Relationships Between Sunfleck Dynamics and Red Fir Seedling Distribution

Abstract

Red fir (Abies magnifica) Seedings show a patcy distribution in much of the species' range. Patterns of irradiance in the understory of a red fir forest in the central Sierra Nevada Mountains of California were measured in an effort to determine whether a correlation existed between the irradiance patterns on the forest floor and the distribution of seedlings. Replicate pairs of plots ona south exposure with significant differences in seedling density were compared with each other and with plots on a contiguous north exposure. No significant differences between plots or south exposures were found for either physical characteristics, soil chemistry, water, or liter layer traits. On the south aspect, plots with few seedlings had mean daily irradiances 2.1 times higher than plots with many seedlings, primarily due to 3.5 times more frequent occurrence of sunflecks at irradiance levels above 1025 μE°m — 2 °s — 1 . Analysis of solor tracks on fish—eye—lens photographs of overstory canopies and measurement of sunflecks on the forest floor indicated that plots with few seedlings had larger sunflecks, particularly during midday hours. In contrast, seedling distribution on the north exposure was not correlated with patterns of irradiance or temperature, apparently because of less intense direct—beam irradiance on these slopes. Apparently the high irradiance on open south slopes is inhibitory to red fir seedlings compared with the low—intensity irrandance on more shaded south slope sites and on all north slope sites. Phonon flux densities >500 μE°m — 2 °s — 1 are beyond the light saturation point for net photosynthesis, as determined in the laboratory from field—collected seedlings. Absorption of light >500 μE°m — 2 °s — 1 by seedlings will substantially increase the energy load without corresponding increases in carbon gain. We suggest seedlings are absent from high—light plots because of temperature and water stresses induced by higher irradiances early in the growing season.