PS 39-2 - Disturbance and resource availability have different effects on survival and growth during invasion

Background/Question/Methods

Invasion is influenced by many factors, including resource availability, the structure of the resident plant community and disturbance. Nutrient enrichment may stimulate competition and exclude the invader, while drought stress can reduce both biotic resistance and invader success. More diverse communities may also better resist invasion, whereas already invaded communities may be more invasible. Disturbance, however, may alter these effects, allowing invaders to take advantage of excess nutrients and avoid competition. To explore these questions, we studied absinthe (Artemisia absinthium) invasion in a native grassland near Saskatoon, Canada. We applied herbicide, prescribed burning, and fertilizer to 10 blocks of native prairie in a split-split plot design (40 plots) in summer 2021, which coincided with an extreme drought. Into each of these plots, we transplanted 2 absinthe seedlings and monitored their survival and growth over the growing season. We also estimated plot composition as percent cover and measured biomass by clipping for each of the plots.

Results/Conclusions

Despite the drought, absinthe survival was high, except in undisturbed, fertilized plots where survival dropped by 50% indicating increased biotic resistance when the intact community is fertilized. Survival probability also increased with the proportion of non-native species, suggesting some facilitation among invaders. Absinthe total biomass increased in both burned and herbicide plots, with the greatest productivity in the combined disturbance plots, indicating that intense disturbances facilitate absinthe growth. There was a negative relationship between absinthe biomass and bare ground, which also increased with disturbance intensity, suggesting that moisture limitation may limit the benefits of disturbance for invasion, at least during drought. Combined, these results indicate that disturbance intensity can increase invasion but that disturbance effects on initial invader survival may be limited to highly competitive environments. If the disturbance also increases stress, this may also limit the benefit to invasion. In communities already dominated by non-native species, however, disturbance may be less important due to already reduced biotic resistance.