Dryland seed mixes with increased functional diversity show influence of species level competitive interactions

Background/Question/Methods

Drylands have highly adapted and resilient plant communities where community assembly is often dictated by abiotic limitations and landscape heterogeneity. Biotic interactions also play a key role in dryland plant community assembly. Competitive interactions tend to be the focus of community assembly studies, however, considering both competitive and facilitative interactions has great research potential. Understanding how these biotic interactions influence assembly could benefit ecological restoration. For example, an understanding of dryland facilitative interactions might improve native seed mix efficacy. By increasing species level diversity, different niches and ecological functions are filled, potentially improving establishment. We sought to determine if higher functional diversity within a seed mix would be correlated with increased facilitative interactions and how this varied under different soil and nutrient conditions. Native seed materials were collected by the BLM Seeds of Success within the Chihuahuan ecoregion. Four seed mixes of different levels of diversity (high+ nitrogen fixer, high, medium, low) were created from ten species, with perennial grass (Bothriochloa barbinodis) as the target species. Mixes were grown in three native soils (sandy, loamy, silty) and potting soil, under two nutrient conditions (ambient, enhanced) for 92 days. Aboveground biomass was compared after individual plants were harvested, dried, and weighed.

Results/Conclusions

The low plant diversity treatment resulted in the greatest average mass of the target species suggesting competition is more apparent than predicted, with competitive effects likely being driven by other species included in the higher functional diversity seeds mixes, such as Digitaria californica. Soil type had a strong effect on overall average biomass, with the potting soil (serving as the control) having greater average biomass than native collected soils. Within native soils, sandy soil had the greatest average biomass, followed by loamy and silty soils respectively. Nutrient addition had a strong effect on average biomass across soil textures and was a strong abiotic driver. Plants from native soils had significantly lower mass in ambient nutrient conditions than potting soils. Biomass differences between functional diversity treatments were most apparent in enhanced nutrient condition. Inclusion of nutrients had the strongest treatment impact; it can influence the effectiveness of seeding treatment if water availability is controlled for and propagules aren’t a limiting factor. Species level competition, irrespective of overall functional diversity levels, may be driving biotic interactions. Facilitative interactions—such as those that occur with inclusion of nitrogen fixing species—may not be as prevalent in higher functional diversity treatments as predicted.