Suitable Seedbeds Research Articles

Predicting soil workability and fragmentation in tillage: a review

AbstractSoil workability and friability are required parameters to consider when creating suitable seedbeds for crop establishment and growth. Knowledge of soil workability is important for scheduling tillage operations and for reducing the risk of tillage‐induced structural degradation of soils. A reliable evaluation of soil workability implies a distinctive definition of the critical water content (wet and dry limits) for tillage. In this review, we provide a comprehensive assessment of the methods for determining soil workability, and the effects of soil properties and tillage systems on soil workability and fragmentation. The strengths and limitations of the different methods for evaluating the water content for soil workability, such as the plastic limit, soil water retention curve (SWRC), standard Proctor compaction test, field assessment, moisture‐pressure‐volume diagram, air permeability and drop‐shatter tests are discussed. Our review reveals that there is limited information on the dry limit and the range of water content for soil workability for different textured soils. We identify the need for further research to evaluate soil workability on undisturbed soils using a combination of SWRC and the drop‐shatter tests or tensile strength; (i) to quantify the effects of soil texture, organic matter and compaction on soil workability; and (ii) to compare soil water content for workability in the field with theoretical soil workability, thereby improving the prediction of soil workability as part of a decision support system for tillage operations.

Spatial patterns with memory: tree regeneration after stand‐replacing disturbance in Picea abies mountain forests

AbstractQuestionsWhat are the spatial patterns of adult trees and recruits in natural Picea abies forests? How are these patterns related to each other? Does the relationship differ before and after stand‐replacing disturbance? What are the ecological processes behind these spatial patterns?LocationMountain Picea abies forests in the Šumava Mts. (Bohemian Forest), Czech Republic, affected by a high‐severity outbreak of bark beetle (Ips typographus) that caused large‐scale dieback of the forest canopy.MethodsWe measured the spatial coordinates and heights of all recruits and the coordinates and DBH of all adult trees in nine plots across a wide range of recruit densities. We distinguished pre‐ and post‐disturbance recruits, trees killed by the disturbance and trees already dead before it. To analyse spatial relationship among these groups, we used univariate and bivariate pair‐correlation functions. To provide further insight into the mechanisms behind the observed patterns, we fitted Thomas and Matérn point processes to the observed data.ResultsRecruits formed tight clusters (2–9 m), whereas trees were distributed randomly or weakly clustered at short distances (1–2 m). Both pre‐disturbance and post‐disturbance recruits were highly clustered (cluster radii < 2 m) around trees before and after the stand–replacing disturbance. This fine‐scale pattern was likely driven by a combination of: (1) seed accumulation in tree wells during winter; (2) nurse effects of tree trunks extending the vegetation period, suppressing competitive vegetation and enhancing nutrient supply from decomposed litter; and (3) suitable seedbeds on some decaying wood. The Thomas point process fitted the observed pattern of decreasing recruit density with increasing distance from mature trees better than the Matérn process.ConclusionsTree spatial pattern in mountain P. abies forests showed high resilience to stand‐replacing disturbance. After a self‐thinning of recruits tightly clustered around parental trees, their spatial pattern will mirror the pattern of trees that formed the stand before the disturbance. This ‘memory’ of tree spatial patterns is an important biological legacy and should be viewed as a fundamental property of natural P. abies forests.

The importance of ecological constraints on the control of multi‐species treeline dynamics in eastern Nunavik, Québec

