Cooperation in the Coniferous Forest: Douglas Fir & Western Red Cedar

Root-Sharing Partners in Drought

In the Pacific Northwest's coniferous forests, Douglas fir (Pseudotsuga menziesii) and Western red cedar (Thuja plicata) engage in a remarkable belowground partnership that helps both survive summer drought.

The Mechanism: Hydraulic Redistribution

1. Root Architecture: Douglas fir has shallow, widespread roots; Western red cedar has deep taproots accessing deep soil water.
2. Drought Stress: Summer drought dries upper soil. Douglas fir roots experience water deficit.
3. Root Proximity & Grafting: Roots of the two species grow close, sometimes forming root grafts (natural anastomoses) or sharing mycorrhizal networks.
4. Water Transfer: At night, when transpiration stops, water moves from cedar's deep roots (high water potential) → shared network → fir's shallow roots (low water potential).
5. Mutual Benefit: Fir gets water; cedar gets shade from fir's canopy, reducing its own transpiration and heat stress.

Why It Matters

• Facilitation, not just competition: Classic ecology emphasizes competition; this shows positive interaction between canopy dominants.
• Drought Resilience: Mixed stands survive dry years better than monocultures.
• Ecosystem Stability: Hydraulic redistribution lifts water for understory plants, microbes, and soil processes.
• Climate Adaptation: As summers get hotter/drier, such partnerships may determine forest persistence.

Broader Context

This is one example of plant-plant facilitation via mycorrhizal networks ("Wood Wide Web"). Others: nitrogen-fixers sharing N; nurse plants shading seedlings; nurse logs hosting seedlings. Forests are cooperative communities, not just collections of competing individuals.