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Revista Chilena de Historia Natural 64: 399-412, 1991 Comparative ecology of temperate rainforests of the Americas along analogous climatic gradients

Ecologfa comparativa de los bosques lluviosos templados de Norte y Sudamerica a lo largo de gradientes climaticos analogos

PAUL B. ALABACK

U.S. Forest Service, Pacific Northwest Research Station Post Office Box 20909, Juneau, AK 99802 USA. (Fax: 907-586-7848)

ABSTRACT

A definition for and its applications to world forest type distribution is presented. Southern and the Pacific coast of North America are the largest expanses of temperate rainforest. Although both regions have important similarities in climate, dramatic differences in the composition of flora and fauna are evident. An- alogous forest types occur at similar levels of summer rainfall in both zones. In Chile sea-level glaciers and associated subantarctic vegetation types begin at 10-15 degrees lower latitude than in North America. Biomass accumulations achieve world record status in coniferous forest types in both regions, although forest types are rare in Chile. Structure and composition of forests in both zones reflect chronic disturbance by and rain, and periodic catastrophic dis- turbance by intense storms, landslides, or vulcanism. Ecological analogs exist for many common species. Detailed comparative studies are needed between the principal rainforest regions of the world to more clearly identify how biogeographical, climatic and historical factors have influenced evolution of forest structure and function. The prospect of global climatic change also underscores the need for comparative data on ecosystem processes along analogous climatic gradients, especially at high latitudes where the most pronounced changes are expected to occur. Key words: Temperate rainforests, climate, biogeography, plant ecology, , Nothofagus, Picea.

RESUMEN

Se presenta una definicion de bosque lluvioso templado y su aplicaci6n a Ia distribucion geografica mundial de bosques. El sur de Chile y Ia costa noroeste de Norteamerica tienen las áreas de bosques lluviosos templados más grandes en todo el mundo. Ambas regiones son semejantes en clima, pero son muy diferentes en cuanto a Ia composicion de especies de plantas y animales. En ambas regiones los tipos de bosques analogos están en lugares que reciben Ia misma cantidad de lluvias durante el verano. En Chile, los glaciares adyacentes a! mar, y Ia mayorfa de los tipos de bosques análogos comienzan 10-15 grados de latitud antes que en Norteamerica. La acumulacion de biomasa más alta de todo el mundo ocurre en bosques de coniferas de ambas regiones, aunque los bosques de coniferas son escasos en Chile. La estructura y composicion de los bosques de ambas regiones son una funci6n de perturbaciones frecuentes causadas por vientos y lluvias y de perturbaciones más raras causadas por tempestades fuertes, derrumbes, avalanchas, o volcanismo. Exis- ten análogos ecologicos para muchas especies de plantas en ambas regiones. Estudios comparativos de bosques lluviosos del mundo son necesarios para entender cómo Ia biogeografia, el clima y Ia historia han influido Ia evolucion del funcionamiento y estructura de estos bosques. La posibilidad de cambios en el clima en un futuro proximo también subraya Ia necesidad de obtener datos comparativos sobre los procesos de eco- sistemas a lo largo de gradientes análogos de clima, sobre todo a altas latitudes, en donde se producirian los cambios más significativos de clima y de funcion ecologica. Palabras claves: Bosques lluviosos, clima, biogeografia, Nothofagus, Picea, ecologia, Alaska.

INTRODUCTION and management throughout their geo- graphic range. They include some of the The term temperate rainforest has been longest-lived and massive species as applied to a broad range of forest types well as the largest remaining virgin land- in both popular and scientific publications. scapes outside of the tropics (Franklin These forests have been the focus of in- & Warring 1980, Veblen et al. 1976). tense public debate on both conservation They also occur at high latitudes where their sensitivity tc and history of rapid climatic change make them ideal subjects (Received 23 March 1990.) for monitoring global climatic change. 400 ALABACK

