Forest Health Indicators for Measuring Forest Degradation



Some issues around using health as an indicator

Forest Degradation is defined as the reduction in the capacity of a forest to provide goods and services.

Issues around forest health can be attributed as causes of degradation, ie. abiotic, biotic agents and invasive species are all causes of degradation. The question is, can they then be indicators of the criteria forest health?

Describing forest health could be considered somewhat similar to describing forest condition.

In terms of forest health, where there are long term effects the area degraded can be captured through measuring growing stock and species composition. In some instances this is a temporary state and may not lead to degradation. Where there is a temporary or short term effect by abiotic, biotic, or invasive species, this is part of the natural cycle and is reported separately in FRA.

The most critical part in applying health indicators in assessing forest degradation is the users’ capabilities to decide whether or not a specific health issue will lead to a “lasting” degradation. Besides training there is also a significant “perception” component in it.

 

During the compilation of the document, some doubts developed about using health indicators for assessing forest degradation. In the case of the other indicator groups such as biomass, diversity (landscapes and species etc.), goods and services and soils, we look at measurable parameters and record them. For health – and this is the big difference – we actually look at the direct and indirect causes of damage that is visible in the forest; e.g. insects, fire, storm etc. that have inflicted damage onto the forest stand. However these “damages” can be much better measured in terms of the other indicators, rather than by applying the one on health.

 

However in the document it is stated, “forest health is defined as the perceived condition of the forest”, and therefore it is subject to perceptions, unless you can effectively measure the many possible factors determining or influencing the health condition of the forest or its deterioration. “The health indicators need to be selected in light of decisions made for the other indicators”. If forest health were to be dropped as a criterion for measuring forest degradation, we may then lose an important part of the story.

If it is a reduction/decline in biomass, the level of species diversity, the productivity of certain goods, the provision of certain services or a fragmentation process, there is always the question what caused that symptom and the response may be one or several factors or driving processes. The value of using forest health as criterion for measuring forest degradation is that some of these possible direct causes are being (more explicitly) addressed IF the symptoms are clearly visible/detected.  Just measuring biomass, timber yields, quantities of NWFP etc. will not tell you too much.  

How much we know about the correlation between causes and symptoms is for sure not much and definitely not for the most important forest types. Here is something we should perhaps look at to consider as an element in the discussion.

FRA 2010: Information on issues relating to forest health and vitality

Insect and disease problems are often either cyclical or chronic and they require long-term investment in data collection. The information supplied by countries for insect pests has been reported as annual averages over five years to help compensate for long-term cyclical events, however five year reporting periods do not adequately reflect the status of some long cyclical outbreaks. Due to the longer duration of some disturbance events it is difficult to accurately assess the area affected annually. Some countries appear to have reported the cumulative area affected rather than the additional area of forest affected within that year. Thus the figures for the different types of disturbances are not always directly comparable.

The quality of data on forest significantly affected by insect pests and diseases is poor, in part because of the lack of clarity in interpreting what constitutes a ‘disturbance’. Insect and disease outbreaks in developing countries are primarily surveyed and reported for planted forests, and corresponding surveys of forest decline and dieback are rarer in these countries.

Other issues that were highlighted:

Disturbance caused by insect pests may only be reported according to the amount of affected wood removed, rather than by the area infested. Reports may only include the actual area reforested after salvage. In addition, a forested area may be defoliated by more than one insect and this often results in an overlap of reported figures. For diseases it can be difficult to convert figures from the total area showing damage to an area newly affected in a year. For both insects and diseases, new reporting methods may have been adopted by countries between reporting periods, making trend analysis difficult.

One solution could be to simply remove the group of health indicators and instead measure biomass decline (dead, fallen or damaged trees), reduced provision of goods (e.g. less timber productivity) and services (soil protection) or biodiversity. If these are caused by biotic/abiotic health factors, these can then be added as brief descriptions to the respective indicator. In addition, in most cases forest degradation is the result of several agents which more often than not are not easy to separate from each other, particularly when the main event that caused or initiated the damage may have occurred sometime in the past.

