Stand-level Dieback Research Articles

Convergent structural responses of tropical forests to diverse disturbance regimes

Size frequency distributions of canopy gaps are a hallmark of forest dynamics. But it remains unknown whether legacies of forest disturbance are influencing vertical size structure of landscapes, or space-filling in the canopy volume. We used data from LiDAR remote sensing to quantify distributions of canopy height and sizes of 434,501 canopy gaps in five tropical rain forest landscapes in Costa Rica and Hawaii. The sites represented a wide range of variation in structure and natural disturbance history, from canopy gap dynamics in lowland Costa Rica and Hawaii, to stages and types of stand-level dieback on upland Mauna Kea and Kohala volcanoes. Large differences in vertical canopy structure characterized these five tropical rain forest landscapes, some of which were related to known disturbance events. Although there were quantitative differences in the values of scaling exponents within and among sites, size frequency distributions of canopy gaps followed power laws at all sites and in all canopy height classes. Scaling relationships in gap size at different heights in the canopy were qualitatively similar at all sites, revealing a remarkable similarity despite clearly defined differences in species composition and modes of prevailing disturbance. These findings indicate that power-law gap-size frequency distributions are ubiquitous features of these five tropical rain forest landscapes, and suggest that mechanisms of forest disturbance may be secondary to other processes in determining vertical and horizontal size structure in canopies.

Stand structure and dendroecology of an old-growth Nothofagus forest in Conguillio National Park, south Chile

The regeneration pattern and dynamics of two emergent southern beech trees ( Nothofagus alpina, Nothofagus dombeyi) were inferred from population age structure and spatial patterns in a forest dominated by the deciduous N. alpina in Conguillio National Park, south Chile. Diameter–frequency, diameter–age relations, and analysis of radial growth patterns of N. alpina (“raulı́”) and evergreen N. dombeyi (“coihue”) were examined at a montane elevation site in the Andean Range. The present age structure indicates that the oldest N. alpina established before 1463 and the oldest N. dombeyi between 1480 and 1520. The forest was uneven-aged as were the populations of raulı́ and coihue. N. alpina dominated the overstory layer, with few N. dombeyi dominant trees, and had fairly continuous recruitment between 1463 and 1984. Since 1885, tree recruitment in the forest has been dominated by both N. alpina and N. dombeyi. The master tree-ring chronology of N. alpina exhibited abrupt growth changes from 1650 to 1725. Increased growth rates occurred after a major volcanic explosion of Volcan Llaima around 1640, followed by a relatively sharp decline in growth from 1730 to 1850; growth remained high from the 1850s to the 1990s. Releases in radial growth, indicative of moderate- and small-scale disturbances occurred in most of the oldest trees during the last 400 years. Multiple periods of release and suppression indicate that N. alpina may take advantage of gaps to reach the main canopy. Gap turnover times in this N. alpina– N. dombeyi forest were estimated at >500 years, although gap formation was highly episodic and possibly associated with regionally strong windstorms, volcanic activity, and stand-level dieback. Gap and treefall characteristics in the forest are similar to results from other mixed Nothofagus forests in northern Patagonia, southern Chile and Tierra del Fuego. As the vegetation of the surveyed forest was homogenous over the last five centuries, it is concluded that regeneration and coexistence of long-lived Nothofagus spp. depends primarily on fine- and moderate-scale single treefall gaps.

