Dynamics and management of some coastal dune woodlands near The Hague,The Netherlands

Changes are reported of Crataegus-Betula dune woodlands between 1950–1980 from Meijendel, a calcareous coastal dune system near the city of The Hague, The Netherlands, which is used as a water catchment area. It concerns both woodlands in degradation stage and, more common, woodlands on the increase. Changes were recorded with help of successive vegetation maps, air photos and permanent plot observations. Woodland increase usually occurs on places with groundwater at or near the surface, either because of natural circumstances or as a result of artificial groundwater recharge through infiltration as part of the water catchment activities. The management of both types of woodland is discussed.Nomenclature follows Heukels-van der Meijden (1983), Flora van Nederland, 20th ed. Wolters-Noordhoff, Groningen. Nomenclature of mosses follows Margadant & During (1982), Beknopte Flora van Nederlandse Blad- en Levermossen, Thieme, Zutphen.The authors have pleasure in thanking their colleagues from the Dune Water Works who supplied data on hydrology.     

Plant community structure in relation to grazing and environmental changes along a north-south transect in the western Kalahari

Sixteen vegetation types were described from a north-south transect in the western Kalahari. Pronounced differences were found between communities on the nutrient poor red Kalahari sand, covering most of the area and those on fine soils and white calcareous sand. Syntaxa resulting from severe overgrazing by livestock were in most cases clearly distinguished from the less disturbed vegetation. The communities on red sand consisted mainly of shrub savanna dominated by perennial tufted grasses, whereas in the vegetation on calcareous material and on overgrazed land, forbs, dwarf shrubs and shrubs played a more important role. The major communities on red sand showed a clear geographical zonation roughly corresponding to the gradient in mean annual rainfall and its interannual variation. In the northern and central Kalahari these syntaxa were dominated by species of Sudano-Zambezian origin and in the southern Kalahari by species showing Karoo-Namib affinities.     

川中丘陵人工幼草本层动态研究初报

 川中丘陵为长江中上游防护林体系建设重点区域。本文对荒山荒坡种植柏木、桤木及补播牧草后的植被动态进行了研究。结果表明：在幼林期,随树木郁闭度的增加,林下草本植物的盖度也随之提高,生物量增大,林草之间具良好的共生效益;补播草种可大大加快幼林地植被覆盖。     

Characteristics of inorganic nitrogen assimilation of plants in fire-prone Mediterranean-type vegetation

A range of approaches was used to investigate how species within a fire-prone Banksia woodland in South West Australia exploited inorganic soil nitrogen sources and how this changes through the development of the fire chronosequence. Nitrate and ammonium were present in soil solution throughout the chronosequence but nitrate predominated in recently burnt sites. Mean shoot nitrate reductase activities were high for all species in recently burnt sites and showed little increase when nitrate was supplied via the transpiration stream. Nitrate reductase of shoots of most species was low at sites not burnt for several years, but following transpirational induction with nitrate, developed activities similar to those at recently burnt sites. The principal amino compounds transported in the xylem were species specific, including asparagine, glutamine and citrulline-dominated species, and changed little in relative composition across the chronosequence. Species most active in leaf nitrate reduction transported the largest amounts of nitrate in their xylem sap and proportional amounts of nitrate in xylem tended to be greatest in recently burnt sites. Most of the species examined appeared to be shoot rather than root nitrate assimilators, but marked differences were recorded in potential of leafy shoots of different species to reduce nitrate. As a general rule, shallow-rooted herbaceous, non-mycorrhizal or VAM-positive species had the highest capacity to reduce nitrate, whereas woody species with ericoid mycorrhizae or combined vesicular arbuscular/ectomycorrhizal associations exhibited little capacity to reduce nitrate in roots or shoots. It seems likely that this latter group utilize ammonium or even organic forms of nitrogen rather than nitrate. Some putative nitrogen-fixing species were active in reducing and transporting nitrate, others were virtually inactive in these respects.     

Spatial pattern,regeneration and growth rates of Brachystegia spiciformis and Julbernardia globiflora

Past work on tree-to-tree interactions in semi-arid savannas, through the use of nearest neighbour analysis, has shown both a regular dispersion pattern and a positive correlation between the size of a tree and the distance to its nearest neighbour. From these results, the importance of competition has been inferred. In the present study, tree-to-tree interactions in a mesic savanna woodland at Marondera, Zimbabwe, dominated by relatively small-sized individuals of Brachystegia spiciformis and Julbernardia globiflora, were examined. Results show that the regular dispersion pattern is uncommon; that there are few significant positive correlations between size of individual and distance to nearest neighbour; and that growth rate is not consistently correlated with size and distance of nearest neighbour. Results of a study of regeneration relative to canopy cover suggest a possible mechanism for these findings: unlike semi-arid savannas, young plants often grow in under-canopy environments so that early spacing of young trees does not occur. Significant positive correlations between the size of a tree and the distance to its nearest neighbour were found in a stand of larger, presumably older, trees, suggesting that such a relationship only develops in mesic miombo woodland through a thinning process as the trees mature. Familial clumping, which is common in the study area, may mask spatial patterns.Abbreviations RBAI Relative basal area increment     

Effect of winter fire on primary productivity and nutrient concentration of a dry tropical savanna

Burning increased the mean annual canopy and belowground biomass of a dry tropical savanna by 40% and 12%, respectively, while littermass was reduced by 85% in comparison to control savanna. Mean annual aboveground and belowground net primary production were 471 and 631 g m^-2 in control, and 584 and 688 g m^-2 in burned savanna, respectively. Fire caused an increase in mean aboveground net production of 24% and in belowground net production of 9%.Concentration of carbon, nitrogen and phosphorus in vegetation of unburned plots ranged between 34.01–38.59%, 0.85–1.53% and 0.04–0.11% and in soil from 0.95–1%, 0.011–0.13% and 0.017–0.02%, respectively. Fire increased the mean concentrations of N and P by 16% and 42% in vegetation and 18.18% and 17.65% in soil, respectively. Thus winter fire can be an important tool for the management of dry tropical savanna with respect to biomass production and nutritive quality.     

Regeneration of cleared Acacia zanzibarica bushland in Kenya

Abstract. Woody biomass production in natural forests of arid and semi-arid regions is low. The fuelwood demand of settlements often exceeds the sustained yield and regeneration capacity of natural forests, which results in deforestation. Regeneration and woody biomass development was studied in cleared Acacia zanzibarica bushland in Bura, eastern Kenya. The area was cleared in 1982 and studied in 1988. The site had been colonized primarily by Acacia zanzibarica and A. reficiens. Mean density was 1333 trees/ha, mean total woody biomass (dry weight) 1954 kg/ha, equal to 2.53 m³/ha. Mean annual increment was 293 kg/ha, or 0.3 8m³/ha. Expressed as rain use efficiency, the natural dry matter productivity of the woody component equals 0.83 kg ha^-1 yr^-1 mm^-1. The regeneration potential and some management implications are discussed.     

Ectomycorrhizae in coastal miombo woodland of Tanzania

Summary Ectomycorrhizae were found in root samples of the treesAfzelia quanzensis Welw. andBrachystegia spiciformis Benth. (Caesalpiniaceae), collected in the coastal miombo type woodland 50 km west of Dar-es-Salaam, Tanzania. Root nodules with a structure resembling that of nitrogen-fixing root nodules of other leguminous plants were observed in theA. quanzensis material. The climate of the locality is rather dry, and strongly seasonal. In the tropics, ectomycorrhizae have previously been found only in humid or rain forest climate zones.