Water relations of pine seedlings in contrasting overstory environments

Overstory conditions influence understory microclimate and resource availability, leading to gradients in evaporative demand and moisture availability that influence seedling water relations. Partial canopies may either reduce seedling moisture stress by ameliorating environmental conditions, or increase moisture stress by reducing soil moisture availability. This study used stable isotope ratios of oxygen (δ18O) and carbon (δ13C) and mass-based foliar nitrogen concentrations to investigate changes in transpiration (E), stomatal conductance (gs) and intrinsic water use efficiency (iWUE) of pine seedlings across an overstory gradient from open canopy gap environments to closed canopy forest. Foliar δ18O increased sharply from basal areas of 0–10 m2 ha−1 in Pinus banksiana, Pinus resinosa, and Pinus strobus seedlings, followed by a more gradual increase with further increases in basal area. Foliar δ13C followed a similar, but less pronounced pattern in P. banksiana and P. strobus seedlings, and had no apparent relationship with overstory basal area in P. resinosa seedlings. The slope of the δ18O:δ13C relationship was positive for every species. Foliar nitrogen concentrations were not correlated with overstory basal area. These results suggest seedling E declined as overstory basal area increased due to reductions in gs, while iWUE increased slightly from open gaps to partial canopy environments. Open gap environments appear to provide sufficient moisture to sustain high leaf-level gas exchange rates in the species we studied, while relatively small increases in overstory basal area apparently promote rapid declines in gs, leading to greatly reduced seedling water loss and small increases in iWUE.