Effects of large herbivore browsing on the functional groups of woody plants in a southern African savanna

This study examined the effect of different large herbivore species and stocking rates in savanna ecosystems of Zimbabwe on the richness and abundances of woody plant functional groups and woody plant functional attributes. Seven fence-lines with different herbivore species and stocking rates on either side of the fence were sampled. Plots were placed on both sides of each fence at each of 18 randomly selected positions. The size and species of each woody plant was recorded for each plot. It was found that the number of species with different functional attributes of spinescence, leaf longevity, fruit type and dispersal mechanism and in the functional groups of palatability were not different on the different sides of the fence. However, there were differences in plant abundances for 26 out of the 35 tests carried out on plant abundances with different functional attributes and functional groups. It was hypothesised that the time needed to change woody plant species richness is hundreds of years in these systems, whereas the time needed to change woody plant abundances is decades.     

Host plant growth form and diversity: Effects on abundance and feeding preference of a specialist herbivore,Acalymma vittata (Coleoptera: Chrysomelidae)

Summary Abundances of the specialist herbivore, Acalymma vittata (Fab.) (Coleoptera: Chrysomelidae), were assessed in small experimental plots with three levels of plant diversity (cucumber monoculture, cucumber/corn, and cucumber/tomato) and two levels of host plant growth form (horizontal on the ground and vertical, staked up or growing up other plant species). Host plant growth form more strongly affected beetle abundances than did plant diversity; greater numbers were found on vertically growing than on horizontally growing cucumber plants. The combination of cucumber monoculture and vertical growth form supported significantly greater herbivore abundances than did any other type of plot, emphasizing a strong interaction between diversity and growth form. Beetles were not more common in monocultures with horizontal growth forms than in mixed species plots, and beetles did not respond differently to plots with corn and plots with tomatoes.Feeding experiments demonstrated that the plant diversity under which a host plant is grown strongly influenced herbivore feeding preference. Beetles given a choice of cucumber leaves grown in monoculture and in plots with tomatoes exhibited individual differences in their food selection behavior, however, a significantly greater number of beetles preferred monoculture leaves. Those individuals preferring monoculture leaves and those individuals preferring leaves from plots with tomatoes did not differ in either absolute or relative amounts of feeding damage per leaf.Neither plant size nor the date on which plots were colonized by beetles explained the differences in herbivore abundance. It is suggested that differences in movement patterns and plant quality contributed to the greater numbers of beetles on plants growing vertically in monocultures.     

Can livestock grazing maintain landscape diversity and stability in an ecosystem that evolved with wild herbivores?

The livestock industry is converting mountain ecosystems in central Argentina into rocky deserts. However, it helps to conserve plant biodiversity, presumably because the ecosystem has evolved under wild herbivores now locally extinct. We hypothesized that low or moderate livestock stocking rates, instead of the high stocking rates currently used for commercial production, might mimic pre-hispanic herbivore pressures. Thus, the mosaic of physiognomies necessary to maintain landscape diversity and soil integrity could be preserved. To test this hypothesis we tracked physiognomic changes in 200 plots (16 m² each) under different stocking rates, including livestock exclusion, for five years. Contrary to our expectations, we found that both low and moderate stocking rates failed to maintain landscape diversity. As observed for livestock exclusion, low to moderate stocking rates promote retraction and, finally, elimination of short grazing lawns and their replacement by tall tussock grasslands, or possibly by woody vegetation. In turn, heavy grazing pressure maintains the desired short lawn patches in the landscape, but also promotes a concomitant loss and eventual elimination of woodlands, together with an expansion of bare rock as a consequence of soil erosion. These results indicate that the present mosaic of physiognomic types is difficult to maintain and raise questions about the past stability of the ecosystem. We suggest that during the last 400 years of livestock production in the ecosystem, short lawns have been maintained by means of heavy stocking rates and anthropogenic fires ignited to eliminate tussocks. However, this relative stability of lawns has been attained concomitantly with a progressive loss of soils and woodland area. We also discuss some possible explanations for the maintenance of short lawns before the introduction of livestock, based on main climatic and floristic shifts during the late Holocene.     

