| Woody plant encroachment is a well documented global phenomenon involving the increasing density and distribution of woody plant species. In the western United States, juniper encroachment is of particular concern as these woodlands occupy between 30 and 40 million hectares of the western United States. Formerly juniper communities were restricted in distribution to rocky outcroppings with shallow soils, but within the last 150 years have encroached onto former grassland and shrub-steppe communities, increased in density in woodlands, and, in southwestern New Mexico, Juniperus monosperma ( Cuprsseaceae; one seed juniper) has expanded its range to include river or riparian habitats. The expansion of one seed juniper into riparian habitats is of particular interest because this species is drought tolerant and riparian habitats are generally not limited by water availability. To further the understanding of juniper encroachment into riparian ecosystems, a variety of techniques were used to: (1) estimate the period of establishment, (2) to statistically quantify spatial distribution patterns, and (3) to demonstrate source water acquisition in a variety of habitats. Dendrochronological techniques revealed that the average age of riparian junipers was 38 years. Ripley's K-statistic demonstrated a significant clustering pattern of junipers beneath the canopies of large phreatophytic trees. The stable isotopes of hydrogen (deltaD) and oxygen (delta18O), showed that upland junipers used monsoon rains, while in riparian habitats, juniper used water from all available sources, including groundwater. Depth to groundwater did not exceed 3.5 meters at any site during any season suggesting that declining water tables may not be important in explaining juniper encroachment. Floods have apparently not been effective at removing juniper from floodplain habitats, although flood frequency and intensity has increased since the early 1970s. Results from this study suggest that encroachment of juniper into riparian ecosystems has occurred recently, that phreatophytic trees may facilitate encroachment and that one-seed juniper has the potential to affect ecohydrological interactions, but the magnitude of the effects at this time are not clear. |
