The use of degradation gradients for the assessment and ecological interpretation of range condition

Abstract

Multivariate procedures were used to subdivide the southern African climatic climax grasslands of the 700–800‐mm rainfall zone into relatively homogeneous grazing areas. A degradation gradient was constructed for each of these areas. This was done by means of centred PCA ordination, using static data representing compositional differences induced mainly by grazing. The first axis of these ordinations represented a gradient of degradation. The second and higher axes were combined in a single value, namely, the Euclidean distance from the first axis. The degradation gradients were then used as a basis for ordinating new sites into the old ordinations, assessing their condition in an objective and quantitative way. The Euclidean distances from the first axis were used to determine the appropriateness of the degradation gradients for the assessment of the condition of a particular site. Knowledge of species response to grazing, community dynamics and recovery potential obtained through the ordination and regression studies was used to interpret the condition assessments.

Meerveranderlike analises is gebruik om die klimaatsklimaks‐grasvelde van die 700–800‐mm reenvalsone in suidelike Afrika in relatief homogene weidingsgebiede onder te verdeel. ‘n Degradasiegradiënt is vir elk van hierdie gebiede ontwikkel. Gesentreerde hoofkomponente‐analises op data wat samestellingsverskille verteenwoordig wat hoofsaaklik deur beweiding veroorsaak is, is hiervoor gebruik. Die eerste as van hierdie ordenings het die degradasiegradiënt verteenwoordig. Die tweede en hoër asse is in ‘n enkel waarde gekombineer, naamlik, die Euclidiese afstand vanaf die eerste as. Die degradasiegradiente is daarna as ‘n basis gebruik vir die inordening van nuwe persele in die ou ordenings, om daardeur hul toestand op ‘n objektiewe en kwantitatiewe wyse te bepaal. Die Euclidiese afstande vanaf die eerste as is gebruik om die geskiktheid van die degradasiegradie'nt as ‘n basis vir toestandbepaling van ‘n spesifieke perseel/veldeenheid te evalueer. Die kennis van spesiesreaksies op beweiding, gemeenskapdinamika en herstel‐potensiaal, wat opgebou is deur middel van die ordeningstudies en regressies, is vir die interpretering van die toestand‐bepalings gebruik.

Additional index words:

• Grazing impact
• key species
• ordination
• recovery potential
• vegetation dynamics