Describing a multitrophic plant-herbivore-parasitoid system at four spatial scales

Herbivore-parasitoid interactions must be studied using a multitrophic and multispecies approach. The strength and direction of multiple effects through trophic levels may change across spatial scales. In this work, we use the herbaceous plant Ruellia nudiflora, its moth herbivore Tripudia quadrifera, and several parasitoid morphospecies that feed on the herbivore to answer the following questions: Do herbivore and parasitoid attack levels vary depending on the spatial scale considered? With which plant characteristics are the parasitoid and the herbivore associated? Do parasitoid morphospecies vary in the magnitude of their positive indirect effect on plant reproduction? We evaluated three approximations of herbivore and parasitoid abundance (raw numbers, ratios, and attack rates) at four spatial scales: regional (three different regions which differ in terms of abiotic and biotic characteristics); population (i.e. four populations within each region); patch (four 1 m² plots in each population); and plant level (using a number of plant characteristics). Finally, we determined whether parasitoids have a positive indirect effect on plant reproductive success (seed number). Herbivore and parasitoid numbers differed at three of the spatial scales considered. However, herbivore/fruit ratio and attack rates did not differ at the population level. Parasitoid/host ratio and attack rates did not differ at any scale, although there was a tendency of a higher attack in one region. At the plant level, herbivore and parasitoid abundances were related to different plant traits, varying the importance and the direction (positive or negative) of those traits. In addition, only one parasitoid species (Bracon sp.) had a positive effect on plant fitness saving up to 20% of the seeds in a fruit. These results underline the importance of knowing the scales that are relevant to organisms at different trophic levels and distinguish between the specific effects of species.     

Threatened woody flora as an ecological indicator of large herbivore introductions

Rewilding and translocations of large herbivores for conservation purposes have increased in recent times, with numerous introductions inside and outside their native range. This study aims to analyze the use of threatened plant taxa as a possible ecological indicator of large herbivore introductions. We examined the effects of a threatened large ungulate, the Barbary sheep (Ammotragus lervia), on both endangered and vulnerable woody taxa after its introduction in 1970. Contrary to our hypothesis, the herbivore impact on threatened woody species was higher than that found on widespread woody plants. The results reveal that 35.7% of the threatened species showed the highest possible level of herbivore damage in contrast to 6.5% for the widespread species. Threatened species were preferred over common plants, probably due to their greater palatability. Overall plant cover, including neighboring species, was also an important factor determining browsing damage and, thus, habitats with low ground cover should be particularly considered in conservation plans. Herbivore damage on common taxa should be taken with caution since they could mask unsustainable herbivore densities for threatened woody taxa or protected habitats. The use of threatened woody taxa through the studied ecological indicators (herbivore damage, plant preferences, habitat use and regeneration success) represented a useful tool to assess the sustainability of large herbivores introductions and to establish a priority conservation ranking for threatened plant species. These findings highlight the deleterious effects of overabundant ungulate populations regardless its origin (exotic or native) and the need of monitoring threatened woody taxa to better estimate the suitability and sustainability of large herbivore introductions.     

Testing the efficiency of three 15N-labeled nitrogen compounds for indirect labeling of grasshoppers via plants in the field

In field studies of plant–insect herbivore interactions it is often difficult to establish which herbivore has fed on a particular plant. We investigated the suitability of three different ¹⁵N‐labeled nitrogen compounds (ammonium, nitrate, and glycine) for indirect marking of three grasshopper species [Omocestus viridulus (L.), Chorthippus parallelus (Zett.), and Chorthippus biguttulus (L.) (Orthoptera: Acrididae)] through labeling their food plants in the field. In two short‐term experiments grassland plots of 1 m² were separately labeled with either one of the different nitrogen compounds. Grasshoppers were caged on three food‐plant species [Dactylis glomerata L., Holcus lanatus L. (Poaceae), and Trifolium repens L. (Fabaceae)] present in these plots for 72 h. Significantly enriched δ¹⁵N values in grasshoppers were found in all plant/grasshopper combinations. Enrichment in grasshoppers was positively correlated with the enrichment of plants and labeling with nitrate resulted in highest ¹⁵N enrichment. In a long‐term experiment, individuals of C. biguttulus were placed in a cage covering an area of 1 m² for 37 days, with sampling of grasshoppers at regular intervals. δ¹⁵N values of the grasshopper and a common food plant, D. glomerata, increased steadily over time, up to 40‐fold by the end of the experiment. Our results demonstrate that ¹⁵N‐labeling of plants is an appropriate tool for the investigation of insect–plant interactions under natural conditions.     

