Effects of structural complexity enhancement on eastern red-backed salamander (Plethodon cinereus) populations in northern hardwood forests

Summary

Vermont Forest Ecosystem Management Demonstration Project (FEMDP) researchers are investigating a range of silvicultural treatments applicable to sustainable forest management in the northern hardwood areas of the northeastern United States and southeastern Canada. Forest management practices can affect microhabitat characteristics important to terrestrial plethodontid salamanders, such as closed canopies that maintain a cool, moist, forest microclimate, uncompacted soils, fallen coarse woody debris and an undisturbed leaf litter layer. Within the detrital food web, terrestrial salamanders are top predators that regulate populations of soil invertebrates, such as hymenoptera (e.g. ants), collembola (springtails) and araneida (mites and spiders).

Managing forests for structural complexity in northern hardwood forests has been proposed to promote microhabitat characteristics important to eastern red-backed salamanders Plethodon cinereus and other organisms. The effects of three different, structure-based, silvicultural systems on red-backed salamander populations at two research sites in Vermont (USA) were evaluated. Treatments included two uneven-aged approaches (single-tree selection and group-selection) and one unconventional approach, termed 'structural complexity enhancement' (SCE) designed to promote development of late-successional structure, including increased levels of coarse woody debris. Red-backed salamanders were surveyed 15 months after after logging. Abundance estimates, corrected for detection probability, were calculated from survey data. These estimates differed between study areas and were influenced by forest structural characteristics. No difference in abundance was found as a response to treatment as a whole, suggesting that all of the silvicultural systems evaluated maintained salamander populations after timber harvest. However, abundance was tied to certain habitat attributes and responded most positively to the density of fallen, well-decayed coarse woody debris. SCE and the two uneven-aged techniques maintained important microhabitat characteristics for red-backed salamanders in the short term. Over the long-term, forestry practices such as SCE that enhance coarse woody debris availability and recruitment may be beneficial to salamanders, as well as other coarse woody debris-dependent organisms.

Study sites: The study was conducted in northwestern Vermont (northeast USA) at the Mount Mansfield State Forest (located on the western slopes of the northern Green Mountain at elevations of 470 to 660 m) and the University of Vermont's Jericho Research Forest (in the foothills between 200 to 250 m). The forest comprises mature (70–100 year old), multi-aged stands dominated by sugar maple Acer saccharum, American beech Fagus grandifolia and yellow birch Betula alleghaniensis. Eastern hemlock Tsuga canadensis is co-dominant at Jericho Research Forest.

Silviculture treatments: Three treatments were applied: single-tree selection, group-selection, structural complexity enhancement (SCE), with unharvested controls (see original paper for treatment details). Treatments were randomly applied to 2 ha units and replicated twice at the Mount Mansfield State Forest. Two additional replicates of the SCE and control only, were applied at the Jericho Research Forest (both sites combined: n = 2 replicates for single-tree selection and group-selection, n = 4 replicates for SCE and control). Treatment units were separated by an unlogged buffer of at least 50 m to minimize cross contamination of treatment effects. Within each treatment unit were five randomly placed 0.1 ha permanent sampling plots buffered by at least 15 m distance from a unit's edges. Experimental logging was undertaken in January–February 2003.

Habitat characteristics: Data on habitat characteristics were collected in the 0.1 ha plots. The decay status of logs was recorded for all coarse woody debris (CWD) ≥10 cm diameter (with a 1–5 classification system) and whether logs were > or ≤50% suspended from the ground. Only logs considered available as salamander habitat (≤50% suspended from the ground) were included in analysis. Volume (m3/ha) and density (logs/ha) of CWD by decay class for the available logs was calculated. Log diameters were placed in one of five size classes: ≥10 to <20 cm; ≥20 to <30 cm; ≥30 to <40 cm; ≥40 to <50 cm; ≥50 cm. All live and dead trees ≥5 cm diameter at breast height (dbh) and over 1.37 m tall were measured and species and decay stage, recorded.

Salamander surveys: In late-May and early-June 2004 (approx. 15 months after logging), daytime searches of 8-min duration were undertaken to survey 4 (25.8 m x 2 m) transects nested within the 0.1 ha sampling plots. To control for factors which may influence salamander activity and distribution, the order in which treatment units were surveyed was rotated, sampling an equal number of plots from each treatment type on a given day. Non-destructive methods were used to locate salamanders under natural cover and to search leaf litter and vegetation for salamanders foraging on the forest floor. Red-backed salamander numbers observed per transect were recorded and transects resurveyed 6–10 days later.
Detection probability covariates: Air and soil temperatures at the centre of each plot, precipitation within 24 h of a survey (0 = no rain, 1 = rain), and sky conditions (0 = clear, 1 = partly cloudy, 2 = overcast, 3 = fog, 4 = light rain, 5 = steady rain) were recorded.

