Green Horizon Newsletter - Issue No. 3, April, 1995

A Newsletter of the Kenyan Forest Health Centre, Forest Department, Ministry of Environment and Natural Resources, Nairobi, Kenya

Mangroves at the Kenya coast face the latest environmental threat

Mangroves forests have long flourished along the Kenya Coast.  They grow well on the saline soils and cover 62,380 hectares of coastline.  Of this, 46,229 hectares are in Lamu district alone.

The forests, however, do not grow on every inch of the coastline and have breaks at certain places.  But they do stretch from the farthest end of the south coast from Vanga on the Kenya-Tanzania border to the Northernmost part of the Kenya-Somali border at Kiunga.

Recently, these coastal resource has come under attack from a wood-boring insect.

Damage was first noticed at Kiunga, Lamu in early 1993 where a pest was reported to have caused massive deaths of natural stands of Sonneratia alba.  Ever since, the damage is said to have spread to Gazi and Tudor.

Surveys carried out by the Forest Health Management Centre to collect specimens and monitor the mangroves also showed that the rate of spread was high.

Specimens of the insect were taken to the UK International Institute for Biological Control by Dr. Sean Murphy.

The insect has been identified by Dr. Jeremy Holloway of the IIBC as Salagena sp. (Cossoidea: Metarbelidae) which is native to Kenya and East Africa as a whole.

Increase in population numbers to the current damaging levels is thought to be caused by changes in agriculture/forest practices.

Uses of Mangrove trees.  Among the many uses of the mangrove tree are wood for boat building for the coastal activities such as fishing.  Others include poles for building, wood for furniture making, fodder for animals, charcoal, wood fuel, control of soil erosion and being habitat to birds.

The mangrove forests also form important hatchery grounds for sea creatures such as oysters, prawns, crabs, fish and lobsters.  Other uses are medicinal.

Damage.  The borer notches away the bark of the mangrove branch leaving an area between one and three inches.  It then burrows into the wood.  The frass generated is ploughed outward to close the tunnel.  Consequent overflowing frass is concentrated round the notched debarked area giving it a muddy brown colour.

Several borers may colonise a single branch.  For example, four borers may infest a single branch of four feet.  Dieback of the branch results.

Recurrent infestation on all branches of the tree may cause mortality.

Holes made by the insect may provide entrance for numerous disease causing organisms such as fungi and bacteria.

A mangrove tree may therefore die from a combination of these factors.

Mangroves: The experience of other countries.

In Kenya, there is little documented information on insect damage to the mangroves which have apparently been infestation free until the current invasion by stem-boring moth larva.  But in some parts of the world, infestation of mangrove stands and attacks on them have been reported and in some cases, solutions implemented and many stands saved.

In central Java for example, nearly 80% of newly planted (3 month old) seedlings of Bruguiera spp. were killed by Chaetocnema sp. while serious damage by crabs was reported in Malaysia, Brazil and Java.

In Malaysia, conversion of Avicennia sp. forests has been proposed to increase the productive forest  area in Peak where yields have declined in second generation Rhizophora sp. stands.  Planting R. apiculata and R. mucronata propagagules at 1.5 metre spacing under canopy resulted in 32% and 37% survival respectively after three months.  Losses were mainly due to crab attack, often leading to mortality.  Damage by scolytids was also rather marked.  Caging to exclude crabs increased R. apiculata survival 55%, losses being mainly due to scolytids.  Results suggest that large scale planting in Avicennia forests is feasible.

In Brazil, a tree climbing crab, Aratus pisonii is known to eat mangroves while in central Java and Indonesia, seedling of Rhizophora mucronata have been severely attacked by crabs.  (Sesarma sp.) which cut off the shoots of young seedlings immediately after planting.

