SPECIES MIXTURE AND SOIL-RESOURCE AVAILABILITY AFFECT THE ROOT-GROWTHRESPONSE OF TREE SEEDLINGS TO ELEVATED ATMOSPHERIC CO21

The effects of CO2 enrichment on root proliferation of loblolly pine ( Pinus taeda L.) and sweetgum (Liquidambar styraciflua L.) seedlings we re studied under varied water and nitrogen (N) regimes and in competit ive interaction. Seedlings of each species were grown from seed as mon ocultures or as 50:50 pine-sweetgum mixtures in 22-L pots filled with forest soil. Seedlings were exposed to either ambient (400 ppm) or CO2 -enriched (ambient plus 400 ppm) air for 32 weeks in continuously stir red tank reactors. Detailed sampling of very fine roots (<0.5 mm diam. ) showed a general increase (up to 2-fold) in root length density (RLD , cm . cm(-3)) with elevated CO2; however, the effects of CO2 on RLD d iffered according to species, culture type, water, and N availability. In monoculture, low water with low N conditions produced the largest RLD responses to elevated CO2: 75% increase for sweetgum and 31% incre ase for pine. In mixed culture, by contrast, the largest RLD responses to CO2 were observed under high water, high N regimes: pine showed a 110% increase and sweetgum a 96% increase. The total RLD of the standi ng crop in mixture under elevated CO2, high water, and high N was 2.6 cm . cm(-3) compared with 1.6 cm . cm(-3) in ambient CO2, with sweetgu m accounting for >75% of the total RLD in both cases. These findings s uggest that resource-rich rather than resource-poor soil environments could be the circumstances under which belowground interference from s weetgum would intensify in pine-sweetgum mixtures with rising atmosphe ric CO2.