Seasonal balance and vertical pattern of photosynthetically active radiation within canopies of a tropical dry deciduous forest ecosystem in Mexico

Major components of the flux density of global photosynthetically active radiation (PAR) were measured above and within canopies in a tropical deciduous forest on the Pacific coast of Mexico. At each of 69 locations grouped along a topographic sequence the PAR reflected from the top of the canopy. the vertical profile of transmittance, and the reflectance from the ground, were measured as many as four times in the year, including the extremes of the wet and dry seasons. With these observations an annual balance of the portion of PAR radiation reflected and absorbed by the canopy and ground was assembled and the detailed spatial and temporal dynamics of PAR within canopy layers were estimated. Canopy stature declined along the topographic sequence and the shape of the transmittance profiles reflected this. In locations of declining moisture availability the fraction of PAR absorbed by the ground increased and the fraction absorbed by non-foliar tissues decreased. Seasonal variation in canopy structure was the dominant influence on the partitioning of radiation - spatial variation was less important. Of a total annual PAR input of 15 200 mot m(-1), about 95% of incident PAR was absorbed, 50% by leaves, 25% by non-foliar tissues and 20% by the ground. The remaining 5% was reflected by the top of the canopy.