Mixed-severity Disturbance Regimes Research Articles

Response of habitat quality to mixed severity disturbance regime in Norway spruce forests

Abstract Natural disturbances change forest habitat quality for many species. As the extent and intensity of natural disturbances may increase under climate change, it is unclear how this increase can affect habitat quality on different spatial scales. To support management tools and policies aiming to prevent habitat loss, we studied how habitat quality develops in the long run depending on the disturbance severity using a space‐for‐time substitution approach. We explored the effects of time since disturbance (0–250 years) and disturbance severity (20%–100% canopy removal) on structure‐based habitat quality indicators in European primary Norway spruce Picea abies forests using 1000 m2 circular plots in hierarchical design (a total of 407 plots in 35 stands). Disturbance history was reconstructed from tree cores. Habitat quality indicators were modelled as a function of the severity of the most severe disturbance and the time since this disturbance. We hypothesised that high within‐stand habitat heterogeneity is formed by different successional stages after disturbances of various intensities. The results showed a U‐shaped response of habitat quality to post‐disturbance habitat succession on the plot scale. The decline deepened with disturbance severity. The U‐shape response occurred in: large tree occurrence, amount of standing and lying deadwood, diversity of understory and understory openness. The spatial diversity in disturbance parameters increased spatial diversity of habitat quality on a stand level as expected. This high within‐stand habitat heterogeneity also decreased with increasing age of the most recent disturbance. This suggests that the absence of young successional stages results in the absence of some important elements for biodiversity, for example sun‐exposed snags. Synthesis and applications. Our results demonstrate that currently intensifying natural disturbance regime can consequently result in a lower habitat heterogeneity. In managed spruce forests after natural disturbances, we recommend at least the partial retention of biological legacies to preserve habitat heterogeneity and to avoid uniform and dense plantations resulting in a greater homogenisation. To emulate the natural disturbances pattern, spruce forests should be managed with a wide range of harvested patches of the size limited by a local natural disturbance regime creating spatial heterogeneity.

Historical mixed-severity disturbances shape current diameter distributions of primary temperate Norway spruce mountain forests in Europe

Natural disturbances strongly influence forest structural dynamics, and subsequently stand structural heterogeneity, biomass, and forest functioning. The impact of disturbance legacies on current forest structure can greatly influence how we interpret drivers of forest dynamics. However, without clear insight into forest history, many studies default to coarse assumptions about forest structure, for example, whether forests are even or unevenly aged. The aim of this study was to analyze the effects of past disturbances on the current diameter distributions of Norway spruce (Picea abies (L.) Karst.)-dominated landscapes throughout the Carpathian Mountains. Our dendroecological dataset comprises tree cores from 339 plots (7,845 total tree cores), nested within 28 primary forest stands, known to vary greatly in the severity of historical disturbances. Our analyses revealed that historical disturbances had a strong and significant effect on the current diameter distribution shapes at the plot level. We demonstrated that mixed-severity disturbance regimes were more frequent and create a complex pattern of diameter distributions at the plot and stand scale. Here, we show that high severity disturbance was associated with unimodal diameter distributions, while low and moderate severity was associated with the reverse J-shaped distribution. This is a result of complex disturbance patterns, with structural biological legacies. Our results will have important management implication in the context of tree size heterogeneity, biomass storage, and productivity as influenced by natural disturbances. Lastly, these results demonstrate that structural changes may arise as consequences of changing disturbance regime associated with global change.

Natural dynamics of temperate mountain beech-dominated primary forests in Central Europe

Natural disturbances are key factors in the formation of forest ecosystem structure. Evaluation of the spatial and temporal extent of disturbance regimes is critical for understanding forest dynamics, forest structural heterogeneity, and biodiversity habitats. Quantifying disturbance regimes is therefore imperative for appropriate management of forests and protected areas. However, natural disturbance regimes have rarely been assessed using dendrochronological methods on a regional scale across primary mixed beech-fir forest stands - one of the dominant forest vegetation types in Europe. To study the natural disturbance regimes of beech-dominated mixed-forest stands, we established 42 permanent study plots with an area of 0.1 ha across three primary forest stands in the Western Carpathians, a region that still contains large areas of primary forest. We reconstructed each stand-level disturbance history using a tree-ring based approach. The temporal synchronicity of disturbance events was then evaluated by delineating stand-level disturbance events using a kernel density function, and through the detection of plot-level disturbances with severities greater than 10 percent. The results obtained from the chronologies showed substantial variability in time and space, especially in the mid-19th century. Low- and moderate-severity plot-level disturbance events were most common, but high- and extremely high-severity plot-level disturbance events also occurred. The observed spatial and temporal variability suggests that the beech-dominated forests were primarily driven by mixed-severity disturbance regimes, with windstorms as the main disturbance agent.This reconstruction of the disturbance regime provided unique insight into the scale of mortality processes in these beech-dominated mixed forests. This information can help guide ecological forestry in areas where both wood production and biodiversity preservation are simultaneous goals, such as by employing more spatiotemporally-complex silvicultural systems that resemble natural disturbance patterns and facilitate heterogeneous forest structures.

