Coordination between leaf water relations and leaf gas exchange in two ecotypes of Fagus crenata tree

Abstract

Fagus crenata Blume is a dominant species in late-successional, cool-temperate forests of Japan. F. crenata has two ecotypes in Japan, one in the Japan Sea region and the other in the Pacific Ocean region. The trees at Nakoso in the Pacific Ocean region suffer higher evaporative demand during the growing season than do those at Appi in the Japan Sea region. This study aimed to determine how leaf morphology and leaf physiology are coordinated to cope with air drought conditions. We investigated intra-crown leaf plasticity for adult trees growing in both regions. The specific hypothesis was that the sun leaves of F. crenata at Nakoso would have a lower osmotic potential and a larger vascular structure within their leaves than those at Appi and, as a result, would have a relatively higher leaf mass per area (LMA) and lower carbon assimilation capacity. Relative to trees at Appi, the sun leaves at Nakoso had a thicker lamina, a larger bundle sheath within the lamina, and a smaller compartment area circumscribed by a bundle sheath extension. Even though LMA in the sun leaves at Nakoso was 1.74 times larger than that at Appi, the area-based photosynthetic rates did not differ between sites. Leaf morphological acclimation to high evaporative conditions at Nakoso included reduced mass-based nitrogen (N) content and the N/C ratio within the lamina as well as reduced mass- and nitrogen-based assimilation rates. Our study indicates that drought tolerance at the leaf level leads to variations in leaf water relations and in leaf morphology. This suggests a trade-off or coordination between leaf water relations and leaf morphology or leaf gas exchange for drought tolerance.