Herbivory, plant resistance, and climate in the tree ring record: interactions distort climatic reconstructions

Abstract

To understand climate change, dendrochronologists have used tree ring analyses to reconstruct past climates, as well as ecological processes such as herbivore population dynamics. Such reconstructions, however, have been hindered by a lack of experiments that separate the influences of confounding impacts on tree rings, such as herbivores and the interactions of multiple factors. Our long-term experiments with scale insects on resistant and susceptible pines demonstrate three major points that are important to the application of this commonly used tool. (i) Herbivory reduced tree ring growth by 25-35%. (ii) The impact on ring growth distorted climate reconstruction, resulting in the overestimation of past moisture levels by more than 2-fold. Our data suggest that, if distortion because of herbivory has been a problem in previous reconstructions, estimates of the magnitude of recent climate changes are likely to be conservative. (iii) Our studies support a detectible plant resistance x herbivore x climate interaction in the tree ring record. Because resistance and susceptibility to herbivory are known to be genetically based in many systems, the potential exists to incorporate plant genetics into the field of dendrochronology, where it may be used to screen distortions from the tree ring record.

Figure 1

(A and B) Before the removal of scales, the susceptible and pretreatment scale-removed groups exhibited reduced growth relative to the resistant group. After scale removal, growth among previously scale-infested trees increased to equal that of the resistant trees. Groups within time periods were compared by using a one-way ANOVA on all rings within the described time period. Differing letters indicate P < 0.001 using Tukey's honestly significantly different (HSD) test for pair-wise comparisons.

Figure 2

Resistant, scale-free trees express twice the climatic sensitivity of scale-infested as denoted by the difference in slope. Resistant and Susceptible model slopes differ significantly at P < 0.05. Model parameters are listed in Table 1.

Figure 3

(A–C) Comparison of mean distance to 1st, 2nd, and 3rd nearest neighbors, soil slope, and age. Although each of these factors can impact the radial growth rates and climatic sensitivity of tree rings, none of these values differed among the three tree groups. Similar letters indicate lack of statistical significance (P > 0.05) using a MANOVA (A) or one-way ANOVA (B and C).

Figure 4

Reconstruction of climate by using models based on resistant and susceptible trees. Note the severe distortion of amplitude shown by the susceptible model. The trees applied to the models to generate the reconstruction were selected to mimic the proportion of resistant and susceptible trees observed in this system.