Many critical scientific questions relating to a common vegetational history. A close landscape level processes may be best examination of sites with a temperate studied in temperate rainforests since their rainforest climate located at the far comers fauna and landscapes are more intact than of the earth where vegetation evolved in most other ecosystem types. Important more or less independently might provide decisions on conservation and management some unique insights as to how climate options for these pristine ecosystems influences forest evolution and function. will also require that we understand in Consideration of the disturbance regimes much greater detail how temperate rain- and functional characteristics of these forests respond to both logging and cli- disparate forest types are also needed to matic pertubations. determine what adaptations to the rain- Research emphasis has been much dif- forest environment are shared between ferent in temperate rainforests of the these floristically distinct locales. In par- northern and southern hemispheres. In ticulari would like to ask the question how the northern hemisphere much attention do the two largest temperate rainforests has been paid to the relation of structure regions in the world compare; and to what to function and of the unique attributes degree do historical factors influence pre- of "old-growth" forests (Alaback & Juday sent day ecosystem functioning, as we 1989, Franklin et al. 1981, Graham & Cro- understand them, and what are the research mack 1982, McKee et a/. 1982, Spies opportunities for comparative studies in eta/. 1988, Spies & Franklin 1988, Waring this forest zone? & Franklin 1979). Succession research has focused on post-logging, fire and A definition for temperate rainforest wind caused disturbances (e.g. Alaback 1982, Cline et a/. 1980, Halpern 1989, The term temperate rainforest is not new Harcombe 1986, Kellman 1969) although to ecology, but has been applied to a range some work has been done on revegetation of vegetative types throughout the world following mass movements and vulcanism (Alaback & Juday 1989, de Laubenfels (Dale 1989, Miles & Swanson 1986, Smith 1975, Koppen 1918, Kuchler 1949, Veblen & Commandeur 1986) and fluvial dynamics et a/. 1983, Webb 1978). One of the most (Fonda 1974, McKee et a/. 1982). Much distinctive features of rainforest climate is of the classic work on post-glacial succes- cool summers and wet weather year around. sion was conducted in the northern tem- An ecological consequence of this unique perate rainforest zone of climate is frequent disturbance by wind, (Cooper 1937, Crocker & Major 1955, and the lack of fire as an important factor Reinersetal. 1971). in forest dynamics. In the southern hemi- In southern Chile research has focused sphere lightning caused fires appear to be on gap dynamics of old-growth forests rare events (Veblen eta/. 1983, Wardle et and succession following mass movement al. 1983). Throughout the northern hemi- events (e.g. Armesto & Figueroa 1987, sphere fire plays a key role, either as a Veblen 1989, Veblen eta/. 1981). Little infrequent catastrophic event in humid work has been done on post-glacial suc- regions, or as a chronic event in drier cession, and on the relation of forest climates. Only in the coastal rainforest structure to function (Kunkel 1955, Ve- is fire of minor importance (Harris & blen 1982, Weinberger 1974, Young 1972). Farr 1974). Although many sites spread throughout Rainforests are difficult to distinguish the temperate zone are commonly referred floristically or physiognomically from relat- to as temperate rainforests there have been ed forest types (Cockayne 1971, de Lau- few attempts to compare the ecological benfels 1975, Webb 1968, 1978). The up- characteristics of these widely scattered per canopy is often composed of a large forests. Most comparisons have been made number of species with few in a position of temperate rainforests with climatically of dominance, and form clumped disparate environments, or of forests with patterns reflecting gap-phase disturbance COMPARISON OF TEMPERATE RAINFORESTS 401 regimes (Armesto et a/. 1986). Epiphytes Cloudforests may be less productive and are often associated with rainforests, but shorter in stature than corresponding high they also frequent timberline and arctic latitude rainforests because of their expo- environments with a high frequency of sure to at the high elevations in which fog. they typically occur (e.g. Howard 1969, De Laubenfels (197 5) has proposed pre- Byer & Weaver 1977). sence of a dense and continuous under- Many phytogeographers have distinguish- story layer and the absence of annual ed the needle-leaved forests of the northern herbaceous as key factors distin- hemisphere from the broad- guishing rainforests from seasonal forests. leaved forests of the , Understory plants in many mesophytic implying they are ecologically distinct temperate forests, however, are light limit- entities. Although it is generally true that ed so that their relative abundanc~ may the northern hemisphere forests have reflect more the structure and disturbance cooler winters and that are well history of the upper canopy than regional adapted to these cool temperatures (Spru- climate (e.g. Alaback 1982). Understory gel 1989), climate alone does not consis- density does not work well in distinguishing tently distinguish these forest types. Some seasonal and rainforest types in Pacific conifers occur in southern hemisphere Northwest temperate forests. In Chilean types (e.g. Podocarpaceae, , rainforests understory density varies dra- Araucariaceae) while conifers are exclusive matically with climate largely because of dominants of forests with analogous cli- the requirements of the bamboo (Chusquea mates in the northern Pacific coast of spp, Veblen 1989). North America (Waring & Franklin 1979). As a whole temperate rainforest eco- For northern ecologists it seems to be an systems are quite distinct from those of apparent contradiction to see lush broad- the tropics. Temperate zone forests have leaved evergreen rainforests (suggestive of a proportionately fewer species in the up- subtropical environment) growing adjacent per canopy. Canopy trees tend to have to icefields as in southern Chile and New smaller more coriaceous or even needle Zealand. In part because of this commonly shaped in the temperate rainforest made distinction between northern and formation, although conifers can occur in sothern rainforests few comparisons have some lowland tropical rainforests (New been made of the principal components Guinea, Queensland, Fiji, Borneo, Malaya). of the world temperate rainforest forma- Temperate rainforests have proportionately tion. fewer lianas when compared with the For purposes of discussion the temperate tropics (Webb 1978, De Laubenfels 1975). rainforest climatic zone could be defined Dense mats of mosses and liverworts often with the following four factors: carpet the forest floor and the upper can- opy. • 1) greater than 1,400 mm annual preci- Cloudforests, or high elevation forests pitation, 1o,. or more occurring during in the wet tropics share many characteristics the summer months in common with high latitude rainforests • 2) cool frequently overcast summers, (c.f. Chapman & White 1970, Cockayne July (or austral January) isotherm 1971, Robbins 1961). The lack of seasonal- < l60C ity in growth and higher overstory diversity • 3) fire infrequent, anJ not an impor- may be key distinguishing characters. tant evolutionary factor Soils in cloudforests tend to be better • 4) dormant season caused by low tem- drained and with less accumulation of peratures, may be accompanted by organic matter than high latitude low eleva- transient snow. tion rainforests (Cockayne 1971, Robbins 1961) although much variation occurs in Applying this definition to the temper- relation to rainfall and bedrock type ate forest zone results in a striking global (Chapman & White 1970, Holdgate 1961). pattern (Fig. 1). Large land masses in ge- 402 ALABACK

