Germination niche of an emergent invasive grass species, Arthraxon hispidus.

Background/Question/Methods

Joint-head grass (Arthraxon hispidus) is an emergent invasive species in the United States. Despite its long tenure since its introduction in the 1870s, A. hispidus has reached an invasive status in 7 states over the last decade. With a large and growing distribution, it is suspected that this species can tolerate a plethora of environmental conditions. To evaluate the fundamental niche of A. hispidus at a critical life-history stage, we exposed batches of seeds to varying environments and measured their germination or emergence. Characterizing this species germination niche is a first step towards understanding its foundational biology, identifying its environmental impacts, and designing effective control and restoration strategies. Seeds collected from several populations were exposed to various abiotic stressors such as salinity (0 - 600mM NaCl), osmotic pressure (0 - -1.46 MPa), burial depth (1 - 8 cm), and pH (5 - 10). We studied these effects using a randomized complete block design with three replications. A functional three-parameter logistical model was fitted to germination percent for experiments involving osmotic potential and salinity gradients. Emergence data were fitted with an exponential decay model.

Results/Conclusions

Germination experiments revealed that A. hispidus has a broad range of environmental tolerance at this critical life stage. A. hispidus is tolerant to highly saline conditions (LD50 = 354 MD & MO, 297 VA mM NaCl), with a high level of germination ( >75%) at 256mM NaCl. Although, germination was completely inhibited at salinity concentrations greater than 427mM NaCl. Most of the soil in the United States is classified as non-saline; therefore, soil salinity will not inhibit the A. hispidus germination. Germination occurs within salinity levels found in brackish systems, but it is unknown if seedlings will establish due to salinity pressure. A. hispidus germinates from moderate to more severe osmotic pressures depending on the population (LD50 = -0.25 VA, -0.37 MO, -0.67 MD MPa). A Maryland population averaged 16% germination at -0.83 MPa, but other populations were inhibited. Burial greatly reduced emergence in A. hispidus. The average emergence of a 1-cm burial was 46%. Emergence was reduced to 13% at 4 cm and completely inhibited at 8 cm. The pH did not affect the germination of A. hispidus. Germination was ≥80% for all pH treatments. Therefore, we concluded that pH will not inhibit germination in the contiguous United States.