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6 Weather Variability, Ecological Processes, and Optimization of Soil Micro-environment for Rangeland Restoration

Monaco, T.A., Editor CAB International PDF

6

Weather Variability, Ecological

Processes, and Optimization of

Soil Micro-environment for

Rangeland Restoration

Stuart P. Hardegree, Jaepil Cho, and

Jeanne M. Schneider

US Department of Agriculture, Agricultural Research Service, USA

Introduction

Precipitation, solar radiation, wind speed, air temperature, and humidity are principal drivers controlling energy and water flux in plant communities. Climate is defined as the long-term average representation of these variables, and their seasonal pattern.

Rangelands are generally characterized by an arid or semi-arid climate with plant communities dominated by grassland, shrubsteppe, and savanna vegetation. Gross climatic variability generally determines the suitability of both native and introduced plant materials for rangeland restoration and rehabilitation (Shown et al., 1969; Shiflet,

1994; Barbour and Billings, 2000; Vogel et al., 2005; USDA, 2006). Unfortunately, the micro-environmental requirements for germination, emergence, and seedling establishment are much more restrictive than the longer term climatic requirements for maintenance of mature plant communities

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5 Light-mediated Germination

Gallagher, R.S., Editor CAB International PDF

5

Light-mediated Germination

Thijs L. Pons*

Department of Plant Ecophysiology, Institute of Environmental

Biology, Utrecht University, Utrecht, the Netherlands

Introduction

The light response of seeds can control the time and place of germination of a seed, a crucial factor in the survival of the resulting seedlings, and the growth and fitness in subsequent developmental stages. The ultimate effect of light on seeds depends on genotype, and on environmental factors during ripening of the seeds, during dormancy and during germination itself. These environmental factors may include light, or factors other than light such as soil temperatures and soil chemical factors (see Chapter 6 of this volume). The picture is further complicated by the fact that the light climate itself has various aspects that have different effects on seeds, such as irradiance, spectral composition and duration of exposure of the seeds. All the above-mentioned factors can interact in one way or another in their effect on seeds. Moreover, the factors are not constant in time and are difficult to characterize at the seed’s position in the soil, thus complicating further the analysis of what is actually happening with a seed in a natural situation and the interpretation of a possible ecological significance of light responses.

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4 Seed Predators and Plant Population Dynamics

Gallagher, R.S., Editor CAB International PDF

4

Seed Predators and Plant

Population Dynamics

Michael J. Crawley*

Department of Biology, Imperial College London,

Ascot, Berkshire, UK

Introduction

The enormous seed production of most plants, coupled with the general paucity of seedlings and saplings, is vivid testimony to the intensity of seed mortality. The degree to which this mortality results from seed predation (the consumption and killing of seeds by granivorous animals) is the subject of the present review (for earlier references, see Crawley, 2000). To people who are unfamiliar with Darwinist thinking, it appears obvious that seed mortality is so high, because ‘plants need only to leave one surviving offspring in a lifetime’. In fact, of course, every individual plant is struggling to ensure that its own offspring make up as big a fraction as possible of the plants in the next generation, and for each individual plant there is a huge evolutionary gain to be achieved by leaving more surviving seedlings, i.e. more copies of its genes, in the next generation. The mass mortality of seeds is part of natural selection in action, and the group selectionist argument that plants only need to replace themselves is simply wrong. Since the numbers of seeds produced are so large, it only takes a small percentage change in seed mortality to make

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17_Chapter

Dr. B.C. Punmia ; Ashok Kr. Jain, Arun Kr. Jain Laxmi Publications PDF

17

PRECISE

LEVELLING

CHAPTER

17.1 INTRODUCTION

Precise levelling is used for establishing bench marks with great accuracy at widely distributed points. The precise levelling differs from the ordinary levelling in the following points:

(i) High grade levels and stadia rods are used in precise levelling.

(ii) Length of sight is limited to 100 m in length.

(iii) Rod readings are taken against the three horizontal hairs of the diaphragm.

(iv) Backsight and foresight distances are precisely kept equal, the distances being calculated from stadia hair readings.

(v) Two rodmen are employed and backsight and foresight are taken in quick succession.

(vi) The adjustments of the precise level are tested daily and the correction applied to the rod readings. The rod is standardized frequently.

The precise levelling can be classified under the following three heads, depending upon the permissible errors:

K or 0.017 ft

M

Second order : permissible error = 8.4 mm

K or 0.035 ft

M

Third order : permissible error = 12 mm

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ch3-1

Dr. R. K. Bansal Laxmi Publications PDF

3

Properties of Surfaces and Solids

3.1. CENTROIDS AND CENTRE OF MASS

The point at which the total area of a plane figure (like rectangle, square, triangle, quadrilateral, circle etc.) is assumed to be concentrated, is known as the centroid of that area.

The centroid is also represented by C.G. or simply G.

Centre of mass is the point at which the total mass of a body, is assumed to be concentrated. A body is having only one centre of mass for all positions of the body.

3.1.1. Centroid of Simple Plane Figures. (i) The centre of gravity (C.G.) of a uniform rod lies at its middle point.

(ii) The centre of gravity of a triangle lies at the point where the three medians* of the triangle meet.

(iii) The centre of gravity of a rectangle or of a parallelogram is at the point, where its diagonals meet each other. It is also the point of intersection of the lines joining the middle points of the opposite sides.

(iv) The centre of gravity of a circle is at its centre.

3.1.2. Centroid of Areas of Plane Figures by the

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