Survival and growth of Juniperus seedlings in Juniperus woodlands

Abstract

Juniperus woodlands are widely distributed in western North America. Few studies of seedling emergence, longterm survival, growth or mortality of the dominant Juniperus spp. in these woodlands have been carried out. Consequently, regeneration dynamics in these woodlands are poorly understood. Juniperus ashei is the dominant woody plant in the majority of woodland and savanna communities of the Edwards Plateau region in central Texas. We examined the emergence, mortality and growth of various spatial and temporal cohorts of J. ashei seedlings over an eight or nine-year period. Greatest emergence was found during the cool, mostly winter months and under the canopy of mature J. ashei trees. Emergence was significantly inversely related to temperature and significantly linearly related to rainfall, but only if the monthly rainfall and emergence were offset by one to four months. Greatest survival occurred below the J. ashei canopy, but greatest growth was at the canopy edge. Emerging seedlings were not from the current year's seed crop, but from one or more previous year's seed crops. Greatest mortality occurred mostly during the summer months and in the grassland habitat. There was a significant inverse logarithmic or exponential relationship between mean monthly temperature and mean monthly mortality. A large number of J. ashei seedlings or immature plants with reduced growth were found beneath the canopy of mature trees. These plants seem to serve as a seedling bank, providing the source of recruitment into the population should the overstory trees be removed. Survival of the two canopy cohorts with known emergence dates declined with time ͑negative exponential function͒ and was 1.0-3.4% after eight or nine years depending on the cohort. The pre-existing cohort seemed to have constant mortality ͑and presumably replacement͒, with about 8% of the population dying each year. Higher growth rates for seedlings were found at the edge of the established woodland canopy, which suggests that conditions in the edge habitat or possibly in canopy gaps are best for growth beyond the seedling stage.

Changing pollen types/concentrations/distribution in the United States: Fact or fiction?

“…Similar increases have been documented with other Juniperus spp in the central, western, and southwestern US; this includes the highly allergenic species Juniperus ashei [ 49 ]. Various theories have been proposed to explain the expansion of Juniperus spp populations, including fi re suppression, overgrazing, and climate change.…”

The water relations of two evergreen tree species in a karst savanna

“…It forms dense to open communities, on the eastern Edwards Plateau primarily with live oak. In the absence of fire, Ashe juniper stands (''cedar brakes'') grow outward and invade adjacent grassland patches (Van Auken et al 2004). Grazing does not control its spread, as cattle and deer avoid eating the bitter and spiny seedlings.…”

Structure of Quercus gambelii communities in the Lincoln National Forest, New Mexico, USA

“…In addition, identifying factors that seem to control recruitment into these populations has also been elusive (Van Auken et al, 2004). In recent years, it has been documented that large and small-scale disturbances such as fire, windthrow, tree falls, and anthropogenic factors play an important role in regulating community structure by opening forest canopies and allowing the establishment of early successional species (Lorimer and Krug, 1983;Abrams and Orwig, 1996;Foster et al, 1996;Baker et al, 2005).…”