Trade-offs in juvenile growth potential vs. shade tolerance among subtropical rain forest trees on soils of contrasting fertility

Plant adaptation to gradients of light availability involves a well-studied functional trade-off, as does adaptation to gradients of nutrient availability. However, little is known about how these two major trade-offs interact, and thus, it remains unclear whether and how the nature of the growth–shade tolerance trade-off differs on soils of contrasting fertility. We asked whether juvenile growth–shade tolerance trade-offs differed in slope and elevation between tree assemblages on nutrient-rich basalt and nutrient-poor rhyolite soils in an Australian subtropical rain forest. We measured the growth of, and the range of light environments occupied by, juveniles (40–120 cm tall) of eight basalt specialists, six rhyolite specialists, and one generalist that was common on both substrates. In situ minimum light requirements were estimated from the 5th percentile of the distribution of naturally regenerated juveniles in relation to daily light transmittance. Stem growth was measured for 12–16 months across a wide range of light environments to estimate the light compensation point of growth of each species. Light compensation points of growth showed nearly a 1 : 1 correspondence with in situ minimum light requirements of species, indicating that whole-plant carbon balance is a key driver of ecological success in low light. Minimum light requirements were negatively correlated with relative growth rate in low light, but correlated positively with growth in high light. Soil type had no effect on either the slope or the elevation of this trade-off, all species aligning around a common growth–shade tolerance trade-off, but our results do show a wider range of growth rates and shade tolerance on the nutrient-rich basalt soil than on the nutrient-poor rhyolite. Our results suggest that adaptation to light availability involves fundamentally similar trade-offs on these two substrates of differing fertility. However, a wider range of growth rates and shade tolerance on the nutrient-rich basalt soil than on the nutrient-poor rhyolite may help to explain the higher species richness and greater structural complexity of forest stands on the former substrate.