Forest and woodland expansion into forestry plantations informs screening for native agroforestry species, Maputaland South Africa

Abstract

Expansion of natural forest or woodland in forestry plantations can potentially provide insight about the behaviour of plant communities in human-modified ecosystems. Here, we investigate the expansion of native woody vegetation into abandoned areas within timber plantations, whereby management decisions (and consequently different conditions of land abandonment) had led to variation in vegetation composition and structure. These differences were assessed to identify native species suitable for agroforestry systems within forestry plantations. Elements of sub-tropical dry forest (the reference forest) had expanded into (i) clear-felled and then abandoned plantations, and (ii) unharvested abandoned plantations. Two-way indicator species analysis, Non-metric multidimensional scaling, and Indicator species analysis were used to describe compositional intergradation between natural forest and secondary vegetation, and to assess correlation with environmental variables of fire and stand structure. Areas of vegetation expansion contained 53 native woody species from 26 families, about half the number sampled in the reference forest. The understory composition of unharvested plantation sites closely resembled regrowth forest, whereas clear-felled plantations had developed a species composition resembling woodland, comprising savanna species. Substantial intergradation among compositions of woodland, plantations and regrowth forest implied that regrowth forest was a likely propagule source for native species, but that past plantation management practices acted as an environmental filter resulting in slightly different vegetation types. Useful woody species Sclerocarya birrea, Vangueria infausta, Trichilia emetica, Strychnos spinosa, Annona senegalensis, and Hyphaene coriaceae were considered ecologically suitable for testing in silvo-pasture agroforestry systems owing to their occurrence in disturbed, open-canopy, fire-exposed environments (see also the graphical abstract in the supplemental files).