When and where we should, and shouldn't, plant trees.

New Zealanders love to plant trees. After centuries of dedicated deforestation of our lowlands, it seems like everyone now wants to plant trees. Auckland Council wants to plant a million trees. Not to be outdone, Wellington plans two million. Now, the government is gearing up to plant a billion trees. Planting trees is clearly the “In Thing” in Aotearoa New Zealand.

Nature is the best at planting trees

When there are nearby seed sources, nature will eagerly plant trees all by itself. We just need to turn off the obstacles, like rabbits and weeds and grazing stock. If it’s native forest we want, we often don’t need to plant, and in a lot of these places we shouldn’t plant.

Letting nature plant the trees is ecologically the best way to make native forest, by far. The right trees end up growing in the right places, arranged in intricate patterns by species and by genetics within species, to make diverse and resilient forests.

We wouldn’t allow an unregulated network of private aviaries to breed up and release masses of native birds throughout New Zealand. That would do all sorts of undesirable things to the genetics of wild bird populations, as well as having a high risk of spreading diseases. It would also be illegal under the Wildlife Act. A lot of New Zealand tree planting is exactly the same as this, and yet everyone seems to think it’s the right thing to do. I’d argue that most of the time it’s not.

Of course, it’s not that simple. Sometimes it’s not enough to wait for nature to plant the trees. Similarly, we do sometimes need to breed and release captive birds. The critically endangered kakī, or black stilt, would be extinct now if it wasn’t for its captive release programme. Similarly, there are situations where some tree species, and some habitats, need help if native forest is to return. Planting is helpful, and necessary, in some conditions.

In an effort to encourage a more nuanced view of the benefits and problems with tree planting, I offer here one ecologist’s suggestions for the conditions that should be met before trees should be planted, if the goal is to return native forest to a site. I don’t claim to have all the answers, but I would like people to think twice before they plant.

Why do we want to plant trees?

First, though, it’s worth a detour into why everyone in New Zealand wants to plant all these trees. It’s not all about nature. For example, much of the current government’s planned investment in tree planting is to expand commercial plantations of exotic timber species to make forestry jobs for the regions. That’s a good thing, if successful, so long as there’s also a commitment to mop up wildings.

Another big motivation for tree planting is to help suck CO₂ from the air, to buy us time to gradually ease our way out of our addiction to petrol cars and fossil fuelled agriculture. Trees use sunshine and water to eat CO₂ and their wood is mostly carbon, as is a lot of forest soil. Forests are CO₂ sponges.

There are a lot more benefits to riding bikes than planting trees, for the climate, personal health, and social cohesion. Instead, at the moment, we’re driving cars to community tree planting days. Social change takes time, and I suppose tree planting is seen as one way to buy us this time.

Planting trees alongside streams (riparian tree planting) is done to soak up excess nutrients running from fertilised farmland into our waterways. The science so far is mixed about how much thin strips of trees can help, to protect streams from the runoff of excess nutrients from farms. Strips of trees help intercept phosphorus, sediments, and microbial contaminants, but they don’t stop much nitrogen. This is analogous to planting trees to soak up some of the massive amounts of CO₂ released from our fossil fuel burning. In both cases, trees certainly help, but we need to be sure that tree planting doesn’t distract and delay us from implementing real solutions at the source of the pollution.

Underneath much of New Zealand’s tree planting enthusiasm is what I see as a desire, even a perceived obligation, to “fix” nature. It’s an appealing story line: we broke it so we’ll fix it. As a bonus, we’ll fix it quickly! The problem is that inappropriate tree planting can cause as much ecological harm as good, if it’s natural forest and biodiversity that we want.

Botanist Miles Giller wrote an excellent article on that topic in last year’s volume of the Canterbury Botanical Society Journal, titled “Restoration planting – Are we saving or sabotaging our precious native remnants?”. Miles is worried about the long-lasting ecological impacts of the wrong trees, and the wrong genetics, being planted around the last remnants of lowland Canterbury vegetation. It’s well worth a read.

