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 greenhouse and open- top chamber studies on the effects of N and elevated atmospheric CO 2 on ponderosa and loblolly pine (Pinus ponderosa Laws. and P. taeda L.) to evaluate common patterns of response. Growth response to elevated CO 2 ranged from zero to more than 1000%, depending largely upon N status. In both species, growth response to CO 2 was greater under moderate N deficiency than under extreme N deficiency or N sufficiency/excess. Elevated CO 2 generally caused lowered tissue N concentrations in many (but not all) cases, which in turn resulted in smaller 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 elevated CO 2 when N was in the extreme deficiency range but not when N was in the moderate deficiency range. In two separate studies, ponderosa pine responded negatively to high N inputs. and in each case this response was mitigated by elevated CO 2 . Collectively, these results show that (i) N deficiency is a continuum rather than a step function, (ii) responses to elevated CO 2 vary across this continuum of N deficiency. and (iii) elevated CO 2 greatly enhances growth response to N additions when N is initially-in the extremely deficient range.