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Article |
Department of Biology, Middle Tennessee State University, Murfreesboro, Tennessee 37132 USA
Received for publication February 22, 2006. Accepted for publication November 10, 2006.
ABSTRACT
Dormancy break and germination of seeds are governed by climatic cues, and predicted changes in climate may impact the ecology and conservation of species. Paysonia perforata and P. stonensis are rare brassicaceous winter annuals occurring primarily in fields on floodplains, where corn or soybeans are recommended for habitat maintenance. We tested the effects of precipitation, based on two predictions of changes in climate, on seed germination in these two species and placed the results into a management framework. Seeds of both species, collected during peak dispersal in late April/early May, were given various periods of light (or darkness) followed by darkness (or light) at summer temperatures before placement in darkness during late summer/early autumn in both laboratory and field. The light requirement was met earliest at 10 wk (mid-July) on alternating wet/dry substrate (simulating current climatic conditions). However, seeds of P. perforata and P. stonensis were photostimulated earliest at 2 wk (mid-May) and 6 wk (mid-June), respectively, on a continuously moist substrate (simulating predicted future conditions). The soil seed bank could be depleted if plowing coincides with photostimulation of seeds. Fields should be prepared after dispersal but before seeds are photostimulated and harvesting completed before seed germination in early September. Because seeds are highly photostimulated in late summer, disturbance from harvesting must be low to prevent burial. Cultivation of soybean, particularly for forage, is better matched to the seed biology and life cycle of Paysonia than that of corn under current and predicted climates.
Key Words: adaptive management strategy • Lesquerella • photoecology • precipitation change • seed dormancy • seed germination • seedling emergence • soil seed bank
Disturbance plays a key role in the maintenance of habitat for many plant species, particularly ruderals, for example, fire ephemerals, desert annuals, and arable weeds (Grime, 2001
). Some plants share many of the features of ruderals such as an annual life cycle, but are considered rare because their distribution is geographically narrow. In frequently disturbed habitats such as fields, roadsides, and pastures, the ruderal community may consist of rare species and geographically widespread, common ones (weeds) (e.g., USFWS, 2005
). Now, management of rare plants often involves anthropogenic forms of disturbance, e.g., mowing to maintain the early successional state of the habitat (Walck et al., 1999
). Populations of rare and common annuals are maintained via disturbance by removing competing vegetation and bringing seeds to the soil surface to facilitate germination.
A primary reason nondormant seeds of most species do not germinate while buried in soil is that they require light for germination (Baskin and Baskin, 1989
). However, the timing for the fulfillment of the light requirement plays a critical role in determining the germinability of seeds in darkness and the formation of a soil seed bank, which is advantageous in communities of annual plants (Fenner and Thompson, 2005
). If seeds are exposed to light, whether sunlight or leaf-filtered light, during the dormancy-breaking period, they might "remember" this light and germinate in darkness when otherwise they would not do so if they had not received light (Baskin and Baskin, 1971; Pons, 1984
; Walck et al., 1997
). Seedlings from most seeds buried beyond a few centimeters in soil would fail to reach the soil's surface and die. This failure would be a serious concern if the timing of an anthropogenic disturbance for managing populations of a rare plant coincided when its seeds were photostimulated, potentially resulting in the unintentional depletion of the soil seed bank.
Temperature and precipitation are critical factors influencing dormancy break and germination (Baskin and Baskin, 1998
). These two variables are likely to be altered as a result of global climatic change. Knowing the impact of changes for these parameters on specific attributes of a species is central to designing any adaptive management strategy (Hulme, 2005
). For example, the Canadian climatic model predicts a high degree of warming (5.5°C) and a decrease in precipitation (20%) by 2100 in the southeastern United States, whereas the Hadley model simulates less warming (2.3°C) but an increase in precipitation (20%) (National Assessment Synthesis Team, 2000
; Burkett et al., 2001
; Smith et al., 2005
). Likewise, annual rainfall trends in several areas of the Southeast have shown a very strong increase of ca. 25% over the past 100 years. Predicted increases in precipitation will occur primarily in summer and autumn. Moreover, shifts in precipitation may have a greater impact on ecosystems than rising CO2 levels and/or temperatures (Weltzin et al., 2003
). Therefore, plant species likely to be impacted by climatic change are those that produce seeds requiring warm (summer) temperatures to overcome dormancy and germinating in autumn, such as winter annuals.
