Biomass allocation in five semi-arid afforestation species is driven mainly by ontogeny rather than resource availability

Abstract

Key message The changes in the relative biomass allocation to roots in juvenile stands of fast-growing (Leucaena leucocephala Lam., Moringa oleifera Lam., and Jatropha curcas L.) and slow-growing (Anacardium occidentale L. and Parkia biglobosa Jacq.) afforestation species are driven mainly by ontogeny rather than resource availability. However, silvicultural management aiming at increasing availability of water and particularly nutrients enhances biomass production in all species. Context Understanding the patterns of biomass allocation among tree species in response to ontogeny and to variation in resource availability is key to the successful restoration of degraded land using forest plantations. Aims This study assessed the effects of resource availability and ontogeny on biomass accumulation and partitioning in five semi-arid afforestation species. Methods The aboveground and belowground biomass production of fast-growing Leucaena leucocephala Lam., Moringa oleifera Lam., and Jatropha curcas L. and slow-growing Anacardium occidentale L. and Parkia biglobosa Jacq. was monitored following the application of manure (1 kg plant(-1)) and/or supplemental irrigation (0.5 L per sapling daily) during the first two rainy seasons and the intervening dry season on degraded cropland in Northern Benin. Results Biomass accumulation in the fast-growing species was positively impacted by fertilization and irrigation during both rainy seasons. The slow-growing species responded positively to the silvicultural treatments during the dry and second rainy season. The application of fertilizer alone increased the biomass of P. biglobosa by up to 335% during the dry season. Fifteen months after planting, manure-treated L. leucocephala accumulated the most biomass (2.9 kg tree(-1)). The root fraction decreased with increasing tree size in all species. The comparison of root versus shoot allocation in trees of equal size indicated that the treatment-induced shifts in biomass partitioning were controlled by ontogeny, which explained 86-95% of the variation in root-shoot biomass relationships. Conclusion While ontogeny was the main driver of biomass partitioning, increased resource availability induced a larger production of biomass, overall leading to greater aboveground production in all species.