Frequency Of Natural Disturbances Research Articles

Predicting biomass of hyperdiverse and structurally complex central Amazonian forests – a virtual approach using extensive field data

Abstract. Old-growth forests are subject to substantial changes in structure and species composition due to the intensification of human activities, gradual climate change and extreme weather events. Trees store ca. 90 % of the total aboveground biomass (AGB) in tropical forests and precise tree biomass estimation models are crucial for management and conservation. In the central Amazon, predicting AGB at large spatial scales is a challenging task due to the heterogeneity of successional stages, high tree species diversity and inherent variations in tree allometry and architecture. We parameterized generic AGB estimation models applicable across species and a wide range of structural and compositional variation related to species sorting into height layers as well as frequent natural disturbances. We used 727 trees (diameter at breast height ≥ 5 cm) from 101 genera and at least 135 species harvested in a contiguous forest near Manaus, Brazil. Sampling from this data set we assembled six scenarios designed to span existing gradients in floristic composition and size distribution in order to select models that best predict AGB at the landscape level across successional gradients. We found that good individual tree model fits do not necessarily translate into reliable predictions of AGB at the landscape level. When predicting AGB (dry mass) over scenarios using our different models and an available pantropical model, we observed systematic biases ranging from −31 % (pantropical) to +39 %, with root-mean-square error (RMSE) values of up to 130 Mg ha−1 (pantropical). Our first and second best models had both low mean biases (0.8 and 3.9 %, respectively) and RMSE (9.4 and 18.6 Mg ha−1) when applied over scenarios. Predicting biomass correctly at the landscape level in hyperdiverse and structurally complex tropical forests, especially allowing good performance at the margins of data availability for model construction/calibration, requires the inclusion of predictors that express inherent variations in species architecture. The model of interest should comprise the floristic composition and size-distribution variability of the target forest, implying that even generic global or pantropical biomass estimation models can lead to strong biases. Reliable biomass assessments for the Amazon basin (i.e., secondary forests) still depend on the collection of allometric data at the local/regional scale and forest inventories including species-specific attributes, which are often unavailable or estimated imprecisely in most regions.

Natural disturbance regimes as templates for the response of bird species assemblages to contemporary forest management

AbstractAimIn managed forest landscapes, the tolerance of species to contemporary alteration of forest cover is often assumed to reflect their resilience to natural disturbances. We tested this central tenet of ecosystem‐based management by comparing the structure of forest bird assemblages among four regions with contrasting historical natural disturbance regimes.LocationCanada's boreal and northern hardwood forests.MethodsUsing point count data from four study regions across Canada, we first determined the relative sensitivity of individual bird species to the contemporary reduction of old forest cover at stand and ‘landscape‐context’ (1‐km radius) scales with log‐linear models. The richness of species most sensitive to loss of old forest (hereafter ‘sensitive species’) was then modelled as a function of landscape‐scale changes in old forest cover. Differences in the rate of decline in the richness of sensitive species with contemporary cover of old forest were compared among regions using ANCOVA. We then compared broken‐stick regression models with linear models to detect thresholds, if present, in this relationship in each region.ResultsBird assemblages from regions with relatively infrequent natural disturbances hosted more species sensitive to contemporary reduction in old forest cover. Those species were also more abundant than in regions with frequent natural disturbances, and the rate of decline in their richness with the loss of old forest was steeper in regions with infrequent natural disturbances than in those where they were frequent. However, we did not detect thresholds in this rate of decline in any study region.Main conclusionsOur findings are consistent with the contention that historical natural disturbance regimes shape the response of biota to contemporary landscape alterations through evolutionary adaptation. We argue that forest management conducted within the natural range of variability in stand and landscape structure specific to a region is likely to be ecologically sustainable.

