Novel Future Forests in New Zealand’s Fragmented Lowland Environments

Maintaining Lowland-Forest Diversity and the Opportunity of Exotic Canopies

Effective strategies to maintain biodiversity place high priority on retention and protection of existing values. Though, in many instances, our forested landscapes have already been fragmented and modified to a point that those forests no longer sustain a diversity of organisms. 

A degraded fragment of native forest.

Native woody habitats have typically contracted, and conversely, exotic woody communities have expanded to occupy niches once filled by native species.  Thus, if the maintenance of native biodiversity is the goal—we must look to these new and developing exotic dominated ecosystems for opportunities to meet our aspirations for the maintenance of native biological diversity. Given any reasonable prospect for transitioning habitats from exotic to native dominance, a thorough understanding of the interaction between native and exotic communities becomes highly desirable.

Species-Area Relationship, Showing Diminishing Diversity With Increasing Habitat Loss. Source: Walker et al. (2008): Accessed from http://www.doc.govt.nz/Documents/science-and-technical/sfc284entire.pdf

In fragmented landscapes, traditional approaches that focus solely on the protection of existing native habitats are unlikely to adequately maintain either healthy-functioning forests or functioning forest landscapes. Under such urgent circumstances, it would seem prudent to closely consider supplementary means of sustaining forest ecosystems in highly-fragmented landscapes. 

Forest Sustainability in Ecologically Dysfunctional Landscapes

The effect of habitat pattern on ecological process is well established in the landscape ecology literature.  Adequate habitat pattern is a prerequisite for the sustenance of ecological processes which are vital for diversification at genetic, species, habitat, and landscape scales. Habitat fragmentation and isolation weakens both pattern and process, bringing on a cascade of effects, including reduction in the ability of seeds to be dispersed between patches. 

Native Lowland-Forest Fragment.

Many of New Zealand’s contemporary lowland forests are of a fragmented configuration, or are missing representative mature forest canopy species. As such, ecological isolation is a chronic issue in contemporary lowland forest ecosystems. Exotic forests might provide supplementary habitats to facilitate forest regeneration in highly fragmented landscapes.

The absence of propagule sources has significant implications for the composition of future forests and ecological isolation from native forest seed sources is a key factor constraining a successional trajectory towards an old-growth native forest composition in many areas of lowland New Zealand.  This factor applies to successions from either native or exotic nurse crops.

Exotic Canopies as Surrogate Habitats in Fragmented Landscapes

We need to better understand how forests comprising exotic species might supplement remnant native forest habitats, and contribute towards the maintenance of biological diversity. In place of former native forest, the New Zealand landscape contains exotic forests, many of which have the potential to facilitate native forest regeneration. Prominent examples include forests of Ulex europaeus (gorse; Fabaceae), and Salix spp. (willow spp.; Salicaceae). Exotic canopies inter-dispersed amongst fragmented landscapes might present valuable opportunities for the regeneration and restoration of species and better-connected native forest communities. 

Mixed Landscape Comprising Native and Exotic Forests.

Exotic gorse is a major invasive species in New Zealand, specialising in colonising disturbed sites, and with long seed dominancy, and high reproductive efficiency under New Zealand conditions. The species has expanded to form a common element in today’s early-seral forests across mainland New Zealand. However, while invasive, in recent decades the species has been recognised for its role in providing sheltered conditions, suitable for the establishment of some native woody species.  Large tracts of land across New Zealand have attained native dominance through successional development from gorse stands.

Seral Forests Attaining Indigenous Dominance from Gorse Forest.

Stands of invasive bitter willow (Salix eleagnos) have been observed to facilitate the recruitment of both broadleaved angiosperms and podocarps. Shade cast by the exotic bitter willow excludes an exotic grass sward which provides an open substrate for the establishment of native seedlings. Nevertheless, based on existing knowledge, the successional trajectory of this exotic forest type towards native dominance is still being unraveled. 

Example of intact bitter willow canopy excluding exotic grass through shading.

Mature forest canopy species regenerating under manipulated bitter willow canopy.

Novel Forests from Exotic Canopies

Where forest regeneration occurs within the understories of these exotic forests, specific canopy types potentially lead to different patterns in both understorey composition and the successional trajectory that might ensue. For example, successions through exotic gorse and native Kunzea follow different successional pathways, and lead to different forest types. Comparative studies¹ of gorse and Kunzea have shown that stands of each support a different composition of species, and that a gorse nurse typically has a lower species richness compared to a Kunzea nurse.

Exotic Canopies as a Means of Restoring Native Forest Cover

These observations suggest that secondary successions from naturalised vegetation communities might provide valuable opportunities for facilitating native forest regeneration. As such, sites of existing exotic dominance might provide opportunities for the restoration of native forest cover. 

Expansive exotic forest, predominantly crack willow.

However, existing evidence suggests that those future forests will not always be of a similar composition to those successions that are derived from native nurse crops, and further research is required to better understand the role of exotic nurse crops in facilitating native forest regeneration. One particularly prominent challenge when deciding whether to utilize exotic canopies for restoration of native vegetation communities is balancing the risks associated with further invasive spread of exotic species against the establishment and successful recruitment of native flora. 

Realistically, even successions from native nurse crops will in many cases be lacking representative canopy species due to isolation from seed sources.  This then raises the question: what should we realistically expect from forest regeneration from exotic canopies in highly-modified lowland environments?  When considering the role of exotic communities in recruiting native forest species, is it realistic to make value judgements based on compositional comparisons with intact primary forest remnants?  Or is a more realistic frame-of-reference secondary native successions, that face the same contemporary constraints on successions as those exotic communities developing in today's degraded lowland environments?

References

¹ Allen, R. B., Partridge, T. R., Lee, W. G., & Efford, M. (1992). Ecology of Kunzea ericoides (A. Rich.) J. Thompson (kanuka) in east Otago, New Zealand. New Zealand Journal of Botany, 30, 135–149. 

Lee, W. G., Allen, R. B., & Johnson, P. N. (1986). Succession and dynamics of gorse (Ulex europaeus L.) communities in the Dunedin Ecological District, South Island, New Zealand. New Zealand Journal of Botany, 24(2), 279–292.

Williams, P. A., & Karl, B. J. (2002). Birds and small mammals in kanuka (Kunzea ericoides) and gorse (Ulex europaeus) scrub and the resulting seed rain and seedling dynamics. New Zealand Journal of Ecology, 26(1), 31–41.