State of the Greater Fundy Ecosystem - Effects of Forestry on amphibians

GREATER FUNDY ECOSYSTEM RESEARCH PROJECT

UNB Faculty of Forestry and Environmental Management

State of the Greater Fundy Ecosystem

Effects of Forest Conversion on Amphibians in and around Fundy National Park

Ruth Waldick, Bill Freedman and Richard Wassersug
Dept. of Biology, Dalhousie University
Halifax, N.S., B3H 4J1

In western North America, various studies have shown that clear-cutting causes dramatic reductions or local extirpations of amphibians (Freedman et al., 1994). Few studies of the effects of silvicultural activities have been made in eastern North America, and none have been conducted in Canada.

GOAL

The present study was conducted to examine effects on amphibians of the conversion of natural forests into plantations in southern New Brunswick.

METHODS

Species of amphibians were studied in Fundy National Park and its vicinity. Amphibian presence and relative abundance were used to evaluate the status of their populations within stands of natural, mixed-species forest and in conifer plantations.

Amphibian populations were surveyed between March and September in 1993 and 1994. Aquatic and terrestrial habitats were studied in stands of mixed-species, natural forest, and in Black Spruce (Picea mariana) plantations of various age (4 to 16 years old). A total of 64 plots of 10 m x 10 m were searched in various stands for amphibians beneath coarse woody debris (Gibbons and Bennett, 1974; Gibbons and Semlitsch, 1981; Campbell and Christman, 1982; Bury, 1983). Pit-fall traps were used to assess the relative abundance of species in 4 categories of stands: (1) mature, natural, mixed-species forests (n = 5 stands), (2) closed-canopy plantations (13-16 years old; n = 3), (3) partial-canopy plantation (7-8 years old; n = 3), and (4) open-canopy plantations (3-5 years old; n = 3).

The following characteristics of terrestrial habitats at the 14 study sites were measured: percentage shading of the ground surface, quantity and type of leaf litter, and the quantity and decay state of coarse woody debris. Relative humidity at the ground surface was recorded at 12 sites.

The occurrence of amphibians in ephemeral breeding ponds was evaluated at 16 sites (5 mixed-forest sites, 5 open-canopy plantations, 4 partial-canopy plantations, 2 closed-canopy plantations). Egg, larval, and adult amphibians were counted and the timing of their activities was determined during the breeding season and throughout the period of larval development, using a variety of sampling methods (drift-fences, vocalization surveys, and direct counts; Waldick, 1994). In 1994, the direction of migration of amphibians approaching 6 breeding ponds was monitored using pit-fall traps and drift-fencing. Water samples were periodically collected and analyzed for major nutrients by Environment Canada in Moncton. The abundance and presence of amphibians was related to physical characteristics of the ponds (i.e., pond hydroperiod, maximum depth, distance to nearest mixed-species forest).

RESULTS

Terrestrial habitats

The diversity and relative abundance of amphibian species were highest in natural forests, with the most abundant species being Red-backed Salamander, Yellow-spotted Salamander, Spring Peeper, and Wood Frog (Table 1). The densities of amphibians beneath, or in, coarse woody debris were significantly higher in natural forests than any of the plantations (p< 0.04). Spring Peeper was not encountered in plantations outside the breeding season, and Red-backed Salamander was located in only a localized area of one plantation (closed-canopy). These species were relatively abundant in natural forests. Red-spotted Newt and American Toad, species known to be relatively tolerant of desiccation (drying up) (Feder and Burggren, 1992), were the most common amphibians in the plantations.

Red-backed salamander (Photo: B. Townsend/FNP)

Yellow-spotted salamander (Photo: M. Rosen/FNP)

Environmental factors differed among the four groups of stand types that were studied. Relative humidity was similar among the plantations (range 52-60%), but consistently lower than in natural forest (62-84%). The decay state of coarse woody debris was more advanced in closed-canopy plantations and natural forests than in open- or partial-canopy plantations (Table 2). In open- and partial-canopy plantations, woody debris was typically dehydrated and had undergone modest levels of decomposition. The amount of leaf litter on the ground surfacealso was less in plantation habitats than in natural forests (Table 3). Coniferous leaf litter dominated the closed-canopy plantations, while herbaceous angiosperm litter dominated the open- and partial-canopy plantations, and coniferous and angiosperm tree foliage was abundant in the natural forests. The quantities of conifer litter were similar between closed-canopy plantations and natural forests, but the latter sites also had large amounts of angiosperm leaf litter.

Coarse woody debris and leaf litter are important habitat components for salamanders (Jaeger, 1980; Parmelee, 1993). Salamanders were found in association with moderately to highly decayed coarse woody debris and leaf litter, which were relatively abundant in stands of natural forest. Red-spotted Newts occupied woody debris in later stages of decay, and were mostly found in closed-canopy stands (both natural and plantation forest). Red-backed Salamander also uses decayed wood, but it requires angiosperm leaf litter as a foraging substrate (Jaeger, 1980). Unlike Red-spotted Newt and Yellow-spotted Salamander, Red-backed Salamander was absent from the plantations studied, including closed-canopy sites. Interestingly, searches of coarse woody debris conducted just before a clear-cut of one site found Red-backed Salamanders in densities similar to other stands of natural forest. However, searches made three months after the harvest failed to locate any individuals.

