6.2 Relationship between xylem anatomy and embolism vulnerability
Xylem anatomy varied considerably between ring-porous Q. alba and diffuse-porous species L. tulipifera and A. saccharum. A. saccharum and L. tulipifera mean lumen areas were indistinguishable, but significantly smaller than Q. alba(Fndf,ddf = 124.37, p = <0.001, Table 3, Fig. 4). By comparison, mean vessel densities were different across all species (Fig. 5c); however Q. alba stems had consistently lower vessel density than L. tulipifera and A. saccharum (Fndf,ddf = 208.982, p = <0.001, Table 3, Fig. 5). Additionally, we detected no influence of local climate or age on mean lumen area or vessel density, such that xylem traits were generally conserved at the species-level (region, age, or interactions not significant).
Xylem anatomy had moderate explanatory power for tissue level embolism vulnerability. Across species, stems with larger vessel lumen area (Fig. 4d) and smaller vessel densities (Fig. 5d) approached 50% loss of hydraulic function at less negative Ψx (R 2 = 0.431,p = <0.001 for lumen area, andR 2 = 0.450, p = <0.001 for vessel density). Patterns with P12 were similar, but generally weaker than in relation to P50. Specifically, tissues with larger mean vessel lumen area tended to approach 12% loss of hydraulic function at lower Ψx relative to tissues with smaller mean lumen area (R 2 = 0.250, p = 0.005, Fig. 4b). Tissues with greater vessel densities were generally more embolism-resistant at P12 (R 2 = 0.322, p= <0.001, Fig. 4b). However, this pattern was contradicted byQ. alba , where stems with greater vessel densities were more vulnerable to 12% loss of hydraulic function (R 2 = 0.26, p = 0.002, Fig. 5b).