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Linking Dendrometers with Remote Sensing for Assessing Forest Phenology

When the days start to lengthen in the spring, it’s not just people who begin to switch wardrobes. Plants experience seasonal changes in both their structure and function with the changing of the seasons. For deciduous species, this manifests as a leaf-out in spring and a dropping of leaves in autumn, among other things. For evergreen species, biochemical changes in the plant’s leaves allow them to maintain their foliage year round while still regulating seasonal changes in photosynthesis and transpiration. The timing of these changes, also known as vegetation phenology, drives functional changes in forest carbon and water cycles. Therefore phenological timing is important for understanding the fate of ecosystems in a changing climate. Unfortunately, vegetation phenology is challenging to both measure and predict.

By combining multiple measurement approaches, we can gain a more holistic picture of phenological change. Specifically, recent research paired dendrometer measurements of stem radius change with remote sensing data, eddy-covariance measurements, and environmental parameters to characterize the spring transition. In doing so, it revealed differences in the timing and environmental drivers of spring phenology for deciduous and evergreen species separately.

Over winter, stem radiuses expand and contract following changes in air temperature with observed stem shrinkage at night and expansion during the day. In summer, the stem radius cycle reverses as trunks expand at night to refill water stores within the tree and shrink during the day due to transpiration loss of stored water. The timing of the transition from a ‘freeze-thaw’ stem radius cycle to a ‘transpiration’ cycle can be used to determine the onset of transpiration in spring.

Linking stem radius measurements to remote sensing and environmental parameters shows that the spring transition occurs in two distinct phases with different environmental controls. The first phase is a reactivation of photosynthesis in evergreens and is triggered by thawed stems, warm air temperature, and moist soil. During this phase, the evergreens also start to transpire. The trees ‘wake up thirst’ and uptake snowmelt water as the snowpack melts and water infiltrates the soil. The second phase is a change in evergreen photoprotective pigment levels and the leaf-out and onset of transpiration for deciduous species. It is triggered by soil thaw. The water source of transpiration is also snowmelt that recharged stem water stores during the first phase.

Ultimately, better quantification of forest phenology will aid in our understanding of forest carbon and water cycles. Linking dendrometers with remote sensing,

isotope tracing, and other measures of forest phenology can help us do that.

About the Authors:

Zoe Pierrat is a PhD Candidate in the Department of Atmospheric and Oceanic Science at the University of California Los Angeles. She uses remote sensing combined with dendrometer and other field-based measurements to understand carbon fluxes and environmental controls on plant productivity in forest ecosystems. Zoe’s undergraduate degree was in physics and the thing she enjoys the most about dendro life is the amazing collaborations she has made with scientists in a wide variety of different fields. In fact, Zoe and Magali’s collaboration started by accident when they unexpectedly met at a fieldsite in Canada. Outside of science, Zoe enjoys hiking, biking, running, and showering her cat Nova with love and attention.

Magali Nehemy is a Postdoc at the Global Institute for Water Security at the University of Saskatchewan, Canada. Magali’s research investigates how trees exploit subsurface water storages, the source of transpiration, and the connectivity between transpiration sources and streamflow. To do this, she uses field-based research and quantitative analyses combining stable isotope tracing with tree hydraulic monitoring (including dendrometers and sapflow sensor). During her masters she used tree-ring analysis to investigate the relationship between tree growth and water table levels in a fen. Magali’s interest in dendro and forest science started earlier during her undergraduate degree in forest engineering in Brazil. Magali, like Zoe, enjoys conducting interdisciplinary research and collaborating with colleagues across different fields. Besides being outside for field work (with her best field crew, Maple her dog), Magali enjoys mountain biking, hiking and has recently attempted some downhill skiing.

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