Water-use characteristics of Pinus rigida growing in the New Jersey Pine Barrens

Background/Question/Methods The Pine Barrens are the largest pine-oak forest system in the United States, located in southern New Jersey. The Pine Barrens consist of two major topographic regions, the lowlands and the uplands. The lowlands make up 80-85% of total Pine Barren region and are recognized by their high water tables, while the xeric uplands cover the additional 15-20% of the Pine Barren region. Pitch pine (Pinus rigida) covers 90% of the upland canopy and only reaches a height of only 4-8 m here. The lowland regions of the Pine Barrens are more diverse, supporting a number of plant species. Pitch pine is one of the few plant species able to successfully tolerate the strong topographic gradient in water availability within the Pine Barrens. This unique physiological tolerance that permits pitch pine to live in both the xeric upland and mesic lowland pine-oak forest creates a unique opportunity to examine the physiological functioning of this important species as it relates to the water use of forest trees. The objective of the current study was to study the physiological functioning of Pinus rigida with respect to its use of water in both the mesic lowlands and the xeric uplands of the New Jersey Pine Barrens. The study was conducted in the New Jersey Pine Barrens within the boundaries of the Brendan T. Byrne State Forest (39°40'N, 74°40' W). Ninety pitch pine trees located in six quadrats were used in the study. Three 50 m x 50 m quadrats were located in the xeric uplands, while three additional 50 m x 50 m quadrats were located in the mesic lowlands. Wood cores were collected for tree-ring growth analysis. In addition, wood and needle samples were analyzed for δ¹³C isotopic discrimination in order to examine water use efficiency of the pitch pine trees. Additionally, measurements of leaf-level gas exchange, stomatal density, and foliar nitrogen concentrations were collected throughout the 2009-growing season.

Results/Conclusions  We found no significant differences in foliar N concentrations or steady-state net photosynthesis in upland and lowland trees. We did find a significantly greater number of stomates on upland needles than those growing in the lowlands. In addition, water-use efficiency as estimated by δ¹³C values was 13% higher in upland pitch pines compared to those in the lowlands. In addition, mean annual tree-ring growth was significantly higher in lowland trees than upland trees.