Relationships between fennel (Foeniculum vulgare Mill.) seed mass, germination rate, and soil water deficit in California

Background/Question/Methods

Water availability changes across seasons and latitudinal and longitudinal gradients in California; much of the state regularly experiences soil water deficit. Many plants respond to soil water deficit by allocating resources differently, thereby affecting functional traits such as specific leaf area or seed mass. Seed mass often correlates with germination success, but the direction and strength of that relationship can vary within and among species. Studies on the relationship between seed mass and germination within a species can improve our understanding of plant distribution across water availability gradients. This is of particular importance in the context of the spread of non-native species in California. In this study we explored potential links between annual water deficit and fennel (Foeniculum vulgare) seed mass and germination rate. We hypothesized that F. vulgare seed mass is positively correlated with water availability, and germination rate is positively correlated with seed mass. We collected and weighed F. vulgareseed from coastal populations across a latitudinal gradient in coastal California, as well as coastal and inland populations in southern California. We estimated annual soil water deficit for each location. We then ran separate germination trials for seeds collected across each of the gradients. 

Results/Conclusions

Soil water deficit was negatively correlated with latitude along collections sites from north to south, but did not differ between the inland and coastal southern California populations. Water deficit was strongly negatively correlated with F. vulgare seed mass in coastal populations. Germination rates in the southern California F. vulgare populations were positively correlated with seed mass. In contrast, germination rates were negatively associated with seed mass of F. vulgare populations along the latitudinal gradient. While seed mass does appear to correlate with water availability in CA coastal populations of F. vulgare, it does not have a consistent relationship with germination across populations. In future studies we hope to tease apart the genetic and environmental drivers of germination in F. vulgare to better understand its potential for further spread in California.