Can spatiotemporal heterogeneity in soil moisture affect species coexistence?

Background/Question/Methods

Plants experience changing environments in space and time, or environmental heterogeneity. This environmental heterogeneity may play a pivotal role in influencing invasibility, the ability of a population to grow from low density, and may assist in understanding coexistence. Early verbal theory suggested that environments with high heterogeneity would have a higher risk of invasibility. However, newer quantitative theory suggests that if heterogeneity is spatiotemporal, then high heterogeneity could result in low invasibility. In a spatiotemporally heterogeneous environment the quality of a patch in which a plant is located can change over time (i.e. high quality patches in one year might become lower quality patches in the next). Plant seeds must evaluate and decide whether to germinate in the patch or lay dormant in the hopes that the patch will have favorable conditions later. In an existing field experiment with heterogeneous (patches of two soil types) and homogeneous soil (patches each having an even mixture of two soil types), we measured the soil moisture levels in each patch to determine whether differences in soil moisture across patch types changed over time, i.e. whether soil moisture exhibited spatiotemporal heterogeneity. To assess whether plants track patch quality, we measured germination and survival weekly.

Results/Conclusions

Our analysis shows that soil moisture exhibited spatiotemporal heterogeneity, such that the ranking of the patches’ quality changed over time. Germination patterns also exhibited significant spatiotemporal heterogeneity. Finally, seeds exhibited the ability to track changes in patch soil moisture. This study suggests that the effects of environmental heterogeneity on invasibility will depend on environmental drivers, such as whether it is a dry or wet year, and a seed’s ability to track these changes. Thus the spatiotemporal variation in soil moisture could influence invasibility, influencing coexistence. These results provide insight on the contradiction commonly found between theory and empirical work. For example, high heterogeneity often leads to low diversity in empirical work. If plants track moving high quality patches this could help keep invaders out, thus reducing diversity and potentially explaining this contradiction between theory and empirical work. Further research will include testing a seed's ability to track patch quality by isolating soil moisture as a driver by autoclaving field collected soil and exposing seeds to 6 different water treatments.