Working together: Fire and post-fire rehabilitation create homogeneous plant communities

Background/Question/Methods

Fire number and size has increased in the western United States over recent decades in part due to invasive annual grasses altering fire regimes. Though some research has been done on how post-fire emergency stabilization and rehabilitation (ESR) affect plant communities, it has been limited to sites with only a single fire. How fire and ESR histories may interact to affect plant communities has not been researched, although this information is necessary for land managers to make informed decisions about what seeding methods may be most effective at limiting invasive annual grasses. We selected sites that varied in fire occurrence (0-6 fires) and ESR seeding methods (aerial, drill, or unseeded) and collected plant cover and density data in 2014 and 2015. We analyzed data using a Multiple Response Permutation Procedure with a coding variable that identified the number of fires, most recent post-fire treatment, and data collection year to examine the effect of between year variation in plant communities on the interpretation of potential interactive effects of fire and ESR treatments. Nonmetric Multidimensional Scaling (NMS) was used to visualize differences among treatments and years, and to identify explanatory variables that correlated with ordination axes.

Results/Conclusions

Plant community composition within the same fire-treatment-collection year were similar and the difference among fire-ESR combinations was significant for plant cover (A=0.203, p<0.0001) and density (A=0.160 p<0.0001). Plant communities within a fire-ESR group did not differ between collection years, after adjusting for multiple comparisons using a false discovery rate method. In general, communities in unburned sites differed significantly from burned sites, but the difference in plant communities among burned sites was inconsistent. Sites aerially seeded after six fires and unseeded sites after one fire were occasionally different from other burned sites, otherwise there was little difference among plant communities among burned sites. The NMS ordination explained most of the variation in plant community composition among sites (cover r² = 0.67; density r²=0.61). Environmental variables correlated with axis one (cover r² = 0.29; density r²=0.24) while treatment and fire variables correlated with axis two (cover r² = 0.21; density r²=0.20). Important treatment factors included the number and time since treatment as did time since fire and time between the two most recent fires. Though environmental variables play a crucial role in recovery after fire, a site’s ESR and fire history should be considered when determining the rehabilitation needs.