Site preparation effects on Douglas-fir and lodgepole pine water relations following planting in a pinegrass-dominated clearcut

Abstract

Plantation establishment on drier sites is often hindered by harsh microclimatic conditions and inappropriate species selection. This study examined first-growing-season water relations of outplanted Douglas-fir ( Pseudotsuga menziesii var. glauca [Beissn.] Franco) and lodgepole pine ( Pinus contorta var. latifolia Engelm.) on a pinegrass-dominated clearcut in southern British Columbia. Seedlings were spring-planted in two consecutive years in an untreated control and in scalped, ripped, and herbicide site preparation treatments. After mid June, mid-morning stomatal conductance ( g sm) and transpiration ( E m) of both species, and mid-morning twig xylem pressure potential ( Ψ txm) of lodgepole pine, were consistently greater in the site preparation treatments than in the control. In contrast, there was relatively little difference in g sm, E m or Ψ txm of either species in the different site preparation treatments. Stomatal response of both species was largely a function of root zone extractable water ( θ e), seedling-height saturation deficit, and solar irradiance. Site preparation effects on g m were largely manifested through increased θ e. Lodgepole pine g sm and Ψ txm were often higher and soil-plant liquid path resistances lower than those of Douglas-fir, despite similar E m levels for the two species. Lodgepole pine showed greater response in g sm and E m than Douglas-fir to improved environmental conditions resulting from site preparation. Lodgepole pine water relations responses were consistent with a strategy of dehydration postponement, while Douglas-fir responses were consistent with a strategy of dehydration tolerance.