Relictual Amphibians and Old?Growth Forests
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Relictual Amphibians and Old-Growth Forests HARTWELL H. WELSH, JR. Pacific Southwest Forest and Range Experiment Station Forest Service, U.S. Department of Agriculture Arcata, CA 95521, U.S.A. Abstract: Terrestrial and aquatic herpetofauna were sam- Resumen: La herpetofauna terrestre y acuática se pled by pitfall traps, time-constrained searches, and area- muestrearon mediante trampas excavadas, cacheos de constrained searches (stream sites only) over a three-year tiempo restringido y búsquedas en áreas restringidas (sola- period to examine the importance of forest age to amphibi- mente en los riachuelos) por un período de tres años para ans and reptiles. Fifty-four terrestrial and 39 aquatic sites in examinar la importancia de la edad de un bosque para los Douglas-fir-dominated, mixed evergreen forests were lo- anfibios y los reptiles. Cincuenta y cuatro lugares terrestres y cated in southwestern Oregon and northwestern California treinta y nueve lugares acuáticos en los bosques perennes Mean age of trees on sites ranged in age from 30 to 560 years. mixtos dominados por (Douglas-fir), fueron localizados en Thirty-one species of amphibians and reptiles were detected el suroeste de Oregón y el noroeste de California La edad de from the 93 localities. Only three species were found prima- los lugares variaba entre 30 y 560 años. Se detectaron 31 rily on older forest sites: the Del Norte salamander (Ple- especies de anfibios y reptiles en las 93 localidades. Sola- thodon elongatus), the Olympic salamander (Rhyacotriton mente se encontraron tres especies, principalmente en los olympicus), and the tailed frog (Ascaphus truei). Paleoeco- lugares de bosques más ancianos: la salamandra del norte logic evidence indicates an historical association between (Plethodon elongatus), la salamandra olympica (Rhyaco- these three amphibians and the extant elements of ancient tyriton olympicus) y la rana coluda (Ascaphus truei). La primeval coniferous forests of the Pacific Northwest. The life evidencia paleoecológica indica una asociacion histórica histories and habitat requirements of these species suggest entre estos tres anfibios y [los elementos contemporarios de that these forms are scarce in younger forests because the ancianos y pristinos bosques de coníferas del Noroeste del microclimatic and microhabitat conditions they require Pacifica Los requerimientos de la vida y el hábitat de estas generally exist only in older forests. The long-term viability especies sugieren que estas formas son más escasas en of these species in northern California and southern Oregon bosques jóvenes debido a que las condiciones microclima- may depend upon developing forestry practices that protect ticas y de microhkbitat que requieren existen generalmente these critical microclimates and microhabitats. sólo en bosques mas antiguos La viabilidad a largo plazo de estas especies en el norte de California y el sur de Oregon puede depender en el desarrollo de prkcticas forestales que protegen estos criticos microclimas y microhábitats, Introduction species comprising these ancient forests (both living The old-growth, Douglas-fir-dominated, coniferous- and extinct) have been termed the Arcto-Tertiary Geo- hardwood forests of the Pacific Northwest originated flora by paleobotonists (Axelrod 1976; Daubenmire millions of years in the past. These forests are remnants 1978). These forests are presently the focus of a conflict of a more species-rich Tertiary forest with a previously over natural resources between those who support con- continuous circumpolar, subarctic distribution. The tinued commercial harvesting for wood products and those who support protecting the remaining old growth to conserve watershed quality, fisheries populations, Paper submitted February 8, 1989; revised manuscript accepted wildlife habitat, and recreational space (see Thomas et March 6, 1990. al. 1988 for an overview). Critical to this issue are two 309 Conservation Biology Volume 4, No. 3, September 1990 310 Relict Amphibians and Old-Growth Forests Welsh questions: how much old-growth forest should be re- locations within the region (Fig. 1). Sites in each of the tained, and by what criteria ought this be determined? three areas were selected from young, mature, and old The basis for such criteria, however, have been estab- forests based on stand characteristics (Franklin et al. lished de facto by federal law. The Endangered Species 1986). Fifty-four terrestrial study sites, ranging from 21 Act of 1973 (16th U.S. Congress) establishes that viable to 150 ha, and 39 second- or third-order streams were populations of all native plants and animals will be main- sampled for amphibians (Fig. 1). tained. The National Forest Management Act of 1976 (16th U.S. Congress) requires that regulations be estab- lished that address the concept of maintenance of bio- Methods logical diversity on federal forest lands (see MacCleery 1982 ). These laws mandate that resource use take into Herpetofauna Sampling