Changes in heathland vegetation under goat grazing: effects of breed and stocking rate

Questions: How are heathland vegetation dynamics affected by different goat grazing management? Location: Cantabrian heathlands in Illano, Asturias, northern Spain. Methods: During 4 years, vegetation dynamics (structural composition, canopy height and floristic diversity) were studied under three goat grazing treatments with three replicates: high stocking rate (11.7 goats ha^?1) with a local Celtiberic breed, and high (15 goats ha^?1) and low (6.7 goats ha^?1) stocking rates with a commercial Cashmere breed. Results: The relative cover of woody plants, particularly heather species, decreased more while herbaceous cover increased more under local Celtiberic than under Cashmere breed grazing. Within Cashmere treatments, the cover and height of live shrubs decreased more and the herbaceous cover increased more under high than under low stocking rate. Redundancy analysis showed a significant effect of treatment × year interaction on floristic composition. Greater species richness was recorded under local goat grazing, but Shannon diversity index fell in the fourth year on these plots because of dominance by two grass species. Conclusions: Local Celtiberic goat grazing at such a high stocking rate (11.7 goats ha^?1) hinders the development of sustainable systems on these heathlands, both in environmental and productive terms, owing to the limitations in soil fertility. Nevertheless, Celtiberic goats could be useful for controlling excessive shrub encroachment and reducing fire hazard. Cashmere goat grazing at high stocking rate promoted the highest Shannon diversity by generating a better balance between woody and herbaceous plants, while shrub dominance was not altered under the low stocking rate.     

Using plant functional types to compare vegetation structure of alien-invaded and uninvaded Acacia nilotica savannas

Four plant functional types (PFTs) were used to compare the vegetation structure of an alien-invaded Acacia nilotica savanna with one of negligible invasions. Heights, canopy covers and species richness of three native PFTs (woody plants, grasses and herbs) and one alien PFT (woody plants) were measured in 14, 1-m² quadrats sampled in a stratified-random pattern in a 400-m² plot demarcated in each savanna. In the uninvaded plot, mean heights of native PFTs were stratified. In the invaded plot, the mean height of aliens extended into the native woody stratum with the lower range of native woody PFT heights reduced to the grass stratum. Discriminant analysis of canopy covers and species richness of the four PFTs revealed significant differences in composition between plots with the alien PFT being the most important variable correlated with these differences. Univariate analysis confirmed the dominance of alien woody plants in the invaded plot but also showed significant reductions in the canopy covers and species richness of native herbs and grasses compared to those in the uninvaded plot. These results suggest that PFTs can rapidly measure small-scale, spatial differences in the physiognomy, composition and species richness of A. nilotica savannas when invaded by alien woody plants.     

Encroachment of woody plants and its impact on pastoral livestock production in the Borana lowlands, southern Oromia, Ethiopia

Encroachment of woody plants has been among the major threats to the livelihoods of Borana pastoralists and their ecosystem. An approach that integrated vegetation survey and pastoralists’ perception was followed to study the impacts of encroachment of woody plants in the Borana lowlands, Ethiopia. Density of woody species was determined in 192 plots of 500 m². Canopy cover of woody plants was estimated in 123 quadrates of 400 m². Pastoralists’ perception was assessed through group discussions and a semi‐structured questionnaire. Results showed that plant density was 3014 woody plants ha^?1. Cover of woody plants was 52%, indicating an increasing trend from ≤40% cover reported in the early 1990s. It was concluded that the increase of woody plants density and cover has crossed the critical threshold and has entered into the encroached condition. Principal components analysis (PCA) and redundancy analysis (RDA) also showed that woody plants were negatively correlated with herbaceous biomass. Commiphora africana, Acacia melliphera, A. drepanolobium, A. brevispica and Lannea rivae were among the dominant encroachers. RDA revealed that soil nutrients were positively correlated with woody plants density and cover. The pastoralists perceived that encroachment of woody plants had decreased the production of their grazingland. A ban on fire was perceived as the major factor that caused encroachment of woody plants. Re‐utilization of fire and strengthening of traditional rangeland management strategies are recommended.     

