Towards a unifying theory for stand-level dieback

Trees may die in forest stands over relatively large areas for obvious reasons, such as fire, flooding, severe drought, strong air pollution, and insect outbreaks. However, in many cases, stand-level dieback occurs without an obvious reason and then its etiology becomes more complicated, involving a combination of factors. The dieback structure, its symptomatology, and its spatial and temporal patterns may be used to identify these factors at least at the generic level. Moreover, observations of the dieback-associated vegetation dynamics can be used to draw inferences about the dieback process. In addition to biotic diseases and environmental changes or short-term recurring stresses, local site characteristics and the life history of the stand must be considered in a more universally applicable theory for stand-level dieback. This is further explained by a simplified model related to the decline disease concept of Houston.     

Forest tree responses to extreme drought and some biotic events: Towards a selection according to hazard tolerance?

The aim of this paper is to illustrate how some extreme events could affect forest ecosystems. Forest tree response can be analysed using dendroecological methods, as tree-ring widths are strongly controlled by climatic or biotic events. Years with such events induce similar tree responses and are called pointer years. They can result from extreme climatic events like frost, a heat wave, spring water logging, drought or insect damage… Forest tree species showed contrasting responses to climatic hazards, depending on their sensitivity to water shortage or temperature hardening, as illustrated from our dendrochronological database. For foresters, a drought or a pest disease is an extreme event if visible and durable symptoms are induced (leaf discolouration, leaf loss, perennial organs mortality, tree dieback and mortality). These symptoms here are shown, lagging one or several years behind a climatic or biotic event, from forest decline cases in progress since the 2003 drought or attributed to previous severe droughts or defoliations in France. Tree growth or vitality recovery is illustrated, and the functional interpretation of the long lasting memory of trees is discussed. A coupled approach linking dendrochronology and ecophysiology helps in discussing vulnerability of forest stands, and suggests management advices in order to mitigate extreme drought and cope with selective mortality.     

Stand-level dieback etiology and its consequences in the forests of Papua New Guinea

Stand-level dieback inNothofagus, mangrove andEucalyptus deglupta forests is examined. InNothofagus and mangrove forests, groups of largely even-aged trees die simultaneously, possibly triggered by drought stress which may directly affect the trees inNothofagus forests or indirectly increase water salinity in mangroves. Lightning strikes may be an additional dieback trigger in mangroves. Crown decline only was observed inE. deglupta forests and this may be associated with water-logging during periods of exceptionally high rainfall. Examination of rainfall records shows that severe droughts occur periodically in association with El Nino Southern Oscillations in the Pacific Ocean.     

The End Ever Nigh: Contemporary population change on Pitcairn island

Approximately 50,000 ha of native ‘ōhi’a (Metrosideros polymorpha) rain forest on the windward side of the island of Hawaii experienced a pronounced dieback of the tree canopy during the 1960s and early 1970's. The forests affected were located between 600 and 1,500 m elevation where the median annual rainfall was greater than 25000mm. Dieback stands were found on a variety of different substrates ranging from 500–10,000 years in age. Most of the stands that experiencel dieback had canopy cover greater than 60% prior to 1960. In 1976 and 1977 we established 43 relevés in dieback and non-dieback forest stands to assess canopy tree vigor, composition and structure of the vegetation, and to describe substrate characteristics.Metrosideros population structure and tree vigor were resampled for 25 of these relevés in 1982 and 1985–1986. The results of air photo analysis and ground sampling showed that dieback has not expanded very much within the study area since 1977. However, 5 of the relevés sampled in 1977 continued to decline in tree canopy vigor between 1977 and 1982 while 2 others showed a slight recovery in vigor over this same period. Seedling and sapling regeneration has been extremely vigorous in most of the sites that experienced a breakdown of the canopy while, in stands with an intact, dense tree canopy no such regeneration occurred. It appears that most of the stands which experienced canopy dieback have the potential to become closed forest communities again, dominated byMetrosideros. The natural recovery process may be disrupted in some areas due to additional competition for light and nutrients from invading populations of both native and alien plant species that have become established following canopy dieback.     

