Shrub Grassland Research Articles

Soil carbon and nitrogen in relation to shrub size and death in a semi-arid grassland

Nutrient accumulation as fertile islands beneath invasive trees and shrubs in grasslands may provide opportunities for carbon sequestration. In a southwestern USA grassland, our objectives were to describe 1) the accumulation beneath Prosopis velutina (velvet mesquite) and isolated grass plants, and 2) the loss of accumulated nutrients 40 y after P. velutina death. We compared organic carbon (OC), total nitrogen (TN) and δ 13C in soil organic matter among large living, large dead, and small living P. velutina and open grassland, and between grass plants and bare ground. Soil samples were collected at 0–5, 5–10, 16.8–23.2, and 36.8–43.2 cm depths, and separated into five size/density fractions: particulate organic matter (Macro- and Micro-POM), mineral associated organic matter (Micro-MAOM), Silt, and Clay. We expected that differences in OC, TN, and δ 13C among soil fractions would suggest mechanisms and rates of accumulation with P. velutina persistence and loss following P. velutina death. Soil OC and TN accumulation was ~ 80–750% greater for large P. velutina than open grassland for whole soil and 4 of 5 fractions at 0–5 cm depth, but only 50–250% greater at 5–10 cm depth for whole soil and 3 of 5 fractions. Total OC and TN accumulation at 0–10 cm depth was 6.12 kg C m − 2 and 0.55 kg N m − 2 , respectively. Accumulation did not occur in whole soil or any fraction at 16.8–23.2 cm and 36.8–43.2 cm depths, or in the Micro-MAOM fraction at any depth. Accumulation under small P. velutina was less than large plants, and not significantly different from open grassland. Beneath isolated grass plants, accumulation of TN occurred at 0–5 cm depth, and OC accumulation at 0–5 and 5–10 cm depths in whole soils only, but change in δ 13C did not accompany accumulations. Forty years after death of large P. velutina, 67–106% of accumulated OC and TN were lost from whole soil and soil fractions at 0–5 cm depth. At 5–10 cm depth, loss (78–93%) was only detected in whole soils. Greater accumulation of OC and TN in the POM than the Silt and Clay fractions is consistent with the large physical size of recent organic matter inputs from P. velutina, but no differences in loss rates among fractions following P. velutina death suggests density dependent rates of organic matter consumption. Declines in δ 13C accompanied OC accumulation and increases occurred during loss. A 20–30 y mean residence time (MRT) for whole soil and the Clay fraction over those 40 y is suggested by changes in OC and δ 13C, but MRT based on changes in OC and δ 13C differ for Macro-POM (1–3 y versus 20–25 y, respectively) and Silt (> 50 y versus 20–30 y, respectively). Activities that remove P. velutina should focus on small plants and protect large plants thereby maintaining their carbon sequestration potential.

Land-cover changes in an urban lake watershed in a mega-city, Central China

Urbanization can exert a profound influence on land covers and landscape characteristics. In this study, we characterize the impact of urbanization on land cover and lacustrine landscape and their consequences in a large urban lake watershed, Donghu Lake watershed (the largest urban lake in China), Central China, by using Landsat TM satellite images of three periods of 1987, 1993 and 1999 and ground-based information. We grouped the land covers into six categories: water body, vegetable land, forested land, shrub-grass land, open area and urban land, and calculated patch-related landscape indices to analyze the effects of urbanization on landscape features. We overlaid the land cover maps of the three periods to track the land cover change processes. The results indicated that urban land continuously expanded from 9.1% of the total watershed area in 1987, to 19.4% in 1993, and to 29.6% in 1999. The vegetable land increased from 7.0% in 1987, 11.9% in 1993, to 13.9% in 1999 to sustain the demands of vegetable for increased urban population. Concurrently, continuous reduction of other land cover types occurred between 1987 and 1999: water body decreased from 30.4% to 23.8%, and forested land from 33.6% to 24.3%. We found that the expansion of urban land has at least in part caused a decrease in relatively wild habitats, such as urban forest and lake water area. These alterations had resulted in significant negative environmental consequences, including decline of lakes, deterioration of water and air quality, and loss of biodiversity.

