Bioclimate, Leaf Temperature, and Primary Production in Red Mangrove Canopies in South Florida

A model of primary production and transpiration of forest canopies was developed from the energy—budget equation of individual leaves to clarify some of the physical processes affecting primary production. The model calculates hourly vertical profiles of temperature, transpiration, respiration, and gross and net photosynthesis of both sunlit and shaded leaves; and calculates appropriately weighted totals for the hourly water loss and gross and net photosynthesis for each level in the canopy. The model includes variations in leaf resistances caused by changes in absorbed solar radiation and changes in leaf water deficit and also takes into account the interdependence of the infrared profiles and the leaf—temperature profiles within the canopy. The model was tested with data collected on red mangrove (Rhizophora mangle Roxb.) forests. Canopy structure and the daily courses of solar and infrared radiation, air temperature, humidity, and wind, and ground temperatures were measured and used as input data for the model. The model produced realistic leaf temperatures, leaf resistances, transpiration rates, and primary production rates and was used to indicate the relative importance of environmental variables in influencing leaf temperature transpiration, and primary production. Maximum leaf temperatures occurred at the top of the canopy in June and the overcast day in January, but at the bottom of the canopy on the clear day in January. The model presently calculates water uptake as a constant rate to leaves and does not include a redistribution of water within the plant. The calculated transpiration was about 20% of the total water loss of the stand, the remaining loss coming directly from the moist substrate under the canopy. The inclusion in the model of stomatal movements reduced daily transpiration under conditions of mild water stress by 0.04 cm day ‐ 1 . Total evapotranspiration was 0.67 cm day ‐ 1 , and transpirational water loss was 0.12 cm day ‐ 1 . Levels in the middle of the canopy had the highest transpiration rate because of their high leaf area, but leaves at the top had the highest transpiration rate per unit leaf area. Leaf water deficits great enough to initiate stomatal closure occurred early in the morning at the top of the canopy in June on clear days, later in the morning at the top of the canopy in June on cloudy days, early in the afternoon at the top of the canopy in January on clear days, and not at all at the top of the canopy in January on cloudy days.Leaf water deficits did not develop within the canopy in either June or January. Net photosynthesis calculated with the model was 5.6 g organic matter m ‐ 2 day ‐ 1 for sunny days and 3.5 for cloudy days in June. Gross photosynthesis per unit leaf area was greater at the top of the canopy than at the bottom, but the middle levels of the canopy had the greatest production. The efficiency of water utilization increased from top to bottom of the canopy. A weighted monthly estimate for production was 3.4 for June and 2.2 for January, giving an average annual net production rate of 2.8 g organic matter m ‐ 2 day ‐ 1 . The model predicts that the maximum photosynthesis for mangrove stands will occur with a leaf—area index of about 2.5 if no acclimation to shade within the canopy occurs. A leaf area greater than about 2.5 may decrease production. The environmental variables with the greatest influence on primary production were air temperature and humidity. Production was decreased by increasing air temperature and increasing humidity. Increasing total solar radiation increased production up to a point, then decreased it. Increasing the diffuse fraction of the total solar radiation increased production. Increasing infrared radiation decreased production. Production and transpiration increased with increasing leaf—area index with steeply inclined leaves and leaf absorptance. Production was decreased by increasing leaf—area index with nearly horizontal leaves, leaf width, and the a p value. The canopy distribution of red mangrove appears to be nearly optimum to maximize the efficiency of water utilization rather than production. This indicates that the canopy is adapted to maximize production under conditions of saturated water supply.