Suggested way forward:

• The assessment of forest degradation will use indicators related to assessable symptoms through criteria/indicators related to biomass, biodiversity, goods and services and soils.

• Each criterion/indicator will further be described/classified according to the major causes responsible for degradation, if they can be identified.

• This description of the causes of degradation can either be done in short “free-style” text or through categories of attributes e.g.

o Biotic factors causing degradation (i.e. bark beetle XX, fungi XX, cattle grazing etc.)

o Abiotic factors (i.e. wild fire, snow break, storm etc.)

o Invasive species (i.e. species X, Y, Z)

In this way forest health is covered in the assessment and we do not lose this important part of the story. In addition, policy-makers and managers obtain information and guidance on how to address forest degradation in their country.

Forest Health Indicators for Measuring Forest Degradation

Drafted by Michael Kleine (IUFRO Headquarters)

With contributions by

• Joseph Cobbinah (IUFRO WP 7.03.09 “Protection of forests in the tropics”, Ghana) and

• Yang Zhong-qi (IUFRO WP 7.03.08 “Forest protection in Northeast Asia”, China)

Revised Version dated 18 December 2010

1. Defining Forest Health and Scope of Guidelines

Generally, forest health is defined as the perceived condition of a forest derived from concerns about such factors as age, structure, composition, function, vigour, presence of unusual levels of insects or diseases, and resilience to disturbance (Helms, 1998).

Forest health is a measure of a forest ecosystem's capacity to supply and allocate water, nutrients and energy in ways that increase or maintain ecosystem productivity while maintaining resistance to biotic and abiotic stresses. Perception and interpretation of forest health are influenced by individual and cultural viewpoints, land management objectives, spatial and temporal scales, the relative health of the stands that comprise the forest, and the appearance of the forest at a given point in time (Adapted from: IUFRO Silvavoc Terminology Project).

There are many factors that can cause the disruption of the health of forests. These can be categorised into living (biotic) and non-living (abiotic) factors. Biotic agents include fungi, bacteria, viruses, insects, mites, parasitic plants, weeds, and larger animals. Non-living factors are related to weather (e.g. fire, wind, snow, hail, and lightning); water (e.g. flooding, waterlogging and drought); soil conditions (e.g. deficiency of nutrients or poor drainage); mechanical agents (e.g. heavy machinery); and chemicals (e.g. pesticides, salt, industrial waste, and atmospheric pollution) (FAO 2003).

Forest ecosystems are continuously influenced by abiotic and biotic agents and processes at different spatial scales (i.e. individual trees, stands, forest landscapes, entire forest types etc.), intensities of impact and combination of agents. As long as these influences are within the natural variation of an ecosystem they will not cause severe long-term forest degradation (i.e. loss of capacity to produce certain goods and services). Considering this in the assessment of forest health conditions a distinction is made between

a) Areas disturbed/damaged/significantly affected by abiotic or biotic agents, and

b) Areas that have lost their capacity to produce desired goods and services because of excessive infestation/disturbance by biotic agents and/or abiotic processes.

The guidelines on forest health indicators are, therefore, subdivided into two main sections. The first section deals with the assessment of damaged forests affected by health problems, while the second sections provides guidance on how to assess what proportion of the area damaged/disturbed should be considered as degraded.

2. Forest Health-related Indicators

Table : Proposed forest health indicators

|Indicator |Measurement method |Relevant case studies or |Scale of measurement |

| | |data source | |

|Area and percent of forest affected by | | |Stand or landscape |

|biotic processes and agents | | | |

| | | | |

| | | | |

|Area and percent of forest affected by | | |Stand and landscape |

|abiotic processes | | | |

| | | | |

| | | | |

|Area and percent of forest affected by | | |Stand and landscape |

|invasive species (e.g. plants, insects) | | | |

Indicator formulation

Forest health is assessed with the following three indicators, one each for biotic and abiotic agents and processes and a separate indicator for invasive species:

• Area and percent of forest significantly affected by biotic processes and agents beyond reference conditions (e.g. insects, fungi, grazing animals)

• Area and percent of forest affected by abiotic agents. (e.g. storms, snow, hail storms)

• Area and percent of forest affected by invasive species beyond reference conditions (e.g. plants, insects)

It needs to be decided whether we are looking at the current area affected (if so a measure may be taken of the average of 5 years), or a single year. Alternatively we may want to be looking at the area annually affected.