Reciprocal interactions between native and introduced populations of common milkweed,Asclepias syriaca,and the specialist aphid,Aphis nerii

Following its introduction into Europe (EU), the common milkweed (Asclepias syriaca) has been free of most specialist herbivores that are present in its native North American (NA) range, except for the oleander aphid Aphis nerii. We compared EU and NA populations of A. nerii on EU and NA milkweed populations to test the hypothesis that plant–insect interactions differ on the two continents. First, we tested if herbivore performance is higher on EU plants than on NA plants, because the former have escaped most of their herbivores and have perhaps been selected for lower defence levels following introduction. Second, we compared two A. nerii lines (one from each continent) to test whether genotypic differences in the herbivore may influence species interactions in plant–herbivore communities in the context of species introductions. The NA population of A. nerii developed faster, had higher fecundity and attained higher population growth rates than the EU population. There was no overall significant continental difference in aphid resistance between the plants. However, milkweed plants from EU supported higher population growth rates and faster development of the NA line of A. nerii than plants from NA. In contrast, EU aphids showed similar (low) performance across plant populations from both continents. In a second experiment, we examined how chewing herbivores indirectly mediate interactions between milkweeds and aphids, and induced A. syriaca plants from each continent by monarch caterpillars (Danaus plexippus) to compare the resulting changes in plant quality on EU aphid performance. As specialist chewing herbivores of A. syriaca are only present in NA, we expected that plants from the two continents may affect aphid growth in different ways when they are challenged by a specialist chewing herbivore. Caterpillar induction decreased aphid developmental times on NA plants, but not on EU plants, whereas fecundity and population growth rates were unaffected by induction on both plant populations. The results show that genetic variation in the plants as well as in the herbivores can determine the outcome of plant–herbivore interactions.     

Linking woody species diversity with plant available water at a landscape scale in a Brazilian savanna

Question: How is the diversity of woody species in a seasonally dry savanna related to plant available water (PAW)? Location: Savannas in central Brazil. Methods: Two‐dimensional soil resistivity profiles to 10‐m depth previously measured along three 10 m × 275 m replicate transects revealed differences in belowground water resources among and within transects: (1) driest/most heterogeneous; (2) wettest/least heterogeneous; and (3) PAW‐intermediate. All woody plants along these transects were identified to species, and height and basal circumference measured. Species diversity was evaluated for the whole transect (total diversity), 100‐m² plots (alpha‐diversity) and dissimilarity among 100‐m² plots within transects (beta‐diversity). Correlation analyses were conducted between PAW and vegetation variables at the 100‐m² scale. Results: The driest/most heterogeneous transect had the lowest total species diversity, while the wettest/least heterogeneous transect showed the lowest beta‐diversity. Floristic variation was correlated with PAW in all transects. In the most heterogeneous transect, species density was positively correlated with PAW in the 0‐400 cm soil layer. Evenness and Simpson's diversity were negatively correlated with PAW in the 700‐1000 cm soil layer. Conclusion: Woody species diversity was related to PAW at a fine spatial scale. Abundant PAW in the top 4 m of soil may favour many species and increase species total diversity. Conversely, abundant PAW at depth may result in lower evenness and total diversity, probably because the few species adapted to obtaining deep soil water can become dominant. Environmental changes altering soil water availability and partitioning in soil layers could affect the diversity of woody plants in this savanna.     

Differential effects of goat browsing on herbaceous plant community in a two-phase mosaic

