Fire regimes and the conservation of sandstone heath in monsoonal northern Australia: frequency, interval, patchiness

Heathlands are scattered across fire-prone monsoonal northern Australia mostly in dissected sandstone terrain. Such communities, although floristically depauperate by comparison with heathlands in southern Australia and southern Africa especially, share in common a relatively high proportion of fire-sensitive, obligate-seeder shrub species. The paper explores the implications of frequent fires, and associated short inter-fire intervals, on populations of obligate-seeder shrub species occurring in extensive heathlands occupying the western rim of the Arnhem Plateau, in the Northern Territory. Two studies are presented. With reference to published data concerning the maturation times of regional obligate-seeder shrubs, the first study reports on minimum and maximum intervals between fires determined from a 16-year fire history, 1980-1995, for the Plateau landform unit in Kakadu National Park, interpreted from LANDSAT MSS imagery. While species with maturation times of 5 or more years are common in the regional heath flora, minimum fire interval data for each 1 ha pixel indicate that 69% of heath habitats had been burnt at least once by fires recurring within 3 years, and 64% had a maximum fire interval of 5 years; 11% burnt only once or remained unburnt. The second study reports on the effects of an unreplicated experimental fire, involving observations on ca. 4000 individual shrubs, on ensuing heath floristic composition and abundance, undertaken 3 years after a wildfire had burnt the same site. Despite the experimental fire being highly patchy, substantial declines in the occurrence and density of many obligate-seeder shrub species were attended by increases in many herbs, including flammable grasses. Three years after the experimental fire the number of obligate-seeder shrubs was still less than half that pre-fire despite significant recruitment of some species in latter years. Collectively, these and other published data indicate that minimum fire return intervals of at least 4-5 years are required for conserving rapidly maturing tropical sandstone heath obligate-seeder shrubs, and longer still on sites comprising species with longer maturation times. For conservation management purposes individual fires should be small (especially in relation to the extent of any one tract of heath), patchy, and recurring intervals between fires should be varied as far as practicable.     

Vegetation patchiness and implications for landscape function: The case of Pteronia incana invader species in Ngqushwa Rural Municipality, Eastern Cape, South Africa

Patchy vegetation patterns are an expression of soil surface conditions, water redistribution on the soil surface and landscape function. Their origin is attributed by many a scholar to the degradation of the original plant cover due to human disturbances and climatic fluctuations. In this study, aerial photographs were analysed to benchmark the onset of the invasion by Pteronia incana. The soil moisture dependencies of the invader shrub and grasses were also investigated. The invasion assumed varying trajectories on abandoned and grazing lands. The different soil moisture dependencies between P. incana and grass species were noted to underpin the competitive advantage and eventual replacement of the latter by the former. Soil surface crusting inherent to P. incana, the loss of patchiness and associated expansion of bare zones promote runoff generation and connectivity, and erosion intensification, leading to conversion of hillslopes to dysfunctional systems. Despite its runoff enhancing role, to some extent, P. incana tussocks act as sink areas for some of the runoff generated on the bare zones. Recognition of this resource capture capability should provide the starting point for the rehabilitation of degraded hillslopes.     

The effects of uneven, patchy cultivar mixtures on disease control and yield in winter barley

Three winter barley cultivars were mixed in equal proportions but in different ways to give different patterns of spatial heterogeneity. The mixtures were sown in large field plots over three successive years and disease severity and yield were compared between the mixtures and the mean of the components grown as monocultures. Most mixtures significantly reduced disease in all years. The mixture composition, which appeared to generate a discrete pattern of small patches of the component cultivars, gave a yield advantage in 2 years, while the mixture which was pre-mixed most homogeneously gave no significant yield advantage in these trials. The cost of homogeneous mixing is therefore unlikely to be recovered in increased yields, compared with a simpler, imprecise mixing of the components in the seed hopper prior to sowing.     