Frequent severe natural disturbances and non-equilibrium landscape dynamics shaped the mountain spruce forest in central Europe

Disturbances shape forest structure and composition, but the temporal dynamics of disturbance patterns, their influence on dynamics of forest structural complexity, and the potential impacts of ongoing climate changes are not fully understood. We addressed these issues by focusing on (1) long-term, landscape level retrospective analysis of disturbance dynamics of mountain Norway spruce (Picea abies) forest; (2) testing for the prevailing disturbance agent; and (3) the detection of disturbance drivers, particularly site conditions, using a dendrochronological approach.We had a unique opportunity to sample a large area of natural forest (in otherwise altered region) in Bohemian Forest, that was recently affected by extensive and severe wind and bark beetle disturbances raising management concerns. Tree-ring series of a minimum of 35 trees on each of the 26 study plots were analysed for signals of past disturbances that were indicated either by release from suppression (rapid growth increase) or rapid early growth rate (gap origin).A broad range of disturbance severities were detected across the landscape, but severe disturbances (>50% trees responded), with a short rotation period of 174years, best characterize the area. Reconstructed disturbances were associated with historically documented windstorms and bark beetle outbreaks. They were distributed across the landscape and affected its large portions with spatially variable severity. Over the last five centuries, disturbances peaked in the 1820s, but were rare after 1880. This non-equilibrium dynamics resulted into mature landscape structure at the end of 20th century and contributed to the large extent of disturbances in the last two decades. Disturbances were more frequent on wind-exposed locations such as mountain ridges and flatter terrain.Frequent disturbances did not allow the forest to develop into an old-growth phase; rather disturbance events of variable severities created structures important for biodiversity and could also preserve key early-seral phases in the landscape. We anticipate that potential future increase of disturbance intensities will not lead to comparable regime shifts in forests shaped by high-severity disturbances that kill largely mature trees (e.g., wind and bark beetle), because such disturbances are followed by resistant forest structure. Conservation areas should be large enough to allow for a wide range of disturbances. Emphasizing ecological functions needs understanding of biological legacies of natural disturbances and managers could partly rely on natural events to restore natural features into cultivated forests.

Multi‐temporal Forest Cover Change and Forest Density Trend Detection in a Mediterranean Environment

AbstractThe loss of forests along with the various types of shrubs in the Mediterranean region is seen as an important driver of climate change and has been repeatedly related with the observed land degradation and desertification in the region. Nevertheless, the extent of woody perennial vegetation cover (WPVC) and its density remain largely unclear. Here, we apply a series of algorithms and methods operationally used in Australia for large‐scale WPVC mapping and monitoring and demonstrate their applicability in the Mediterranean region using a Spanish area as the trial site. Five Landsat TM and ETM+ images from various dates spanning 14 years are used to map changes in the extent of WPVC and to identify areas with a declining, stabilising or recovering trend. Results show that the applied methodology, which incorporates (i) pre‐processing of the Landsat imagery, (ii) a canonical variate analysis to spectrally discriminate between woody and non‐woody land cover types, (iii) a conditional probability network and (iv) spectral indices for mapping woody cover and density trend, is highly successful and well suited for use in Mediterranean environments. A rigorous accuracy assessment is undertaken producing overall accuracies above 97% for both woody and non‐woody cover types and all dates. Results also show that in the area of study, the majority of WPVC disturbances were due to forest fires, which represent the region's most frequent natural and anthropogenic disturbance. This raises significant concerns about the future of the area's WPVC. Regeneration compensated to some degree for the high disturbance rates. Copyright © 2015 John Wiley & Sons, Ltd.

Application of landscape metrics and a Markov chain model to assess land cover changes within a forested watershed, Taiwan

AbstractAnalysis of land cover changes is fundamental for providing information about watershed land management, monitoring, and planning. This study reveals large‐scale land cover transformation under the effects of frequent natural disturbances within the Taimali watershed in eastern Taiwan during 2005–2011. A landscape analysis approach combining landscape metrics and a Markov chain model is used to understand land cover changes with regard to natural disturbances. Results of the composition metrics analysis show that the landslide area within the region has expanded 7.5 times (15.5 km2), but the forested area has shrunk by 10.6% (20.9 km2). Spatial configuration metrics analysis indicates that patches of forest are becoming small, irregular, and spatially fragmented, while landslide patches are expanding and becoming spatially aggregated. Land cover changes in the area are considered to have occurred mainly through loss and fragmentation of forest cover, and an increase in the number and area of landslides. During 2005–2011, the most noticeable area changes related to transitions from forest cover to landslides (15.8 km2) and channels (6.7 km2). However, the greatest transition probabilities were for human‐made patches changing into forest cover and channel corridors over the study period. Through the Markov chain analysis, three projected proportions of land cover for the periods 2017–2035 are produced. These three predictive scenarios pose different risks to downstream life and property. Copyright © 2015 John Wiley & Sons, Ltd.

