Do interspecific differences in sapling growth traits contribute to the co-dominance of Acer saccharum and Fagus grandifolia?

Abstract

Background and aims: Acer saccharum and Fagus grandifolia are among the most dominant late-successional tree species in North America. The influence of sapling growth responses to canopy gaps on the co-dominance of the two species in an old-growth forest in southern Quebec, Canada was examined. Two predictions were evaluated: (a) F. grandifolia is more shade tolerant than A. saccharum due to greater sapling leaf area and net production per sapling in closed-canopy conditions; and (b) the height growth rate of A. saccharum in canopy gaps is greater than that of F. grandifolia due to increased net production per sapling.

Methods: Sapling crown allometry, net production and height growth rates were compared between and within the two species in closed canopy vs. canopy gaps. Standardized major axis regression was used to analyse differences in crown allometry.

Key results: F. grandifolia had greater crown projection, sapling leaf area and net production rate per sapling than A. saccharum in closed-canopy conditions. In response to canopy gaps, net production per sapling increased to the same degree in both species. The net production per sapling of F. grandifolia thus was much greater than that of A. saccharum in both canopy gap and closed-canopy conditions. The height growth rate of both species increased in canopy gaps, but the degree of increase was greater in F. grandifolia than in A. saccharum.

Conclusions: F. grandifolia regenerated more successfully than A. saccharum in both closed-canopy conditions and canopy gaps, which indicates that the co-dominance of the two species cannot be maintained simply by interspecific differences in shade tolerance and growth in gaps. Previous research showed that although Fagus and Acer shared dominance at this site, their relative dominance shifted with edaphic conditions. This suggests that the widespread co-dominance of the two species in eastern North American forests is maintained by the joint influence of canopy disturbance and species-specific responses to the heterogeneity of moisture and fertility regimes within forested landscapes.

Fig. 1.

Comparisons of crown architectural and growth traits of Acer saccharum and Fagus grandifolia in closed-canopy conditions and canopy gaps: (A) crown projection (A_C), (B) sapling leaf mass (M_L), (C) sapling leaf area (A_L) at 90 cm trunk height in 2005 (H₀₅), (D) branch mass (M_B) at 0·17 m² A_L, (E) net production rate per sapling (NP) at 0·17 m² A_L, (F) NP at 90 cm trunk height in 2004, i.e. before the current-year growth (H₀₄), (G) trunk height growth rate, (H) trunk length (L_T) at 90 cm H₀₅, (I) mean stem length of current-year leader shoot. A trunk height of 90 cm and a sapling leaf area of 0·17 m² are the overall mean values of the examined saplings. Each crown component was estimated by the allometric regression listed in Appendix 1. Interspecific difference in the intercept of regressions within each canopy condition (closed or gaps) was denoted by asterisks (*P < 0·05, **P < 0·01, ***P < 0·001 by ANCOVA). The slope of regression was not significantly different between the two species for each allometry. Interspecific differences in the height growth rate and stem length were also denoted by asterisks (Mann–Whitney U-test).

Fig. 2.

Relationship between branch mass and sapling leaf area for Acer saccharum and Fagus grandifolia in closed-canopy conditions (filled symbols, solid line) and canopy gaps (open symbols, broken line). Regression equations are listed in Appendix 1.

Fig. 3.

Relationship between net production rate per sapling and sapling leaf area for Acer saccharum and Fagus grandifolia in closed-canopy conditions (filled symbols, solid line) and canopy gaps (open symbols, broken line). Regression equations are listed in Appendix 1.

Fig. 4.

Relationship between net production rate per sapling and trunk height before the current-year growth for Acer saccharum and Fagus grandifolia in closed-canopy conditions (filled symbols, solid line) and canopy gaps (open symbols, broken line). Regression equations are listed in Appendix 1.

Fig. 5.

Relationship between the number of leaves per shoot and stem length of the current-year leader shoot for Acer saccharum and Fagus grandifolia in closed-canopy conditions (filled symbols) and canopy gaps (open symbols).