account the potential and actual impact of that use on all resident plants and animals. Criteria for area and unit Three methods were used to sample the species com- spacing for old-growth forest preservation could there- position and relative abundance of the herpetofauna: fore be based on the requirements for viability of those pitfall traps (PF), time-constrained searches (TCS), and species of plants and animals that require old-growth area-constrained aquatic searches (AQS). These meth- forests. This approach to establishing criteria must ad- ods are described briefly below and in more detail in dress two issues: (1) Which species require old-growth forest habitat? (2) How much of this habitat is needed and what spatial configurations are required to allow gene flow and thus assure the viability of those species? An intensive research program has been under way since 1981, under the auspices of the U.S.D.A. Forest Service, to determine which vertebrate species require old-growth forest habitats (Meslow et al. 1981; Raphael 1984; Ruggiero & Cary 1984; Ruggiero et al., in press). The Forest Service's Pacific Southwest Forest and Range Experiment Station (PSW) collected data on forest age and habitat associations of all vertebrates (except fish) in forests of southwestern Oregon and northwestern California from 1984 to 1986. The primary objective of these studies was to examine the importance of forest age to wildlife. During the course of this research 3 of 31 species of reptiles and amphibians that were sampled were found primarily in mature and old-growth habitats: the Olym- pic salamander, the Del Norte salamander, and the tailed frog. This paper reports on these three old-growth- associated forms; on the relationship between presence and abundance of these amphibians and forest age, and on aspects of their natural history that may influence this relationship. I suggest that these species are evolu- tionarily conservative elements of an ancient relictual forest ecosystem. I stress here that other species of her- petofauna in the Pacific Northwest exhibit characteristics suggestive of relict forms, but I lacked sufficient data to include them in my discussion. Study Area These studies were conducted in Douglas-fir (Pseudot- suga menziesii) -- dominated mixed coniferous-hard- wood forests of northwestern California and southwest- ern Oregon. The forests ranged in age from 30 to 560 years and in elevation from 500 to 5,000 feet. Equal numbers of forest sites were selected in three general Figure 1. Study sites in northwestern California and southwestern Oregon. Conservation Biology Volume 4, No. 3, September 1990 Welsh Relict Amphibians and Old-Growth Forests 311 Welsh (1987), Corn and Bury (1989), and Corn and daily mean temperatures were rising and surface mois- Bury (in press). ture was high in northwestern California. Forty-nine sites contained PF trap grids with 36 traps The AQS method consisted of systematic searches of spaced at 15 m intervals in a 6 X 6 arrangement. Traps measured stream reaches in 39 second- or third-order were made of two # 10 tins taped together and buried streams (Strahler 1952) on or near the terrestrial sites. with the lip at ground level and concealed by a cover of Three 5 m reaches 1-3 m wide were selected along bark or cedar shake propped slightly above ground each stream by walking 50 m upstream from the nearest level. Traps were open at 49 sites in October and No- trail or road access for the first reach and 50 m from the vember of 1984 and October of 1985, for 50 and 30 top of the previous reach for subsequent reaches. Each nights, respectively. reach was measured and mapped, then all substrates The TCS method consisted of timing the search ef- were methodically searched, moving all rocks where forts of 2-3 persons as they moved through the forest possible, working upstream with catch nets placed sites at random examining all microhabitats encoun- downstream to capture dislodged animals. AQS were tered, including raking through litter with a potato rack, conducted in July and August of 1984 and 1985. turning rocks and small logs, breaking open larger de- caying logs, probing vegetation, etc. The clock was Determining Forest Age stopped when animals were encountered, and data were Tree ages were determined by increment coring and gathered. Four hours of TCS were conducted at each of where possible, by counting rings on stumps in adjacent the 54 sites between April and June of 1984 and again in logged areas (old-growth terrestrial sites). Forest age 1985. Preliminary analyses of TCS data in 1984 indi- was estimated by averaging tree ages from a minimum of cated that this method was not adequately sampling the three trees at each site (Table 1). Dominant or codom- relatively uncommon and unevenly distributed micro- inant size classes (based on tree counts within 0.2-ha habitat of the Olympic salamander. Consequently, po- circular plots) of Douglas-fir trees were selected for ag- tential sampling sites were preselected within the stands ing and trees were cored at breast height.