COS 28-2 - Herbaceous community assembly at the transition between grassland and woodland: influence of drought and shade stress on the recruitment niche

Background/Question/Methods

Recruitment is a key process influencing plant community composition. The recruitment niche includes the set of abiotic tolerances under which early-life stage transitions (seed germination, seedling emergence and survival) are successful, but empirical descriptions of the recruitment niche are rare.

In the Ozark Highlands (USA), woodlands and adjacent rocky grasslands represent abrupt transitions in abiotic conditions and plant community composition. We asked whether sorting of the species pool into these distinct habitats is predicted by recruitment niche differences with respect to shade and drought stress. Specifically, we tested if: 1) tolerances of early-stages to shade and drought predict species habitat affinities, 2) early-life stage transitions differ in their response to abiotic variation, and 3) seed and seedling traits relate to the recruitment niche.

We selected 48 native herbaceous perennial species representing four functional groups (sedges, grasses, legumes, and forbs) and 16 con-familial triads, each consisting of a generalist, woodland specialist, and glade specialist. We subjected seeds and seedlings of each species to factorial combinations of shade (high, low, none) and moisture (ambient, drought) in lab and greenhouse experiments. We characterized seed (mass, shape) and seedling (mass, height, root:shoot, SLA) traits, and used generalized liner mixed models to test our predictions.

Results/Conclusions

We used backwards selection to identify the best models predicting each early-stage transitions based on cumulative germination, emergence, and survival. Across treatment groups, 52% of seeds germinated, 73% of germinated seeds emerged, and 48% of emerged seedlings survived the establishment period. 1) At the germination stage, we found that moisture availability had the strongest influence, with lower germination under droughty conditions regardless of habitat affinity. Shade treatments differentiated habitat groups, with generalists performing equally well in all light environments, glade-obligate species performing worse when shaded, and woodland species germinating a higher rates when shaded. Abiotic filters had little effect on seedling emergence. Survival peaked in low-shade and high-moisture, but there was no interaction with habitat affinity. 2) We only found evidence for niche differences between groups at the germination stage. 3) Seed viability rate was strongly related to cumulative germination and larger seeds had a higher probability of emergence. Seedling traits responded strongly to treatments, with shade increasing allocation to shoot biomass and height, and decreasing total mass. Our results suggest that germination cues are important for sorting in this system, and that across habitat groups, seedlings show high trait variability in response to stress, which likely increases survival.