Mortality of Adult Joshua Trees (Yucca brevifolia) Due to Small Mammal Herbivory at Joshua Tree National Park, California

Esque, Todd; Haines, Dustin; Defalco, Lesley; Rodgers, Jane; Goodwin, Kimberley; Scoles, Sara

In October of 2001, National Park Service staff and visitors observed damage to the trunks of Joshua trees (Yucca brevifolia) in Joshua Tree National Park (JTNP). The bark-like tissue on the trunks of Joshua trees, known as the periderm, was removed by small mammals thus exposing large, light colored patches of underlying tissue. The U.S. Geological Survey initiated a study to determine the extent of the damage to Joshua tree populations and the consequences of damage to Joshua tree survivorship within the park. During the 2002 hydrologic year (October 1, 2001 through September 30, 2002), JTNP received 20 mm of precipitation, well below the 54-year average of 103 mm. As a result, germination of annual plants failed in winter 2001/02 and very little new growth and flowering of perennial plants occurred. Coincident with low rainfall and plant productivity, small mammals, including antelope ground squirrels (Ammospermophilus leucurus), Botta’s pocket gophers (Thomomys bottae) and black-tailed jackrabbits (Lepus californicus), which normally forage on seeds, fruits, shoots and roots of annual and perennial plants, removed the periderm from Joshua trees. The survivorship of damaged trees was measured at three sites at JTNP during the summers of 2002 and 2003. During that time 95% of undamaged trees survived, but only 42% of trees with periderm damage survived. The survivorship of the damaged trees was associated with the percent of periderm removed by herbivores: 60% of trees with less than 5% periderm damage were alive in 2003, but none of the trees with greater than 25% damage survived. On density plots established in the summer of 2003, 12% of the Joshua tree population had periderm damage, which is approximately 4.4 trees ha-1 . Although incidence of periderm damage was low during the summer of 2002, a subsequent roadside survey of Joshua tree stands demonstrated that by July 2003 there were no areas of the park free from periderm damage. The widespread occurrence of Joshua tree damage, the low survivorship of damaged trees, and the recent drought within the park have contributed to the loss of potentially thousands of trees. Large-scale management action to suppress this natural occurrence would not be feasible. However, simple methods, such as wire screening to exclude small mammals, could be used to protect individual trees of particular cultural value, where such actions do not conflict with other management objectives. While drought and herbivory on Joshua trees are not uncommon in the Mojave Desert, their combined effects over the past 2 years will likely impact the demography of the species at JTNP. While these events pose challenges to natural resource management, they also allow us to learn more about the life history of a long-lived plant species and effective management strategies necessary to preserve natural populations while presenting an excellent opportunity to educate the public about interactions between plants and animals.

Mortality of Adult Joshua Trees (Yucca brevifolia) Due to Small Mammal Herbivory at Joshua Tree National Park, California

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