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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.

tree physiology

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