Sap Flow and Tree Water Deficit.

 

Central European forests, especially those dominated by beech and other deciduous species, are increasingly vulnerable to extreme drought conditions. To better understand these challenges, the researchers, from the University of Gottingen, Germany, studied the intricate relationship between sap flow density (SFD) and tree water deficit (TWD) in a mixed beech forest during the exceptionally dry summer of 2022. These physiological metrics reveal how trees balance water transport and storage under stress.

 

Setting the Scene: A Forest Under Stress.

Germany sap flow research.The study was conducted in the Hainich National Park, an unmanaged, diverse forest in Germany. Researchers monitored three co-occurring species: European beech (Fagus sylvatica), European ash (Fraxinus excelsior), and sycamore maple (Acer pseudoplatanus). Each species has unique wood anatomy and water-use strategies, making them ideal for exploring drought responses.

To measure sap flow, the researchers deployed the Implexx Sap Flow Sensor. Tree water deficit (TWD), defined as reversible stem shrinkage when water is depleted, was measured with a tree trunk dendrometer similar to the DE-1T model.

 

Observing Nature’s Responses.

Over the summer, as soil moisture content (SWC) plummeted, sap flow decreased across all species, while tree water deficit rose. Each species reacted differently. For example, ash trees, with their ring-porous wood, showed the highest water deficit but maintained relatively consistent sap flow. In contrast, beech trees exhibited significant sap flow reduction due to their anisohydric (less water-conserving) stomatal behavior.

 

Key Findings.
  1. Species-Specific Strategies: Ash trees’ deep roots and efficient water transport helped them access scarce resources, but they faced intense stem shrinkage. Beech and maple trees, with diffuse-porous wood, struggled more to sustain sap flow under prolonged dryness.
  2. Environmental Drivers: Soil moisture was the dominant factor affecting both sap flow and water deficit. Interestingly, solar radiation played a crucial role during extreme drought, driving water loss through photosynthesis and transpiration.
  3. Time Lags and Hysteresis: A fascinating delay emerged between sap flow peaks (around midday) and water deficit peaks (afternoon), highlighting the complex interplay between water transport and storage within trees.

 

Why It Matters.

This research underscores the importance of species-specific traits in determining drought resilience. While traditional classifications like isohydric (water-conserving) versus anisohydric may guide predictions, the study reveals the oversimplification of these categories. Multiple factors, including wood anatomy, rooting depth, and environmental conditions, jointly shape trees' responses.

As climate change intensifies, understanding these nuanced responses is vital for developing adaptive strategies. Protecting and managing forests with diverse species compositions may help mitigate drought impacts and preserve ecosystem services.

 

References.
  • Donfack et al. 2024. Linking sap flow and tree water deficit in an unmanaged, mixed beech forest during the summer drought 2022.