It’s therefore important to consider when it’s appropriate to plant trees, and when we really should focus on pest and weed control and leave the planting to nature.

In what conditions should we plant trees?

I see six common situations where tree planting is necessary to establish native forest. These conditions are widespread throughout much of lowland New Zealand, so if you’re into planting trees, you can find the right places to do it.

Particular tree species are needed in particular places

Not all tree species are equal for supplying ecosystem services and other useful products. The right flowering trees can enhance insect pollination services and the populations of predators of pest insects. Some native trees also provide excellent timber. ESR scientists are currently looking into which tree species are best at killing off E. coli bacteria flowing from farmland into streams. Native rata and manuka are proving to be much better at this than commonly planted exotic pines, populars, and eucalypts. When it comes to selecting trees to protect streams from neighbouring farmland, a particular mix of species is likely to be best.

In these situations, it’s not natural native forest that we want. We’re selecting the right species, and often the right genetics within those species, to do particular jobs for us. We plant them where we want them to grow.

No nearby seed sources

The obvious reason to plant native trees is when there are no natural seed sources within many kilometres of a site. There are plenty of places like that in NZ, especially the drier eastern lowlands. Kick starting native forests in such places needs some planting. In these situations it’s especially important to plant in a diversity of local genetics within species, as well as a diversity of species. It’s this diversity that gives natural forests their resilience.

How many seeds naturally land at a site will be determined by how far away patches of parent plants are, how many seeds those parents are making (how old and healthy the patches are), and how effectively their seeds are being dispersed. Natural seed dispersal to new sites can be improved by looking after the health of nearby native vegetation, such as by weeding and controlling pest herbivores and seed predators, as well as looking after the local pollinators and seed dispersers.

Seeds travel longer distances inside bigger birds, like kererū and tūī. It’s worth casting our eyes forward in time should mammalian predator-free New Zealand be achieved by 2050, or in the decades afterwards. An increase in the extent and abundance of these big birds will result in the long-distance dispersal of many more seeds. Not only will seeds be dispersing further, but there will also be more seed produced as many native trees rely on birds for some, or most, of their pollination. Their seed set will increase when there are more pollinating birds (like tūī and korimako).

Predator control in a landscape should therefore increase the speed and distance at which native forest will naturally return. In some places, some of the money and effort invested in tree planting might be better spent on predator control (and weed and browser control), even in landscapes with only scattered fragments of remaining native vegetation.

Having said that, there are still plenty of sites that are now far removed from any nearby native vegetation. Establishing native vegetation at these sites requires planting.

Break grass dominance

Even in areas with nearby natural seed sources, thick “rank” grass can be a formidable obstacle for native seedlings. That’s especially the case in drier parts of the country. In the summer time these thick stands of exotic pasture grasses can suck all the moisture from the upper layer of soil and kill off tree seedlings trying to establish.

The chokehold of grass on a site can be broken in a few ways. You can mow or hand weed and then lay down weed mat around planted trees to suppress the return of the grass. You can spray herbicide, repeatedly, over the first few years, if you can be careful not to damage the wild and planted native plants at the site. Good results have also been achieved in artificial shading trials, but that’s most practical for small sites.

If there is a nearby source of native seed, nature will plant a diverse natural forest if you can deal to the rank grass. It’s therefore best in this situation to only plant a limited selection of early successional trees, like kanuka, to shade out the grass. Diverse natural forest will follow a decade or two later. Remember, there’s no hurry. Good things take time.

For large sites, it’s also possible to plant islands of kanuka. Those islands of forest will gradually expand out into the grass and take over the site while other trees naturally establish under the older canopies.

When there are natural seed sources nearby, planting should always be seen as priming the pump for the natural processes to return the forest. The full diversity of plants does not need to be planted, and should not be planted.

