The 'black box' of plant demography: how do seed type, climate and seed fungal communities affect grass seed germination?

Large and non-spherical seeds are less likely to form a persistent soil seed bank

There is some evidence that seed traits can affect the long-term persistence of seeds in the soil. However, findings on this topic have differed between systems. Here, we brought together a worldwide database of seed persistence data for 1474 species to test the generality of seed mass-shape-persistence relationships. We found a significant trend for low seed persistence to be associated with larger and less spherical seeds. However, the relationship varied across different clades, growth forms and species ecological preferences. Specifically, relationships of seed mass-shape-persistence were more pronounced in Poales than in other order clades. Herbaceous species that tend to be found in sites with low soil sand content and precipitation have stronger relationships between seed shape and persistence than in sites with higher soil sand content and precipitation. For the woody plants, the relationship between persistence and seed morphology was stronger in sites with high soil sand content and low precipitation than in sites with low soil sand content and higher precipitation. Improving the ability to predict the soil seed bank formation process, including burial and persistence, could benefit the utilization of seed morphology-persistence relationships in management strategies for vegetation restoration and controlling species invasion across diverse vegetation types and environments.

Smut fungus (Langdonia walkerae) incidence is lower in two bunchgrass species (Aristida stricta and A. beyrichiana) after fires early in the year

PREMISE

In frequently burned southeastern USA pine-grassland communities, wiregrass (Aristida stricta and A. beyrichiana) are dominant bunchgrasses whose flowers are infected during flowering by a smut fungus (Langdonia walkerae). We hypothesized that because prescribed fire timing affects wiregrass flowering patterns, it could affect smut incidence (occurrence of smut on plants) and severity of infection in inflorescences and spikelets. Because soil order could influence plant susceptibility, we hypothesized that these patterns would differ between soil orders. We hypothesized differences between species as representative of geographic variation in this ecosystem.

METHODS

We surveyed the incidence and severity of L. walkerae in wiregrass populations (85 populations at 14 sites) that had been prescription burned at different times during the previous year. We used binomial regressions to test whether incidence and severity differed by burn day, soil order, or species, with site as a random effect.

RESULTS

Fires that occurred in the winter were associated with significantly lower incidence than fires later in the year (as the months progressed into summer). Plants growing on Spodosol soils were significantly less likely to be infected than those on other soils. More variation in incidence, however, was explained by site, suggesting that site-specific characteristics were important. Smut severity in inflorescences and spikelets was greater overall in populations of A. stricta than in southern populations (A. beyrichiana).

CONCLUSIONS

Our findings indicate that fire timing and soil order affect L. walkerae incidence in wiregrass plants, but neither appears to be associated with greater severity. Patterns of smut infection are related to site history and geographic variation.

Seed type and origin-dependent seedling emergence patterns in Danthonia californica, a species commonly used in grassland restoration

Danthonia californica Bolander (Poaceae)is a native perennial bunchgrass commonly used in the restoration of prairie ecosystems in the western United States. Plants of this species simultaneously produce both chasmogamous (potentially outcrossed) and cleistogamous (obligately self-fertilized) seeds. Restoration practitioners almost exclusively use chasmogamous seeds for outplanting, which are predicted to perform better in novel environments due to their greater genetic diversity. Meanwhile, cleistogamous seeds may exhibit greater local adaptation to the conditions in which the maternal plant exists. We performed a common garden experiment at two sites in the Willamette Valley, Oregon, to assess the influence of seed type and source population (eight populations from a latitudinal gradient) on seedling emergence and found no evidence of local adaptation for either seed type. Cleistogamous seeds outperformed chasmogamous seeds, regardless of whether seeds were sourced directly from the common gardens (local seeds) or other populations (nonlocal seeds). Furthermore, average seed weight had a strong positive effect on seedling emergence, despite the fact that chasmogamous seeds had significantly greater mass than cleistogamous seeds. At one common garden, we observed that seeds of both types sourced from north of our planting site performed significantly better than local or southern-sourced seeds. We also found a significant seed type and distance-dependent interaction, with cleistogamous seedling emergence peaking approximately 125 km from the garden. These results suggest that cleistogamous seeds should be considered for greater use in D. californica restoration.