During the past century, forest structure on south-facing slopes of Mount Constitution,
Orcas Island, Washington, has changed from open-grown Douglas-fir (Pseudotsuga
menziesii) mixed with prairie to primarily closed canopy forest. Density of open-grown
Douglas-fir was approximately 7 stems/ha in the 19th century, while current density of
trees in closed-canopy mature forest is 426 stems/ha. Trees occur at intermediate
densities in areas of transition from savanna-like stands to closed canopy. Analysis of fire
scars indicates that at least seven fires have occurred on Mount Constitution since 1736,
but only one fire has occurred since 1893, which suggests that the recent increase in stem
density has been caused primarily by fire exclusion. The high stem densities currently
found in this landscape put the relict (120-350+ years old) Douglas-fir at risk from
contemporary fires, which would likely be high-intensity crown fires. Given the
transition of forests on Orcas Island during the 20th century to closed canopy structure,
undisturbed open-grown coniferous forest is now extremely rare in the San Juan Islands.