TEMPERATE RAINFORESTS - 60------Fig. 1: World distribution of temperate rainforests. Ellipses indicate principal regions with temperate rainforests. This figure represents potential maximum {historical) distribution and does not take account of current land-use practices. Distribuci6n mundial de bosques lluviosos templados. Las elipses indican regiones principales con bosques lluviosos. Esta f~gura representa Ia distribuci6n hist6rica (maximo) y no considera las reducciones en area debido a! impacto re- ciente del hombre.

neral have continental climates that in- Geology and vegetation history fluence much of the coastal zone, or at least produce rain-shadows preventing the Southern Chile and the northern Pacific development of rainforest. The principal coastline show a strikingly similar geologic rainforest zones are along the northern history, resulting in a complex maize of Pacific coast of North America, from a fjords, channels, and extensive low eleva- narrow band along the Olympic Peninsula tion ice fields (Fig. 2). Miller et al. (1977) in State ( 460N, to a broad summarized the timing and extent of gla- band reaching the coastal cordillera in cial advance and retreat in the two regions and southeastern Alaska and concluded that they were sufficiently ( 61 ON); and along a similar latitudinal similar to treat them as analogous physical range and physiographic pattern in south- environments (specifically the Mediterra- ern Chile from approximately Valdivia in nean sites). It should be pointed out, the coast and Conguillio in the however, that a major contrast with the (380S) south to western two regions lies in the dynamics of the (550S). All other rainforest zones are in geologic plates. Southern Chile abuts one isolated patches in mid-montane regions of the world most active subduction zones, or in smaller islands, with the possible resulting in high frequency volcanic and exception of western south island, New associated mudflow and landslide dis- Zealand. This global pattern closely matches turbances (Veblen eta/. 1981). By contrast the temperature driven classification of volcanic activity is rare within most of the Koppen (1949) type CFb, except that northern Pacific rainforest zone. Major CFb includes more of central Chile, north- volcanic eruptions are confined to interior eastern Asia (Sakhalin Island, central mountain ranges such as the Cascades and Honshu) and the high elevation Notho­ the non-forested arctic terrain · in the fagus forests of New Guinea. Aleutian islands and western Alaska. COMPARISON OF TEMPERATE RAINFORESTS 403

J ,; TEMPERATE RAINFORESTS I II 30 ,/I I ' ''.,, 0-G NORTH AMERICA I 25 ',. e--·e SOUTH AMERICA (f) w 43 u 20 w 0.. (f) 15 w w a:: 10 1- 5 NORTH 0~------~----~------+------~ 40 45 50 55 60

LATITUDE, a

56 Fig. 3: Tree diversity patterns within the rainforest regions of Chile and North America. Includes total CHILE number of taxa within each geographic region per degree oflatitude. - RAINFOREST Modelos de la diversidad de arboles dentro de las regiones de bosques lluviosos de Chile y de Norteamerica. Se indica el numero total de especies dentro de cada region por grado de latitud.

