UBC Okanagan Study Reveals Visual Clues as Trees Rehydrate from Spring Drought Stress

Discovering Nature's Subtle Signals: Branches as Indicators of Tree Health

In the quiet transition from winter to spring in Canada's northern forests, trees undergo a critical process known as stem rehydration. After months of dormancy and potential water loss, trees refill their internal water reserves, primarily from snowmelt and early rains, preparing for the growing season. A groundbreaking study from the University of British Columbia Okanagan (UBCO) has identified a simple yet powerful visual cue in this process: the orientation of tree branches. When trees rehydrate, branches lift upward; when stressed by drought, they droop downward. This discovery offers forest managers, researchers, and ecologists a low-cost way to assess tree water status without sophisticated equipment. 55 56

Led by Assistant Professor Dr. Magali Nehemy in UBCO's Department of Earth and Environmental Sciences, the research highlights how these branch movements correlate directly with stem water content changes. Conducted in Ontario's Harp-4 catchment—a mixed forest site with sandy soils and seasonal snow cover—the study bridges plant physiology and hydrology, revealing insights vital for forests facing shifting climate patterns.

Dr. Magali Nehemy: Pioneering Forest Hydrology at UBC Okanagan

Dr. Magali Nehemy, an ecohydrologist who joined UBC Okanagan in 2025, brings a wealth of expertise to her work on forest-water interactions. Holding a PhD from the University of Saskatchewan, her research spans isotope hydrology, tree ecophysiology, and phenology. At UBCO's Irving K. Barber Faculty of Science, Nehemy's lab explores how trees source water amid environmental stresses like drought and altered snowmelt. 57

Nehemy's previous contributions include studies on Amazon rain recycling and boreal forest snowmelt water use, emphasizing her focus on global forest resilience. "Spring rehydration is one of the key transitions in forest ecosystems," she notes. "It marks the moment when trees begin to restore internal water reserves and prepare for the growing season." Her interdisciplinary approach, collaborating with institutions like the University of Göttingen and University of Saskatchewan, underscores UBC Okanagan's role in advancing Canadian environmental science. 55

The Science of Stem Rehydration: A Step-by-Step Process

Stem rehydration, or the refilling of a tree's xylem vessels and tissues with water, follows winter shrinkage caused by freeze-thaw cycles and low soil moisture. In snow-dominated temperate forests like those across Canada, this begins with snowmelt in March-May, providing meltwater that roots absorb and transport upward.

• Step 1: Snowmelt infiltrates soil, raising groundwater levels and soil moisture.
• Step 2: Roots uptake water via osmosis, driven by gradients in water potential.
• Step 3: Water moves through xylem, expanding stem radius as turgor pressure increases.
• Step 4: Branches respond to hydraulic signals, adjusting posture via turgor changes in petioles and twigs.
• Step 5: Transpiration resumes, marking full spring activation.

Tree Water Deficit (TWD), calculated from dendrometer data as the difference between maximum and minimum daily stem radius, quantifies this. Positive TWD indicates stress; reduction signals recovery. Climate change disrupts this by advancing snowmelt 1-2 weeks earlier in many regions, potentially desynchronizing water supply with tree demand. 56

Innovative Methods: Combining Dendrometers and Time-Lapse Imagery

The study employed point dendrometers (DC3 model by Ecomatik)—precision sensors measuring stem radius fluctuations every 15 minutes—on hemlock trees. Nearby balsam fir (Abies balsamea) branches were monitored via a high-resolution trail camera (Zopu SL122M Pro) capturing images at 15-minute intervals, compiled into time-lapse videos.

Data processing used R packages like treenetproc for normalization (relative to maxima) and gganimate for visualization. Branch displacement was quantified with Tracker software. Environmental data (temperature, precipitation) from Dorset Environmental Science Centre contextualized patterns. This non-invasive setup revealed diel (daily) and multi-day cycles, distinguishing hydration from temperature effects. 56

Key Findings: Branches Lift with Rehydration, Droop Under Stress

Upward branch movement preceded or coincided with stem expansion during snowmelt (mid-March) and rains (April), reducing TWD. Downward drooping matched shrinkage and rising TWD in dry spells (e.g., late April). Freeze-thaw nights caused rapid stem contraction but minimal branch change, confirming posture tracks hydration, not temperature. 107

Conifers showed pronounced shifts; leafless deciduous trees (e.g., beech) none until leaf-out. "When stems expanded—indicating rehydration—branches lifted. When water stress increased, branches drooped," explains Nehemy. Figures from the paper illustrate this: time-lapse sequences show branches rising hours before stem peaks, suggesting faster branch response. 55

Visual Clues in Action: From Field Observations to Ecosystem Insights

Branch posture offers a perceptual model: hydrated trees hold branches horizontally/upward due to turgor; stressed ones droop. Enhancing visibility with markers (e.g., ping-pong balls) could refine this for trail cams. For more details, explore the full open-access paper. 107

This aligns with Charles Darwin's 19th-century observations on plant movements, modernized for hydrology.

Climate Change Challenges: Shifting Snowmelt in Canadian Forests

Canada's forests, covering 347 million hectares, rely on snowmelt for 30-50% annual water in boreal zones. Warmer winters reduce snowpack (e.g., 2026's historic low in Okanagan), advance melt, and heighten drought risk. BC's Thompson-Okanagan saw deterioration in early 2026, with 14% provincial drought coverage. 71 80 Earlier rehydration could mismatch with budburst, stressing trees.

Okanagan Context: Drought, Wildfires, and Forest Vulnerability

The Okanagan, a semi-arid valley in BC, faces "severe" drought cycles, low groundwater, and wildfire boosts from dry fuels. UBCO studies show post-wildfire streamflow surges but long-term declines, emphasizing forests' water retention role. This branch clue aids monitoring local conifers like ponderosa pine amid 2026 concerns. 68 72

Forests prevent floods/droughts; loss makes watersheds "leakier," per UBCO research. 20

Practical Applications for Forestry and Ecosystem Management

• Low-cost scouting: Foresters scan branch postures for stress hotspots.
• Climate adaptation: Track rehydration shifts to adjust thinning/planting.
• Wildfire prep: Identify vulnerable stands pre-dry season.
• Research efficiency: Pair visuals with sensors for scalable monitoring.

Not a replacement for dendrometers, but complementary for vast Canadian forests.

Limitations, Future Directions, and UBC Okanagan's Research Legacy

Study limited to one site/species; needs multi-species validation. Future: markers, LiDAR, diverse biomes. UBC Okanagan leads with watershed leakiness studies and wildfire hydrology, positioning it as a hub for climate-resilient forestry. 56

Check UBC's news release for videos. 55

Careers in Forest Hydrology: Opportunities at Canadian Universities

This study exemplifies demand for ecohydrologists. Roles in research, policy, and management abound at UBC, with positions in faculty, postdocs, and research assistants. Canada's forests need experts to combat climate threats.

Photo by AHMAD HASAN on Unsplash