Site description
The TLW has a total relief of 300 m from the highest point on Batchawana Mountain (626m) to the watershed outlet on Norberg Creek. The site is almost entirely underlain by Precambrian silicate greenstone (i.e. metamorphosed basalt) with some small outcrops of more felsic igneous rock (Semkin & Jeffries, 1983). A two-component glacial till, composed of ablation till superimposed on a compact basal till, overlies the bedrock. Till thickness varies from <1 m at high-elevation locations (with frequent surface exposure of bedrock) to 1-2 m at lower elevation, and with the occasional occurrence of extremely deep till sequences up to 70 m (Elliot, 1985; Buttle, Webster, Hazlett & Jeffries, 2018). Tills contain a small but measurable amount of CaCO3 (0-2%) that increases with depth and is higher at lower elevation locations (Craig & Johnston, 1983).
TLW soils are predominantly podzols (spodosols) with well-developed L, F, H (Oi, Oe, Oa) horizons and accumulation of organic matter (10%), iron and aluminum in the B horizon (Hazlett, Semkin & Beall, 2001; Hazlett & Foster, 2002). The pH of the mineral soil is 4.0 at the surface, increasing to 5.5 at depth. Organic soils occupy wetlands and riparian areas of the watershed (Creed, Beall, Clair, Dillon & Hesslien et al., 2008; Wickware & Cowell, 1985).
The TLW forest is composed of uneven-aged tolerant hardwood forest dominated by mature to over-mature sugar maple (Acer saccharum ) that are 150 - 200 years old. Minor components include yellow birch (Betual alleghaniensis ), red maple (Acer rubrum ), ironwood (Ostrya virginiana ) white spruce (Picea glauca ) and white pine (Pinus strobus ). In riparian areas, eastern white cedar (Thuja occidentalis ) balsam fir (Abies balsamea ), tamarack (Larix laricina ) and black ash (Fraxinus nigra ) can be found (Morrison, 1990). Stands regenerate through gap dynamics and, due to natural tree mortality, are roughly in equilibrium in terms of aboveground net phytomass accumulation (Morrison, 1990). The watershed is relatively undisturbed. The most recent operational logging occurred in the 1950’s when the area was selectively logged for high-quality yellow birch, sugar maple and likely white spruce and white pine (Jeffries et al., 1988). The biggest impact was a reduction in the overstory yellow birch component. There have been no significant natural disturbances (forest fires, insect outbreaks) across the watershed since the establishment of the study, with the exception of an experimental harvest in 1997 (see below).
The high relief and leeward position relative to Lake Superior influences the quantity of precipitation at the site. The mean annual precipitation for the period 1980 to 2017 was 1203 mm (unpublished data). Approximately a third of the precipitation falls as snow (Semkin et al., 2012), with snow cover developing typically in late October – November and melting during the March – April period. Average annual temperature over the 1980 to 2017 period was 4.5 °C (unpublished data). The TLW is showing evidence of climate change. While there have not been substantive changes in average annual precipitation, there have been other indicators of change. These include (Buttle et al., 2018; Hazlett, unpublished data): increasing annual air temperatures (0.3o C per decade, primarily driven by higher autumn temperatures), increasing potential evapotranspiration (PET), increasing growing degree days, lengthening of the growing season, more ice-free days on lakes (driven by later in the year dates of ice formation on), and less precipitation falling as snow (e.g., in December).