Impacts of herbicides used for control of invasive Pinus contorta on the potential for reinvasion and germination of restoration species

Herbicides that persist in the forest litter and soil following their use for managing invasive plant species may negatively affect restoration efforts as well as minimize reinvasion via their residual phytotoxic activity. This study determined the impact of an herbicide mixture comprising triclopyr, dicamba, picloram, and aminopyralid (TDPA) for the control of dense infestations of a woody invader, Pinus contorta Douglas ex Loudon, on the germination of reinvading P. contorta and three New Zealand native species (Chionochloa rubra Zotov, Nothofagus cliffortiodes (Hook. f.) Oerst., and Leptospermum scoparium J.R. Forst. & G. Forst.) used in restoration. Given the essential role of ectomycorrhizal fungi in facilitating conifer reinvasion, the impact of residual herbicides present in mineral soil on the ectomycorrhizal infection of P. contorta seedling roots was also examined. Germination trials were conducted using intact forest litter-soil cores collected at 27, 112 and 480 d (after herbicide spraying) from sprayed and adjacent unsprayed dense P. contorta infestations. At the same time, mineral soil was also collected for the ectomycorrhizal infection study. Post-spray herbicide residue bound in the litter significantly decreased survival, germination rate, root and shoot growth, and also caused malformation of P. contorta seedlings. Similar results were recorded for native species' germination; however, overall viability of native seed was poor, resulting in low germination rates. There was no difference in levels of ectomycorrhizal infection rates of P. contorta between treatments. Results indicate residual levels of TDPA herbicide in forest floor litter negatively affect P. contorta reinvasion, native recruitment, and active restoration management. Ectomycorrhizal fungi, however, are unaffected by this herbicide mixture and therefore remain a risk to facilitating reinvasion as residual herbicide declines.