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Competitive conifers prefer an uneven canopy

LIDAR survey of forests in Borneo and Japan shows that in the canopy conifers and broadleaf trees are battling to extract the most energy from the sun.
Rainforest canopy
Image: Aiba et al.

The common view of the rainforest is of a landscape dominated by a canopy of trees. It’s a layer that works to extract as much energy from sunlight as possible, to the extent that only a small percentage of light penetrates the canopy in a typical rainforest. It makes the canopy one of the key battlegrounds for competition between plants. Aiba, Akutsu and Onoda have a paper Canopy structure of tropical and sub-tropical rain forests in relation to conifer dominance analysed with a portable LIDAR system. In this paper they look to see if the shape of the canopy can tell us about competition between trees.

The competition they’re particularly interested in is between conifers and angiosperms. Conifers are shade intolerant, They need light but they need it in a particular way. Conifers are conical and while their leaves may needle-like, their crowns are round and deep. They need light to penetrate beyond the top of the plant. Angiosperms in contrast can spread out to fill gaps. Aiba et al thought this could be a key to competition between conifers and angiosperms.

There are places where conifers dominate, particularly on high ground where the climate is colder. However conifers don’t need the cold. They would often be happy where the angiosperms were. Are angiosperms blocking out conifers in the canopy? If they are the canopies of angiosperm forests should be reasonably flat as the trees fill out the space. In contrast mixed forests and conifer forests should be more uneven.

A map showing Kinabalu and Yakushima
The areas studied, click to enlarge. Source: Google Maps.

This kind of mapping is something that fits well with lidar. Lidar is a lightwave version of radar that has a much better ability to focus. It allows fast and accurate mapping. The team decided to map around Mount Kinabalu in Borneo. Kinabalu park has a varied geology and the mountain is dominated by conifers. They also examined Yakushima, southern Japan. This is an island with a peak at 1936 metres with a mix of conifers, tropical and sub-tropical variation.

The system they used to map the forest was ground based. It means that they didn’t get sweeping vistas of the canopy, but it did have some advantages. Doing the survey from the ground meant that it was possible to keep a close eye on the local geology. Secondly, it was a lot cheaper.

What they found was that they were right, broadly speaking. In the mixed forests, either at higher elevations or on poorer soils, where the conifers had an advantage, the canopies were less even. However they did find some locations where this wasn’t the case. Some mixed forests on Kinabalu didn’t have uneven canopies. Aiba et al. think that there must be some limiting factor on the growth of the conifers. They rule out wind as Kinabalu isn’t in a cyclone zone, so they propose some hydraulic factor, like drought, might be the cause.

They also found some angiosperm forests on Kinabalu had canopies like mixed forests, not even like they expected. The reason they propose for this is the opposite to the even canopies they found in mixed forests. The trees of South-East Asia include emergent species. These are trees that can penetrate the canopy and rise above it. Aiba et al. suggest that if the environmental factors are not a limitation then broadleaf emergent species can grow to a greater height than if there was something limiting them. They compare the lack of strong winds at Kinabalu with the typhoons of Yakushima where wind would make a single layer canopy more effective.

What I like about the paper is that it doesn’t give a sense of an idealised or well-behaved landscape.

Parker et al. (2004) walked straight lines at a constant speed for measurement, but this was often impractical in the difficult terrain on the mountain slopes.

That’s not perfect, but that’s because the world doesn’t provide perfect environments for botanists. In that case it’s much more helpful to record the troubles you had than simply editing them out for a neater paper, if anyone is going to refer to this work and build on it.

It’s not simply the results that matter with this paper, it’s also the method. When I saw the title I assumed the portable lidar was portable in the sense it was carried on an aeroplane. The idea of zapping a forest from ground-level to examine the canopy didn’t occur to me. Aircraft are glamorous, but this approach of looking from inside the forest seems to have a lot of potential. I’ve skipped a lot of the discussion of basal area for the tree stands in the paper. I thought a good enough air-based lidar would be able to measure basal area, but it looks like I’m wrong. Aiba et al. note that lidar cannot distinguish between leaves and other parts of a plant. It’s all surface. In this case getting on the ground would give a better insight what is happening at ground level. Aiba et al comment on the basal area of the trees being greater in mixed-forests than in broadleaf forest, but the caution that this might be down to the below-ground environment as much as the obvious above-ground architecture differences between conifers and angiosperms.

All in all this looks a very effective way of looks at the limits of the trees, where angiosperms are limited by temperature and soild while conifers are limited by access to light. You can pick up the paper free, as it’s open access.

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