Seed rain into upland plant communities in Hong Kong, China

Hong Kong is an extreme example of tropical landscape degradation, with no substantial remnants of the original forest cover and a highly impoverished disperser fauna. Seed availability is a potential limiting factor in vegetation recovery in such landscapes. To assess the quantity and quality of the seed rain of woody taxa, seed traps were placed in the major upland vegetation types: fire-maintained grassland, shrubland, and secondary forest. Within the grassland site, traps were placed under isolated trees, isolated male and female shrubs of Eurya chinensis, and in the open. Seeds were collected every 2 weeks for 2 years. The seed rain was highest under female shrubs in grassland (6455 seeds m−2 year−1), where it was almost entirely confined to their fruiting period. Next highest were isolated trees (890 seeds), followed by male isolated shrubs (611 seeds), shrubland (558 seeds), forest (129 seeds) and open grassland (47 seeds). The number of seed taxa was highest in shrubland (59), followed by isolated trees (42), forest (42), female isolated shrubs (28), male isolated shrubs (15), and open grassland (9). The seed rain differed in species composition between the forest, shrubland, and grassland sites, while the differences within the grassland site were largely in terms of quantity. Birds (particularly bulbuls, Pycnonotus spp.) are known or inferred to be the major dispersal agents for 85% of the seed taxa trapped, 99% of the total number of seeds trapped, and 99.8% of the seeds trapped in the grassland site. Few taxa and \(<1\%\) of the total seeds were dispersed by wind and no seed taxa were definitely dispersed by fruit bats. The results suggest that even in the most degraded landscape the seed rain is adequate for the development of woody vegetation cover, but that human intervention will be needed for the restoration of plant diversity.

Trends in the nutrient enrichment of U.S. rivers during the late 20th century and their relation to changes in probable stream trophic conditions

We estimated trends in concentrations of total phosphorus (TP) and total nitrogen (TN) and the related change in the probabilities of trophic conditions from 1975 to 1994 at 250 nationally representative riverine monitoring locations in the U.S. with drainage areas larger than about 1,000 km2. Statistically significant (p &lt; 0.05) declines were detected in TP and TN concentrations at 44% and 37% of the monitoring sites, and significant increases were detected at 3% and 9% of the sites, respectively. We used a statistical model to assess changes in the probable trophic‐state classification of the sites after adjusting for climate‐related variability in nutrient concentrations. The probabilistic assessment accounts for current knowledge of the trophic response of streams to nutrient enrichment, based on a recently proposed definition of “eutrophic,” “mesotrophic,” and “oligotrophic” conditions in relation to total nutrient concentrations. Based on these trophic definitions, we found that the trophic state improved at 25% of the monitoring sites and worsened at fewer than 5% of the sites; about 70% of the sites were unchanged. Improvements in trophic‐state related to declines in TP were more common in predominantly forested and shrub—grassland watersheds, whereas the trophic state of predominantly agricultural sites was unchanged. Despite the declines in TP concentrations at many sites, about 50% of all monitoring sites, and more than 60% of the sites in predominantly agricultural and urban watersheds, were classified as eutrophic in 1994 based on TP concentrations. Contemporaneous reductions in major nutrient sources to streams, related to wastewater treatment upgrades, phosphate detergent bans, and declines in some agricultural sources, may have contributed to the declines in riverine nutrient concentrations and associated improvements in trophic conditions.

