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We conducted our study in the Second College Grant (SCG), a township and Dartmouth College property located in northern New Hampshire, along the Maine border. This remote property, which is within a temperate-boreal transition zone at 44.9 °N, 71.1 °W, is ~11,000 ha of forested land that is managed for recreation, wildlife, and forest production. Dominant hardwood tree species of the SCG include American beech (Fagus grandifolia), sugar maple (Acer saccharum), red maple (Acer rubrum), quaking aspen (Populus tremuloides), white birch (Betula papyrifera), and yellow birch (Betula alleghaniensis). Dominant softwood species include balsam fir (Abies balsamea), red spruce (Picea rubens) and white spruce (Picea glauca) (Nagel et al., 2017).

Sampling was conducted across a large geographic area; thus, a range of elevations were sampled (471-667 m above sea level). Mean elevation (± standard deviation) of the sampled areas within the SCG was 564 ± 61.5 m a.s.l. This study occurred over two winters and one summer. Data from a weather station roughly 40 km from the SCG in Berlin, New Hampshire, revealed that annual mean daily low and high temperatures from 1990-2022 were 0.2 ± 0.9 °C and 12.2 ± 0.8 °C. Mean annual precipitation was 108.3 ± 15.6 cm with roughly 20% of that falling as snow, and average annual snow depth was 8.1 ± 3.5 cm (Menne et al., 2012a,b). Winter sampling occurred during late January to mid-March in 2020-2021 and 2021-2022, while summer sampling occurred in late July and late August in 2021. During the winter collections the average daily winter temperature was -7.5 ± 6.5 °C with an average daily temperature range of -21.5 to 5 °C. During the summer collections the average daily temperature was 17 ± 3 °C with an average daily temperature range of 12.5 to 21.5 °C (unpublished weather station data, Dartmouth College Woodlands). Mean snow depth measured for the two winters during mid-winter (late January) was 35 ± 5.5 cm with a range of 23-52 cm. Late-winter measurements (mid-March) showed a decreased mean snow depth of 32.5 ± 6.5 cm with a range of 12-49 cm. Comparisons of our study years with climate data from the nearby NOAA station indicated that weather conditions were within the norms for 1990-2022.

Within the SCG, there are four parcels of land dedicated to research, designed, and managed by the Adaptive Silviculture for Climate Change (ASCC) experiment group (Nagel et al., 2017; Palik et al., 2022). Each parcel comprises four different treatments based on silvicultural intervention and planting to better understand the role of forest management in climate change resilience. The research discussed herein was conducted only within control (unmanaged) treatments (Fig. 1C, D) which are representative of mid to late-successional northern forests of the area. There are four control treatment blocks (one within each parcel), each measuring about 10.5 ha in size.

Pitfall trap and sampling design

Ground-active arthropods were collected using pitfall traps modified to withstand snow and improve trap catch during both the winter and summer (Fig. 2). The exterior structure of the trap, the walls and roof, were built from black corrugated plastic sheets pinned to the ground with 15.2 cm long garden stakes. Each of the 4 wall sections of the structure measured 61 cm in length and 7.62 cm in height. The roof was a 25.4 x 25.4 cm square. The ‘X’ shaped design maximized arthropod catch by intercepting passing arthropods, forcing them to walk along the walls and towards the cup containing preservative fluid (Boetzl et al., 2018).

Two cups, inner and outer, that formed the pitfall were made from durable plastic containers (Placon®). The larger outer cup (946 ml, 14 cm height) held the preservative fluid. We limited bycatch of larger non-target animals by using a shallow inner cup (355 ml, 5.7 cm height) that occupied the upper volume of the outer cup. The inner cup had two, 2.54 cm diameter holes cut at the bottom of the cup at opposite edges. The opening of both cups, and therefore of the pitfall trap itself, measured 11.43 cm in diameter. This is a unique pitfall trap design that incorporates components of existing designs (Merriam et al., 1983; Boetzl et al., 2018).

In the winter we used 500 ml of propylene glycol-based, glycerin-free antifreeze mixed with a few drops of odorless dish soap as the preservative fluid in each trap. This substance is harmless to vertebrates and did not freeze at temperatures present within the subnivium. For summer collections we used a salt solution instead of antifreeze to avoid vandalism from bears or racoons. The summer preservative for each trap included 297 ml of water, 3 ml of Tween 80 (Thermo Fisher Scientific Inc), and nine water softener salt pellets (Morton®). See Appendix A within supplemental materials for more details on the traps.

We sampled the control treatments of the four different parcels during two consecutive winters, 2020-2021 (“Winter 1”) and 2021-2022 (“Winter 2”), and during the summer of 2021. Paired pitfall traps roughly 5 to 10m apart were constructed at sampling sites, with a total of 32 pitfall traps used across the four blocks. Two of these traps in Block 4, however, were lost and removed from further consideration. Winter traps were deployed in November, but each was covered with a plastic lid that was later removed by pulling on an attached string that was tied to a nearby stake (Fig. 2). Lid removal activated the traps without disturbing the subnivean. Sampling began in January of each year once significant (~0.5 m deep) snowpack had formed and continued to mid-March for a total of 56 days each winter. The traps were only collected once during the winter, at the end of the 56-day period, to avoid disturbing the snowpack. We collected the traps before snowpack termination occurred at the end of winter. Summer sampling consisted of two separate, two-week-long sampling periods, the first in late July (“Summer 1A”) and the second in late August (“Summer 1B”). See Appendix B within supplemental materials for more details on sampling.

Sample processing, arthropod identification, and biomass calculation

We sorted all arthropods into morphospecies with the aid of a dissecting microscope. Representatives from each morphospecies were photographed, assigned a code, and later identified taxonomically to the family level. In many instances we were only able to identify immature arthropods to the order level. When possible, all rove beetles (family Staphylinidae) from every sample were identified to the species level. Spiders (order Araneae) from a subset of 16 randomly selected traps per collection (Winter 1 and 2, Summer 1A and 1B) were also identified to the species level when possible, primarily using Ubick et al. (2017) and Paquin & Dupérré (2003) and following taxonomy accepted in the World Spider Catalog (WSC) (2023). These two groups, spiders and rove beetles, were selected for identification to species because initial sample processing indicated that they were abundant in winter sample collections and relatively large compared to other winter collected taxa. As both groups are known to be top predators, they may also play a uniquely important role in the winter arthropod community. We freeze-dried and weighed representatives for nearly all morphospecies and then calculated a mean dry mass per individual for each morphospecies. However, about 35% of the morphospecies were too rare to accurately calculate mean biomass; for these rare taxa, we used mass estimates from similarly sized and taxonomically related morphospecies. Error in the estimation of these weights are presumed to have had negligible effects on calculations of overall biomass given their low relative abundances.