Seasonal variation of grassland basal cover

Abstract

Preliminary results of a long‐term study of seasonal variation in grassland basal cover are presented. Seasonal stability appears to have been taken for granted but has not yet been shown experimentally. The experimental area is situated in the National Botanic Gardens, Pretoria, at 28°18'E and 25°44'S on a gentle, southwest‐facing slope with an annual rainfall of 630 mm. The vegetation cover is an open, bunch grassland. The construction of a portable bridge for establishing 1000 relocatable points is described. Basal cover, measured by wheel‐point and bridge‐point methods, showed a statistically significant seasonal increase through summer and a decrease again after the rainfall began decreasing with the onset of winter. An initial small rise and fall in basal cover at the beginning of spring was shown by the wheel‐point survey only. Inspection of individual species behaviour showed that the seasonal changes in basal cover are a result of small changes in basal cover of many species. Possible sources of error in the application of the point technique which were revealed during the study arc given.

UITTREKSEL

‘n Uiteensetting van die voorlopige resultate wat tot dusver in ‘n langtermyn‐ondersoek oor die seisoensverandering van die hasaalbedekking van ‘n grasveld verkry is, word aangebied. Alhoewel dit nog nie eksperimenteel bewys is nie, word stabiliteit van hasaalbedekking blykbaar as van selfsprekend aan‐vaar. Die proefgehied is in die Nasionale Botaniese Tuin, naby Pretoria, 28°18'O en 25°44'S, teen ‘n geleidelike, suid‐westelike helling geleë en het ‘n jaarlikse re'enval van 630 mm. Die plantegroei bestaan uit ‘n oop, kort, polgrasveld. Die opstelling van ‘n draagbare brug vir die vasstelling van 1000 vaste punte, word beskryf. Basaalbedekking soos verkry d.m.v. wielpunt‐ en brugpuntopnames, toon ‘n statisties betekenis‐vol seisoenstoename gedurende die somer, en ‘n afname wat met die afname in reënval teen die begin van die winter gepaard gaan. Slegs die wielpuntopname het aanvanklik ‘n effense styging en daling in basaalbedekking tydens die begin van die lente getoon. Die ondersoek van die gedrag van individuele spesies toon dat die seosoensverandering in basaalbedekking die gevolg van klein veranderings in basaalbedekking van ‘n groot aantal spesies is. Moontlike oorsake van foute in die toepassing van die puntmetode, soos gedurende die ondersoek gevind, word gemeld.