Improve the odds against weeds

Weeds are a complicating factor when deciding how far is too far to wait for natural processes to disperse in native tree seeds. It might be OK to wait for nature to disperse trees to a restoration site a couple of kilometers away from a healthy forest reserve, when there’s only pasture in-between. It can be a different matter when that site is surrounded by woody weeds growing in private gardens or in nearby scrub. In this situation, the native seeds dispersing in will be heavily outnumbered by the local weeds.

In this case, the best thing to do, if at all possible, is to remove the weed sources on the nearby land. Spending one hour killing one nearby adult tree of a woody weed can save many hundreds of hours of weeding. This can be complicated by private land ownership. Some people treasure their large mature garden trees, even when they’re dispersing thousands of weed seeds into surrounding properties each year.

The other option is to plant native seed sources at, or next to, the restoration site. In this case, planting bigger, older trees is better as they’ll take less time to mature and start seeding. The whole site doesn’t necessarily need planting, just enough to tip the balance back to native regeneration.

Reduce genetic pollution from gardens

The more complicated problem with nearby gardens is the planted natives that start seeding into a restoration sites. That’s great when the local gardeners have planted eco-sourced local natives, but it’s usually quite the opposite. Instead all manner of eccentric botanical cultivars and ornamnental hybrids are planted, sourced from all over New Zealand.

This does odd things to wild vegetation in urban areas. Seedlings at these sites are often dominated by hybrid swarms of Pseudopanax species and Coprosma species, and by many non-local natives that have temporarily escaped their usual natural enemies and other constraints to become as abundant as weeds.

At the extreme, when a small reserve is surrounded by a larger sea of houses and gardens, the risk is that the local genetics in those reserves will get washed away in the flood of immigration by pollination and seed dispersal. In nature, as in human societies, local knowledge tends to win only when there’s a level playing field.

If the goal is to restore forest to a site surrounded by gardens, and that this site should retain its local genetic integrity and diversity, then the gardens will need to be considered. Lots of nearby non-local natives, or even a large number of clonally propagated local cultivars, will make it challenging to establish and sustain a native forest that safeguards the original local genetics of the area.

It’s OK to throw your hands up and give up at this point. Urban wild places are typically novel ecosystems of their own—a melting pot of local and exotic species, just like the cities around them.

However, if you’re trying to sustain, and perhaps expand, a valuable old-growth urban reserve, then you’ve got to think carefully about genetic integrity and the real threat of genetic pollution from surrounding gardens.

One such place is the old growth Riccarton bush in Ōtautahi Christchurch, one of just three surviving patches of lowland Canterbury Plains podocarp forest. A huge effort late last century was spent removing North Island lacebark from the reserve after it was purposefully planted in there and started taking over. I’m not aware of any genetic studies on what is happening to the genetic makeup of the trees in Riccarton Bush, but they must be pollinated with pollen from nearby garden cultivars of the same species.

The solution to genetic pollution is to treat non-local native cultivars in nearby properties the same as you would weeds. Whenever possible, remove these seed and pollen sources and encourage people living near wildlands to plant only eco-sourced natives or exotics. Widespread gardening and planting must be making it difficult to source genetically local eco-sourced seed from forests anywhere near NZ cities and towns.

Of course, the very last thing you should do is surround an important wildland with inappropriately sourced planted trees. Yet, that’s often what happens.

Create new populations of critically threatened species

Some endangered plant species, just like endangered wildlife, can benefit greatly from being established in new areas of suitable habitat. Some nationally and regionally endangered New Zealand plants are now used in gardening and restoration projects. The shrub (Muehlenbeckia astonii)[https://inaturalist.nz/taxa/410120-Muehlenbeckia-astonii] is a widely planted example. There must easily now be many thousands more planted Muehlenbeckia astonii than grow in the wild.

When endangered trees are being planted for conservation reasons, it’s especially important that they’ve got a diversity of the right genetics. Some species can be propagated as clones grown from cuttings. In this case it’s important for the long term resilience of a population that many different clones are planted, preferably sourced from many different individuals from a wild source population (and done with the right collection permits). There’s a great example of that having been done in Wellington where the original wild population, the source of the cuttings was later destroyed by a housing development, but all the genetics of that population remain safe in restoration plantings.

