Woody plant phenology in the West Africa savanna

In the savanna of West Africa the seasonality of rainfall, with a drought period of at least four months, strongly influences the vegetation. Rainfall is a very critical abiotic variable and therefore plant species must be well adapted to survive in this habitat.
In our research, phenological patterns of 120 woody plant species have been examined based on the presence of green leaves. According to the patterns found, these species can be classified in phenological groups, which represent different strategies for survival. Two extreme strategies are found to resist drought: (1) by using the waterstorage in the deeper soil layers and river beds and by restricting drought-damage through scleromorphic features, and (2) by avoiding the drought through foliage shedding in the dry period.
The first strategy is represented by the riparian and upland evergreens , and the semi-evergreens . The evergreens bear leaves the whole year, gradually replacing old leaves by new ones. The riparian evergreens are strictly bound to riverbeds and grow in or immediately adjacent to them. The semi-evergreens shed their leaves and start sprouting during a short period (one-two weeks) once a year. Because the evergreens and the semi-evergreens are in leaf in the dry period they have to protect themselves to drought damage by scleromorphic features.
Contrary to these species are the deciduous species which are bare for at least some months per year. When the dry season starts their leaves dry out and are subsequently shed. They start sprouting before or at the beginning of the first rains. Although much less in number, some deciduous trees also have scleromorphic features to resist drought-damage. The strategy of sprouting just before the rainy season begins indicates that certain water resources remain available to these deep-rooting woody plants throughout the year, providing them with a fully operating photosynthetic apparatus when favourable conditions arrive.     

Herbaceous ecosystems on the Guayana Shield, a regional overview

Aims Ecosystems dominated by herbaceous plant communities are amongst the most diversified landscape units in the Guayana (Guiana) Shield region. This paper aims to present a synthetic overview of the wide array of herbaceous ecosystems found in the region in an attempt to furnish a more concrete baseline for a better understanding of the pattern of variation, and to clarify some of the differences that occur in the vegetation of the area. Location The Guayana Shield region, and the area of north‐eastern South America extending between the Orinoco River to the North and the Amazon River to the South (c. 8° N to 1° S). Methods Floristic and ecological field data gathered from over 300 study sites located at different altitudinal levels in the Venezuelan Guayana and the northern Brazilian Amazon are evaluated and interpreted in the light of personal observations and existing literature. Results The diversification pattern includes physiognomic as well as floristic variation and shows two opposing tendencies in relation to their altitudinal location: grass‐dominated meadows (savannas) predominate in the macrothermic lowlands, whereas non‐gramineous, broadleaved herbaceous communities are dominant in the mesothermic highlands of the Guayanan mountains. In detail, the biogeographic region of Guayana (Guiana) consists of three easily recognized altitudinal levels, each of which contains a highly characteristic set of landscape types with their associated ecosystems and endemic plant communities. Between 0 and c. 500 m a.s.l. the extensive macrothermic (mean annual temperature, MAT > 24 °C) lowlands are found, where an enormous – still largely unexplored – diversity of forest types represents the main plant cover of the plains, peneplains, glacis and piedmont slopes. Sparsely distributed within this forest cover are numerous herbaceous ecosystems, ranging from true grass savannas to unusual and poorly understood meadows developed on extremely oligotrophic white sandy soils dominated by non‐gramineous genera of Rapateaceae and Xyridaceae. In the intermediate submesothermic (MAT 24–18 °C) Guayana uplands, extending roughly between 500 and 1500 m a.s.l., the grasslands of the Gran Sabana in south‐eastern Venezuela reach their upper altitudinal limit. At the same altitudinal level, however, several distinct herbaceous communities are found, in which other genera of Rapateaceae, together with Bromeliaceae and Xyridaceae, are predominant. Finally, at the uppermost altitudinal level, i.e. in Pantepui (which includes the characteristically flat topped mountain summits (tepuis) of the Guayana highlands) between 1500 and 3000 m a.s.l., encompassing a range of meso‐ to submicrothermic temperature regimes (MAT 18–8 °C), the extensive herbaceous ecosystems are developed either on deep organic soils (peat) or on open sandstone surfaces. These high‐tepui meadows present a considerable physiognomic diversification and are formed by a variety of endemic genera of the Rapateaceae, Bromeliaceae, Xyridaceae, Eriocaulaceae and Cyperaceae families, often dominated by locally endemic species on each of the larger tepui massifs. In contrast, grass dominated plant communities are very rare and restricted to only a few high‐tepui sites. Main conclusions A marked floristic and ecologic differentiation of herbaceous ecosystems in the Guayana Shield region can be recognized. The ecological differentiation results primarily from the wide spectrum of variations in the substrate found at the various altitudinal levels of the Guayana Shield region. A possible explanation for the present‐day pattern of herbaceous vegetation types may be the following: Non‐gramineous meadows representing ancient species pools of Guayana‐centred families had evolved successful colonization strategies in occupying extremely nutrient poor sites at all altitudinal levels. In contrast, the more modern grass savannas, which preferentially occupy the peripheral Guayana landscapes, are restricted to richer soil conditions with better internal drainage and water retention conditions.     

