Changing directions: tree hydraulic redistribution from canopy soil

Abstract

One of the most important and noticeable components of the canopy environment is the large biomass of epiphytes composed of vascular and non-vascular plants (e.g., mosses, lichens and ferns) which accumulates and decompose over time forming what we know as canopy soils. These arboreal soils are important because they provide habitat and can retain water and nutrients for epiphytes and their associated biota. While it is true that large trees can access water from deep in the ground during dry periods, the physical difficulty of moving water to a high crown is thought to be a major limitation to height in trees. Thus, a reservoir accessible to the crown without fighting gravity may allow stomata to remain open for photosynthesis while reducing cavitation risk and regulating plant temperature to sustain positive carbon balance under dry conditions. A wide range of tree species in temperate and tropical rainforests have the ability to sprout adventitious roots from branches under these water reservoirs; however, water uptake from canopy soil has not been confirmed or measured. This study approach how water from canopy soil can be redistributed to other organs along the tree body by reverse or bi-directional flow mitigating the effects of drought conditions. To answer this enigma, this research assessed canopy root anatomy, water redistribution and physiological performance of trees with canopy soil under adverse conditions of water availability. Three approaches were used to accomplish these objectives: lab, greenhouse and field experiments. Initially, microscopy anatomical comparisons were made between canopy and terrestrial roots, to ensure that these were functional and could potentially facilitate water absorption from canopy soil. If the canopy roots vascular system is directly connected to the host tree, aboveground water can be then reallocated to other organs of the host tree. In order to assess canopy water uptake and redistribution, greenhouse and field trials were designed, using poplar trees (Populus trichocarpa) and applying air-layering technique to develop canopy soil systems and roots. Dye technique, stable isotope (deuterium- δD) and sap flow sensor methods were used to track water movement from aboveground pools to other tree organs. Finally, greenhouse trials were designed to simulate plant water relations (in a simplified system) between canopy soils and host trees under low water availability conditions, to evaluate the advantages and significance of canopy soil, as an extra-water source that could mitigate the effects of drought. Data show that canopy and terrestrial roots are anatomically similar and present all necessary features to be hydraulically functional. Dye-experiments and sap flow data confirmed that there is water uptake by canopy roots in this poplar system and suggest potential bi-directional water flow (i.e. up- and downwards sap flow) within the tree. This is supported by the isotopic data that confirmed that water from canopy soil is reallocated not only at the canopy level but also could reach belowground organs. Data also suggest that trees with canopy roots can mitigate drought conditions by taking up water from canopy soils and maintain their water and carbon balance for a longer period than plants without them. It is expected that temperatures in the Pacific Northwest will rise, and drought will become more severe. Such conditions are predicted to increase heat stress-related tree mortality in the western US. The role of canopy soils and roots may be crucial to plant biodiversity in critical habitats, and likely account for an important but understudied source of water and nutrients in trees, especially under conditions where trees become heat stressed.