Results
All relations reported below are based on linear SMA regressions on ln-transformed values. We found strong similarity in scaling relationships across seedlings of woody species between total stomatal area and xylem tissue area from the whole-plant level to single-leaf level. Across the 53 species, plant total stomatal area scaled to stem xylem area (slope = 1.29, r 2 = 0.81, P < 0.001; Fig. 2a), and to stem xylem conductance area (slope = 1.22, r 2 = 0.77, P < 0.001; Fig. 2b); leaf total stomatal area scaled to midvein xylem area (slope = 1.30, r 2 = 0.79, P < 0.001) (Fig. 3). The slopes of the regression lines were substantially and significantly larger than 1 (the 95% confidence intervals did not bracket zero, Table 1).
Fig. 2 Size-covariation of stomata and xylem at the whole plant level, across seedlings of 53 European woody species varying in leaf-habit, growth-form and relative growth rate (RGR). (a) Convergent scaling of plant total stomatal area and stem xylem transect area. (b) Convergent scaling of plant total stomatal area and stem xylem conductance area. Growth forms: T tree, S shrub, SS subshrub,C+Sc scrambler or climber. Regression coefficients of standardized major axis (SMA) are documented in Table 1.
Fig. 3 Convergent scaling of leaf total stomatal area and midvein xylem transect area, across seedlings of 53 European woody species varying in leaf-habit, growth-form and relative growth rate (RGR). Lines indicate significant scaling relationships. Growth forms: T tree, Sshrub, SS subshrub, C+Sc scrambler or climber. Regression coefficients of standardized major axis (SMA) are documented in Table 1.
Table 1. Ln – Ln scaling relationships were analyzed with standardized major axis regression (SMA) analyses, with additional reference to the contributions of different growth forms, leaf habits and relative growth rates (RGRs) to these relationships. Y-intercept and slopes as well as slope homogeneity with 1 are reported for pairwise relationships with significant results. 95% confidence intervals (CI) are in parentheses. Growth-form: T tree, S shrub, SS subshrub,C+Sc scrambler or climber; Leaf-habit: D deciduous,E evergreen. Y/X – RGR SMA regression of Y/X (ratio of Y values to X values) and RGR. ***, P < 0.001; **P < 0.05; ns, not significant.
Moreover, leaf total stomatal number had a strong scaling relation with leaf midvein xylem area (slope = 1.32, r 2 = 0.79, P < 0.001) as well as average leaf area (slope = 0.97, r 2 = 0.97, P < 0.001) (Fig. 4a). For the former, the slope of the regression line was significantly larger than 1 (the 95% confidence intervals did not bracket zero), while slope of the latter was convergent to 1 (Table 1). In contrast, average stomatal area was independent of leaf midvein xylem area (r 2 = 0.01, P > 0.05) or average leaf area (r 2 <0.01, P > 0.05) (Fig. 4b; Table 1). Similarly, plant minor vessel number scaled with stem xylem area (slope = 0.88,r 2 = 0.88, P < 0.001) and total leaf area (slope = 0.69, r 2 = 0.62,P < 0.001; Fig. 4c) (Table 1). Average minor vessel area was independent of stem xylem area (r 2< 0.01, P > 0.05) or total leaf area (r 2 < 0.01, P> 0.05) (Fig. 4d; Table 1).
Fig. 4 (a) Covariation of leaf total stomatal number and midvein xylem area (or average leaf area, insert). (b) Relationship between average stomatal area and midvein xylem area (or average leaf area, insert). (c) Covariation of plant total stomatal number and stem xylem area (or total leaf area, insert). (d) Relationship between minor vessel area and stem xylem area (or total leaf area, insert). Lines indicate significant scaling relationships. Growth forms: T tree, S shrub,SS subshrub, C+Sc scrambler or climber. Regression coefficients of standardized major axis (SMA) are documented in Table 1.
Furthermore, plant total stomatal number strongly scaled with plant minor vessel number (slope = 1.51, r 2 = 0.62,P < 0.001; Fig. 5a); while the average area of stomata and minor vessels did not show any relation with each other (r 2 = 0.05, P > 0.05; Fig. 5b) (Table 1).
Fig. 5 (a) Covariation of plant total stomatal number and plant minor vessel number. (b) Relation between average stomatal area and average minor vessel area. Lines indicate significant scaling relationships. Growth forms: T tree, S shrub, SS subshrub,C+Sc scrambler or climber. Regression coefficients of standardized major axis (SMA) are documented in Table 1.