Effects of elevated atmospheric carbon dioxide, soil nutrients and water conditions on photosynthetic and growth responses of Alnus hirsuta

The objective of this paper is to clarify the effects of multiple environmental conditions, elevated atmospheric CO2 concentration ([CO2]) and soil conditions on the physiological and morphological properties of Alnus hirsuta Turcz., an N2-fixing species, to predict its responses to environmental changes. We examined the responses of photosynthetic properties, leaf characteristics, biomass and N allocation of A. hirsuta to elevated [CO2], soil N and phosphorus availability, and soil drought by using the results of two experiments. The effects of P availability were more marked than those of N availability and soil drought. The photosynthetic responses of A. hirsuta to elevated [CO2] under high P were considered to be ‘photosynthetic acclimation’, while A. hirsuta presented the obvious ‘photosynthetic downregulation’ to elevated [CO2] under low P. Soil P availability affected the growth responses to elevated [CO2] through effects on these photosynthetic properties and biomass allocation. Though elevated [CO2] caused no marked change in the allometric relationships in biomass, with some exceptions, the responses of N allocation among tissue to elevated [CO2] differed from those of biomass allocation. These results suggest that it is necessary to evaluate N mass allocation as well as biomass when we consider the N2-fixing ability of Alnus under elevated [CO2].