The indicators describe the area or percentage of the forest area at the national level that has been degraded through ill health. The area figure or percentage can also be provided separately for individual forest types if such a classification is applied within the context of a specific country (refer to result table below).

Data and methods of data collection

Area figures in hectare can be obtained in various ways and depend on the type of forests and the extent and intensity of the biotic/abiotic agents to be assessed. From the discussions in Section 1 it is obvious that only severe impact of biotic/abiotic agents will lead to forest degradation, thus the extent of area affected should be rather easy to measure, either through

(a) Traditional field inspections or

(b) Remote sensing methods.

Regular field inspections by forestry staff are common practice in most countries. These are carried out either by district offices of governmental forest departments or forest managers of private forest companies or community-operated forest organisations.

In the case of insect attacks usually, areas affected are closely monitored, thus the extent of area and likely further spread of a disease is known at an early stage of disease outbreak. If, under specific circumstances, the outbreak leads to destruction of the stands and degradation for a longer period of time, the area need to be recorded as degraded.

Most fire affected areas are identified by remote sensing methodologies, in place in many countries. Whether or not fire leads (or has already led) to degradation needs to be assessed through field inspections. The same applies to areas damaged by storms, snow or floods.

The impacts of overgrazing are more difficult to assess. Negative effects such as absence of natural regeneration and/or undesired changes in species composition are the result of such long term processes. Usually, local forestry staff and stakeholders are aware of such influences and can assist in identifying areas where degradation is most severe.

Methods useful for invasive species mapping include

* Mapping using multi-resolution remote sensing data

* Discriminating invasive species using vegetation indices

* Mapping of three-dimensional spectral and structural properties of forests

* Stand-level assessments (aggregated at FMU or district-level)

* Field inspections

* Data from forest management plans.

The results of data collection on forest degradation caused by health-related factors for different forest types can be summarised as shown in the example below:

|Health factors (examples for |Area affected in ha |Percent of forest type |Percent of total forest area (at |

|different forest types) | | |national level) |

|Forest Type 1 (e.g. natural tropical high forest) |

|Forest fire | | | |

|Storm | | | |

|Invasive species | | | |

|Forest Type 2 (e.g. mangove forests) |

|Tsunami | | | |

| | | | |

|Forest Type 3 (e.g. forest plantations) |

|Shoot borer | | | |

|Cyclone | | | |

|Hailstorm | | | |

Limits on use of the indicators

The proposed indicators on forest health for the assessment of forest degradation are expressed in terms of area or percent of forest area affected by a particular disease or agent. In using these indicators the following considerations need to be taken into account:

• In the first or second year after the occurrence of an insect calamity or a storm event, the primary cause of the damage inflicted on the forests can easily be identified and the area measured. This means that for calamities occurring shortly before the assessment, it will be no problem to use the respective indicator. With the passage of time following the calamity or disastrous damage, other biotic and/or abiotic agents may continue the destruction of the forests, leading to a situation where a single cause of degradation cannot be identified. In such cases, it is more important to describe forest degradation by means of above-ground biomass; growing stock, species composition (e.g. loss of biodiversity) and/or specific goods and services from forests, rather than forest health. Rules need to be included that clarify the use of a certain health indicator in relation to the other indicators.

• The indicators related to biomass, biodiversity, and goods and services provide information on the actual conditions of a forest at the time of assessment. In contrary, the health indicators focus on the causes of visible damage. When compiling data at the national level, these differences need to be accounted as to avoid that the same forest area is counted within two different indicators.

• In the case of slow degradation processes e.g. through over-grazing and invasion of exotic species it needs to be further discussed whether or not to use the health indicators. Alternatively, it might also be an option to categorise such areas under indicators on biomass, biodiversity or goods and services. In addition, the health-related causes of the decline in stocking etc. could be described in a brief note that comes along with the indicator.