Successful conservation strategies entail understanding the physiological responses of organisms to present and future environments (Wikelski and Cooke, 2005
), and these need to be set within a management framework. Our work focused on the genus Paysonia (Brassicaceae), recently segregated from Lesquerella. It contains 10 species in southeastern (five) or southwestern (five) North America (Rollins, 1993
; O'Kane and Al-Shehbaz, 2002
). All southeastern taxa are winter annuals and considered rare throughout their ranges. In presettlement times, these plants probably grew on floodplains where flooding prevented the formation of a closed canopy of trees, shrubs, and/or herbaceous plants or in rocky glade-like habitats. After settlement, they spread into anthropogenically disturbed habitats. Today, they occur in large numbers in fields on floodplains in which cotton, soybeans, corn, millet, or winter wheat are cultivated (Rollins, 1955
; Webb and Kral, 1986
; Baskin and Baskin, 1990
, 2000
; Baskin et al., 1992
; USFWS, 1996
, 2005
; Tennessee Division of Natural Areas, unpublished data).
Cultivation of annual crops is recommended as an artificial way to maintain the habitat for Paysonia. No-till farming is thought to adversely affect Paysonia species due to the lack of soil disturbance and the extensive use of herbicides (USFWS, 1996
, 2005
; Shea, 2001
). Management advice was based on the aboveground life cycle of plants: field disturbance associated with farming should take place after seed maturation and dispersal in late April and May and before seed germination in September and October. Great fluctuations in populations can occur with thousands of plants present in some years followed by few or no plants in subsequent years, underscoring the dependence on soil seed banks, in which seeds remain viable for at least 4–7 years. Moreover, populations respond positively to disturbance, and as such, plowing has become an important factor in the ecology of these species, but ill-timed plowing has been noted as a serious threat (Baskin and Baskin, 1990
, 2000
; Baskin et al., 1992
; Rollins, 1993
; USFWS, 1996
).
To our knowledge, this is the first study to address how predicted changes in future precipitation regimes (sensu Hadley model) will influence seed ecophysiological responses in relation to management decisions. The primary objective of our research was to investigate the impact of two moisture regimens that simulated current and future climatic conditions on dormancy break and germination on two species of Paysonia. A series of white light exposures were given to determine the timing for the fulfillment of the light requirement under the moisture regimens. In addition, the effects of leaf-filtered light on germination and burial depth on seedling emergence were examined. Last, a conceptual model was developed to correlate our results with the planting and harvesting regimens of crops recommended to be planted with both species.
MATERIALS AND METHODS
Study species and plant material
Paysonia perforata (Rollins) O'Kane & Al-Shehbaz occurs along Spring, Bartons, and Cedar creeks in Wilson County, Tennessee (USA), and P. stonensis (Rollins) O'Kane & Al-Shehbaz along the East and West forks of Stones River in Rutherford County, Tennessee (Rollins, 1993
; Shea, 2001
; USFWS, 2005
). Paysonia perforata is federally endangered, and P. stonensis was once a candidate; both taxa are endangered in Tennessee (USFWS, 1999
, 2005
; Bailey, 2004
).
Fresh seeds of P. perforata were collected from the same population in Wilson County, Tennessee on 4 May 2002, 27 April 2004, and 2 May 2005. In Rutherford County, Tennessee, P. stonensis seeds were collected on 4 May 2002 and 2 May 2005 from the same population and on 30 April 2004 from a site ca. 7 km away. Seeds were stored dry at room temperature between collection and the start of experiments.
