EFFECTS OF CARBON-DIOXIDE AND NITROGEN ON GROWTH AND NITROGEN UPTAKE IN PONDEROSA AND LOBLOLLY-PINE

The purpose of this paper is to summarize the results of a series of g reenhouse and open-top chamber studies on the effects of N and elevate d atmospheric CO2 on ponderosa and loblolly pine (Pinus ponderosa Laws , and P. taeda L.) to evaluate common patterns of response. Growth res ponse to elevated CO2 ranged from zero to more than 1000%, depending l argely upon N status, In both species, growth response to CO2 was grea ter under moderate N deficiency than under extreme N deficiency or N s ufficiency/excess. Elevated CO2 generally caused lowered tissue N conc entrations in many (but not all) cases, which in turn resulted in smal ler increases in N uptake than in biomass. Growth response to N ranged from -50 (in ponderosa pine) to more than 1000%, depending upon the N status of the control medium. Growth response to N was enhanced by el evated CO2 when N was in the extreme deficiency range but not when N w as in the moderate deficiency range. In two separate studies, ponderos a pine responded negatively to high N inputs, and in each case this re sponse was mitigated by elevated CO2. Collectively, these results show that (i) N deficiency is a continuum rather than a step function, (ii ) responses to elevated CO2 vary across this continuum of N deficiency , and (iii) elevated CO2 greatly enhances growth response to N additio ns when N is initially in the extremely deficient range.