Elevated CO2 and different forms of N alters stem hydraulic conductivity in conifer seedlings

Background/Question/Methods

Rising atmospheric CO₂ has been shown to affect plant water status but the underlying mechanism(s) are not understood. Therefore, factors that control water status of plants are critical in models designed to predict hydrologic exchanges between biosphere and atmosphere. Plant water status is governed by hydraulic properties along the soil-plant-and-atmosphere-continuum which includes the axial conductivity in the stem.  In this study we examined stem conductivity of four conifer seedlings in response to different CO₂ levels. Plant hydraulics has also been shown to be affected by the availability and/or form of nitrogen (N). Therefore, we also examined the interacting effects of N-form (ammonium vs. nitrate) and CO₂. One year-old seedlings of Douglas-fir (Pseudotsuga menziesii), Norway spruce (Picea abies), Colorado spruce (Picea pungens), and Fraser Fir (Abies fraseri) were treated at either near ambient (400 ppm), or elevated (600 or 800 ppm) CO₂ for six months. Thermal and photo periods were identical for all treatments but seedlings received a modified Hoagland solution which contained either ammonium, nitrate, or ammonium nitrate. We measured stem specific conductivity (k_s) and leaf specific conductivity (LSC). k_s and LSC was calculated by dividing the stem hydraulic conductivity by stem cross sectional area and total needle area, respectively.
Results/Conclusions

Elevated CO₂ significantly affected k_S but this effect was highly species dependent. For example, in Colorado spruce, increasing CO₂ levels from 400 to 800 ppm, increased k_s by an average of 23 % across all N treatments but decreased k_s in Douglas fir by 24 %.  Overall, there was no significant effect of N form on k_s however, the CO₂ induced changes in k_s was highly depended on N form. For example, in Colorado spruce k_s increased by 102% in nitrate but decreased by 20% in ammonium treated seedlings with the doubling of CO₂ concentration. Statistically LSC responses to CO2 were similar to k_s i.e., the effect of elevated CO₂ on LSC was highly dependent on species and within species LSC depended on N form.  Despite this statistical similarity, k_s and LSC responses within species were quite dissimilar suggesting that the CO₂-induced changes in k_s are not mediated by an adjustment in canopy size and/or canopy water conductance. The results suggest that as CO₂ levels increases, there is a significant adjustment in k_s which is apparently not coordinated with the canopy conductance. Preliminary data indicates that k_s responses to rising CO₂ levels are more closely coupled to root hydraulic conductance.