Herbaceous phytomass and nutrient concentrations of four grass species in Sudanian savanna woodland subjected to recurrent early fire.

Fire is an integral ecological factor in African savanna ecosystems, but its effects on savanna productivity are highly variable and less understood. We conducted a field experiment to quantify changes in herbaceous phytomass and nutrient composition in a Sudanian savanna woodland subjected to annual early fire from 1993 to 2004. Fire effects were also assessed on two perennial and two annual grass species during the following growing season. Early fire significantly reduced above-ground phytomass of the studied species (P = 0.03), their crude protein (P = 0.022), neutral detergent insoluble crude protein (P = 0.016) and concentrations of Ca, Fe and Mn (P < 0.05). Perennial grasses had higher above-ground phytomass but lower total crude protein and fat than annual grasses. Nonstructural carbohydrates tended to be higher for annuals, while fibre and lignin contents were high for perennials. Except Na and Fe, the concentration of mineral elements varied between species. Fire did not affect measures of digestibility and metabolizable energy, but its effect differed significantly among species. In conclusion, the results illustrate that long-term frequent fire will counterbalance the short-term increase in soil fertility and plant nutrient concentrations claimed to be accrued from single or less frequent fire. Resume Le feu est un facteur ecologique a part entiere dans les ecosystemes de savane africaine, mais ses effets sur la productivite de la savane sont tres variables et mal compris. Nous avons realise une experience de terrain pour quantifier des changements de biomasse herbacee et de composition des nutriments dans une foret de savane soudanienne sujette a des feux precoces annuels, entre 1993 et 2004. Les effets des feux ont aussi eteevalues sur deux especes d’herbes perennes et sur deux especes annuelles au cours de la saison de croissance suivante. Le feu precoce reduit significativement la biomasse vegetale aerienne des especes etudiees (P = 0.03), leurs matieres azotees (P = 0.022), les proteines brutes insolubles au detergent neutre (P = 0.016) et les concentrations de Ca, Fe et Mn (P < 0.05). Les herbes perennes avaient une plus grande biomasse aerienne mais un taux plus faible de matieres azotees totales et de lipides que les herbes annuelles. Les hydrates de carbone non structures avaient tendance aetre plus eleves pour les herbes annuelles, alors que les contenus en fibres et en lignine etaient eleves chez les perennes. A l’exception du Na et du Fe, la concentration des mineraux variait selon les especes. Le feu n’affectait pas les mesures de digestibilite et d’energie metabolisable mais son effet differait significativement selon les especes. En conclusion, les resultats illustrent que des feux frequents utilises a long terme vont contrebalancer l’augmentation a court terme de la fertilite du sol et les concentrations de nutriments dans les plantes, dont on dit qu’elles sont accrues par des feux uniques ou moins frequents.

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