Photosynthesis and leaf nitrogen in five Amazonian tree species during early secondary succession

Abstract

Field measurements of maximum net photosynthesis (Pmax), leaf nitrogen (N) content (leaf N per area and percent N), and specific leaf area (SLA) were made for Amazonian tree species within and across early successional sites of known ages after abandonment from slash-and-burn agriculture. We examined five species across a successional sere near San Carlos de Rio Negro, Venezuela, to test whether plasticity was associated with successional status and to determine whether changes in foliar properties during secondary succession can be attributed to shifts in species composition, in resource availability, or both. Average leaf N concentration was high (nearly 3%) for a pioneer species (Cecropiaficifolia) early in succession (1-3 yr after abandonment) but was always lower for the other early and mid- to-late succession species, especially later in succession (1-2% at 5-10 yr after abandonment). Net photosynthetic capacity (P /area and P I mass) varied as much as sixfold, being higher in pioneer species such as Cecropia and Vismia on recently abandoned sites and lower in late successional species such as Miconia and Licania on 10-yr abandoned agricultural sites. Total daily light availability also varied widely (14-fold) from its peak 1 yr after farm abandonment to low levels 9 yr into succession. During the first 5 yr of secondary succession, there were significant (P < 0.05) differences in Pmax and leaf N concentration among species in any given year. In most species, Pmax values declined with increasing time since abandonment within any given site. There were important differences in photosynthetic plasticity among species: Pmax tended to be much greater in earlier than later successional species soon after abandonment. Also, the difference in Pmax among species narrowed (or reversed) over time since abandonment, largely because of decreasing Pmax in pioneer species. The results suggest that changes in both species composition and in resource availability combine to produce the common pattern of decreasing leaf N concentration and photosynthetic rates during early rain forest succession after agriculture. Early successional species showed strong (r2 - 0.57, P = 0.0001) mass-based photosynthesis-N relationships but weak (r2 = 0.40 or lower, P = 0.000 1) area-based relationships both across the secondary successional sere after agriculture and across sites varying in types of disturbance. Both mass- and area-based photosynthesis-N relationships were poorer or not significant (P > 0.05) for mid- to late-successional species. Higher instantaneous Pmax/N and greater slopes of the photosynthesis-N relationships in early than late successional species suggest that pioneer species may show greater carbon assimilation capacity with elevated leaf N concentration on early successional sites than co-occurring species. The data suggest that early and late successional species may differ in the mode and degree of leaf-level physiological plasticity across succession.