More ways than one: Mixed-severity disturbance regimes foster structural complexity via multiple developmental pathways

Abstract Mixed-severity disturbance regimes are prevalent in temperate forests worldwide, but key uncertainties remain regarding the variability of disturbance-mediated structural development pathways. This study investigates the influence of disturbance history on current structure in primary, unmanaged Norway spruce (Picea abies) forests throughout the Carpathian Mountains of central and eastern Europe, where windstorms and native bark beetle outbreaks are the dominant natural disturbances. We inventoried forest structure on 453 plots (0.1 ha) spanning a large geographical gradient (>1,000 km), coring 15–25 canopy trees per plot for disturbance history reconstruction (tree core total n = 11,309). Our specific objectives were to: (1) classify sub-hectare-scale disturbance history based on disturbance timing and severity; (2) classify current forest structure based on tree size distributions (live, dead, standing, downed); (3) characterize structural development pathways as revealed by the association between disturbance history and current forest structural complexity. We used hierarchical cluster analysis for the first two objectives and indicator analysis for the third. The disturbance-based cluster analysis yielded six groups associated with three levels of disturbance severity (low, moderate, and high canopy loss) and two levels of timing (old, recent) over the past 200 years. The structure-based cluster analysis yielded three groups along a gradient of increasing structural complexity. A large majority of plots exhibited relatively high (53%) or very high (26%) structural complexity, indicated by abundant large live trees, standing and downed dead trees, and spruce regeneration. Consistent with conventional models of structural development, some disturbance history groups were associated with specific structural complexity groups, particularly low-severity/recent (very high complexity) and high-severity/recent (moderate complexity) disturbances. In other cases, however, the distribution of plots among disturbance history and structural complexity groups indicated either divergent or convergent pathways. For example, multiple disturbance history groups were significantly associated with the high complexity group, demonstrating structural convergence. These results illustrate that complex forest structure – including features nominally associated with old-growth – can be associated as much with disturbance severity as it is with conventional notions of forest age. Because wind and bark beetles are natural disturbance processes that can induce relatively high levels of tree mortality while simultaneously contributing to structural complexity and heterogeneity, we suggest that forest management plans allow for the stochastic occurrence of disturbance and variable post-disturbance development trajectories. Such applications are especially appropriate in conservation areas where biodiversity and forest resilience are management objectives, particularly given projections of increasing disturbance activity under global change.

Mixed-severity natural disturbances promote the occurrence of an endangered umbrella species in primary forests

Abstract Primary forests are characterized by high vertical and horizontal stand diversity, which provides habitat for a diverse range of species with complex habitat requirements. Detailed knowledge of related ecological processes and habitat development of primary forest species are essential to inform forest management and biodiversity conservation decisions, but relationships are not well documented. We collected dendrochronological data and inventoried numerous structural elements in permanent plots throughout the primary temperate forests within the Carpathian Mountains. We fit and compared multiple predictive models to quantify the importance of 200 years of natural disturbance dynamics on the occurrence probability of an umbrella species – the capercaillie (Tetrao urogallus). We showed that a mixed-severity disturbance regime ranging from low through moderate to high severity disturbances is required to generate diverse forest habitats suitable for capercaillie. The variation in natural disturbance severity and its timing promoted key structural habitat elements, such as low natural regeneration density, low mature tree density, high ground vegetation cover, availability of forest gaps, and abundance of standing deadwood. This study demonstrates the importance of natural disturbance in maintaining the variety of conditions necessary to support primary forest specialist species. Managers of protected areas should be mindful that natural disturbances generate habitat for the capercaillie in mountain Norway spruce forests. Further intervention is unnecessary. Conservation planning and forest reserve design should shift focus to the large-scale spatial requirements needed to ensure that a wide range of forest developmental phases are represented in protected areas.