Influence of variation of soil spatial heterogeneity on vegetation restoration

Numerous hypotheses and conceptional models dealing with the grassland deserti- fication or degradation processes recognize that the invasion of shrubs in grasslands is the most striking feature of the variation of vegetation patterns in the grassland degradation or desertifica- tion processes in arid and semiarid regions. This is because the invasion of shrubs in grasslands increases the heterogeneity of the temporal and spatial distribution of primary vegetation and soil resources. As a result, the biological processes of the soil-vegetation system are increasingly concentrated in the "fertile islands" under shrub canopies, and the soil between shrubs gradually turns into bare area or moving sand under the influences of prolonged wind and water erosion. Most of relative researches support this bio-ecological interpretation for the degraded process of grassland. However, as viewed from the other aspect, the shrub vegetation distributed in patches also serves as the "trigger spots" for the grassland restoration or desertification reversion, and this has been demonstrated by the practices of combating desertification in China. Nearly 50 years of succession of artificial sand-binding vegetation in the Shapotou area and the regional restoration of eco-environment are the theoretical verification and successful example for the desertification reversion. The establishment of artificial vegetation in the region began with the installation of sand fences and planting xerophytic shrubs relying on less than 200 mm of annual precipitation under the non-irrigation condition, this made the moving sand, an originally uni- formly distributed soil resource, occur the variation of spatial heterogeneity. Through the redis- tribution of precipitation and dustfall by the canopy of xerophytic shrubs, litter accumulation and cryptogamic crust development, soil-forming processes under shrub canopies were accelerated; in the meantime, it also created a favorable condition for the invasion and colonization of annual and perennial plant species. However, with the depletion of soil moisture in the deep layer in the sand stabilization area the coverage of shrubs in the sand-binding vegetation lowered from the highest value of 33% to 6%, the dominant position and leading effect of shrubs in the communi- ties were weakened, furthermore they were gradually taken out from the vegetation composition. This correspondingly weakened the spatial heterogeneity of soil resource distribution. The propagation of numerous cryptogams on fixed sand surface and the colonization of annual and perennial plant species promoted the succession and restoration of the vegetation towards herb-dominated vegetation, which are similar to the primary vegetation types of the adjacent steppified desert and desert steppe. This paper, taking nearly 50 years of succession of sand-binding vegetation in the Shapotou region as an example and using the geostatistical

Forage and bed sites characteristics of Indian muntjac (Muntiacus muntjak) in Hainan Island, China

Microhabitat factors associated with forage and bed sites of Indian muntjac (Muntiacus muntjak) were examined and compared in Hainan Island from 2001 to 2002. Habitat characteristics at forage and bed sites were measured by tracking five radio‐collared adult muntjacs (three females, two males) and locating from fresh sign or muntjac flushed during transect surveys. Among six available habitat types, Indian muntjac preferred shrub grassland and thorny shrubland, and used dry savanna in proportion to its availability. Muntjac avoided woods, cultivated grassplot and deciduous monsoon forests. Comparing forage sites with bed sites, food availability was greater at forage sites. For bed sites, taller trees with larger canopies, taller shrubs, denser shrub canopy covers and concealment covers were essential factors. No seasonal difference (wet season vs. dry season) was found in food abundance at either forage sites or at bed sites. During the wet season, canopy closure for both types of sites was higher and at bed sites concealment cover was higher than during the dry season. Principal component analysis indicated that tree height, d.b.h. and maximum canopy diameters were important factors in habitat selection. The different microhabitat characteristics at forage and bed sites can be a clue to understanding the survival strategy of Indian muntjac, a small‐bodied ungulate, in savanna woodlands.

C3shrub expansion in a C4grassland: positive post‐fire responses in resources and shoot growth

Changes in land management and reductions in fire frequency have enabled woody species to increase in grasslands worldwide. Nevertheless, fire is rarely eliminated from grasslands, and for shrubs to survive, they must be able cope with fire and replace aboveground structures. Because new shoots may have more available solar radiation, greater root : shoot ratios, and thus more resources available belowground after fire compared to undisturbed shrub communities, we hypothesized that carbon, nutrient, and water relations may be enhanced in stems compared to those in an undisturbed grassland. However, this same post-fire resource pulse stimulates the grasses and may intensify competitive interactions between shrubs and grasses. To test these predictions, we measured seasonal patterns in net photosynthesis (A), predawn xylem pressure potentials (XPP), leaf nitrogen (N) content, and productivity of Cornus drummondii shoots from shrub patches (islands) of different sizes in mesic grasslands burned annually, burned infrequently, and protected from fire. Seasonal average A was 20% higher (P = 0.016) in burned than in unburned shrubs, regardless of island size. Shrubs in burned sites also produced shoots with higher leaf N than unburned shrubs, and N content was higher in leaves from small islands compared to large islands (P < 0.0001). Burning caused a decrease in late summer predawn XPP in small islands (-3.1 MPa), whereas burned large islands did not differ from unburned shrubs. Post-fire productivity of new shoots was significantly greater compared to shoots in unburned sites. These results indicate that a transient period of high resource availability after fire allows for increased growth and rapid recovery of grassland shrubs. Thus, although fire has a negative effect on aboveground biomass of shrubs, the post-fire increases in resource availability, which enhance growth in the dominant grasses, are also important for recovery of woody species.