Forest regeneration dynamics following bark beetle outbreak in Norway spruce stands: Influence of meso-relief, forest edge distance and deer browsing

Management of Norway spruce monocultures in Europe is becoming increasingly difficult due to frequent natural disturbances. Their transformation could be especially challenging if several disturbances interact. In 2003 a spruce bark beetle outbreak damaged large tracts of spruce bottomland successional forest in southeast Slovenia where overabundant ungulate populations are present. In openings (5.4–7.5ha in size) in four salvaged forest compartments, we studied the effects of meso-relief, forest edge, seed trees and fencing on vegetation succession and tree regeneration. In 2005 and 2013 we sampled seedling density on 240 plots according to height class and species. In the second inventory we also assessed distance to seed trees and on a subsample of the plots the coverage of vascular plants. The results indicated sufficient natural regeneration and niche partitioning among species groups, with Norway spruce seedlings being more successful at stand edges, pioneers and anemochorous broadleaves in gap centres, and zoochorous broadleaves on plateaus as opposed to sinkholes. Seedling density of all anemochorous broadleaves was higher closer to seed trees. Spruce seedling abundance was negatively associated with the cover of herbaceous vegetation and that of pioneer trees with shrubs. Fencing resulted in significantly greater density, height and coverage of seedlings as well as a higher share of broadleaves. While bark beetle outbreak triggered regeneration of broadleaves, overbrowsing acted in the opposite direction by facilitating spruce. Thus, to accelerate successional development and to prevent a spruce dominated alternative ecosystem state, a significant reduction in deer abundance is needed. To safeguard seed sources, promote seed dispersal by encouraging perches and shelters, and preserve potential habitats, some damaged trees should be retained during salvaging operations.

Edge influence on vegetation at natural and anthropogenic edges of boreal forests in Canada and Fennoscandia

Summary Although anthropogenic edges are an important consequence of timber harvesting, edges due to natural disturbances or landscape heterogeneity are also common. Forest edges have been well studied in temperate and tropical forests, but less so in less productive, disturbance‐adapted boreal forests. We synthesized data on forest vegetation at edges of boreal forests and compared edge influence among edge types (fire, cut, lake/wetland; old vs. young), forest types (broadleaf vs. coniferous) and geographic regions. Our objectives were to quantify vegetation responses at edges of all types and to compare the strength and extent of edge influence among different types of edges and forests. Research was conducted using the same general sampling design in Alberta, Ontario and Quebec in Canada, and in Sweden and Finland. We conducted a meta‐analysis for a variety of response variables including forest structure, deadwood abundance, regeneration, understorey abundance and diversity, and non‐vascular plant cover. We also determined the magnitude and distance of edge influence (DEI) using randomization tests. Some edge responses (lower tree basal area, tree canopy and bryophyte cover; more logs; higher regeneration) were significant overall across studies. Edge influence on ground vegetation in boreal forests was generally weak, not very extensive (DEI usually < 20 m) and decreased with time. We found more extensive edge influence at natural edges, at younger edges and in broadleaf forests. The comparison among regions revealed weaker edge influence in Fennoscandian forests. Synthesis. Edges created by forest harvesting do not appear to have as strong, extensive or persistent influence on vegetation in boreal as in tropical or temperate forested ecosystems. We attribute this apparent resistance to shorter canopy heights, inherent heterogeneity in boreal forests and their adaptation to frequent natural disturbance. Nevertheless, notable differences between forest structure responses to natural (fire) and anthropogenic (cut) edges raise concerns about biodiversity implications of extensive creation of anthropogenic edges. By highlighting universal responses to edge influence in boreal forests that are significant irrespective of edge or forest type, and those which vary by edge type, we provide a context for the conservation of boreal forests.

