Scaling Aspen-FACE experimental results to century and landscape scales

Background/Question/Methods

The Aspen-FACE experiment (Rhinelander WI) generated 11 years of empirical data on the effect of CO₂ enrichment and elevated ozone on the growth of field-grown trees (maple, birch and four aspen clones) in northern Wisconsin, but it is not known how these short-term plot-level responses might play out at the landscape scale over multiple decades where competition, succession and disturbances interact with the tree-level responses.  In this study we used a landscape disturbance and succession model (LANDIS-II) to scale these site level results to broader temporal and spatial scales.  First, we temporally scaled the single decade FACE results to multiple decades using a single-site version of the LANDIS-II succession algorithms (LANDIS-II-Site).  This allowed us to study site-level temporal trends without the confounding effects of spatial processes such as contiguous disturbances and seed source neighborhoods.  We then scaled FACE results both temporally and spatially using LANDIS-II.  In both stages we used the models to conduct simple, replicated 2x2 factorial simulation experiments that were directly analogous to the Aspen-FACE experiment.  The FACE treatments were CO₂ (350-380 (control) and 550 ppm (elevated)) and O₃ (30-50 ppb (control) and 60-80 ppb (elevated)) concentrations and their combination, replicated three times.  

Results/Conclusions

The following general principles were identified from the results.  1) The productivity of taxa under future conditions is the primary determinant of short-term taxon dominance.  2) Over a longer-term, longevity and shade tolerance may supersede productivity as the determinant of importance, depending on the disturbance regime.  This result offers hope that, even in the face of atmospheric changes, managers may have some control over future forest composition and carbon sequestration through modification of disturbance regimes.  3) Changes in the abundance of taxa were mostly gradual and none of the taxa we studied were extirpated from the landscape, even under treatments for which they were clearly poorly adapted.  This suggests that as atmospheric conditions change, abrupt extirpations should be rare.  4) Similarly, different taxa fared relatively well under different treatments.  This suggests that maintaining species and genetic diversity is an adaptive strategy in the face of global change.  5) Accounting for spatial processes is important because seed dispersal and establishment may limit the ability of some species to colonize available habitat.