Annual Report: CIRAD 2007

There is a perpetual exchange of matter (carbon, water) and energy between terrestrial ecosystems and their environments. The type of cover and its microclimate influence these flows, which in turn impact the global climate. These flowas can be measured and modelled and the resulting data have many applications in the fields of agronomy, ecology and climatology. Different flows—light absorption, photosynthesis and evapotranspiration— have been recorded and modelled for a wide variety of vegetation covers through the FLUXNET global network for more than ten years at its 250 measurement sites. Spatial or temporal extrapolations can then be performed to, for instance, assess the effects of different climatic scenarios. After standardization, the results are allocated to different databases. They are ultimately used in meta-analyses—the entire network thus has a global scope, which is essential to be able to deal with environmental issues that arise.

CIRAD actively contributes to FLUXNET concerning flows in tropical tree plantations (coconut, eucalyptus, natural rubber, coffee, oil palm) and some associated natural ecosystems (savannahs). In particular, over a three-year period, CIRAD measured carbon, water and energy flows between a 25 ha coconut plot and the atmosphere under almost optimal growing conditions in Vanuatu—fertile soil with a humid tropical climate throughout the year. Measurements were continuously recorded 10 times/s and then integrated on a 0.5 h time step. All flow variations could then be studied on different scales: instantaneous, such as the movement of a cloud; seasonal, such as dry or rainy periods; or interannual.

The results of these carbon flow analyses were compared to records obtained by CIRAD’s partners in tropical rainforests. The results were surprising—the productivity of a coconut plantation under almost optimal growing conditions was close to that of a natural tropical evergreen rainforest. This “productivity” concept encompasses three general ideas: the gross primary production of plants (photosynthesis of the cover), the net primary production (sum of the visible annual plant growth and the litter production), and the net primary production of the ecosystem (difference between carbon uptake in the ecosystem by photosynthesis and its loss by respiration), which represents the ecosystem’s carbon balance, ie the key factor in carbon sequestration. Coconut plantations fix high quantities of carbon (39 t/ha a year), which is close to that of tropical rainforests (35 t). These plantations are also excellent biomass and litter producers (around 16 t/ha a year of carbon). Moreover, their carbon balance is high, ie 7 t/ha a year of carbon fixed (after deducting copra harvest yield), which is higher than but comparable to the mean carbon balance of tropical evergreen rainforests (4 t).

These meta-analyses also indicated how the natural or artificial carbon production of an ecosystem varies according to the leaf surface of the prevailing plants, duration of the vegetative growth period, climate and soil fertility. The highest production was noted in tropical evergreen rainforests, and the results were comparable to those obtained for the studied coconut plantation.

Few studies have been carried out to date on tropical tree plantations, despite their economic and ecological importance—a broad range of strategies may be implemented for the management of these plantations, from high input commercial approaches to agroforestry and organic farming approaches. It is thus essential to conduct further research to describe and model these new systems, including their sustainability and versatility with respect to climate change, and to compare the resulting information in terms of two key criteria, ie human development and biodiversity conservation.

Contact

Olivier Roupsard, e-mail ; Christophe Jourdan, Functioning and Management of Tree-based Planted Ecosystems (UPR)

Partners

• Centre d’écologie fonctionnelle et évolutive (CEFE, France)
• Institut national de la recherche agronomique (INRA, France)
• Languedoc-Roussillon Regional Council (France)
• Henri Poincaré University (France)
• Vanuatu Agricultural Research and Training Center (VARTC, Vanuatu)

For further information

• Luyssaert S. et al., 2007. The CO2-balance of boreal, temperate and tropical forests derived from a global database. Global Change Biology, 13 : 2509-2537.
• Roupsard O. et al., 2006. Partitioning energy and evapo-transpiration above and below a tropical palm canopy. Agricultural and Forest Meteorology, 139 : 252-268.
• Roupsard O. et al., 2008. Cross-validating Sun-shade and 3D models of light absorption by a tree-crop canopy. Agricultural and Forest Meteorology (sous presse) .
• Roupsard O., Bonnefond J.M., Luyssaert S., 2007. Productivity of a tropical plantation of coconut tree (Cocos nucifera L.), compared with tropical evergreen humid forests. Asiaflux Newsletter, 23 : 4-9.