Secondary vegetation¹ after practice of Slash-and-Burn Cultivation: Characteristic and optimization, the Case of Central Menabe in Madagascar

Raharimalala Hanitriniaina Olga Jeannine

Madagascar forests are among the most biologically and unique ecosystems. Strong pressure from human activities causes, however, a rapid destruction of natural habitats and a massive loss of biodiversity. Slash and burn is one of the causes of this pressure. This is a traditional practice in many areas and predominantly in Madagascar, particularly in the South-West. Secondary vegetation that has developed on surfaces formerly burned and abandoned has an economic value for the local population, either as firewood, root crops, pasture or medicinal plants, or potential commercial uses. To limit the pressure on natural forest, re-use of previously burned and abandoned surfaces is needed. The aim of this thesis is to study the potential of secondary succession for the practice of slash and burn in order to optimize their use. Different ages of culture abandonment were studied in plots ranging from 1 year of culture abandonment to more than 40 years, and classified into six classes (1-5 years, 6-10 years, 11-20 years, 21-30, 31-40 years and more than 40 years).
In this region, farmers prefer to use yellow soils (name used because of their color) rather than red soils because pH is slightly basic in the yellow and the ratio C/N higher. The species richness increases with the age of abandonment and stabilizes at more than 40 years. Woody species increased from 10 years of culture abandonment and herbaceous species decreased slightly after 30 years.

With respect to biomass, our measures showed that herbaceous biomass increases up to 20 years of abandonment and can reach up to 2.1t/ha (dry biomass, i.e. air dryed biomass during three weeks), then this quantity decreases to 1.2t/ha (dry biomass) after 40 years of abandonment. This decrease is due to the growth of large trees and bushes and the decrease of luminosity. For woody species, the oven dry biomass increases with the age of abandonment. The amount of oven dry biomass increases from 0.43t/ha for the class 1 to 5 years, to 66.9t/ha after 40 years of abandonment.
The nutrients concentrations released by ashes are higher for the leaves as compared to the trunk and the branches. The amount of nutrients released increases with the age of abandonment because of overal increase of biomass. Nine species reach a dry biomass over 1t/ha after 30 to 40 years. Poupartia sylvatica and Tarena sericea contribute not only to the largest amount of biomass but also of nutrients released to the soil
With respect to soil fertility, for most chemical variables, a significant increase is observed between six and ten years after abandonment. After this period, nitrate and ammonium concentrations increase steadily. As for magnesium, calcium and potassium, they show a tendency for a humped-shaped pattern, whereas the concentration remains constant for Ctot and N tot, and decreases for pH and CEC.

We conclude that a fallow period shorter than 10 years would lead to an unsustainable agricultural system and recommend a turn-over period of 11 to 30 years since last cultivation to permit soil recovery to a sufficient fertility level to start a new cycle of cultivation

Key words : Slash and burn -  secondary succession – biomass – nutrient – inflammability – fertility-  Central Menabe -  Madagascar

Addresses:
Doctoral candidate

Raharimalala Hanitriniaina Olga Jeannine
Laboratory of ecological systems
EPFL-ENAC
Bâtiment GR, Station 2
CH-1015 Lausanne, Suisse
olga.raharimalala[at]epfl.ch

Research partners

- Pr Rajoelison Lalanirina Gabrielle
Département des Eaux et Forêts
Ecole Supérieure des Sciences Agronomiques, Université d'Antananarivo
B.P. 175 Ankatso,
101 Antananarivo
Madagascar
e-mail : g.rajoelison@simicro.mg

- Pr. Dimitrakopoulos Alexandrian
Departement of Forestry and Natural Environment
P.O Box 228, Aristotle University, 540.06 Thessaloniki, Greece
e-mail: alexdimi@for.auth.gr

Dr. Jean-Pierre Sorg
Groupe de foresterie pour le développement
 ETH-Zentrum
Universitätstr 22
8092 Zürich
E-Mail: jean-pierre.sorg@env.ethz.ch

Directors of  thethesis

Pr Alexandre Buttler
EPFL-ENAC-ECOS
Bâtiment GR, Station 2
CH-1015 Lausanne, Suisse
alexandre.buttler[at]epfl.ch

and

Pr Jean-Michel Gobat

Laboratoire sol et vegetation
Université de Neuchâtel
Laboratoire d'écologie végétale
Faculté des Sciences
Emile Argand 11
Case postale 158
CH-2009 Neuchâtel
(Switzerland)
Jean-Michel.Gobat(at)unine.ch

¹The secondary vegetation succession develops following a significant disturbance (anthropic or natural) of the primary forest. The principal factors of differentiation of the secondary succession are the climate, altitude, the type of ground, the human actions and the natural disasters (crumblings, volcanicity, etc) (definition by FAO).

Baobab Trees

Secondary vegetation after fire

Secondary vegetation