Spatial distribution of regeneration in West-Carpathian uneven-aged silver fir forests

The objective of our study was to examine whether distribution of regeneration in uneven-aged fir (Abies alba Mill.) forests is related to the spatial pattern of trees. In 12 sample plots of size 0.45–1.00 ha (in total 8.65 ha, with stand basal areas ranging from 27.6 m² ha^–1 to 39.5 m² ha^–1), all live and dead trees above 5 cm in d_1.3 were mapped and their diameters measured. In eight plots, all live and dead fir saplings were mapped. In three plots, the number of live fir saplings and seedlings was registered on small systematically distributed circular plots. The values of an analytically developed index of stand influence were compared in patches occupied and unoccupied by live or dead fir regeneration. Contrary to preliminary assumptions, only in a few cases did saplings and trees 5–15 cm in d_1.3 appear more often in gaps and looser stand patches. Rather, in many plots, the opposite tendency was observed. The seedling density showed a weak but positive correlation with the index of influence. If the spatial pattern of regeneration reflects the spatially varying mortality of juvenile trees, then no evidence was found that stand competition was the most important factor inducing this mortality. On the contrary, on the basis of the results obtained, we can presume that the survival rate of juvenile firs was higher in patches with a relatively higher local basal area. Thus, it was hypothesised that, first, dispersion of regeneration in uneven-aged fir forests is controlled by easy-to-change edaphic factors such as humus form and acidity of the upper soil horizons, and second, that these soil features are linked with the spatial pattern of trees.     

Determining the spatial scale for conservation purposes - an example with grizzly bears

This study suggests procedures for determining the spatial scale for conservation guidelines for animals, giving an illustration with an analysis of grizzly bear habitat selection. Bear densities were sampled by identifying hairs at bait stations in British Columbia. Habitat variables were measured using remote sensing. Spatial scale was changed by varying the window size over which the variables were averaged. First, the spatial pattern of bears was studied, measuring the patchiness in bear densities at a variety of spatial scales, by calculating the correlation in bear densities between adjacent windows. This was repeated for the habitat variables. Finally, the overall interaction between bears and habitats was analysed, measuring the strength of habitat selection at different spatial scales. There are three domains of scale: at 2-4 km, bears and habitats are patchy, at 5-10 km, bears select for habitats, and at 40+ km, habitats are patchy and bears select for habitats. At scales of 40+ km, bears selected for: (i) higher slopes, or (ii) higher slopes, and some combination of more avalanche chutes, fewer roads and trees, higher elevations, and less logged land. Within 15 km areas, bears selected for 6 km areas that are either at higher elevations, or at higher elevations and had fewer trees. The relationship of conservation guidelines at different spatial scales should be determined by measuring and comparing hierarchical to non-hierarchical selection. The scales that bears select for habitats roughly correspond to the scales used in present grizzly bear conservation plans in British Columbia.     

Spatial and temporal dynamics of Apera spica-venti seedling populations

Apera spica-venti is an important weed problem in winter cereals in Europe. Spatial and temporal dynamics of A. spica-venti were investigated to test the hypothesis that this species has a spatially aggregated distribution. A. spica-venti distribution was quantified over several crop rotation sequences in three commercial fields. From 1999 to 2006, the spatial pattern of A. spica-venti was sampled yearly at the same grid points. Geostatistical techniques were used to characterise the spatial and temporal variability of A. spica-venti density. The spatial pattern was analysed by Lloyd's index of patchiness. From year to year, differing aggregation of A. spica-venti resulted in weed patchiness. The Spearman Rank Correlation Coefficient (r_s) was used to discover the strength of A. spica-venti occurrence in several years. For two sites there were significant correlations of weed occurrence between years but the relationship was less strong for the third field. Based on rank correlation coefficients, the temporal dynamics were marked by an overall continuity of distribution patterns. Knowing that spatial distributions of weeds vary little in time can reduce sampling efforts, and increases feasibility of site-specific weed control.