General germination procedures
Germination tests were conducted in three light- and temperature-controlled incubators set at 12/12 h alternating temperatures of 25/15, 30/15, and 35/20°C. These thermoperiods represent the mean daily maximum and minimum air temperatures in central Tennessee: May and October, 25/15°C; June and September, 30/15°C; and July and August, 35/20°C (NOAA, 2002
). Optimum temperatures for dormancy break of Paysonia seeds are 25/15–35/20°C and germination are 25/15–30/15°C (Baskin and Baskin, 1990
, 2000
; Baskin et al., 1992
). A 14-h photoperiod extended 1 h before the beginning to 1 h after the ending of the daily high temperature period. The white light source was 20 W cool white fluorescent tubes with a photon flux density (400–700 nm) at seed level from 48 (top) to 72 (bottom shelf) µmol · m–2·s–1.
Unless otherwise indicated, seeds were placed in plastic petri dishes (6 cm diam.) on sand (2002 experiments) or limestone-derived topsoil (2004–2005 experiments) moistened to saturation with 5 or 8 mL of distilled water, respectively (also see Fitch et al., in press
). The topsoil was obtained from an agricultural field in central Tennessee and is similar to soil on which both species naturally grow. It was sifted to remove debris and reduce particle size to 1.4 mm. Three replicates of dishes containing 25 or 50 seeds each were used for each test condition. Sample sizes depended on availability varying among years and differing only between the two species in 2002. Dishes were wrapped in plastic film to reduce water loss, and those containing seeds incubated in darkness were wrapped additionally with two layers of aluminum foil. Seeds incubated in darkness were not checked for germination until the end of the dark treatment. The criterion for germination was the emergence of the radicle. Germinated seeds were counted and removed from dishes in the light treatments every 2 wk, and distilled water was added to the dishes as needed. Viability of nongerminated seeds was determined by pinching them with forceps under a dissecting microscope to see if they contained firm, white (viable) embryos or soft, light brown (nonviable) ones. Tetrazolium tests confirmed that white embryos were viable and that brown ones were not.
Laboratory light experiments
Seeds of both species collected in 2002 were incubated on a continuously moist substrate starting on 13 May 2002 (N = 25, P. perforata; N = 50, P. stonensis). They were exposed to white light treatments during a simulated sequence of natural seasonal temperature regimes: May (4 wk at 25/15°C) 
June (4 wk, 30/15°C) July (4 wk, 35/20°C) August (4 wk, 35/20°C). Seeds were given either 0 to 16 wk of light followed by 16 to 0 wk of darkness (L D) or 0 to 16 wk of darkness followed by 16 to 0 wk of light (D L). At 2-wk intervals, seeds exposed initially to light were wrapped with aluminum foil and kept in darkness for the remainder of the 16 wk, and those incubated initially in darkness were unwrapped and kept in light for the remainder of the 16 wk. At the end of the August regimen, all seeds were incubated in darkness for 2 wk at a September regimen (30/15°C). One set of controls was kept continuously in light during the temperature sequences (May September) and a second set in darkness.
Experiments on 2004 and 2005 seeds of both species started on 10 May 2004 and 25 May 2005, respectively (N = 50 for both species in 2004; N = 25 for both species in 2005). Temperature and light regimens were the same as those described for the 2002 protocol, with three exceptions: (1) the L D and D L treatments were tested in 2004 but only L D was tested in 2005, (2) an October regimen (25/15°C) was used at the end of the 16-wk incubation, and (3) two moisture treatments were applied. The temperature and light regimens were set up in duplicate: substrate and seeds of one set were kept continuously moist throughout the 18-wk experiment (future climate) and those of a second set were alternately wet and dry (current climate). At the beginning of the experiment, the substrate was moistened with water and then allowed to air dry for 13 d until the end of the 2-wk incubation. Thereafter, they were exposed to a 1 d wet/13 d dry cycle. Petri dish lids and plastic wrap were not used during the dry phase of the wet/dry treatment, but were used during the wet phase. Seeds were fully imbibed within 2–4 h after wetting, and the substrate dried to its original mass within 24 h. The substrate in all dishes was moistened and lids and plastic wrap restored before seeds were placed into darkness. A light control was kept continuously moist in the October regimen.
