Evergreen Coniferous Forests of the Pacific Northwest

Reprint Series 29 June 1979, Volume 204, pp. 1380-1386 IE Evergreen Coniferous Forests of the Pacific Northwest R. H. Waring and J. F. Franklin Copyright © 1979 by the American Association for the Advancement of Science maintaining the conifer gene pool in the Pacific Northwest (2). Environmental features have also been proposed as major factors in conifer dominance. Chaney et al. (3) suggest Evergreen Coniferous Forests that arid periods caused hardwood loss- es. Daubenmire (4) identifies cool sum- of the Pacific Northwest mers, coupled with the inability of deciduous hardwoods to utilize frequent warm days in spring and fall. Regal (5) pro- Massive long-lived conifers dominating these forests posed that gymnosperms survive as are adapted to a winter-wet, summer-dry environment. dominants only in environments that are, in some way, harsh or rigorous; however, he concedes uncertainty as to how R. H. Waring and J. F. Franklin the coniferous forests of the Pacific Northwest conform to this hypothesis. The steep mountainous topography of coastal northwestern America might sug- Along the Pacific Coast of north- ocarpus), chinquapin (Castanopsis), gest youthful, thin soils as a factor, but western America, the dominant vegeta- maple (Acer), oak (Quercus), and alder regional soils are, in fact, at least as deep tion consists of dense forests of ever- (Alms)-which can achieve some local and as fertile as those of other temperate green conifers, which clothe the land- importance. However, conifer-hard- forest regions. Hence, while harsh cli- scapes from northern California to the wood roles in the Pacific Northwest mates, thin soils, and periodic wildfires panhandle of Alaska (Fig. 1). The forests largely are the reverse of those in eastern are probably factors in the development are unrivaled both in the size and longev- North America. For example, north- of the conifer forests of intermountain ity of individual trees and in the accumu- western hardwoods often play pioneer western North America in the rain shad- ow of the coastal mountains, these factors fail to explain conifer dominance in Summary. The massive, evergreen coniferous forests in the Pacific Northwest are the coastal region. unique among temperate forest regions of the world. The region’s forests escaped Knowledge of the structure and func- decimation during Pleistocene glaciation; they are now dominated by a few broadly tion of the northwestern coniferous for- distributed and well-adapted conifers that grow to large size and great age. Large est has been greatly extended by studies trees with evergreen needle- or scale-like leaves have distinct advantages under the conducted as part of the International Bi- current climatic regime. Photosynthesis and nutrient uptake and storage are possible ological Program. In this article, we ap- during the relatively warm, wet fall and winter months. High evaporative demand dur- ply these data to examine the relative ing the warm, dry summer reduces photosynthesis. Deciduous hardwoods are re- merit of evergreen conifer and deciduous peatedly at a disadvantage in competing with conifers in the regional climate. Their hardwood habitats under existing climat- photosynthesis is predominantly limited to the growing season when evaporative de- ic conditions. Because regional climatic mand is high and water is often limiting. Most nutrients needed are also less available regimes have been similar for several at this time. The large size attained by conifers provides a buffer against environmen- epochs, we propose that the advantages tal stress (especially for nutrients and moisture). The long duration between destruc- enjoyed by evergreen conifers in such tive fires and storms permits conifers to outgrow hardwoods with more limited stature environments have been key factors in and life spans. competitively eliminating much of the original hardwood flora. In addition, we document the large biomass and produc- lations of biomass of individual stands. roles [such as red alder (Alnus rubra)] or tivity values of the forests and suggest Furthermore, the massive evergreen occupy habitats whose environmental how massiveness is advantageous. canopies of these forests contrast with features significantly differ from the re- the deciduous hardwood or mixed hard- gional norm [for example, Oregon white wood-conifer stands typical of the North oak (Quercus garryana) on droughty Paleobotanical Record Temperate Zone. habitats]. The degree of conifer dominance is im- What factors favored the evolution of During the early and middle Miocene pressive. In the Cascade Mountains and these massive, conifer-dominated forests 18 to 28 million years ago, more than 40 Coast Range of the Pacific Northwest, in contrast to the deciduous hardwood genera of woody dicotyledons extended the biomass of conifers is 1000 times that forests in other temperate regions? Sci- from Oregon north through Alaska and of the hardwoods. Of the 25 coniferous entists since Von Humboldt in the mid- Siberia to Japan (6, 7). A pure coniferous species in these forests many represent 1800’s have speculated on this