Indications of Deep Soil Water Usage by Limber Pine (Pinus Flexilis) and Skunkbush Sumac (Rhus Aromatica) in Northeastern Colorado: An Oxygen Isotope Study

By Roberts, L. M.; McCulley, R. L. et al. | The American Midland Naturalist, July 2004 | Go to article overview

Indications of Deep Soil Water Usage by Limber Pine (Pinus Flexilis) and Skunkbush Sumac (Rhus Aromatica) in Northeastern Colorado: An Oxygen Isotope Study


Roberts, L. M., McCulley, R. L., Burke, I. C., Lauenroth, W. K., The American Midland Naturalist


ABSTRACT.-

We examined whether limber pine and skunkbush sumac individuals co-existing on the Pawnee Buttes of northeastern Colorado have the ability to access and utilize deep soil water resources. The [delta]^sup 18^O signature of source water to the plants (deep soil water and precipitation) and plant cell water were measured in June 2000. The [delta]^sup 18^O signatures of the two woody plant species were not significantly different from each other. However, the average [delta]^sup 18^O signature of skunkbush sumac (-9.4[per thousand]) differed from the [delta]^sup 18^O signature of deep soil water (-13.0[per thousand]), while the average [delta]^sup 18^O signature of limber pine (-11.3[per thousand]) did not. Variability in the [delta]^sup 18^O signature across plots and between individuals within a plot was relatively high for both species. These results suggest limber pine and some individuals of skunkbush sumac are able to access and utilize deep soil water resources at this site. This ability may confer an ecological advantage to these plants given the semi-arid climate of the site.

INTRODUCTION

Isolated conifer woodlands, often interpreted as relict Pleistocene populations, occur on escarpments in the western Great Plains (Wells and Stewart, 1987; Schuster el al, I995). In northeastern Colorado, these escarpment woodlands are primarily composed of limber pine {Finns flexuu'James) ', Rocky Mountain juniper (Juniperus scopulorum Sarg.), skunkbush sumac [Rhus aromatica Aiton wir lrilobala (Nutt.) A. Gray] and the grass little bluestem (Andmpogon scoparium Michx.). Studies of the population dynamics of limber pine in this region indicate that most trees are <200 y old (Schuster el al, 1995; Dreyfuss, 2002). Given the potential longevity of limber pine (maximum >1500 y old; Schulman, 1954), either these isolated woodlands are of recent origin or frequent disturbance, such as fire and/or drought, has prevented limber pine in this region from attaining ages commonly observed in other locations east of the Rocky Mountains (Schuster el al., 1995).

The western Great Plains climate is semi-arid, with low and highly variable precipitation (Borchert, 1950; Lauenroth and Burke, 1995). Water limitation is a primary factor determining vegetation structure and dynamics in this region (SaIa el al., 1981; Lauenroth el al., 1999). Evidence from limber pine population age structures on the Pawnee Buttes of northeastern Colorado indicates that the severe drought experienced across the Great Plains in the 1930-194Os significantly reduced individual recruitment at this location (Schuster el al., 1995). While such episodic droughts undoubtedly play a role in determining the population dynamics of these relict communities, the ability of these woodlands to exist in the current, pervasive semi-arid conditions at these sites suggest individuals of limber pine on the escarpments are able to withstand seasonally dry conditions and large year-to-year variability in the amount of precipitation received.

One potential hypothesis explaining continued limber pine regeneration and growth in this semi-arid environment is that these trees are able to access and use deep soil water resources. The source of water present in plant cells can be determined by using stable isotopes of oxygen if the primary sources of water to the plant have different isotopic signatures (Ehleringer and Dawson, 1992). Previous work from a shortgrass steppe plant community within 60 km of the Pawnee Buttes has shown that the two primary sources of water in this region (precipitation and groundwater) are isotopically distinct during the growing season, and that at least two types of woody plants [plains cottonwood (Populus sargentii Dode) and four-wing saltbrush (Atriplex canescens Pursh. Nutt.)] common to the shortgrass steppe access deep soil water reserves (Dodd el al., 1998). However, whether limber pines and other woody species growing on buttes 61 m above the surrounding plains access deep soil water resources is unknown. …

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Indications of Deep Soil Water Usage by Limber Pine (Pinus Flexilis) and Skunkbush Sumac (Rhus Aromatica) in Northeastern Colorado: An Oxygen Isotope Study
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