3. Assessing Forest Health in the Context of Forest Degradation

Guidelines for how to report on forest damage in the FRA process

In assessing forest health aspects in the context of forest degradation, only those impacts will be considered that cause long-term damage to the forest over larger areas and thus exceed the natural variation typical for a particular forest ecosystem. As a consequence, the forest loses its capacity to provide goods and services. In these cases, the forest cannot recover to a desired state within an acceptable time frame without significant efforts in terms of rehabilitation and investments of additional resources (i.e. manpower, funds). Such efforts would exceed regular forest management and protection activities.

The above approach requires a thorough analysis of major health impediments on forests within a country and – based thereon – a decision whether or not these health issues lead to irreversible or permanent forest degradation. The extent (spatial) and level or intensity of the damage (threshold) naturally play an important role in this decision.

In principle, forest health will be assessed on the basis of

a) Visible signs of affected forest stands caused by biotic (e.g. insects, fungi, grazing animals, invasive species) or abiotic agents and processes (e.g. fire, storm or tsunami);

b) Extent of area and percentage of forests affected by visible damage.

For the decision on whether a specific health issue leads to more or less permanent, degradation (in the absence of efforts to recover the forest) and thus should be recorded in the assessment as “degraded forest”, the natural dynamics of a particular forest type as well as the management objectives or primary function designated to a particular forests need to be known. The following examples describing health aspects in the context of various regions, forest types, and management objectives are intended to provide some guidance for decision-making.

3.1 Naturally regenerated forests

Primary management objectives: Forest Conservation

According to the Global Forest Resources Assessment the annual outbreak of forest insects that damage forests worldwide is estimated at about 35 million ha with the majority in the boreal and temperate zones (FAO, 2010). Although this represents about 7 to 8 % of the total forest area only, there are specific sub-regions and countries which are heavily affected by biotic pests and diseases which under certain circumstances lead to severe forest degradation.

Naturally regenerated forest ecosystems in tropical and sub-tropical regions are generally diverse and complex in terms of species composition and age. Because the inherent high diversity does not enhance high population build up of biotic agents, the extent of natural tropical forests affected by biotic pests and diseases of epidemic proportions is minimal. As an example, the African oak (Milicia excelsa and M.regia) is not only a prime timber species of the humid tropical ecosystem of tropical Africa but is also considered as one of the most generally useful timber species of tropical Africa. In the highly diverse tropical forests where it occurs the species is largely free from pest problems. However, all previous efforts at establishing monoculture plantations of this high valued timber species have been hampered by attacks of a gall - forming insect pest (Phytolyma lata) closely followed by infestation of a pathogen that causes dieback (Cobbinah, 1986; Cobbinah and Wagner,1995)

In contrast, natural boreal and temperate forests are more regularly subject to large-scale outbreaks of pest and diseases with severe ecological and economic consequences. For example, currently British Columbia (Canada) is affected by a Mountain Pine Beetle (Dendroctonus ponderosae) outbreak of epidemic proportions. It is expected that up to 80% of the country’s mature lodgepole pine forests will be dead by 2013, an area of more than 17 million ha (Natural Resources Canada, 2010).

Whether or not a specific pest and disease outbreak in naturally regenerated forests should be included into the degradation assessment depends on the management objectives (forest function) defined for a particular naturally regenerated forests area. In nature conservation areas (e.g. national parks, virgin jungle reserves) which primarily aim at preserving forests and protecting ecological processes, disease outbreaks are considered part of the natural processes and thus should not be included. For example, a policy of non-intervention has been applied in the Sumava National Park (Czech Republic) following a bark beetle Ips typographus calamity leaving extensive areas of mountain spruce forests (Picea abies) dead (Jonasova and Prach, 2004). In general, in conservation areas where forests should recover naturally, a damaged area should not be considered as a degraded forest. The same would also apply to forests damaged by fire, storms or other abiotic factors.