Treelines are temperature-sensitive ecotones that should be able to expand in response to global warming; however, they are also controlled by ecological constraints. These constraints can create bottlenecks for tree regeneration, hindering treeline advances. Near Kangiqsualujjuaq (Nunavik, subarctic Québec), previous studies suggested successful recruitment of Larix laricina above the altitudinal treeline, while Picea mariana establishment remains scarce. We studied regeneration of both species to identify factors responsible for such contrasting responses. • We measured seeds and wings to evaluate species dispersal potential. We compared seed viability and tolerance to shrub leachates with germination trials. To evaluate seedbed preferences, we compared seedling occurrence on the different seedbeds with seedbed relative abundance in the field. • Seed germination was similar between L. laricina and P. mariana, whereas dispersal potential was higher for the latter. Germination of P. mariana seeds was more strongly inhibited by shrub leachates than were L. laricina seeds. In the field, we found only a few Picea seedlings, but numerous seedlings of Larix had established disproportionally on several seedbeds. While Betula glandulosa, mosses, and Vaccinium uliginosim impeded Larix establishment, numerous seedlings were found on lichens, mineral soil, and liverworts. The low occurrence of suitable seedbeds for Picea, mainly mineral soil, could explain the seedling scarcity of this species. • This study highlighted that allelopathy and unsuitable seedbeds could contribute to regeneration failure of P. mariana in eastern Nunavik and emphasizes the need to consider ecological preferences of species before predicting treeline expansion under a warmer climate.

香川県直島の山火事跡再生二次林における野生ツツジ群落の保全

We performed trimming shrubs and sweeping up litters in a post-fire secondary forest to develop the skill for conserving the regenerated scrub forest using native Rhododendrons, not depending on forest fires. In results, we found several initial effects regarding preserving native Rhododendron communities and promoting the flower-buds differentiation. It is necessary to protect native Rhododendrons from being suppressed by other shrubs because most of species growing on this site had traits of vigorous sprouting. Especially, Eurya japonica that grows in higher density and Ligustrum lucidum, Robinia pseudo-acacia and Lespedeza cyrtobotrya that sprout longer branches need to be controlled.The effect of shrub trimming on seedling establishment was not clear at the next year after the shrub trimming. However, the effect of sweeping up litters will be expected by making adding efforts like keeping existing seedlings away from damages by sweeping, and sowing new seeds on suitable seedbeds after sweeping. The effect on flowering rates in the next spring after the shrub trimming was not observed because the timing of trimming was too late to stimulate the flower buds differentiation.

Low input tillage/cropping systems for limited resource areas

Agriculture in limited resource areas is characterized by small farms which are generally too small to adequately support the needs of an average farm family. The farming operation can be described as a low input cropping system with the main energy source being manual labor, draught animals and in some areas hand tractors. These farming systems are the most important contributor to the national economy of many developing countries. The role of tillage is similar in dryland agricultural systems in both the high input (HICS) and low input cropping systems (LICS), however, wet cultivation or puddling is unique to lowland rice-based systems in low input cropping systems. Evidence suggest that tillage may result in marginal increases in crop yield in the short term, however, in the longer term it may be neutral or give rise to yield decreases associated with soil structural degradation. On marginal soils, tillage may be required to prepare suitable seedbeds or to release adequate Nitrogen through mineralization, but in the longer term, however, tillage reduces soil organic matter content, increases soil erodibility and the emission of greenhouse gases. Tillage in low input cropping systems involves a very large proportion of the population and any changes in current practices such as increased mechanization will have a large social impact such as increased unemployment and increasing feminization of poverty, as mechanization may actually reduce jobs for women. Rapid mechanization is likely to result in failures, but slower change, accompanied by measures to provide alternative rural employment, might be beneficial. Agriculture in limited resource areas must produce the food and fiber needs of their community, and its future depends on the development of sustainable tillage/cropping systems that are suitable for the soil and climatic conditions. These should be based on sound biophysical principles and meet the needs of and be acceptable to the farming communities. Some of the principle requirements for a sustainable system includes the maintenance of soil health, an increase in the rain water use efficiency of the system, increased use of fertilizer and the prevention of erosion. The maintenance of crop residues on the surface is paramount for meeting these requirements, and the competing use of crop residues must be met from other sources. These requirements can be met within a zonal tillage system combined with suitable agroforestry, which will reduce the need for crop residues. It is, however, essential that farmers participate in the development of any new technologies to ensure adoption of the new system.

