Does the growth response of woody plants to elevated CO2 increase with temperature? A model-oriented meta-analysis.
This paper summarizes the data on nutrient uptake and soil responses in opentop chambers planted with ponderosa pine (Pinus ponderosa Laws.) treated with both N and CO2. Based upon the literature, we hypothesized that 1) elevated CO2 would cause increased growth and yield of biomass per unit uptake of N even if N is limiting, and 2) elevated CO2 would cause increased biomass yield per unit uptake of other nutrients only by growth dilution and only if they are non-limiting. Hypothesis 1 was supported only in part: there were greater yields of biomass per unit N uptake in the first two years of growth but not in the third year. Hypothesis 2 was supported in many cases: elevated CO2 caused growth dilution (decreased concentrations but not decreased uptake) of P, S, and Mg. Effects of elevated CO2 on K, Ca, and B concentrations were smaller and mostly non-significant. There was no evidence that N responded in a unique manner to elevated CO2, despite its unique role in rubisco. Simple growth dilution seemed to explain nutrient responses in almost all cases. There were significant declines in soil exchangeable K+, Ca2+, Mg2+ and extractable P over time which were attributed to disturbance effects associated with plowing. The only statistically significant treatment effects on soils were negative effects of elevated CO2 on mineralizeable N and extractable P, and positive effects of both N fertilization and CO2 on exchangeable Al3+. Soil exchangeable K+, Ca2+, and Mg2+ pools remained much higher than vegetation pools, but extractable P pools were lower than vegetation pools in the third year of growth. There were also large losses of both native soil N and fertilizer N over time. These soil N losses could account for the observed losses in exchangeable K+, Ca2+, Mg2+ if N was nitrified and leached as NO 3 − .