Oak dune forests in Denmark and their ecology

Floristic and structural data of oak dune forests in western Jutland (Kaergård and Blåbjerg) were collected in 1998 and compared with previous studies of Danish oligotrophic oak forests. A hierarchical divisive clustering resulted in four main clusters, containing two coastal oak vegetation types, Quercus robur– Carex arenaria and Q. robur– Pleurozium schreberi communities, both restricted to the oak dune forest areas in western Jutland, and two more widespread oak forest types, Q. robur– Vaccinium myrtillus and Q. robur– Holcus mollis communities. The low-growing Q. robur– C. arenaria shrub and forest vegetation occurs primarily in the outer dunes, with the tallest trees on the leeward sides of the dunes, while a Q. robur– P. schreberi community is found in the inner parts of the dune system. The similarity to a considerable number of the acidophilous vascular plant species closely relates the Danish oak dune forest communities to the Pinus sylvestris or P. sylvestris– Q. robur vegetation described in neighbouring countries, suggesting that P. sylvestris probably had a prominent role in the Danish oak dune forests before human exploitation caused its extinction in the 18th century. The low-growing oak shrub in the outer dunes, aged between 52 and 85 years, generally seems to have remained unchanged in floristic composition and structure since 1964 when the previous study on the shrub was performed. In the dune valleys, oak stand-level dieback is widespread and was latest recorded in 1985. Such retrogressive processes are probably due to a combination of low winter temperatures and other adverse climatic conditions. In the central parts of the oak dune forest, the oak trees, aged between 55 and 73 years, grow to a higher stature, but also here stand-level dieback is widespread leading to retrogression. Only in the innermost parts, a regular forest cover seems to prevail with trees of 68–104 years of age and the development of the field vegetation cover progresses into a typical forest community. This is exemplified by the decrease of light-demanding species and the establishment of true forest species like Oxalis acetosella and Dryopteris dilatata. The structure and poor-species diversity make the oak dune forest unique both in Denmark and on an international scale. We recommend further studies of the remnants of the original, and in the past probably more widespread dune forests in Denmark, and drawing up of a detailed management and conservation plan. The possibility of permitting the native and now reintroduced P. sylvestris to re-colonise the oak dune forest area and be part of the natural vegetation succession processes should be seriously considered.

Noise-induced Regularity of Spatial Wave Patterns in SubalpineAbiesForests

In subalpine forests dominated byAbiesspecies in Japan and northeastern United States, trees show traveling wave of regeneration with many striped zones of tree dieback, moving downwind at a constant rate. Previous theoretical studies have demonstrated that a very simple model can generate wave-like spatio-temporal patterns of tree regeneration in a lattice-structured habitat with each site occupied by a cohort of trees. A cohort taller than the average height of its windward neighbor experiences stand-level dieback in the next time step and the height becomes zero. Otherwise the cohort increases its height at a constant rate. Starting from a random initial pattern, this simple deterministic model can generate a saw-toothed pattern that moves downwind at a constant rate, but the distance between adjacent dieback zones has a large variance. In this paper, we study the effects of “noises” in tree dieback rules in two forms which help to generate more regular patterns: (1) additional random disturbances at a low rate, which change the size of “clusters” (defined as a group of cohorts between adjacent dieback zones) by splitting a large cluster into two or by merging a small one with a neighbor, and (2) the stochastic rule of tree dieback, represented by the probability of dieback in unit time being a sigmoidal function of the difference in the tree height between the site and the windward neighbors. These noises are effective both for one-dimensional and two-dimensional models, but spatial patterns are much more regular in the two-dimensional model than in the one-dimensional model.

Factors predisposing forests to canopy collapse in the southern Ruahine Range, New Zealand

The introduced brushtail possum Trichosurus vulpecula Kerr is recognised as the primary agent of defoliation and stand-level dieback in New Zealand broadleaved forests (except Nothofagus). The distribution and magnitude of canopy collapse of forest in 33 500 ha of the southern Ruahine Range, New Zealand was mapped from 1995 polychrome aerial photographs. Relationships between canopy collapse and forest type, altitude, aspect and slope were analysed using generalised additive models. Canopy composition was the strongest factor predicting the extent of collapse and modification: broadleaved-conifer forest is most affected, with 68–87% of the area of six such forest types replaced by scrub-low forest and tree-fernland; and surprisingly, three Nothofagus-dominated types have up to 28% of their former area now in shrubland or tussock grassland. The susceptibility to collapse of Nothofagus forest was positively correlated with distance to non- Nothofagus forest types. Subalpine scrub, which is dominated by species not favoured by possums, has increased in area by 32%, replacing former upper montane forest. Physiographic factors were much less important in multiple regressions. However, in general terms, forests on steeper slopes, in the upper montane-subalpine zone, and on warm westerly and northerly aspects were more susceptible to collapse and modification than elsewhere. It is postulated that possums were the primary agent responsible for collapse of non- Nothofagus broadleaved forest, and that red deer Cervus elaphus and goats Capra hircus were responsible for inhibiting canopy replacement by eliminating regeneration in forest understoreys. Secondary effects, such as outbreaks of defoliating insects and mechanical damage from wind also contributed to canopy collapse and were possibly triggered by possums opening up the canopy. Collapse of Nothofagus canopies possibly results from a breakdown in the recruitment phase of stand turnover, by the smothering effects of deer-induced shrubs and tussock grasses on seedlings and saplings.

