Barking up the wrong tree?

As I sit here, my body is aching. Last evening was the second game of our touch rugby season. As Mel says in the Lethal Weapons movies “I’m getting too old for this stuff” (or maybe he used another word…). I have played for the last 33 years (now there is a number to scare you!) and this season I was all set to just have the odd cameo off the bench and concentrate on refereeing. Touch is a young person’s game, fast and furious, quick jinks, abrupt acceleration, and sudden turns. Old age and wisdom are certainly useful but it doesn’t help to chase down a 20 year old who has hit the gap. Over the last few years my team has changed as player’s knees and backs give out and last season I was the oldest by a decade. Heck, even my youngest son was playing in the team. But now I find myself back out there, scrambling to hold a defensive line, swooping in to be the dummy half, taking the touch. More than that, many of the former players have recovered from their knee operations and are playing with me. I had a comment from our ref last night that it was good to see that the band had gotten back together. Just like the old days (although the shirts seem smaller and tighter now). And that is certainly what it looks like.

But that would be wrong. You see, the recently added 30 year olds in the team have almost all succumbed to their own leg injuries and we have frantically called on the old fellows to fill the gap. So the obvious explanation, that the old guys wanted to play again, is not actually the reality, a response to an injury crisis. Sometimes an obvious hypothesis can be quite misleading. In ecology it is often easy to make a simple intuitive leap from some data to an explanation. And often the explanation is wrong and good science is required to show why. OK, an example.

Bark on a kahikatea tree

Bark on trees largely protects the tree from the travails of the world.  The thickness of bark varies between species and often within species as well. Presumably this variation is, at least partly, in response to environmental conditions. The thickness of bark is clearly a major decision for a tree as a lot of resources go into producing and maintaining this covering. Adding extra thickness requires diverting resources from other key parts of the tree. Bark may help the tree to remain upright and stable, manage its water potential and protect the xylem and phloem from damage. Another key function is that in fire-prone habitats you tend to have thicker bark which allows survival of a tree from small fire events (where the outside of the bark may be consumed but in doing so it protects the fragile inner world of the tree. In less fire-prone environments, like rain forests, bark is very thin, presumably because there is a low risk of fire. So a common explanation for the large variation in bark thickness is that it is linked to the likelihood of fires in an area, and the data from fire-prone systems seems to support this.

Beech forest in a cool and low soil fertility environment

New Zealand is not seen as a fire-prone environment. We have a lot of temperate rainforests and a wide-range of tree groups have representatives in these ecosystems. Given that we don’t have a lot of fires and are often too damp and cool for fires to spread very far, a prediction would be that bark thickness should generally lean towards the thin end of measurements. However, New Zealand plant ecologists have noticed that there seems to be considerable variation in our bark thickness in our rainforests. Tim Curran and postgraduate student Monique Wright, from Lincoln University and several colleagues from around New Zealand and Australia joined Sarah Richardson of Landcare Research to look at this hypothesis in detail. Bark thickness from 6000 individuals from 82 tree species from New Zealand temperate rainforests was measured as were a number of other plant and habitat traits, such as stem specific density and total phosphorus. Bark thickness generally ranged from 0.3mm to 15mm. In a paper published in the American Journal of Botany they found that, overall, there was a lot of variation in bark thickness within species, and not what was predicted, as low fire risk areas, bark thickness should have been uniformly thin. Thicker bark was generally found in cool and dry sites. An even stronger signal was that barks were thicker on less fertile soils. These findings potentially question some of the work that shows a link between fire and bark thickness in more fire-prone habitats elsewhere. It may be that more fire-prone habitats are often in dry areas with low fertility soils and that we are not seeing a direct link between fire and bark thickness but between habitat and climate and bark thickness. Rather than fire driving the thickness of bark (which seems like an obvious link with benefits for the trees), it could be that the fire protection of thick bark is just an exaptation or useful side effect of living on soils with poor fertility. In ecology we need to question our hypotheses and not always settle for the ‘simple and obvious’ hypothesis.



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