In ecosystems dominated by woody overstory vegetation coexisting with herbaceous understory vegetation, the stability of these systems may be linked to a more equitable distribution of biomass among species than is currently found for these systems due to anthropogenic alterations. The benefits of system stability as a result of increased understory biomass and increased community functional diversity arise from the net effects of overstory-understory interactions through environmental and physiological triggers. In the montane aspen (Populus tremuloides) forest and sagebrush (Artemisia tridentata)-steppe, the herbaceous component is hypothesized to enhance persistence of the woody component by acting as a stabilizer on the system through adjusting annual production to prevailing environmental conditions, especially soil moisture availability. For the herbaceous component, growth is highly variable from year to year, closely tracking resource availability. This variable growth dampens the magnitude of soil moisture oscillations and associated nutrient availability experienced by the woody component. We address this question through a combination of field measurement of soil moisture, plant performance, annual biomass production, and soil moisture simulation modeling for Intermountain West, USA montane aspen and sagebrush-steppe ecosystems.

Results/Conclusions

Functional diversity was related to the amount of soil water available to the overstory species in quaking aspen and sagebrush-steppe ecosystems in the Intermountain West. In 2007, soil moisture remaining in the top 0.5 m in an aspen-herbaceous community was 0.08 g/g and 0.04 g/g in an aspen-sagebrush community. The peak biomass of the herbaceous understory tracks snowpack size, related to soil moisture. With a 389.2 cm snowpack (2007), peak biomass was 76.4 g/m²; for a 656.2 cm snowpack (2008), understory biomass was 153.9 g/m².  In a sagebrush-steppe community, dead branch units on sagebrush (a measure of xylem hydraulic failure) decreased linearly with increasing understory cover (r²=0.84), suggesting greater understory cover has a protective effect on the woody overstory. Simulated resource use for woody and herbaceous species support the contention that herbaceous understory growth does track annual resource availability and through its resource utilization patterns (exponential vs. uniform) may exert a protective, buffering effect on the woody overstory growth. Buffering of woody species growth combined with greater access to deep water resources when herbaceous species are dormant may provide greater persistence of woody species in these communities