Carbon allocation to defense, storage, and growth in seedlings of two temperate broad-leaved tree species

Optimal carbon allocation to growth, defense, or storage is a critical trait in determining the shade tolerance of tree species. Thus, examining interspecific differences in carbon allocation patterns is useful when evaluating niche partitioning in forest communities. We hypothesized that shade-tolerant species allocate more carbon to defense and storage and less to growth compared to shade-intolerant species. In gaps and forest understory, we measured relative growth rates (RGR), carbon-based defensive compounds (condensed tannin, total phenolics), and storage compounds (total non-structural carbohydrate; TNC) in seedlings of two tree species differing in shade tolerance. RGR was greater in the shade-intolerant species, Castanea crenata, than in the shade-tolerant species, Quercus mongolica var. grosseserrata, in gaps, but did not differ between the species in the forest understory. In contrast, concentrations of condensed tannin and total phenolics were greater in Quercus than in Castanea at both sites. TNC pool sizes did not differ between the species. Condensed tannin concentrations increased with increasing growth rate of structural biomass (GRstr) in Quercus but not in Castanea. TNC pool sizes increased with increasing GRstr in both species, but the rate of increase did not differ between the species. Accordingly, the amount of condensed tannin against TNC pool sizes was usually higher in Quercus than in Castanea. Hence, Quercus preferentially invested more carbon in defense than in storage. Such a large allocation of carbon to defense would be advantageous for a shade-tolerant species, allowing Quercus to persist in the forest understory where damage from herbivores and pathogens is costly. In contrast, the shade-intolerant Castanea preferentially invested more carbon in growth rather than defense (and similar amounts in storage as Quercus), ensuring establishment success in gaps, where severe competition occurs for light among neighboring plants. These contrasting carbon allocation patterns are closely associated with strategies for persistence in these species' respective habitats.