Atmospheric CO2 enrichment and soil N fertility effects on juvenile ponderosa pine: growth, ectomycorrhizal development, and xylem water potential.

摘要

Interactive effects were investigated of increased atmospheric CO2 and soil N fertility on above- and below-ground growth, mycorrhizal colonization, and water relations of juvenile ponderosa pine (Pinus ponderosa). One-year-old seedlings were planted in undisturbed field soil within open-top chambers (at Placerville, California) which permitted creation of atmospheres with 700 or 525 鎙 litre-1, or ambient CO2 concentrations. High and medium soil N treatments were imposed by incorporating sufficient (NH4)2SO4 to increase total N by 200 鎔 g-1, respectively, while unamended soil, which had a total N concentration of approximately 900 鎔 g-1, constituted the low N treatment. Following each of 2 consecutive field growing seasons, whole seedlings of every combination of CO2 and N treatment were harvested to permit assessment of shoot and root growth and quantification of ectomycorrhizal development. Late in the second growing season, a simulated drought episode was imposed by withholding irrigation,and during this predawn and midday xylem water potential and soil water potential were measured. The initial harvest showed that coarse and fine root weights were increased by CO2 enrichment during the first growing season. This result was most apparentin the 525 鎙 litre-1 CO2 treatment and high soil N, which produced the greatest root volume as well. Shoot/root ratio decreased with increasing CO2 at the first harvest. After 2 growing seasons, increased CO2 had promoted increased seedling diameter, shoot and root volume, and shoot and coarse root weight, again most prominently in high N. However, at this stage stimulation of seedling growth was most in the 700 鎙 litre-1 CO2 atmosphere, and shoot/root ratio was unaffected by either CO2 or N. At bothharvests, seedlings grown in the enriched atmospheres generally had higher mycorrhizal counts and greater percentages of colonized root length, but differences among treatments in ectomycorrhizal development were nonsignificant regardless of quantification method. During the imposed drought episode, xylem water potential of seedlings grown in increased CO2 descended below that of seedlings grown in the ambient atmosphere as soil water potential decreased, most notably in the predawn measurements. Theseresults suggest that CO2 enrichment stimulates shoot and root growth of juvenile ponderosa pine under field conditions, a response somewhat dependent on soil N availability. However, below-ground growth is not increased proportionally more than that above ground, which may predispose this species to greater stress when soil water is limited.