Primary management objectives: productive/protective functions

Many naturally regenerated forests are primarily managed for timber or environmental services such as soil and water protection (e.g. watershed management). Dieback in these “managed” naturally regenerated forests due to infestation by biotic agents or damage through wind and snow when occurring on a larger scale require substantive measures for preventing further spread of the disease and also regenerating the damaged forest. In well managed forests such rehabilitation measures are undertaken immediately following catastrophic events. In Austria, for example, forest owners are required by law to restock damaged/destroyed areas within five years either through natural regeneration or artificial planting. In this way, timber production forests do not permanently lose their ability to produce wood and thus should not be recorded as “degraded forests.”

In countries where strict regulations on reforestation are not enforced or financial constraints prevent reforestation and forest rehabilitation activities, damaged areas should be included as “degraded forests”. In the following section some cases are highlighted where biotic and abiotic processes have led to serious forest degradation.

The Pine wood nematode (Bursaphelenchus xylophilus) was first found in China in 1982. Since then it has spread into 17 provinces mainly in southern China. It has killed 500 million pine trees and 333 000 ha of pine forests were destroyed, leading to over 100 billion RMB or 15 billion US$ of economic losses. Currently, the pine wood nematode threatens the landscape quality and scenery of many famous mountains and great rivers in China, as well as many places of historic value and cultural heritage, e.g., Huangshan, Lushan, and Zhangjiajie Mountains. The ecological environment of the Three Gorges Dam area is particularly endangered by the pine wood nematode (Yang, 2010). Biotic calamities of such proportions described here have led to severed forest degradation. In this case, a natural pest has exceeded the average expectation and so the forests are degraded and need to be recorded.

In early spring of 2008 China experienced a severe disaster caused by ice and snow, a rare event in China’s history. Altogether 19 provinces were affected by the disaster mainly in southern China. In total, about 5.46 million ha of bamboo forests and 5.18 million ha of other forests (mainly pine plantations) were destroyed, causing a total loss of about 16 billion RMB or 2.4 billion US$ (Yang, 2010). Because of the immense scale of the disaster, it is unlikely that all the areas will be reforested in the short-term. Despite enormous efforts in rehabilitation, larger areas will not be able to fulfil their timber production function and thus need to be classified as degraded forests.

Adequate natural regeneration is an indicator of well-managed and healthy forests. Loss of natural regeneration - if not mitigated - leads to a loss of services from forests. In India, for example, overgrazing by livestock has resulted in loss of natural regeneration, particularly of broadleaved species. Natural regeneration is either absent or inadequate in 53 percent of the country’s forests. Further, the number of Indian states in which the extent of natural regeneration was high, decreased between 1987 and 1995 indicating progressive degradation of forests (Kant et al. 2008). A similar situation can be found in the dryland zones of Sub-Saharan Africa where anthropogenic over-utilisation of forests has led to land degradation and partly also to desertification. Comparative studies from Ethiopia using enclosures as a tool for forest rehabilitation indicate a drastic reduction in the number of woody and shrub species due to overgrazing and fire wood collection (Blay et al. 2004). Over time, the forest loses its capacity to regenerate naturally and thus will be unable to provide desired goods (e.g. fodder and fire wood) and services (e.g. soil protection, regulation of local climate). Forests that are in the above-described state need to be recorded as degraded forest.

3.2 Planted forests

Primary management objectives: timber production

In planted forests with generally simpler stand structures and fewer species and thus less biological diversity, impacts of biotic agents can have more serious consequences compared with diverse natural ecosystems. For example, the fight against calamities caused by various biotic factors is a major concern in the Republic of Korea. Due to continuous reforestation activities from the 1960s to the late 1980s large areas of the country are currently covered by young forests, including single-species plantations. These forests are prone to attacks by insects (e.g. pine moth Dendrolimus spectabilis, pine needle gall midge Thecodiplosis japonensis), fungi (e.g. oak wilt disease Ceratocystis fagacearum), and wood nematodes (pine wilt disease Bursaphelenchus xylophilus). Over the past decades large areas have been affected to a varying degree by these diseases, resulting in degradation of forests (Shin et al. 2007). Because of enormous efforts in combating these diseases, not all areas would be classified as degraded forests. Local experts need to identify those affected areas that have lost their capacity to provide the intended goods and services.

