Abstract
Pine-oak ecosystems are globally distributed even though differences in anatomy and leaf habit between many co-occurring oaks and pines suggest different strategies for resource use, efficiency and stomatal behavior. The New Jersey Pinelands contain sandy soils with low water- and nutrient-holding capacity providing an opportunity to examine trade-offs in resource uptake and efficiency. Therefore, we compared resource use in terms of transpiration rates and leaf nitrogen content and resource-use efficiency including water-use efficiency (WUE) via gas exchange and leaf carbon isotopes and photosynthetic nitrogen-use efficiency (PNUE) between oaks (Quercus alba, Q. prinus, Q. velutina) and pines (Pinus rigida, P. echinata). We also determined environmental drivers [vapor pressure deficit (VPD), soil moisture, solar radiation] of canopy stomatal conductance (GS) estimated via sap flow and stomatal sensitivity to light and soil moisture. Net assimilation rates were similar between genera, but oak leaves used about 10% more water and pine foliage contained about 20% more N per unit leaf area. Therefore, oaks exhibited greater PNUE while pines had higher WUE based on gas exchange, although WUE from carbon isotopes was not significantly different. For the environmental drivers of GS, oaks had about 10% lower stomatal sensitivity to VPD normalized by reference stomatal conductance compared with pines. Pines exhibited a significant positive relationship between shallow soil moisture and GS, but only GS in Q. velutina was positively 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 but lower PNUE demonstrating a trade-off between using water and nitrogen efficiently in a resource-limited ecosystem.
Highlights
In an ecosystem where both water and nutrients are limiting, we found that oaks and pines exhibited differing strategies for resource acquisition and resource-use efficiency
Conductances per unit leaf area compared with pines, while pines had greater water use efficiency based on gas exchange data
This confirms our hypothesis and could suggest that the large vessels of oaks are needed to overcome the frictional path length hydraulic resistance of pulling water from deeper groundwater sources. These results differ from the findings of Guehl et al (1995) who found that Quercus robur L. had higher iWUE than Pinus pinaster Aiton when derived from either gas exchange or stable isotopes
Summary
Forest ecosystems dominated by pines and oaks are distributed globally (Richardson and Rundel, 1998; Nixon, 2006) including locations in the Eastern United States (Nowacki and Abrams, 2008), Southwestern North and Central America (Kolb and Stone, 2000; Oak pine resource-use efficiencyGómez-Mendoza and Arriaga, 2007), the Mediterranean (Guehl et al, 1995; Díaz, 2006), and Alpine Europe (Weber et al, 2007; Zweifel et al, 2007; Sterck et al, 2008; Eilmann et al, 2009). Pine-oak forests tend to occur in water-limited systems where either strong seasonality of precipitation or well-drained, sandy soils limit the occurrence of more mesic species (Nowacki and Abrams, 2008). On the other hand, have small, evergreen needle-like leaves and rely on smaller diameter tracheids for water transport. These large differences in leaf habit and water transport capacity of co-occurring oaks and pines could suggest that they have different strategies for obtaining and using nutrients and water, in ecosystems with sandy soils that exhibit low waterand nutrient-holding capacity in the upper soil layers
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