Dynamics of foliar nitrogen of evergreen and deciduous plant species in a wet tropical forest, South Assam, India

Nitrogen is one of the important elements that may limit plant growth. The present study describes the variations in foliar N during a leaf's life span as well as the degree of N resorption for 22 plant species (15 evergreen and 7 deciduous, including 9 canopy, 10 sub-canopy and 3 shrub species) in a wet tropical forest of southern Assam, India. Generally, N concentrations and mass decreased after an initial peak, then stabilized or fluctuated as leaves matured, and declined during senescence. N concentrations were significantly (P < 0.05) higher in deciduous species than in evergreens. There was considerable inter-specific variation in the magnitude of N resorption, but N resorption efficiency (NReff) was not found to differ amongst growth or leaf persistence forms. On average, 52.6 % of foliar N was resorbed during senescence. NReff showed a strong positive correlation with leaf dry mass loss. No significant relationship was found between NReff and leaf life span. However, leaf life span showed a negative correlation with N resorption proficiency (NRpro) and N concentration of mature leaves, and also exhibited a positive correlation with peak nitrogen mass. Therefore, leaf life span is important for N conservation. Evergreen and deciduous species showed different phenological adaptations in terms of dry mass loss, foliar N concentration and N resorption from senescing leaves which probably enable them to share available resources and coexist in these multi-species forests.