If you instead planted out a big stand of one easily propagated clone, for example, because they were cheap from a nursery, there’s a good chance the planting, or it’s inbred offspring, will later get badly knocked back by a pest or disease outbreak, or dieback in an extreme weather event. Genetic diversity is nature’s way of preparing for challenges.

Dos and don’ts of tree planting

Do

Do a species survey of your site before planting. You can use (iNaturalist NZ)[htt[s://inaturalist.nz]] to identify all the species you don’t know. Make sure that your planting methods don’t damage the natural values already at a site. A lot of tree planting in NZ involves site preparation with herbicides, especially in rural areas choked with rank grass. Look carefully before you decide to spray.

Do control the worst weeds and pests at and near a site. In many places, that’s all you’ll need to do. No planting required. Remember that good things take time.

At sites in heavily deforested landscapes, many kilometres from the nearest native forest, do plant a diversity of tree species that would have once grown in the area. Make sure to include a diversity of the local genetics within those species. Also consider later planting appropriate understorey plants. Be sure to document everything you plant, and where its seed was sourced from.

At sites choked with thick rank grass that are close to areas of native forest, do plant the appropriate early successional trees to shade out the grass. In many places, just the local kanuka or manuka species will do. Keep out stock and pest herbivores and let nature plant everything else. You’ll have diverse native forest regenerating in a few decades.

Do eco-source all plants from seed obtained from local, original native habitats, preferably from nearby old growth remnants. If there have been plantings in a local habitat before, especially planting of trees from undocumented sources, then it cannot be used for eco-sourcing seed. Many smaller reserves in New Zealand are now a messy mixture of original plants and planted plants or unknown origin, which means that they cannot be used for eco-sourcing. Remember that even if the mother tree making the seed is an original local tree, you don’t know who were the father trees that made the pollen.

If private gardens and other weed sources are closer to a site than native forest, do attempt to remove or reduce those weed sources. Weeding out young weeds while not removing nearby parent weeds wastes a lot of effort.

If you cannot reduce the neighbouring weed seed sources, consider planting clearly defined islands of eco-sourced natives at your site to provide closer seed sources to fuel native regeneration across the rest of the site.

Do try to mimic natural succession sequences. Plant early successional trees like kanuka and let natural processes do their thing as much as possible.

Don’t

Don’t plant if you can’t properly eco-source. Seriously. Don’t do it.

Don’t plant species that will naturally arrive by themselves in the next few decades.

Don’t plant a lot of clones cultivated from cuttings, unless they’re of short-lived early successional trees that you don’t expect to persist at the site. Wild forests need genetic diversity to deal with native pathogens and herbivores and environmental changes.

Don’t plant then walk away. Site weeding and pest control is more important than planting. If you aren’t able to commit to sustained weeding and pest control, then don’t plant.

Wrapping up

Natural forests are wild forests. They naturally grow in complexity over decades and centuries. From the hundreds of thousands of seeds that naturally disperse to the forest floor, the right species with the right genetics naturally end up growing in the right places. We short-circuit that process when we plant.

There’s also no rush to get big old trees established. Different species thrive at different stages of forest succession.

In some conditions, tree planting can be useful to prime the pump for the return of native forest. Planting should always be viewed as priming the pump. There is no need to completely supplant the wild natural processes and plant out a complete arboretum of nursery-grown trees. That causes more ecological damage than good, and compromises the integrity of a forest.

Focus on helping the natural processes that make forests, rather than replace them. Controlling weeds and pests, both predators and browsers, remain much more important than planting, for returning native nature to most of New Zealand.

[Coming up next: the ecological implications of replacing natural forest succession with tree planting for future forest composition.] [See also the previous article, how to decide where and when tree planting is really necessary.]