The Wet Alder complex is dominated by Sitka alder, but often includes mountain alder and low shrubs such as thimbleberry and black twinberry. Common understorey herbs are lady fern, spiny wood fern, bracken, bluejoint, and fireweed. It occurs predominantly on wet, rich sites, and seepage areas. In southern interior British Columbia, the Wet Alder complex is most common in the ESSF and ICH biogeoclimatic zones, and areas that are transitional between the two. It also has limited occurrence in the MS zone.

Sitka and mountain alder are often well-developed in gaps in undisturbed forests, and  following logging, they spread vegetatively in response to increased light availability. This is particularly true following winter logging, which inflicts little damage on existing alder clumps. However, summer logging, mechanical site preparation, and burning also promote development of this complex by stimulating root sprouting and creating a favourable seedbed. Both Sitka and mountain alder produce abundant airborne seed and readily colonize disturbed areas, especially where mineral soil is exposed.

The Wet Alder complex develops relatively quickly following harvest: within 3-10 years it can form a continuous thicket that drastically reduces the amount of solar radiation reaching conifer seedlings and the soil surface. The rich, moist conditions where alder thrives are also conducive to abundant herb growth, particularly in areas that are not fully occupied by shrubs. Sitka alder reaches a maximum height of about 5 m and mountain alder can grow as tall as 10 m. Understorey vegetation can reach 2-3 m in height.

Where abundant, the Wet Alder complex can negatively affect seedling performance by competing for light and soil resources, by inhibiting soil and air warming, and by causing physical damage through vegetation- and snow-press. However, it is important to remember that the Wet Alder complex often develops on sites where poor seedling performance is related primarily to the effects of cold, wet, fine-textured soils on seedling root development, rather than to vegetation competition. The presence of some alder can benefit seedlings by increasing the availability of nitrogen and by improving long-term site productivity.

The Wet Alder complex after 22 years of development.