元江干热河谷稀树灌草丛植被碳储量及净初级生产力

稀树灌草丛作为干热河谷区特殊的植被类型,其碳储量等一直缺乏必要的研究。以元江干热河谷稀树灌草丛植被为对象,利用典型样地法研究该区稀树灌草丛植被的碳储量与净初级生产力。结果表明:元江稀树灌草丛植被的碳储量为32.13 t C/hm~2,其中乔木、灌木和草本各层次的碳储量为26.70、4.04、1.40 t C/hm~2,分别占到总碳储量的83.02%、12.57%、4.4%。乔木层中地上部分碳储量占到66.70%。另外,元江稀树灌草丛的净初级生产力为3.88 t C hm~(-2)a~(-1),其中林分的净初级生产力为1.90 t C hm~(-2)a~(-1),凋落物量为1.98 t C hm~(-2)a~(-1);林下植被层对林分净初级生产力的贡献达到了46.92%。说明元江稀树灌草丛具有较高的碳储量和碳汇能力。结果为稀树灌草丛碳循环及碳汇功能研究提供了基础,同时也为干热河谷区植被的保育与可持续经营提供了科学依据。     

Lessons Learned about Research and Management: A Case Study from a Midwest Lowland Savanna,U.S.A.

Integrating research and practice is a fundamental challenge for restoration and conservation. Scientists recognize restoration as an opportunity to test basic ecological principles, and that core scientific practices such as establishing control plots, replication, monitoring, and data management are fundamental to learning from restoration and management. However, practical constraints inherent to many projects often work against fully integrating research and management. In 2005, we had the opportunity to evaluate a lowland savanna that had been undergoing restoration by The Nature Conservancy since 1997. We used the data generated as a case study to evaluate how the restoration process might have changed had scientists been involved from the outset. Prescribed fire is the primary tool used to restore upland savanna and was chosen by The Nature Conservancy to manage the lowland savanna. In open areas dominated by Reed canary grass (Phalaris arundinacea), fire was associated with reduced cover of this highly invasive species and an increase in native species richness. However, other results suggested that management options beyond the use of fire are needed to restore this lowland savanna. In retrospect, an approach that integrated the skills of researchers and managers at the outset of the study would have altered many of the restoration goals and processes. Pretreatment data, greater replication, and systematically varying treatments could have provided a more robust assessment of treatment effects. We recognize, however, that impediments in staffing, funding, and time were barriers to following this course of action.     

Expansion of gallery forests into central Brazilian savannas

Upland tropical forests have expanded and contracted in response to past climates, but it is not clear whether similar dynamics were exhibited by gallery (riparian) forests within savanna biomes. Because such forests generally have access to ample water, their extent may be buffered against changing climates. We tested the long‐term stability of gallery forest boundaries by characterizing the border between gallery forests and savannas and tracing the presence of gallery forest through isotopic analysis of organic carbon in the soil profile. We measured leaf area index, grass vs. shrub or tree coverage, the organic carbon, phosphorus, nitrogen and calcium concentrations in soils and the carbon isotope ratios of soil organic matter in two transitions spanning gallery forests and savanna in a Cerrado ecosystem. Gallery forests without grasses typically show a greater leaf area index in contrast to savannas, which show dense grass coverage. Soils of gallery forests have significantly greater concentrations of organic carbon, phosphorus, nitrogen and calcium than those of savannas. Soil organic carbon of savannas is significantly more enriched in ¹³C compared with that of gallery forests. This difference in enrichment is in part caused by the presence of C₄ grasses in savanna ecosystem and its absence in gallery forests. Using the ¹³C abundance as a signature for savanna and gallery forest ecosystems in 1 m soil cores, we show that the borders of gallery forests have expanded into the savanna and that this process initiated at least 3000–4000 bp based on ¹⁴C analysis. Gallery forests, however, may be still expanding as we found more recent transitions according to ¹⁴C activity measurements. We discuss the possible mechanisms of gallery forest expansion and the means by which nutrients required for the expansion of gallery forest might accumulate.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.     

