Grass Layer Research Articles

Tree thinning as an option to increase herbaceous yield of an encroached semi-arid savanna in South Africa

BackgroundThe investigation was conducted in a savanna area covered by what was considered an undesirably dense stand of Colophospermum mopane trees, mainly because such a dense stand of trees often results in the suppression of herbaceous plants. The objectives of this study were to determine the influence of intensity of tree thinning on the dry matter yield of herbaceous plants (notably grasses) and to investigate differences in herbaceous species composition between defined subhabitats (under tree canopies, between tree canopies and where trees have been removed). Seven plots (65 × 180 m) were subjected to different intensities of tree thinning, ranging from a totally cleared plot (0 %) to plots thinned to the equivalent of 10 %, 20%, 35 %, 50% and 75 % of the leaf biomass of a control plot (100 %) with a tree density of 2711 plants ha-1. The establishment of herbaceous plants (grasses and forbs) in response to reduced competition from the woody plants was measured during three full growing seasons following the thinning treatments.ResultsThe grass component reacted positively to the tree thinning in terms of total dry matter (DM) yield, but forbs were negatively influenced. Rainfall interacted with tree density and the differences between grass DM yields in thinned plots during years of below average rainfall were substantially higher than those of the control. At high tree densities, yields differed little between seasons of varying rainfall. The relation between grass DM yield and tree biomass was curvilinear, best described by the exponential regression equation. Subhabitat differentiation by C. mopane trees did provide some qualitative benefits, with certain desirable grass species showing a preference for the subhabitat under tree canopies.ConclusionWhile it can be concluded from this study that high tree densities suppress herbaceous production, the decision to clear/thin the C. mopane trees should include additional considerations. Thinning of C. mopane with the exclusive objective of increasing productivity of the grass layer would thus invariably involve a compromise situation where some trees should be left for the sake of the qualitative benefits on the herbaceous layer, soil enrichment, provision of browse and stability of the ecosystem.

Spatial–temporal variation in soil respiration in an oak–grass savanna ecosystem in California and its partitioning into autotrophic and heterotrophic components

The spatial upscaling of soil respiration from field measurements to ecosystem levels will be biased without studying its spatial variation. We took advantage of the unique spatial gradients of an oak-grass savanna ecosystem in California, with widely spaced oak trees overlying a grass layer, to study the spatial variation in soil respiration and to use these natural gradients to partition soil respiration according to its autotrophic and heterotrophic components. We measured soil respiration along a 42.5 m transect between two oak trees in 2001 and 2002, and found that soil respiration under tree canopies decreased with distance from its base. In the open area, tree roots have no influence on soil respiration. Seasonally, soil respiration increased in spring until late April, and decreased in summer following the decrease in soil moisture content, despite the further increase in soil temperature. Soil respiration significantly increased following the rain events in autumn. During the grass growing season between November and mid-May, the average of CO2 efflux under trees was 2.29 lmol m � 2 s � 1 , while CO2 efflux from the open area was 1.40 lmol m � 2 s � 1 . We deduced that oak root respiration averaged as 0.89 lmol m � 2 s � 1 , accounting for 39% of total soil respiration (oak root + grass root + microbes). During the dry season between mid-May and October, the average of CO2 efflux under trees was 0.87 lmol m � 2 s � 1 , while CO2 efflux from the open areas was 0.51 lmol m � 2 s � 1 . Oak root res- piration was 0.36 lmol m � 2 s � 1 , accounting for 41% of total soil respiration (oak root + mic- robes). The seasonal pattern of soil CO2 efflux under trees and in open areas was simulated by a bi- variable model driven by soil temperature and moisture. The diurnal pattern was influenced by tree physiology as well. Based on the spatial gradient of soil respiration, spatial analysis of crown closure and the simulation model, we spatially and temporally upscaled chamber measurements to the ecosystem scale. We estimated that the cumulative soil respiration in 2002 was

