Glossary - Afforestation, Reforestation and Forest Restoration in Arid and Semi-arid Tropics - page 248

Air-layering A technique of plant propagation wherein a branch of an adult tree is
debarked and tied with moist cloth and moss, leading to development of shoots and
roots which are then carefully removed as an independent plant.

Aldrin A highly poisonous insecticide (chemical formula C 12 H 8 C l 6 ). It is the most
effective insecticide against common insects like the termite, but its long break-down
period makes it a risky pollutant in the soils, and therefore its use in agriculture and
forestry has been banned in most countries. Check the legal restrictions obtaining in
your country before using it on a large scale.

Alidade A metallic ruler with a bevelled edge and two vertical attachments con-
sisting of a slit on one end and a hairline on the other, used for sighting objects and
marking their directions in plane-table surveying. A tacheometric alidade is provided
with a telescope fitted with a stadia diaphragm so that the length of the rays can be
read off a graduated staff and need not be measured.

Alkaline soil Soils containing sodium carbonate in quantities detrimental to plant
growth. During rains, alkaline solution of organic matter may accumulate in depres-
sions which on drying leaves black deposits. For this reason, alkaline soils are also
known by the name 'black alkali'.

Alkalinity Soluble salts present in arid soils in quantity detrimental to growth of
plants.

Amenity plantations Plantations raised for bio-aesthetic, recreational, or envi-
ronmental purposes. Such plantations are often raised in parks, along roads, near
residential areas, or in other open spaces near places of human habitation, recreation,
and work.

Appraisal, mid-term A systematic and comprehensive assessment of work done,
in a project nearly half-way through its execution. A mid-term appraisal aims at
determining how effectively the aims and objectives of the project are being fulfilled,
and if not, what corrective actions are called for.

Arid Climate or place characterised by low level of atmospheric moisture and insuf-
ficient rainfall to support plant growth. Dry climates can be classified into the three
categories of semi-arid, arid, and hyper-arid, depending upon a measure known as
Thorntwaite's index of aridity. This index is defined as the ratio of the total precip-
itation to the total potential evapo-transpiration which depends upon atmospheric
humidity, temperature, and wind. In this topic, a place having less than 500 mm
annual rainfall and less than 20 rainy days in a year (typically having aridity index
of 0.20) is defined as arid.

⇒ Hyper-arid, Semi-arid.

Auxins Organic substances that promote cell elongation and root initiation in plants.
These are often used to improve the rooting rate in cuttings in nurseries.

Back bearing In surveying, the bearing, or angle measured with respect to the
North, measured from a station to another station which has already been surveyed