CONSTRUCTION COST OF LOBLOLLY AND PONDEROSA PINE LEAVES GROWN WITH VARYING CARBON AND NITROGEN AVAILABILITY

We grew loblolly and ponderosa pine seedlings in a factorial experimen t with two CO2 partial pressures (35 and 70 Pa), and two nitrogen trea tments (1 . 0 and 3 . 5 mol m(-3) NH4+), for one growing season to exa mine the effects of carbon and nitrogen availability on leaf construct ion cost, Growth in elevated CO2 reduced leaf nitrogen concentrations by 17 to 40%, and increased C:N by 22 to 68%. Elevated N availability increased leaf N concentrations and decreased C:N, Non-structural carb ohydrates increased in high-CO2-grown loblolly seedlings, except in fa scicles from low N, and in ponderosa primary and fascicle leaves grown in high N, In loblolly, increases in starch were nearly 2-fold greate r than the increases in soluble sugars, In ponderosa, only the soluble sugars were affected by CO2. Leaf construction cost (g glucose g(-1) dm) varied by 9 . 3% across all treatments, All of the variation in lo blolly leaf construction cost could be explained by changes in non-str uctural carbohydrates. A model of the response of construction cost to changes in the mass of different biochemical fractions suggests that the remainder of the variation in ponderosa, not explained by non-stru ctural carbohydrates, is probably attributable to changes in lignin, p henolic or protein concentrations.