In Ghana, naturally regenerated forests have declined from 8.22 million ha to less than 1.7 million hectares in a century while the demand for industrial and domestic wood has increased and continues to increase. With the implementation of plantation development programmes since 2001 and a general increase in forest plantation areas, pest problems will as matter of cause be expected to increase. The situation thus calls for adequate preparation and readiness to contain imminent outbreaks that could lead to forest degradation.

Earlier efforts at establishing indigenous plantations have been hampered by pests in the past. Outbreaks, for example, have been recorded in native species plantations of Milicia excelsa, African mahoganies, and Mansonia altissima. Because of the success of exotic species such as Tectona grandis, Cedrela odorata about 90 percent of the tree plantations in Ghana are of exotic species. These exotic plantations have so far been free of any serious and damaging pests. They appear to be free of major pests and diseases that damage and kill them in their country of origin. Recently some sporadic and isolated attacks and infestation have been observed, however, on Teak and Cedrela (Bosu et al. 2004). Theses insects and pathogens are being closely monitored. Among the indigenous species selected as priority species for afforestation under the plantation development programme insect attacks have already been observed in the nurseries where seedlings are being raised. Some 50 percent of Terminalia ivorensis seedlings in nursery were attacked by an unidentified leaf gall. Also seedlings of Nauclea diderichii and Terminalia superba were attacked by defoliating insects (Bosu et al. 2004).

Influence by a pest in timber production forests that is recurrent, but – under certain conditions - does not result in long-term forest degradation and should not be recorded. For example, the Gypsy moth in Canada could be controlled over many decades, although occasionally larger outbreaks and expansion of its range occurs, causing up to 20% mortality in some forest stands ().

Extensive damage caused by hurricanes, cyclones or hailstorms have been reported from various tropical regions. In 1988 for example, a massive hurricane destroyed large portions of Jamaica’s pine plantations. A severe hailstorm in Swaziland followed by an invasion of the fungus Sphaeropsis sapinea resulted in extensive damage on hundreds of hectares of Pinus patula plantations (Evans 1992). However, whether or not such severe events lead to forest degradation depends on the level of forest protection and rehabilitation measures that are undertaken following such catastrophic events.

3.3 Forests and invasive species

Many invasive species (such as insects, woody plants, shrubs or grasses) - though not all – threaten the integrity of ecosystems throughout the world. They not only affect species diversity of native areas but also their biological integrity, as well as causing adverse social and economic impacts. Alien invasive species in India such as Parthenium hysterophorus (Asteraceae, commonly known as congress grass), Lantana camara (Verbenaceae, shrub species), and Ageratum conyzoides (Asteraceae, Billy goat weed) are gradually replacing native vegetation, invading agricultural fields, creating fodder scarcity, and also causing a number of health problems for local people (Kohli et al. 2006).

A number of invasive plant species have been recorded in all the major forest types in tropical Africa. Chromolaena odorata and Lantana camara are by far the most widespread invasive plant species in the tropical rainforest region though a recently introduced species Broussonetia papyrifera has shown considerable invasive potential. In the savannah and semi arid zones of eastern Africa, Proposis juliflora, Acacia mearnsii, Maeopsis eminii, and Senna spectabilis are among the most troublesome invasive woody plants. In general, the invasive species colonise disturbed forest ecosystems and cause synergistic degradation effects. Disturbance of natural ecosystems by biotic and abiotic factors and stresses due to climate change accelerate the establishment of invasive species into the new habitat. They are better able to efficiently use the resources of stressed ecosystems than their native competitors (Bosu et al. 2009).

Overall, invasion by exotic plants or insects tends to be a gradual process. In most cases experts within a country are aware of the progress made by invasive species and the risks associated with their expansion. Early management intervention might eliminate the problem, but often this does not happen owing to a lack of resources.