Fig. 2: Distribution of rainforests in North America nities along the Chilean coast (Soto 1991 ). and Chile. Included in the general rainforest Perhaps because of the narrowness of the regions are non-forest types such as alpine, bog, and permanent snowfield regions. Glaciers noted peninsula of southern South America ' on each coastline represent the equatorial-most glacial advances or some related biogeo- tidewater glaciers. graphical factors have made more effective barriers to species migration in South Distribucion general de bosques lluviosos en Norteame- rica Y en Chile. Las areas de bosque tambien incluyen en America than in corresponding regions in part~ otros tipos de vegetacion, como praderas, pantanos, North America, despite less ice advance glacmres, etc. Los glaciares indicados en cada costa representan la latitud mas baja de un glaciar que alcanza in South America and widespread vulca- el nivel del mar. nism (Markgraf 1989, 1990)*. Diversity of rainforest shrubs and herbs does not show a cldar pattern in relation The presence of glaciers and icefields to climate. Many species persist along may be a key factor constraining species the entire range, sometimes becoming more richness in the coolest portions of each important at high latitudes due to the di- coastline (Fig. 3). Contemporary forest minished density of overstory dominants communities have existed in many pole- (Alaback & Juday 1989). ward parts of these coastlines for less than Although the floras of Chile and the 3,000 years. At the highest latitudes in northern Pacific coast of North America both coastlines only 5 plant species can evolved independently a significant number become forest trees. Tree species richness of species occur principally in these two of the northern Pacific coast monotonical- ly increases until reaching the limit of the locations or have close analogs in each (Hul- rainforest zone or the forest zone in the ten 1968, Moore 1983). The fossil record south. In South America by contrast tree shows no evidence that the common species richness remains at around 5 species tree genera in southern South America until passing the principal icefield regions after which tree species richness rapidly * MARKGRAF V (1990) Quaternary history of tem- increases, generally exceeding the level perate ecosystems in North and South America. of forests in comparable climates in the Presented at the Workshop Comparative studies of North and South American temperate ecosystems: northern Pacific. A similar pattern has theory, approaches and limitations. Termas de Chi- been observed in freshwater algal commu- ltan, Chile, January 5-10, 1990. 404 ALABACK

ever occurred in North America or vice Chilean sites resemble the temperature versa. Yet several herbaceous species occur patterns of the Washington coast but with only in the high latitude temperate rain- the rainfall patterns of coastal Alaska. forests of North America and southern In both regions rugged topography Chile (Heckard 1963, Moore 1972, Raven causes a wide range in temperature and 1963). Over 20 species of plants occur in rainfall over short distances (U.S. Weather both Tierra del Fuego and the Pacific Bureau 1989, Weinberger 1974). The high coasts of North America. Of these at least degree of local variation in precipitation 7 occur only in these two locales (the in the rugged terrain of both coastlines others having large distributions in Europe makes interpretation of rainfall data dif- or Asia) including species of Osmorhiza, ficult at best. In Juneau at sea level, for Fragaria, and Carex. A leading hypothe- example, annual rainfall varies by a factor sis is that these species are products of of 2-3x within a 4 km radius. In both long distance dispersal by birds (e.g. regions however, heavy rainfall and low terns, plovers, other shore birds). The temperatures combine to produce among ability of these species to fluorish iri both the highest runoff rates in the temperate environments provides further evidence of zone (Di Castri 1976). This high runoff the comparability of the environments in rate leads to rapid rock weathering and both rainforest regions. soils formation, and to high frequencies of landslides and other mass wasting types Climate of erosion (Alexander 1988, Bishop & eta/. Numerous authors have been struck by the Stevens 1964, Veblen 1981). climatic similarity of Chile and western Both Chilean and northern Pacific coast- North America (Mooney et al. 1977, lines appear to have a steady decline in Moldencke 1976). In the rainforest region growing season temperatures which is the comparison is not nearly as precise as closely related to species richness and tree it is for the Mediterranean zones (lnstituto productivity (Farr & Harris 1979, Hold- Geognifico Militar 1979, Franklin & Dyr- gate 1961). In the southern hemisphere ness 1973, Farr & Harris 1979). Precipita- climates tend to be more maritimes so that tion magnitude and seasonal pattern appear cool climates begin at much lower latitudes to be comparable, although the lack of than in North America. The rainforest climatic stations in exposed coastline of North America begins at nearly 100 locales in Chile make precise comparisons higher latitude than it does in South Ame- highly tentative (Fig. 4). Overall, the rica. In Chile tidewater glaciers begin at only 4SOS, or nearly 120 lower in latitude

