Dynamics of foliar nitrogen of evergreen and deciduous plant species in tropical forest of southern part of Assam, India
Nitrogen is one of the important elements that may limit plant growth. Present study describes the variations in foliar N mass during a leaf lifespan and N resorption in deciduous and evergreen species in a wet tropical forest of southern Assam, India. We aimed to study the differences in leaf N dynamics in deciduous and evergreen species rooted in the same habitat to specially understand the adaptation traits that allow them to coexist. The study also tested the extent to which nutrient resorption was correlated with other leaf parameters like leaf life-span, leaf dry mass and leaf area. A total of 22 plant species (15 evergreen and 7 deciduous), which includes 9 canopy, 10 sub-canopy and 3 shrub species were selected for the study. Fifteen leaves from different twigs were sampled at weekly intervals during expansion period then at monthly intervals till leaves senesced to determined variation in leaf dry mass and nitrogen content during its entire life span for each species. Nitrogen (N) accounts were calculated on leaf mass basis.
In general, the N concentration decreased from initial peak, stabilized or fluctuated as leaves matured and declined during senescence, after stable period. N concentration was significantly (P < 0.05) higher in deciduous species than evergreens. Seasonal patterns of N mass contents during a leaf life span were characterized by a phase of increasing pattern as leaves expanded followed by relatively stable stage as leaves mature, and subsequent declines during senescence. There was a considerable interspecific variation in the magnitude of N resorption but N resorption efficiency (NReff) was not found different among various growth or leaf persistence forms. NReff showed a strong positive correlation with leaf dry mass loss. No significant relationship was found between NReff and leaf life span, but leaf life span was found to have negative correlation with NRpro and N concentration of mature leaves, and also have a positive correlation with peak nitrogen mass (PNM). Both evergreen and deciduous species showed different phenological adaptations in terms of dry mass loss, N concentration in leaves, N resorption from senescing leaves and N uptake which probably enabled them to share the resources and coexist.