Restoring Landscapes Post-Farming: Is Replanting Really the Answer?

By Nina de Jong

Sheep grazing on farmland at Tawharanui, New Zealand. Farming drastically transforms the hydrology and ecology of a landscape. Photo by Koon Chakhatrakan on Unsplash (2020).

Volunteer restoration groups in Aotearoa are a huge part of national conservation and restoration progress. Generally, we have a “top-down” approach to environmental management, where the Department of Conservation largely manages public land. However, resource constraints often lead to communities taking on restoration projects, overseen and approved by DOC. In many reserves, community groups do the bulk of the management and provide on-the-ground labour. These groups engage in a huge range of activities including pest trapping, native bird and invertebrate monitoring, building recreational and conservation infrastructure such as tracks, toilets, nurseries, and tree planting.

Tree planting is usually central to restoration, and arguably for a good reason. Replanting can accelerate ecosystem regeneration processes [1]. It is seen as important for carbon fixation [2], and aligns more with human time scales — natural regeneration is far slower [3]. There are also strong social dimensions to tree planting. It’s a great activity for getting stuck in and making change that you can see. It’s extremely satisfying to overlook a field that a group of you have replanted, and growing native plants in the nursery is delightful and industrious. Restoration groups are important places for forming friendships and communities, and the practices centred around replanting keep meetings regular and give groups direction and purpose. I can say this from personal experience! However, replanting is not the only approach restoration groups can and should take, both for the landscape they are involved with and the group themselves. How we are replanting, and perhaps in some cases, whether we should replant at all, might be important to rethink.

Over recent generations, land in Aotearoa that was previously used for farming has been transferred to regional parks and state reserves, and now this land is managed with environmental conservation in mind. In many instances, new land that is adopted into conservation management has been drastically altered from its pre-human condition by farming. Like any land-use activity, farming involves making profound changes to the environment. Pākehā settlers and the colonial state that bought or confiscated millions of acres from Māori, had clear priorities: to make the land suitable for farming, and become educated in profitable farming techniques [4]. To do this, firstly they had to burn off the forest to create better conditions for agriculture. If the land was poorly draining, farmers would drain the land so that it wasn’t as muddy, reducing the extent of wetlands and increasing the probability of river flooding. To reduce river flooding, they then needed to place stopbanks and train river channels so that land would flood less frequently, and the river would carry water more efficiently out of the farm. Drained land and naturally drier land is more drought prone, and so irrigation from rivers back onto the fields might be necessary. As farming is practiced, soil fertility is reduced. Without continued input of nutrients from ecosystem processes like decomposition, nitrogen-fixing, or regular flooding, fertilisers must be added to the land to improve production. This creates nutrient-rich run-off that flows into waterways, promoting algal blooms, anoxic conditions and local extinction of aquatic species… and of course you can follow these threads on and on.

These processes are not straightforward and don’t always occur in the same way or in the same order. However, the important pattern is that colonial farming practices develop a completely different set of hydrological and ecological processes for the land they are practiced in. The changes made under agricultural production do not disappear when the farming stops and planting starts. History accumulates in the memory of a landscape, which means reversion back to “pristine”, pre-human ecosystems will never happen. This has consequences for remediation of land. Restoration approaches have to understand the land’s history, and take this history into the new, remembered, ecosystem of the future. For example, replanting at the edges of streams is often considered an effective way to absorb and filter nutrient run-off from farms before it reaches the water. However, on farms where drains have been laid or where groundwater flow is deeper than where roots have grown to, run-off can flow straight past riparian planting and into waterways [5]. If this history is not addressed, the same issue remains and the replanting is ineffective.

Clearing vegetation is only one part of the drastic transformations that landscapes undergo when land conversion occurs. The complex networks of connections between soils, waterways, animals, plants, fungi, nutrients, and everything in between means that restoration projects have to tackle lots of things at once. Successful replanting relies on the state of other aspects of the environment, such as water availability, pest species, and exposure. For example, some plants that do best in wet places, such as kahikatea, may struggle to grow in a landscape that has been drained, where they can’t compete with other species [6]. A lack of pest control can prevent the success of planting, as rabbits, possums and deer can easily continue to browse new plants to death, and invasive plants can smother native trees [7,8,9]. It might be better to focus just as much, or more, on these other issues than to give all our energy to replanting.

Across the world, reforestation projects often use only one or only a few species when replanting, leading to low biodiversity plantations that do not resemble a naturally regenerating land [10]. In Aotearoa, replanting schemes use mānuka and kānuka far in excess of any other plants. This is because they’re thought of as classic “pioneer” species that grow quickly in exposed paddock environments, creating shelter and shade for trees more typical of mature forest to grow beneath. The economic value of mānuka honey has also led beekeepers to plant monospecific crops for honey-making. However, this approach may not be best for restoration if creating a “natural” forest is the goal. Mānuka and kānuka have very dense canopies, especially when they are young. Although some protection from the elements is important for many forest trees, the darkness beneath dense mānuka or kānuka canopies prevents most plants from surviving, especially where the trees are still low and bunched up. As a result, instead of the multi-storeyed, species-rich, complex forest structures that you see in naturally regenerated kānuka or mānuka dominated pioneer forest, beneath these monospecific canopies, there is usually bare, dry ground. Other plants often used in pioneer planting such as harakeke, do not actually have a canopy at all as it is a large flax bush. Tī kōuka has only a small canopy that is unlikely to provide extensive shade and shelter. The conditions required for the biodiverse, multi-layered forests everyone wants to see might not be achieved by planting large, dense swathes of just a few species.

