Rooting volume, nutrient availability, and CO₂-induced growth enhancements in temperate forest tree seedlings

We examined growth and allocation responses to CO₂ enrichment for three species of co-occurring temperate forest tree seedlings grown in pots of varying rooting volumes and nutrient supply. Under both current and projected future CO₂ atmospheres, tree seedling growth was substantially greater with greater total nutrient supply (either due to increased nutrient addition rate or increased rooting volume) for all species. Increasing rooting volume alone, holding total nutrient supply constant, increased growth for gray and yellow birch and decreased growth for red maple. Root/shoot ratios were less and specific leaf masses were greater for plants grown in smaller pots, suggesting that the smaller pots did restrict root growth with consequences for whole-plant carbon allocation. After 12 wk of growth at light levels simulating those found in small gaps in temperate forests, each species exhibited growth, allocational and/or architectural differences due to increased CO₂. Of 11 traits measured, 9 were significantly altered by CO₂ regime. Gray birch responded in architectural and allocational parameters only; total carbon accumulation after 12 wk of growth was not affected by CO₂ regime. Red maple and especially yellow birch grew larger in elevated CO₂, and were less responsive in architectural and allocational parameters than gray birch. Increasing N concentration did not increase CO₂-induced growth enhancements, except for increased leaf production in gray birch. In fact, CO₂-induced increases in branch production were greatest at low nutrient concentration. Pot size had no effect on CO₂induced growth responses, except that CO₂-induced enhancement in branch production was greater in smaller pots. With few exceptions, conditions within pots did not influence responses to elevated CO₂, despite the many growth and architectural responses manifested by these tree seedlings in response to CO₂, nutrient regime, and pot size.