RANGES improves satellite‐based information and land cover assessments in southwest United States

Because of its influence on hydrology climate, and global biogeochemical cycles, land cover change may be the most significant agent of global environmental change. Land degradation results not only from land cover conversion, but also land cover function. For example, human activities in the southwest U.S., such as grazing regimes and fire frequency, are accelerating functional changes to fragile rangeland ecosystems, causing increased proportions of shrubs in grasslands, decreases in overall vegetation density and the introduction and spread of non‐native invasive species.

Pollen‐based biome reconstructions for Colombia at 3000, 6000, 9000, 12 000, 15 000 and 18 000 14C yr ago: Late Quaternary tropical vegetation dynamics

AbstractColombian biomes are reconstructed at 45 sites from the modern period extending to the Last Glacial Maximum (LGM). The basis for our reconstruction is pollen data assigned to plant functional types and biomes at six 3000‐yr intervals. A reconstruction of modern biomes is used to check the treatment of the modern pollen data set against a map of potential vegetation. This allows the biomes reconstructed at past periods to be assessed relative to the modern situation. This process also provides a check on the a priori assignment of pollen taxa to plant functional types and biomes. For the majority of the sites, the pollen data accurately reflect the potential vegetation, even though much of the original vegetation has been transformed by agricultural practices. At 18 000 14C yr BP, a generally cool and dry environment is reflected in biome, assignments of cold mixed forests, cool evergreen forests and cool grassland–shrub; the latter extending to lower altitudes than presently recorded. This signal is strongly recorded at 15 000 and 12 000 14C yr BP, the vegetation at these times also reflecting a relatively cool and dry environment. At 9000 14C yr BP there is a shift to biomes thought to result from slightly cooler environmental conditions. This trend is reversed by 6000 14C yr BP; most sites, within a range of different environmental settings, recording a shift to more xeric biome types. There is an expansion of steppe and cool mixed‐forest biomes, replacing tropical dry forest and cool grassland–shrub biomes, respectively. These changes in biome assignments from the modern situation can be interpreted as a biotic response to mid‐Holocene climatic aridity. At 3000 14C yr BP the shift is mainly to biomes characteristic of slightly more mesic environmental conditions.There are a number of sites that do not change biome assignment relative to the modern reconstruction, although the affinities that these sites have to a specific biome do change. These ‘anomalies’ are interpreted on a site‐by‐site basis. Spatially constant, but differential response of the vegetation to climatic shifts are related to changes in moisture sources and the importance of edaphic controls on the vegetation. The Late Quaternary reconstruction of large‐scale vegetation dynamics in Colombia allows an understanding of the environmental controls on these to be developed. In particular, shifts in the character of the main climatic systems that influence Colombian vegetation are described. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Floristic and life form diversity in post-pasture successions on a Mediterranean island (Corsica)

The landscapes of the French island of Corsica are the result of the grazing tradition which has always occupied a pivotal position in the island’s economy. Furthermore, in the last fifty years, a reduction in the pressure from human activity has lead to changes in plant cover. This article intends to study the evolution of dynamic mechanisms during the secondary successions and the effect of changes in vegetation on plant diversity. Our study was based on a simultaneous study of 375 plots carried out between May and September from 1996 to 1998. We studied the floristic composition of each stage both qualitatively (life form, adaptive strategy and seed dispersal mode) and quantitatively (species richness, equitability, Shannon index). The results show a change in species and environmental conditions, with a progressive move from heliophilic grassland and low shrub species with an S or SR strategy, anemochoric and with a high tolerance for soil quality to a final stage of dense forest composed of competitive (C or CS) zoochoric or barochoric phanerophytes. Moreover, the turning point in Corsica in terms of floristic diversity lies between 20 and 55 years after land abandonment. Successions are due to a series of rapid interactions. Thus, when a site is given reserve or protected area status, we need to take into account the fact that it will evolve according to the biological traits of the species. From a research point of view, we need to look at process diversity and the role of each of the species present during the successions.