Replanting is an important and popular aspect of landscape restoration. Photo by Lachlan Cormie on Unsplash (2019).

The scientific understandings of actual natural regeneration and succession in native ecosystems are far from complete. Plants used in “pioneer” planting are used to recreate what research has suggested occurs in natural landscape regeneration, but this isn’t the only way a forest can develop. Not all mature forest trees require a sheltered nurse canopy to grow beneath. Iconic forest species such as kahikatea, totara, and other conifers need high light environments to regenerate [11]. If all the abandoned fields across the country were planted with “pioneer” plants, we could lose opportunities for these conifers to establish. The plants that arrive first in a regenerating plot often determine what comes next. For example, the native species established beneath gorse growing in a paddock are different from the species found growing beneath mānuka [12], and the species that regenerate under kānuka and silver fern are not the same species that regenerate beneath mamaku [13]. Letting land regenerate naturally can reveal the interesting patterns of how plants appear in the landscape. It is fascinating to see how many different directions ecosystems might take, and how this could lead to different plant communities [14]. If we create “garden” forests, where we try to recreate natural regeneration with our best guess at the plants that should be there, we might never know what sort of community would have developed otherwise.

Replanting landscapes can be very important, for example where erosion is a major problem or where there is social pressure for action and results. However, not every landscape must be replanted, because every landscape is different. Other aspects of restoration, such as raising the water table, or reducing pest plants and animal populations, might be more important to restoring the wellbeing of a landscape. Sometimes it can feel like replanting is a way of covering over our mistakes — we can cover a paddock with trees and pretend it was never a paddock at all. However, the landscape doesn’t forget, and it isn’t transformed back to a pre-human ecology by planting. It might be slower, less satisfying, more unpredictable and even more work (if other aspects of restoration are more difficult), to just let plants naturally return to the land. However taking things slowly, learning as we go and working with landscapes rather than just re-managing them over again, might be an interesting alternative for restoration.

References

[1] Omeja, Patrick A., Colin A. Chapman, Joseph Obua, Jeremiah S. Lwanga, Aerin L. Jacob, Frederick Wanyama, and Richard Mugenyi. "Intensive tree planting facilitates tropical forest biodiversity and biomass accumulation in Kibale National Park, Uganda." Forest Ecology and Management 261, no. 3 (2011): 703-709.

[2] Bastin, Jean-Francois, Yelena Finegold, Claude Garcia, Danilo Mollicone, Marcelo Rezende, Devin Routh, Constantin M. Zohner, and Thomas W. Crowther. "The global tree restoration potential." Science 365, no. 6448 (2019): 76-79.

[3] Holl, Karen D., and T. Mitchell Aide. "When and where to actively restore ecosystems?." Forest Ecology and Management 261, no. 10 (2011): 1558-1563.

[4] Nightingale, Tony. “Government and agriculture.” Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/government-and-agriculture/print (accessed 21 May 2021)

[5] McKergow, Lucy A., Fleur E. Matheson, and John M. Quinn. "Riparian management: A restoration tool for New Zealand streams." Ecological Management & Restoration 17, no. 3 (2016): 218-227.

[6] Ogden, John., Stewart, Glenn. H. “Community Dynamics of the New Zealand Conifers.” In Ecology of the Southern Conifers, 81-119. Melbourne University Press, 1995.

[7] Husheer, Sean W. "Introduced red deer reduce tree regeneration in Pureora Forest, central North Island, New Zealand." New Zealand Journal of Ecology (2007): 79-87.

[8] Gillman, L. N., and J. Ogden. "Seedling mortality and damage due to non‐trophic animal interactions in a northern New Zealand forest." Austral Ecology 28, no. 1 (2003): 48-52.

[9] Standish, Rachel J., Alastair W. Robertson, and Peter A. Williams. "The impact of an invasive weed Tradescantia fluminensis on native forest regeneration." Journal of Applied Ecology 38, no. 6 (2001): 1253-1263.

[10] Seddon, Nathalie, Beth Turner, Pam Berry, Alexandre Chausson, and Cécile AJ Girardin. "Grounding nature-based climate solutions in sound biodiversity science." Nature Climate Change 9, no. 2 (2019): 84-87.

[11] Lusk, Christopher H., Murray A. Jorgensen, and Peter J. Bellingham. "A conifer–angiosperm divergence in the growth vs. shade tolerance trade‐off underlies the dynamics of a New Zealand warm‐temperate rain forest." Journal of Ecology 103, no. 2 (2015): 479-488.

[12] Sullivan, Jon J., Peter A. Williams, and Susan M. Timmins. "Secondary forest succession differs through naturalised gorse and native kānuka near Wellington and Nelson." New Zealand Journal of Ecology (2007): 22-38.

[13] Brock, James MR, George LW Perry, William G. Lee, Luitgard Schwendenmann, and Bruce R. Burns. "Pioneer tree ferns influence community assembly in northern New Zealand forests." New Zealand Journal of Ecology 42, no. 1 (2018): 18-30.

[14] Wilson, H. D. "Nature not nurture; minimum interference management and forest restoration on Hinewai reserve, Banks Peninsula." Canterbury Botanical Society Journal 37 (2003): 25-41.