An explanation of natural forest dieback based on the “pipe model” analogy

A decade of investigation of stand-level dieback ofmetrosideros in the Hawaiian montane rain forests has led to the theory that one of the major underlying causes is cohort senescence. A literature review shows that while there is little evidence of a senescence mechanism in polycarpic trees, the low vigor associated with the senescent state is correlated with a declining balance of photosynthesis to respiration. Specific Leaf Burden (SLB, cm³ of sapwood per g of leaf mass) is here defined as an index of respiring to photosynthesizing tissue. Using the assumptions of the pipe model theory, it is shown that SLB increases with stem elongation and is independent of tree diameter. Thus, the trees of a cohort that are the same height although variable in diameter will have the same carbon balance, as indexed by the SLB, and could be expected to decline synchronously, as stands ofMetrosideros do. Changes in growth patterns, deviating from the pipe model, that minimize the SLB as trees grow larger are identified. Research undertaken to test the assumptions of the pipe model and the conclusion of uniform SLB in a cohort of agingMetrosideros is described.     

Forest decline in the Federal Republic of Germany —appearance,extent, potential causes

Present forest damage phenomena are observed for many tree species, on all soils and sites as well as in areas with considerable climatic variation. They cannot be explained without assuming a significant contribution of air pollution in particular as consequence of the two- to fourfold higher deposition under forest stands than on adjacent open areas. So far only loss and yellowing of needles or leaves are suitable operational symptoms for forest damage surveys. These indicate that in the Federal Republic of Germany for the period 1984–1987 in the important stands over 60 years of age a reduction of trees without damage symptoms from 31% to 24% occurred in spite of prevailing growth promoeing weather. This points to a considerable destabilization of forest ecosystems. As forests are complex ecosystems rather than just an assemblage of leaves, rootes or trees any interpretation of phenomena has to consider the complex and permanently changing interaction between the elements of the ecosystem themselves as well as its basic changing interaction between the elements of the ecosystem themselves as well as its basic environmental factors in atmosphere and soils. Field experience shows that for a given situation mostly several of the cause explaining hypotheses appear to we valid. Thus the phenomena are reflecting the influence of an array of stress factors acting directly on the foliage of the tree or indirectly through the soil and often in a synergic pattern. The model of considering anthropogene increased acid deposition with subsequent acidification of soils, damage of fine roots and impediment of water and nutrient supply of the tree is conclusive and isoprogressively being verified by experiments. For counteracting the observed destabilization and deterioration of forests a consequent policy of reduction of air pollution in its various forms and avoidance of forest management measures increasing system related acidification of forests are urgently required.     

Tree decline in industrialized areas of North America

Tree decline has been reported from several locations in North America in recent decades. The decline is widespread in areas remote from the classical point sources of air pollution. This paper describes the geographic and spatial extent of the decline, the symptomatology of affected trees, and the known and possible causes. In addition, a few case histories of tree decline in the vicinity of sulphur dioxide and fluoride emitting sources are described.     

The hydrogeological role of trees in water-limited environments

Field experiments have already proven that many tree species in water-limited environments (WLE) depend on groundwater. Typically, such trees survive dry seasons and droughts by uptake of water, directly from the groundwater body or from the capillary fringe, by rooting systems that may extend to several tens of meters depth. Such trees are also very efficient in finding soil moisture in the unsaturated zone, reducing groundwater recharge. Considering that WLE are typically characterized by low recharge, and that trees may use a significant amount of groundwater, this groundwater “consumption” should not be neglected in groundwater balancing, modeling and resources management. In practice, groundwater uptake by trees in WLE is either underestimated or disregarded because of limited knowledge about that phenomenon. This review discusses the current understanding of the hydrogeological role of trees in water-limited environments, the partitioning of tree transpiration into groundwater and unsaturated zone contributions and the integration of that partitioning in numerical groundwater models. Problems involved in this research are highlighted and possible future research directions are discussed.     