Two common terrestrial pathogenic fungi have been found to cause butt and heart rot on old growth stands of Rhizophora apiculata.  These are Phellinus pachyphloeus and Phellinus rimosus.  A foliage disease on young leaves was found to be a sooty mould caused by Meliola sp. brown spot disease was caused by Pestalotopsos sp.

In India, the spotted deer (Axis Axis) and the barking deer (Muntais muntjak) have been found to dig out the roots of and ultimately destroy pure stands.  The deer also graze on the leaves.  Macaca mulatta, a monkey has also been seen feeding on leaves and fruits in India.

Site Factors Associated with Cypress aphid attack

Physiologically stressed trees are more disposed to pest attacks.  So the maintenance of tree growth vigor will reduce the susceptibility and vulnerability of the trees.  Thinning for purposes of increased stand vigor and other silvicultural practices that will promote growth should be encouraged.

Stand susceptibility depends on species, site and length of time the insect has been present.  Stands more susceptible to attack usually occupy poor sites (dry, rocky ridges with shallow soils or dry sandy soils).  These sites are characterized by slow growth.  In other situations, stands characterized by high basal areas and slow diameter growth have been known to fall in the category of those easily attacked.  A combination of several silvicultural practices such as thinning and salvage cuts have been found to reduce the basal area and enhance growth as a result of reduced competition among the trees in the stand.

Slope elevation, age of stand, annual rainfall, number of rainy days including distribution and temperatures are also factors to be considered when evaluating susceptibility of stands to attack.  Other factors just as important include the soil pH as this, among others, affects the vigor of tree growth.

Alternative species should also be evaluated and those which have been found effective by researchers pinpointed.  Secondly, a preliminary survey ought to be carried out in the cypress growing areas to establish which other species have been introduced.  Finally, growth of the introduced species should be evaluated and recommendations made on which among them could replace cypress.

Some New methods of Monitoring Environmental Conditions

Environmental health is a global concern.  The world's forests, like all valuable resources, must be taken care of by means of constant monitoring of conditions if their health is to be maintained.  One of the most common ways of doing this is through aerial surveys.

An aerial survey is an appraisal of a given area using an aircraft.  Aerial surveys are used to detect and monitor the location, extend and intensity of forest damage by a forest pest.  Using a light aircraft, the crew fly over forested areas and sketchmap the damage.  Later, ground surveys can be carried out to verify the information and find out the host, damage and causal agent .  One big advantage of aerial surveys is that they allow for quick assessment of forest damage particularly over large areas within a very short time.

The electronics age has brought its benefits to environmental matters as well evolving other more sophisticated ways of acquiring data.  Such methods include Global Information Systems (GIS), Airborne videography and Global Positioning Systems (GPS) among others.

Global Information Systems (GIS).  In the strictest sense, a GIS is a computer system capable of assembling, storing, manipulating and displaying geographically referenced information, i.e. data identified according to their locations.  Practitioners also regard the total GIS as including operating personnel and the data that go into the system.

A Geographical Information System represents a system, commonly computer-based, for handling spatial (of space) data.  All complete or full geographic information systems perform the following major functions:

Data Input: normally consists of a mixture of manual and automatic digitizing operations together with associated data cleaning and editing.

Data Storage and Retrieval: Initial creation of the spatial data base together with subsequent update operations and query handling.

Data Manipulation: Creation of composite variables through processing activities directed toward both spatial and non-spatial attributes of system entities.

Report Generation: Creation of both tabular and cartographic reports reflecting selective retrieval and manipulation of entities within the data base.

Airborne Videography.  In this, a small video camera and video tape are used in the aircraft to record the desired information.  These can be viewed in the office and important features noted.  Airborne videography is more accurate than manual sketchmapping.  Preservation of the video tapes can be done for many years and stand conditions at the spot can be compared frequently.

Global Positioning Systems (GPS)   This instrument is used for navigating both from the ground and in the air.  GPS can be an efficient means of updating GIS database.  It involves the use of a receiver which gets signals from satellites.  This information is then used for mapping.