Forest Expansion (forest + expansion)

Distribution by Scientific Domains

Selected Abstracts

Impact of the invasive alien grass Melinis minutiflora at the savanna-forest ecotone in the Brazilian Cerrado

William A. Hoffmann
ABSTRACT Exotic grasses are a serious threat to biodiversity in the cerrado savannas of central Brazil. Of particular concern is the possible role they may have in impeding tree regeneration at gallery (riverine) forest edges and increasing fire intensity, thereby driving gallery forest retreat. Here we quantify the effect of roads and distance from gallery forests on the abundance of the African grass Melinis minutiflora Beauv. and test for an effect of this species on woody plant regeneration and leaf area index. Melinis was present at approximately 70% of the sites near gallery forest edges, with its frequency declining sharply at greater distances from the edge. Melinis frequency was 2.8 times greater where roads were present nearby. Leaf area index (LAI) of the ground layer was 38% higher where Melinis was present than where it was absent. LAI was strongly correlated to fine fuel mass (r2 = 0.80), indicating higher fuel loads where Melinis was present. The abundance of tree and shrub species in the ground layer was negatively related to LAI and to the presence of Melinis. The greater fuel accumulation and reduced tree regeneration caused by Melinis may cause a net reduction in forest area by increasing fire intensity at the gallery forest edge and slowing the rate of forest expansion. [source]

Abiotic constraints on the establishment of Quercus seedlings in grassland

Brett T. Danner
Abstract High evaporative demand and periodic drought characterize the growing season in midwestern grasslands relative to deciduous forests of the eastern US, and predicted climatic changes suggest that these climatic extremes may be exacerbated. Despite this less than optimal environment for tree seedling establishment, deciduous trees have expanded into adjacent tallgrass prairie within the last century leading to a dramatic land cover change. In order to determine the role of light and temperature on seedling establishment, we assessed carbon and water relations and aboveground growth of first-year Quercus macrocarpa seedlings exposed to one of three conditions: (1) intact tallgrass prairie communities (control), (2) aboveground herbaceous biomass removed (grass removal), and (3) shade plus biomass removal to reduce light (PFD) to levels typical of the grassland-forest ecotone (shade). In the 2000 growing season, precipitation was 35% below the long-term average, which had a significant negative effect on oak seedling carbon gain at midseason (photosynthesis declined to 10% of maximum rates). However, net photosynthesis and stomatal conductance in the shade treatment was ca. 2.5 and 1.5 times greater, respectively, than in control treatment seedlings during this drought. During this period, leaf and air temperatures in control seedlings were similar whereas leaf temperatures in the shade treatment remained below air temperature. A late-season recovery period, coincident with decreased air temperatures, resulted in increased net photosynthesis for all seedlings. Increased photosynthetic rates and water relations in shaded seedlings compared to seedlings in full sun suggest that, at least in dry years, high light and temperature may negatively impact oak seedling performance. However, high survival rates for all seedlings indicate that Q. macrocarpa seedlings are capable of tolerating both present-day and future climatic extremes. Unless historic fire regimes are restored, forest expansion and land cover change are likely to continue. [source]

Spatial patterns of tree recruitment in a relict population of Pinus uncinata: forest expansion through stratified diffusion

J. Julio Camarero
Abstract Aim, To infer future changes in the distribution of isolated relict tree populations at the limit of a species' geographical range, a deep understanding of the regeneration niche and the spatial pattern of tree recruitment is needed. Location, A relict Pinus uncinata population located at the south-western limit of distribution of the species in the Iberian System of north-eastern Spain. Methods,Pinus uncinata individuals were mapped within a 50 × 40-m plot, and their size, age and reproductive status were estimated. Data on seed dispersal were obtained from a seed-release experiment. The regeneration niche of the species was assessed based on the associations of seedling density with substrate and understorey cover. The spatial pattern of seedlings was described using point-pattern (Ripley's K) and surface-pattern (correlograms, Moran's I) analyses. Statistical and inverse modelling were used to characterize seedling clustering. Results, Pine seedlings appeared aggregated in 6-m patches. Inverse modelling estimated a longer mean dispersal distance (27 m), which corresponded to the size of a large cluster along the north to north-eastward direction paralleled by an eastward trend of increasing seedling age. The two spatial scales of recruitment were related to two dispersal processes. The small-scale clustering of seedlings was due to local seed dispersal in open areas near the edge of Calluna vulgaris mats: the regeneration niche. The long-range expansion might be caused by less frequent medium-distance dispersal events due to the dominant north-westerly winds. Main conclusions, To understand future range shifts of marginal tree populations, data on seed dispersal, regeneration niche and spatial pattern of recruitment at local scales should be obtained. The monitoring of understorey communities should be a priority in order to predict correctly shifts in tree species range in response to global warming. [source]

