Quantity and Germinability of Oryzopsis hymenoides Seed in Lahontan Sands

Abstract

The location, quantity, and germinability of seed (caryopses) reserves of Oryzopsis hymenoides (R. & S.) Ricker in the soil were investigated at 4 locations in the Carson Desert of western Nevada. Numerous germinable seeds were recovered from the surface 15 cm of Lahontan sands. There was no clear relation between the number of seeds and depth of burial. On sites with pronounced dunes more seeds were found in the dune sands than in the interspaces. Seeds were recovered with and without evidence of sand abrasion of the indurate lemma and paleas. Seeds without wear marks were much more highly germinable (80%) than the more numerous seeds with wear marks (6%). The germhmble seed reserve consisted of seeds that germinated without pretreatment when incubated at 20°C; a much larger portion that required dissection to remove the indurate lemma and palea; and a fnction with embryo dormancy that had to be overcome with gibberellin enrichment of the germination substrate. Large numbers of opened, empty lemma and paler were found in the sands. Rodent enhancement of the germination of Oryzopsis hymenoides seeds appears to be a more valid hypothesis than mechanical abrasion from saltation. Oryzopsis hymenoides (R. & S.) Ricker is the predominant herbaceous species in many plant communities at lower elevations of pluvial lake basins of the Great Basin. This perennial bunchgrass reaches its greatest abundance on sandy soils or on active dune areas. In the northern Carson Desert large areas of relatively pure stands of Oryzopsis hymenoides grow on Lahontan sands (Billings 1945, 1949). This geologic formation is derived from sand-textured sediments that were dumped by rivers into the pluvial Lake Lahontan during the Pleistocene and since have been transported by winds across what is now a desert landscape (Morrison 1964). Oryzopsis hymenoides seeds (caryopses) are highly polymorphic, but all forms are generally dormant (Huntamer 1934). The nature of the dormancy has been attributed to the persistent lemma and palea and pericarp that inhibit oxygen transfer to the embryo (Huntamer 1934, Fendall 1964, Clark and Bass 1970, Shaw 1976, and McDonald and Khan 1977). The lemma and palea have to be removed and the pericarp pricked in order for the seeds to germinate (Plummerand Frischknecht 1952). Considering the nature of thedormancy, 3 hypotheses have been offered to explain how dormancy is broken in the natural environment. The first hypothesis postulates that temperature fluctuations and microbial degradation cause the lemma and palea to split and allow germination (Stoddart and Wilkinson 1938). The second hypothesis takes into account the affinity of this grass for sandy soils and suggests that the relatively small and dense seeds of this species move along the surface of the sands by saltation until the lemma and palea are broken by the abrasive action of the sand grains (e.g. Robertson 1977). The third hypothesis, as recently proposed by McAdoo et al. (1983), is that the collecting, mampulating, and caching of seeds by rodents enhances the natural establishment of Oryzopsis hymenoides. Authors are range scientists, USDA. ARS, Renewable Resources, University of Nevada. Rena. 920 Valley Road. This study is a contribution from the USDA. ARS and the Agriculture Experiment Station, University of Nevada, Reno. Journal Series No. 507. Manuscript received March 29, 1982. 62 One of the few completed studies of natural seedling establishment of Oryzopsis hymenoides determined that 85% of successful Oryzopsis hymenoides seedlings emerged from depths of 3 to 7 cm in Lahontan sands (Kinsinger 1962). Considering the relatively small size of the seeds (5 mm diameter), emergence from greater than 5 cm is surprising (Young et al. 1969). Our purposes were to estimate the quantity and location of Oryzopsis hymenoides seeds in Lahontan sands and to study the germinability of recovered seeds todetermine the natural means by which the inherent dormancy of the seeds is overcome. Materials and Methods Field studies were conducted in the northern Carson Desert about 80 to 100 km northeast to east of Reno, Nev., (latitude 390 45’ N, longitude I 190 0’ W). Billings (1945) described the general vegetation, soils, and climate of the Carson Desert. We chose 4 sites with similar potential plant communities growing on Lahontan sands (Table 1). The sites differed in the density of Oryzopsis hymenoides plants. Frenchman’s Station (military bombing range) was in near pristine condition with no grazing; Desert Queen Valley was occasionally grazed by cattle; Summit was moderately grazed; and Eagle Valley was severely grazed. Based on the closest stations with long-term weather records (Lovelock and Fallon, Nev.) and the gaugesthat we maintained on the sites, annual precipitation is estimated at greater than 100 mm, but consistently less than 120 mm. The Lahontan sands (Typic tropopsaments) have a decided microtopography with dunes and interspaces. The dunes rise less than 0.5 m above the interspaces. We stratified our sampling based on this microtopography. At each location, in September 1977, we selected a relatively uniform area and laid out 4,50 by 50-m blocks in a randomized design. In each block, open-bottom metal boxes 32 by 32 cm were driven into the sands to a depth of 15 cm. The inside surfaces of each box were divided into segments by horizontal lines every 2.5 cm to facilitate sampling. The sand was carefully removed in 2.5-cm deep increments and placed in bags for transportation to the laboratory. A box was randomly located in an interspace and dune in each replication for a total of 8 samples at each location on each sampling date. The samples were spread on paper on greenhouse benches to air dry and screened through a 0.99-mm screen to recover all Oryzopsis hymenoides seeds and seed parts. The 15-cm maximum sampling depth was based on the soil depth in the most shallow interspace soils. The Lahontan sands range from 15 to 100 cm deep, but are deeper on dunes. The texture is rather uniformly 94 to 96% sand with 4 to 6% silt and clay. The pH in the soil profile ranges from 8.0 to 8.9. Because of the low silt and clay contents and the nearly complete lack of soil structure, seeds were easily recovered by screening. The recovered seed material was divided into classes as follows: (a) entire seeds, (b) seeds with wear marks or holes in the lemma and palea, and (c) empty lemma and paleas. The entire seeds and with wear areas were squeezed between thumb and forefinger to see if they were empty. Empty lemmas and paleas disintegrated while filled seeds were too hard to crush. Seeds that passed this test were JOURNAL OF RANGE MANAGEMENT 36(l), January 1983 Table 1. Characteristics of experimental sites.’