The vegetation in Sitka National Historical Park is a coastal temperate rainforest typical of southeast Alaska. Western hemlock closed-canopy forest type is found on all the stable landforms, including most of the park. Blueberry and devil's club dominate the shrub layer. Occasional Sitka spruce are found in the overstory, frequently taller than the hemlocks. Much of the park is second-growth hemlock (100-125 years in age) that shows an overstory of relatively uniform height. In areas where the canopy has been opened through blowdown of trees, there is a well-developed shrub understory. Under the denser canopies there are fewer shrubs and limited forbs and ferns.
Extensive blowdowns may also result in a dense canopy as is the case northwest of the fort site. This area was hit by an intense wind storm in 1967. A dense even-aged stand of young hemlocks have grown up, greatly reducing sunlight penetration to the forest floor.
The largest trees are in the forest in the northeast corner of the park. This forest exhibits old-growth characteristics such as multiple canopy layers, trees of varying diameters, snags (dead standing trees), and woody debris. One Sitka spruce is close to 500 years of age. Old cut stumps in this area suggest that selective logging may have been practiced here in the past.