INTERACTING EFFECTS OF SOIL FERTILITY AND ATMOSPHERIC CO2 ON LEAF-AREA GROWTH AND CARBON GAIN PHYSIOLOGY IN POPULUS X EURAMERICANA (DODE) GUINIER

Two important processes which may limit productivity gains in forest e cosystems with rising atmospheric CO2 are reduction in photosynthetic capacity following prolonged exposure to high CO2 and diminution of po sitive growth responses when soil nutrients, particularly N, are limit ing. To examine the interacting effects of soil fertility and CO2 enri chment on photosynthesis and growth in trees we grew hybrid poplar (Po pulus x euramericana) for 158 d in the field at ambient and twice ambi ent CO2 and in soil with low or high N availability. We measured the t iming and rate of canopy development, the seasonal dynamics of leaf le vel photosynthetic capacity, respiration, and N and carbohydrate conce ntration, and final above- and belowground dry weight. Single leaf net CO2 assimilation (A) increased at elevated CO2 over the majority of t he growing season in both fertility treatments. At high fertility, the maximum size of individual leaves, total leaf number, and seasonal le af area duration (LAD) also increased at elevated CO2, leading to a 49 % increase in total dry weight. In contrast, at low fertility leaf are a growth was unaffected by CO2 treatment. Total dry weight nonetheless increased 25% due to CO2 effects on A. Photosynthetic capacity (A at constant internal p(CO2), (C-i)) was reduced in high CO2 plants after 100 d growth at low fertility and 135 d growth at high fertility. Anal ysis of A responses to changing C-i indicated that this negative adjus tment of photosynthesis was due to a reduction in the maximum rate of CO2 fixation by Rubisco. Maximum rate of electron transport and phosph ate regeneration capacity were either unaffected or declined at elevat ed CO2. Carbon dioxide effects on leaf respiration were most pronounce d at high fertility, with increased respiration mid-season and no chan ge (area basis) or reduced (mass basis) respiration late-season in ele vated compared to ambient CO2 plants. This temporal variation correlat ed with changes in leaf N concentration and leaf mass per area. Our re sults demonstrate the importance of considering both structural and ph ysiological pathways of net C gain in predicting tree responses to ris ing CO2 under conditions of suboptimal soil fertility.