Land-Cover/Land-Use Changes in the Derekolli Catchment of the South Welo Zone of Amhara Region, Ethiopia

A land-cover analysis carried out in the catchment of Derekolli stream, using image analysis and GIS technologies, in conjunction with data collected through field surveys, revealed two types of changes, i.e., land-cover modification and conversion. The shrubland, which apparently formed the climax vegetation of the study site, and accounted for 16.4 % of the watershed in 1957, disappeared at the rate of 1.6 and 0.31 per cent per year from 1957 to 1986, and from 1986 to 2000, respectively. This change involved a gradual thinning of the shrub and its modification to shrub grassland, and then grassland, due to the selective cutting of the woody biomass for fuelwood and charcoal production. A significant conversion from natural vegetation cover to cropland was observed only between 1957 and 1986, where the cultivated land expanded by 7 per cent. There was very little change in the cropland area since 1986, as most of the land suitable for cultivation was already in use and the limit for expansion had almost been reached. The other type of conversion, i.e., the change from cultivated land to urban area, was insignificant since the land taken up by the emerging town, together with the roads accounted for less than 1.5 per cent of the total area of the catchment.

Effect of Habitat Characteristics on the Probability of Parasitism and Predation of Bell's Vireo Nests

Because habitat characteristics may influence rates of brood parasitism by brown-headed cowbirds (Molothrus ater) and nest predation, we investigated the effects of nest-patch and habitat-patch characteristics on brood parasitism and predation of Bell's vireo (Vireo bellii) nests in grassland-shrub habitats in central Missouri, USA. We developed multivariate logistic regression models explaining parasitism and predation from variables measured at 142 and 130 nests, respectively. We ranked models based on Akaike's Information Criterion (AIC c ) and reported models that differed by <1 AIC c unit from the minimum AIC c value as candidates for the best model. We identified 5 candidate models explaining parasitism that included 9-11 variables and R 2 = 0.358-0.386. Nest-patch width, territory shrub cover, distance to crop field, site, year, site x territory shrub cover, nest height x side concealment, patch width x length, and habitat type x patch length were in all models. We identified 5 candidate models explaining predation with 3-5 variables and R 2 = 0.165-0.199. Parasitism status, patch width, and patch length or patch length x site appeared in all models. Overall, our models provide support for hypotheses that increased nest concealment, amount of nest cover, and nest height affect on nest predation and parasitism. Nest-patch width was a factor in most models but in the opposite direction predicted by these hypotheses. Interactions among some variables also produced habitat and site-specific exceptions to predictions by these hypotheses. We hypothesize that the best overall method to increase nest success in existing grassland shrub habitats is to increase the density of large shrub patches.

Reconstruction of a subalpine grass-dominated ecosystem, Lake Rutundu, Mount Kenya: a novel multi-proxy approach

Palaeoecological reconstructions based on a single proxy are limited, but by combining pollen, biogeochemistry and grass cuticle analysis, ecosystem structure and function can be better understood. Lake Rutundu is a small, subalpine lake on the northeast flank of Mt Kenya. During the last glacial, pollen evidence suggests a shrub grassland dominated by Afroalpine taxa and Poaceae, representing a dry, cold, open environment. The δ 13C values of terrestrial biomarkers imply a high proportion of C 4 plants. Grass cuticle analysis allows resolution of the different C 4 subtypes and shows that the vegetation was dominated by tall C 4 panicoid grasses, prone to frequent fires. During the Holocene, Poaceae pollen declined while subalpine shrubs increased. The δ 13C values of terrestrial biomarkers imply a C 3-dominated vegetation. Together with an expansion of rainforest at lower altitudes, this suggests wetter conditions more favourable to C 3 plants. Increased percentages of C 3 pooid grass cuticles confirm a reduction in moisture stress.

A reconstruction of Colombian biomes derived from modern pollen data along an altitude gradient

