The arrival of non-native annuals in the warm deserts of the South-West USA was an ecological “game changer”, causing altered community structure and even changes in fire regimes. Besides being arid, deserts have very unpredictable precipitation regimes and periods of drought are only infrequently broken by years with relative abundant rainfall. In response to the stochastic climate, short-lived plants needed to develop survival strategies to ensure survival of populations through longer drought periods. Long lasting soil seedbanks of dormant seeds are understood to be such a strategy. We ask how long-lasting seedbanks and other seed reservoirs are changed with the invasion of exotic annuals. For this we collected soils for three years from two climatically distinct yet physiognomically similar desert sites (Mojave and Sonoran Deserts) and quantified viable seeds by providing optimal conditions for germination (several separate watering pushes). Soil samples were collected prior to onset of winter rains in the proximity of creosote shrubs (Larrea tridentata), the dominant shrub species in both deserts. The soil was collected from transects ranging from the shrub’s subcanopy to open areas between shrubs. Samples were watered in the greenhouse, and germinated seedlings were identified and quantified.
Results/Conclusions
Seed densities in the samples of two desert sites differ strongly in the proportion of natives to exotics: the Mojave Desert had higher densities of exotic plants than of natives, whereas in the Sonoran Desert density of natives exceeds that of exotics. The distribution of soil seed densities varied between origin of species. Exotic species density in the Mojave Desert was lowest under the shrub canopy and increased with distance from the shrub. In the Sonoran Desert exotic species density peaked under the shrub canopy. In contrast, native species showed similar patterns in both deserts: density peaked under the shrub canopy and decreased with distance from the shrub. Germination of exotic plants was triggered differently in their response to watering: in both deserts most of the exotic plant seedlings (95-97%) appeared following the first germination push. In contrast, only 83% (Mojave) and 77% (Sonoran) of seedlings of native plants germinated initially. These results indicate that seed strategy differences between exotic and native annuals are causing deserts to change in terms of seed reservoir distribution that is correlated to changes in the spatial arrangement of annual communities. The high initial germination of exotics leads to reduction of long term seed banks.