Making risk real: Urban trees and the ontological politics of risk

Over the past several decades, risk has become a distinct field of social inquiry as scholars in a variety of disciplines have developed theories about the ‘nature’ of risk and the role it plays in contemporary society. Collectively, these theories enrich our understanding of the politics of risk, the dynamics of risk perception, and the way risk shapes and is shaped by space, culture, social change, and modes of governing in the neoliberal era. In this paper, however, we argue these theories are helpful but not entirely suited to understanding risk when it becomes the subject of something Whatmore (2009, p. 587, 2013) calls “environmental knowledge controversies”. These controversies are generative events where more-than-human agencies and the political and knowledge making practices of heterogeneous actors reshape our sense of the real. To address this issue, we draw on the concepts of enactment, multiplicity, and ontological politics to explore how different kinds of risk and tree were made more or less real during a contentious debate over the risk posed by a group of urban trees in Newcastle, Australia. This case study suggests we can think of risk and hazardous entities like trees as effects that also affect because they elicit interventions that transform bodies and spaces in more or less enduring ways. Attending to the enactment, multiplicity, and ontological politics of risk, we argue, provides an alternative way to navigate moments of political contestation over the assessment and management of risk that has implications for how these processes are conceived and conducted in the future.     

Factors predisposing rata-kamahi(Metrosideros umbellata-Weinmannia racemosa) forests to canopy dieback,Westland, New Zealand

In many areas of WestlandMetrosideros umbellata (southern rata) andWeinmannia racemosa (kamahi) have undergone apparently excessive canopy dieback over the las 30–40 years. Browsing by the introduced brushtail possum is the predominant trigger involved. However, the roles of predisposing factors have received little attention. Dieback patterns indicate that the susceptibility of Westland rata-kamahi forest depends on their composition and structure. Stands containing abundant seral shrub hardwoods and a high proportion of old canopy trees are highly predisposed to dieback. Less dieback occurs in mature stands lacking seral species or in young apparently vigorous stands. Forest composition and structure are largely determined by site stability, i.e., the frequency and magnitude of physical disturbances such as mass movements. The underlying stability of different parent materials and landforms therefore largely determines rata-kamahi dieback patterns.     

Etiology of “Shimagare” dieback and regeneration in subalpineAbies forests of Japan

The “Shimagare” phenomenon relates to a striped pattern of stand-level dieback in the subalpineAbies (fir) forest of central Japan. Dieback fronts move gradually into pure stands ofAbies. As the dieback zones move forward they become regenerated with even-aged cohorts ofAbies which come up in form of waves. The phenomenon occurs mostly on the south slopes near mountain tops, exposing dieback fronts to south (i.e. downwards). Several facts suggest that desiccation stress and mechanical damage accelerated by the prevailing wind trigger the directional dieback ofAbies stands. The dieback is, however, promoted by over-crowded and even-sized stand structure. When less shade-tolerantBetula (birch) trees invade in an early stage of regeneration, mixed stands develop. These hardwood-mixed stands, with less-crowded and various-sizedAbies populations, hardly decay simultaneously. Here, alsoAbies trees become larger and live longer. Thus, the tendency of mass dieback ofAbies is not a characteristic of the species as such, but rather one of dense pure stands of the species.     