500.------, than tidewater glaciers in Alaska (Fig. 1!.-1!. LONG BEACH 45• N 2). Despite these cool summer temperat- 0-0 JUNEAU 58" N ures, however, winter temperatures are ~ 400 - EASTERN USA z much higher throughout the Chilean coast- 0 300 line than in most of the northern Pacific ~ rainforest. Growing season temperatures, o_ 200 appear to be much more closely matched u w between the two regions (Fig. 5). The a::: 100 CL monthly temperatures for Long Beach, Washington and Puerto Montt, Chile for JFMAMJJASOND example are nearly identical. MONTH (northern equiv.) Presumably growing season temperatures are more important to productivity and Fig. 4: Rainfall patterns of selected sites in the rainforest region of North America and South related ecosystem nutrient budgets than America. Based on 20 year averages. are winter temperatures making compa- rison between these regions an appropriate Modelos de Ia distribucion de las lluvias de algunas locali- dades dentro de las regiones de bosques lluviosos de Chile one. Winter phtosynthesis can, however, y de Norteamerica. En base a promedios de veinte aiios. play a significant role in carbon uptake COMPARISON OF TEMPERATE RAINFORESTS 405

30,------, in .A.-.A. PUERTO MONTT 41 o S podzol formation temperate rainforests 25 'Y-'Y PUNTA ARENAS 5Y S (Alexander 1988, Bowers 1987, Holdgate ~ +-+ PUNTA DEL A 52° S 1961). Heavy accumulations of organic matter and nutrient immobilization lead w 20 0:::: to the development of a hardpan layer, ::::) 15 impeding water drainage (Ugolini & Mann ~ 1986). Without chronic disturbance, soils 0:::: 10 w in southeast Alaska are hypothesized to ()_ develop into waterlogged acidic, peat- ::::2: 5 w like soils which can only support stunted f- 0 0-0 JUNEAU 58" N bog-like forest plants. The causes of bog EASTERN USA formation and the determinants of the ecotone between bog and forest have JFMAMJJASOND been long debated in both hemispheres MONTH (northern equiv.) (Hennon 1986, Holdgate 1961, Neiland 1971). A leading hypothesis is that the Fig. 5: Mean monthly temperatures in the rain- balance between bog and forest may forest region of North America and South America. reflect changes in regional climates (Win- Based on 20 years averages. kler 1988). In both regions cupressid tree Temperaturas promedio mensuales de algunas localidades species have rapidly diminished their former dentro de las regiones de bosques lluviosos de Chile y Norteamerica. En base a promedios de veinte anos. geographic range or are experiencing disease and decline ( cupressoides, nootkatensis). For Chamae­ in cool northern climates and thus would cyparis it appears that the forest-bog eco- be expected to be of even greater import- tone is a key factor. Another cupressid, ance in Chile (Waring & Franklin 1979). Pilgerodendron uvifera although uncom- Forest productivity data for the Chilean mon, plays a similar ecological role in coast could provide a valuable reference Chile. on how a more attenuated distribution of temperature could influence carbon uptake. Forest structure and composition Some models of global climatic change along a climatic gradient predict a climate more like southern Chile for northern rainforests sites, making this Along the more than 15 degrees of latitude comparison a particularly important one. in each coastline temperate rainforest As in many tropical regions, the limit species composition and structure changes of rainforest along both coastlines is mostly continuously both in response to changes a function of precipitation rather than in temperature and moisture. Along the temperature (De Laubenfels 1975). Forest North American coast a steady decline in extends to the highest latitudes where accumulated temperature is closely related heavy rainfall persist (e.g. the contrast to declines in forest productivity and tree the climate of Punta Delgada, Chile and height (Farr & Harris 1979) and to in- Adak Island, Alaska). Wind may also play creases in soil organic matter and litter a role in restricting forest growth as in the depth. Similar trends occur along the case of the Aleutian Islands in Alaska Chilean coast (Holdgate 1961, Weinberger where planted Picea seedlings only survive 1974). Much of the temperature differences in sheltered microsites (Alden & Bruce are due to not only diminishing global 1989), or in the exposed moorelands of insolation but also due to higher summer the southern Chilean coast (Holdgate cloudiness and rainfall. Moisture-laden 1961, Young 1972). clouds are particularly effective in decreas- A key consequence of the unique cli- ing insolation at high latitudes because mate of temperate rainforests is the rapid of low solar altitudes. rate of soils morphogenesis. High rates The key characteristic that distinguishes of runoff result in rapid leaching and major forest formations and that of the 406 ALABACK temperate rainforest zone itself is the & Dryness 1973, Roemer et a/. 