Spaced Seeds Research Articles

Ecological succession and the rehabilitation of disturbed terrestrial ecosystems

Soil and vegetation development on surface-mined coal sites in a mixed grass prairie region were studied as (a) naturally revegetated chronosequences of 1, 7, 17, 30 and 45-year old sites, and (b) on sites that were regraded, received topsoil, and were fertilized and seeded. In both cases, vegetation and soils of adjacent, relatively undisturbed areas were also studied for comparison. Species richness was lowest (26 species) at the 1-year old site and highest (114 species) at unmined sites; species richness increased youngest to the oldest sites. Stand ordinations revealed that the site age was the most important factor influencing species richness and abundance. On the younger sites, the environment was characterized by unleached parent materials high in electrical conductivity, SO4, Mg, Ca, and Na and plants growing there were non-native, annual pioneers. At about 30 years, the edaphic conditions had moderated somewhat, and the species complex had a greater number of native species. However, even after 45 years, there were clear differences among mined and unmined sites both in species composition and edaphic conditions. Rates of nutrient accumulations over the 45-year time gradient were as follows: organic C 131 kg ha−1 yr−1, N 25 kg ha−1 yr−1, P 0.1 kg ha−1 yr−1, and K 4.9 kg ha−1 yr−1. C/N ratios showed the widest range for 1-year old sites (5–40), but 70% of the 45-year old sites showed values below 15, comparable to unmined sites. Distributional diagrams of plant species revealed their topographic and habitat preferences which may be helpful in formulating species prescriptions for revegetation. Stand-environmental complex ordinations with 53 variables in the matrix showed topographic variables to be the most important, followed by site age. This study attempted to evaluate simultaneously two major environmental factors—time and topography, as these influence the development of soils and vegetation concomitantly. The use of canonical correspondence analysis facilitated the resolution of these two major environmental factors in illustrating the relationships between the species and the environments in which they grow. Simulations with the CENTURY model of C and N dynamics on north- and south-facing slopes provided useful insights into the recovery time of mined systems. All carbon pools (except passive) were larger on the north-facing slopes than on the south, as was the accumulation of organic N. Rehabilitation of mined areas is now required by law and regulations applicable in the region under discussion mandate the following steps: regrading of ridges to blend in with the surrounding topography, spreading topsoil, fertilizing and seeding. The course of vegetation and soil development of such sites was studied intensively for four years and revealed that the first year vegetation, irrespective of desirable species seeded, was dominated by the pioneer forb, Kochia scoparia. Its replacement by seeded grasses was quick. The rapid replacement of Kochia by grasses appears to be related to the production of allelochemics by Kochia and its acting as a nurse crop in the first year. Although allelochemics may be produced by Kochia as a defense mechanism for herbivory, the compounds seem to produce ionic imbalances (P, Mn, Zn) which hasten its own demise through autotoxicity. Like the abandoned mined sites, dispersal of seeds in space (immigration) seemed to be more important than from seeds in the topsoil (seed banks- dispersal in time). But in contrast to abandoned mined sites, the managed sites showed (a) a very rapid replacement of pioneers, (b) three to five times faster rates of leaching of ions capable of diminishing plant growth, (c) 5–8 times faster rates of mineralization of ions that promote growth, and (d) successional trends followed the facilitation and auto-inhibition pathways. A comparison of abandoned and managed systems clearly shows that by following the legal mandates, the rehabilitation process can be achieved many decades sooner with human assistance.

Induced dormancy and colour polymorphism in seeds of the bull thistleCirsium vulgare(Savi) Ten.

AbstractTo colonize newly disturbed areas, weedy species must disperse their seeds in space and/or in time.Cirsium vulgarehas poor dispersal in space, but can form a persistent seed bank. Fresh seeds are capable of germinating in either light/dark or constant dark conditions under favourable diurnal temperatures of 25°C: 10°C. Despite this lack of dormancy, a large proportion of seeds do not germinate in the autumn after dispersal. Variation in germination requirements according to seed colour was also examined. Fresh seeds were placed under one of two overwintering treatments at 5°C, alternating light/dark or constant darkness. Some seeds of all colours: white, light, intermediate, dark and black germinated under these conditions. Six months later, ungerminated seeds subjected to the light/dark treatment did not require light for germination when placed under optimal temperatures. However, most seeds that had overwintered in constant darkness required light for germination. In the field, this induced dormancy would prevent seeds from germinating if buried or located in deep shade. Such seeds have the potential of forming a persistent seed bank. The proportion of dormant seeds did not appear to be associated with seed colour. Seeds incubated at 10°C following overwintering under light/dark conditions germinated to a higher percentage than seeds overwintered in darkness, regardless of the light conditions for germination. The results of this study help to explain the contradiction between delayed germination in the field and the lack of seed dormancy.

