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 CO2 on ponderosa and loblolly pine (Pinus ponderosa Laws. and P. taeda L.) to evaluate common patterns of response. Growth response to elevated CO2 ranged from zero to more than 1000%, depending largely upon N status. In both species, growth response to CO2 was greater under moderate N deficiency than under extreme N deficiency or N sufficiency/excess. Elevated CO2 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 CO2 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 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 additions when N is initially in the extremely deficient range