SHAPING THE LANDSCAPE: FIRE–GRAZER INTERACTIONS IN AN AFRICAN SAVANNA

The effects of fire–grazing interactions on grass communities are difficult to identify because fire and grazing influence each other on a landscape scale. Persistent heavy grazing can prevent the spread of fire by breaking up the grass layer. In contrast, frequent burning might inhibit the persistence of grazed patches by attracting grazers to the post-burn green flush. We explored the effect of burning on grazing activity, and the persistence of grazed patches, in a landscape-level experiment in a South African savanna. We created 17 grazed patches by mowing grass in a 20 m diameter plot, with an adjacent un-mown control. We used dung counts as a measure of grazer visitation, and grass height as a measure of grazing intensity, at each of the sites over a year. Nearly all mowing treatments resulted in a rapid increase in grazing activity relative to controls (on average, 4–6 times more dung was found on mown sites). Subsequent fates of the grazed patches depended on their location with respect to fire. Burned areas drew animals off nearby unburned grazed patches, which then recovered lost biomass. Patches >1.5 km from burns remained grazed short. Frequent large fires might prevent areas of heavy grazing from persisting in the landscape, and thus limit the spread of grazing-adapted grasses. Spatial information on fire frequencies in the park was used to explore the influence that the “magnet effect” of fire can have on grass communities. We mapped the distribution of tall, bunch grasslands and grazing-lawn grasslands using a 1999 Landsat TM satellite image. The extent of grazing lawns was directly related to fire return interval. Areas with a fire return of <4 years had less lawn grass than would be expected from the proportions of lawn grass in the park. A logistic regression analysis, which used various environmental variables known to influence grazing, showed fire history to be an important predictor of grazing-lawn distributions. This work shows that, by influencing where, when, and for how long animals graze a patch, fire can influence the competitive balance between grazing-tolerant, and grazing-intolerant grass species and affect their distributions in the landscape. We discuss the implications of this research for the management of natural grazing systems and rangelands.

Pothole erosion of a dissected Lake‐shore platform in Peat, Loch More, Halkirk, Caithness

An unusual combination of environmental factors has brought about the excavation of a series of classical cauldron‐shaped potholes in a dissected lake‐shore platform of peat on the shore of Loch More, Halkirk, Caithness. The potholes have both been eroded by, and now contain, ‘cobbles’ of peat. Some potholes are obviously currently active during periods of heavy rain, while others are abandoned or at least less active, being filled with a growing layer of grass. The platform has been deeply dissected by water erosion along contraction joints which formed as the peat dried out ('nagging') and it now generally resembles a karst‐like miniature ‘bad‐lands’ topography with small ‘mesas’ and ‘buttes’.

Diet and habitat preference of the Cape York short-nosed bandicoot (Isoodon obesulus peninsulae) in north-east Queensland

Diet and habitat preferences of the Cape York bandicoot (Isoodon obesulus peninsulae) were studied along a rainfall gradient from dry open woodland to wet Allocasuarina–Eucalyptus forest in the Lamb Range, Queensland. I. o. peninsulae was an omnivore-insectivore with invertebrates contributing 35–56% of faecal contents. Roots represented the most important plant food. Grass, forbs, fruits and hypogeous fungi were also eaten but in small quantities. The species was most abundant at the drier end of the rainfall gradient. Preferred habitats in open woodland were characterised by a high grass tree (Xanthorrhoea johnsonii) abundance and high shrub cover in the understorey. In contrast, areas with a tall and dense grass layer in conjunction with a high litter cover were avoided. I. o. peninsulae did not seem to share its habitat with the sympatrically occurring I. macrourus even though the habitat appeared suitable for the latter. More studies are required to evaluate the causes of differing habitat preferences of sympatric bandicoot species.

