History matters in ecology, so be careful what history you make.

I had a good reminder this week that the answer to everything in ecology is “it depends”. I’d been invited to give a couple of half hour introductory ecology lectures at a one-day community conservation symposium on New Zealand’s West Coast. About 100 people from community groups along the West Coast came together to mingle and learn. My first slot was about West Coast forest ecology. In it, I emphasized how history can make a big difference to ecosystems. Which ecological communities are found where depends not just on different sites’ environmental conditions, but also their history. “It depends.”

The classic example relevant to this audience was the West Coast Beech Gap. Southern beech species dominate the forests at the southwest and northwest of New Zealand’s South Island. However, they are absent in between, giving way to a very different and more diverse forest of podocarp conifers and broadleaf angiosperm trees.

These forests are different ecosystems in many ways. For example, the trees use different types of mycorrhizal fungi in the soil. Beech trees use ectomycorrhizal fungi that coat the outside of their roots, while popdocarps and most other New Zealand flowering plants use endomycorrhizal fungi that penetrate deep into their roots.

Most of the trees in these podocarp-broadleaf forests make bird-edible fruits, great for sustaining bird life, but also great for feeding invasive brush-tailed possums and rats. Beech trees, in contrast, make beech nuts that disperse (poorly) in the wind and are eaten by the few birds that eat seeds rather than fruit (like New Zealand’s kakariki parakeets).

Beech forests are generally worse habitats for possums and rodents than podocarp-broadleaf forests. That would be good for native birds except for one thing. Beech forests also mast seed, meaning that every few years they produce massive seed crops. In these years, mice populations, and sometimes also rat populations, erupt to plague proportions and hammer the beech forest birds and invertebrates. Mice and rats are also prey for stoats which increase in numbers and hammer the birds when they go hungry after the rodent plague collapses. Everything then starts to recover, before the next beech mast hits.

These differences, and others, mean that the beech-free Beech Gap forests are ecologically very different from the beech forests of the south and north. How did this come to be? The surprising conclusion of the many ecological studies on this is that it’s an historical accident. The glaciers from the past glacial maximum (“ice age”) drove away all forest from this middle section of the West Coast. When the glaciers retreated by about 10,000 years ago, the slow dispersing southern beech trees were beaten by a mix of much better dispersing tree species with bird-edible fruit.

There’s no expectation that the southern beeches would not be just as competitively dominant here as they are in the south and the north. It’s just slow for any tree to encroach into a dense, well-established temperate rain forest. Therefore, here we are ten thousand years after the glaciers retreated (which is not a lot of tree generations) and the beech gap remains.

The same thing happened in New Zealand’s Stewart Island to the south. Stewart Island is another diverse podocarp-broadleaf forest growing in environmental conditions where beech should dominate. Beech, again, was beaten there, this time failing to disperse across Foveaux Strait after the ice retreated. More recently, some Stewart Island residents have chosen to plant beech trees in their gardens. Watch this space in a thousand years.

What forest is where, and the ecology of that forest for animals and fungi, can therefore depend on historical accident. Initial conditions matter, often a lot, for which species form ecological communities at a site.

That was also one of the messages from my second half-hour talk, this time on forest succession. I cautioned community groups to be very careful what they plant when reestablishing forest.

What species people choose to plant now will have a big influence on the composition, and ecology, of these forests for centuries to come. With that in mind, I suggested that it is always worth thinking twice about whether planting is necessary at all. Will natural forest regenerate by itself, if weeds and grazing and browsing pests are controlled? I expect the answer is a resounding “yes” in almost all of the West Coast.

I also used an example from my own research of how initial conditions can affect forest regeneration. Together with Peter Williams and Susan Timmins, we contrasted how native forest succession occurs through the invasive weed gorse (Ulex europaeus) and the native early successional tree kanaka (Kunzea species). We studied this in the northwest and east of the South Island, the bottom of the North Island. In all cases, a different and less species rich forest was establishing through gorse than through kanaka. Beech, for example, comes up through kanuka much easier than through gorse, as it appears do podocarp trees. What forest types will be where in New Zealand, in centuries to come, will depend on where exactly the gorse and kanuka are now.

In the question time after my succession talk, I got a third and unanticipated example of “it depends”. Several people in the audience had native forest regenerating through gorse on their land and it was not proceeding like I had described from other parts of the country. A rich diversity of local trees, including podocarps, were coming up through their gorse.

We got to talking and came up with a possible reason for this difference. Where I worked, gorse forms a thick dry leaf litter, often 10—15 cm thick of dried gorse leaves and twigs. Unpublished research I did with a Lincoln University undergraduate student, Jordan Farrant, indicated that it was the thick gorse leaf litter depth that is the primary impediment to woody seedlings like beech. At least that’s what Jordan and I found in Hinewai Reserve on Banks Peninsula on the east coast of the South Island. The rain fall on the West Coast is hugely higher than the east coast, and the audience assured me that the gorse leaf litter rots away very quickly on their properties.

It’s fun to be wrong in science, and I hope to follow up these observations with proper surveys of forest succession under West Coast gorse compared with native successions. It could well be that the effects of gorse on forest succession are quite different in these high rainfall sites. There are still likely to be effects though. Gorse fixes nitrogen while kanuka doesn’t, gorse grows shorter and faster than kanuka, and it uses different mycorrhizal fungi. I still expect there to be some differences in the tree species doing the best in each succession, but perhaps those differences will be more subtle.

Nature is an elegantly untidy and complex dance of environment and history. What exactly happens where, and when, depends on a myriad of things. The big themes, and limits, are hard wired in by physics and chemistry, but within that there’s a universe of possibilities. That’s why wild forests, especially old growth wild forests, are so rich in tangled layers of stories of place. Those temperate rain forests of the West Coast beech gap tell stories of a much colder time when almost unimaginably massive glaciers flowed from the mountains to the sea, followed by a race among trees to reforest open ground. Ten thousand years later, that race still isn’t over, as beech slowly encroaches in from the edges and other slow-dispersing trees, like kowhai, creep inland generation-by-generation up the sides of the rivers.

This is all directly relevant to ecological restoration projects. Do we really want the future stories of our restored forests to be about what someone’s favourite plants were two centuries ago, or what plants were on sale at that time from the local nurseries? Shouldn’t they be about nature’s wild dance of naturally dispersed seeds and local environmental conditions? In some thoroughly deforested places, we need to prime the pump by planting some tree species. In most places, nature is fully capable of making new forests. We should let it.

Please don’t take the wild away from our new forests, unless you absolutely have to.

[See also the previous articles, how to decide where and when tree planting is really necessary and when and where we should plant trees]