Forest progression modes in littoral Congo, Central Atlantic Africa

Charly Favier
Abstract Aim, To understand the persistence of a forest,savanna mosaic in places where rainfall data suggest that forest take-over should take place. To study the various modes of forest encroachment, and the role of human activities to hamper it. Location, Data were collected on several forest,savanna ecotones in the coastal region of the Republic of Congo. The sites were chosen to illustrate the differing principal modes of forest expansion, corresponding to different levels of anthropic pressure. Methods, The study sites were situated on five transects perpendicular to the ecotone (total sampled area: 1.7 ha) and 10 forest clumps in savanna (with diameters from 3 to 20 m). Along the transects botanical identification, diameter measurement and cartography were performed, while leaf area index was measured at a high resolution (every metre) along two of them. Collected data were analysed using a continuous quantification approach, which is much more useful than classical quadrat analysis. Time calibration of progression rates was performed using a simple model of the growth of the characteristic pioneer species, Aucoumea klaineana. Results, The two main different modes are reflected in different successional patterns. The edge diffusion is slow (its rate is evaluated to c. 1 m year,1) and is characterized by a progressive increase in large-diameter tree density and shade-tolerant tree density away from the ecotone. Conversely, savanna to forest phase transition by coalescence of clumps exhibits high tree density remnants distributed in established forest. The composition of these remnants is compatible with that of the forest clumps in savannas. Main conclusions, Three functional groups of pioneer trees are distinguished: some occupy the edge (edge pioneer), others establish clumps of forest in savanna (clump pioneers) and the longer-living A. klaineana ensures the transition to ,mature' forest. The two different observed patterns (linear edge progression and clump coalescence) can be understood with the use of a model of forest,savanna dynamics, ,FORSAT'. The two control parameters are the annual rainfall and the frequency of man-made fires in each savanna. In particular, an increase in the fire frequency can lead to a shift from the coalescence regime to the edge progression one. [source]

Rain forest invasion of eucalypt-dominated woodland savanna, Iron Range, north-eastern Australia: II.

Rates of landscape change
Abstract Aim, To explore rates of rain forest expansion and associated ecological correlates in Eucalyptus -dominated woodland savanna vegetation in north-eastern Australia, over the period 1943,91. Location, Iron Range National Park and environs, north-east Queensland, Australia. This remote region supports probably the largest extent of lowland (< 300 m) rain forest extant in Australia. Rainfall (c. 1700 mm p.a.) occurs mostly between November and June, with some rain typically occurring even in the driest months July,October. Methods, Interpretation of change in lowland rain forest vegetation cover was undertaken for a 140 km2 area comprising complex vegetation, geology and physiography using available air photos (1943, 1970 and 1991). A GIS database was assembled comprising rain forest extent for the three time periods, geology, elevation, slope, aspect, proximity to streams and roads. Using standard GIS procedures, a sample of 6996 10 × 10 m cells (0.5% of study area) was selected randomly and attributed for vegetation structure (rain forest and non-rain forest), and landscape features. Associations of rain forest expansion with landscape features were examined with logistic regression using the subset of cells that had changed from other vegetation types to rain forest, and remained rain forest over the assessment period, and comparing them with cells that showed no change from their original, non-rain forest condition. Results, Rain forest in the air photo study area increased from 45 km2 in 1943 to 78.1 km2 by 1970, and to 82.6 km2 by 1991. Rainfall (and atmospheric CO2 concentration) was markedly lower in the first assessment period (1943,70). Modelled rates of rain forest invasion differed predominantly with respect to substrate type, occurring faster on substrates possessing better moisture retention properties, and across all elevation classes. Greatest expansion, at least in the first assessment period, occurred on the most inherently infertile substrates. Expansion was little constrained by slope, aspect and proximity to streams and roads. On schist substrates, probability of invasion remained high (> 60%) over distances up to 1500 m from mature rain forest margins; on less favourable substrates (diorite, granites), probability of expansion was negligible at sites more than 400 m from mature margins. Main conclusions, (i) Rain forest expansion was associated primarily with release from burning pressure from c. the 1920s, following major disruption of customary Aboriginal lifestyles including hunting and burning practices. (ii) Decadal-scale expansion of rain forest at Iron Range supports extensive observations from the palaeoecological literature concerning rapid rain forest invasion under conducive environmental conditions. (iii) The generality of these substrate-mediated observations requires further testing, especially given that landscape-scale rain forest invasion of sclerophyll-dominated communities is reported from other regions of north-eastern Australia. [source]

