Instantaneous photosynthetic responses to temperature of deciduous and evergreen Nothofagus species

Deciduous species of Nothofagus tend to replace evergreen Nothofagus at the highest altitudes. We investigated whether deciduous Nothofagus species have higher maximum rates of net photosynthesis (P-max) and specific leaf area (SLA) than evergreen species and whether there is an increasing photosynthetic advantage (e. g. higher P-max) in deciduous species relative to evergreen species with increasing altitude that might explain their replacement of evergreen species. Net photosynthesis was investigated in (1) five deciduous and five evergreen species of Nothofagus grown in a common environment and (2) two co-occurring species, N. gunnii (Hook. f.) Oerst. (deciduous) and N. cunninghamii (Hook.) Oerst. (evergreen), across a range of altitudes in Tasmania. In the first experiment, the maximum rate of net photosynthesis per leaf mass (P-max, mass) and SLA were higher in deciduous species, whereas the maximum rate of net photosynthesis per leaf area (P-max, area) did not differ between leaf habits. However, in the field-based study, both mass- and area-based Pmax were higher in N. gunnii than N. cunninghamii across all sites. The high Pmax, mass of deciduous species may provide a competitive advantage at higher altitudes by maximising carbon gain during the growing season. However, in the study of sympatric populations of N. gunnii and N. cunninghamii there was no evidence of increasing photosynthetic advantage of the deciduous N. gunnii with increasing altitude.