Foliage physiology and biochemistry in response to light gradients in conifers with varying shade tolerance

To examine the predictability of leaf physiology and biochemistry from ligh t gradients within canopies, we measured photosynthetic light-response curv es, leaf mass per area (LMA) and concentrations of nitrogen, phosphorus and chlorophyll at 15-20 positions within canopies of three conifer species wi th increasing shade tolerance, ponderosa pine [Pinus ponderosa (Laws.)], Do uglas fir [Pseudotsuga menziesii (Mirb.) France], and western hemlock [Tsug a heterophylla (Raf.) Sarg.]. Adjacent to each sampling position, we contin uously monitored photosynthetically active photon flux density (PPFD) over a 5-week period using quantum sensors. From these measurements we calculate d FPAR: integrated PPFD at each sampling point as a fraction of full sun. F rom the shadiest to the brightest canopy positions, LMA increased by about 50% in ponderosa pine and 100% in western hemlock; Douglas fir was intermed iate. Canopy-average LMA increased with decreasing shade tolerance. Most fo liage properties showed more variability within and between canopies when e xpressed on a leaf area basis than on a leaf mass basis, although the rever se was true for chlorophyll. Where foliage biochemistry or physiology was c orrelated with FPAR, the relationships were non-linear, tending to reach a plateau at about 50% of full sunlight. Slopes of response functions relatin g physiology and biochemistry to ln(FPAR) were not significantly different among species except for the light compensation point, which did not vary i n response to light in ponderosa pine, but did in the other two species. We used the physiological measurements for Douglas fir in a model to simulate canopy photosynthetic potential (daily net carbon gain limited only by PPF D) and tested the hypothesis that allocation of carbon and nitrogen is opti mized relative to PPFD gradients. Simulated photosynthetic potential for th e whole canopy was slightly higher (<10%) using the measured allocation of C and N within the canopy compared with no stratification (i.e., all foliag e identical). However, there was no evidence that the actual allocation pat tern was optimized on the basis of PPFD gradients alone; simulated net carb on assimilation increased still further when even more N and C were allocat ed to high-light environments at the canopy top.