The age of wisdom

Recently I have had cause to rue the age of the arboriculture industry here in New Zealand; rue not being word one normally would or indeed should use, but it seemed better than bewail or lament. Anyway, I was feeling a bit stink about the arboricultural industry in NZ.

My feelings came about after a radio interview where I successfully avoided answering any listener questions about tree health. Why is my tree yellow, my tree is old, is it ethical to remove a mature tree, is it intergenerational theft… technically I mentioned intergenerational theft, but I still couldn’t provide an answer, so that didn’t help. Part of my reluctance to be drawn into answering questions was the expectation that there was a single and simple answer – for most questions I could think up 5 different answers followed by 15 reasons why my answer might not apply. Yes but no but maybe.

I left the interview feeling a bit shell-shocked, arborists don’t have all the answers – don’t they know that we don’t know… do they know that we don’t know – why don’t we know? Apparently, it wasn’t a bad interview and I was told that it stimulated interest, but a car crash also stimulates interest, so I’m not sure if being interesting is a meaningful measure of success. Anyway, as a result, I had chance to ponder the bigger question; why don’t we know?

This is where my thinking took me.

New Zealand finds itself in a fascinating point of arboricultural time. Many of our exotic trees still exist in their first cycle – i.e. the oldest exotic trees are still alive; they have not been around for long enough to live, decline and naturally die, meaning we don’t know how long they will actually live for in this part of the world. The native trees that have been planted in the urban environment are also in their first cycle – meaning we don’t know how long they will live for in the urban setting, (we have a reasonable understanding of life expectancy of native trees in the natural environment, but not so much in the built word). So we find ourselves in the best of times, but also in the worst of times.

Like most kiwi arborists, I have spent several years working abroad. I have worked on and with many of our exotic trees in their country of origin. I’m relatively confident that I have (or at least had) a reasonable understanding of what the tree was doing and how it was likely to respond. But more importantly, I was very confident that if I didn’t know, there were people that did or because trees of the same age and stage were all around. But it’s not quite like that back here.

My gut feeling is that most European and North American trees age about four times faster here compared to where they come from, but not all and I’m only guessing. The oldest exotic tree in New Zealand only about 200 years old – it’s a pear tree. The oldest pear tree in the UK is said to be around 250 years old. Using the Ancient Tree classifications, both trees have a veteran form so maybe pear’s age at about the same rate. But oaks on the other hand… at around 150 years our oaks take on a ‘veteran’ form where in the UK the form may be more typical of a young or mature tree. I suspect our oaks will implode long before they can reach 200 and will be completely consumed long before ‘ancient’. It is a similar situation for Pinus and Cupressus sp. although their decaying carcasses may last a little longer. Yew’s on the other hand might go the distance but we won’t know for another 1000 years. So how long do exotic trees live in NZ? We don’t know.

In the bush (the native forest), we have standing kauri trees (Agathis australis) that could be over 2,500 years old. Totara (Podocarpus totara) in theory could stick around for over 1000 years and our southern beech (Nothofagaceae) should make it to at least 500 but in the urban environment, who knows? Most of our native trees do not do well in cities but pohutukawa (Metrosideros excelsa) not only survive but seem to thrive. How long can urban Pohutukawa live for? Who knows?

Yes we can guess and we do extrapolate tree mortality, but we don’t know – we don’t know, because the trees and/or the urban environment in which our trees grow has not existed for long enough.  [Note; pre European settlements (Māori pā) were more in tune with nature and lacked hard surfaces like roads and courtyards – so settlements and large villages existed but their effect on the growing environment was very different]. All of this unknown could be depressing, especially if you are the type of person that needs black and white, cast in stone answers. We don’t know, because we don’t know – it is the worst of times.

And then there is climate change, in-fill housing, the average life expectancy of urban infrastructure and the fickle nature of elected officials; working out natural tree mortality rates in the urban environment is probably pointless… it is not all doom and gloom.

I was coming at this from a position of being behind; we had to look north to see what happened or we had to wait another 100 years or so. But what if we were so far behind we were effectively in front? There can be a point in a race where the front runners come up behind the rear of the field – the rear is a lap behind, but it’s still physically in front of the leaders. Maybe we are not behind, maybe we are in front.

As far as the exotics go, we might not know what they could do, but because they age four times faster, we should be able to find out what they will do four times faster. Our native trees, well that’s another story, but our natives are exotics in other countries maybe we can learn from there.

Practices like retrenchment pruning and veteranisation weren’t really done back in the day – at least not intentionally or by self-respecting arborists. Admittedly there is not a lot of it done here, but what there is can be easily monitored and we’ll know if it’s working relatively quickly. We could dispel some of the voodoo around coronet-cuts and find out the relative stub size where wound closure will never happen. There is so much to do, so much learning to share, we might not know, but we will.

Time is on our side – it is the best of times.

• Written for Tree Matters (NZ)