Dynamics of Juniper Invaded Grasslands and Old-Growth Woodlands at Wupatki National Monument, Northern Arizona, USA

Hassler, Frank

Land managers at Wupatki National Monument in northern Arizona, USA have become increasingly concerned about expansion of Juniperus monosperma (Engelm.) Sarg.(one-seed juniper) into the semi-arid grassland ecosystem. I reconstructed recruitment rates and tree densities by means of dendrochronological analysis within the Monument and south of it on the Coconino National Forest. Size attributes (height, drip line, stem diameter) of 216 juniper trees were measured. A root collar cross section of select trees were removed for dendrochronological analysis. The most consistent correlation between age and size among all trees was a simple linear regression of stem diameter: y = 0.6890578 + 0.0706339 x, where x= years of age and y = stem diameter in cm. This regression had an R2 value of .785. The correlation between age and size of trees in more droughty environments was more difficult to predict than trees in less stressful environments. This strong correlation of stem diameter to age likely reflects the fact that growth patterns of J. monosperma in arid woodlands are different than those of other tree species competing for resources in a traditional forest environment. I recommend that land managers use the diameter of the largest stem of J. monosperma to evaluate tree age and stand age structure. Juniper trees became established in the young juniper "savanna" stands within the Monument within the last 200 years. In these past grasslands, trees of all ages were found to be equally susceptible to wildfire-induced mortality, with an average of 55% mortality. Prescribed fire can therefore be an effective management tool for maintaining grassland and open savanna conditions. Conversely, denser, juniper woodland stands on the adjacent National Forest appear to have been established for 700 years or more. Little evidence of low severity surface fires was found in these areas. Therefore, it seems that the fire regime and overstory structure of these older juniper woodlands has not been significantly altered by fire suppression and grazing in the last 150 years. Subsequently, there is no ecologicallybased motivation for thinning, prescribed fire or other high-intensity “restoration” activities and management of these stands.

Dynamics of Juniper Invaded Grasslands and Old-Growth Woodlands at Wupatki National Monument, Northern Arizona, USA

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