Germination percentages were determined based on the number of viable seeds and were arcsine square-root transformed for statistical tests. Means were compared by three- (2002 and 2005 studies) or four-way (2004 study), completely randomized analyses of variances (ANOVAs) followed by protected least significant difference tests (PLSDs, P = 0.05) or t tests (SPSS, 2003
). The fixed factors light regimens, time exposures, and/or moisture treatments were examined, but species was not included because treatment effects were of primary concern. In the L D treatments, germination percentages were relativized by the following formula:
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Field light experiment
Six plots were established on 25 May 2004 in a 10-ha field of Rutherford County, Tennessee. This field was sprayed with glyphosate about 3 May 2004, and millet [Setaria italica (L.) P. Beauv.] and soybean [Glycine max (L.) Merr. ‘Laredo'] were planted for silage/haylage about 10 May 2004. During the 2004 growing season, soybean and Johnson grass [Sorghum halepense (L.) Pers.] were the dominant plants in the field. When the plots were established, maximum heights of soybean and Johnson grass were ca. 12 and 30 cm, respectively, and the ground mostly received full sun. The field was a dense stand of plants with 40 and 130 cm maximum heights for soybean and 90 and 250 cm for Johnson grass on 10 June and 3 July 2004, respectively. Crops were harvested on 12 July 2004. After this date, the field was devoid of vegetation and the soil exposed to full sun.
Two treatments with three plots each were established: vegetation was removed from an approximately 2 x 2 m area, or vegetation was left intact until the field was harvested. The plots were located 3 m apart in two rows with treatments alternating with each other. Fifty 2004 seeds of both species were placed in each of 72 fine-mesh nylon bags. The bags were placed either on the soil surface underneath a metal screen in the field plots or buried 7 cm deep in pots filled with topsoil. Bags were placed randomly in the middle of plots, and those in the vegetation intact plots were between rows of soybean (ca. 30 cm apart). Pots were placed together in a random pattern in a yard in Murfreesboro, Tennessee where they received full sun and direct precipitation.
Seeds in bags were exposed to various light–dark treatments in the field, with those in the vegetation-intact treatment receiving leaf-filtered light from approximately 25 May 2004 until 12 July 2004 and then sunlight for the remainder of the experiment. Those in the vegetation-removed treatment received sunlight throughout the entire study. Seeds in both treatments were given 0 to 12 wk of light followed by 12 to 0 wk of darkness (L D) or 0 to 12 wk of darkness followed by 12 to 0 wk of light (D L). At 3-wk intervals (14 June, 5 July, 26 July, 17 August 2004), seeds exposed initially to light (leaf-filtered or sunlight) were buried in soil and kept in darkness for the remainder of the 12-wk period. Seeds initially buried in soil were exhumed, placed on the soil in the plots, covered with a metal screen, and kept in light for the remainder of the 12-wk period. The burial and exhumation of seed bags took place in the study field. Bags of seeds in the vegetation-intact treatment were placed into or exhumed from pots on 14 June 2004 and 5 July 2004 underneath the vegetation, i.e., seeds were not exposed to direct sunlight during the process of burial or exhumation. Before bags of seeds were buried or exhumed, they were examined for germinated seeds, and if any were present, the number was recorded. During the untimely harvesting of the field, several bags in the vegetation-removed treatment plots were destroyed.
At the end of the 12-wk period (17 August 2004), all seeds were buried in pots of soil until 5 September 2004 when they were exhumed and examined for germination. Between 17 August 2004 and 5 September 2004, the soil in pots was watered daily to provide optimal conditions for germination. One set of controls was kept continuously in light in the field until 17 August 2004 and then in the yard until 5 September 2004. A second set of controls was kept in darkness for the duration of the study. The 5 September 2004 was chosen as the termination date because many of the seeds in the light control had germinated by this date. Three replications (bags) were used per treatment.
Air temperatures and precipitation were recorded at a Murfreesboro, Tennessee weather station, ca. 2 and 7 km from the field and yard, respectively. From 1 June 2004 to 31 August 2004, mean maximum and minimum daily temperatures were 29 and 18°C, respectively, and total precipitation was 33.7 cm (Fig. 1). The 30-year normal maximum and minimum temperatures for the same period recorded in Nashville, Tennessee (located ca. 42 km from Murfreesboro) are 31 and 20°C, respectively, and total precipitation is 28.3 cm (NOAA, 2002
).
