Phenotypic Plasticity within the Native Perennial Grass, Poa secunda (Sandberg bluegrass), an Important Component of the Western United States Sagebrush Grassland Vegetation
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. I investigated the following questions about Poa secunda, a common native perennial grass in sagebrush ecosystems: 1) Can P. secunda respond plastically to directly-manipulated nutrient availability? 2) Does P. secunda respond to the presence of B. tectorum? and 3) Are these plastic responses adaptive? For the nutrient experiment, ten seeds from twenty families were sown individually into greenhouse pots, where one P. secunda family is defined as one individual plant. Within each family, five individuals received a low nutrient treatment, and five a high nutrient treatment. Plants were harvested fifty days after emergence. Roots were rinsed, clipped from leaf mass, and digitally scanned. For the 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 sixty-seven days after emergence (early harvest), and after one growing season (late harvest) for the competition experiment. Roots were rinsed, clipped from their leaf mass and digitally scanned in a similar manner as the nutrient experiment. For both the nutrient and the competition experiments, measurements included total biomass, root to shoot ratio, leaf number, specific root length (SRL), percent allocation to different root diameter size classes, and plasticity in allocation to these same root diameter classes. In addition change in percent allocation to different root diameter size classes was calculated for plants in response to both low nutrients and competition from B. tectorum. Measures were correlated with plant performance measured as CPI (competitive performance index) and total biomass to determine if they were adaptive. Finally, to determine if plastic changes were simply a result of plant size or represented a change in strategy due to different growing environments, traits were assessed for "active" or "passive" plasticity by accounting for ontogenetic drift. In response to low nutrient treatment, P. secunda showed a 27% decrease in biomass (P<0.0001), and a higher root to shoot ratio (P<0.0001). Plants showed an increase in specific root length (P<0.0001) and a decrease in leaf number under low nutrients (P<0.0001). 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 to larger root categories (0.3mm through 0.6mm, and >2.0mm; P=0.04). Families differed in their response to low nutrients, and the nutrient by family interaction was significant for total biomass (P=0.0059). The nutrient experiment results demonstrated that P. secunda has the ability to allocate biomass to the production of more fine roots in response to low resource environments. In response to competition with B. tectorum for early harvest plants, P. secunda showed a 46% decrease in total biomass (P<0.0001) and an increase in root to shoot ratio of 14% (P<0.0001). Plants had a higher specific root length and a decrease in leaf number under competition. 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 B. tectorum, and significant decreases in allocation to course root diameter classes (0.4mm through >2.0mm; P<0.0001). Families differed in their response to traits measured, and the treatment by family interaction was significant for leaf number (P=0.0071) and percent allocation to different root diameter size classes (P=0.0001). The shift towards the production of finer root class 0.2-0.3mm and a higher root to shoot ratio may be an adaptively plastic response to competition with B. tectorum. For late harvest plants, P. secunda produced 88% less biomass (P<0.0001) and had 44% higher root to shoot ratio (P<0.0001) in response to competition. Families different in their root to shoot ratio (P<0.0001) and in their percent allocation to different root diameter size classes (P<0.0001). The treatment by family interaction, and the location by treatment interaction was significant for percent allocation to different root diameter size classes (P<0.0001). Plants allocated overall more biomass to fine root diameter size classes (0.1mm through 0.4mm) under competition (P<0.0001). In contrast to early harvest, allocation to coarse roots was adaptive in the late harvest. When ontogenetic drift was assessed, results show that P. secunda had a higher root to shoot ratio and allocated more resources to coarse roots relative to their size for some traits measured. This shift in allocation from producing less fine to more coarse roots may suggest a shift towards an adaptively plastic response to competition with B. tectorum. These results also suggest that P. secunda may have the ability to adaptively respond to invasion from B. tectorum through phenotypic plasticity.
Great Basin, ontogenetic drift, phenotypic plasticity, Poa secunda, restoration, Sandberg bluegrass
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