The historical disturbance regime of mountain Norway spruce forests in the Western Carpathians and its influence on current forest structure and composition

In order to gauge ongoing and future changes to disturbance regimes, it is necessary to establish a solid baseline of historic disturbance patterns against which to evaluate these changes. Further, understanding how forest structure and composition respond to variation in past disturbances may provide insight into future resilience to climate-driven alterations of disturbance regimes.We established 184 plots (mostly 1000m2) in 14 primary mountain Norway spruce forests in the Western Carpathians. On each plot we surveyed live and dead trees and regeneration, and cored around 25 canopy trees. Disturbance history was reconstructed by examining individual tree growth trends. The study plots were further aggregated into five groups based on disturbance history (severity and timing) to evaluate and explain its influence on forest structure.These ecosystems are characterized by a mixed severity disturbance regime with high spatiotemporal variability in severity and frequency. However, periods of synchrony in disturbance activity were also found. Specifically, a peak of canopy disturbance was found for the mid-19th century across the region (about 60% of trees established), with the most important periods of disturbance in the 1820s and from the 1840s to the 1870s. Current stand size and age structure were strongly influenced by past disturbance activity. In contrast, past disturbances did not have a significant effect on current tree density, the amount of coarse woody debris, and regeneration. High mean densities of regeneration with height >50cm (about 1400 individuals per ha) were observed.Extensive high severity disturbances have recently affected Central European forests, spurring a discussion about the causes and consequences. We found some evidence that forests in the Western Carpathians were predisposed to recent severe disturbance events as a result of synchronized past disturbance activity, which partly homogenized size and age structure and made recent stands more vulnerable to bark beetle outbreak. Our data suggest that these events are still part of the range of natural variability. The finding that regeneration density and volume of coarse woody debris were not influenced by past disturbance illustrates that vastly different past disturbance histories are not likely to change the future trajectories of these forests. These ecosystems currently have high ecological resilience to disturbance. In conclusion, we suggest that management should recognize disturbances as a natural part of ecosystem dynamics in the mountain forests of Central Europe, account for their stochastic occurrence in management planning, and mimic their patterns to foster biodiversity in forest landscapes.

Linking spatiotemporal disturbance history with tree regeneration and diversity in an old-growth forest in northern Japan

Knowledge of long-term spatiotemporal effects of disturbances on forest structure, tree regeneration and species composition is key for understanding forest dynamics and predicting future forest responses to climate change. Here, we explore the spatiotemporal impact of disturbances of different severities on tree recruitment and diversity in species-rich oak-fir-maple forest in Hokkaido, a typhoon-prone area in northern Japan, over the past 230 years. The forest disturbance history was reconstructed by growth-release analysis from more than 45,500 tree rings of 385 trees belonging to 15 species. A mixed severity disturbance regime was prevalent over the study period. Altogether, 310 major and 293 moderate growth releases were identified. These were both temporally and spatially localized, with 80% of events detected in only four time periods: 1775–1784, 1815–1839, 1880–1909 and 1950–1979. Disturbances were followed by major recruitment pulses, each lasting around 30 years. Dendrochronological reconstructions alone indicate that severe (i.e. high proportion of releases), infrequent disturbances control tree regeneration and forest development (from oak-dominated forests to mixed-stand with higher proportion of shade-tolerant tree species). However, a combination of temporal and spatial pattern analysis revealed that less severe disturbances, creating small gaps, promote higher density and diversity of recruitment (altogether 19 tree species recorded) compared with severe disturbances. The latter create large forest gaps which became overgrown by dwarf bamboo and suppress tree regeneration. These results provide evidence that severe disturbances interacting with a strong biotic understory filter (as dwarf bamboo), can disrupt forest ecosystem dynamics by significantly reducing the extent and diversity of tree recruitment. Our findings are important as most climate models predict an elevated intensity of typhoons in Northeast Asia. We conclude that a combination of temporal and spatial analyses, as presented here, is necessary to disentangle the complex drivers of long-term forest dynamics.

The legacy of disturbance on individual tree and stand-level aboveground biomass accumulation and stocks in primary mountain Picea abies forests

