Non-Peer ReviewedIn order to study downy brome seed bank dynamics, a burial experiment was established near Maple Creek in August 1987. Downy brome seed was buried at four depths (2.5 to 20 cm), and placed on the soil surface, in nylon mesh bags. Seed was exhumed at monthly intervals in the fall of 1987 and throughout the 1988 growing season. Seed germination in the field in the fall of 1987 was greatest from the buried seed (almost 50 %) while the surface seed exhibited little germination.  This germination occurred in response to fall precipitation. Seed on the soil surface was not in contact with moist soil for a sufficient time to stimulate germination, so 80-90 % of this seed had not germinated by freeze-up in 1987. However the surface collected seed had germination of 34 % when transferred to an incubator in the laboratory, indicating environmentally induced dormancy in the field. There was little recruitment from the seed bank
occurred in the spring of 1988 in spite of favorable moisture and temperature conditions. Seed on the soil surface remained viable until the 14th month but the buried seed at all depths had either germinated or deteriorated by the 9th month. This study was repeated in 1988, with similar results to date

Derksen, D.A.

Douglas, B.J.

Thomas, A.G.

English

eCommons@USASK

Downy brome seed bank dynamics in southwestern Saskatchewan

University of Saskatchewan Research Archive

Downy Brame Seed Bank Dynamics in Southwestern Saskatchewan Brian J . Douglas, A. Gordon Thomas, and Doug A. Derksen. Research Station, Agriculture Canada, P.O. Box 440, Regina, SK Funding for this project was provided by the Canada/Saskatchewan Subsidiary Agreement in Agriculture Development (ERDA) for winter wheat production. Abstract In order to study downy. brome seed bank dynamics, a burial experiment was established near Maple Creek in August 1987. Downy brome seed was buried at four depths (2.5 to 20 em), and placed on the soil surface, in nylon mesh bags. Seed was exhumed at monthly intervals in the fall of 1987 and throughout the 1988 growing season. Seed germination in the field in the fall of 1987 was greatest from the buri~d seed (almost 50%) while the surface seed exhibited little germination. ' This germination occurred in response to fall precipitation . Seed on the soil surface was not in contact with moist soil for a sufficient time to stimulate germination, so 80- 90% of this seed had not germinated by freeze-up in 1987. However the surface collected seed had germination of 34% when transferred to an incubator in the laboratory, indicating environmentally induced dormancy in the field. There was little recruitment from the seed bank occurred in the spring of 1988 in spite of favorable moisture and temperature conditions . Seed on the soil surface remained viable until the 14th month but the buried seed at all depths had either germinated or deteriorated by the 9th month . This study was repeated in 1988, with similar results to date. Introduction Downy brome ( Bromus tectorum ) an aggressive, introduced annual grass, 1s now well established as a serious weed of winter wheat in southwestern Saskatchewan. Downy brome was likely introduced into Saskatchewan through contaminated winter wheat seed from the U.S. in the early 1960's. During the intervening period downy brome has spread throughout the southwest region to infest 90 townships across 28 rural municipalities in 1988. Downy brome reproduces only by seed and survives from year to year as seed on the soil surface or buried in the soil (Evans and Young 1970). After ripening requirements have been reported for downy brome (Laude 1956; Thill et al. - 159 -1980 ) , but this can be overcome quickly under field conditions so seed usually germinates readily afte r maturity. Caryopses of downy brome are large, lightweight, and elongated with a relatively long awn. These characteristics make intimate soil contact and bu ri al difficult, so seed tends to remain on the surface of a bare seedbed (Evans and Young 1984). However downy brome can germinate without the ba se of the caryopsis in a moisture supplying substrate (Young et al . 