The effects of litter depth on germination rates in the serpentine endemic Boechera constancei

Background/Question/Methods

Litter accumulation may negatively impact individual plant species by altering key ecosystem properties such as light availability, microbial communities, and nutrient cycling rates. The impacts of litter depth may be particularly strong in harsh environments, such as serpentine soils, where nutrient availability is low and reduced vegetative cover increases light availability. Here, we report the effects of litter accumulation on the germination of the rare, serpentine endemic Boechera constancei (Brassicaceae). In the serpentine outcrops of Plumas National Forest, litter accumulation is highly variable and B. constancei inhabits a range of litter depths (0-40 mm).  However, observational data suggests that populations of B. constancei demonstrate greater population viability in areas with lower litter accumulation. We tested whether increased litter depth impedes germination, and whether populations vary in tolerance to litter depth. Specifically, we quantified the impacts of treatments representing the observed range of litter depth occupied by B. constancei  [0g(minimum), 8g(median), 27g(maximum)] on the germination of seed collected from 9 populations.  Each treatment and population combination was replicated 4 times in a randomized block design (108 treatments). Seeds were placed under dark cold stratification for 4 weeks and then in a greenhouse for 2 weeks. Germinants were counted each week.

Results/Conclusions

Differences in germination rates were analyzed using a two-way ANOVA.  While both source population (p<0.001) and litter treatment (p<0.001) influenced germination rates, the interaction term was not significant (p=.278). Post-hoc Tukey’s honest significance tests revealed that both germination rates under 8g and 27g litter were significantly lower than under no litter (p<0.001).  The moderate and high litter treatments did not differ significantly (p=0.97).  However, both treatments affected germination in dark as well as in lit conditions, suggesting a chemical component to germination inhibition rather than light availability. These results indicate that this species would benefit from litter reduction treatments, such as prescribed burning, as 50% of B. constancei populations are in areas where litter depth is enough to impact germination. Results from this study will be used to develop a species management plan for the conservation of B. constancei. In addition, data collected from this experiment will be available for use in model simulations to illustrate the demography of this plant species.