Composition of the understory in 132 woody weed populations and implications for succession

The species composition of the understory can be a key indicator of successional trajectories in the absence of disturbance at forested sites. We surveyed species composition and percent cover in the understory of 132 closed-canopy stands of 41 woody weed species throughout New Zealand as a first step in understanding potential successional trajectories in these weed populations. Twenty-seven weed species had zero, or very few, conspecific seedlings or saplings present beneath their own canopy. Fourteen weed species had medium to high numbers of conspecific seedlings and/or saplings present. Some weed species had variable understory regeneration, with high numbers of conspecific seedlings and saplings present at some sites, but none at others. Twenty-eight weed species had native understory cover of ≥ 50% at one or more sites. Native understory cover was higher at sites close to remnants of native vegetation compared to sites distant from native vegetation. Overall, many more native than non-native species were present in woody weed understories. Melicytus ramiflorus (mahoe) was the most common native species, present at 67% of sites. At least 76 other native species were recorded at five or more sites. Our results demonstrate that (1) woody weed species vary in the extent to which they regenerate under their own canopy, and (2) closed canopy woody weed stands frequently have a predominantly native understory. Further research to determine whether the composition of the understory can be used to predict successional trajectories in woody weed populations would be valuable.