Semi-arid rangelands in the Western US are increasingly threatened by noxious invasive species whose phenologies differ from those of resident communities, as often evident in the extension of the invasives’ growing period into the dry season. In California, species of concern include Taeniatherum caput-medusae (medusahead) and Aegilops triuncialis (barbed goatgrass), which are late-season noxious grass species that displace established cool-season forage species. A central question in the invasion ecology of these invaders is to what degree their success is mediated by late-season soil moisture availability, which is controlled not only by environmental factors and species composition, but also by management factors (e.g., grazing timing) and anthropogenic climate change. Here, we examine soil moisture dynamics in Mediterranean-climate rangelands in California during the late-season dry-down period. We used time domain reflectometry (TDR) to measure volumetric soil moisture in the top 20 cm and top 45 cm of soil underlying vegetation patches of differing composition during the dry-down period. To evaluate the influence of any soil moisture differences on plant performance, we also measured stomatal conductance of weeds and non-weeds in mixed and monotypic patches.

Results/Conclusions

We found notable temporal differences in soil moisture dynamics in monotypic patches of noxious invasive grasses as compared to patches of mixed vegetation. For example, at the beginning of the dry-down period (early April), soil moisture was generally greater in monotypic weed patches than in monotypic forage grass patches, but this relationship reversed by late May as the weed species continued to remove water from the soil profile while the forage grasses senesced. Measurements of stomatal conductance suggest that such soil moisture differences influenced plant performance. For example, in late April, foliar conductance rates in individual weeds in monotypic weed patches were about twice the rate of those same species grown in a mixed matrix with forage grasses. Overall, our findings so far suggest that late-season soil moisture availability favors the establishment of monotypic weed patches and/or that the establishment of these patches increases late-season moisture availability, through such as mechanisms as self-mulching.