Multiple resource limitation and ontogeny combined: a growth rate comparison of three co-occurring conifers

The combined effects of light, soil fertility, and ontogenetic changes on plant growth rates are poorly understood, yet these three factors play fundamental roles in structuring plant communities. We sought to determine how lodgepole pine (Pinus contorta Dougl. ex Loud. var. latifolia), interior spruce (Picea glauca x engelmanii (Moench) Voss), and subalpine fir (Abies lasiocarpa (Hook.) Nutt.) sapling growth responds to the combination of light, soil fertility, and ontogeny and how these three dominant conifer species in sub-boreal forests of British Columbia differ in their responses. Using maximum likelihood methods, we found that 0.20-4 m tall sapling growth rates changed during ontogeny and were limited by both light and soil resources. The strongest differences among species' growth rates were due to tree size, with smaller differences due to soil fertility, and there were no differences among species in the shape of their growth responses to light. Rank order in growth rates for small saplings (pine > spruce > fir) inversely corresponded to classic shade-tolerance ratings, thus supporting the carbon balance theory. Interior spruce height growth rates increased relative to lodgepole pine with increasing soil fertility, clearly matching the landscape-scale increase in canopy dominance of interior spruce over lodgepole pine with increasing soil fertility.