The impact of herbivores on herbaceous plant communities is usually attributed to direct consumption of plants. We hypothesized that goats affect herbaceous plants both directly (consumption by foraging) and indirectly, by changing environmental conditions through modification of woody plant structure. We assessed the effects of goats browsing on environmental conditions, landscape structure, and herbaceous plants to link the direct and indirect effects of goats on herbaceous communities. Our model system was the Mediterranean woodland in Mt. Carmel, Israel. This is a two-phase mosaic landscape, composed of herbaceous (open) and woody patches. We delineated 10 plots of 1000 m², goats were introduced to five plots and five plots remained without goats. We monitored plant species richness and composition in two adjacent patch types (woody and open) in each plot. For each patch type, in all plots, we collected data on environmental conditions. We analyzed landscape structure using landscape metrics derived from a high-resolution vegetation map. We found that goats modified the structure of woody plants and hence the landscape mosaic. This alteration was associated with changes in environmental conditions, with more light penetration and higher temperatures. The impact of goats on the herbaceous plant community depended on patch type. In open patches, goats affected the herbaceous community mostly by direct consumption, whereas in woody patches they affected the herbaceous community mainly by modification of abiotic conditions. Our results stress the importance of considering landscape and patch structure in analyzing the effect of herbivory on plant communities.     

Is leaf-level photosynthesis related to plant success in a highly productive grassland?

We addressed the question: “Are short-term, leaf-level measurements of photosynthesis correlated with long-term patterns of plant success?” in a productive grassland where interspecific competitive interactions are important. To answer this question, seasonal patterns of leaf-level photosynthesis were measured in 27 tallgrass prairie species growing in sites that differed in species composition and productivity due to differences in fire history. Our specific goals were to assess the relationship between gas exchange under field conditions and success (defined as aerial plant cover) for a wide range of species, as well as for these species grouped as dominant and sub-dominant grasses, forbs, and woody plants. Because fire increases productivity and dominance by grasses in this system, we hypothesized that any relationship between photosynthesis and success would be strongest in annually burned sites. We also predicted that regardless of fire history, the dominant species (primarily C₄ grasses) would have higher photosynthetic rates than the less successful species (primarily C₃ grasses, forbs and woody plants). Because forbs and woody species are less abundant in annually burned sites, we expected that these species would have lower photosynthetic rates in annually burned than in infrequently burned sites. As expected, the dominant C₄?grasses had the highest cover on all sites, relative to?other growth forms, and they had the highest maximum and seasonally averaged photosynthetic rates (17.6 ± 0.42 μmol m^?2 s^?1). Woody species had the lowest average cover as well as the lowest average photosynthetic rates, with subdominant grasses and forbs intermediate in both cover and photosynthesis. Also as predicted, the highest overall photosynthetic rates were found on the most productive annually burned site. Perhaps most importantly, a positive relationship was found between leaf-level photosynthesis and cover for a core group of species when data were combined across all sites. These data support the hypothesis that higher instantaneous rates of leaf-level photosynthesis are indicative of long-term plant success in this grassland. However, in contrast to our predictions, the subdominant grasses, forbs and woody species on the annually burned site had higher photosynthetic rates than in the less frequently burned sites, even though their average cover was lower on annually burned sites, and hence they were less successful. The direct negative effect of fire on plant cover and species-specific differences in the availability of resources may explain why photosynthesis was high but cover was low in some growth forms in annually burned sites.     

Managing open habitats by wild ungulate browsing and grazing: A case‐study in North‐Eastern Germany

Question: Can wild ungulates efficiently maintain and restore open habitats? Location: Brandenburg, NE Germany. Methods: The effect of wild ungulate grazing and browsing was studied in three successional stages: (1) Corynephorus canescens‐dominated grassland; (2) ruderal tall forb vegetation dominated by Tanacetum vulgare; and (3) Pinus sylvestris‐pioneer forest. The study was conducted over 3 yr. In each successional stage, six paired 4 m²‐monitoring plots of permanently grazed versus ungrazed plots were arranged in three random blocks. Removal of grazing was introduced de novo for the study. In each plot, percentage cover of each plant and lichen species and total cover of woody plants was recorded. Results: Wild ungulates considerably affected successional pathways and species composition in open habitats but this influence became evident in alteration of abundances of only a few species. Grazing effects differed considerably between successional stages: species richness was higher in grazed versus ungrazed ruderal and pioneer forest plots, but not in the Corynephorus sites. Herbivory affected woody plant cover only in the Pioneer forest sites. Although the study period was too short to observe drastic changes in species richness and woody plant cover, notable changes in species composition were still detected in all successional stages. Conclusion: Wild ungulate browsing is a useful tool to inhibit encroachment of woody vegetation and to conserve a species‐rich, open landscape.     

