Tamarisk Removal in Grand Canyon National Park: Changing the Native–Non-native Relationship as a Restoration Goal

The removal of non-native species is often an important restoration goal in natural areas, including national parks. There is often a positive correlation between non-native and native plant species richness, so that areas with the greatest number of native species support the greatest number of non-natives. This pattern may create both challenges and opportunities for restoration. Native species may be inadvertently damaged or killed during mechanical or chemical treatments targeted toward non-natives. However, greater numbers of native species may better colonize subsequent openings. We used data from an ongoing active management project in Grand Canyon National Park to test three predictions of plant community response to invasive tamarisk (Tamarix spp.) removal. We investigated patterns of native and non-native species abundance and richness, compositional shifts, and the relationship between native and non-native species before and after treatments. In general, native species richness and abundance did not increase, but species composition following tamarisk removal tended to become more similar to that in uninvaded stands. Native and non-native species richness and abundance were positively correlated before tamarisk removal. While treatment successfully reduced cover of tamarisk after one to three years, our results suggest that it may not increase richness of native species during the same period. Furthermore, investigating native and non-native species relationships may be a useful tool for exploring hypotheses and consequences of restoration via invasive species removal.