Biomes are reconstructed in Colombia from modern (core-top) pollen data derived from 22 sites along an altitudinal gradient (2000–4100m) that encompasses the tree line. The ‘biomization’ methodology is described in a stepwise manner that details the reconstruction of vegetation along an altitudinal gradient. In the majority of the cases, the results are comparable to site-specific descriptions of the vegetation. At altitudes between 2000 and 3000m, cool mixed forest and cool evergreen forest biomes are important. Between 3000 and 3700m the cool grassland/shrub biome is dominant. Above 3700m, the affinity score to arboreal biomes is low with a concomitant increased affinity to the cool grassland biome that co-dominates along with the cool grassland/shrub biome. Hence, the technique is shown successful at reconstructing modern vegetation, this is particularly so when affinity scores to the range of biomes, rather the single most dominant biome, are used to describe vegetation composition. The results are able to delimit the altitudinally-induced changes in vegetation, from Andean forest associations, open forest and shrub grassland complex through to high altitude grasslands. However, this ability of the modern pollen data to predict the potential vegetation, via the biomization technique, is complicated by several site-specific factors that impact on the vegetation. Of particular importance are localised environmental conditions, particularly moisture regime. An additional area of interest is the impact that human activity has had on the vegetation. By manipulation of the input matrices, which are assigned a priori, and consultation of the range of biome affinity scores, it is possible to identify which sites reflect an anthropogenic signal and how this is manifested within the pollen data. By investigating the range of affinity scores, these factors can be identified and the relatively subtle changes in reconstructed vegetation determined, in particular, how the composition and quantity of the arboreal component changes about the tree line. The results are discussed in the context of using tree line position, and altitudinally-induced floristic changes, for palaeoclimatic reconstruction.

Mesquite establishment in arid grasslands: an experimental investigation of the role of kangaroo rats

Kangaroo rats are thought to play a role in promoting the establishment of woody shrubs in arid grasslands by caching mesquite seeds. We examined this hypothesis experimentally by comparing mesquite establishment on plots from which kangaroo rats have been continuously removed for either 10 or 21 years to control plots with kangaroo rats present. Contrary to the above hypothesis, mesquite establishment was lowest on plots that contained kangaroo rats and highest on plots from which kangaroo rats have been absent for 21 years. We suggest that herbivory, either by kangaroo rats or lagomorphs, who were found to be more active on plots with kangaroo rats, more negatively affects mesquite seedling establishment than the potential positive effects of kangaroo rat seed-caching behavior in arid grasslands.

Genetic variation for carbon isotope composition in honey mesquite (Prosopis glandulosa).

Carbon isotope composition (delta(13)C) is a useful surrogate for integrated, plant water-use efficiency (WUE) when measured on plants grown in a common environment. In a variety of species, genetic variation in delta(13)C has been linked to the distribution of genotypes across gradients in atmospheric and soil water. We examined genetic variation for delta(13)C in seedlings of honey mesquite (Prosopis glandulosa Torr.), an invasive grassland shrub that thrives in the southwestern USA. Fifteen maternal families, representing progeny of 15 adult trees, were studied in three common garden experiments in a greenhouse. The 990-km east-west transect along which the adult trees were located encompasses a wide precipitation gradient, and includes mesic grassland, semiarid grassland, and Chihuahuan desert ecosystems. Genetic variation for delta(13)C in mesquite was substantial, with the rank order of half-sib families based on delta(13)C relatively stable across experiments, which were conducted under different environmental conditions. Conversely, rankings of families by mean seedling height (an index of growth rate) varied markedly among experiments. Seedlings derived from Chihuahuan desert adults emerged more quickly and had more negative delta(13)C (indicative of lower WUE) than seedlings derived from the other regions. Although delta(13)C and seedling height were not correlated, these results suggest that mesquite genotypes at the drier, western extreme of the species' range are adapted for quicker emergence and possibly faster growth than genotypes from mesic areas. Together, these traits may facilitate exploitation of infrequent precipitation events.

Vegetational type of Stonechats Saxicola torquata habitat during breeding season

The vegetation types of home ranges and the activity of Stonechats Saxicola torquata were investigated in a native shrub-grass land during their breeding season in eastern Hokkaido, Japan. They made 69% of nests in the vegetation type 1 (patches of tall plants mixed with low grasses). More than 70 % of their home range was covered by type 1 and type 2 (tall plants are scattered uniformly in low grasses). Two pairs and one male out of three pairs studied used type 1 more than expected during all breeding periods and one pair and one male also used type 2 in the incubation period and the fledgling period. Two males used type 3 (grove of Salix) and one male type 5 (tall plants grow densely) during the incubation and the fledgling periods respectively, while the uses of type 4 (low grasses) was low in all individuals. Type 1 and type 2 preferred by Stonechats as habitats may provide perches to hunt insects on the ground of open area. The feature of habitats in this study areas was similar to those in other regions.