Population persistence of a Tertiary relict tree Tetracentron sinense on the Ailao Mountains, Yunnan, China

The persistence of the Tertiary relict tree Tetracentron sinense Oliv. on the eastern slope of the Ailao Mountains, Yunnan, SW China, was here studied in terms of population structure (size, age) and regeneration patterns. T. sinense occurred in unstable habitats by stream banks, on steep slopes, on scree slopes, or on roadsides near streams in narrow valleys, all places subject to frequent natural disturbances, whereas none were found on stable gentle slopes free of major disturbances at similar altitudes. Further, no established saplings of T. sinense were found in forests having high bamboo (Yushania crassicollis Yi) coverage in their understory. The size and age structure of T. sinense were multimodal. The reproduction of the tree was either by means of abundant minute wind-dispersed seeds or by resprouts in unstable habitats. These populations depended on disturbance or gap regeneration to survive. T. sinense, along with other tree life-forms including evergreen broad-leaved species and conifers, dominated in the forest canopy layer, even reaching the emergent layer in places. Results of the study provide insight into the ecological characteristics and survival mechanisms of this East Asian paleoendemic tree species. The study will provide a scientific basis for recommendations for the conservation of this species and for other Tertiary relict plants having similar regeneration dynamics.

Niche partitioning of fish assemblages in a mountain stream with frequent natural disturbances – an examination of microhabitat in riffle areas

Abstract – Microhabitat usage by different species of fish was studied to examine the potential for niche partitioning in riffle areas of a mountain river characterised by frequent natural disturbances. We randomly sampled 96 riffle‐area plots (mostly 1 m × 1 m) in a stream in south‐western Taiwan for fish abundance and microhabitat characteristics during the dry season from November 2008 to March 2009. Density and electivity values associated with microhabitat usage indicate the presence of niche partitioning among four major species of fish based largely on flow velocity and water depth. Hemimyzon formosanum, a herbivore, prefers higher velocity and shallower water but avoids sandy substrate. Onychostoma alticorpus, a herbivore, prefers lower velocity and deeper water. Rhinogobius nantaiensis, an omnivore, prefers higher velocity, shallower water and gravel‐sized substrate but avoids boulder substrate. Acrossocheilus paradoxus, an omnivore, prefers lower velocity and deeper water but avoids pebble‐sized substrate. The existence of differentiation of microhabitat preference and overlap of microhabitat usage suggests that interspecific competition is a factor affecting the structure of this fish assemblage, which may remain at early successional stages because of frequent disturbances in the stream. Study results also show that density and electivity detect preference differently. Density detects microhabitat preference for two herbivores while electivity detects microhabitat preference for two omnivores.

Relationship between Razor Clam Fishing Intensity and Potential Changes in Associated Benthic Communities

Macrobenthic community changes were assessed in relation to different levels of fishing intensity following a before—after, control—impact experimental design with a control, a moderately impacted area, and a highly impacted area. Samples were collected before, immediately after, and 1, 2, 3, 7, 14, 30, and 120 days after fishing. Communities in fished areas showed short-term significant effects on abundance, number of species, biomass, and diversity 1–3 days after fishing, with fished areas presenting lower mean values than the control. The opposite pattern was found for the carnivore-to-suspension feeding ratio. The main taxonomic groups showed different responses to fishing. Only gastropod abundance and the abundance of the polychaete families Pectinariidae and Sabellidae showed persistent negative effects of fishing. The effects on these polychaete families confirms the sensitivity of sedentary and suspension feeders to fishing. Some variables presented slightly higher negative effects with fishing intensity, but differences were subtle. Between 30 days and 120 days, a period of storms occurred, resulting in a general decrease of biological variables in all areas. This decrease was usually more pronounced than after fishing. The general short-term effects of fishing observed may reflect the resilience of communities that are able to cope with frequent natural disturbance events.

