Latitudinal patterns in defense and growth traits in Alaska paper birch (Betula neoalaskana)
The latitudinal herbivory-defense hypothesis (LHDH) predicts that plants near the equator will be more heavily defended against herbivores than are plants at higher latitudes. Although this idea is widely found in the literature, recent analyses have called this biogeographic pattern into question. We sought to evaluate this hypothesis in a terrestrial ecosystem at high, northern latitudes where herbivory by mammals such as snowshoe hare can be intense. We collected seeds of Alaska paper birch (Betula neoalaskana) from nine locations along two north-south transects between 55° N and 62° N latitudes in western, interior Canada. The birch seeds were planted in pots in a common garden located in Madison, Wisconsin, United States. From the resulting seedlings, we determined levels of chemical defense by assessing the density of resin glands (papyriferic-acid-producing glandular trichomes) that have been shown to be negatively correlated with browsing. To explore variation in inherent growth rate, we harvested a subset of the birch seedlings to assess plant growth traits such as height, mean individual leaf area, and root-to-shoot ratio. We also used these growth parameters to examine tradeoffs between growth and defense.
Our data do not support the predictions of the LHDH. Instead, we found a positive correlation between chemical defense and latitude (r = 0.783; p = 0.013). We also found that birch seedlings originating from higher latitudes were shorter, smaller-leaved, and had a higher root-to-shoot ratio than their counterparts from lower latitudes. This suggests that lower annual temperatures at higher latitudes may be reducing nutrient mineralization and soil fertility and selecting for a low inherent growth rate that makes northern B. neoalaskana populations particularly sensitive to selection for antiherbivore defense, a result that is consistent with the resource availability hypothesis of the evolution of antiherbivore defense. Growth-defense tradeoffs were observed in negative correlations between resin gland density and height (r = -0.836; p = 0.005) and between resin gland density and leaf size (r = -0.915; p = 0.001). Additionally, we found a strong positive correlation between resin gland density and percent annual area burned (PAAB) by fires around each collection site (r = 0.884; p = 0.002) and also between PAAB and latitude (r = 0.697; p = 0.037). We propose that these interconnected relationships are likely driving the positive correlation we observed between defense and latitude in B. neoalaskana.