Leaf Gas Exchange and Nutrient Use Efficiency Help Explain the Distribution of Two Neotropical Mangroves under Contrasting Flooding and Salinity

Abstract

Rhizophora mangleandLaguncularia racemosacooccur along many intertidal floodplains in the Neotropics. Their patterns of dominance shift along various gradients, coincident with salinity, soil fertility, and tidal flooding. We used leaf gas exchange metrics to investigate the strategies of these two species in mixed culture to simulate competition under different salinity concentrations and hydroperiods. Semidiurnal tidal and permanent flooding hydroperiods at two constant salinity regimes (10 g L−1and 40 g L−1) were simulated over 10 months. Assimilation (A), stomatal conductance (gw), intercellular CO2concentration (Ci), instantaneous photosynthetic water use efficiency (PWUE), and photosynthetic nitrogen use efficiency (PNUE) were determined at the leaf level for both species over two time periods.Rhizophora manglehad significantly higher PWUE than didL. racemosaseedlings at low salinities; however,L. racemosahad higher PNUE andgwand, accordingly, had greater intercellular CO2(calculated) during measurements. Both species maintained similar capacities forAat 10 and 40 g L−1salinity and during both permanent and tidal hydroperiod treatments. Hydroperiod alone had no detectable effect on leaf gas exchange. However, PWUE increased and PNUE decreased for both species at 40 g L−1salinity compared to 10 g L−1. At 40 g L−1salinity, PNUE was higher forL. racemosathanR. manglewith tidal flooding. These treatments indicated that salinity influences gas exchange efficiency, might affect how gases are apportioned intercellularly, and accentuates different strategies for distributing leaf nitrogen to photosynthesis for these two species while growing competitively.

Mangroves in Contrasting Osmotic Environments: Photosynthetic Costs of High Salinity Tolerance

“…However, those differences disappear if the amount of N invested in glycinebetain is subtracted from the total amount of N. In both species, the NUE decreases in hypersaline sites. In an experimental study, Cardona-Olarte et al [16] did not find differences in WUE based on gas exchange of seedling grown in nutrient solutions with salinities between 10 and 40 ppt; however, PNUE decreased from about 85 μmol/mol N at 10 ppt to nearly 60 at 40 ppt.…”

“…Photosynthesis decreases significantly with salinity of interstitial water in several mangrove species [7][8][9][10][11]. Some species appear to be more sensitive to soil salinity than others, a characteristic that may be associated with specific metabolic and structural properties such as synthesis of compatible solutes, root permeability, salt excretion, and compartmentation of excess ions [12][13][14][15][16][17].…”

Water levels primarily drive variation in photosynthesis and nutrient use of scrub Red Mangroves in the southeastern Florida Everglades

“…Although mangrove species are tolerant to flooded conditions, they are still susceptible to flooding damage if plants become completely submerged for days to weeks (Wanless 1998, Mendelssohn and McKee 2000, McKee 2011). Inundation stress typically decreases plant carbon uptake and storage, due to interactions between inundation duration and mangrove transpiration rates; hence A net and growth rates are usually depressed in mangrove forests subjected to longer flooding duration (He et al 2007, Cardona-Olarte et al 2013). For example, greenhouse studies have revealed a 20% reduction in maximum A net when mangrove seedlings and saplings were subjected to short-term intermittent seawater flooding (6 to 22 days, Krauss et al 2006).…”

“…However, it is not completely understood how the interaction between water level dynamics and salinity affect in situ rates of leaf gas exchange of mangrove in this region. Experimental evidence using R. mangle seedlings from south Florida showed that inundation created a greater degree of physiological stress than did salinity levels; however, salinity accelerated the adverse effects of inundation stress on leaf function over time (Cardona-Olarte et al 2013). In contrast, other studies have reported no clear effect of water levels or flooding duration on rates of mangrove gas exchange, although inundation duration decreased variability in leaf gas exchange measurements (Hoppe-Speer et al 2011).…”

Gas exchange and isotopic signature of mangrove species in Southern Brazil

“…Mangrove environments may characterize a physiological challenge for plants because of the salinity and, consequently, highly negative water potentials of soil pore water (Krauss et al, 2008;Cardona-Olarte et al, 2013). Therefore, the main issue to mangrove tree species is the tradeoff between water loss and carbon gain, since water acquisition is more energetically expensive than in non-saline soils (Reef & Lovelock, 2015).…”