Trees move minerals from the soil to the surface, influencing their movement and loss

What is it about?

Elements such as silicon and calcium that are weathered from rocks can be moved from deep in the soil to the surface through uptake by plant roots and subsequent litterfall. At the surface, these elements are more vulnerable to being lost from an ecosystem by being eroded or washed into waterways, where they can eventually influence downstream processes such as ocean carbon uptake. It is difficult to estimate how important transport by trees is for element loss (or for recycling within the ecosystem) without good measurement methods. Here we compared two methods for estimating tree uptake of seven elements by measuring their concentrations in either leaf litter or plant sap. We measured three dominant tree species (oaks and maples) in a temperate forest catchment in central Pennsylvania and used total litterfall and sap flow to scale these concentrations into annual uptake rates. For most elements, both methods generated similar estimates that showed that the amount taken up by trees was similar to the amount lost in streams every year. However, for potassium and manganese, tree uptake was much greater than stream loss, suggesting that incorporation into tree litter helps to retain these elements within the ecosystem.

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Why is it important?

Elements move frequently between ecosystems, especially from terrestrial systems into rivers or oceans. There, they can be important substrates for a variety of processes (such as carbon uptake by marine diatoms). Therefore, it is important to understand what controls how much of a given element moves from the ecosystem where it first weathered out of rocks and into new systems. In forests, trees can play an important role in moving elements from the soil to the surface, where they are vulnerable to be washed downstream.