Jump‐starting podocarp forest establishment in a wetland dominated by invasive willow

Abstract

Kahikatea (Dacrycarpus dacrydioides) is a tall, wetland‐tolerant native podocarp tree that has potential for restoration of New Zealand wetlands currently dominated by invasive Grey willow (Salix cinerea). We investigated methods to jump‐start establishment of Kahikatea saplings in the Ramsar‐listed Whangamarino Wetland. Treatments involved planting saplings after full herbicide clearance of the willow canopy (FULL), partial herbicide clearance of the canopy (PARTIAL), and manual cutting of light wells (MANUAL). Saplings were planted as individuals or clusters of seven. Treatment plots were established in 2015 and monitored until 2020. Despite Kahikatea being considered a “light‐demanding” species, saplings grew best in the PARTIAL treatment which had the lowest ambient light levels (x̄=32% of non‐intercepted sunlight). PARTIAL treatment saplings were estimated to require 24–36 years to reach the height of the surrounding willow canopy if they maintained similar growth rates. Preconditioning saplings by soaking roots before planting provided a growth advantage in the PARTIAL treatment plots, whereas nonconditioned plants grew better in the higher‐light MANUAL and FULL plots. No benefit was found for cluster planting. Restoration toward a native Kahikatea‐dominated forest will be contingent on saplings reaching willow canopy height before the faster‐growing willows reestablish, or periodic management of willows to maintain light levels above the compensation point for continued growth. Ultimately, the utility of this approach depends on what on‐going facilitation is required and whether seedlings subsequently produced by these emergent podocarp “islands” are able to jump‐start a long‐term change in the successional trajectory of the surrounding willow‐dominated wetland.