Effects of fire on surface carbon, energy and water vapour fluxes over campo sujo savanna in central Brazil

Summary Tower‐based eddy covariance measurements were used to quantify the effect of fire on subsequent carbon dioxide fluxes and water and surface energy balance characteristics for campo sujo savanna located near Brasília in Central Brazil (15°56′ S, 47°51′ W). Campo sujo is a xeromorphic, open shrub savanna with very scattered but definitely visible shrubs and tree‐like shrub elements. We studied two areas, one exposed to a prescribed fire late in the dry season, and a second that had not been burned for the previous 4 years. The fire on 22 September 1998 consumed an estimated 26 mol C m−2. Immediately after the fire, evapotranspiration rates decreased and the savanna became a stronger net source of CO2 to the atmosphere. This was attributed to the removal of the still slightly physiologically active grass layer and higher soil CO2 efflux rates as a consequence of elevated surface soil temperatures post‐burning. On the commencement of the first rains in early October 1998, this situation was reversed, with the burned area rapidly becoming a stronger sink for CO2 and with higher evapotranspiration rates than a nearby unburned (control) area. This difference persisted throughout the wet season (until at least June 1999) and was attributable to greater physiological activity of the regrowing vegetation in the burned area. Early in the growing season, higher soil evaporation rates may also have contributed to faster water use by the previously burned area. Overall, we estimate an annual gross primary productivity for the burned area of 135 mol C m−2 year−1, with that for the unburned area being 106 mol C m−2 year−1. Estimated ecosystem respiration rates were more similar on an annual basis (96 and 82 mol C m−2 year−1 for the burned and unburned areas, respectively), giving rise to a substantially higher net ecosystem productivity for the previously burned area (38 vs 24 mol C m−2 year−1). Stimulation of photosynthetic activity in the rapid post‐fire growth phase means that the negative effects of fire on the ecosystem carbon balance were more or less neutralized after only 12 months.

Conundrums in mixed woody–herbaceous plant systems

AbstractAims To identify approaches to improve our understanding of, and predictive capability for, mixed tree–grass systems. Elucidation of the interactions, dynamics and determinants, and identification of robust generalizations that can be broadly applied to tree–grass systems would benefit ecological theory, modelling and land management.Methods A series of workshops brought together scientific expertise to review theory, data availability, modelling approaches and key questions.Location Ecosystems characterized by mixtures of herbaceous and woody plant life‐forms, often termed ‘savannas’, range from open grasslands with few woody plants, to woodlands or forests with a grass layer. These ecosystems represent a substantial portion of the terrestrial biosphere, an important wildlife habitat, and a major resource for provision of livestock, fuel wood and other products.Results Although many concepts and principles developed for grassland and forest systems are relevant to these dual life‐form communities, the novel, complex, nonlinear behaviour of mixed tree–grass systems cannot be accounted for by simply studying or modelling woody and herbaceous components independently. A more robust understanding requires addressing three fundamental conundrums: (1) The ‘treeness’ conundrum. What controls the relative abundance of woody and herbaceous plants for a given set of conditions at given site? (2) The coexistence conundrum. How do the life‐forms interact with each other? Is a given woody–herbaceous ratio dynamically stable and persistent under a particular set of conditions? (3) The net primary productivity (NPP) conundrum. How does NPP of the woody vegetation, the herbaceous vegetation, and the total ecosystem (woody + herbaceous) change with changes in the tree–grass ratio?Tests of the theory and conceptual models of determinants of mixed woody–herbaceous systems have been largely site‐ or region‐specific and have seldom been broadly or quantitatively evaluated. Cross‐site syntheses based on data and modelling are required to address the conundrums and identify emerging patterns, yet, there are very few data sets for which either biomass or NPP have been quantified for both the woody and the herbaceous components of tree–grass systems. Furthermore, there are few cross‐site comparisons spanning the diverse array of woody–herbaceous mixtures. Hence, initial synthesis studies should focus on compiling and standardizing a global data base which could be (1) explored to ascertain if robust generalizations and consistent patterns exist; and (2) used to evaluate the performance of savanna simulation models over a range of woody–herbaceous mixtures.Savanna structure and productivity are the result of complex and dynamic interactions between climate, soils and disturbances, notably fire and herbivory. Such factors are difficult to isolate or experimentally manipulate in order to evaluate their impacts at spatial and temporal scales appropriate for assessing ecosystem dynamics. These factors can, however, be evaluated with simulation models. Existing savanna models vary markedly with respect to their conceptual approach, their data requirements and the extent to which they incorporate mechanistic processes. Model intercomparisons can elucidate those approaches most suitable for various research questions and management applications.Conclusion Theoretical and conceptual advances could be achieved by considering a broad continuum of grass–shrub–tree combinations using data meta‐analysis techniques and modelling.