Regeneration silviculture of birch: A review

The birches are most commonly regenerated using even-aged systems, primarily clearcutting. Clearcut sizes range from 16 ha down to 0.4-ha patches. Seed trees or shelterwoods are sometimes used to provide additional seed and site protection. Uneven-aged systems are not recommended, with the exception of group selection for yellow birch. Coppice systems do not provide full stocking except in short-rotation biomass culture, but coppice provides acceptable supplemental regeneration. Natural regeneration by seeding prevails because it is economical and highly reliable where the water balance is favorable. Yellow birch sometimes regenerates in advance, but otherwise the canopy, understory, and seedbed must be manipulated to meet germination and seedling-establishment requirements. Suitable seedbeds are prepared by burning, by mechanical site-preparation equipment, and sometimes by the logging process itself. Draining often is all that is necessary to regenerate peatlands. Seedlings should be protected from grazing or browsing. Relatively few stands are regenerated artificially. Direct seeding (either broadcast- or spot-sowing) is usually reliable. Planting is more common and gives the opportunity to introduce exotic or genetically improved stock. In species trials, silver birch has consistently grown faster - and yellow birch slower - than eeitherhairy birch or paper birch. Phenotypically selected natural stands and controlled pollination produce progeny that grow in volume from 30 to 80% faster than average. Thorough site preparation, fertilization, mulching, weed control, and protection from animals benefit seedling growth. Both bare-root and container-grown stock are planted successfully. Birch is highly regarded for afforesting mine spoils, drained or worked-out peatlands, abandoned and abused agricultural lands, heaths, previously flooded lands, and areas with high air pollution. Fertilizers are often applied to improve site quality. Birch nurse crops benefit conifers by protecting them from frost, insects, and pathogens; by improving the soil through nutrient cycling; and by increasing stand wind-firmness on shallow soils. The optimum proportion of birch in mixed stands is 17–25%. Overly dense birch should be controlled with chemicals, by hand, or with machinery to allow optimum conifer development.

Ash Yellows in Zion National Park: Impact, Idenity of Pathogen Mode of Spread, and Prospects for Management

Many velvet ash (Fraxinus velutina) in Zion Canyon have declined in vigor, and some are dying. This species is aesthetically and ecologically important in the canyon, because it is one of only three species that commonly grow to large size on the canyon floor. None of the three species is reproducing abundantly, because suitable seedbeds are scarce and young plants are severely browsed by deer. A disease known as ash yellows, which is caused by unnamed mycoplasmalike organisms (wall-less, obligately parasitic prokaryotic organisms, commonly called MLOs) was found in Zion Canyon in 1988, and is suspected to cause or contribute to the decline of velvet ash there. Until this project began, nothing was known about the ecology or epidemiology of ash yellows in the West.

POPULATION PERSISTENCE AND REPRODUCTIVE ECOLOGY OF A FOREST HERB: ASTER ACUMINATUS

Aster acuminatus Michaux, a patchily distributed herb of eastern forests, is commonly represented in all stages of forest succession. To determine its mechanisms of persistence, we carefully examined the distribution patterns of this species as affected by its reproductive ecology and demography. Using A. acuminatus as a model, we hypothesized that 1) availability of propagules and location of adequate recruitment sites impose first‐order control over plant distributions in stands of all ages by determining where plants might be found and 2) light and other resources exert a second‐order control over herb distributions by determining where established plants cannot persist. Results from our study at the Hubbard Brook Experimental Forest in New Hampshire demonstrated this species' ability to exploit resource‐rich disturbances through immediate and profuse ramet and seed production. This ensured rapid colonization of new sites and provided genetic exchange for longer‐term survival. Relict colonies which survived the disturbance served as the sole propagule‐producing loci for the first year of colonization. After several years, resources became more limiting and the aster shifted to a more conservative asexual reproduction strategy which provided population stability until the next disturbance. The clumped distribution of A. acuminatus in mature forests is explained by strict adherence to asexual ramet replacement in which there is no year‐to‐year change in aster importance. Location of persistent colonies probably is dictated by location of suitable seedbeds during the first few years of recovery after disturbance. Whereas numerous sites may be colonized initially, timing of colonization and quality of site impose constraints on which colonies persist.