1988). duration and intensity of cool summer Understory plants vary from dense thickets rain (Fig. 6, Veblen eta/. 1983). Although of woody broadleaf evergreen shrubs to summer rains occur in other temperate low growing herbaceous carpets depend- forest types they are usually associated ing on soil moisture and exposure. with intense storms of short duration and A rich diversity of evergreen trees do- so do not lead to a cool climate. In tem- minate the Valdivian rainforest including perate rainforests long periods of fog, various combinations of Nothofagus dom­ drizzle, and light rain are common. This beyi, N. nitida, Aextoxicon, Laurelia, may be why the cloud forests of the Drimys, Weinmannia, Podocarpus and tropics share so much in common both Fitzroya. The dense lush multilayered climatically and biologically with temperate forest is reminiscent of the physiognomy rainforests. Within the temperate rainforest of a subtropical rainforest. In Fitzroya zone duration of summer rains are closely sites biomass can approach 1,000 mt/ha, associated with forest structure and com- whereas typical Nothofagus sites only position. attain 200-300 mt/ha (Alaback 1989). In the most equatorial portion of the On many of these sites Chusquea bamboos rainforest zone short summer droughts form an almost impenetrable thicket un- and temperatures over 250C are common, derneath a patchy upper canopy. Under- but moisture is still freely available, making story tree and shrub species also contri- it a nearly ideal environment for tree bute to this thicket of undergrowth. growth. Forest productivity and biomass Herbaceous species are less prominent accumulation is greatest in this zone, which than in the coniferous forests of the includes the 0 lympic rainforest in Wash- northern Pacific. Summer rain is ap- ington and southern British Columbia and proximately 40% of annual precipitation the V aldivian rainforest in Chile. In the C. Perez, pers. comm.). Pacific Northwest forest biomass com- In the North Patagonian rainforest and monly exceeds 1,200 mt/ha as compared the rainforest of the northern with 150-300 mt/ha in eastern North Pacific the overstory is less diverse and America or typical broadleaf sites in Chile productivity lower than in the Valdivian & (Franklin Waring 1980). Dominant rainforest. Within this zone the greatest tree species in North America are Picea change in overstory species richness and sitchensis, , Pseudo­ productivity occurs in both hemispheres. tsuga menziesii plicata and (Franklin In North America Picea sitchensis, Tsuga heterophylla, Abies amabilis and Thuja _J 30 _J plicata are the dominant species (Alaback SOUTHERN CHILE NORTH PACIFIC & ~ ~ Juday 1989, Gagnon and Bradfield ~ 25 ~ z ~ ~ <( / 1986, Roemer et a/. 1988). Pure stands cr / t.IAGELLANIC RAINFOREST s BOREAL 20 I RAI CREST Picea Tsuga cr I of and are common with few w I :a; ~ trees reaching intermediate crown posi- 15 :a; 1 - NoRTH RAINFOREST :::> tions. The understory is dominated by U1 I SPRUCE RAINFOREST 10 j 1- deciduous ericaceous woody shrubs and z / VALOMAN RAINFOREST w DOUGLAS-FIR SPRUCE FOREST evergreen herbs which play a key role in u 5 cr REDWOODS providing winter habitat for mammals w Cl.. 0 and birds (Alaback 1982). Forest biomass 38 43 48 53 58 63 is generally 25-50'1o that of the Olympic LATITUDE, 0 or related forest in the Pa- cific Northwest (Alaback 1990). Summer Fig. 6: Distribution of forest types within the rainfall ranges from 10 to 20 percent of rainforest region of North America and Chile in relation to climate. annual precipitation. In the most poleward sites a distinctly Distribuci6n de los tipos de bosques dentro de