Root plasticity of Poa secunda in response to nutrient availability and invasion

Background/Question/Methods

Native plant communities of the Nevada Great Basin are experiencing a rapid shift from native perennial grasses to invasive annual grasses. The ability to respond to competition through root plasticity may be an important trait for survival of native plants in invaded systems. We investigated whether Poa secunda, a common native perennial grass in sagebrush ecosystems, could:  1) respond plastically to directly-manipulated nutrient availability, and 2) respond in a similar way to the presence of Bromus tectorum.  For our nutrient experiment, ten seeds from twenty families were sown individually into greenhouse pots. Within each family, five individuals received low nutrient treatment, and five high nutrient treatment.  For our competition experiment, twenty seeds from forty-eight families were sown individually into small and large greenhouse pots and one seed of B. tectorum was added to half of the pots.  Plants were harvested fifty days after emergence for nutrient experiment, sixty-seven days after emergence (early harvest), and after one growing season (late harvest) for competition experiment.  Roots were rinsed, clipped from leaf mass, and digitally scanned. Measurements included total biomass, root:shoot ratio, and change in percent allocation to different root diameter size classes.

Results/Conclusions

In response to low nutrient treatment, Poa secunda showed a 27% decrease in biomass (P<.0001), and a higher root:shoot ratio (P<.0001).  Percent allocation to different root diameter size classes changed in response to low nutrients, with significant increases in allocation to fine root diameter classes 0.0mm through 0.2mm, and decreased allocation in larger root categories 0.3mm through 0.6mm, and >2.0mm (P=0.04).  In response to competition with Bromus tectorum for early harvest plants, Poa secunda showed a 46% decrease in total biomass (P<.0001) and an increase in root:shoot ratio of 14% (P<.0001). Percent allocation to different root diameter size classes changed with competition status, with significant increases in allocation to fine root diameter classes 0.0-0.1mm, and 0.2mm through 0.4mm when grown with Bromus tectorum, and significant decreases in allocation to course root diameter classes 0.4mm through >2.0mm (P<.0001). For late harvest plants, Poa secunda produced 88% less biomass (P<.0001) and had 44% higher root:shoot ratio (P<.0001).  Plants also allocated more biomass to fine root diameter size classes 0.1mm through 0.4mm under competition (P<.0001).  Overall, results suggest that Poa secunda has the ability to allocate biomass to the production of more fine roots in response to invasion in low resource environments.