Attenuation of cosmic ray flux in temperate forest

Forests alter secondary cosmic radiation (CR) to the ground by (1) diminishing it owing to absorption by trees, (2) inducing spatial and temporal variability because biomass distribution is heterogeneous, and (3) lengthening the apparent mean attenuation length at ground level because nucleons are shielded over muons. We model CR flux through three‐dimensional simulated forests with properties drawn from old‐growth plots in Nova Scotia, Canada (Acadian forest) and the Olympic Peninsula, Washington state (coastal rain forest). For exposure durations of ≥10 5 , conservative mean shielding in rain forest is 7.3 ± 2.3%, canopy and floor biomass included. Acadian/boreal forest has mean shielding 2.3 ± 0.6%. These long‐timescale mean values are similar to previous estimates from treating forest biomass as a layer of constant thickness. Ground flux varies significantly between sites within a forest, ranging from 1 to 100% of nonforested flux for short timescales if some trees are large (diameters ≥1.5 m) because the position of the sample site relative to individual large trees is important. Temperate rain forests have large trees and disturbance/regeneration intervals approaching 10 3 y; hence, CR flux and resultant terrestrial cosmogenic nuclide (TCN) concentrations vary by 1.5% after 8000 y but only 0.2% after 80,000 y. These results are for a forest that is statistically uniform through time; changes in biomass heterogeneity through time and/or space, owing to climate, wind‐throw, or localized recruitment, would increase the inherent variability of TCN production. TCN dating experiments on timescales much shorter than 80 secondary successions of the forest will have significant uncertainty in effective production rates but catchment‐wide average erosion experiments will not.