The overarching goals of the project were to examine the temporal dynamics of invasive species impacts -- how those impacts change over the course of an invasion, and how they persist (or not) after invader removal as a 'legacy' of invasion. Using the widespread invader Cytisus scoparius (Scotch broom), we quantified invader impacts on an important economic forestry species in the Pacific Northwest, Douglas-fir, using a suite of field and greenhouse studies to understand how this native tree responds to changing impacts over invasion age and time since invader removal. Our particular focus was the relationship between Doug-fir and its soil-based mutualist partners, ectomycorrhizal fungi, characterizing the disruption of that mutualism and the soil-based factors associated with those disruptions.

In the first large-scale field experiment of this project, we examined Scotch broom impacts on soils, Doug fir mortality and mycorrhizal mutualisms over a 45 year chronosequence. We found that while Scotch broom has large impacts on soils and Doug fir --  increasing soil N, depleting soil P, and causing 25% greater mortality of Doug-fir -- those impacts do not increase with invasion duration. Instead, we found that Scotch broom impacts may depend on other site factors: for example, the degree of soil N enrichment by Scotch broom, a nitrogen-fixer, is driven by initial site fertility rather than by how long that site has been invaded. More fertile sites experience a smaller impact on soil N dynamics by the invasion of this nitrogen-fixer. We have found in prior experiments that Doug-fir's soil-based mutualists (ectomycorrhizae) are depressed in soils that have been invaded by Scotch broom, and some taxa of fungi are essentially lost with invasion; we have characterized these changes in our previous work using a genetics approach and are currently using the same approach to analyze samples across the 45 year chronosequence of invasion, which will be a unique contribution to the field of mycorrhizal community ecology and invasion ecology. 

In our second large-scale field experiment we asked, How long does the legacy of Scotch broom last after its removal, and how does Doug-fir respond to that legacy? We conducted a removal experiment over 56 months, sequentially removing stands of Scotch broom at regular intervals, then planting all plots with Doug-fir in 2016. Tracking soil N, we found a predictable spike in soil N availability directly following removal (likely due to input of N-rich plant material to soil during removal); this transitory spike was then followed by a decline. A year after removal we found that soil N had declined to levels lower than control plots where Scotch broom was intact. Even with this decline, however, invaded soils remained highly enriched in N even two years after invader removal compared to nearby forest soils that had never been invaded by Scotch broom. Doug-fir planted into these invaded plots benefited from increased N immediately following removal, but benefited less (grew smaller) in plots where Scotch broom had been absent for longer (had less N). Native understory species, in contrast, grew the same across the time sequence, irrespective of how long Scotch broom had been removed from the plots and irrespective of N availability. Other non-native grasses and forbs invaded following the removal of Scotch broom, likely capitalizing on increased N availability. The increase in the abundance of these secondary invaders is likely the mechanism behind Doug-fir's negative response to time since invader removal.  While conducting these experiments we interacted with local foresters and land managers, who showed us that locally adapted (wild type) Doug-fir seedlings from drought prone sites survived better in poor quality sites than seedlings that were bred for fast growth. We collected a subset of these trees to analyze the mycorrhizal communities on their roots, hypothesizing that the wildtype Doug-fir genotype may associate with a different mycorrhizal community compared to the genetically improved genotype, allowing it to perform better in drought-prone soils. We then found in a greenhouse experiment that mycorrhizal colonization on wild type seedlings was less impacted by competition with Scotch broom than the genetically improved seedlings, again suggesting that wild type trees may be more stress tolerant than the genetically improved lines. It appears that mycorrhizal fungi associated with the wild type Douglas-fir may increase stress tolerance and resilience to drought and invasion, which has useful implications for forestry practices in this region.

This research is focused on an important applied problem: the invasion of an aggressive plant species that affects Douglas-fir timber production, agricultural and range production, and wildlife. The losses in these sectors result in income and jobs lost to the regional economy of the Pacific Northwest. Our objective is to inform management and planting strategies to enhance reforestation success in the face of this invasion and its impacts on a key economic native tree species.

Last Modified: 11/12/2019
Modified by: Karen A Haubensak