determine how the presumed physiognomic conversion from grassland or savanna to woodlands might have occurred.Some upland landscapes are dominated by closed-canopy woodlands in southern Texas, whereas others have a two-phase pattern of discrete shrub clusters scattered throughout a grassland. More mesic sites are dominated by closed-canopy woodlands. We hypothesized the two-phase landscapes represented an intermediate stage in the conversion of grassland to woodland. As new shrub clusters were initiated and existing clusters expanded and coalesced, a gradual shift from grassland to savanna to woodland would occur. To address this hypothesis, we inventoried herbaceous interspaces for woody colonizers, quantified the composition and distribution of shrub clusters on upland sites, and compared the structure of clusters to that of adjacent, more mesic areas with continuous woody plant cover. To assess the physiognomic stability of the two-phase landscapes, cluster size, density, and cover were quantified for 1941, 1960, and 1983 from aerial photographs.A lone mesquite (Prosopis glandulosa) plant occurred in > 80% of the upland clusters, where it was typically the largest individual in terms of basal area, height, and canopy area. The number of woody species per cluster ranged from 1 to 15 and was strongly related to mesquite basal diameter (R2 = 0.86). Cluster diversity, evenness, and size were also significantly correlated with mesquite size. The data suggest that mesquite plants invaded grasslands and served as recruitment foci for bird-disseminated seeds of other woody species previously restricted to other habitats. The result was a landscape composed of discrete chronosequences of woody plant assemblages organized about a mesquite nucleus.Within the two-phase portion of the landscape, 50% of the clusters were within 5 m of another and 95% were within 15 m of another. Analysis of the size class distribution of clusters suggested that most had yet to realize their growth potential. Moreover, the herbaceous clearings between clusters contained high densities of woody seedlings, mostly (>70%) mesquite, which occurred in 85% of the clearings, with a mean density of 350 plants/ha. Coalescence will become increasingly probable i...

Sources of water used by natural mesquite vegetation in a semi-arid region of northern Mexico

“…We hope that the results from our study have provided some explanation. These results also bring some explanation regarding what has been written in some historical reports, suggesting that the spread of mesquite has been lateral from drainage and riparian zones into upland prairies (Johnson, 1963;Archer et al, 1988).…”

“…Fire suppression, grazing by domestic livestock, rising CO 2 levels, and changes in climate have been invoked to explain expansion of woody species such as mesquite into areas that once supported grassland (Brown & Archer, 1987;Archer et al, 1988;Schlesinger et al, 1990;Polly et al, 1994;McPherson, 1997). It is clear that conversion of grassland to woodland has profoundly altered the nature of surface-atmosphere interactions in this region.…”

Dinâmica espaço-temporal (1962-2006) das fitofisionomias em unidade de conservação do Cerrado no sudeste do Brasil

“…Além das fi sionomias mencionadas, que puderam ser enquadradas na classifi cação de Ribeiro & Walter (1998) 1, tabela 1). Outras pesquisas sobre dinâmica da cobertura vegetal com dados de sensoriamento remoto, em zonas de interface entre fl orestas e savanas ou campos, mostram resultados semelhantes aos obtidos neste estudo (Archer et al 1988, Carmel & Kadmon 1999, Bowman et al 2001, Oliveira & Pillar 2004, Pinheiro 2006, Durigan & Ratter 2006e Marimon et al 2006. Nessas pesquisas há um consenso de que o avanço das fi sionomias fl orestais sobre formações campestres é explicado pela situação climática favorável às primeiras, e que a eliminação do pastejo e queimadas são fatores importantes para a expansão de formações fl orestais.…”

“…ser explicados os processos ecológicos que estão condicionando a reversão da invasão por esta espécie. Embora sejam relevantes as evidências de que a ação antrópica e os incêndios sejam fatores limitantes para a expansão de tipos vegetacionais fl orestais sobre vegetações campestres ou savânicas (Archer et al 1988, Carmel & Kadmon 1999, Bowman et al 2001, Moreira 2000, Oliveira-Filho & Ratter 2002, Pinheiro 2006, Durigan & Ratter 2006, as explicações para tais alterações não estão esgotadas, devendo ser analisados outros possíveis fatores, tais como tipo de solo, microclima e disponibilidade de luz, como condicionantes desse 6 7 8 9…”

Isozyme and RAPD studies in Prosopis glandulosa and P. Velutina (Leguminosae, Mimosoideae)

“…Some species, like P. juliflora and P. flexuosa, are being used for reforestation of severely degraded ecosystems (Felker, 1984;Cony, 1995). By contrast, some other species colonize and invade areas altered by human activity, particularly P. glandulosa (honey mesquite) in Texas, Mexico, and New Mexico, P. velutina (velvet mesquite) in Arizona, and P. ruscifolia (vinal) in the Chaco Region, Argentina (Morello et al, 1971;Burkart, 1976;Simpson and Solbrig, 1977;Hennessy et al, 1983;Brown and Archer, 1987;Archer et al, 1988;Scanlan and Archer, 1991). In North America, mesquite encroachment damages grasslands, especially when combinated with other factors such as overgrazing, drought, climate shifts and fire (Fisher, 1977;Schlesinger et al, 1990).…”