Changes in forest structure and composition on Changbai Mountain in Northeast China

Long-term ecological studies are essential for understanding the dynamics of natural systems. However, describing long-term dynamics in old-growth forests is often hindered by a lack of archival data. In this study, we investigated (1) changes in forest structure and tree species composition and (2) population dynamics of the dominant tree species over a 29-year period (1981–2010) in two old-growth forests with stand ages exceeding 200 years on Changbai Mountain in Northeast China. Results showed that the stem density of the broad-leaved/conifer mixed forest at 740 m asl increased slightly from 510 to 515 trees per hectare and the total basal area increased significantly from 36.19 to 46.53 m2 ha−1, suggesting that the forest was still maturing over the study period. However, the coniferous forest at 1,680 m asl displayed structural changes in the form of a significantly decrease in both stem density and total basal area over the study period. No changes in tree species composition in the two forests were observed. Wind was considered to be a frequent natural disturbance affecting the forest structure and dynamics, especially for the coniferous forest at the higher altitude. Dominant tree species in the two forests exploited different biological characteristics in maintaining their respective populations.

Characteristics of Dominant and Subordinate Led Social Groups of White-tailed Deer in Illinois

This study of dominant and subordinate led social groups of white-tailed deer (Odocoileus virginianus) was designed to investigate longevity and associations among members as well as the reproductive success that determines the durability of these groups. Characteristics of 25 dominant and 17 subordinate female led social groups of white-tailed deer were studied on three areas in Illinois. Group size for dominant led social groups ranged between 3.8 and 5.2 deer/y and for subordinate led groups only 2–2.5 deer. Dominant females survived significantly longer (8.2 y) then did subordinate females (5.4 y) and fawns born to dominants were significantly more sedentary after independence. Fawn recruitment (fawns alive at 1 y) was also significantly higher for fawns born to dominant females. Members of a dominant female's social group generally confined themselves to the home range of the dominant female but as they aged were seen less often with her. Dominant females occupied stable habitats free of environmental problems while subordinates occupied ranges with frequent natural and human induced disturbances. By association, fawns of dominant females inherit a stable home range that fosters improved longevity and successful fawn recruitment

Reproductive biology and early life history of the hermaphroditic feather star Dorometra sesokonis (Echinodermata: Crinoidea)

Reproduction and larval/post-larval development of the one of the smallest known comatulid Dorometra sesokonis were studied on the coral reefs of Sesoko-jima Island, Okinawa, Japan. Breeding individuals were found every month from May 2003 to April 2004 indicating continuous reproduction. Individuals possessed both mature ovaries and testes, and were therefore characterized as simultaneous hermaphrodites. This species was observed to be an external brooder in which the embryos developed on the surface of parental genital pinnules until the doliolaria larval stage, and subsequently released doliolariae settled on substrata within 4 days after hatching. Larvae then metamorphosed into the post-larval cystidean stage, which lasted for more than a month. The reproductive features of this species are unique among crinoids, and, together with juvenile morphological features observed in adults, can be interpreted as adaptations to their cryptic habitat, which is subject to frequent natural disturbances.

Application of least-cost path model to identify a giant panda dispersal corridor network after the Wenchuan earthquake—Case study of Wolong Nature Reserve in China

With growing levels of human-activity and frequent natural disturbances throughout the world, it is increasingly important that both research and management efforts take into account the widespread landscape fragmentation and its consequences for biodiversity conservation. The magnitude 5.12 Wenchuan earthquake in China caused dramatic impacts on giant panda ( Ailuropoda melanoleuca) habitat in the nature reserves within Minshan and Qionglai mountains. With the combined stresses of the natural disaster and the extensive human activities during postquake reconstruction, giant panda habitat in this region may become more fragmented in the future. In order to preserve the giant panda population after the earthquake and protect the species against habitat fragmentation, this article explores a method of identifying giant panda migration corridors involving habitat suitability assessments and a least-cost path model. Focusing on postquake Wolong Nature Reserve, our results demonstrate that it contains 430.3 km 2 of suitable habitat (21.1% of total area), 463.8 km 2 of marginally suitable habitat (22.8%) and 1141.9 km 2 of unsuitable habitat (50.1%). We further show that several giant panda dispersal corridors exist in the reserve, including four corridor groups that cross the provincial highway and five corridors that do not intersect areas of human activity. This study will contribute to management and conservation efforts in Wolong Nature Reserve and beyond after the Wenchuan earthquake.