3.4 Conclusion on forest health-related degradation

In conclusion, assessors of forest-health related degradation are generally faced with two principal situations:

a) visible damage to forest stands (either dead or dying) of large proportions (i.e. affecting large areas) caused by biotic agents and processes or abiotic agents; and

b) gradual degradation due to overgrazing fuel wood harvesting, shifting cultivation, unplanned fire, climate change and/or invasive species.

As explained above, whether to include an area as degraded forest depends not only on the severity and long-term effects of the damage, for example, on biodiversity, forest growth, or soil conditions but also on the primary management objective(s) assigned to a particular forest.

The next step would be to decide whether or not the damage has led to degradation.

4. References

Blay, D., Bonkoungou, E., Chamshama, S.A.O. and Ben Chikamai. 2004. Rehabilitation of degraded lands in Sub-Saharan Africa: Lessons learned from selected cases studies. Editors: Peter Wood and Atse M. Yapi. Forestry Research Network for Sub-Saharan Africa (FORNESSA) and Internal Union of Forest Research Organizations’

Bosu P.P.,S. Adu-Bredu and E.E.Nkrumah. 2004. Observations of insect pest activities within selected nurseries in the Ashanti, Ghana. In Pest Management in Tropical Plantations pp 154-162. Eds Cobbinah,J.R.,D.A.Ofori, P.P.Bosu Proceedings IUFRO WP 7-03-09 Confrence, Kumasi, Ghana .

Bosu P.P.,M.M.Apertorgbor and Refera Alemayehu. 2009. Ecology and management of Tropical forest invaders. In Invasive Plants and Forest ecosystems Ed R.K.Kohli, S.Jose,H.P.Singh and D.R.Batish pp 355=378 CRC Press, Boca.

Cobbinah J.R. 1986. Factors affecting the distribution and abundance of Phytolyma lata (Homoptera:Psyllidae) Insect Sci & Applic 7:111-115.

Cobbinah J.R.and M.R.Wagner. 1995. Phenotypic variation in Milicia excelsa to attack by Phytolyma lata. Forest Ecology & Management 75:147-153

Evans, J. 1992. Plantation Forestry in the Tropics. Second Edition. Oxford Science Publications. Clarendon Press. Oxford.

FAO, 2003. An illustrated guide to the state of healt of trees. Recognition and interpretation of symptoms and damage. Eric Boa, Diagnostic and Advisory Service, CABI Bioscience, Egham, Surrey, United Kingdom.

FAO, 2010. Global Forest Ressources Assessment 2010 – Key findings. Rome, Italy.

Helms, J.,1998. The Dictionary of Forestry. Society of American Foresters.

IUFRO Silvavoc Terminology Project. SilvaTerm Database



(visited on 20 October 2010)

Jonasova, M. and K. Prach. 2004. Central-European mountain spruce (Picea abies (L.) Karst.) forests: regeneration of tree species after bark beetle outbreak. Ecological Engineering (23) 15-27. Elsevier.

Natural Resources Canada,

(accessed on 29 October 2010).

Promode Kant, Preet Pal Singh, Ghazala Shahabuddin and Rajeshwar Singh Jasrotia, 2008. India: Bringing a third of the land under forest cover. In: Don K. Lee (editor), 2008. Keep Asia Green, Volume III “South Asia”. IUFRO World Series Volume 20-III. Vienna, p. 220.

Ravinder K. Kohli, Daizy R. Batish, H.P. Singh and Kuldip S. Dogra, 2006. Status, invasiveness and environmental threats of three tropical American invasive weeds (Parthenium hysterophorus L., Ageratum conyzoides L., Lantana camara L.) in India. In: Springer Biological Invasion “Special Issue: Beijing International Symposium on Biological Invasions", Volume 8, Number 7, 1501-1510. DOI 10.1007/s10530-005-5842-1.

Shin, J.H., Park, P.S., and D. K. Lee, 2007. Forest Restoration in Korea. pp-55-80. In: Don K. Lee (editor), 2007. Keep Asia Green Volume II “Northeast Asia”. IUFRO World Series Volume 20-II. Vienna, p.170.

Yang, Zhong-qi, 2010. Verbal Communication.

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