Substrate heterogeneity and number of plant species in Everglades savannas (Florida,USA)

Environmental heterogeneity, especially that related to topography, has been proposed to influence numbers of plant species in different sized areas. Despite little variation in elevation, large numbers of vascular plant species occur in some habitats. This study explored possible relationships between number of plant species and substrateheterogeneity in two species-rich habitats, subtropical pine savannas and short-hydroperiod prairies, in the Long Pine Key region of Everglades National Park (Florida, U.S.A.). We examined relationships between numbers of vascular plant species and topographic heterogeneity by measuring numbers of species and elevations in differ-ent sizes of nested plots that spanned five orders of magnitude (0.1 m² to 1000 m² ) and that were located along two transects extending from pine savannas into short-hydroperiod prairies in different areas of Long Pine Key. We also classified substrates and soil depths in 1 m 2 sized submodules within the nested plots. Pine savannas occurred at higher elevations than adjacent short-hydroperiod prairies. Although differences occurred in substrate types and distribution within 1 m 2 plots, numbers of species were not associated with these differences. Vari-ances in elevations were similar in the smallest plots, but increased with area more rapidly in pine savannas than in short-hydroperiod prairies. Plot size explained about 85% of the variation in species numbers, which increased from 2040 per 1 m 2 to 80120 per 1000 m² . An interaction between habitat and scale explained 5% of the variation; more species occurred in shorthydroperiod prairies than pine savannas at scales <10 m² , but the re-verse occurred at scales >10 m². The number of species in pine savannas at scales of 1 m²and 10 m²was positively associated with variation in elevations; no significant relationships were obtained in short-hydroperiod prairies, which lack the fine-scale topographic variation of pine savannas. Our data indicate that substrate het-erogeneity, measured as variation in elevations, is not likely to be involved in the co-occurrence of many species within small areas of these savannas, but may influence numbers of species at larger scales of observation, es-pecially in pine savannas. Why many plant species occur within very small areas in these savannas remains un-answered.     

Unpreferred plants affect patch choice and spatial distribution of European brown hares

Many herbivore species prefer to forage on patches of intermediate biomass. Plant quality and forage efficiency are predicted to decrease with increasing plant standing crop which explains the lower preference of the herbivore. However, often is ignored that on the long-term, plant species composition is predicted to change with increasing plant standing crop. The amount of low-quality, unpreferred food plants increases with increasing plant standing crop. In the present study the effects of unpreferred plants on patch choice and distribution of European brown hare in a salt-marsh system were studied. In one experiment, unpreferred plants were removed from plots. In the second experiment, plots were planted with different densities of an unpreferred artificial plant. Removal of unpreferred plants increased hare-grazing pressure more than fivefold compared to unmanipulated plots. Planting of unpreferred plants reduced hare-grazing pressure, with a significant reduction of grazing already occurring at low unpreferred plant density. Spatial distribution of hares within this salt-marsh system was related to spatial arrangement of unpreferred plants. Hare-grazing intensity decreased strongly with increasing abundance of unpreferred plants despite a high abundance of principal food plants. The results of this study indicate that plant species replacement is an important factor determining patch choice and spatial distribution of hares next to changing plant quality. Increasing abundance of unpreferred plant species can strengthen the decreasing patch quality with increasing standing crop and can decrease grazing intensity when preferred food plants are still abundantly present.     

Effects of restriction of wild herbivore movement on woody and herbaceous vegetation in the Okavango Delta Botswana

We conducted herbaceous and woody vegetation surveys across Botswana's southern Okavango Buffalo Fence, which separates wildlife management from tribal grazing areas, to determine whether the restriction of herbivore movement by fencing has influenced vegetation composition, diversity and structure. We sampled herbaceous and woody vegetation at twenty paired sites every 2 km along the fence. For the herbaceous layer, ten 0.25 m² quadrats were laid every 10 m perpendicular to the fence; while for the woody vegetation, variable quadrat plots were used. Paired t‐tests were run. Results show little difference in forb or grass composition between the two sides. However, the cover and diversity of many woody species were greatly reduced across most height classes on the wildlife management side. Overall woody cover on the wildlife side of the fence was nearly half that of the tribal grazing side (t = 2.83, P = 0.011, df = 19), while overall wood diversity was also significantly less on the wildlife side (t = 3.29, P = 0.004, df = 19). We conclude that the concentration of wildlife due to the fence, while improving habitat for some herbivore species, is having a detrimental effect on plant diversity in general.     