1971). In most downy brome infested areas the bulk of the germination occurs during the fall, if moisture is adequate (Wicks et al. 1971; Mack and Pike 1984; Hassan and West 1986) but many seeds in t he soil do not germinate until the spring . Germination in the f all depends on appropriate microenvironmental conditions, particularly the presence of plant litter or trash cover which effectively moderates t emperature and humidity fluctuations (Evans and Young 1970). In climates with modetate winter temperatures, downy brome may germinate all winter long (Mack and Pike 1983 ) . Environmental conditions over the winter can lead to an acquired dormancy (Hull and Hansen 1974; Evans and Young 1975; Young and Evans 1975) . The germination pattern shifts from one of simultaneous germination in the fall to continuous germination in the spring (Newman 1963). This may be an effective " strategy " for survival in unpredictable environments (Mack and Pike 1983) . Previous studies on downy brome seed bank dynamics have been restricted to the Intermountain regions of Washington Sate and the desert areas of Nevada (Evans and Young 1984; Mack and Pike 1983, 1984). Wicks et al. (1971) studied downy brome emergence and longevity in Nebraska. Recent evidence has shown that there is a wide variation in germination behavior of downy brome from different regions in the u.s . (Milby and Johnson 1987) but there are no comparable studies for Canadian populations (Upadhyaya et al. 1986) . The critical factors controlling downy brome populations are that (Young et al. 1969) : 1 ) large numbers of viable caryopses are carried over from one year to the next in litter and soil. 2) reductions in the density of downy brome populations can result in a net increase in caryopses production. 3) the simultaneous germination characteristics of freshly harvested downy brome seed can be environmentally conditioned to continuous germi nat ic~ . Since downy brome is a recent ecological intruder in southwestern Saskatchewan, there is no information available on population dynamics in these climatic and soil zones. Control of downy brome would be simple if all the seed germinated each year, but the viable, ungerminated caryopses that remain on and in the soil are safe from our present weed control methods with the exception of long periods of fallow (Finnerty and Klingman 1962; Young et al. 1969; Evans et al. 1970) . - 160 -These studies were conducted to provide important information on recruitment from the seed bank, mortality of seed, and seed longevity under the climatic conditions of southwestern Saskatchewan. Materials and Methods On August 15,1987 350 nylon mesh bags containing 100 downy brome seeds were buried near Maple Creek at 0, 2.5, 5.0, 10.0, and 20.0 em from the soil surface. The experiment was conducted in a split-plot design with five replications. The experiment was repeated at the same site 1n 1988, but 200 seeds were placed in each bag. Seed used for the 1987 experiment for both the burial and as a "check" for the germination in the incubator were collEcted in 1987, while seed for the 1988 experiment was collected from the same site, but in 1988. Seed bags placed on the soil surface were covered with straw to simulate the microenvironmental conditions in a typical conservation tillage system. One seed bag from each plot was exhumed at monthly intervals from each experiment. Seed germination in situ was· determined for the first three sampling dates. At later sampling dates, there was little germination in the field and it was difficult to determine the time of germination so these data were no longer collected after the April 1988 sample. The remaining intact seed in each bag was germinated in an incubator at 20/15 c and a 16 h photoperiod for 3 weeks. Results and Discussion During August of 1987 precipitation was well above normal for the burial area, so the seed was buried in moist soil (Table 1). Rainfall in the latter half of August (after the seed was buried) totaled 33 mm. This provided adequate soil moisture to stimulate germination of the seed. One month after burial (September 198?) mean germination of the buried seed in the field was 40% (Figure 1). In contrast, the surface seed averaged only 17% germination in situ. This difference in germination behavior can be at least partially attributed to environmental factors and the lack of "safe sites" on the soil surface. Soil at the burial site is very sandy, permitting rapid infiltration of any rainfall into the soil profile, with minumum persistence of water on the soil surface. Thus moisture levels on the soil surface were not high enough for a sufficient length of time to stimulate high germination levels. Although the September to November period was extremely dry in 1987 (only 29% of normal precipitation) seed germination in the field averaged 34% in the buried bags, but only 11 % for the surface seed for the November sample. Moisture in the spring of 1988 stimulated germination in only a small number of seeds, irrespective of the depth of burial. - 161 -Results to date from the 1988 burial have shown a similar response of germination in the field to fall rains, with much greater germination in the buried seed than in the surface seed (Figure 1) . The proportion of intact, ungerminated seed in each bag was greater on the surface than in the buried bags, indicating