Print PageIn most environments, plants face the difficult choice of maximizing growth and reproduction while simultaneously defending against the onset of disease, pathogens, and herbivory. Plants therefore, must balance the allocation of resources such as photosynthates and mineral nutrients to growth, reproduction, defense or internal storage. Under equal conditions, plants that allocate a larger proportion of resources to growth must do so at the expense of allocating fewer resources to storage. The critical balance between growth and storage leads to the hypothesis that in high-resource environments, plants that express high growth rates are more susceptible to episodic disturbance than plants that express lower growth rates. This hypothesis was tested by measuring the radial growth of mature tamarisk trees (Tamarix spp) along the Colorado River in southeastern, Utah. All of the trees had been subjected to episodic defoliation over three consecutive growing seasons (2007 – 2009) by the recently introduced saltcedar leaf beetle (Diorhabda carinulata) resulting in mortality in approximately 50% of the stand. Specifically, it was hypothesized that recently killed trees would express a higher radial growth rate in years prior to the onset of defoliation than co-occurring trees that continued to survive the defoliation events.
Results/Conclusions
Mean annual radial growth (measured from annual ring widths) in the 15 years prior to defoliation (1992 – 2006) was 34% higher in killed trees compared to living trees (P < 0.0001). Differences in radial growth resulted in a basal area increment that was 100% higher in killed trees compared to living trees prior to defoliation (P < 0.0001). Moreover, the mean annual radial growth in killed trees was significantly more sensitive to environmental conditions including mean annual river flow (an analog for available soil moisture) and the regional Palmer Drought Severity Index than living trees. Results from this investigation suggest that living trees allocate a relatively large proportion of resources to storage, thereby allowing these trees to produce new leaves after each subsequent defoliation event. On the other hand, recently killed trees likely allocate a larger proportion of resources to growth at the expense of maintaining smaller storage reserves than living trees. Defoliation by the saltcedar beetle therefore may be expected to reduce the overall net primary productivity of surviving tamarisk trees and may result in a reduction in genetic variability if these allocation patterns are adaptive.