Radiotelemetric Study of Activity and Movements of Racers (Coluber constrictor) Associated with a Carolina Bay in South Carolina

Racers (Coluber constrictor) were monitored with radiotelemetry in an old field/forest habitat surrounding a Carolina bay in South Carolina. Racers were active on approximately 70% of days; inactive snakes usually were in ecdysis. Active racers were relocated on the ground surface (40%), above ground in shrubs and trees (35%), and underground (25%). Habitats in which racers were relocated were thickets and grassland shrubs (50%), woodland/forest (25%), and grassland (25%). Active snakes moved an average of 104 m/d within home ranges averaging 12.2 ha estimated as minimum convex polygons. The home range of each snake included at least a portion of the bay and overlapped with home ranges of other snakes. In comparison with other studies of racers in which data were collected, analyzed, and presented in the same manner as in this study, home range size and daily movement of South Carolina racers were significantly greater. Possible explanations for the greater movement include trophic level differences and a proximate response to a local drought.

Landscape and Climatic Control of Woody Vegetation in a Dry Tropical Ecosystem: Turkana District, Kenya

The spatial organization of a dry woodland/ savanna/shrub-steppe ecosystem in a 9000 km2 region of arid Northern Kenya was explored by analysing the abundance and distribution of woody vegetation in relation to landscape gradients and gradients in rainfall. Woody species assemblages were clustered into four major groups. Three of these (and most of the sites) were dominated by species of Acacia. Acacia tortilis was the community dominant in riparian and riverine zones, A. senegal on hilly and rocky sites, and A. reficiens on non-riparian sites with fine soils. The fourth group, found at highest elevations, was distinguished by a very low abundance of Acacia. These soil/landform associations were systematically distributed along land system gradients (mountains, pediment, valley, bajada), thus relating woody species groups to large-scale landscape characteristics. Tree woody canopy cover ranged from < 1.0% to 100% over the region as a whole. Cover was directly related to precipitation when the effects of water concentration in riparian zones were removed. However, canopy cover was not greatly influenced by either species composition or landscape characteristics. Fire reduced woody canopy cover, however, fires appeared to affect a relatively small portion of the ecosystem. Recent anthropogenic disturbances such as wood-cutting and livestock corralling were encountered in patches, but the cumulative long-term effects of patch scale disturbances could not be discerned in vegetation patterns at the regional scale. Vegetation physiognomy (woodland, bush, bushed grassland, etc.) was controlled by both water availability and landscape pattern. Woodland and forest occurred almost exclusively in riparian and riverine situations while the driest parts of the region supported dwarf shrub grassland with few trees. Over the region as a whole, climate and landscape gradients converged in diverse ways, giving rise to structurally variable associations of woody plant species. Vegetation structure in tropical savannas and dry woodlands is often interpreted in terms of competition between woody and herbaceous life forms for soil moisture. The outcome of this competitive interaction is thought to be influenced by disturbances that shift the system from one stable state to another. However, our findings suggest that dry tropical ecosystem structure is hierarchically constrained by physical factors: by climate at regional to continental scales; by topographic effects on rainfall and landscape water redistribution, and geomorphic effects on soil and plant available water at the landscape to regional scales; and finally by water redistribution and disturbance at local and patch scales.

A study of the abundance and patchiness of cicada nymphs (Homoptera: Tibicinidae) in a New Zealand subalpine shrub-grassland

Abstract Cicada emergence skins in a subalpine shrub grassland have been sampled during 1969–75 to determine the abundance and spatial distributions of nymphs feeding on plant roots. A guild of six cicada species is primarily associated with two forms of vegetation: shrubs (Dracophyllum and Cassinia) and tall tussock (Chionochloa). Skin locations were mapped relative to dominant vegetation species, litter zones, and soil and rock pavements over a range of aspects, altitudes, and vegetation types, and sampling methods were scaled at four levels: the locality, plot, quadrat, and individual plant. There were significant differences in skin counts over four years, and different measures of mean skin densities are given for the four sampling scales. The two primary vegetation types had cumulative 1969–72 mean densities of 5.2 ± 4.0 and 12.9 ± 10.0 skins/quadrat (2.3 m2) , and the 1969–72 mean productivities of the upper 25% of quadrats (adjusted for percent ground cover) were, respectively, 5.5 and 35.5 skins/...