Trees are defined as plants with a woody stem that rise over 10 feet (3.2 meters) in height. Trees are the dominant plant form of the Lowland Pacific Northwest.  Conifers have several advantages in our climate. They retain their leaves year around and so can continue to photosynthesize during warmer winter days. The needle-like leaves of conifers have a thick waxy coating which reduces the amount of evaporation from the leaf, and they are full of resins and alkaloids which make them unpalatable to many insects. The spiraled pyramidal shape of the crown allows the trees to grow densely together, and this creates a dark understory which eliminates much of the competition.

Only a few trees, such as hemlocks, yews or vine maples can successfully establish seedlings in the dark understory and so if left undisturbed for several hundred years, eventually a thick forest of western hemlock would establish a dominant community. However, disease, windstorms, and fires open up the forest canopy to produce a matrix of trees and plants.

Conifer trees form a life supporting connection with fungi, called mycorrhiza where the trees root hairs become connected to the tiny mycelia fibers of various species of fungi.  In this partnership the fungi gets plant sugars and in exchange provides the trees with water, nutrients and some anti-bacterial protection.  This partnership is especially crucial to young seedling trees, whose need for water and nutrients often exceeds the capacity of their tiny root systems.  Some fungi only associate with certain trees, while others will associate with many kinds of trees and any given tree can have several species of fungi attached.  It is estimated that as much as 40% of the product of tree photosynthesis ends up being pumped out the roots into the underground matrix of fungi. Radioactive tagged elements attached to tree sugars in the crowns of trees have been found more than 100 meters away in the soil, and also in other trees, so apparently the underground matrix of fungi transports materials from tree to tree. In this way, perhaps, a young tree is "fed" by the older trees around it, until such time it can produce resources for the network.

The life spans and size of conifers is impressive, with large old growth cedars reaching over 1,000 years in age, and the tallest of Douglas firs exceeding 360 ft. However, there are few places left in the lowlands where trees this size and age can be found. Extensive logging has reduced most of the forests here to second or third growth trees.  Most the trees around us are less than 75 years old, and many of the lowland forests are owned by timber interests which  grow genetically modified Douglas fir for harvesting every 30-40 years.  Since Douglas fir is the tree of choice for its fast growth and strong, light wood, and since this tree requires direct sunlight in order to grow, vast acres of forest are cut all at once, called clearcuts. These are then replanted with nursery grown seedlings in a monoculture crop. The resulting tree farm forms a forest that has little diversity, and thus offers much less habitat for wildlife species.

With the exception of Vine maple, and English holly, most deciduous trees are forced to the edges or within clearings where there is sufficient light in order to make a living.  Deciduous trees are shorter lived than conifers, some transitional species such as Alders rarely live more than a century, although some large maples can live for a couple hundred years.  The leaves that fall from deciduous trees are quickly recycled in the forest soils and support a large diversity of insects. Usually by the time fall arrives, much of the leaf mass in any given tree is full of holes, where nibbling insects have taken their toll.  Because of the larger diversity of insects that eat them, deciduous forests tend to support a larger diversity of bird species than the darker, and less palatable conifer forests.