Photosynthetic performance and resource utilization of two mangrove species coexisting in a hypersaline scrub forest

In a hypersaline mangrove scrub forest in northern Florida, coexisting trees of Laguncularia racemosa and Avicennia germinans were either fertilized with nitrogen or phosphorus, or not fertilized (controls). We aimed to test whether nutrient additions differentially altered photosynthetic performance and resource utilization in these two species. In control trees, photosynthetic rates were higher in L. racemosa than A. germinans. However, leaf nitrogen concentrations were higher in A. germinans than L. racemosa. Avicennia germinans responded to fertilization with nitrogen by increasing leaf nitrogen concentrations and rates of photosynthesis such that they were equivalent to photosynthesis in L. racemosa. Laguncularia racemosa did not show a response to nitrogen additions. Neither species showed strong responses to phosphorus fertilization. Avicennia germinans had high photosynthetic water-use efficiency (photosynthesis/transpiration), but low photosynthetic nitrogen-use efficiency (photosynthesis/leaf nitrogen). In contrast, L. racemosa had comparatively low photosynthetic water use efficiency and high photosynthetic nitrogen use efficiency. Leaf level characteristics lead us to hypothesize that coexistence of A. germinans and L. racemosa should occur where nitrogen levels are low and salinity is moderate, or at least moderate for some period of the year.