Plumbing the depths of tall trees: quantifying the hydraulic structure and function of deep roots

A project undertaken at the School of Plant Biology, The University of Western Australia, annd supervised by Dr Pauline Grierson and Dr Tim Bleby

The massive stems and towering canopies (up to 100 m height) of gigantic gum trees like karri (Eucalyptus diversicolor) are truly awe-inspiring. But did you know that if you could view these trees “upside-down”, with roots on top and shoots underneath, then the root systems of these trees would appear just as impressive? Large trees like karri grow vast networks of roots over large horizontal and vertical distances in search of water and nutrients. Having deep roots is especially important because they allow trees to tap deep water sources during times of drought, which is essential for survival in seasonally dry environments. Despite their importance, deep roots are rarely studied. Very little is known about their anatomical structure or the mechanisms behind their ability to transport water from deep underground to aboveground stems and leaves.

Using a unique combination of cave field sites to access deep roots at 40-60 m depth, and a range of novel ecophysiological, anatomical and hydraulic techniques, the aim of this project is to elucidate the hydraulic structure and function of deep karri roots. We hypothesise that compared to shallow roots and stems, xylem tissues (the microcopic ‘pipes’ used to transport water) in deep roots have anatomical specialisations designed to reduce flow resistance, thus enabling water to be transported over large distances with great efficiency, but this may come at the cost of greater vulnerability to cavitation (the blockage of ‘pipes’ by air bubbles). We also hypothesise that deep roots in consistently moist deep soil contribute relatively large amounts of water for transpiration during drought, and they tend to display more consistent and reliable patterns of water transport compared to roots in shallow soil layers subject to regular wetting and drying cycles.

Our broad objective is to better understand water transport in tall trees and the role of deep-rooted vegetation in the water cycle. Knowledge gained from this project will contribute to better management of forested catchments Australia-wide that support industry and supply drinking water for major cities and towns, including the iconic karri forests of south-west WA that are coming under stress as a result of declining winter rainfall.

Figure 1. A 60 m tall karri tree (Eucalyptus diversicolor) growing in forest above caves near Augusta, WA.

Figure 2. Woody roots and fine roots of karri growing from the ceiling of a cave at 50 m depth.

Figure 3. Woody roots of karri growing along the moist floor of a cave at 50 m depth.