las regio- nes de bosques lluviosos de Norteamerica y de Chile en open and depauperate forest develops. relaci6n al clima. In Alaska this subarctic rainforest is best COMPARISON OF TEMPERATE RAINFORESTS 407 represented in Prince William Sound with forest canopies, although forests with a a mixture of Picea sitchensis, Tsuga hete­ high component of Picea are normally rophylla, and T. mertensiana (Alaback & subject large scale catastrophic disturbances. Juday 1989, Borchers et al. 1989, Cooper In the Valdivian Andes Nothofagus species 1942, Eck 1984). Growing sites are poor by contrast appear to require catastrophic and mature canopies of 20-35 m are com- disturbance for regeneration (e.g. Veblen mon (Farr & Harris 1979, Young 1972). et al. 1983). The mixed forest of myrtles, Thickets of Rubus spectabilis and other laurels and podocarps appears the most tall woody shrubs are less common than ecologically analogous to the Tsuga forests in the spruce rainforest type. The under- of North America (Armesto & Figueroa story is dominated by ericaceous woody 1987). shrubs and carpets of bryophytes and The first work on plant succession in ferns on the forest floor. the temperate rainforest zone focused on In the Magellanic or subantarctic rain- succession following deglaciation (Cooper forest the overall physiognomy is much 1937, Croker & Major 1955, Reiners et more similar to the northern conifer forests a/. 1971). A simple chronosequence was than in any other type. The absence of used to characterize this succession from Chusquea bamboos makes for a much bryophytes (e.g. Rhacomitrium spp.), to a more open, mossy forest. The under- shrub stage (Salix spp.), to a Picea sit­ story is often low and patchy with erica- chensis forest, finally to a climax Tsuga ceous species such as Gaultheria and Per­ heterophylla forest. This primary succes- nettya and extensive carpets of bryophytes sion can proceed remarkably quickly, on the forest floor. , with a dense mature Picea forest establish- or N. pumilio are overstory ed on land scraped bare by glaciers only dominants. Severe wind and cool tem- two centuries ago. peratures are common during the growing In North America Alnus sinuata is wid- season in both locales. The landscape in ely recognized for playing an important both hemispheres is a dynamic fine-grained role in soils nutrition following deglacia- mosaic of bog or moorland, icefields and tion by fixing atmospheric nitrigen (Cro- rainforest. These are among the least well cker & Major 1955). Following establish- known rainforest types. ment of Alnus forests, Picea seedlings grow slowly in the understory, eventually Forest dynamics overtopping the sort-lived Alnus. The nitrogen-rich soils then allow for rapid Temperate rainforests are extremely dy- growth of Picea and the establishment of namic systems with disturbances occurring a pure Picea forest within two centuries at several spatial and temporal scales. In of deglaciation (Cooper 1937). In Chile the Valdivian and Olympic rainforest Gunnera chilensis appears to play a similar zones catastrophic disturbance appears to role to Alnus, although few field studies be an important component of stand have been conducted to verify how im- structure, whether caused by fire, land- portant its nitrogen fixation is to soils slides or vulcanism (Franklin & Waring development and plant succession follow- 1980, Veblen et a/. 1981). At higher la- ing glacial retreat (Veblen et al. 1989). titudes and along exposed coastlines con- Recent work on post glacial succession tinual disturbance by wind may become a has emphasized plant life history strategies more important factor (Armesto & Fi- and competitive relationships to examine gueroa 1987, Harris 1989, Taylor 1990). if succession truly follows the facilitation Forest stand structure is often an inte- model of Connell & Slatyer ( 1977) or gration of both scales of distrubance due simply relay floristic (Egler 19 54). Pre- to the longevity of dominant trees both liminary results of some of this work in Chile and North America. Picea and suggests that post glacial succession is a Tsuga are least moderately shade tolerant far more complex and multi-faceted process and can regenerate effectively under partial than first believed. For example in Glacier 408 ALABACK

