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).