Towards a Nurse-Crop Prescription for Native Forest Restoration

Exotic grass as a competitive barrier to forest regeneration

Exotic grassland covers much of contemporary lowland New Zealand. This predominance of exotic grassland means that exotic grass swards are often present in areas where regeneration of native forest is desired. When these exotic grasslands are retired from grazing – being set aside for restoration purposes – a dense, tall, rank grass sward rapidly develops. Despite best intentions, retirement of pastoral land for restoration of forest cover commonly leads to forest clearings, forest margins, or planted sites being choked with a dense cover of exotic grass. This dense rank grass cover creates highly competitive conditions which either reduces or inhibits the regeneration of woody forest flora.   

A dense exotic grass sward surrounding the margin of a native forest remnant.

While the protection of existing forest remnants is critical to the maintenance of native biodiversity in New Zealand’s lowlands, the expansion of existing sites is also important. Once fenced from stock, the competitive barrier presented by a dense exotic-grass sward means that further intervention, such as restorative planting of native species, would be necessary to facilitate the expansion of the forest boundary. However, given the high financial cost and labor requirements of establishing native species, in practice – the process of forest expansion is normally left to the natural regeneration processes of seed dispersal, seed germination, seedling establishment, and subsequent recruitment.  In the presence of exotic grass, the strong competitive effects on regeneration processes mean that reliance on natural regeneration processes for site expansion is a sub-optimal approach. 

Illustration of the proliferation of exotic grass cover when pasture is retired from productive land use.

Native restorative plantings amongst a dense cover of exotic grass.

Approaches to overcome the competitive barriers of exotic grass

The concept of "nurse crops" has become well established in forest restoration circles. Creating a shaded environment by rapidly establishing a canopy is a particularly useful way of excluding light-demanding competitors, such as exotic grass species. The shelter provided by the nurse canopy moderates microclimatic conditions, favoring the establishment of seral and late-successional woody species of greater shade-tolerance.  However, the high financial cost of planting and initially maintaining a native nurse crop puts native restorative planting beyond the reach of many Land Managers. 

Example of shade cast by a nurse crop, resulting in thinning of the exotic grass cover, thus exposing a soil substrate for potential seedling establishment.

Kahikatea (Dacrycarpus dacrydioides) seedling establishment under the shade of a mixed native-exotic forest canopy, free from the competition of the surrounding exotic grass cover.

Towards an affordable, rapid, exotic nurse-crop restoration prescription

I suggest that a non-invasive, short-lived, fast-growing, exotic species might provide an affordable, robust, repeatable approach for Land Managers to create a shaded environment that is unsuitable for the persistence of exotic grasses. Such nurse species could be used around native forest remnants, riparian zones, and restorative planting sites, close to native forest remnants where native forest plants can disperse to establish themselves, and where native forest regeneration can then proceed. This would be an alternative approach to planting costlier, slower-growing, native nurse species.

Through trials, a restoration prescription for a restorative exotic nurse could be developed.  Trials would be required to determine the optimum planting density, approach to management, and applicability of the prescription to different sites and climatic conditions. Based on the trial results, a restoration prescription would be developed for Land Managers wishing to establish native forest regeneration on sites currently dominated by exotic grass species, located in close proximity to native forest seed sources.

Tree lucerne (Chamaecytisus proliferous var palmensis) is an exotic shrub or small tree, which grows to about 5 m tall, is relatively short lived, low cost, and easily established. It has a preference for good drainage, tolerates coastal situations, provides a winter flower source for bees, and a seasonal food source for New Zealand pigeon (Hemiphaga novaeseelandiae). The species has received some previous attention as a nurse crop for establishing both amenity trees and native forest species. As such, tree lucerne is an obvious candidate species for a forest restoration nurse crop prescription in New Zealand’s lowland pastoral landscapes, although its invasiveness requires careful consideration.

If you are interested in knowing more, or being involved in the development or funding of the exotic nurse crop restoration prescription, please contact Dr. Adam Forbes for further information.