Interpreting some outstanding features of the flora and vegetation of Madagascar

Six features are covered. (1) The high endemism, which is not discussed in detail, is all-pervasive, and has resulted from the isolation of Madagascar from Africa some 125 million years ago and their present separation by 430 km. (2) The great richness in plant species (especially relative to Africa), seen particularly in the families of woody species in the wetter vegetation-types, involves both sympatry and allopatry within genera, and is explicable in terms of much less extreme drying out than in Africa during the Pleistocene and effective ‘species-pumping’ rather than mass extinctions during that period. (3) The abundance and species-richness of palms, pandans, tree-ferns, bamboos, and certain families of dicot trees (notably Lauraceae, Monimiaceae, Myrsinaceae and Myristicaceae) in the lowland rain forests also appears to be a result of both past and present wetness of the climate, while it is hypothesized that the low stature of most lowland rain forests, paucity of large-girth trees, and small size and sparsity of broad-leaved herbs, are a result of most rain forest soils being old and relatively nutrient-poor. (4) Within the dry evergreen forest region where rainfall is moderate (900–1600 mm yr⁻¹) a sub-set of trees with fire-resistant bark seems to have evolved at sites prone to frequent ground fires, some perhaps spreading out of adjacent palm savanna on seasonally flooded sites. (5) Both the evolution of thicket rather than grassy woodland in the driest areas (300–600 mm yr⁻¹), and the abundance of evergreen trees and shrubs on ordinary soils – not confined to run-on sites – are explicable in terms of there being a finite chance of rain throughout the year rather than one short wet season, coupled with relatively high values for air humidity throughout the year. The same factors probably explain the abundance and variety of succulents in the thicket; they are found throughout and not just on rocks. (6) Concerning physical defence against herbivores, the rain forests, dry evergreen forests and deciduous forests all show a complete lack of plants with physiognomic features plausibly related to browsing by extinct giant birds (a strong contrast with New Zealand), but in the semi-deciduous thicket there are many tiny-leaved, mostly non-spiny shrubs and small trees, whose dense branching and impenetrability have plausibly evolved as a defence against browsing by elephant birds. The Didiereaceae of the thicket are spiny (unlike members of the same family in Africa), and are giant analogues of the ‘ocotillo’ (Fouquieria splendens) in western North America rather than of Cactaceae; their spines appear to be protecting the leaves more than the stems against arboreal primates, spine length paralleling leaf length.     

Woody overstorey effects on soil carbon and nitrogen pools in South African savanna

Abstract Woody plant encroachment in savannas may alter carbon (C) and nitrogen (N) pools over the long‐term, which could have regional or global biogeochemical implications given the widespread encroachment observed in the vast savanna biome. Soil and litter %C and %N were surveyed across four soil types in two encroached, semi‐arid savanna landscapes in northern South Africa. Litter at sampling points with a woody component had a higher %N and lower C : N ratio than litter at solely herbaceous points. Severely encroached areas had lower C : N ratios throughout the soil profile than less encroached areas. Soil %C and %N were highly influenced by soil texture but were also influenced by the presence of a woody overstorey, which increased surface soil %C on three soil types but decreased it on the most heavily encroached soil type. Soil C sequestration may initially increase with bush encroachment but then decline if bush densities become so high as to inhibit understorey grass growth.     

Tree heterogeneity,resource availability,and larger scale processes regulating arboreal ant species richness

Abstract Processes acting on different spatial and temporal scales may influence local species richness. Ant communities are usually described as interactive and therefore determined by local processes. In this paper we tested two hypotheses linked to the question of why there is local variation in arboreal ant species richness in the Brazilian savanna (‘cerrado’). The hypotheses are: (i) there is a positive relationship between ant species richness and tree species richness, used as a surrogate of heterogeneity; and (ii) there is a positive relationship between ant species richness and tree density, used as a surrogate of resource availability. Arboreal ants were sampled in two cerrado sites in Brazil using baited pitfall traps and manual sampling, in quadrats of 20 m × 50 m. Ant species richness in each quadrat was used as the response variable in regression tests, using tree species richness and tree density as explanatory variables. Ant species richness responded positively to tree species richness and density. Sampling site also influenced ant species richness, and the relationship between tree density and tree species richness was also positive and significant. Tree species richness may have influenced ant species richness through three processes: (i) increasing the variety of resources and allowing the existence of a higher number of specialist species; (ii) increasing the amount of resources to generalist species; and (iii) some other unmeasured factor may have influenced both ant and tree species richness. Tree density may also have influenced ant species richness through three processes: (i) increasing the amount of resources and allowing a higher ant species richness; (ii) changing habitat conditions and dominance hierarchies in ant communities; and (iii) increasing the area and causing a species–area pattern. Processes acting on larger scales, such as disturbance, altitude and evolutionary histories, as well as sampling effect may have caused the difference between sites.     

Effect of fire regime on plant abundance in a tropical eucalypt savanna of north‐eastern Australia

Abstract Changes in plant abundance within a eucalypt savanna of north‐eastern Australia were studied using a manipulative fire experiment. Three fire regimes were compared between 1997 and 2001: (i) control, savanna burnt in the mid‐dry season (July) 1997 only; (ii) early burnt, savanna burnt in the mid‐dry season 1997 and early dry season (May) 1999; and (iii) late burnt, savanna burnt in the mid‐dry season 1997 and late dry season (October) 1999. Five annual surveys of permanent plots detected stability in the abundance of most species, irrespective of fire regime. However, a significant increase in the abundance of several subshrubs, ephemeral and twining perennial forbs, and grasses occurred in the first year after fire, particularly after late dry season fires. The abundance of these species declined toward prefire levels in the second year after fire. The dominant grass Heteropogon triticeus significantly declined in abundance with fire intervals of 4 years. The density of trees (>2 m tall) significantly increased in the absence of fire for 4 years, because of the growth of saplings; and the basal area of the dominant tree Corymbia clarksoniana significantly increased over the 5‐year study, irrespective of fire regime. Conservation management of these savannas will need to balance the role of regular fires in maintaining the diversity of herbaceous species with the requirement of fire intervals of at least 4‐years for allowing the growth of saplings >2 m in height. Whereas late dry season fires may cause some tree mortality, the use of occasional late fires may help maintain sustainable populations of many grasses and forbs.