By using high-resolution sensors on a tree's stems and time-lapse photography, UBC Okanagan researchers can watch for the visual clues some trees present when water stressed.
With the arrival of spring a few weeks ago, new buds and colours on the trees started to appear.
Along with that new growth, a UBC Okanagan researcher has determined that some trees in spring also provide simple, visual clues-raised or lowered branches-to indicate that they are rehydrating or water-stressed.
Dr. Magali Nehemy studies forest hydrology with UBCO's Department of Earth and Environmental Science in the Irving K. Barber Faculty of Science . Her latest paper, published in Hydrological Processes , examined how tree branches shift as they start to rehydrate when winter ends.
"Spring rehydration is one of the key transitions in forest ecosystems," says Dr. Nehemy. "It marks the moment when trees begin to restore internal water reserves and prepare for the growing season."
Using high-resolution sensors on the tree's stems and time-lapse photography, the researchers recorded branch movements of balsam fir while the trees rehydrated during snowmelt and rainfall events. During drier periods, the branches gradually drooped downward, indicating a deficit in the tree's water.
The study took place from early March to mid-May on a grove of balsam fir in Ontario's Muskoka region. Using sensors, the researchers took stem radius measurements every 15 minutes. This data reveals cycles of contraction and expansion related to water loss and replenishment within the tree. At the same time, images captured by time-lapse cameras showed subtle but consistent changes in branch positions. The two signals closely tracked each other, she says.
"When the stems expanded-indicating rehydration-branches lifted. When water stress increased, branches drooped."
As climate change impacts the amount and timing of snowmelt and water availability in northern forests, understanding these plant-water interactions is becoming increasingly important. Simple processes visible to the naked eye-such as whether branches are lifting or drooping-may offer another window into how forests respond to changing environmental conditions, she explains.
"Interestingly, freeze-thaw cycles on cold spring nights caused sharp changes in stem size but had little effect on branch orientation," she adds. "This suggests that branch posture reflects longer-term water status rather than short-term temperature fluctuations."
The findings also raise questions about how different species respond. In the same forest, nearby deciduous trees without leaves showed little or no branch movement, while evergreen conifers showed clear posture changes.
The idea that plants move in response to their environment has fascinated scientists for centuries, dating back to Charles Darwin's studies of plant movement in the 19th century. Yet many aspects of these movements remain poorly understood.
"Branch movement is not a replacement for scientific instruments such as dendrometers or plant water sensors," says Dr. Nehemy. "But it could offer a visual, low-cost indicator of tree hydration, and this is especially useful for field observations or ecosystem monitoring."
