Abstract
Although climate warming is generally expected to facilitate upward advance of forests, conifer seedling regeneration and survival may be hindered by low substrate moisture, high radiation, and both low and high snow accumulation. To better understand substrate-related factors promoting regeneration in the alpine treeline ecotone, this study compared 2 substrates supporting conifer seedlings: rotten downed wood and adjacent soil. Study locations, each with 3 levels of incoming radiation, were randomly selected at forest line–alpine meadow borders in Pacific Northwest wilderness areas extending along an east–west precipitation gradient. Associations among substrate type, seedling density, radiation, site moisture, site temperature, plant water potential, and plant stomatal conductance were assessed. Wood microsites, flush with the ground and supporting Abies spp conifer seedlings, extended up to 20 m into alpine meadows from the forest line. Although wood microsites thawed later in the spring and froze earlier in the fall, they had warmer summer temperatures, greater volumetric water content, and more growing degree hours, and seedlings growing on wood had higher water potentials than seedlings growing on adjacent soil. At drier eastern sites, there was a positive relationship between seedling density and volumetric water content. Further, there was a positive relationship between seedling stomatal conductance and volumetric water content. Our study indicates that in the Pacific Northwest. and likely elsewhere, seedlings benefit from wood microsites, which provide greater water content. Given predictions of increased summer drought in some locations globally, wood microsites at forest line–alpine meadows and forest line–grasslands borders may become increasingly important for successful conifer regeneration.
Highlights
The theme ‘‘Dynamic Planet’’ highlighted at the Perth III Conference is exemplified by studies of upper forest limitation (Future Earth 2014)
Wood microsites and adjacent soil substrates at forest line–alpine meadow borders were assessed under conditions of low, medium, and high percent transmitted radiation, areas initially described by 3 levels of overstory directly under forest-line trees, approximately 3–10 m beyond the forest line into alpine meadow, and open areas generally located 5–20 m beyond the forest line into alpine meadow (Figure 1)
Western wood microsites with low and medium radiation were cooler than their corresponding soil substrates; in high-radiation sites, substrate types were not significantly different (1.768C versus 1.938C, P, 0.001; 1.68C versus 1.788C, P 1⁄4 0.002; and P 1⁄4 0.99, respectively)
Summary
The theme ‘‘Dynamic Planet’’ highlighted at the Perth III Conference is exemplified by studies of upper forest limitation (Future Earth 2014). With climate warming, there is a general expectation that forests will advance, but factors including low substrate moisture, low substrate temperatures, high radiation, fire, and both low and high snow accumulation may limit seedling regeneration and survival, restricting upward advance (Baig and Tranquillini 1980; Callaway 1995; Bansal and Germino 2008; Harsch et al 2009; Tranquillini 2012; Mu€ller et al 2016). Soil temperature is associated with treeline dynamics (K€orner and Paulsen 2004), but lack of moisture, considered the primary cause of seedling mortality (Germino et al 2002; Moles and Westoby 2004), may restrict seedling regeneration and limit forest upward advance (Gieger and Leuschner 2004; Maher et al 2005; Harsch and Bader 2011; Moyes et al 2015; Mu€ller et al 2016).
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