Rooting of Young Interior Douglas-fir Seedlings in Response to a Commercial Liquid Extract of the Marine Macroalga Ascophyllum nodosum

Interior Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. glauca (Beissn.) Franco) inhabits the interior mountain ranges and plateaus of western North America from Canada to Mexico. Ecosystem-based bioclimate envelope modelling for British Columbia, Canada, predicts an expansion of the interior Douglas-fir climate region within 50 years. Thus, reforestation with interior Douglas-fir will likely increase ahead of this shift. In containerized nursery culture, abundant lateral rooting in the upper third of peat plugs can be difficult to attain. Our goal is to provide an inexpensive treatment to improve root-system quality in the nursery, and thus increase initial root growth after planting, which is vital for seedling survival and will become more critical under a changing climate. Earlier, we reported an increase in spring root growth of Pinus contorta Dougl. ex Loud. var. latifolia Engelm. and Picea glauca (Moench) Voss seedlings in response to a liquid extract of Ascophyllum nodosum (L.) Le Jolis (ANE). The objective of the initial trial, reported here, was to determine whether and, at what rate, ANE would increase rooting in young interior Douglas-fir seedlings. We tested a seedlot donated by the British Columbia Ministry of Forests, Lands and Natural Resource Operations that is under investigation in their Assisted Migration Adaptation Trial. We imbibed seed overnight, placed them in dark, cold stratification (4°C) for 21 d before moving them to a germination cabinet (30/20°C D/N, 8-h photoperiod, 85% RH) for 7 d. Next, we suspended resultant germinants in vials filled with ANE in deionized water at the following rates (mL/L): 0, 0.5, 1.0, 1.5, and 2.0. We grouped vials for each rate into separate racks, before randomly arranging racks on heating mats (24°C) under full-spectrum grow lights (90 μmols·m⁻²·sec⁻¹, 16-h photoperiod). We observed germinants/seedlings for rooting after 28 and 56 d. After 28 d, rooting was not evident at any ANE rate. After 56 d, rooting exhibited a response curve to ANE from stimulation through inhibition. Root-system length was increased by 135% at the optimal rate (1.0 mL/L) compared with control. Moreover, secondary roots were initiated on 100% of primary roots at the optimal rate compared with 11% in control. Furthermore, secondary root number was increased 680% at the optimal rate compared with control.