weed impacts

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.

Ecological impacts of three weed species of similar life form, Asparagus scandens, Plectranthus ciliatus and Tradescantia fluminensis, were investigated in six lowland forest remnants in New Zealand. All three species form dense, ground-covering mats of vegetation, and are tolerant of a broad range of light environments. Relationships between canopy openness, weed volume, native plant abundance and native species richness were investigated. Volume of all three weed species increased as canopy openness increased.

In New Zealand, the European shrub gorse (Ulex europaeus) is becoming the initial post-disturbance shrub, replacing the native myrtaceous manuka (Leptospermum scoparium) and kanuka (Kunzea ericoides) scrub in this role. Change in the dominant vegetation is likely to affect the native invertebrate community.

The dominant native woody species forming early-successional vegetation on formerly forested sites in lowland New Zealand were kānuka (Kunzea ericoides) and mānuka (Leptospermum scoparium) (Myrtaceae). These have been replaced extensively by gorse (Ulex europaeus, Fabaceae), a naturalised species in New Zealand. Because gorse typically gives way to native broadleaved (angiosperm) forest in about 30 years, it is often considered desirable for facilitating native forest restoration.