J. van Andel

Relationship between vegetation growth rates at the onset of the wet season and soil type in the Sahel of Burkina Faso: implications for resource utilisation at large scales

In the Sahel, poor soil quality and rainfall levels have a great influence on pasture production and hence on secondary output. In areas where rainfall is the limiting factor for primary production, recovery of primary and secondary production after the dry season depends on soil type. On sandy soils a large fraction of rainfall infiltrates and becomes available for plant growth, stimulating fast herbage growth, while on clayey and loamy soils low infiltration rates generate runoff, leading to slower herbage growth rates. The very different moisture retention characteristic of sands and clays is another possible cause for the observed differences in growth rates. In this paper we investigate the herbage growth rate from the onset of the rainy season. We hypothesise that, in areas where rainfall is the limiting factor for primary production, the vegetation growth rate on clayey soils is lower than that on sandy soils. We will test this hypothesis using long-term rainfall, soil types and satellite derived normalised difference vegetation index data. This research shows that the growth rates on sandy soil are significantly greater than that on clayey soils during the early part of the rainy season. We also show that these differences can be detected at large scales using satellite imagery. We also conclude that, at this scale, movement strategies of pastoralists would be intrinsically linked to not only rainfall patterns and distribution, but also to the underlying soil types in the region as this affects the quality and quantity of fodder available.

ANALYSIS OF THE GERMINATION SYNDROME OF DUNE ANNUALS

Summary Freshly collected seeds of seven co-existing winter annuals were imbibed and exposed to different constant temperatures with alternating periods of six weeks, each with cold and warm pretreatments. The length of the experiment and the temperature changes after six and twelve weeks respectively, made it possible to approach the seasonal temperature fluctuations in the field situation. Great differences between these species from the open coastal sand dune area of Holland appeared in their reaction to the different temperature treatments. Not only the length of the necessary after-ripening period differed, but also the optimum germination temperatures and the percentage of dormant seeds at the end of the experiment. Germination of most species immediately following seedset is prevented by innate dormancy, while the temperature which best counteracts innate dormancy differs from species to species. In most species, the dormancy is quickly broken when a cold period follows a warm period of only six weeks, as one might expect from winter annuals. Although, apart from Erophila verna most of the species germinate without a cold shock as well. It is concluded that the general drought avoidance syndrome results from species-specific physiological mechanisms (cf. A ngevine & C habot 1979).

Population ecology of plant species in disturbed forest and grassland habitats

Disturbed habitats can be defined only with reference to undisturbed systems. Therefore, we compare species in undisturbed and disturbed forest and grassland habitats. In the present chapter, patch dynamics and environmental heterogeneity are considered characteristic of undisturbed steady state systems. For successional communities the direction of succession and the fate of populations of characteristic species are useful criteria to distinguish between undisturbed and disturbed development of a community. Population ecological processes of co-occurring species are useful determinants of disturbance in both steady state and successional communities.

VARIABILITY IN GROWTH AND REPRODUCTION IN F1-FAMILIES OF AN EROPHILA-VERNA POPULATION

SummarySeeds from two morphologically different groups in an Erophila verna dune population were used in a growth experiment to analyse and quantify flexibility and plasticity of growth and reproductive characteristics.A strong correlation between plant morphology and individual seed weight was shown in parents and progeny. Seeds of plants with narrow leaves were twice as heavy (H-plants) as those from plants with broad leaves (L-plants) Families of H-plants allocated ca. 10% more dry matter to the roots. These characters were fixed, presumably due to selfing. Plants of both types are characterized by a graded control function between the vegetative and the reproductive phase. Most of the families showed a single switch from vegetative to reproductive growth, two families showed even two switch periods.Between-family differentiation is interpreted as a result of multiniche selection in populations of Erophila verna. Within-family variation is disadvantageous due to low fecundity.

Growth Response to Different Levels of Soil Moisture and Soil Fertility in Four Winter Annual Species during Their Life-Cycle

Summary The effect of three different soil moisture levels and two soil fertility levels on the growth, biomass allocation and reproductive effort of four winter annual plant species from the open dune habitat i.e. Aira praecox, Cerastium arvense, Erophila verna , and Veronica arvensis , has been investigated. Soil moisture has a considerable effect on the growth and reproduction of all species except Veronica arvensis , in which an increasing soil fertility seems to be of overriding importance. In the experimental period (27-30weeks), only Cerastium semidecandrum and Erophila verna completed their life-cycle and produced seeds. In these species absolute seed biomass and number per plant are markedly depressed by a low soil moisture level. In contrast to Erophila verna , the values of these parameters increase in Cerastium semidecandrum when the soil fertility is high. Mean seed weight was not affected by the treatments. Regarding the different niches of these winter annual species in the dune habitat, we discuss in how far the niches of these annuals represent different strategies or may be considered one particular strategy sensu G rime (1979).