Partitioning overstory and understory evapotranspiration in a semiarid savanna woodland from the isotopic composition of water vapor

The relative contributions of overstory and understory plant transpiration and soil evaporation to total evapotranspiration (ET) in a semiarid savanna woodland were determined from stable isotope measurements of atmospheric water vapor. The savanna overstory was dominated by the deeply rooted, woody legume Prosopis velutina (“mesquite”), and the understory was dominated by a perennial C 4 grass, Sporobolus wrightii. “Keeling plots” (turbulent mixing relationships) were generated from isotope ratios (δD and δ 18 O ) of atmospheric water vapor collected within the tree (3–14 m) and understory (0.1–1 m) canopies during peak (July) and post-monsoon (September) periods of 2001. The unique regression intercepts from upper and lower profiles were used to partition the ET flux from the understory layer separately from that of the whole ecosystem. Although ET partitioning was problematic during the first sampling period in July, our results in September provided support to the validity of this method for measuring and understanding the dynamic behavior of water balance components in this semiarid savanna woodland. During the post-monsoon period (22nd September), transpiration accounted for 85% of ecosystem ET. Transpiration by the grass layer accounted for 50% of the understory ET over the same period. The total ecosystem ET estimated by eddy covariance (EC) on 22nd September was 3.5 mm per day. Based on partitioning by the isotope method, 2.5 mm per day (70%) was from tree transpiration and 0.5 mm per day (15%) was from transpiration by the grass layer. Independent estimates of overstory and understory ET partitioning from distributed understory EC measurements were remarkably consistent with our isotope approach.

TREE LAYER SPATIAL STRUCTURE CAN AFFECT SAVANNA PRODUCTION AND WATER BUDGET: RESULTS OF A 3-D MODEL

The spatially explicit and mechanistic model TREEGRASS was used to test the effects of tree density, tree spatial distribution, and tree size distribution on radiation absorption, net primary production (NPP), and water fluxes for a West African savanna. The model has previously been shown to adequately predict radiation absorption, production, and soil water balance in humid savannas. Using field measurements quantifying the spatial variations in grass NPP, the model was found to predict correctly the effect of tree abundance on grass NPP. Model simulations showed that annual photosynthetically active radiation (PAR), absorption efficiency, NPP, and transpiration of the tree layer increased with increasing tree density and decreased with increasing tree aggregation; the grass layer showed the opposite trends. Total NPP (tree plus grass) remained stable for all tree densities and decreased slightly for aggregated tree distributions. Total transpiration increased with tree density and was not affected by t...

Protected areas as biodiversity benchmarks for human impact: agriculture and the Serengeti avifauna.

Protected areas as biodiversity benchmarks allow a separation of the direct effects of human impact on biodiversity loss from those of other environmental changes. We illustrate the use of ecological baselines with a case from the Serengeti ecosystem, Tanzania. We document a substantial but previously unnoted loss of bird diversity in agriculture detected by reference to the immediately adjacent native vegetation in Serengeti. The abundance of species found in agriculture was only 28% of that for the same species in native savannah. Insectivorous species feeding in the grass layer or in trees were the most reduced. Some 50% of both insectivorous and granivorous species were not recorded in agriculture, with ground-feeding and tree species most affected. Grass-layer insect abundance and diversity was much reduced in agriculture, consistent with the loss of insectivorous birds. These results indicate that many species of birds will become confined to protected areas over time. We need to determine whether existing protected areas are sufficiently large to maintain viable populations of insectivorous birds likely to become confined to them. This study highlights the essential nature of baseline areas for assessing causes of change in human-dominated systems and for developing innovative strategies to restore biodiversity.