Establishing a perimeter position: speciation around the Indian Ocean Basin

Abstract Historical biological interactions among peripheral landmasses on the periphery of the Indian Ocean Basin (IOB) are generally poorly understood. While interactions based on early Gondwanan vicariance have been used to explain present day lineage distributions, several recent studies have instead inferred dispersal across the IOB. This inference is often advanced because lineages under study have species inhabiting IOB islands. Here we examine the roles of continental vicariance vs. trans-IOB dispersal in the distribution of an avian genus found around the perimeter of the IOB. A molecular phylogeny does reveal evidence of a relationship that would require the inference of trans-IOB dispersal between eastern Africa and Sri Lanka. However, molecular clock data, ancestral area analyses and paleoclimatic reconstructions suggest that vicariance related to tropical forest expansion and retraction is more likely to have facilitated African,Asian interchange, with an initial colonization of Africa from Asia quickly followed by a recolonization of Asia. Subsequent dispersal from Asia to Sri Lanka and islands east of the Sunda Shelf are inferred; these latter islands were colonized in a stepping-stone fashion that culminated in colonization of the Sunda Shelf, and a recolonization of mainland Asia. We propose that circum-IOB distributions, which post-date early Gondwanan breakup, are most likely the result of continent-based vicariant events, particularly those events related to large-scale habitat alterations, and not trans-IOB dispersals. [source]

Challenging Neo-Malthusian Deforestation Analyses in West Africa's Dynamic Forest Landscapes

Melissa Leach
Many influential analyses of West Africa take it for granted that ,original' forest cover has progressively been converted and savannized during the twentieth century by growing populations. By testing these assumptions against historical evidence, exemplified for Ghana and Ivory Coast, this article shows that these neo-Malthusian deforestation narratives badly misrepresent people,forest relationships. They obscure important nonlinear dynamics, as well as widespread anthropogenic forest expansion and landscape enrichment. These processes are better captured, in broad terms, by a neo-Boserupian perspective on population,forest dynamics. However, comprehending variations in locale-specific trajectories of change requires fuller appreciation of social differences in environmental and resource values, of how diverse institutions shape resource access and control, and of ecological variability and path dependency in how landscapes respond to use. The second half of the article présents and illustrates such a "landscape structuretion" perspective through case studies from the forest,savanna transition zones of Ghana and Guinea. [source]

Landscape Patterns of Tropical Forest Recovery in the Republic of Palau,

BIOTROPICA, Issue 4 2001
Bryan A. Endress
ABSTRACT A GIS (geographic information systems) database was constructed from aerial photographs, a vegetation map, and topographic map data of the Ngeremeduu Bay Drainage Area (NBDA), Palau, to examine relationships between upland land cover dynamics, environmental variables, and past land use. In 1992, 82.9 percent of the NBDA was forest, 16.6 percent was grassland, and 0.5 percent consisted of village areas. Between 1947 and 1992, there was a 11.2 percent reduction of grassland area primarily due to a 10.9 percent increase in forest cover. These land cover changes led to larger, more continuous stretches of forest and numerous, highly fragmented grassland patches. Significant relationships (P 0.001) were found between the spatial distribution of forest and grassland cover and slope, elevation, soil pH, and percent soil organic matter. These patterns, however, may have resulted from past farm site selection rather than from ecological relationships. Our results indicate that areas of forest expansion were significantly (P 0.001) associated with the location of abandoned agricultural communities. In addition, over 92 percent of areas of forest expansion occurred within 100 m of established forest. These results suggest that the proximity of established forest facilitate forest recovery following human disturbance. [source]