Relating leaf photosynthetic rate to whole-plant growth: drought and shade effects on seedlings of four Quercus species

Understanding the impacts of combined resource supplies on seedlings is critical to enable prediction of establishment growth, and forest dynamics. We investigated the effects of irradiance and water treatments on absolute growth, and relative growth rate (RGR) and its components, for seedlings of fourQuercus species differing in leaf habit and with a wide variation in seedmass. Plants were grown for 6.5months at three levels of irradiance (100, 27, and 3%daylight), and treated during the last 2.5 months with two watering treatments (frequent watering v. suspended watering). Both shade and drought reduced seedling growth rates, with a significant interaction: under full irradiance the drought treatment had a stronger impact on RGR and final biomass than under deep shade. For three species, seed mass was positively related to absolute growth,with stronger correlations at lower irradiance. The evergreen species grew faster than the deciduous species, though leaf habit accounted for a minor part of the interspecific variation in absolute growth. Seedling biomass was determinedpositively either byRGRor seedmass;RGRwaspositively linkedwith net assimilation rate (NAR) and leafmass fraction (LMF), and seed mass was negatively linked with RGR and LMF, but positively linked with NAR. Seedling RGR was not correlated with light-saturated net photosynthetic rate, but was strongly correlated with the net carbon balance estimated, from photosynthetic light-response curves, considering daily variation in irradiance. These findings suggest an approach to applying short-term physiological measurements to predict the RGR and absolute growth rate of seedlings in a wide range of combinations of irradiance and water supplies. Additional keywords: biomass allocation, carbon balance, growth analysis, leaf habit,Mediterranean oak, relative growth rate, seed mass, specific leaf area.