Biogeography of Alaska paper birch (Betula neoalaskana): Latitudinal patterns in chemical defense and plant architecture

Biogeographic theory predicts that levels of plant defense are highest near the equator and decrease toward the poles.  Data that support this theory come mainly from studies of marine systems and/or invertebrate herbivores.  To test a fundamental paradigm of biogeography in a terrestrial system driven by vertebrate herbivores, we collected seeds of Alaska paper birch (Betula neoalaskana) from nine locations along two north-south transects in northwestern Canada. We determined levels of plant chemical defense by assessing resin gland density on the resulting seedlings since resin gland density is negatively correlated with snowshoe hare (Lepus americanus) preference.  Our data do not support the predictions of biogeographic theory.  Instead, we found a positive correlation between plant defense and latitude.  We also found that more northern Alaska paper birch were rootier and shorter than their more southern counterparts.  This research provides valuable information regarding biogeographic patterns and plant-mammal interactions on a continental scale.