Bay the original work was done only on on the timing and nature of primary the eastern arm of the Bay where glacial succession either following deglaciation or retreat occurred rapidly (Cooper 1937). hillslope failures in the region (Veblen In the western arm where calcareous parent eta/. 1981). materials predominate a wide range of Secondary succession also proceeds rap- plant successional trajectories develop idly in the northern temperate rainforest following deglaciation. The speed of zone. Following windthrow or logging deglaciation and physical processes as- most of the pre-disturbance propagules sociates with it such as scour, fill, and survive intact allowing growth release of other soils changes lead to divergent suc- tree seedlings and layering or resprout- cessional pathways. It also appears that ing of shrubs and herbs. The most charac- late sere species occur at the earliest teristics aspect of the succession is an stages of the succession, imposing some extended period of species impoverished constraints on the facilitation model. understory vegetation following the esta- Erosional events are distinctly different blishment of a dense overstory canopy in the two regions. In Alaska and British layer. Less than 1it. of the understory Columbia, because of the lower winter biomass of old growth forests are maintain- temperatures heavy wet snow accumulates ed in these younger forests for I 00 years much more readily than in Chile resulting or more (Alaback 1982, 1984). As stands in frequent snow avalanches on steep mature shade tolerant shrubs and tree slopes. The mainland coast is particularly seedlings invade the understory, often prone to avalanching as it combines steep leading to a dense secondary canopy glaciated topography with heavy snow- layer. Only after the principal overstory fall. Landslides occupy only a small frac- stratum senesces, and begins to follow tion of the landscape and are often associat- a pattern of gap-phase dynamics does the ed with stream channels (Swanston & full structural diversity of shrubs, herbs, Swanson 1976). In southern Chile ava- and tree seedlings develop (Alaback 1990). lanche caused erosion appears to be less Many herbaceous species (e.g. Cornus common, but increased ruggedness of the canadensis, Rubus pedatus, R. lasiococcus, terrain and heavy rainstorms appear to Oxalis oregona) which are important sour- result in high frequency slope failures. ces of highly nutritious forage are most Few studies have been conducted on abundant in chronically disturbed old succession following landslides or mud- growth or in the oldest age class of seral flows in the northern temperate zone dominants (500-1 ,000 years old) (Ala- (Dale 1989, Miles & Swanson 1986, Smith back 1982, 1990, Spies & Franklin 1988). & Commandeur 1986). Chronosequences Few data are available on how the are difficult to establish because of the structure of secondary forests compares individualistic nature of each disturbance with primary forest in southern Chile and event and the wide range of soil condi- how long it takes for this forest to become tions which may result from different ecologically equivalent to the original kinds of disturbances or within different forest. The ecological value of secondary terrains contained in a single landslide forests should become a key question as area. The succession resembles post glacial human activity extends into these south- succession in its dominance by nitrogen ern forests in coming decades. fixing species such as Lupinus and A /nus Aggressive \\Oody understory species can and early establishment of Picea. Simi- cause regeneration problems with chronic larly in Chile the nitrogen fixer Gunnera disturbances, impeding the succession chilensis is an early colonizing species, towards climax species. Chusquea forms along with the climax species, Nothogagus impenetrable thickets following canopy dombeyi. Much of the climax old grwoth gap creation, impeding regeneration of Nothofagus and Fitzroya is presumed to Nothofagus (Veblen 1982, Veblen et a/. originate from landslide or mudflow 1983). In North America Rubus spectabilis events. Yet little information is available can similarly exclude other woody seedl- COMPARISON OF TEMPERATE RAINFORESTS 409 ings under partial canopies, especially develops. Overall productivity of these along floodplains and other riparian sites. ecosystems should be heavily influenced The presence of these species by themselves by soils fertility. An excess amount of can play a pivotal role in determining water should lead to drainage of water, ecosystem response to disturbance. oxygenation of soils, and soil temperature In the North Patagonian and Magellanic as being limiting growth factors. rainforest the greatest similarity of eco- During the past several decades research system response to disturbance probably on the functional ecology of temperate occurs, however neither region has been rainforests in North America has generated intensively studied. Both have chronic several general hypotheses about how cli- wind disturbance and overstory species mate influences forest growth and structure. with a range of shade tolerance. Shade Forest area, for example has been used tolerant trees and shrubs rapidly reoccupy to estimate potential productivity and site forest sites by resprouting or by releasing growing potential of both secondary and established seedlings that survived the primary forests (Waring & Franklin 1980). disturbance (e.g. Alaback 1990). In Alaska The growth form and physiology of trees Tsuga mertensiana resembles Nothofagus in the Pacific Northwest has also been in that it seldom regenerates under itself, attributed to unique climatic conditions. and appears to require a large scale dis- But without good replication it is difficult turbance to be maintained in the stand. to determine how important these factors Little work has been done on dynamics really were to the evolution of present of Tsuga mertensiana or Nothofagus be­ day ecosystems. The analogous climatic tuloides. More detailed information on gradients in North and South America gap sizes and survivorship under partial provide· a unique opportunity to test canopies will be needed to answer questions many of these ideas, while at the same time about what kind of disturbances are needed providing some insights as to how historical to maintain present stand structure in both factors influence ecosystem structure and regions. function. Thus despite the striking contrast in Predictions and summary biota the physical environment and history The unique cool and wet climate of the of the Northern Pacific coast and the ex- temperate rainforest should create distinct treme southern coast of Chile provide a ecological stresses to which these forests unique opportunity for comparative eco- must adapt. This climate should lead to logical studies. The extremely wet and rapid soils development and immobiliza- cool climatic conditions also provide unique tion of nutrients, eventually leading to bog physiological conditions for plants and formation, unless the soil horizons are soils. Key questions that might be addres- disturbed. Low growing season temperatures sed in such studies might be: what is the should also contribute to paludification by relative importance of biogeographical allowing greater accumulation of organic factors and physical environment in re- matter. Soils disturbance should be benef- gulating the evolution of forest ecosystem icial to maintain long-term productivity structure and function; of how might on such sites. the structure and dynamics of temperate Shade tolerant species should become rainforests change with an increase in increasingly important along the latitudinal precipitation and temperature (as has often gradient of the rainforest. This implies been proposed under a global warming rapid ecosystem recovery to disturbance, scenario)? and important influences of surviving propagules on forest regeneration and structure. The dense vegetation and high ACKNOWLEDGMENTS cloud cover should work together to Funding for this research was provided by the USDA suppress understory diversity in secondary Forest Service Pacific Northwest Research Station, and forests until a patchy overstory canopy by the USDA Office of International and Community 410 ALABACK

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