AN EXPERIMENTAL DISTURBANCE ALTERS FISH SIZE STRUCTURE BUT NOT FOOD CHAIN LENGTH IN STREAMS

Streams experience frequent natural disturbance and are undergoing considerable anthropogenic disturbance due to dam construction and water diversion. Disturbance is known to impact community structure, but its effect on food chain length is still a matter of considerable debate. Theoretical models show that longer food chains are less resilient to disturbance, so food chain length is predicted to be shorter following a disturbance event. Here we experimentally test the effect of disturbance on food chain length in streams by diverting stream flow. We found that our experimental low-flow disturbance did not alter food chain length. We did see an effect on body-size structure in our food webs suggesting that food chain length may be an insensitive indicator of disturbance. We suggest that habitat heterogeneity and food web complexity buffer the effect of disturbance on food chain length. The theoretical predictions of disturbance on food chain length are only likely to be seen in homogeneous systems that closely approximate the linear food chains the models are based upon.

The Effect of Fire Reintroduction on Endemic and Rare Plants of a Southeastern Glade Ecosystem

Abstract Open habitats dominated by herbaceous plants on thin, rocky soils occur within the forests of eastern North America. Although these habitats vary in origin, structure, geology, and species composition, all contribute greatly to regional biodiversity by harboring endemic and/or rare plants. Little is known about how disturbances affect plant populations in these ecosystems. Fire once was a frequent natural disturbance in the Ketona dolomite glades of Alabama, an ecosystem harboring eight endemic taxa and numerous other species of conservation concern. We designed an experiment to determine how the reintroduction of fire into the glades and surrounding longleaf pine forests affects populations of rare glade plant species. Experimental and control plots were established within the glades. Experimental plots were burned in April 2004, and all plots were surveyed during two subsequent growing seasons (2004 and 2005). Populations of three of 14 species of conservation concern declined significantly after the initial fire but recovered the next year. Among other herbaceous species, only five and two differed in population size in 2004 and 2005, respectively. In 2004, more species were more abundant in control than burned plots, but this difference was not detected in 2005. Multivariate community‐level analyses of species presence–absence suggested that the effects of fire were negligible by the 2005 survey. Populations of young trees that had invaded the glades declined dramatically as a result of treatment fires. These results suggest that the reintroduction of fire will not harm glade species and may help prevent encroachment of the surrounding forest.

Spatial patterns of tropical forest trees in Western Polynesia suggest recruitment limitations during secondary succession

Spatial analysis can be used to relate the patterns of tree species to their regeneration syndromes – pioneer to late-successional – and is a first step in refining hypotheses about the species traits and biotic and abiotic factors that give rise to forest community dynamics. This study examines the spatial pattern of the most abundant trees in three 0.45-ha plots in species-poor lowland rain forests on oceanic islands in Tonga, Western Polynesia, that experience frequent natural disturbance and have a 3000-y history of shifting cultivation. We contrast secondary vs. remnant late-successional forest, with particular attention paid to the spatial dispersion and clustering of tree species, and the presence of spatial dependence in the density of seedlings and saplings. Shade-tolerant species were not strongly clustered at any scale. They did not appear to be dispersal limited, in late successional forest, and only some showed patterns consistent with density-dependent mortality (more clumped when small). Shade-tolerant species were more clumped in secondary forest, and may be dispersal-limited there because vertebrate dispersers prefer primary forest. Shade-intolerant species were clumped in gaps in late-successional forest, but some were also clumped in secondary forest, indicating that they too may be dispersal limited during secondary succession. We also compared the species composition of seedlings and saplings in the centre of plots with trees in the surrounding area and inferred that active dispersal (by vertebrate frugivores) contributed as much as 50% to site species richness.

Successional replacement mediated by frequency and severity of wind and snow disturbances in a Picea‐Abies forest

