— Red Alder: A State of Knowledge — Biology and Ecology of Red Alder

Red alder is the most common hardwood in the Paciﬁc Northwest with a range stretching from coastal southeast Alaska to southern California and east to isolated populations in Idaho. Soil moisture during the growing season inﬂuences where it grows and its growth rates; it can tolerate poor drainage but not droughty, hot sites. Due to its tolerance of wet soil conditions, alder is common in riparian areas. Alder can be injured by spring and fall frosts and is not found at elevations above 1100 m anywhere in its range. The species produces small, very light seeds that disperse over long distances; it is favored by disturbance and often increases in abundance after logging or burning. stems alder rots do not result in high volumes of damage overall but can be locally important; ring shake can also be a serious problem in some stands. Insects and diseases are not generally a problem in young stands although insect defoliators, Nectria cankers, and alder bark beetles can cause problems. Meadow mice, voles, and beaver can also hinder stand establishment in some areas. Deer or elk may rub small alder saplings during the fall with their antlers but do not usually browse its leaves and twigs. Sapsucker damage is sporadic, but if stems are repeatedly damaged, their future log value will be greatly reduced. Mortality and top damage have been observed after ice or early snowstorms. Red alder is now being managed for a wider range of objectives and on many more sites than in the past; this is due to recent increases in wood value of alder relative to other species as well as its ability to ﬁll specialized niches (e.g., add nitrogen to forest ecosystems, immunity to laminated root rot) and produce sawlogs on relatively short rotations. Management experience with the species, however, is still limited to a fairly narrow range of sites and management scenarios. Thus, information on the biology and ecology of the species should help guide managers until more direct experience is available. Abstract This paper synthesizes information on the development of natural pure red alder stands and dynamics of mixed alder-conifer stands. Early research on red alder growth and yield focused on developing stand volume and normal yield tables for alder in the Paciﬁc Northwest. Recent site-index estimation and height-growth curves were developed on a 20-year site base age. These height-growth and site-index curves were a signiﬁcant improvement over earlier work and are widely used today. Red alder exhibits rapid early height growth with much more rapid height growth than for conifer associates. On good sites, trees may be 9 m at age 5, 16 m at age 10 and 24 m at age 20. Height growth then quickly declines, and by age 15 alder will reach more than half its total height and nearly all of its mature height by the age of 40. Alder height growth essentially stops by age 50. Long-term successional sequences of red alder stands are not well understood. Red alder frequently occurs in mixed stands with conifer associates including Douglas-ﬁr, western hemlock, Sitka spruce and western redcedar. Mixed species dynamics is more complex and variable than for pure stands and depends on a number of factors including forest composition, establishment and survival of understory of alder mortality, and composition of shrubs, Red alder is short lived (approximately 100 years) compared with its conifer associates. In some sites, alder may succeed to brush communities with few if any conifers to replace the alder. In other sites, succession to more shade tolerant trees such as western hemlock and Sitka spruce is certain but the sequence of events is unknown. On some nitrogen deﬁcient sites red alder has been shown to increase growth of conifer associates but in other cases red alder has been shown to suppress and even kill less shade tolerant conifers. Other conifer species such as western hemlock, Sitka spruce and western redcedar can survive as lower strata species and eventually grow above the alders but it may take several decades for this to occur. Height growth and long-term stand dynamics in mixed stands are highly variable and long-term succession is difﬁcult to predict. More work is needed to understand the long-term successional sequence of events.