that more of the buried seed had germinated or decomposed, compared to seed placed on the surface by the spring of 1988 (Figure 2). Seed which failed to germinate in the field in response to environmental conditions (i.e. intact seed) was germinated in an incubator for three weeks. Germination of the buried seed was less than 5% from collections in the fall of 1987 (F igure 3). The surface seed exhibited a gradual decline in germination from 34 % at one month after burial to 1% after 15 months of burial. Some loss in seed viability was evident even after a short period of burial (one month) as the control seed (stored at 2 C) from the same seed source had SO to 65 % germination. Seed from the 1988 burial experiment had a similar decline in germination in the fall of 1988, but the germination percentages were somewhat greater (Figure 3 ) . Survival as seed in the soil is an important mechanism for continuing infestations of winter annual weeds under the climatic conditions of southwestern Saskatchewan (Budd et al. 1954). Downy brome seed longevity depends on the relative contributions of phenotype and environment to germination (Lonsdale 1988). Seed weight can affect dormancy characteristics, as can environmental effects on the parental plant during seed maturation. In studies at Swift Current, downy brome seed buried three inches deep in soil that was periodically cultivated, had less than 2% emergence during the th ird year after burial (Chepi l 1946). Buried seed germinated throughout the year, with peak germination in the spring, presumably in response to rainfall, but no environmental data were collected. In Nebraska, downy brome seed longevity ranged from 2 to 5 years ac~oss the state depending on the environmental conditions particular to each burial site (Wicks et al. 1971). More downy brorne caryopses remained viable on the soil surface than at any other depths . Soil texture and precipitation were determining variables in the results of this study. Germination and emergence on the surface were greater in the fine textured soils (silt loam, silt clay loam) compared to the sandy loam. These differences occurred because of differing water infiltra~! o~ rates among the soil textures. The surface of t he sandy loam soil dried out rapidly compared to the finer textured soils. - 162 -Soil water potential ranging from - 0.17 to - 1 . 28 MPa had little effec t on downy brome germination in a laboratory exper iment (Thill et al. 1979 ) but Mc Donough (1975) reported that germinating seeds reach an equilibrium water po tentia l at - 1.7 MPa. Under field conditions soil texture influences soil structure and surface characteristics with respect to microtopography, water infiltration and retention, and heat capacity and temperature. Although most of the downy brome infestations in southwestern Saskatchewan occur on coarse textured soils, seed bank dynamics on finer textured soils in areas with higher precipitation (eg. adjacent to the Cypress Hills) may differ from the results reported here. In addition relatively small differences in precipitation amounts and distribution can result in large differences in recruitment from the seed bank, the survival of seedlings, and the induction of dormancy. Several conclusions can be made regarding downy brome seed bank dynamics: 1) Downy brome germination occurs in response to rainfall/snow melt. 2) Peak germination of downy brome seed generally occurs in the fall if precipitation is adequate. 3) Spring germination occurs in seed with innate dormancy or after ripening requirements . 4) Seed survival is greatest on the soil surface. 5) Surface seed represents a source for continuing infestations for at least 15 months. 6) Buried seed had either germinated or lost viability after 7 months irrespective of the depth of burial. 7) Recruitment, mortality, and persistence of the seed bank vary from year to year with the weather conditions. 8) Safe sites for downy brome germination are more numerous in conservation tillage winter wheat production than in conventional tillage systems. Table 1. Monthly precipitation (mm) at the seed burial site (Maple Creek North) . Month 1987 1988 Norma l January 13.4 27 . 7 February 23.9 22 . 0 March 17 . 2 17 .0 April trace 3 8. )_ May 13.0 43. 8 June 64 . 5· July 50.7 42.1 August 75 . 3 42.5 September 7.9 24. 4 35.8 October 10.6 19 . 5 November 2. 7 17 .9 December 20.4 23 . 5 - 163 -Literature Cited Budd, A. c., w. s. Chepil, and Germination of weed seeds . 3. and fallow on the weed population Agric. Sci. 34:18-27. J . L. Doughty. 1954. The influence of crops of the soil. Can. J. Chepil, w. s. 1946. Germination of weed seeds. 1. Longevity, periodicity of germination, and vitality of seeds in cultivated soil . Sci . Agr. 26:307- 346. Evans, R. A. and J. A. Young. 