Aquatic Habitats

Larvae of Wood Frog, Spring Peeper, American Toad and Yellow-spotted Salamander occurred in more than 90% of the study ponds. The densities of Wood Frog and Yellow-spotted Salamander at these ponds were negatively correlated with the distance to the nearest natural forest (p <0.006 for Yellow-spotted Salamander; p <0.10 for Wood Frog). Captures of adult animals at drift-fenced ponds showed higher captures and greater species diversity in traps facing nearby natural forest than those facing adjacent, open-canopy plantations (Table 4). This suggests that few amphibians reside in the plantation habitat, but they migrate from nearby forest to take advantage of breeding habitat available in ditches and borrow pits in clear-cuts.

Wood Frog (Photo: B. Townsend/FNP)

In open- and partial-canopy plantations, high exposure (due to little or no canopy cover) reduced the hydroperiod to less than 100 days. In fact, 84% of the studied ponds that were located in plantations dried during both study years before most or any larvae were able to metamorphose. In contrast, only 40% of the ponds in natural forest dried in this way. Successful metamorphosis of the more slowly developing species (i.e., Yellow-spotted Salamander and Spring Peeper) only occurred in ponds that persisted for at least 120 days. Only 13-19% of the plantation ponds persisted long enough to permit metamorphosis of larvae of Spring Peeper and Yellow-spotted Salamander, compared with 60% of ponds in natural forest (Figure 1).

Figure 1. Desiccated amphibian larvae on the bottom of a pond in a plantation
that dried out before the animals could complete metamorphosis
(Photo: R. Waldick)

Pond hydroperiod was strongly correlated with the abundance of larvae of Yeollow-spotted Salamander, Spring Peeper, and Wood Frog. For Yellow-spotted Salamander, 49% of the variability in larval density was attributable to pond hydroperiod and cover of submerged macrophytes (p = 0.016 and p = 0.009 in 1993 and 1994, respectively). For Wood Frog and Spring Peeper, hydroperiod, pond shading, and cover of macrophytes collectively accounted for 15% of the variation in larval density (this trend was not significant; p >0.05).

Our study shows that the conversion of natural, mixed-species forests into conifer plantations reduces the amount of habitat available for amphibians. The Red-backed Salamander was most intensely affected by the forest conversion. Frogsandtoads are also largely absent from terrestrial habitats of the plantations, except as transients during the breeding season. Yellow-spotted Salamander and Red-spotted Newt were the only species that showed signs of recovery in plantations, once the tree canopy had closed after about 16 years. Recruitment failure was an important problem for species breeding in temporary ponds in the plantations, because the hydroperiod was too short to allow metamorphosis to be completed.

IMPLICATIONS FOR MANAGEMENT

The short hydroperiods and resulting amphibian mortality caused by desiccation in ponds in plantations would be avoided by constructing these artificial waterbodies to be deeper, and by leaving a buffer of shading trees around the edge. Terrestrial habitats could be improved by allowing some angiosperm trees to persist in the plantations, so the favourable properties of their leaf litter can be maintained to some degree. Retention of some snags and non-crop trees will also allow for future inputs of coarse woody debris, which would otherwise largely disappear from areas managed intensively as plantations (Freedman et al., 1996). One option for providing these critical elements of terrestrial habitat is to leave islands of natural-forest refuge habitats within the plantations. We estimate that this function could be provided by islands about one hectare in area, with two of these provided for every ten hectares of plantation, and each remnant having 2-4 dug-out breeding pools located nearby. If a mitigation of this sort is adopted, however, it will be necessary to monitor its actual efficacy in conserving viable amphibian populations within the plantations.

ACKNOWLEDGEMENTS

This research was supported by an NSERC scholarship to Ruth Waldick, an NSERC operating grant to Bill Freedman, contract support from the Fundy Model Forest, and financial and logistical support from Parks Canada. Thanks also to Guy Bruin and Tom Pollock for chemical analysis of water samples (Environment Canada, Moncton). Special thanks are also extended to our field assistants, park wardens, and other staff at Fundy National Park for their assistance and interest in this study.

Further reading:

Waldick, R.C. 1994. Implications of forestry-associated habitat conversion for amphibians in the vicinity of Fundy National Park, New Brunswick. Unpublished M.Sc. Thesis, Dalhousie University. Halifax, N.S.

Clay, D. and J. Brownlie. 1996. Status of amphibians and reptiles of Fundy National Park and its greater ecosystem. In: D. Clay (ed.) Resources of Fundy National Park: A primer of ecosystem studies. Part II. Chapter I. Pks. Can. Eco. Sci. Rev. Rept. No.2. 38 pp.

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