PHOTOSYNTHETIC RATES AND NUTRIENT-USE EFFICIENCY AMONG EVERGREEN AND DECIDUOUS SHRUBS IN OKEFENOKEE SWAMP

We compared photosynthetic rates and indices of nutrient-use efficiency for evergreen and deciduous shrubs growing in the Okefenokee swamp. Despite statistically significant differences between evergreen and deciduous species, photosynthetic variables and leaf-level nutrient-use efficiencies did not sort cleanly into two groups representing differences in leaf longevity-instead, traits varied continuously with specific leaf mass (SLM). Lyonia lucida, an evergreen (SLM: 161.1 g/m(2)), and Itea virginica, a deciduous shrub (SLM: 75.1 g/m(2)), represent the extremes of the gradient. Net photosynthesis on a leaf area basis (A(area)2) was higher for evergreen than deciduous shrubs but was lower for evergreens when calculated on a leaf mass basis (A(mass)). There was, however, considerable overlap among species with different leaf longevities. Discriminant analysis indicated that SLM was the most important variable differentiating species. Thus, SLM rather than leaf longevity pet se is a better predictor of leaf function. Instantaneous photosynthetic nitrogen-use efficiency (PNUE) or phosphorus-use efficiency (PPUE) did not predict whole plant nutrient-use efficiency, measured as percent nutrient retranslocation from leaves prior to abscission. Evergreen species had significantly higher P and N retranslocation efficiency than deciduous species, yet slightly lower PNUE and PPUE. The evergreen Cyrilla racemiflora retranslocated >90% of leaf P. High retranslocation efficiency for P indicates that this resource may be an important limiting nutrient in this swamp, and high retranslocation efficiency and a longer growing season may explain the higher productivity of evergreen than deciduous shrubs in the nutrient-limited swamp habitat. Midday stomatal closure for L. lucida (evergreen) and I. virginica (deciduous) corresponded with decreasing xylem pressure potential in conjunction with high evaporative demand. The development of large differences between root, stem, and leaf water potential indicates that high resistance to water flux may limit leaf conductance. Phosphorus limitations increase hydraulic resistance in some herbaceous plants and may contribute to water stress in the swamp habitat. Thus, in addition to the direct effects of nutrient limitations, reduced water uptake and transport may be an important stress for swamp shrubs.