Factors Influencing Understory Seedling Establishment of Engelmann Spruce (Picea engelmannii) and Subalpine Fir (Abies lasiocarpa) in Southeast Wyoming

Although trees of Picea engelmannii were more abundant and had a greater basal area in the overstorey than those of Abies lasiocarpa at two study sites, saplings and seedlings of A. lasioscarpa were more numerous in the understorey. Field measurements in the Medicine Bow Mountains of southeast Wyoming showed that seedlings of A. lasiocarpa had greater photosynthetic rates at low understorey light levels and required lower levels of incident radiation for saturation of photosynthesis than those of P. engelmannii. However, both conifers occurred in understorey locations where total daily solar radiation was equally low (\u3c 2 MJ.m-2.day-1). Seedlings of P. engelmannii were substantially more abundant at locations with thinner litter layers compared with those of A. lasiocarpa. Also, laboratory studies showed that the smaller seeds of P. engelmannii germinated more rapidly and at lower temperatures than seeds of A. lasiocarpa although growth of tap roots on A. lasiocarpa seedlings was 50-200% greater. The deeper penetrating tap root of A. lasiocarpa seedlings may enable this conifer to establish more abundantly on thick, rapidly drying litter layers that are characteristic of mature spruce-fir forests. In contrast, establishment of P. engelmannii seedlings may be limited to microsites without a thick litter layer such as disturbed areas or decomposing wood, where surface drying may occur more gradually throughout the summer