Native forest regeneration and vegetation dynamics in non-native Pinus patula tree plantations in Madagascar.
Today, exotic invasive alien species are considered to be one of the greatest threats to biodiversity. However, not all exotic species cause negative consequences in the ecosystems in which they settle. In some cases, artificial plantations of exotic species can act as a catalyst for the regeneration of native species. In southern central Madagascar, which has been affected by diverse disturbances (cyclones, logging, cultivation), Pinus patula plantations have become environments conducive to the regeneration of native species. This study aims to assess the level of understory plant diversity in these artificial plantations, describe the vegetation dynamics, and examine the effects of agricultural practices and logging on vegetation diversity and structure. Floristic inventories and biomass measurements were made on three types of vegetation that have resulted from previous human-induced disturbance: (i) cassava crops established after pine logging; (ii) fallows corresponding to cassava crops left fallow for 1-2 years; and (iii) post-logging regrowth after pine logging 3-10 years previously. Botanical surveys were conducted on 205 10×10 m2 quadrats placed along 40 transects. A total of 125 species, 34 of which endemic, from 46 families were inventoried. The Shannon Weaver index H' values were significantly different between post-logging regrowth (H′=4.01±0.67), and cultivated fields (H′=1.46±0.23) and fallows (H′=3.31±0.78) (p<0.001). Agricultural practices, such as tillage regime, number of crop-fallow cycles (>2 cycles) and number of fire occurrences (>4 fires) had a greater impact on the reconstitution of vegetation than environmental parameters such as slope and topographic position. The Shannon-Weaver diversity index (H') and vegetation structure parameters (tree density, biomass) were inversely correlated with the intensity of disturbance linked to these agricultural practices. These results suggest that a low level of disturbance, such as that resulting from Pinus tree logging, can act as a catalyst for the establishment of native woodlands. Excessive disturbance from sylvicultural and agricultural practices, such as frequent burning, heavy tillage and high number of crop/fallow cycles, may hinder, but does not block, the establishment of native woodlands.