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Simulated leaf-canopy shade experiment
Three sets of seeds (N = 50 for both species) collected in 2004 in different dormancy break conditions were incubated under red, far-red, and white lights and in darkness: fresh seeds (after 1 mo of dry storage at room temperatures), dry-stored seeds (after 6 mo of dry storage), and moist-stored seeds. Fresh seeds were incubated on moist topsoil, and dry-stored seeds on moist Whatman (Florham Park, New Jersey, USA), No. 1 filter paper, at 25/15°C for 2 wk under each light regimen. Moist-stored seeds were placed on wet topsoil starting 1 mo after collection, and they were incubated in darkness at the following sequence of temperatures: May (4 wk, 25/15°C) June (4 wk, 30/15°C) July (4 wk, 35/20°C) August (4 wk, 35/20°C). After August, seeds were incubated under red, far-red, and white lights and in darkness for 2 wk at October (25/15°C) temperatures.
Seeds exposed to white, red, or far-red light were placed into plastic (20 x 33 x 38 cm) containers that blocked light from the sides and bottom. Containers with seeds exposed to red or far-red light were covered on top with light filters. Red light was produced by using one layer of No. 19 Roscolux fire filter (Rosco, Stamford, Connecticut, USA) and far-red light using one layer of No. 83 Roscolux medium blue filter (Rosco) plus one layer of No. 14 Roscolux medium straw filter (Rosco). The ratio of far-red (725–750 nm) to red (650–675) photon irradiance was ca. 0.1 for the red filter and 7.0 for far-red (Hou and Simpson, 1990
). All light treatments were placed on the bottom shelf of the incubator under eight fluorescent white tubes. Seeds of Lactuca sativa L. ‘Grand Rapids' (incubated on sand at 25/15°C for 2 wk) were used as a bioassay; they germinated to (mean ± 1 SE) 100 ± 0, 12 ± 5, 100 ± 0, and 95 ± 1% under red, far-red, and white lights and in darkness, respectively, indicating that our light filters worked effectively. If the seed germination responses in the two Paysonia species are sensitive to far-red light (i.e., plant shading), they should germinate similarly to the bioindicator seeds.
Germination percentages were determined based on number of viable seeds and were arcsine square-root transformed for statistical tests. A two-way ANOVA examined the effects and interaction of type of light and seed condition, and then was followed by PLSDs.
Effect of burial depth on seedling emergence
On 29 October 2004, 100 dry-stored 2004 seeds of both species each were placed into five 10.0-cm-diameter plastic petri dishes on moist filter paper. All dishes were incubated in light at 25/15°C and checked for germinated seeds every other day. Only seeds in which the radicle had emerged ca. 1 mm were used in this experiment. Burial depths tested ranged from 0–4.0 cm, in increments of 0.5 cm. Three replicates of 10 seeds each were used. Plastic containers (59 mL) with holes punched in the bottoms to allow water absorption were first filled with topsoil to the specific depth at which seedlings were placed. Seedlings were transferred from dishes to the containers and planted equally spaced from each other and from the side of the container in a circular pattern. Then the containers were filled with additional topsoil. After seeds were buried (0.5–4.0 cm depth) or planted on the surface (0 cm depth), the containers were placed randomly in a 1.5 cm deep tray in the 25/15°C incubator with a 14-h photoperiod in white light. The containers were watered from the bottom and checked for seedling emergence at the soil surface every 2 wk for 2 mo. Seedling emergence percentages were determined based on number of seeds placed into each container. They were compared by a two-way ANOVA with depth and time as factors and then by PLSDs.
