Resource acquisition and resource use efficiency of oaks and pines growing on a sand plain ecosystem
Ecosystems growing on sandy soils with low water- and nutrient-holding capacity including the New Jersey Pinelands found on the Atlantic Coastal Plain provide the ideal opportunity to examine trade-offs in resource acquisition and resource-use efficiency in co-occurring trees. Oak-, pine-dominated and mixed forests are found throughout the region as well as globally especially in places where water tends to be limiting. Because oaks and pines differ in their hydraulic transport systems (large vessels vs. tracheids), leaf habit and morphology, they should exhibit differing strategies for growth in a resource-limited environment. Specifically, we sought to compare resource use in terms of transpiration rates and leaf nitrogen content as well as resource-use efficiency including water-use efficiency (WUE) estimated via gas exchange and leaf carbon isotope data and photosynthetic nitrogen-use efficiency (PNUE) estimated via gas exchange and leaf nitrogen content between oaks (Quercus alba, Q. prinus and Q. velutina) and pines (Pinus rigida and P. echinata). Additionally, we sought to determine the environmental drivers (vapor pressure deficit, soil moisture, photosynthetic photon flux density) of canopy stomatal conductance estimated via heat dissipation sapflow sensors as well as stomatal sensitivity across a soil moisture and solar radiation gradient in oaks and pines.
Oaks used more water per unit leaf area, but total tree-level transpiration rates were higher in pines because they had about five times higher leaf areas than oaks. Pines had significantly greater WUE based on gas exchange, however estimates based on carbon isotopes were not significantly different between oaks and pines. In terms of nitrogen, oak leaves had higher N concentrations but lower leaf mass and lower nitrogen content per unit leaf area compared with pines. Net photosynthetic assimilation rates were similar between genera, therefore oaks had greater PNUE than pines. In terms of the environmental drivers of stomatal conductance, oaks displayed lower stomatal sensitivity to VPD normalized by reference stomatal conductance compared with pines. Pines exhibited a negative relationship between shallow soil moisture and stomatal conductance but only stomatal conductance in Q. velutina was negatively related to soil moisture. In contrast, stomatal sensitivity to VPD was significantly related to solar radiation in all oak species but only pines at one site. Therefore, oaks rely more heavily on groundwater resources but have lower WUE, while pines have larger leaf areas and nitrogen acquisition rates but lower PNUE demonstrating a trade-off between using water and nitrogen efficiently in a resource-limited ecosystem.