Tree seedling growth in natural deep shade: functional traits related to interspecific variation in response to elevated CO2

The mechanisms for species-specific growth responses to changes in atmosphe ric CO2 concentration within narrow ecological groups of species, such as s hade-tolerant, late-successional trees, have rarely been addressed and are not well understood. In this study the underlying functional traits for int erspecific variation in the biomass response to elevated CO2 were explored for seedlings of five late-successional temperate forest tree species (Fagu s sylvatica, Acer pseudoplatanus, Quercus robur, Taxus baccata, Abies alba) . The seedlings were grown in the natural forest understorey in very low an d low light microsites (an average of 1.3% and 3.4% full sun in this experi ment), and were exposed to either current ambient CO2 concentrations, 500, or 660 mul CO2 l(-1) in 36 open-top chambers (OTC) over two growing seasons . Even across the narrow range of successional status and shade tolerance, the study species varied greatly in photosynthesis, light compensation poin t, leaf dark respiration (R-d), leaf nitrogen concentration, specific leaf area (SLA), leaf area ratio (LAR), and biomass allocation among different p lant parts, and showed distinct responses to CO2 in these traits. No single species combined all characteristics traditionally considered as adaptive to low light conditions. At very low light, the CO2 stimulation of seedling biomass was related to increased LAR and decreased R-d, responses that wer e observed only in Fagus and Taxus. At slightly higher light levels, inters pecific differences in the biomass response to elevated CO2 were reversed a nd correlated best with leaf photosynthesis. The data provided here contrib ute to a mechanistic process-based understanding of distinct response patte rns in co-occurring tree species to elevated CO2 in natural deep shade. I c onclude that the high variation in physiological and morphological traits a mong late-successional species, and the consequences for their responses to slight changes in resource availability, have previously been underestimat ed. The commonly used broad definitions of functional groups of species may not be sufficient for the understanding of recruitment success and dynamic changes in species composition of old-growth forests in response to rising concentrations of atmospheric CO2.