Differential impacts of soil microbes on native and co‐occurring invasive tree species

Abstract

AbstractThe Janzen‐Connell effect is often generated by interactions between trees and soil microbes while the enemy release hypothesis states that invasive species are released from regulation by natural enemies. Thus, the strength of the Janzen‐Connell effect could differ between native vs. nonnative plants. We tested this hypothesis with congeneric pairs of invasive and native tree species in Europe: boxelder (Acer negundo) vs. Norway maple (A. platanoides), and Northern red oak (Quercus rubra) vs. pedunculate oak (Q. robur). We conducted greenhouse experiments using soil sterilization treatments, field experiments on distance‐dependent germination, and field surveys of early life stages of the focal species. Greenhouse and field experiments demonstrated patterns consistent with Janzen‐Connell effect that is most likely caused by negative distance dependence in seed germination and stem growth which was found in both genera of the native, but not the invasive trees. Soil sterilization experiments suggested that these effects are driven by interactions with soil biota. Field surveys revealed the Janzen‐Connell pattern in the distribution of seedlings and saplings of the native, but not the invasive Acer species. Our findings indicate that weakened Janzen‐Connell effect might contribute to successful invasions of certain nonnative plant species.