Salamander abundance: Terrestrial salamander are difficult to survey as many are primarily subterranean and only a fraction of a population may be near the surface and available for counting at a given time. Due to this, raw count data was adjusted to account for the factors influencing detection probabilities. For this purpose, a model was developed to estimate abundance adjusted for detectability from the survey data.

Treatment effects on course woody debris: Unsuprisingly, CWD volume and CWD density were correlated with each other. Average densities of available less-decayed CWD (logs/ha) were significantly different between treatments, size classes and treatment × size class interaction - SCE had the greatest density of CWD (logs/ha) in the two smallest diameter size classes, while all three treatments had significantly greater densities in the smaller size classes compared to the unlogged control. The Jericho Research Forest SCE had twice as many logs in the smallest size class compared to Mount Mansfield State Forest. The unlogged control areas were not significantly different.

Average densities of more-decayed fallen CWD (logs/ha) had significantly different diameter distributions; the majority of logs were in smaller size classes. There were no significant differences between treatment types treatment × size class interaction, or site × treatment × size class interaction. Densities of more-decayed CWD (decay classes 3–5) were not significantly different between treatments or the control.

Overstory structure: Post-harvest relative density of overstory trees was highest (as expected) in the unharvested control (relative density 6.6) in comparison with SCE (6.2), single tree selection (4.4) and group-selection (4.8) (values read from original graphs).

Salamander survey results: A total of 714 red-backed salamanders were counted during the surveys: 477 at the Mount Mansfield State Forest and 237 at the Jericho Research Forest (see Table 1, attached for numbers within each treatment). The number of salamanders counted per transect ranged from 0 to 7. The average number found per transect in each treatment and the control were fairly similar ranging from 1.11 to 1.86. Four other salamander species were observed during the study. Eastern newt Notophthalmus viridescens was the next most common species observed on all treatments and the control. This was the only other species observed at Jericho Research Forest. At Mount Mansfield State Forest, northern dusky salamanders Desmognathus fuscus were observed in the control and SCE stands. A spotted salamander Ambystoma maculatum was observed in SCE and a spring salamander Gyrinophilus porphyriticus in the control stand.

Detection probability covariates: There was a high amount (CV > 30%) of within survey variability regarding precipitation, sky conditions and air temperature. 75% of surveys were conducted following precipitation in the past 24 h with 17% occurring during fog, drizzle or rain. Air temperature ranged from 7.5 to 29 °C (average 17 °C). Soil temperature ranged from 8 to 16 °C (average 11 °C) but was not significantly different between treatments. Overall, salamander detection probability was estimated to be 42%.

Conclusions: Contrary to expectations, red-backed salamander abundance did not differ significantly between SCE and the other treatments or controls. The low-impact silviculture treatments employed (designed to enhance post-harvest structural retention) had apparently little or no negative impact on salamander abundance and were effective in maintaining suitable red-backed salamander habitat following timber harvest, at least in the short term. The lack of an immediate post-harvest treatment effect may be attributed, in part, to similar densities of well-decayed CWD (decay classes 3–5) across all treatment and control units. These was due to residual logs present prior to treatment. That the density of well-decayed CWD had the greatest influence on red-backed salamander abundance is strongly supported by the modeling results. Salamander abundance in northern hardwood forests appears more closely associated with CWD density rather than volume. Greater log densities across a range of sizes may provide more retreats. This would support greater densities of salamanders compared to the presence of only a few large logs (even if collectively representing a larger volume) guarded by resident territorial pairs.

Forest floor microclimate is sensitive to changes in canopy cover. Loss of canopy cover typically results in more extreme environmental conditions in exposed patches, such as temperature and solar radiation increases and soil moisture decrease. The low-impact forestry methods used were designed to limit such effects e.g. group-selection cutting patches were small (each approximately 0.05 ha in size), which is smaller than the 0.1–0.5 ha groups typical for the region, and all three methods were designed to increase retention of post-harvest tree basal area. Relative tree density had only a small effect on the salamander abundance, indicative of the high levels of structure retained by all the treatments.Climate conditions may have contributed to the lack of observable treatment effects. Vermont had higher than average precipitation (129%) and lower than average temperatures (−1.2 °F) during the summer of 2004. Weather conditions and moisture influence salamander activity, and thus, detection probability. In this study, salamanders may have responded to the cool, moist weather by increasing their surface activity in open areas. Had conditions been drier and warmer, overstory relative tree density may have been more important in moderating microclimate conditions and treatment effect on salamander densities may have been detected.

Overall, results from this study indicate that structure-based forestry methods, such as SCE, can enhance CWD densities, at least over the short-term, for the benefit of salamanders and other coarse woody debris-dependent organisms.


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