Preservation Within Log Coffins Before and After Barrow Construction

A limited number of South Scandinavian barrows from the Bronze Age contains several burials with well-preserved organic material including remnants of the dead. In some cases there are indications of an accumulation of the burials over some time before the barrow was erected. This raises the question of how the decomposition of the organic matter was hindered until the barrow construction. Experiments with deposition of piglets in models of Bronze Age oak log coffins with and without covering barrows were conducted, and the development of oxygen content in the coffins was monitored. The experiments resulted in different oxygen depletion patterns, which were compared with the preservation of the piglets upon excavation of the barrows. Coffins covered with a thin layer of grass sods quickly developed an anoxic environment and the decomposing processes were slowed considerably, whereas the free standing coffins remained oxygenated and the piglets decomposed almost completely within a period of three months. The results allow conclusions on the decomposing processes in the barrows and suggest that an interim cover of the burials was necessary to preserve the bodies until the barrow could be constructed.

The influence of tree density on the grass layer of three semi-arid savanna types of southern Africa

The influence of bush density on the grass component of the herbaceous layer was investigated for three savanna veld types (Molopo Thornveld, Mixed Vaalbos Thornveld, and the Eastern Grass and Bushveld) in the Northern Cape and North-West Province of South Africa. Relationships between bush density and the density, species composition and phytomass production of the grass component were determined over a four-year period. Bush encroachment had an adverse influence on the density and production of the grass component, but not on the species composition. Fluctuations in seasonal rainfall appeared to be the most important factor governing changes in the species composition of the grass component. Bush enchroachment (2 500 tree equivalents ha−l) decreased the potential grazing capacity by as much as 331%, 149% and 58% in the Molopo Thornveld, the Mixed Vaalbos Thornveld and the Eastern Grass Bushveld, respectively, In comparison to sites with tree densities of less than 400 tree equivalents ha−1. In the encroached condition grazing capacities of 15, 13 and 10 ha LSU−1 are recommended for the Molopo Thornveld, the Mixed Vaalbos Thornveld and the Eastern Grass and Bushveld, respectively. The implementation of bush control measures in all problem areas is proposed in order to restore the productivity of these areas in the long term.

Influence of different meteorological parameters on the development of an ozone gradient between the soil surface and a level of 250 cm

In the present study the percentage ratio of the ozone concentration within and above a grass layer of 5 cm height were measured with respect to the ozone concentration at a level of 250 cm. In addition the influence of several meteorological parameters (global radiation, wind speed, temperature, relative air humidity) was determined. To this end ozone concentrations were measured at levels of 1 cm, 5 cm, 10 cm, 50 cm and 250 cm above ground at the meteorological station of the University of Hohenheim (Stuttgart, Germany) during a period from August to October 1999. The registered vertical profiles of ozone clearly show lower ozone concentrations within the vegetation layer than at the 250-cm level. It could be noticed that the ozone concentrations at the different levels measured have a typical ratio compared to the ozone concentration at the 250-cm level, the absolute amount of which had no influence on the percentage ratios close to the soil surface. At the 1-cm level the ozone concentration amounted to about 53% of the one measured at the 250-cm level, whereas this amount increased to 92% at the 50-cm level already. The percentage ratio of the ozone concentration is influenced by different meteorological factors analysed for the 1-cm level. The most important factor of influence is the wind speed which causes a logarithmic increase of the concentration ratio. A rise of the relative air humidity by about 10% results in a decline of the concentration ratio of about 4%. The influence of global radiation can be observed mostly in the mornings and evenings whereas the influence of temperature remains rather slight. As reasons for the concentration ratios observed turbulent mixing processes as well as the stomatal behaviour of plants are discussed.

