Using patchy plant invasions to understand how diffuse interactions modify facilitation and competition

Indirect interactions among plants promote conditionality in competitive outcomes that affect plant community structure and function. I utilized spatially patchy distributions of two invasive exotic plants, Centaurea stoebe and Bromus tectorum, to explore conditionality in plant interactions and the implications of this conditionality for community invasibility. Additionally, I expanded this research to investigate how these two invaders interact with each other as they overrun native ecosystems. Throughout intermountain prairie of western Montana Centaurea was found at high abundances in open prairie, but was a relatively minor component of the plant community under isolated Pinus ponderosa. In contrast, Bromus was also common in open prairie, but it was most dominant under Pinus canopies.I then experimentally investigated the complex dynamics potentially driving apparent biotic resistance by Pinus to one exotic species but facilitation of a second. I found that Pinus directly inhibited Centaurea growth through shade and litter effects and attenuated the competitive effects of Centaurea. While Pinus litter strongly suppressed Centaurea establishment, Festuca and Bromus where much less effected. The native plant community and Bromus were thereby indirectly facilitated. Additionally, the allelochemical (+/-)-catechin that is exuded by Centaurea roots was more phytotoxic to Festuca in open prairie than under Pinus canopies and in prairie soils than in conifer soils when tested in a greenhouse. Plant-soil feedbacks were important as well. When Centaurea was grown in full sunlight it "cultivated" the soil such that legacy effects inhibited recruitment of Festuca long after Centaurea had been removed, but these feedback effects did not occur when Centaurea cultivated soil in experimentally shaded plots. Bromus was directly facilitated by Pinus shade and soil but these effects were highly moderated by the native grass Festuca idahoensis. While many relatively straightforward pair-wise studies have shown direct facilitative effects of one species on another, these results demonstrate another form of biotic conditionality strong facilitative effects manifest in pair-wise experiments can be eliminated or diminished by the presence of other competitors. In general, my results illustrate the importance of the competitive and facilitative interactions that occur among natives and exotics ultimately structuring plant communities on natural landscapes.