Canopy dieback in the upper montane rain forests of Sri Lanka

In 1978 a patch of dying upper montane rain forest had been observed on the slope of Thotupolakanda above Horton Plains. Patches in the upper montane rain forests of Sri Lanka were also recorded in 1980/81. All these sites were on windswept western slopes and ridges with stunted tree growth and shallow soil. From 1971 to 1983 Sri Lanka experienced some exceptionally dry years. The severe droughts of 1976 might have cause or “triggered” that dieback. After heavy rains between 1983 and 1986, these forests showed signs of recovery. Some trees had survived and sprouted again. Tree saplings were found, but regeneration was very low and further impeded by serious frost in February 1986. The influence of drought and frost clearly indicates the vulnerability of the stunted upper montane rain forest in the highest and wind-exposed regions of Sri Lanka.     

The assessment of landslide susceptibility mapping using random forest and decision tree methods in the Three Gorges Reservoir area,China

Landslide susceptibility mapping is an indispensable prerequisite for landslide prevention and reduction. At present, research into landslide susceptibility mapping has begun to combine machine learning with remote sensing and geographic information system (GIS) techniques. The random forest model is a new integrated classification method, but its application to landslide susceptibility mapping remains limited. Landslides represent a serious threat to the lives and property of people living in the Zigui–Badong area in the Three Gorges region of China, as well as to the operation of the Three Gorges Reservoir. However, the geological structure of this region is complex, involving steep mountains and deep valleys. The purpose of the current study is to produce a landslide susceptibility map of the Zigui–Badong area using a random forest model, multisource data, GIS, and remote sensing data. In total, 300 pre-existing landslide locations were obtained from a landslide inventory map. These landslides were identified using visual interpretation of high-resolution remote sensing images, topographic and geologic data, and extensive field surveys. The occurrence of landslides is closely related to a series of environmental parameters. Topographic, geologic, Landsat-8 image, raining data, and seismic data were used as the primary data sources to extract the geo-environmental factors influencing landslides. Thirty-four layers of causative factors were prepared as predictor variables, which can mainly be categorized as topographic, geological, hydrological, land cover, and environmental trigger parameters. The random forest method is an ensemble classification technique that extends diversity among the classification trees by resampling the data with replacement and randomly changing the predictive variable sets during the different tree induction processes. A random forest model was adopted to calculate the quantitative relationships between the landslide-conditioning factors and the landslide inventory map and then generate a landslide susceptibility map. The analytical results were compared with known landslide locations in terms of area under the receiver operating characteristic curve. The random forest model has an area ratio of 86.10%. In contrast to the random forest (whole factors, WF), random forest (12 major factors, 12F), decision tree (WF), decision tree (12F), the final result shows that random forest (12F) has a higher prediction accuracy. Meanwhile, the random forest models have higher prediction accuracy than the decision tree model. Subsequently, the landslide susceptibility map was classified into five classes (very low, low, moderate, high, and very high). The results demonstrate that the random forest model achieved a reasonable accuracy in landslide susceptibility mapping. The landslide hazard zone information will be useful for general development planning and landslide risk management.     

Pigment analysis and spectral assessment of spruce trees undergoing forest decline in the NE United States and Germany

Measurement of photosynthetic pigments as ground truth for remotely sensed spectra of boreal communities was tested. Chlorophyll and carotenoid concentrations and ratios were obtained from needles of spruce trees which were healthy as well as those undergoing forest decline (Waldsterben) in Vermont (USA) and Baden-Württemberg (FR Germany). In needles of trees exhibiting forest decline symptoms, chlorophyll pigment concentrations were lower, chlorophyll b levels decreased relative to chlorophyll a, total chlorophyll (a + b) was less relative to total carotenoid, and percent of reflectance in the visible range was higher. Pigment and reflectance data differentiated between needles from healthy and declining sites. These results were compared to remotely sensed spectral data obtained by aircraft and satelitee. As a result of these initial comparisons, it appears that using photosynthetic pigments as ground truth for remotely sensed spectral data may be of value in developing techniques for differentiating undamaged and damaged tree canopies on a large spatial scale. Finally, similar pigment and reflectance properties characterized healthy and declining communities in both Vermont and Baden-Württemberg.     

