(1) Budburst and leafexpansion up to maximal leafarea index were followed from 1976 to 1983 in a 120-year-old Qirercib pefraea stand. (2) A phenological model was formulated to predict the date of budburst and the increase in leaf area as a function of cumulative temperature and of day-length. Budburst occurs when the temperature sum of the past 10 d a y exceeds a given threshold, which is a decreasing function of day-length. Leaf area increases with the temperature sum since budburst, and reaches its greatest value when the temperature sum exceeds a given threshold, which is also a function of day-length. (3) The model was tested during the growing season of 1982 and'1983. Simulated dates of budburst and leaf area index development were in close agreement with the measurements.

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Nitrate in the unsaturated zone between the soil surface and the water table was studied in agroforestry Parklands in north western Senegal by examination of samples obtained by hand auger. Depending on location, water tables existed at depths between 10 and 35 m below ground. Previous studies of groundwater in this region had found that large concentrations of nitrate were unconnected with anthropogenic activity. The objective of this study was to determine whether nitrogen fixing vegetation had a role in groundwater nitrate accumulation and whether roots of trees were located deeply enough to access the nitrate. Accordingly, sample profiles were augered close to stems of nitrogen fixing trees, non-nitrogen fixing trees and also in adjacent areas that were unaffected by tree presence. These adjacent areas were typically open pasture or cultivated fields. Tree fine roots were quantified in the samples and examined for the presence of mycorrhizas. Similarly, sand/soil samples were examined and tested for the presence of nitrogen fixing rhizobia that were capable of forming functional nodules on appropriate host plants. Concentrations of nitrate were greatest in soils beneath nitrogen fixing trees and nitrate was more plentiful in profiles augered beneath nitrogen fixing crops than it was elsewhere suggesting that N-fixation was the source of the nitrate. The concentrrations of nitrate that were found in the unsaturated zone were greatly in excess of the WHO recommended limit for nitrate in drinking water. High NO 3 -N/Cl ratios confirm in situ production of nitrate, and indicate that this is a natural baseline occurrence related to N-fixation. The nitrate is moving down the profile and impacts the groundwater unless it can be intercepted by plant roots. NO 3 -N amounts in solution in the soil profile varied between 75 and 1000 kg ha −1 beneath trees and between 120 and 400 kg ha −1 in areas outwith tree crowns. Although these quantities of N occupy the lower end of the range of N values obtained in north American deserts, they comprise a considerable dryland resource where amounts of organic fertilizer are limited and where cost prohibits the use of commercial fertilizers. Roots of both nitrogen fixing and non-nitrogen fixing trees were deep enough to access the nitrate but the small amounts of available water at intermediate depths suggest that large scale uptake of nitrate will only be possible in the wetter zones located close to the water table. Shallow roots tended to be more heavily colonized by mycorrhizas than deeper roots but mycorrhizas were recovered from roots located 22 m below ground. Tree roots and rhizobia had similar patterns of distribution. They were commonest close to the soil surface, less frequent at intermediate depths and tended to increase in frequency close to the water table. * FAX No: +44 131 445 3943.