Convergence of leaf display and photosynthetic characteristics of understory Abies amabilis and Tsuga heterophylla in an old-growth forest in southwestern Washington State, USA.

We compared the morphological and physiological characteristics of understory trees of Abies amabilis (Dougl. ex Loud.) Dougl. ex J. Forbes and Tsuga heterophylla (Raf.) Sarg. growing adjacent to each other in an old-growth forest in southwestern Washington State, USA. We hypothesized that, despite contrasting branching patterns and crown architectures, the two species should exhibit convergence in leaf display and photosynthetic gain per light intercepting area, because these are important properties determining their survival in the light-limited understory. The branching pattern of A. amabilis was regular (normal shoot-length distribution, less variable branching angle and bifurcation ratio), whereas that of T. heterophylla was more plastic (positively skewed shoot-length distribution, more variable branching angle and bifurcation ratio). The two species had similar shoot morphologies: number of leaves per unit shoot length and leaf to axis dry mass ratio. Leaf morphology, in contrast, was significantly different. Leaves of A. amabilis were larger and heavier than those of T. heterophylla, which resulted in lower mass-based photosynthetic rate for A. amabilis. Despite these differences, the two species had similar levels of leaf overlap and area-based photosynthetic characteristics. Needle longevity of A. amabilis was nearly twice that of T. heterophylla. The leaf N contents of current and 1-year-old leaves were lower for A. amabilis than for T. heterophylla. However, the leaf N content of A. amabilis did not change from current leaves to 6-year-old leaves, whereas that of T. heterophylla decreased with increasing leaf age. Abies amabilis had deeper crowns than T. heterophylla and retained branches with low relative growth rates. Longer branch retention may compensate for the lower branch-level assimilation rate of A. amabilis. We inferred that the convergence of leaf display and photosynthetic characteristics between A. amabilis and T. heterophylla may contribute to the persistence of both species in the understory of this forest.