Absorbed dose measurements on LDEF and comparisons with predictions

The radiation environment on LDEF was monitored by cumulative absorbed dose measurements made with TLDs at different locations and shielding depths. The TLDs were included in four experiments: A0015(a) Biostack, P0004 Seeds in Space and P0006 Linear Energy Transfer Spectrum Measurements at the trailing edge (west side) of the satellite; M0004 Fiber Optics Data Link at the leading edge (east side); and A0015(b) Biostack at the Earth side. The shielding depths varied between 0.48 and 15.4 g/cm 2, Al equivalent. Both the directional dependence of trapped protons incident on the satellite and the shielding thickness were reflected in absorbed dose values. The trapped proton anisotropy was measured by TLDs at the east and west sides of LDEF. At the east side doses ranged from 2.10 to 2.58 Gy under shielding of 2.90 to 1.37 g/cm 2 (M0004) while on the west side doses ranged from 2.66 to 6.48 Gy under shielding of 15.4 to 0.48 g/cm 2 (P0006). The west side doses were more than a factor of two higher, where the vertical shielding thicknesses to space were equal. Other west side doses of 3.04 to 4.49 Gy under shielding of 11.7 to 3.85 g/cm 2 (A0015(a)) and 2.91 to 6.64 Gy under shielding of 11.1 to 0.48 g/cm 2 (P0004) generally agreed with the P0006 results. The Earth side doses of 2.41 to 3.93 Gy under shielding of 10.0 to 1.66 g/cm 2 (A0015(b)) were intermediate between the east side and west side doses. Calculations utilizing a model of trapped proton spectra were performed by Watts et al. (1993) and comparisons of dose measurement and calculations may be found in a companion paper (Armstrong et al., 1996).

Germination, growth rates, and electron microscope analysis of tomato seeds flown on the LDEF

The purpose of the experiment was to determine cosmic rays long-term effects on living tissue. A batch of tomato seeds were flown in orbit aboard the Long Duration Exposure Facility (LDEF) for almost 6 y. During this time, the seeds received an abundant exposure to cosmic radiation. Upon the return of the LDEF to Earth, the seeds were distributed throughout the United States and 30 foreign countries for analysis. Our university analysis included germination and growth rates as well as scanning electron microscopy (SEM) and X-ray analysis of the control as well as space exposed tomato seeds. In analyzing the seeds under the electron microscope, usual observations were performed on the nutritional and epidermis layer of the seed. These layers appeared to be more porous in the space exposed seeds than on Earth-based control seeds. This unusual characteristic may explain the increases in the space seeds growth pattern. (Several test results showed that the space-exposed seeds germinated sooner than Earth-based seeds. Also, the space-exposed seeds grew at a faster rate.) The porous nutritional region may allow the seeds to receive necessary nutrients and liquids more readily, thus enabling the plant to grow at a faster rate. Roots, leaves and stems were cut into small sections and mounted. After sputter coating the specimens with argon/gold palladium plasma, they were viewed under the electron microscope. Many micrographs were taken. The X-ray analysis displayed possible identifications of calcium, potassium, chlorine, copper, aluminum, silicon, phosphate, carbon, and sometimes sulfur and iron. The highest concentrations were shown in potassium and calcium. As a result of the electron interaction and X-ray production within the open seeds, the traditional layers of the space-exposed seed gave peaks of Mg, P and S, while the Earth seed gave an iron peak, which was not detected in the space-exposed seed because of electron beam positioning difference. The space-exposed seed and the Earth-control seed specimens displayed high concentrations of copper.

Space-exposed seed experiment developed for students

The SEEDS project was flown in orbit aboard the Long Duration Exposure (LDEF) for nearly six years. During this time in space, the tomato seeds received an enormously abundant supply of cosmic radiation. Upon the return of the LDEF to earth, the SEEDS project was distributed throughout the United States and 30 foreign countries for analysis. The purpose of the experiment was to determine the long term effects of cosmic rays on living tissue. At Morgan, the analysis performed varied from germination and growth rates to electron microscopy and x ray analysis.In analyzing the seeds under the electron microscope, usual observations were performed on the nutritional and epidermic layers of the seed. These layers appeared to be more porous in the space-exposed seeds than in the Earth-based control seeds. This unusual characteristic may explain the increase in the space seeds’ growth pattern. (Several test results show that the space-exposed seeds germinate sooner than the Earth-based seeds. Also, the space exposed seeds are growing at a faster rate.) The porous nutritional region may allow the the seeds to receive necessary nutrients and liquids more readily. Thus, enabling the plant to grow at a much faster rate.