Scaling, structure, and (dys)function in plant hydraulic systems

Water transport has shaped land plants for over half a billion years of evolution, governing their growth, reproduction, and survival. This dissertation investigates the scaling, structure, and function of plant hydraulic systems – and the mechanisms by which hydraulic dysfunction causes mortality – through biophysical modeling, theoretical synthesis, and laboratory and field experimentation. Chapter 2 addresses a methodological challenge in biology: empirical tests of biological scaling relationships are biased when observations cluster unevenly across the predictor range. I introduce a bootstrapping approach that balances data density before linear model fitting, demonstrating with plant height–diameter and metabolic scaling datasets that balanced fits yield more accurate slope estimates and more reliable comparisons with theory. Chapter 3 applies this method to xylem conduit morphology. Hydraulic theory predicts conduits widen from tip to base to minimize resistance, but no theory addresses how collapse prevention shapes vertical variation in wall thickness. I synthesize new theory for thickness-to-span scaling and test predictions on nearly 600,000 conduits from five conifer species spanning above- and belowground organs. Conduits widened with distance from leaf tips and from fine to coarse roots; thickness-to-span ratios increased toward tips, consistently exceeding the critical collapse limit, revealing how sap transport physics jointly shape hydraulic efficiency and structural stability. Chapters 4 and 5 examine hydraulic dysfunction, focusing on yellow-cedar decline, an ongoing mortality event affecting ~4,000 km² of coastal Pacific Northwest forests linked to climate-driven snowpack loss. In Chapter 4, freeze-thaw experiments on seedlings of four co-occurring conifers showed that radial conductivity loss via cortical lacunae – not axial xylem embolism – is the primary injury mechanism. Yellow-cedar's acquisitive fine root strategy – characterized by metabolically active yet mechanically fragile fine roots with unlignified cortical walls reinforced by phi-thickenings – explains its unique vulnerability. Chapter 5 tests this mechanism in adult trees through a year-long snow-removal experiment at a well-drained montane/subalpine site in British Columbia. Despite altered soil conditions, adult yellow-cedars showed no sustained hydraulic decline, suggesting deeper roots compensate when surface fine roots are injured. Decline likely requires the convergence of snowpack loss, extreme freeze-thaw cycles, and shallow root systems – refining conservation priorities toward poorly drained, low-elevation populations.