Emile Fonty

Defense: December, 16th. 2011

Spirotropis and me

Tropical rainforest are reknown for their high tree species richness, in fact, most of the species grow at low density. Thus, the exploitation of tropical tree species costs both an extensive (and expensive) trail network and and heavy environmental impact. Nevertheless, some species show an aggregated pattern of their spatial distribution and can locally dominated the stand. If they reach >50% of the relative density and/or the relative dominance, this species are described as mondominant forests. Understanding mechanisms leading to monodominance, if they could be applied to timber species, may lead to a respectful forest managment.

Spirotropis longifolia DC Baill. (Papilionoidea) is the only monodominant terra firme tree species known in French Guiana. Although spreading across the Guianian shield, its monodominance had never been investigated. This study aim to understand the mechanisms which lead to its monodominance. Three main question will be investigated:

  1. What do the dominated forests look like ? (Floristic composition, diameter class distribution, spatial repartition, …)
  2. Do monodominance of S. longifolia require specific abiotic conditions (soil, disturbance, …) ?
  3. Which reproductive strategy, sexual vs vegetative, lead to monodominance ?

In our inventory plots, S. longifolia show an heavy monodominance (up to 70% of the stand). Its population seems to be spatially organised. In the margin of the dominated stands, S. longifolia look like a standard monostem tree, even though, most of the trees produce basal sprouts and adventitious roots. Further in the stand, some S. longifolia loose their main stem while one or several sprouts reach the canopy. Finally, mature S. longifolia look like a woody clump surrounded by a thick root mount. This ’persistence niche’ may slowed the forest turnover and this increase the dominance of S. longifolia. Furthermore, S. longifolia conserves its ability to sprout even if it is uprooted. Sprouts frequently become autonomous as adventitious roots, growing at their base, reach the soil. Thus, S. longifolia may use this layering ability as a shortcut during forest regeneration to maintain its dominance in the stand.

View online : Download the Thesis

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