INFLUENCE OF CLIMATE AND FERTILIZATION ON NET PHOTOSYNTHESIS OF MATURE SLASH PINE

Net photosynthesis was measured under field conditions in 23-year-old slash pine (Pinus elliottii Engelm. var. elliottii) trees to determine how it was affected by fertilization and climate. There was only a sm all decrease in rates of net photosynthesis from late summer through w inter demonstrating that appreciable carbon gain occurs throughout the year in slash pine. Although fertilization substantially increased le af area and aboveground biomass, it only slightly increased the rate o f net photosynthesis. Simultaneous measurements of gas exchange in fer tilized and unfertilized (control) plots allowed the detection of a sm all, but statistically significant difference in average net photosynt hesis of 0.14 mumol m-2 s-1. Irradiance, and to a lesser extent air te mperature, were the environmental factors that exerted the most contro l on net photosynthesis. The highest rates of net photosynthesis occur red between air temperatures of 25 and 35-degrees-C. Because air tempe ratures were within this range for 46% of all daylight hours during th e year, air temperature was not often a significant limitation. Soil a nd atmospheric water deficits had less effect on photosynthesis than i rradiance or air temperature. Although the depth to the water table ch anged during the year from 10 to 160 cm, predawn and midday xylem pres sure potentials only changed slightly throughout the year. Predawn val ues ranged from -0.63 to -0.88 MPa in the control plot and from -0.51 to -0.87 MPa in the fertilized plot and were not correlated with water table depth. There was no correlation between xylem pressure potentia ls and net photosynthesis, presumably because water uptake was adequat e. Although vapor pressure deficits reached 3.5 kPa during the summer, they had little effect on net photosynthesis. Over a vapor pressure d eficit range from 1.0 to 3.0 kPa, net photosynthesis only decreased 21 %. No differences in responses to these environmental factors could be attributed to fertilization.