Conceptual model
The aboveground life cycle and photoecology for seed germination of both Paysonia species were related to the planting and harvesting regimens of crops (corn, soybean) recommended to be grown with them. Data were compiled from life cycle (Pearson, 1967
; Baskin and Baskin, 1990
; Shea, 2001
; Fitch, 2004
; USFWS, 2005
; Tennessee Division of Natural Areas, unpublished data; E. Fitch et al., personal observations), photoecology of germination (present study), and cropping regimes (USDA, 1997
; Tennessee Agricultural Statistics Service, 2002
; G. Murphy, T. Redd, Middle Tennessee State University, personal communication; J. Walck and S. Hidayati, personal observations).
RESULTS
Laboratory light experiments
Germination across the timed exposures was significantly different between the light and/or moisture regimens for both species, except where noted (n.s. = not significant): light regime (F1,40 
10.184, P 
0.003), time exposures, and their interaction (F9,40 5.661, P < 0.001) for 2002; light (F1,80 115.672, P < 0.001), time (F9,80 22.000, P < 0.001), moisture (F1,80 = 81.574, P < 0.001 for P. perforata, n.s. for P. stonensis), light x moisture (F1,80 9.988, P 0.002), and all other two- and three-way interactions (F9,80 2.156, P 0.034) for 2004; moisture (n.s. for P. perforata, F1,40 = 17.740, P < 0.001 for P. stonensis), time, and their interaction (F9,40 4.509, P < 0.001) for 2005.
Seeds of P. perforata from 2004 germinated to 61% in darkness with 16 wk of light on an alternating wet/dry substrate and those from 2005 to 73% in darkness with 14 wk of light (Table 1). On a constantly moist substrate, 2002 P. perforata seeds germinated to 100% in darkness if exposed to 12–16 wk of light, but 2004 seeds to 29% with 10 wk and 2005 seeds to 36% with 14 wk. Paysonia stonensis seeds from 2004 germinated to 48% in darkness when given 16 wk of light on an alternating wet/dry substrate and those from 2005 to 84% with 14 wk (Table 1). On continuous moisture, 2002 seeds of P. stonensis germinated to 64% in darkness if exposed to 14 wk of light, but 2004 seeds germinated only to 37% with 14 wk and 2005 seeds to 38% with 10–12 wk. The threshold for fulfilling the light requirement for the nondormant portion of the seed set (relativized data) was estimated to be at 20% for both species (Fig. 2). The light requirement for P. perforata and P. stonensis seeds was fulfilled at 10 wk under alternating wet/dry conditions, whereas it was at 2 and 6 wk, respectively, under continuous moisture.
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47.636, P < 0.001; time and interaction, F5,24 6.708, P < 0.001). In the vegetation-intact treatment, seeds of P. perforata and P. stonensis exposed initially to 0–9 wk of light germinated up to 10% in darkness, whereas those exposed to 12 wk germinated to 57% in light or darkness (Table 3). Seeds of both species exposed initially to 0–9 wk of darkness and then light germinated up to 63% in darkness and those exposed to 12 wk of darkness to 53% in light and 2% in darkness. Although results of the vegetation-removed treatment are limited due to untimely crop harvesting, they are similar to the vegetation intact treatment within light regimens.
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4.838, P 0.009; condition, F2,24 453.758, P < 0.001; interaction, F6,24 8.843, P < 0.001). Fresh seeds of both species germinated to 0–4%, regardless of the light regimen (Table 4). Moist-stored seeds germinated to 1–10% under far-red and in darkness and 20–53% under red and white lights and dry-stored seeds to 69–74% under far-red and 82–96% under red and white lights and in darkness.
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16.954, P < 0.001), but emergence among the various depths was similar over all times (time and time x depth, n.s.); therefore, time data were pooled. Emergence for P. perforata seedlings ranged from 71–89% at depths of 0–2.5 cm, but it was 43% at 3.0 cm and 16–23% at 3.5–4.0 cm (Fig. 3). On the other hand, 60–95% of P. stonensis seedlings emerged from 0–2.0 cm depths, 41% at 2.5 cm, and 0–14% at 3.0–4.0 cm.