Effects of nutrients and shade on tree‐grass interactions in an East African savanna

Abstract. Savanna trees have a multitude of positive and negative effects on understorey grass production, but little is known about how these effects interact. We report on a fertilization and shading experiment carried out in a Tanzanian tropical dry savanna around Acacia tortilis trees. In two years of study there was no difference in grass production under tree canopies or in open grassland. Fertilization, however, indicate that trees do affect the nutrient limitation of the grass layer with an N‐limited system in open grassland to a P‐limited system under the trees. The N:P ratios of grass gave a reliable indication of the nature of nutrient limitation, but only when assessed at the end of the wet season. Mid‐wet season nutrient concentrations of grasses were higher under than outside the tree canopy, suggesting that factors other than nutrients limit grass production. A shading experiment indicated that light may be such a limiting factor during the wet season when water and nutrients are sufficiently available. However, in the dry season when water is scarce, the effect of shade on plant production became positive. We conclude that whether trees increase or decrease production of the herbaceous layer depends on how positive effects (increased soil fertility) and negative effects (shade and soil water availability) interact and that these interactions may significantly change between wet and dry seasons.

Impact of grazing on the input of organic matter and nutrients to the soil in a grass-encroached Scots pine forest

In the Netherlands, grazing is used as a measure to counteract grass-encroachment in Dutch pine forests. A 7-year experiment with moderate and heavy grazing intensities versus non-grazed control situations was performed to evaluate the impact of grazing on nutrient cycling in these forest ecosystems. In this paper, changes in organic matter and nutrient input to the soil compartment due to grazing are quantified. Grazing resulted in a 50% reduction of grass shoots, which was partly caused by a lower biomass production (35%). Grass root production and input also decreased by about 30%. Total litter input in the pine stand was only reduced by 20%, due to the lack of impact on tree litter input. The relative contribution of pine litter to the total litter input increased from about 30 to 40%. Grazing also reduced the input of nutrients, but reduction was not equal for all nutrients. Changes in nutrient input were not a result of changed nutrient concentrations in litter, but were due to shifts in the relative contribution of different litter types. Potassium was most reduced (30%) and reduction was mainly due to biomass consumption, while the input of N and P (20–23% reduction) was mainly affected by decreased production of grass roots and shoots. The input of Ca was least affected by grazing: 10% reduction. Reduced input of fresh litter and nutrients and an increased contribution of pine litter may result in lower decomposition rates and presumably decreased nutrient availability. This may affect biomass production and nutrient cycling through the grass layer.

Проект Всероссийского Кодекса фитоценологической номенклатуры

The Code of phytosociological nomenclature of the International Association of Vegetation Science (Barkman et al., 1986) may be applied only to floristic-sociological classification of vegetation. Russian phytosociological tradition was based on the dominant aproach and used since 1900 the nomenclature proposed by R. Sernander. Though for a long time there were no formal rules in the form of the code. In 1989 the author published the Project of the All-Union Code of the Phytocoenological Nomenclature. Since that time many Russian phytosociologists applied the Code in their works. The author received many suggestions on the improvement of the Code that were taken into account in the new variant of the Code. The Code consists of principles, rules and advices. The Code is independent of the Code of the floristic-sociological classification. It regulates the names of syntaxa. Syntaxon is the type of the plant community with the determined rank. The usage of the names of the syntaxa is based on the nomenclature types (types of names) and the priority in the publication of their diagnoses and releves. The Code is retroactive to 1900. The ranks of the syntaxa of the main system (in ­ascending order) are the next: variant, subassociation, association, group of associations, class of associations, subformation, formation, group of formations, class of formations, subtype of vegetation, type of vegetation, group of types of vegetation, class of types of vegetation, subdivision, division. The main ranks are association, formation, type of vegetation, division. The ortogonal system is used mainly for the forest plant communities and based on the similarity of the moss and the grass layers. It has two ranks of syntaxa — cicle and group of cycles. The name of the formation is based on the names of one or two dominating species or on the names of dominating ecobiomorfs. The nomenclature type of the formation is the association. The names of the syntaxa that have the rank higher then formation must be made of the name of the formation that is the nomenclature type of the syntaxon. The suffixes in the names of formation and higher syntaxa are the next: formation — «-eta», group of formations — «-etesum», class of fomations — «-etosa», subtype of vegetation — «-etium», type of vegetation — «‑etion», group of types of vegetation — «-phytum», class of types of vegetation — «-phytosa», subdivision — «‑phytium», division — «-phytion». The names of associations, groups and classes of associations and subformations are binary. The first word in the name is the name of formation to which the association belonges with the suffix «-etum» for association, «‑eta» for group, class of associations and subformations. The second word is made of the names of constant or dominant one or two species or of the names of dominating ecobiomorfs with the suffix «-osum» for association and «-osa» for group of associations, «-osium» for classes of associations, «-osion» for subformation. The type of the name of the association is the published releve in which the abundance or the cover is shown for all species. The name of subassociation consists of the name of association, the word subass. and the word characterising dominant, constant species or structural, ecological, ecobiomorfological peculiarities of the suassociation The suffix of the last word is «-osum». For the suassociation containing the nomenclature type of association the name «typicum» may be used. The name of variant consists of the name of the association, the word var. and the word characterising the variant with suffix «-osum». The last word may be produced similarly as the last word in the name of subassociation. The names of cycle and group of cycles must be made of the second part of the name of the assotiation that is a type of cycle or the group of cycles. The suffix for cycle is «-osum» for the group of cycles «-osa».

