Woody plant encroachment in central Texas decreases the species richness of native herbaceous communities, primarily by limiting the amount of available habitat.  This encroachment also slows the spread of the invasive grass, Bothriochloa ischaemum through habitat fragmentation.  Here, we tested whether the loss of species from communities in areas undergoing encroachment facilitates the establishment of B. ischaemum.  The relationship between species richness and invasibility often varies across spatial scales and depends on community composition.  Here we tested this relationship across an observed gradient in richness and composition.  We identified the species commonly lost with woody plant encroachment in an observational study.  We then established 111 1m radius circular plots in three sites with no B. ischaemum present.  In each site, plots were assigned to triplets with similar composition and proximity.  Within each triplet, one plot was randomly assigned to each of three treatments: (1) unmanipulated (2) random removal of plant biomass, or (3) removal of species lost with encroachment.  We added B. ischaemum seeds to 9 replicate plots per treatment at each site.  We also transplanted greenhouse grown plants into 11 replicate plots per treatment at two sites.  We compared B. ischaemum germination, survival, growth and reproduction between treatments.

Results/Conclusions

B. ischaemum germination rates were significantly higher in species removal plots than unmanipulated controls, with germination rates in biomass removal plots intermediate .  Manipulated plots had significantly higher germination rates than unmanipulated plots, suggesting that B. ischaemum germination is promoted by reduced competition.  These patterns were also reflected in the growth of B. ischaemum transplants.

Overall, our results indicate that while woody plant encroachment may slow the arrival of B. ischaemum seeds, it also indirectly facilitates the establishment of this invasive grass upon its arrival to communities with reduced native species richness.