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This study assessed whether allometric scaling applied to pit sizes in stems of Douglas fir. Pit and pit aperture diameters were measured in xylem from stems of four plantation-grown Douglas fir (Pseudotsuga menziesii) trees from each of two different sites in Denmark. One site had fertile soil, the other had poor soil fertility. Three different heights were accessed for each tree, and 40 pits were measured per height. Results showed that pit diameter varied between 17 and 24 μm and decreased significantly with increasing height above ground. Representing the position in the tree as a power function of distance from the top of the tree (L0.2) rather than the height above ground improved the model fit for pit diameter as expressed by the R2 value. However, the pit diameter relationship was found to be significantly affected by site, suggesting that anatomic dimensions relate not only to tree size but also to growth conditions. This would imply that even though pit size supposedly has a strong biophysical determination, some xylem plasticity may still be induced by environmental factors.