AbstractQuestion:How do wind and snow disturbances affect the successional replacement of Picea glehnii by Abies sachalinensis in a sub‐alpine coniferous forest?Location:Cool temperate, northern Japan.Methods:Tree demography (growth, mortality and recruitment rates) was determined by repeated measurements of stem diameter and height, and multiple censuses in four Picea‐Abies stands undergoing succession. Above‐ground stand bio‐mass, residence time and tree growth trajectories of the component species were estimated to examine successional changes in structure and dynamics. Individual based simulations were used to examine the effects of disturbances that slowed down succession. Multiple regression analyses were used to determine the relative importance of disturbance frequency and intensity on species composition during succession.Results:Above‐ground biomass was larger in P. glehnii than in A. sachalinensis stands, whereas residence time, a proxy of productivity, was much shorter for A. sachalinensis than for P. glehnii populations. During successional replacement, both species increased in initial growth rate and decreased in size‐dependency of growth in canopy gaps. These plastic growth responses were more prominent in P. glehnii than in A. sachalinensis. Disturbance frequency was the most important predictor of species composition in the simulations, and windstorms were more important than snowfall in terms of disturbance intensity.Conclusions:The frequency of natural disturbances does not have the potential to initiate stand dynamics but it does, however, slow down succession. When disturbance is locally frequent because of the direction and pitch of the topography, early‐successional P. glehnii stands may persist for thousands of years.

Dispersal of plant fragments in small streams

Summary1. Streams are subject to frequent natural and anthropogenic disturbances that cause sediment erosion and loss of submerged vegetation. This loss makes downstream transport and retention of vegetative propagules on the streambed very important for re‐establishing vegetation cover. We measured dispersal and retention of macrophyte stem fragments (15–20 cm long) along 300 m long reaches of four small to medium sized Danish lowland streams.2. The number of drifting stem fragments declined exponentially with distance below the point of release. This finding makes the retention coefficient (k, m−1) in the exponential equation a suitable measure for comparisons among different macrophyte species, and between stream reaches of different hydrology and vegetation cover.3. Buoyancy of macrophyte tissue influenced retention.Elodea canadensisstems drifted below the water surface, and were more inclined to be retained in deeper water associated with submerged plants and obstacles in the streambed.Ranunculus peltatusstems were more buoyant, drifted at the water surface, and were more inclined to be trapped in shallow water and in riparian vegetation.4. The retention coefficient of drifting stems increased with the relative contact between the flowing water and streambed, bank and vegetation. Thus, the retention coefficients were highest (0.02–0.12 m−1) in shallow reaches with a narrow, vegetation‐free flow channel. Here there were no significant differences betweenE. canadensisandR. peltatus. Retention coefficients were lowest (0.0005–0.0135 m−1) in deeper reaches with wider vegetation‐free flow channels. Retention ofE. canadensiswas up to 16 times more likely than retention ofR. peltatus.5. Overall, the longitudinal position in the stream system of source populations of species capable of producing numerous stems, the species‐specific retention coefficients of stems, and the retention capacity of stream reaches should be important for species distribution in perturbed stream systems. Retention of stems is probably constrained in headwaters by the small downstream flux of stem fragments because of the restricted source area, and constrained in downstream reaches by small retention coefficients. Macrophyte retention may, consequently, peak in medium‐sized streams.

Indirect effects of fish winterkills on amphibian populations in boreal lakes

We exploited fish winterkills in small, boreal Alberta lakes to determine if anuran amphibians respond to large but natural changes in fish densities. Eight large declines in fish abundance occurred in seven lakes over a 5 year period, while major increases in fish abundance, reflecting recovery after winterkill, were recorded 5 times. Summer pitfall trapping of young-of-the-year (YOY) Wood Frogs (Rana sylvatica LeConte, 1825) and Boreal (Bufo boreas boreas Baird and Girard, 1852) and Canadian (Bufo hemiophrys Cope, 1886) toads indicated that frog abundance responded consistently to such large changes in fish abundance, but especially if fish communities were dominated by small-bodied species (sticklebacks and minnows). As well, changes in YOY Wood Frog and fish abundance were negatively correlated; YOY Wood Frogs were as much as 7.7 times more abundant after winterkills than in non-winterkill years. These increases in metamorphs did not result from an increased immigration of breeding adults to winterkill lakes, suggesting instead that larval survival was greater. Higher abundance of YOY Wood Frogs and toads was associated with smaller body size at metamorphosis. Despite this apparent reduction in individual growth, abundance of juvenile frogs remained significantly elevated 1 year after winterkill. In contrast to Wood Frogs, YOY toads tended to respond positively to recoveries of small-fish populations. Because anuran amphibians can respond to fish winterkill, and because winterkill is a frequent natural disturbance, small fish-bearing lakes can serve as important breeding habitat for amphibians in Alberta's boreal forest.