Root-zone soil temperature: sources of variation and some effects on planted conifer seedlings in high-elevation forest openings in the interior of British Columbia
Creators: name: Balisky, Allen C.
Publisher: name: University of British Columbia
Year: 2008

description: Slow growth of planted seedlings and failure of many artificially regenerated forest openings has tempered initial optimism regarding reforestation in high-elevation environments in British Columbia. Poor rooting of planted stock, partly because of low soil temperatures, has been suggested as a major cause for failure of reforestation efforts in these harsh environments. The first objective of this study was to record the variation in root-zone soil temperature occurring in high-elevation openings and to identify specific microsite attributes most strongly associated with differing soil thermal regimes. This objective was accomplished by means of a field survey in which seven microsite characteristics were measured and related to soil temperature. A manipulative experiment was also conducted involving continuous monitoring of seedling microclimate (i.e., soil temperature, soil moisture, and air temperature) in treatments broadly representative of the range of microsites found in forest openings logged by clearcutting. The second objective was to quantify conifer seedling growth response to the effects of "cool" versus "warm" root-zone temperatures. A field survey of soil temperature (at a depth of -10 cm from the mineral soil surface) in six high-elevation clearcuts in central British Columbia, as related to microsite attributes, showed that variation in root-zone soil temperature was most strongly associated with the combination of thickness of forest floor (duff), cumulative % vegetation cover, and % soil moisture (R2 = 0.73). Combinations including relative surface irradiance (PACL), cumulative % cover-weighted vegetation height, micro-slope and micro-aspect were less important. While the influence of microtopography (presence of a hummock, average dimensions of 1.0m x 1.0 m x 0.4 m) on soil temperature was significant, measurements of micro-slope and micro-aspect showed little if any association with variation in soil temperature. Microsites associated with suboptimally low soil temperatures (&lt;12 °C) during much of the growing season were widespread (70 to 80% of the sampled areas) in high elevation openings. Microclimate monitoring was conducted in treatments consisting of 1) bare mineralsoil mounds, 2) flat, bare mineral soil, 3) flat, duff-covered mineral soil, 4) flat, vegetated mineral soil, 5) vegetated, duff-covered mounds, and 6) flat, vegetated, duff-covered mineral soil. All mounded treatments (hummocks) were natural. Microsites associated with hummocks consistently exhibited higher root-zone soil temperatures than similarly covered flat microsites. A difference of 10 to 15 °C in daily mean soil temperature was common between thermally buffered microsites and thermally sensitive microsites on a clear day during the growing season. Accumulated growing season degree-days (8 °C threshold) were greatest for a bare mineral soil hummock and least for microsites covered with forest floor and vegetation. Natural vegetation cover and an intact forest floor were equivalent in their effects on the accumulation of growing season degree-days. Monthly mean air temperatures (+20cm) in all treatments remained above 10 °C for June, July, and August. Daily mean air temperatures (at a height of +20 cm) ranged between 10 and 25 °C. Treatment effects on air temperature were slight. The lowest root-zone water potentials were measured in vegetated hummocks. Growth of Engelmann spruce (Picea engelmannii Parry ex Engelm.) and lodgepole pine (Pinus contorta Dougl. ex Loud.) seedlings was strongly affected by differences in soil thermal regime. Seedlings of both species growing in the cool treatment (daily maximumfrom 10 to 13 °C, daily mean &lt; 12 °C) exhibited increased shoot extension and shoot/rootratios, but reduced root collar diameter, stem biomass, foliage biomass, and root biomass. Seedlings grown in the warm treatment (daily maximum from 16 to 23 °C, daily mean from 14 to 18 °C) showed rapid initial root egress. Cool treatment seedlings exhibited much less vigorous root growth throughout the growing season. This study indicates that low root-zone soil temperatures are widespread in high-elevation openings. Higher soil temperatures are associated with reduced surface cover and the presence of hummocks. In the context of current reforestation practices, traditional silvicultural activities which effectively reduce the thickness of forest floor and the abundance of vegetation are worth continuing. Utilizing hummocks and ridges as planting microsites would also be an effective measure for improving soil temperatures at the root-zone. However, low root-zone temperatures are a function of root location. Use of stocktypes and planting strategies which allow for rapid, lateral growth of roots in surface organic horizons may be effective options worth pursuing.