Evidence of a Unique Association Between Single Forest Vegetation-Types and Seral Sequences: Praise for the Concept of ‘Vegetation Series’

Abstract

Succession has always been a central issue in ecological theory. Ever since first historical formulations, mosaics of vegetation-types taken to be the result of successional processes were envisaged as the pieces composing the Plant Landscape. Following the seminal ideas of F.E. Clements and A. Tansley, first European vegetation scientists, namely G.E. Du Rietz and R. Tüxen, laid the foundations of a system of describing successional vegetation mosaics as a higher complexity level of the Plant Landscape. Such repetitive mosaic units would correspond to ‘elementary’ successional sequences. These included both metastable seral stages and an ecologically mature stable state: the climax stage. Such a successional unit could then be used, as a reference, to systematize Plant Landscape. Early views on succession, expressed by a linear chronological sequence leading to a single territorial stable climatic climax, have been nowadays greatly challenged. Models encompassing sets of non-equilibrium alternative vegetation states, as a function of environmental disturbance, have been developed throughout. Moreover, phytocoenotical diversity also arises from zonation processes, i.e the spatial differentiation of vegetation mosaics following spatial environmental gradients. We do not consider that subject in this paper. In spite of criticism, continental Europe’s phytosociology schools have been elaborating on contemporary concepts of ‘elementary successional units’ or vegetation series as relevant to Vegetation Science. A widely-followed contemporary formulation of vegetation series is that of S. Rivas-Martínez (An Inst Bot Cavanilles 33:179–188, 1976): for a given biogeographical context, in spatially contiguous units possessing uniform environmental conditions, a single successional sequence develops and includes a sole ecologically mature vegetation type—the climax stage (our paraphrase). The real-world existence of coherent vegetation mosaics as the result of succession in homogeneous habitats, possessing a single ‘climax’ stage, is still challenged by several authors. On the contrary, by testing this very hypothesis by means of a case-study on vegetation mosaics from several types of managed oak forests in central-southern Portugal, we found a verification of S. Rivas-Martínez model. Also, accounting for bibliographic sources of evidence supporting verification of the vegetation series hypothesis, we found that putative falsifications issued mostly from distinct semantic or methodological settings. Thus, we find that the concept is, in spite of this, scientifically sound as a basis for a typological system of Plant Landscape which is relevant for Vegetation Science and useful for Nature Management and Vegetation Mapping.