Evolution of high genetic variability in the rare Hawaiian fern Adenophorus periens and implications for conservation management

Adenophorus periens is a rain forest-dwelling epiphytic fern and is known only from one population on the island of Hawaii (Kahaualea) and from a few scattered individuals on the islands of Kauai and Molokai. A genetic survey was conducted of the Kahaualea population employing protein electrophoresis. The individuals sampled expressed extremely high levels of allozymic variability compared to two more widespread congeneric species ( A. tamariscinus and A. tripinnatifidus). All individuals examined were produced via sexual recombination. Given the extremely young volcanic substrate upon which the Kahaualea forest has developed, the historical factors leading to the high genetic variability of A. periens are somewhat obscure. The outcrossed mating system of this species may play a key role in the maintenance of genetic diversity in combination with its perennial life cycle. Because the lack of neighboring populations precludes the influx of new genetic variability via gene flow, the population must rely solely on the appearance and incorporation of new mutations as sources of new variability. The data suggest that this genetically diverse population is not likely to suffer genetically from minor reductions in population size over the short term. The most obvious potential threats to A. periens are those affecting the demography of the population and include habitat destruction by lava flows, death of host trees through stand-level dieback, chance loss of individuals by a variety of mechanisms, and the adverse effects of gaseous emissions from volcanic activity and geothermal development. The State of Hawaii should undertake an artificial propagation and transplantation program to establish populations in less threatened, suitable habitats.

Forest decline and soil nutritional problems in Pacific areas

Soil nutrient stresses have been determined to contribute to stand-level dieback in two Pacific forest biomes, the Hawaiian Metrosideros rain forest and remnants of the eastern Australian Eucalyptus forest. In the Hawaiian dieback, low levels of N limit indigenous forest development early in primary succession on volcanic soils, while later in primary succession, stresses appear to be associated with soil aging, acidification, loss of cations, decreasing levels of P, increases in soluble Al, and, under poor drainage, sharp increases in soluble Fe. These nutrient limitations put a ceiling on stand development and growth and are considered as one of the three causes predisposing stands to dieback. In the rural or New England dieback of eastern Australia, indigenous eucalypts are adapted to ancient soils with very low levels of P, but pasture improvement with clover and fertilization with superphosphate has imposed different stresses on remnant eucalypts in pastures and nearby forest islands. After fertilization, the trees grow faster initially, but their foliage becomes highly nutritious for insects. Other factors also contribute to a build-up of insects as pests, which now threaten the remaining eucalypts. The rural dieback represents an example of how forests with low canopy species diversity, simplified structure, lack of successional species, and which are prone to dieback under natural conditions, can be destroyed by intensification of agricultural development. The paper closes with a summary of generic factors that were found to cause forest dieback under natural conditions and compares these to the anthropogenically superimposed stresses that led to aggravated tree and forest decline in the Australian rural or New England dieback. The suggestion is made that comparative dieback research at the global level will lead to an improved understanding of natural forest dynamics as an aid in interpreting the new stresses. imposed on forests by human activity.

Community organization and ecosystem theory

The community hypotheses of earlier European and North American vegetation ecologists stimulated much thought-provoking research on the nature of plant community organization. These research efforts can now be seen as providing an improved understanding of species patterning in broader ecosystem units, such as those defined by Krajina's biogeoclimatic zones. Built on this tradition, the author's research concept is outlined, which generally begins with the documentation of species distribution patterns, leading to autoecological studies of key species and then to a synthesis of species functions at the ecosystem level. This approach resulted in an ecosystem theory that explains periodic stand-level dieback in the Hawaiian rain forest as primarily a demographic phenomenon of the leading canopy species, Metrosideros polymorpha. The theory may have application to similar forests that are made up of mosaics of cohort stands. The theory also adds to a better understanding of the ecological concepts of climax and succession and provides for an important plant ecological criterion in the design of natural areas for ecosystem conservation.

Stand-level dieback and regeneration of forests in the Gal�pagos Islands

Stand-level dieback and regeneration of forests in the Gal�pagos Islands

Forest dynamics and stand-level dieback in New Zealand'sNothofagus forests

Emphasis is placed on the role synchronised cohort mortality in the dynamics of beech (Nothofagus) forest in New Zealand. Mature populations are frequently relatively even-aged and have size structures which render them vulnerable to exogenous disturbances from which they cannot recover, except by regeneration. Examples of regeneration systems in pure and mixed stands are described. Mountain beech (N. solandri var.cliffortioides) is a relatively short-lived light demanding species largely dependent upon exogenous canopy destruction for regeneration. Red beech (N. fusca) is a larger longer-lived tree which can maintain its population by autonomous gap-phase regeneration. When these species grow in mixture with silver beech (N. menziesii) in the absence of disturbance, they seem destined to be replaced by it, because silver beech is more shade tolerant and longer-lived. Thus, factors promoting stand synchrony are important for mountain and red beech, because if large scale canopy opening does not occur, sites are more likely to be lost to competitors. Cohort synchrony is associated also with physiological synchrony between individuals. Consequently periodic stresses, such as mast seeding, may affect almost all individuals. Storm damage and drought years are implicated. If only a few individuals (or even parts of individuals) die, this can lead to rapid stand decline, and subsequent large scale regeneration.