Analysis of an estuarine striped bass population: Effects of environmental conditions during early life

Estuarine fish populations are exposed to a variety of environmental conditions that cause both short-term variability and long-term trends in abundance. We analyzed an extensive data set for striped bass (Morone saxatilis) in the San Francisco Estuary to refine our understanding of how environmental variability influences recruitment. We examined the effects of environmental variability during early life stages on subsequent recruitment (age 3 yr), and the degree to which conditions in early life may have contributed to a long-term decline in abundance of adult striped bass in the San Francisco Estuary. Survival from egg to young-of-the-year varied strongly with freshwater flow; this effect apparently occurred within the first week or two of life, a time period that encompasses transport of eggs and larvae from the rivers to rearing areas and the onset of feeding. The rate of freshwater flow to pumping facilities that export freshwater from the system had small or sporadic effects on survival during the first month or two of life. Although many young striped bass between ages 2 and 8 mo were entrained in export pumping facilities, the resulting high mortality was unrelated to total mortality rates determined from field data on young striped bass. This lack of effect was apparently due to strong density-dependent mortality occurring between ages 1 mo and 3 yr (Kimmerer et al. 2000). The available data do not support previously suggested relationships between recruitment and freshwater flow during early life, or between gross estimates of pesticide input and survival of early life stages. We used a simple life-cycle model to show that various combined factors could have led to a decline in adult abundance, particularly a large and increasing adult mortality, but that events early in life probably did not contribute substantially to the decline. These results demonstrate that several decades of monitoring data from numerous life stages are needed to distinguish among alternative hypotheses about environmental influences on populations of estuarine fish.     

Progress and Challenges in Coupled Hydrodynamic-Ecological Estuarine Modeling

Numerical modeling has emerged over the last several decades as a widely accepted tool for investigations in environmental sciences. In estuarine research, hydrodynamic and ecological models have moved along parallel tracks with regard to complexity, refinement, computational power, and incorporation of uncertainty. Coupled hydrodynamic-ecological models have been used to assess ecosystem processes and interactions, simulate future scenarios, and evaluate remedial actions in response to eutrophication, habitat loss, and freshwater diversion. The need to couple hydrodynamic and ecological models to address research and management questions is clear because dynamic feedbacks between biotic and physical processes are critical interactions within ecosystems. In this review, we present historical and modern perspectives on estuarine hydrodynamic and ecological modeling, consider model limitations, and address aspects of model linkage, skill assessment, and complexity. We discuss the balance between spatial and temporal resolution and present examples using different spatiotemporal scales. Finally, we recommend future lines of inquiry, approaches to balance complexity and uncertainty, and model transparency and utility. It is idealistic to think we can pursue a “theory of everything” for estuarine models, but recent advances suggest that models for both scientific investigations and management applications will continue to improve in terms of realism, precision, and accuracy.     

Damage and management of cyclone Sidr-affected homestead tree plantations: a case study from Patuakhali, Bangladesh

Geographically, Bangladesh is considered to be one of the most cyclone-prone areas in the world. The super cyclone Sidr in 2007 was one of the most devastating disasters to have ever occurred in Bangladesh, having wind speeds of 223?km/h with a tidal surge of 6.1?C7.6?m high. In order to draw a pre- and post-Sidr damage and management scenario, a survey was conducted at Dumki upazila, Patuakhali district in Bangladesh with a pretested questionnaire. Primary data were collected through a questionnaire, while secondary data were collected from pertinent offices as well as academic journals. The major focus of this study was to assess the state of pre-Sidr tree plantations (woody and fruit trees), the devastation caused by Sidr on these tree plantations, and post-Sidr recovery and further sustainable management initiatives to counteract a possible future neo-Sidr. The post-Sidr study showed that about 16.84 million woody and fruit trees were uprooted by Sidr. A total of 3,120 rain tree (Samanea saman L.), 7,548 mahogany (Swietenia mahagoni L.), 1,444 royal siris (Albizzia richardiana L.), 702 white siris (Albizzia procera L.), 1,214 mango (Mangifera indica L.), 1,092 coconut (Cocos nucifera L.), 1,380 jackfruit (Artocarpus heterophyllus L.), and 7,640 betel nut (Areca catechu L.) trees were sampled among which 922, 1,662, 382, 210, 106, 38, 362, and 1,652 trees, respectively, were markedly damaged by Sidr. Sexually propagated woody trees with a damaged taproot were vulnerable to heavy wind during the cyclone. Fruit trees were more resistant to the cyclone than other types of trees, and the lack of proper tree training and pruning was one of the principal causes of the severe damage to woody trees. Hence, plantations of woody tree saplings with damaged taproots are strongly discouraged. However, proper training and pruning of homestead trees together with plantation of fruit trees rather than woody trees is suggested as a possible effective management strategy to protect the homestead ecosystem from devastation by future cyclones in Bangladesh.     