topic. Al- forest existed only in the uplands, above the largest, and often the longest-lived, though some have suggested that cold 500 meters in Japan and above 700 me- of their genera (Table 1). Still represent- temperatures during glacial epochs elimi- ters in Oregon. Thus, the areas domi- ed are arboreal evergreen and deciduous nated many hardwood genera (1), this is nated by conifers-mostly fir, spruce, taxa-such as species of tan oak (Lith- clearly not the case. Most hardwood ex- and hemlock-were highly disjunct dur- tinction actually occurred during the ing the early and middle Miocene in Dr. Waring is professor of forest ecology in the Pliocene, much earlier than the Pleisto- School of Forestry, Oregon State University, Cor- northwestern North America and north- vallis 97331. Dr. Franklin is chief plant ecologist at cene when relatively mild environments eastern Asia (7, 8). the Forestry Sciences Laboratory, U.S. Department of Agriculture Forest Service, Corvallis, Oregon and good north-south routes for propaga- By the late Miocene 12 to 18 million 9733 1. tion may have been major factors in years ago, coniferous forests began to 1380 0036-8075/79/0629-1380$01.75/0 Copyright © 1979 AAAS SCIENCE, VOL. 204, 29 JUNE 1979 occupy large areas in the uplands. Floras today. Major components of the original (Fig. 2A). In other temperate forest re- at intermediate elevations throughout widespread deciduous hardwood forest gions, summers are typically hotter and the western United States contained still persist in Japan, China, Europe, and more humid, and the winters are much moderate to large amounts of fir, spruce, parts of the eastern United States. Their colder. During the growing season in the and hemlock (6). For the first time, a extinction from the Pacific Northwest Pacific Northwest, night temperatures coniferous forest extended continuously probably was related to changes in cli- usually remain below 120C, often drop- from the uplands of Oregon northward mate that favor the conifers present ping to 10°C near the coast or along cold through British Columbia and into today. air drainages in the mountain valleys. Alaska (7). Dew may form on cool nights, but clear During the late Miocene or early Plio- warm days cause the water to evaporate cene some 10 to 12 million years ago, a Present Climate quickly, resulting in evaporative de- rich boreal forest of spruce, pine, and mands much higher than those experi- hemlock-with smaller quantities of Climatically the region experiences enced at similar temperatures in other larch, fir, beech, oak, and elm-was es- wet mild winters and warm dry sum- temperate forest regions. Past regional tablished in northeastern Siberia (9). A mers. The dormant season, when shoot comparisons have underestimated evap- similar trend was occurring in Alaska. growth is inactive, is characterized by orative differences by using a simple However, in Oregon, the early Pliocene heavy precipitation with daytime tem- estimate of potential evaporation (11) floras west of the Cascade Mountains peratures usually above freezing. Away that does not consider differences in hu- contained an impoverished deciduous from the coast, the growing season is midity (12). This method for assessing flora with hickory, elm, and sycamore characterized by warm temperatures, evaporation in the Pacific Northwest still represented (3). Thus, more hard- clear days, and little precipitation. Water leads to values 25 to 60 percent too low wood species became extinct during the storage in snowpack, soils, and vegeta- for July and August. late Pliocene than during any period tion-as well as pulses of fog, clouds, or Another significant climatic difference since. cool maritime air which reduce evapo- from other temperate forest regions may By the early Pleistocene, some 1.5 mil- transpiration-obviously are more im- be the absence of typhoons and hurri- lion years ago, and before major glacia- portant during a summer drought. canes that frequent eastern Asia and tion, the flora of the Pacific Northwest The climate varies considerably as a eastern North America. Frequent de- was essentially established as it appears consequence of the interplay between structive storms presumably would in- maritime and continental air masses and hibit evolution toward massiveness, re- mountain ranges. Along the coast where gardless of other potential benefits (13, the maritime influence is strongest, mild 14). temperatures are associated with pro- longed cloudiness and narrow diurnal and seasonal fluctuations (60 to IO0C) in temperature. Winters are extremely wet, and freezing temperatures are rare. Sum- mers are cool and relatively dry, but extended periods of cloudiness and fog often greatly reduce evaporation. Valleys in the lee of the Coast Range are drier, subject to greater temperature extremes and evaporative demand as are the lower elevation sites in the Cascades. On the western slopes of the Cascade Moun- tains, precipitation increases, and temperature regimes moderate until sub- alpine environments, with their cooler temperatures and deep winter snow- packs, are encountered.

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