Inverse relationship between understory light and foliar nitrogen along productivity gradients of boreal forests

The constraints of light and N on sapling growth in forest understories can covary with site productivity, but this reciprocal relationship is not always recognized when describing stand dynamics. To facilitate this, we examined light availability and foliar N status of understory subalpine fir (Abies lasiocarpa (Hook.) Nutt.) along natural productivity gradients of old-growth boreal forests in British Columbia, Canada. Understory light declined with soil fertility, from a high of 30% of full sun on poor sites to as low as 15% on very rich sites. In contrast, understory foliar N concentration (N-%) of subalpine fir increased with soil fertility (ranging from 9.3 to 14.2 g.kg(-1)) and paralleled asymptotic stand height, despite growing in the shade. Trends in foliar N% of the understory were comparable with overstory subalpine fir, and both increased in foliar N% with soil fertility more so than lodgepole pine (Pinus contorta Dougl. ex Loud.). Foliar N per unit leaf area of the suppressed understory was fairly consistent and relatively low (approximately 2.0 g N.m(-2)), suggesting no net change in the resource constraints to growth along the productivity gradient. The inherent linkages between soil fertility, light attenuation, and species N nutrition provides a useful framework for understanding regeneration dynamics over a full range of site potential.