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Photoecology of seed germination
Dormancy release in the two Paysonia species was highly dependent on moisture levels: (1) higher percentages of seeds germinated from alternating wet/dry or dry conditions during dormancy break than from a continuously moist one (Tables 1, 2, 4), and (2) the timing for the fulfillment of the light requirement was earlier under continuous moisture (Fig. 2). Loss of dormancy in seeds of other species that need warm temperatures for dormancy break is optimal at low seed moisture contents (Foley, 1994
; Mohamed et al., 1998
) or on alternating wet/dry conditions (Zager et al., 1971
). Foley (1994)
found a negative relationship between seed moisture and temperatures for maximum dormancy release.
The ecological implications of moisture and temperature conditions for overcoming dormancy taking into account climatic change have not been fully considered, particularly for species, like winter annuals, that break dormancy during summer. Under climatic scenarios for the southeastern United States, environmental conditions during dormancy break will undoubtedly influence the population dynamics of both Paysonia species as well as numerous other species, albeit the amount of adaptability remains uncertain. To this end, the size of the soil seed bank for Paysonia could decrease as seeds lose dormancy more readily with increasing temperatures and decreasing precipitation (Canadian model) or it could increase as more seeds remain dormant with increasing temperatures and precipitation (Hadley model).
The fulfillment of the light requirement occurred earliest at 10 wk for seeds of both Paysonia species on alternating wet/dry substrate under laboratory conditions (Table 1, Fig. 2) and 12 wk in the field (Table 3). However, seeds of P. perforata and P. stonensis were photostimulated earliest at 2 and 6 wk, respectively, on a continuous moist substrate. This would indicate that seeds of both species dispersed in early May could lie on the soil surface until approximately mid-July, and if they became buried before this time, seeds would not germinate to high percentages in darkness during September under current climatic conditions, but those that became buried in August would be expected to germinate from moderate to high percentages in darkness. On the other hand, seeds of both species dispersed in early May could remain on the soil surface only until about mid-May (P. perforata) or early to mid-June (P. stonensis) under future climatic conditions. After such time, seeds would be expected to germinate to relatively moderate to high percentages in darkness in September.
Apparently, the light requirement can be fulfilled only when seeds are mostly nondormant, and then they can germinate in darkness. If warm stratification made no difference in the ability of Paysonia seeds to respond to light, then those given light in late spring and at the beginning of summer should respond to it in the same manner as those given light near the end of summer. Seeds of both species given 3–6 wk of light in the field or 2–6 wk of continuous moisture in the laboratory near the end of summer germinated to 8–73% during warm stratification (Tables 2 and 3, see numbers in parentheses), whereas those given the same amount of light in late spring and at the beginning of summer germinated to only 0–11% (Tables 1 and 3, see numbers in parentheses). Baskin and Baskin (1985)
and Walck et al. (1997)
offered a similar explanation concerning the photoecology of the perennials Aster pilosus, Solidago nemoralis, and S. shortii. In these cases, however, cold stratification overcame dormancy but warm stratification did not. One cannot tell whether the germination response of seeds on continuous moisture conditions in the laboratory was similar to that on alternating wet/dry conditions, because seeds never germinated in light during warm stratification.
Germination percentages of seeds from the two Paysonia species varied among the collections (Tables 1, 2). Studies in 2002 were done on sand, whereas those in 2004 and 2005 were on topsoil. Fitch et al. (in press
) showed that substrate can influence germination of seeds from both species. Seeds of P. perforata came from the same population and were collected at about the same time in 2004 and 2005, whereas those of P. stonensis came from different populations. Thus, population effects and/or preconditioning (cf. Fenner and Thompson, 2005
) probably account for the variation in germination responses between years for P. stonensis, but preconditioning alone is responsible for that in P. perforata.
The ability of seeds to detect the red to far-red ratio of light ensures plants like Paysonia taxa that grow in open habitats and are usually shade intolerant, will not germinate under inadequate conditions of low light for growth and reproduction (Smith, 1982
). Seeds of both species stored dry at room temperatures germinated to 69–74% under far-red light as compared to 82–96% under red and white lights or in darkness, and those stored moist at simulated natural temperatures germinated to 4–10% under far-red light, 20–53% under red and white lights, and 1–4% in darkness (Table 4). Thus, the percentage of seeds germinating under far-red light was only slightly lower relative to that under white light. Although the results of the field experiment involving plots with vegetation left intact vs. removed are inconclusive, the percentages of seeds from both species that germinated under leaf-filtered light vs. sunlight (i.e., before the field was prematurely harvested) did not differ (Table 3; t test, P 0.067). Altogether, results from laboratory and field experiments suggest that a gap in the plant canopy is not required for seed germination.
