Facilitation of afforestation by Lupinus nootkatensis and by black plastic mulch in south-west Iceland

Abstract

Afforestation has proven difficult in south-west Iceland in a region of degraded soils and high winds. Experiments at Keflavik International Airport began in 2002 to examine whether Nootka lupine (lupin; Lupinus nootkatensis) or black plastic mulch facilitates establishment of Sitka spruce (Picea sitchensis), Hooker willow (Salix hookeriana) or downy birch (Betula pubescens) by ameliorating microsite conditions. By 2008, both lupine and black plastic mulch facilitated growth of all species at most plots. However, survival of spruce and birch seedlings decreased where dense lupine was accompanied by dense grass (but not in dense lupine alone). This indirect mechanism (nurse plant stimulation of competitor species) differs from prior models of shifts in balance from facilitation to competition under the stress-gradient hypothesis. Hooker willow performed best in both lupine and plastic mulch. However, in areas without dense grass, Sitka spruce continued successful growth and has potential for longer term afforestation. Planting seedlings into shallow excavations in lupine improved growth of willow and birch but not spruce. For afforestation in south-west Iceland, it is recommended that a mix of tree seedlings be transplanted directly into young lupine stands with sparse grass cover, with shelterbelts of seedlings planted into black plastic mulch along the stand edges.