短花针茅荒漠草原α多样性对绵羊载畜率的响应

以短花针茅(Stipa breviflora)荒漠草原为研究对象,采用巢式样方法分析在不同绵羊载畜率下草地的α多样性以及物种组成的变化,探讨荒漠草原生态系统在放牧利用下α多样性对载畜率的响应。结果表明:物种数和α多样性指数随着载畜率的升高而下降。放牧会减少群落非优势物种的相对多度,非优势物种能够反映α多样性。40 m~2为短花针茅荒漠草原多样性研究的最佳取样面积。种-面积关系以及α多样性指数-面积关系符合对数增长模型:y=aln(x)+b。随着取样面积尺度的增加,α多样性指数沿载畜率梯度差异性逐渐增大,在取样面积为0.16 m~2到0.64 m~2时可以体现中、高载畜率和零、低载畜率水平间的差异性,在160 m~2时可以体现各载畜率水平之间的差异显著性。     

Plant Functional Types of Woody Species Related to Fire Disturbance in Forest–Grassland Ecotones

The study of plant functional types (PFTs) has been widely emphasized when analysing plant community changes in relation to variations in climate and disturbance regime. In this study, we search for PFTs of woody species near forest–grassland boundaries in South Brazil where, due to climate, forests tend to expand over grassland but are being restricted by frequent fires. We aimed at answering the questions: (i) which plant functional types of forest woody species can establish in adjacent grassland subject to fire disturbance and (ii) which plant functional types of forest and grassland woody species are related to short-term community dynamics in frequently burned grassland. Traits were assessed in woody plants in 156 plots (6.75 m²) arranged in 12 transects across forest–grassland boundaries with different fire history in their grassland part. The analysis used a recursive algorithm to search for traits and PFTs maximally associated to spatial distance from forest limit in one analysis, and elapsed time since last fire in another. As a result, nine PFTs of forest woody species were identified that best described community patterns associated to distance from forest. Resprouting ability characterized forest plants able to colonize grasslands. PFT diversity was higher in border plots than inside forest or grassland. Four PFTs of forest and grassland woody species best described woody species community patterns in the grassland associated to elapsed time since fire. Taller individuals of single-stemmed shrubs predominated in late post-fire recovery (3–4 years), while shorter multi-stemmed shrubs in recently burned areas (3 months to 1 year). PFTs of forest trees occurred in border plots or, as established adults, in grassland, remaining unaffected by fire. We conclude that easily measurable structural plant traits, such as those used in our study, are sufficient to evaluate post-fire community dynamics. Forest PFTs in burned grassland are restricted to those with resprouting ability to survive recurrent fire events. Establishment success is highest on protected sites with lesser or low-intensity fire.     

Multi-scale comparisons of cliff vegetation in Colorado

We developed a nested vegetation sampling protocol to sample the plant diversity on south-facing cliffs and cliff bases in Jefferson County, Colorado. The multi-scale plots included three nested spatial scales, 1 m², 20 m², and 40 m². We compared plant species richness and species diversity among large cliffs, medium cliffs, small cliffs, and non-cliff sites using Hill's diversity numbers (N ₀, N ₁, and N ₂) for the 1-m² quadrats. Species richness (N ₀) was calculated for the 20-m² and 40-m² plots. Our results indicate that plant species diversity on the cliff faces did not increase with increasing cliff area. This pattern was consistent at all three sampling scales. A model selection was run to determine if plant species diversity values on the cliff faces were associated with cliff variables. None of the cliff variables measured were good predictors of diversity at the 1-m² scale. However, at the 20-m² scale, canyon differences and a positive relationship with increasing cliff surface roughness explained 70% of the variability in species richness. Although most plant species sampled on the cliff faces were also found in the base plots, 13 species were sampled only on the cliff faces. Additionally, dry south facing cliffs support a mix of xeric and mesic plants indicating that cliffs may provide unique microenvironments for plant establishment. This revised version was published online in August 2006 with corrections to the Cover Date.