Synthetic impact response functions for flood vulnerability analysis and adaptation measures in coastal zones under changing climatic conditions: a case study in Gippsland coastal region,Australia

There is an increasing concern that the current management practices for many coastal regions are unsustainable. Very few countries have planned to deal with the exacerbation of environmental decline in the face of sea level rise. It is therefore necessary to assess socioeconomic and environmental impacts of sea level rises to better understand the vulnerability of coastal zones, as part of devising adaptive and integrated management principles. This paper presents a systematic approach by which relevant stakeholders can be actively engaged in prioritising flood impact issues and deriving information for quantification of impacts for adaptation measures and demonstrates the approach through implementation in the Gippsland coastal region. As outcomes of the project, we have identified key issues of concern for this region for flood impacts and constructed synthetic response functions for quantification of impacts of floods on some of the key issues in the region. The analysis also showed that stakeholders consider that some of the issues are not likely to be significantly affected by floods and thus may not require adaptation measures. The analysis did not provide high agreement on some issues. Different approaches are required to assess the importance of these issues and to establish impact response functions for them.     

Modelling the responses of mature forest trees to air pollution

Systems analysis of the responses of forest trees to air pollutants led to mathematical models which represent the essential life processes of mature trees (photosynthesis, respiration, growth etc.) under ‘normal’ conditions and — especially — under pollution strees. Computer simulations using these models have made it possible to test and analyse the consequences of — known or hypothetical — pollution effects. The simulation runs have led to the identification of three distinct behavioural modes (growth, stagnation, breakdown) and corresponding stable and unstable system states. The results have produced new insights and contributed to the understanding of the dieback process of spruce and beech trees. The presentation will concentrate on the most recent and most complex of the hitherto existing models, theBeech model.     

A model-based approach for a systematic risk analysis of local flood emergency operation plans: a first step toward a decision support system

The recurring heavy precipitations which have struck the south of France these last few years highlight that the affected cities have little pertinent information on how to manage these emergencies, and therefore appear powerless. In response, the development of a local emergency operations plan (EOP) for communities threatened by natural and technological hazards was made compulsory by French law in 2004. This article describes a systematic risk analysis approach for local EOPs. This method is based on a functional model that represents a local EOP as a set of interacting functions, each of them using resources and controls. This decomposition facilitates the identification of the human, technical, and organizational resources that are essential to safeguard the inhabitants of a community threatened by a hazard. This model provides a precise frame for performing an exhaustive and rigorous risk analysis of a local EOP. Based on this model and this risk analysis, potential failures are identified and organized into fault tree for each function. Then assessment checklists of the functions of the EOP are structured via these fault trees. By using these checklists, the analysis of a local EOP becomes more rigorous, exhaustive, and systematic, which makes it possible to broadly study the criticality of the plan. Therefore, this method enhances the potential success of the pre-planned actions during a disaster. This is the first step of a decision support system for city emergency managers who are designing their local emergency plan.