Population Persistence and Reproductive Ecology of a Forest Herb: Aster acuminatus

Aster acuminatus Michaux, a patchily distributed herb of eastern forests, is commonly represented in all stages of forest succession. To determine its mechanisms of persistence, we carefully examined the distribution patterns of this species as affected by its reproductive ecology and demography. Using A. acuminatus as a model, we hypothesized that 1) availability of propagules and location of adequate recruitment sites impose first-order control over plant distributions in stands of all ages by determining where plants might be found and 2) light and other resources exert a second-order control over herb distributions by determining where established plants cannot persist. Results from our study at the Hubbard Brook Experimental Forest in New Hampshire demonstrated this species' ability to exploit resource-rich disturbances through immediate and profuse ramet and seed production. This ensured rapid colonization of new sites and provided genetic exchange for longer-term survival. Relict colonies which survived the disturbance served as the sole propagule-producing loci for the first year of colonization. After several years, resources became more limiting and the aster shifted to a more conservative asexual reproduction strategy which provided population stability until the next disturbance. The clumped distribution of A. acuminatus in mature forests is explained by strict adherence to asexual ramet replacement in which there is no year-to-year change in aster importance. Location of persistent colonies probably is dictated by location of suitable seedbeds during the first few years of recovery after disturbance. Whereas numerous sites may be colonized initially, timing of colonization and quality of site impose constraints on which colonies persist.

Caching and Recovery of Food by the Western Fox Squirrel

Caching of seeds by rodents for future food supplies is recognized by wildlife managers, foresters, nurserymen, and others to be important in plant reproduction. If the entire crop is consumed, reproduction is prevented but if the crop is stored though not all used by the rodent harvesters, plant reproduction may be encouraged by the seed being protected through the winter from normal exposure to severe climatic conditions (Shaw, 1936). Olmsted (1937) has pointed out that, in Connecticut, black oak (Quercus velutina) invades many communities through burial of its acorns by More knowledge is needed of the caching habit and its effect on forest and other vegetation. Of the many forest-dwelling rodents, probably none is of greater significance to their environment than the larger tree squirrels. Red squirrels and most small ground-dwelling rodents gather many seeds into relatively few hoards, but the larger squirrels make many caches, each of one or a few nuts. This wide scattering and planting, if in suitable seed-beds, may at times be a desirable factor in perpetuating the forest. These squirrels, however, need large quantities of food to sustain them, especially as they do not hibernate through the long winters. They depend heavily on the fruit of commercially important hardwood species such as oaks, hickories, walnuts, beech and formerly, the American chestnut. The rapidity with which these nut crops are cleaned up from the forest floor is almost phenome'nal. Hahn (1908) describes the disappearance of a very large acorn crop on the Farm of the University of Indiana in the autumn of 1906, when gray squirrels were also very abundant. He estimated that each of the large white oaks produced from two to eight thousand acorns during that season. Eighty acres [were] heavily wooded with white oaks and nearly a hundred acres more [had] a considerable growth of these trees. Before November 1, the immense crop of acorns had been so completely garnered by the squirrels that none were in sight on top of the leaves and only an occasional one could be found bythe most careful search. In the long run, the fate of the seed crops controls that of the forest. In the fall of 1928 I studied the life history of the western fox squirrel (Sciurus niger rufiventer), a major objective being to determine, if possible, the effect of this species on forest reproduction. This investigation was a part of the program of a junior instructorship which continued through the following spring under the School of Forestry and Conservation, University of Michigan. I am grateful to Dean S. T. Dana and Alvin G. Whitney for arranging the financial assistance that made possible this year of study and field work and to Professors Whitney, H. M. Wight and L. R. Dice for advice and suggestions on this project. Two studies on caching and recovery of nuts to determine the percentage and method of removal from hiding places, were made in Forest Hills Cemetery in Ann Arbor. The squirrels were accus-