Not all tropical plants can stand the heat brought by climate change

How will increasing temperatures affect plants where temperatures are hottest? A paper in the Journal of Ecology looks at plants around Kilimanjaro.
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A good first sentence is important, and the very first line of the summary for this paper caught my eye.

Despite strong climate change in the tropics, little is known about the responses of tropical plants to changing environments.

This surprised me. Botanists love the tropics. The tropics are hotbeds of biodoversity, so they’re a great place to visit if you’re looking for something new. Yet I can’t recall reading anything on tropical responses. A quick check in the recent review in Annals of Botany shows little on tropical plant responses. The studies I’ve read tend to be temperate or montane plants that I’ve read about. In their paper Elevational transplantation suggests different responses of African submontane and savanna plants to climate warming, Ensslin and colleagues do have a mountain to help study climate change, but Kilimanjaro is famously tropical.

Mount Kilimanjaro
A panorama of Mount Kilimanjaro. Picture taken in Moshi, Tanzania. Image r Muhammad Mahdi Karim / Wikipedia

Ensslin’s group set out to test two simple ideas. How do savannah and submontane plants respond to a change in temperate and does the kind of plant make a different to how it adapts.

The first question was answered with the kind of very simple experiment that I like. There were two gardens, one savannah at 890m elevation and and one submontane at 1450m elevation, with a 5 degree centigrade difference between them. The two gardens were 5km apart, but they also not that the upper garden got around twice the level of natural precipitation than the lower garden. TThey then planted seeds and watched what happened.

In this way, the lower garden was the control for the savannah species, while the upper garden was the test. However, for the submontane species it was the upper garden that was the control and the lower garden was the test. It was, if you read the paper, a more complicated set-up than this. For example they corrected for the difference in precipitation, but the basic idea was this simple.

Another example of one of the details that I’ve glossed over is that the plants, especially in the savannah garden were shaded with netting. That meant that pollinators had trouble accessing the plants, so the experimenters couldn’t use seed count as a direct measure of reproduction. Instead they used flower count – which is not exactly the same but close enough to act as a reasonable measure for discussion.

They got two clear results. For the submontane species there was a significant difference in growth and reproductive success between the gardens – with the plants in the hotter garden doing worse. It’s a clear indication that the submontane species do not do well with extra heat.

For the savannah plants there was no significant difference. That creates a new puzzle. If they can grow in a submontane environment why don’t they? The authors suggest the problem is biotic. It might be interaction, but it might also be a failure of pollination or seed dispersal. If something changes that allows them to move uphill then there seems to be no reason why they couldn’t be successful.

As temperatures rise the submontane plants will also have to move uphill. There is plenty of room up to around 5900m, but as Ensslin and colleagues point out Kilimanjaro is a bit of a freak in that regard. A lot of East African mountains run out of up after around 2500m.

As well as comparing savannah plants with submontane plants the team did something else interesting. They compared grasses with forbs. This was complicated a bit by most of the grasses being C4 plants and most of the forbs being C3 plants. They found that the C4 plants didn’t seem to have the usual advantages over C3 plants, but that net shading may have has something to do with that, as might the watering regime – so they highlight that as a problem for further investigation.

To some extent I’m not surprised by the findings. Elsewhere we see that alpine plants suffer with heat so it might be expected to see something similar in submontane plants in Africa. But there’s a difference between expecting it and knowing it, and Ensslin and colleagues have quantified those differences. There are plenty of interactions that will change with the climate, but this paper provides a foundation for others to work from to see what this experiment has missed. You can read their work in the Journal of Ecology.

References

Parmesan, C., & Hanley, M. E. (2015). Plants and climate change: complexities and surprises. Annals of Botany, 116(6), 849–864. https://doi.org/10.1093/aob/mcv169

Ensslin, A., Mollel, N. P., Hemp, A., & Fischer, M. (2017). Elevational transplantation suggests different responses of African submontane and savanna plants to climate warming. Journal of Ecology. https://doi.org/10.1111/1365-2745.12842


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