Comparison of photosynthetically active radiation and cover estimation for measuring the effects of interspecific competition on jack pine seedlings

Using an additive-density study with 10 woody and herbaceous plant species of the boreal forest, we compared equations predicting the biomass growth of jack pine(Pinus banksiana Lamb.)seedlings using measurements of photosynthetically active radiation (PAR) transmission and visual estimates of vegetation cover. Nonlinear regression equations accounted for between 77 and 83% of the variation in the annual biomass increment of jack pine when all competing species were combined. Our objective was to determine whether measuring PAR provided any advantage over cover estimation for predicting the effects of interspecific competition. We found no difference in the predictive ability of visual cover estimates and either a single PAR measurement at the point of maximum canopy development or repeated PAR measurements throughout the growing season. Results were consistent among experimental plots that contained a variety of competing plant species with different growth forms. Measurements of PAR transmission once during the growing season performed as well as PAR measurements taken throughout the growing season. Although the precision of plant cover estimates in vegetation sampling has been a concern for some time, high variation in PAR measurements can be created by sensor location sensitivity, narrow sampling windows, and the need for sensor calibration.