Assessing the impacts of fire on the vegetation resources that are available to the local communities of the seasonal wetlands of the Okavango, Botswana, in the context of different land uses and key government policies

The Okavango wetlands in north western Botswana are the most fire-prone environment in Botswana. Most of these fires are anthropogenic. The fires in this environment are thought to impact the environment negatively and therefore practices that are associated with extensive use of fire have been strongly criticized. Despite this, there has been little work done to understand how these fires impact the wetlands environment and its dynamics, especially the vegetation resources that are used by the local communities in the wetlands. The objective of the study was to identify fire spatial and temporal trends in relation to settlement distribution, through the use of remote sensing, socio-economic and phytosociological surveys. The fire history results show that geographically there has not been any significant change in vegetation structure and that in fact fires may have promoted biodiversity. The results of analysis show an overall variance on vegetation structure of 23% whereas the rest are unaccounted for. There is a strong association between settlements, ethnicities, literacy and fire occurrences. The most fire-prone areas are inhabited by communities that have used fire in the past for various resource use practices.     

Evaluation of symbiotic properties and nitrogen contribution of mucuna to maize grown in the derived savanna of West Africa

The severity and increase of the Imperata cylindrica constraint as a weed, the decline of the traditional fallow systems as a means of soil fertility management and the lack of inorganic fertilizer appear to have created opportunities for adoption of mucuna (Mucuna pruriens) technology by smallholder farmers in some areas in the derived savanna of West Africa. What is not known, however, is the extent to which the establishment and N contribution of mucuna in these areas depend on symbiotic properties such as effective nodulation and mycorrhizal infection. Short term surveys carried out in 34 farmer's arable fields located in four different sites in the derived savanna, southern Benin, West Africa, together with results of greenhouse and field experiments showed that mycorrhizal infection rate of mucuma ranged from 2 to 31% and correlated positively with nodulation and shoot dry matter production. Nodulation occurred in 79% of the fields with numbers of nodules ranging from 0 to 135 plant^–1. Mucuna responded both to inoculation and N fertilizer in degraded soils but growth response depended on the rhizobia strains and mucuna varieties. Mucuna accumulated in 12 weeks about 313 kg N ha^–1 as either a sole crop or 166 kg N ha^–1 when mixed/intercropped with maize, respectively. Across all cropping systems it derived an average of 70% of its N from atmospheric N₂ (estimates made by the ¹⁵N isotope dilution method), representing 167 kg N ha^–1 per 12 weeks in the field. Mucuna interplanted with maize obtained a greater proportion of its nitrogen (74%) from fixation than did mucuna grown alone (66%) suggesting that competition for soil N influences the proportion of nitrogen fixed by mucuna. The total amount of N₂ fixed per hectare was, however, reduced significantly by intercropping mucuna with maize. A preceding mucuna crop provided a maize yield equivalent to 120 kg N kg ha^–1 of inorganic N fertilizer.     

遮光水环境中4种南方植物凋落叶分解特性

驳岸乔木与湖体草本植物遮蔽形成了南方地区湖体部分水域独特的光照缺乏环境,本研究采用室内分解模拟试验,对南方湖泊与驳岸常用木本植物桂花、榕树以及草本植物美人蕉、狐尾藻凋落叶分解过程中分解底物的残留量、木质素和纤维素含量进行分析,探究该环境下木本、草本植物凋落叶分解与木质素和纤维素释放规律。结果表明: 经140 d的分解,桂花、榕树、美人蕉和狐尾藻的失重率分别为46.0%、42.3%、74.4%和68.6%,木本植物分解速率(k)显著低于草本植物。在分解前期(0～7 d),4种植物失重率与k均为整个试验过程中的最高值,4种植物纤维素均呈高释放状态,木本植物桂花(42.9%)、榕树(38.9%)的木质素释放率显著高于草本植物。木本植物桂花的木质素释放率与其k值呈显著正相关,而榕树及草本植物木质素和纤维素释放率与k值相关性均不显著。木质素的释放可能是影响遮光水环境中木本植物分解的关键因子,并调控遮光湖体碳循环过程。     

云南潞江坝怒江干热河谷植被研究

应用法瑞地植物学派的理论和方法,将云南潞江坝1300m以下地区干热河谷植被确定为1群目、3群属、6群丛、4亚群丛。通过对植被外貌、结构、区系组成、分布特点的分析,认为本区现存植被主要由河谷季雨林长期破坏所至,目前较典型的干热河谷灌草丛植被类型属于次生性稀树草原或半自然性稀树草原。     

土壤增温调节中亚热带森林更新初期植物生物量分配格局

全球变暖提高了温带森林生态系统植物的生产力,但对亚热带森林生产力的影响仍然不清楚。由于亚热带森林植物的碳储量巨大,因此了解全球变暖对亚热带森林植物生长的影响至关重要。采用加热电缆模拟土壤增温(+5℃),探讨中亚热带森林几种主要草本植物和木本植物的生长及其生物量分配格局对温度升高的响应。结果表明:增温显著增加五节芒(Miscanthus floridulus)、山油麻(Trema dielsiana)和东南野桐(Mallotus lianus)的高度,但黑莎草(Gahnia tristis)高度显著降低。增温显著增加木本植物的地上、地下和总生物量,而草本植物的地上、地下和总生物量均显著降低。增温对整个群落的地上和地下部分生物量分配模式无显著影响,但木本植物总生物量在各器官之间分配随温度发生改变,增温显著提高木本植物枝生物量比(BMR),降低干生物量比(SMR),而叶生物量比(LMR)和根生物量比(RMR)无显著影响,但显著降低了细根占总根系生物量比率。结果表明木本植物能够通过调节生物量分配模式应对未来全球气候变暖。