Douglas-firs are medium-size to extremely large evergreen trees, 20–100 metres (70–330 feet) tall (although only Pseudotsuga menziesii var. menziesii, common name coast Douglas-firs, reach heights near 100 m)[9] and commonly reach 2.4 m (8 ft) in diameter,[10] although trees with diameters of almost 5 metres (16 feet) exist.[11] The largest coast Douglas-firs regularly live over 500 years, with the oldest specimens living for over 1,300 years.[12] Rocky Mountain Douglas-firs, found further to the east,[13] are less long-lived, usually not exceeding 400 years in age.[14]
There are records of former coast Douglas-firs exceeding 120 metres (390 feet)[15][16] in height, which if alive today would make it the tallest tree species on Earth. Some particular specimens that exceeded 400 feet tall were the Lynn Valley Tree and the Nooksack Giant.
The leaves are flat, soft, linear needles 2–4 centimetres (3⁄4–1+1⁄2 in) long, generally resembling those of the firs, occurring singly rather than in fascicles; they completely encircle the branches, which can be useful in recognizing the species. As the trees grow taller in denser forest, they lose their lower branches, such that the foliage may start as high as 34 m (110 ft) off the ground.[17] Douglas-firs in environments with more light may have branches much closer to the ground.
The bark on young trees is thin, smooth, grey, and contains numerous resin blisters. On mature trees, usually exceeding 80 years,[citation needed] it is very thick and corky, growing up to 36 cm (14 in) thick with distinctive, deep vertical fissures caused by growth. Layers of darker brown bark are interspersed with layers of lighter colored, corky material.[18] This thickness makes the Douglas-fir perhaps the most fire-resistant tree native to the Pacific Northwest.[19]
The female cones are pendulous, with persistent scales, unlike those of true firs. They have distinctive long, trifid (three-pointed) bracts which protrude prominently above each scale and are said to resemble the back half of a mouse, with two feet and a tail.[20] The cones are tan when mature, measuring 6–10 cm (2+1⁄2–4 in) long for coastal Douglas-firs and a couple of centimetres shorter inland.[13]
The massive mega-genome of Douglas fir was sequenced in 2017 by the large PineRefSeq consortium, revealing a specialized photosynthetic apparatus in the light-harvesting complex genes.[21]