Soil penetration of seedlings
A high percentage of seeds emerged and survived only if seedlings were buried 2.0 cm (P. perforata) or 1.5 cm (P. stonensis) (Fig. 3). Seed size (length x width) does not vary appreciably between the species: 1.5–2.5 x 1.2–2.0 mm and 1.8–2.0 x ca. 1.5 mm, respectively (Rollins, 1955
). Tillage at depths >1.5 cm would severely hinder seedling emergence and establishment and subsequently reproduction in Paysonia populations. However, most disking and tillage takes place at depths of 5–10 cm or greater (up to 30 cm), and corn and soybeans are planted at 2.5–5 cm (Flinchum, 2001a
, b
). The depth of plowing would have little effect when seeds are not photostimulated. On the other hand, any plowing when seeds are highly sensitive to light should be done at the shallowest depth possible to limit the loss of seedlings due to their failure to emerge from burial.
The soil used in our experiments was not compacted, but lightly placed on top of the seedlings. Compaction of our soil probably would have reduced seedling emergence further (Bassett et al., 2005
). Emergence of crop species is diminished by wheel traffic from farm equipment via soil compaction (Radford et al., 2000
; Altuntas et al., 2005
). In contrast, Jurik and Zhang (1999)
found that compaction from wheel traffic did not create a physical impediment to seedling emergence of common weeds but altered the microenvironment in ways that stimulated weed germination and emergence. Similarly, farming might enhance Paysonia seedling establishment as long as seeds are not buried to deeply.
Agroecosystem management
Planting along with field preparation, such as plowing, needs to be done following seed dispersal (after mid-May) but before seeds become photostimulated: by mid-July under current climatic conditions and mid-May to mid-June under future conditions (Fig. 4). Harvesting should take place before seeds germinate in early September. Because seeds are highly photostimulated in August under both climatic scenarios, harvesting and other activities in the field require very low disturbance levels. Fields should remain fallow from September until May, when the aboveground life cycle of the plant is evident.
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). However, the primary planting and harvesting times of corn for grain and forage overlap with seed development and dispersal and with seed germination and seedling establishment, respectively, of Paysonia (Fig. 4). On the other hand, the planting time for soybean comes after Paysonia seed dispersal although it may coincide with the onset of photostimulation for germination in the future. Harvesting soybean for forage can be done before Paysonia seed germination, but that for grain interferes with seedling and juvenile stages of Paysonia unless impacts from combines are minimal. In addition, residue discharged from the rear of combines after harvesting of corn and soybean for grain is deposited in rows on fields. When harvest time for both crops is considered, the residue would hinder plant establishment. In contrast, sparse residue remains after forage harvest. Thus, soybean, especially forage cultivation, is better matched to the ecology of Paysonia than corn. Other summer annual crops, such as sudax (Sorghum bicolor x S. sudanense), would work just as well as long as they are planted and harvested similarly to forage soybean.
Future climatic conditions will necessitate adjustments to current farming practices, e.g., changing to alternative crops or altering planting and harvesting dates (Burkett et al., 2001
). Modifications to these practices may affect management of Paysonia and will need to be taken into account. The conceptual model developed for Paysonia (Fig. 4) gives an efficient means for evaluating future conservation options.
FOOTNOTES
1 The authors thank the Tennessee Division of Natural Areas for allowing access to study sites and seed collection. Funding was provided by the Mary C. Dunn Scholarship from the Biology Department and by the Faculty Research and Creative Activity Grant Program at Middle Tennessee State University. Assistance from M. Cofer, M. Rolig, and M. Imboden and information on agriculture in middle Tennessee from G. Murphy and T. Redd is greatly appreciated. 
2 Author for correspondence (e-mail: jwalck{at}mtsu.edu
)
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