Can large trees that grow by the sea hasten erosion by the roots breaking up the rock and clay cliff?

Imagine a farm by the ocean. Much of it is bordered by a ten meter cliff to the sea. The cliff is eroding. Large trees grow on the cliff front. Pines and NZ Pohutukawa trees. The erosion has exposed part of the root system of the trees. Is that root system hastening erosion by breaking up the clay cliff, or preventing erosion by holding it together?



  1. 0 Votes

    Sounds like a question from a school assignment?

    The clue is figuring out which process works more quickly, and what each process is doing. (And yes, there’s an obvious answer, your teacher didn’t mean this to be a PhD research topic.)

    • 0 Votes

      thanks Karma, but this is not school project. The farm is real, in Northland, NZ, and the farmer is blaming the coastal trees for the erosion. I think the issue here is really sea level rise due to global warming, which, it seems to me, has more to do with the coastal erosion than the roots of the trees. Nonetheless, as the roots grow they do expand and, according to the farmer, hasten the break up of the cliff face which is clay/limestone. this has a certain logic to it. It is not the same as trees stabilising land in terms of rainfall from above. My gut instinct is that the farmer is wrong, but it is not that easy to say why – and the answer is not obvious to me, at least. This is not just an idle question. Several beautiful coastal pohutakawas are at stake.

    • 0 Votes

      Tree roots of course do break up very substantial rocks and structures, and break into pipes, which is what the farmer is probably thinking of, in part. But this process is very slow, years or even decades.

      Significant soil erosion can happen in just a single major storm, however.

      One of the great criticisms of “clear cut logging” is that without plants holding back erosion, the fertile soil is quickly stripped away. That is, your farmer may be considering only the frontal erosion by the sea, and ignoring the erosion to the top of the land.

      I’m curious as to why the trees are being blamed. Have they been artificially planted there? If they are natural is the farmer actually proposing cutting down natural trees to reduce erosion? I.e., no one really knows what erosion would happen without the trees? That course of action, in those circumstances seems entirely misguided!

      It seems that there should be an empirical way of answering the question: Is the coast eroding more quickly where the trees are, or not?

      Coastal erosion is a hotly debated topic. Can it be economically stopped? Should it be stopped? By what means? Solutions may favor one special interest group to the detriment of another. For example, building up beaches with a groyne may cause another beach to lose sand.

      You will want to read the Wikipedia article on Coastal Management, because it sounds to me that you and the farmer may have some common ground, so to speak, that you haven’t identified.

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