1970. Plant litter and establishment of alien annual weed species in rangeland communities. Weed Sci . 18:697-703. Evans, R. A. germination of 23:354-357. and J. A. Young. 1975. Enhancing Weed Sci. dormant seeds of downy brome. Evans,R . A. and J. A. requirements for downy infestation and control on Sci . 32, Suppl. 1:13-17. Young. brome sagebrush 1984. Microsite (Bromus tectorum) rangelands. weed Evans,R . A., H. R. Holbo, R. E. Eckert, Jr. , and J. A. Young. 1970. Functional environment of downy brome communities in relation to weed control and revegetation. Weed Sci. 18:154- 162 . Finnerty, D. w. and D. L. Klingman. 1962. Life cycles and control studies Weeds 10:40- 47. of some weed bromegrasses . Hassan, M. soil seed semi - deserts. A. and N. E. West . pools in burned and Ecology 67:269- 272. 1986. Dynamics of unburned sagebrush Hull, A. Delayed Manage. C. , Jr. and germination of 27:366-368. w. T. Hansen, cheatgrass seed. Jr. J. 1974 . Range Laude, H. M. seed of some 9:126- 129. 1956. Germination of western range species. freshly harvested J. Range Manage. Lonsdale, W. M. 1988. Interpreting seed sur v i vors~i~ curves. Oikos 52:361-364. Mack, R. N. and D. Bromus tectorum Ecol. 71:69 - 93. A. Pyke. 1983. The demography of variation in time and space. J. Mack, R. N. and D. A. Pyke. 1984. The demography of Bromus tectorum: the role of microclimate, grazing and disease. J. Ecol. 72:731-748. --·164 -McDonoug h, W. T . seeds . Bo t . Gaz. 1975 . 136: Water pote n tial of 106-108. ge rminat ing Milby, T. H. downy brome north Texas. and F . L . Johnson . 198 7 . Germi nation o f f rom southern Kansas, central Oklahoma, and J. Range Manage. 40: 534-536. Newman, E. germination 51:625- 639 . I. date 1963. of Factors controlling winter annuals. J. the Ecol. Thill , D. C., R. D. Schirman, and A. P . Appleby. 1979. Influence of soil moisture, temperature, and compaction on the germination and emergence of downy brome (Bromus tectorum) . Weed Sci. 27:625 - 630. Thill, D. C. , R . D. Schirman, and A. P. Appleby. 1980 . Influence of afterripening temperature and endogenous rhythms on downy brome (Bromus tectorum ) germination. Weed Sci. 28:321-323. Upadhyaya, M. K. , R . 1986 . The biology tectorum L. Can. J . Turkington, of Canadian Plant Sci . and D. Mcilvride . weeds. 75. Bromus 66:689 - 709. Wicks, G. A. , 0. 1971. Influence downy brome seed. C. Burnside, and C. R. Fenster. of soil type and depth of planting on Weed Sci. 19:82- 86. Young, J. A. and R. A. Evans. 1975. Germinability of seed reserves in a big sagebrush community. Weed Sci. 23:358-364. Young, J. A., R. 1969. Population 17:20-26. A. Evans, and dynamics of R. E. Eckert, Jr. downy brome. Weed Sci. Young, J. A., R . A. Evans, and B. L . Kay. 1971. Germination of caryopses of annual grasses i n s i mula ted litter. Agron. J . 63:551-555. - 165 -(/) c c 0 :.;:; 0 c E L Q) ~ "'0 Q.) Q) (/) 70 60 50 40 30 20 10 Downy Brome Seedbank Dynamics September 1987 - November 1988 ·-· .......... &_____ -"!' --- ' -----·-9~0---b------- A_,¢ ..L 1 - - - - -":! 0+-~--~~----~~~~~--~~--~~--~~~ 70 60 50 40 30 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 September 1988 - November 1988 ·-+---• 20 T 0-( ·-4 10 .//f....._o 0 4 0+--+--~~----~~~~~~~~--~~--~~~ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Duration of Burial (months) Figure 1. Recruitment from the seed bank in the field. oce sd r--.. ~ ....__ -o Q) Q) (/) -o Q) +J 0 c E I I L.. f-' Q) 0'\ Ol -.J c :::> .._ 0 c 0 t 0 a. 0 L,. . 0... 100 90 80 70 60 50 40 Downy Brame Seedbank Dynamics September 1987 - November 1988 o--6------------Q"o 9.,.....,. .1 1 ~ -----·--·- - - - - - '!l' ·-·/ 30+-~--~~--~~~~--r--r--~~--~~--,--,--~ 100 90 80 70 60 50 40 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 September 1988 - November 1988 0-0surface e-eburied 30+-~--~~--~~--~~--~~--~~--~~--~~ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Duration of Burial (months) Figure 2. Proportion of intact, ungerminated seed recove r ed from the buried bags . c 0 :;; 0 c E L-Q) C) c 0 Q) ~ 80 60 20 Downy Brame Seedbank Dynamics September 1987 - November 1988 o 0-------- ----o o o 'o/ ~o/ -o-o~~o---2/o • ·-+-•------------·----+--·--·--· + --.-- , 0 +--¥==Q--Q--~-T--~~--~~--Q-~--~~~~·~--~ 0 80 60 40 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1~ September 1988 - November 1988 ;-o--o ~I . y-; 0-C :k 20 ·-· <>-• 0+-~--T-~--~~--~~--~~--~~----~--~~ 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1~ Duration of Burial (months) I Figure 3. Germination of recovered, intact downy brome seed in ru ace ed 1bator. 

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Downy brome seed bank dynamics in southwestern Saskatchewan

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