Heterozygosity, gender, and the growth-defense trade-off in quaking aspen

Although plant growth is generally recognized to be influenced by allocation to defense, genetic background (e.g., inbreeding), and gender, rarely have those factors been addressed collectively. In quaking aspen (Populus tremuloides Michx.), phenolic glycosides (PGs) and condensed tannins (CTs) constitute up to 30 % of leaf dry weight. To quantify the allocation cost of this chemical defense, we measured growth, defense chemistry, and individual heterozygosity (H_obs at 16 microsatellite loci) for male and female trees in both controlled and natural environments. The controlled environment consisted of 12 juvenile genets grown for 3 years in a common garden, with replication. The natural environment consisted of 51 mature genets in wild populations, from which we sampled multiple ramets (trees) per genet. Concentrations of PGs and CTs were negatively correlated. PGs were uncorrelated with growth, but CT production represented a major cost. Across the range of CT levels found in wild-grown trees, growth rates varied by 2.6-fold, such that a 10 % increase in CT concentration occurred with a 38.5 % decrease in growth. H_obs had a marked effect on aspen growth: for wild trees, a 10 % increase in H_obs corresponded to a 12.5 % increase in growth. In wild trees, this CT effect was significant only in females, in which reproduction seems to exacerbate the cost of defense, while the H_obs effect was significant only in males. Despite the lower growth rate of low-H_obs trees, their higher CT levels may improve survival, which could account for the deficit of heterozygotes repeatedly found in natural aspen populations.