Weighing trees: Measuring interception and evaporation dynamics by monitoring sub- micrometer tree trunk compaction

Interception of rain and snow by tree crowns can be a significant part of the water balance. Rain, intercepted by leaves, that evaporates directly back into the atmosphere may make up 10% to 60% of total precipitation. In most forests, 2 mm or more is intercepted per rainfall event. Measuring interception and its subsequent evaporation is difficult. Measuring rain above and below the canopy is prone to noise. Radiometric measurements are difficult to calibrate. Here, we make use of the fact that water or snow intercepted by the crown compacts the trunk, simply following Hooke's law. By measuring the compaction, the amount of water stored in the crown can be measured. In practice, many problems have to be overcome before a good weight signal is obtained. For starters, 1 kg of water stored in a small tree causes a compaction of only 100 nm over a stretch of 1 m. Such small displacements can actually be measured relatively easily with special potentiometers. We used such displacement sensors together with quartz rods of 1m length, that were bolted to the trunk. Quartz was chosen to reduce the effect of temperature changes. In addition, the trees were insulated and kept at a constant temperature with heating wire. The main source of noise is bending by wind. By radially installing three sensors, wind effects could accounted for as well. Final measurement accuracies of 1 kg to 5g were obtained, depending on tree size. Tree weighing results from Europe and Africa will be presented.