PARENT SESSION
Poster Session # 16: Invasive Species.

Wednesday, August 6 Presentation from 5:00 PM to 6:30 PM. SITCC Exhibit Hall B.

An invasive tree suppresses natives and facilitates conspecifics by altering the light in the understory.

Gurnee, Julie^*,1, Reinhart, Kurt¹, Calloway, Ray ¹, ¹ University of Montana, Missoula, MT

ABSTRACT- Facilitation and ecosystem engineering by plants are important processes that organize natural plant communities, yet the role of these processes in plant invasions is less clear. Riparian communities invaded by Acer platanoides have understories with 3.5 times fewer plant species (m^-2), 23 times less photosynthetically active radiation (PAR), and 57% less red to far-red light (R:Fr) than natural riparian forests in Montana. Here we describe the effect of these PAR and R:Fr levels on the growth and survival of seedlings for six native species and two nonnative species, including A. platanoides. We hypothesized that changes in the understory light environment associated with A. platanoides canopies facilitate conspecifics and negatively impact natives. We found that A. platanoides seedlings had high survival (75-100%) in all light treatments. In contrast, 4 of 6 native species experienced higher mortality at low PAR levels, similar to that beneath Acer canopies. Low PAR levels reduced the biomass of all species; however the reductions in growth were more dramatic for many native species than seedlings of A. platanoides. The light quantity beneath Acer trees had a direct negative effect on the survival and growth of native species, which corresponds with the general decline in species richness beneath Acer trees. These results indicate that ecosystem engineering by an invasive tree alters the light in the understory and negatively impacts native species while indirectly promoting invasion of non-natives.

Key words: A. platanoides, indirect effects, ecosystem engineer, canopy effects