An Ordination Analysis of Vegetation Patterns on Topoclimate Gradients in South-East Spain

Abstract

Aspect-induced topoclimate trends are examined in the semi-arid vegetation of five study areas in Murcia Province, S.E. Spain. Vegetation data are simplified using reciprocal averaging and detrended correspondence analysis ordinations based on quantitative and presence floristic data. Trend surface analysis is then used to fit aspect data to ordination configurations and seek an optimal axis of aspect symmetry which is deflected from a north-south line. Deflection angles for an opti- mal axis approximate 19? in four study areas, 7? in the fifth. In an aggregate set of all study areas there is an approximate 14? deflection which is also found in northern and southern subsets of aggregate data. Trend surfaces account for 64-85% of the variance in adjusted aspect data. Fairly consistent deflection patterns between ordinations and high explanation levels for trend surfaces provide strong evidence for the existence of an optimal axis of aspect symmetry in vegetation data. Species patterns on topoclimate gradients are summarized using a seria- tion technique to isolate species significantly restricted to sectors of the climatic continuum. This is applied to four ordinations and two direct gradients based on unadjusted and optimal aspect scores. A floristic dis- continuity is revealed on the topoclimate continuum, the great majority of species exhibiting a preference for either northern or southern habitats. The discontinuity is interpreted as an indication of two geographical floras preserved together by local topoclimate gradients. The wider potential of an ordination interpretation strategy based on trend surface analysis and seriation is considered, making a case for the use