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.

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.

Stand-level dieback and ecosystem processes: A global perspective

Stand-level dieback and ecosystem processes: A global perspective

Stand-level dieback andMetrosideros regeneration in the montane rain forest of Hawaii

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.

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.

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.

Nutrient Limitation and Metrosideros Forest Dieback in Hawaii

Large numbers of Metrosideros polymorpha trees have died in the montane rain forest on the Island of Hawai'i, but previous research has failed to identify a principal cause. This paper describes an experiment that tests the hypothesis that nutrient deficiency is the principal cause of tree death and stand—level dieback. Treatments were fertilizing, stand thinning, and a combination of the two. No significant change in the mortality rate was caused by the treatments; however, stem diameter growth of surviving trees was found to be nutrient limited at all sites. We conclude that nutrient deficiency is not the principal cause of most Metrosideros dieback, but it may be a contributing factor.

Natural Dieback in Forests

urrently, much public concern and research effort focuses on forest dieback in industrial countries. Factors associated with air pollution, such as acid rain, nitrous oxide, changes in the ozone level, and heavy metal deposition are often considered responsible for tree mortality on both sides of the Atlantic. While it is absolutely clear that air pollution from nearby smelters can cause stand-level tree mortality by toxification (Smith 1981), the acid-rain relationship with forest dieback (Ulrich 1981, 1982) is as yet highly conjectural (Cramer 1984, Johnson 1983, Kandler 1983, Nihlgard 1985, Rehfuess 1983, Schiitt and Cowling 1985). This article is not intended to diminish concern about air pollution's relationship to forest dieback. The objective instead is to point out that stand-level dieback in forests is by no means a new phenomenon and further that, in many cases, it is due to natural causes. While it is sometimes possible to distinguish between manmade and natural causes of standlevel dieback, it is usually difficult to determine the exact causes. Dieback is the death of groups of neighboring trees rather than isolated trees dying in an otherwise green forest matrix. Dieback stands are forest segments having significant loss of canopy. In these stands the majority of trees are either dead or display reduced vigor. The term dieback is

PERSPECTIVES FOR AN ETIOLOGY OF STAND-LEVEL DIEBACK

When trees are dying in large numbers in a forest stand, it is usually assumed they are dying from a disease. Therefore, the study of stand-level dieback has traditionally been the concern of the forest pathologist and pest entomologist. However, stand-level dieback is not always caused by biotic agents, and the term as defined in this paper refers to stands of dead or dying trees whose dieback cause is not obvious and that typically occur in several locations of a larger forest ecosystem. In such stands, adjacent trees as well as spatially separated trees die more or less simultaneously, and loss of foliage usually begins from the top downwards. Dieback trees may be identified either as standing snags or as still living but stag-headed trees. Such dieback stands are common in many indigenous Pacific forests (41), and they have received attention also in several North American (32) and European forests (9). The recent emphasis on the dieback of forests in industrial countries has enlarged the group of researchers to include soil scientists, foresters, and pollution specialists. In both the pathological and air pollution research of dieback, the structure of the forests with dieback, their spatial and temporal patterns, their habitat relationships, and their associated vegetation and successional re­ sponses have received less attention than the more immediate symptoms and assumed causes. However, to arrive at a more complete etiology, such vegetation structural aspects should be analyzed with equal effort. Without such effort, it is difficult to distinguish commonalities and discrepancies in stand-level dieback of different forest biomes. For example, Jimenez et al (21) described stand-level dieback in mangrove forests as and suggested that mass diebacks always have an abiotic rather than biotic cause. Their distinction between mass mortality and normal mortality is helpful in understanding natural dieback, but when larger and smaller areas of

Perspectives for an Etiology of Stand-Level Dieback

Species distribution models (SDMs) are numerical tools that combine observations of species occurrence or abundance with environmental estimates. They are used to gain ecological and evolutionary insights and to predict distributions across landscapes, ...Read More