Selective forces of CO2 and ozone on a forest tree

Background/Question/Methods

Genetic variation in plants influences numerous community and ecosystem properties.  If global change factors, including pollution, lead to shifts in the genetic composition of plant populations, this could have cascading effects on processes such as herbivore community assembly, nutrient cycling, and carbon storage.  Elevated CO₂ may indirectly select for competitive ability (eg. growth rate) due to its fertilizing effect, while O₃  may select for less for rapid growth than for tolerance to its damaging effects.  While a number of studies have examined adaptive responses to pollutants in herbaceous plants, few have examined the potential for adaptive responses in trees.  Making use of data from the AspenFACE experiment, we ask: Can atmospheric pollutants (CO₂ and O₃) exert a measurable selective effect on a woody plant over short timescales?  To address this question, we developed a hierarchical Bayesian model to test whether genotype x treatment interactions are necessary to explain patterns of tree mortality.  We hypothesized that fast-growing clones would tend to have an advantage, particularly at high [CO₂], but that factors other than growth (such as tolerance of elevated [O₃]) would also be important for survival.

Results/Conclusions

We find that genotype x treatment interactions in aspen mortality lead to altered dominance hierarchies and decreased evenness under elevated CO₂ and O₃.  These responses are largely due to differences in growth responses between aspen clones.  An early-phenology clone had the greatest gain in rank in the CO₂ treatment, while the greatest gain in rank in the O₃ treatment came not from the most O₃-tolerant clone, which had high growth and survival in all treatments, but from a late-phenology clone.  While one might have expected a tradeoff between growth and stress tolerance, this was not observed.  These results support the hypothesis that elevated [CO­₂] favors fast-growing genotypes and those with a higher-than average growth response to CO₂, while high [O₃] selects for genotypes that can maintain their growth despite exposure to this leaf-damaging pollutant.  This suggests that managers might be able to increase carbon sequestration in temperate forests by selectively planting genotypes with these traits, which are identifiable in young saplings; however, because of possible tradeoffs between growth and tolerance of drought and /or frost, further study is needed.