Assessment of vegetation indexes useful for browse (forage) prediction in semi-arid rangelands

Considerable significance is placed on the mapping and monitoring of degraded areas in semi-arid regions of the world, including Botswana. Degraded areas include those suffering from bush encroachment, believed to result from heavy cattle grazing over a number of years. However, certain bush encroachment species have been found to be relatively nutrient-rich. The present work considers the extent to which a series of quantified layers through mainly bush encroachment canopies can be identified using conventional and newly derived vegetation indexes and transforms based on Thematic Mapper (TM) imagery. Field work involved the stratification of green biomass into firstly the herbaceous cover layer; secondly the 0.3-1.5 m browse layer; then the 1.5-2.5 m browse layer; and finally the >2.5 m browse layer. Biomass measurements from these layers were statistically associated with conventional vegetation indexes and transforms such as the Normalized Difference Vegetation Index (NDVI), brightness and greenness values, and relatively newly derived darkening indexes involving the mid-infrared bands. When green biomass and transformed pixel data were averaged per classified vegetation unit, weak negative correlations emerged between grass biomass and the transformed pixel data and no significant correlations developed with the woody biomass (browse) layers. However, when point data were used in the analyses, results showed that most indexes and the brightness transform were significantly correlated with the lower browse layer. Only the darkening indexes and brightness function were sensitive to the browse layers individually and the browse plus grass layers. This work shows the limitations of conventional indexes such as the NDVI in terms of browse and herbaceous layer assessment. New indexes for forage assessment based on relationships between the mid-infrared bands, such as those found in the new MODIS TERRA platform, are urgently required for semi-arid areas.

Классификация зоогенной и антропогенной растительности центральносибирского сектора Арктики (полуостров Таймыр, архипелаг Северная Земля)

The grassland vegetation which occurs in places of animal colonies or around human settlements in the Central-Siberian sector of the Arctic is referred to 2 new associations, namely Saxifrago cernuae—Alopecuretum alpini and Poo arcticae—Calamagrostietum holmii ass. nov., and also to 2 variants (inops and Astragalus umbellatus var. nov) of the previously described associa­tion Saxifrago hirculi—Poetum alpigenae (Zanokha, 1995). The first association is distributed within the whole Taymyr Peninsula and also the part of the polar desert subzone in the Bolshevik Isl. The grass cover is formed by Poa alpigena and Alopecurus alpinus in various combinations of their dominance. The ass. Poo arcticae—Calamagrostietum holmii is distributed only throughout the tundra zone of Taymyr; its grass layer consists of Calamagrostis holmii and Poa arctica. The mentioned variants of ass. Saxifrago hirculi—Poetum alpigenae occur locally in the vicinities of Dixon settlement.

