Leaf chlorophyll plasticity in advanced regeneration of co-occurring Acer pensylvanicum and Fagus grandifolia beneath deciduous forest canopy

Abstract

The chemical trait ‘total chlorophyll content per unit leaf area’ (CHL) has long been recognized to confer plasticity within a genetically fixed range of phenotypic variance. The plasticity of leaf CHL (PCHL) was assessed with special reference to saplings of the two morphologically divergent tree species Acer pensylvanicum L. (AP) and Fagus grandifolia Ehrh. (FG) which co-occur in the eastern deciduous forest biome of North America. A randomized block design with repeated measures was used to field-test the hypothesis that low leaf display efficiency favoured selection for higher PCHL in FG than in AP. Species were tested in the shade of closed forest canopy at naturally low soil fertility and following applications of mineral NPK to test the second hypothesis that nutrient-poor soils select for high PCHL. Results support the first hypothesis of higher phenotypic PCHL and higher developmental PCHL in FG than in AP. Moreover, a markedly more dynamic CHL increase coincided with more rapid leaf development in FG compared to AP. Differential leaf phenology likely contributed to high PCHL in FG. Results also support, in the case of FG, the second hypothesis that nutrient-poor soils favour phenotypic PCHL. This is, however, not true for AP and confirms a lower shade tolerance rank for AP. Results strongly suggest that the ecological conditions for advanced regeneration in deep shade on nutrient-poor soils maintain high PCHL in FG populations.