2002-07-08_GENERAL DOCUMENTS - M2002004 (4)

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Table 1. Seed density in seed banks of diverse community types. Investigation Iverson and Wali (this study) Brophy (1980) Lippert and Hopkins (1950) Johnston et al. (1968) Beuchamp et al. (1975) Rabinowitz (1980) r and Aguirre (1978) Major and Pyott (1966) Robinson (1949) Study site North Dakota prairie grazed site, mixed grass ungrazed site, mixed grass ungrazed mixed grass prairie 6- year -old mined site Kansas grazed niixef grass prairie overgrazed short grass prairie grazed short grass prairie 25- year -old' abandoned field Alberta ungrazed rangeland - heavily grazed rangeland Wyoming sagebrush grass Missouri tall grass prairie New Mexico Desert Grassland -: - poor rang•:;; fair range good ra Cali)ornia y ungrazed bunt r. 'grazed bunchgrasi Minnesota oropland England; arable1a ch of 136 samples were as folfows;50g of dry sterilized peat (for water retention)' were placed in_a04 hel container, covered with a filter paper on which 100 g of field sample was placed,; The containers were kept` moist.ywith;deionized. water throughout the 16 -month duration of the experiment. The growth chamber conditions were 15 hr day /9 hr night length, 26° C day /18° night temperature, and 54-72 microeinsteins m' sec photosynthetically active radiation. These environmental condi- tions are known to be favorable for germination fora wide range of species. Plants were identified as early in their life cycle as possible, and removed promptly after identification. Seedlings were identi- fied following Dunham et al. (1947), Gleason and Cronquist 963), Stevens (1963), Best et al. (1971), and Van Bruggen (1976). The experiment was monitored and the data were grouped by 4 time periods: seedling emergence during (a) initial 4 months of the experiment, May - August 1979 (hereafter period `A '); September- December 1979 eriod `B January-April 1980 (period `C (P �; rY P (Pe '); and May- September 1980 (period `D '). , ' - Statistical analysis included calculation of seed density means with 95% confidence intervals both for site and depth, and t -tests for detecting differences between sites. In addition, Shannon- Weiner index of diversity values were calculated. Results and Discussion Seed Numbers From the grazed site samples, a total of 319 seedlings (26 species) germinated during the 16 -month study period (Fig. 1); 46% of the seedlings were horseweed (Conyza canadensis), six -weeks fescue (Festuca octoflora), and rough penny royal. In contrast, signifi- cantly fewer (P(.01) seedlings (140) representing 23 species emerged from the ungrazed site samples with green sage (Artemisia Depth, cm Total seeds/ m 0 -15 7740 Germination from cores for 16 0 -15 3870 months. 0 -7.5 990 Washing /tetrazolium. 0 -7.5 2460 Washing /tetrazolium. 0 -1.25 0 -1.25 0 -1.25 0 -1.25 0 -2.5 0 -2.5 0 -5 0 -12 6470 Germination from cores for 145 days. 0 -2 0-2 0-2 0 -5 0 -5 0 -15 9 -10 . 0-20 440 3640 760 5130 2970 3240 370 485 810 285 8230 12,290 8300 8600 19,240 Method Germination in boxes from 930 cm samples for 76 days. Germination from cores for 3 weeks. Germination in flats from 100 cm samples for 6 weeks. Germination from cores for 2 months Germination from cores for 6 months Germination from cores. Germination from cores for 2 years. Germination from cores for 23 months. dracunculus), yellow sweet clover (Melilotus officinalis), and blue grama (Bouteloua gracilis) accounting for 52% of the total (Fig. 2; Table 2). Numbers of germinating seeds in both fresh and 1 -year old stockpiled topsoils were very low and significantly fewer (PK.01) than the grazed or ungrazed sites; only 14 seedlings from 9 species emerged (Table 2). Blue grama and six -weeks fescue were the only 2 species common to both topsoils; little bluestem (Andro- pogon scoparius) and ticklegrass (Agrostis scabra) were present only in the fresh stockpile. The paucity of seeds in the stockpiled topsoil (also reported by Brophy 1980) may be due to the inversion of soil layers during the stockpiling process wherein upper layers are buried deep while the lower layers are placed on top. Seeds may also lose viability over time during stockpiling. In contrast to the ungrazed site, the grazed site not only had statistically higher seed numbers, but also a high proportion of weed seeds (44% as against 7 %). Our results show a similar trend to that of Lippert and Hopkins (1950) who found the seed bank in an overgrazed shortgrass prairie in Kansas composed of 85% weed seeds, compared with 58% weed seeds in a moderately grazed site. The presence of larger numbers of weed seeds after grazingmay be attributed to (1) the avoidance of weeds by grazers, and (2) a greater seed output by weeds (ruderal- selected species of Grime 1979). These data indicate that notice should be taken as to the grazing history on pre -mined sites, as it has a bearing on the initial aboveground flora. Distribution by Depth On both grazed and ungrazed sites, the surface layers had the highest concentrations of seeds; seed quantity and diversity dimin- ished with depth (Table 2). Seeds are moved to lower depths by physical means such as washing down fissures and by biotic activ- ity such as the storage of seeds by insects or earthworm burrowing �1.