Photosynthesis of Conifers in Relation to Annual Growth Cycles and Dry Matter Production

Abstract Observations that deciduous and evergreen conifers growing in Britain show similar annual growth increments, despite marked differences in short‐term growth rates, led to a comparative study of seasonal photosynthetic capacity in established trees of four coniferous species with contrasting growth habits. The photo‐synthetic data were compared with seasonal changes in environmental parameters and chloroplast ultrastructure. The maximum net photosynthetic rates (at 20°C) recorded for Larix leptolepis were higher than those for the evergreen conifers when expressed on a leaf weight basis but not when expressed per unit leaf area. The photosynthetic efficiency of new needles in the evergreen species showed an overall decline from just after needle maturity until just before budbreak in their second season, after which photosynthetic rates recovered temporarily, approaching previous maximum levels. There was no obvious correlation between seasonal photosynthetic efficiency (at 20°C) on the one hand, and daily air, and (30 cm) ground temperatures on the other, and there was no obvious winter suppression of evergreen photosynthetic rates. Evergreen needles showed starch loss and some membrane changes with the onset of winter, but there was no evidence for wintertime chloroplast clumping or membrane disruption.