Succession and Regeneration Patterns of East African Mountain Forests. A Review

Regeneration and succession processes in montane forests of Kenya and parts of Ethiopia were investigated. The slowly growing camphor tree, Ocotea usambarensis, regenerates mainly by suckers from old root systems. Undamaged seeds were very rarely seen, and, apparently are viable only for a few days. After natural breakdown of an old tree, the gap is filled by fast growing species, mainly Macaranga kilimandscharica and Neoboutonia macrocalyx, in the shade of which the young Ocotea suckers can establish. After the death of the secondary species, whose germination requires full sunlight and is inhibited in the shade, the Ocotea forest recovers. Large scale logging of Camphor trees predominantly destroys the regeneration of Ocotea and leads to secondary forest types, which regenerate in own cycles. The high number of big game was found to be without impact on the natural regeneration of Ocotea. The Cassipourea malosana dominated forest regenerates as a mosaic-climax without a single species becoming dominant. Cedar forests dominated by Juniperus procera regenerate most efficiently after fire. Without recurrent burning, the forests of the lower and middle part of the montane zone, regenerate to climax associations dominated by broad-leaved trees as Olea capensis ssp. hochstetteri, Olea europaea ssp. cuspidata and Podocarpus latifolius. On higher altitudes Juniperus procera always remains dominant even if fire does not occur for a longer time period. The high population density especially of buffaloes seems to inhibit successful regeneration in many parts of the Juniperion, due to intensive browsing and in particular trampling. The African Bamboo, Sinarun- dinaria alpina, shows a distinct growth cycle and can grow for extremely long periods in the vegetative state before flowering, especially so at the borders of its area. After flowering and dying of the bamboo, in most areas a dense Sambucus africana shrub is formed into which bamboo is invading from old rhizomes again. In Ethiopia, regeneration occurs directly, without an interspersed Sambucus stage. The uniform age of the Hagenia stands suggests that regeneration in these forests occurs suddenly after a disturbing event. Germination tests yielded, that Hagenia seeds are not germinating unless the competing undergrowth is removed. As fire is the only natural factor able to clear the grass layer of larger areas, it is assumed, that the reproduction of the subalpine forests depends on occasional burning.

Fluxo de seiva e condutância estomática de duas espécies lenhosas sempre-verdes no campo sujo e cerradão

No presente estudo, comparou-se o curso diário de fluxo de seiva, condutância estomática e potencial hídrico (psi) de duas espécies sempre-verdes, Rapanea guianensis e Roupala montana. Os dados foram obtidos no final da estação seca, quando os efeitos do déficit hídrico seriam mais acentuados. As medidas foram realizadas no campo sujo, onde predomina gramíneas com sistemas radiculares superficiais, e no cerradão, em que a maior densidade de elementos lenhosos com sistemas radiculares profundos poderia levar a um esgotamento das reservas de água do subsolo na estação seca. Valores máximos de psinão diferiram para as duas espécies nas duas fisionomias estudadas. Os valores mínimos de potencial hídrico foliar de R. guianensis foram mais negativos (P&lt;0,05) para as plantas do cerradão, enquanto os valores para R. montana não apresentaram diferenças entre as duas fisionomias. Os resultados indicaram que não existem diferenças significativas (P &gt; 0,05) entre os valores de fluxo máximo e de fluxo total entre o campo sujo e cerradão. Os valores máximos de fluxo de seiva variaram entre 0,13 e 0,22 l. h-1 para R. guianensis e entre 0,15 e 0,54 l. h-1 para R. montana. Entretanto, as duas espécies, tanto no campo sujo quanto no cerradão, apresentaram um rígido controle estomático em relação à alta demanda evaporativa da atmosfera. O fluxo de seiva aumentou bruscamente pela manhã, alcançando rapidamente valores máximos entre 8 e 10 horas, e logo após decresceu severamente, apesar do crescente aumento da radiação solar e da demanda evaporativa da atmosfera.