Associations between shade tolerance and wood specific gravity for conifers in contrast to angiosperm trees: foundations of the conifer fitness-enhancing shade tolerance hypothesis

For decades, researchers have held that wood specific gravity was an indicator or surrogate for both shade tolerance and successional status.  However, recent research in dry tropical forests has shown very different associations regarding wood specific gravity.  Past analyses of the tolerance and wood properties in tree species have focused on pooled coniferous and angiosperm species in temperate regions; fewer analyses have been conducted separately for conifers and angiosperm species. A database was compiled for wood properties and/or tolerance scores of 542 temperate Northern Hemisphere conifer and angiosperm trees.  Plant strategy was defined by shade tolerance (Tshade), drought tolerance (Tdrought), and polytolerance (Tpoly = Tshade + Tdrought) and fundamental wood properties were represented by basic specific gravity (SGbasic), relative stiffness (MOE/SGbasic), and relative strength (MOR/SGbasic). Simple linear regressions tested significance (P < 0.05) of correlations between plant strategy and wood properties. Conifers, unlike angiosperm trees, showed a negative correlation between Tshade and SGbasic and a positive correlation between Tshade and both MOE/SGbasic and MOR/SGbasic.  Only angiosperm trees had a significant correlation between Tpoly and both SGbasic and MOE/SGbasic, but both conifers and angiosperm trees had a significant correlation between Tdrought and both SGbasic and MOE/SGbasic.  Shade tolerance, as a plant strategy, has functional implications for wood properties in temperate Northern Hemisphere conifers but not in associated angiosperms. The implied functional link between wood properties (SGbasic) and shade tolerance hypothetically extends to other fitness-enhancing traits impacted by SGbasic such as growth rates and species maximum height.