Disturbances, both natural and human induced, influence forest dynamics, ecosystem functioning, and ecosystem services. Here, we aim to evaluate the consequences of natural disturbances on the magnitude and dynamics of tree- and stand-level biomass accumulation from decadal to centennial scales. We use tree-ring data from 2301 trees and biometric data from 4909 trees sampled in 96 plots (each 1000m2) to quantify the influence of mixed severity disturbance regimes on annual aboveground biomass increment (AGBI) and total aboveground biomass accumulation (AGB) across a mountainous monotypic Norway spruce (Picea abies (L.) Karst.) primary forest. We hypothesise that the multiple internal and external factors constraining tree growth will cause differences in tree and stand-level biomass trajectories in these natural forests.Although we found that tree-level AGB growth increases with tree size, we also found that tree age and disturbance legacies plays a crucial role for AGB in the investigated Norway spruce forests. Importantly, while younger trees of the same diameter class have an average current AGBI rate that is ∼225% higher than older trees (300–400years), we find trees that have been suppressed for up to 120years can respond vigorously when competition is reduced. On average, post disturbance AGBI was ∼400% greater than pre-disturbance AGBI. Growth of suppressed trees, independent of their age, followed similar trajectories after canopy accession. While aboveground biomass generally increased through time, the time since disturbance and disturbance severity are important co-predictors for stand-level AGBI and AGB. These forests regained most of the above ground living biomass over short interval (∼50years) after low intensity disturbances. The highest stand-level living AGB was observed on plots that experienced >40% canopy removal 160–190years ago, whereas the highest AGBI occurred in plots disturbed recently within the past 40–50years.Our results emphasize the importance of including both individual tree age and disturbance legacies to accurately characterize biomass dynamics and trajectories in forest ecosystems. Importantly, the period of time that a tree is in the canopy, and not tree age, modulates the trajectory of tree level AGBI. Growth rates begin to decline after ∼30years (tree-rings width) and ∼100years (AGBI) in the canopy. We demonstrate that even late-seral forests can rapidly regain biomass lost to low intensity disturbance.

A mixed severity disturbance regime in the primary Picea abies (L.) Karst. forests of the Ukrainian Carpathians

Natural disturbance regimes play key roles in shaping forest structure and development at stand and landscape levels. Disturbances are commonly complex and variable, such that classical dichotomous characterization of disturbance regimes as following large infrequent disturbances or patch dynamics is too simplistic, especially when the resulting damage is more severe than the baseline of a single tree patch dynamic, but not severe enough to represent large infrequent disturbance. Ongoing climate change affects mountain Picea abies forests in Central, East and Southeastern Europe by an increasing frequency of storms and bark beetle outbreaks. We present a unique study based on extensive dataset aimed to reveal the spatiotemporal pattern of the disturbance history and role of the mixed severity disturbances in primary spruce mountain forest landscapes in the Ukrainian Carpathians.We reconstructed canopy disturbance history and maximum disturbance severity using ca. 2396 tree cores in 96 sample plots. Neither large-scale stand-replacement nor fine scale dynamics was the prevailing disturbance over the last four centuries. Rather, we observed a complex spatiotemporal pattern of mixed severity disturbances. Canopy turnover time ranged between 50 and 300years and depended on the maximum severity of the disturbance event. Spatial analyses revealed no similarity in spatiotemporal pattern across disturbance histories or maximum disturbance severities. We observed evidence of a combination of variable severity disturbances that fails to fit the classical scheme of gap or patch dynamics with sharply defined sizes and borders, but is more consistent with a mixed severity disturbance regime across the landscape. Windstorms were likely the most important past disturbance agent. The probability of an epidemic bark beetle attack was low, although the possibility of small local outbreaks cannot be excluded. An additional, potentially overlooked, agent of disturbance could be historic periods of extreme cold.This reconstructed disturbance regime may challenge existing silvicultural systems in the Carpathians, calling for a more complex spatiotemporal forest management approach. However, mimicking a mixed severity disturbance regime can be done at the forest management level by applying a range of disturbance severities at the stand level.

Net carbon transfer between Pseudotsuga menziesii var. glauca seedlings in the field is influenced by soil disturbance

Summary 1. Mycorrhizal pathways are comprised of fungal hyphae that facilitate carbon transfer between plants. We determined whether net carbon transfer occurred between conspecific conifer seedlings in the field, and whether soil disturbance or access to mycorrhizal pathways affected transfer. 2. We established two soil disturbances and planted pairs of different sized Pseudotsuga menziesii var. glauca seedlings (naturally regenerated or planted) into one of four mesh treatments (0.5, 35, 250 μm or directly into soil) restricting mycorrhizal pathways. We pulse‐labelled both seedlings, one with 13CO2 and the other with 14CO2, to quantify net carbon transfer. Ectomycorrhizas were identified using morphological and molecular techniques. 3. Net carbon transfers were detected and were not due to re‐fixation of respired carbon. More transferred carbon accumulated in shoots than roots. In disturbed soil, there was greater net carbon transfer to natural seedlings than planted seedlings; the reverse pattern was observed in undisturbed soil. For planted seedlings only, the magnitude of net carbon gain was positively related to seedling size and height growth rate. Greater net accumulation of carbon occurred in Rhizopogon vinicolor, a long‐distance ectomycorrhizal fungi exploration type (morphological character), than the two other most abundant ectomycorrhizal fungi with contact‐ and short‐distance exploration types. Long‐distance exploration types have the potential to form long‐distance hyphal connections between plant roots, whereas contact‐ and short‐distance are restricted to short‐distance (c. 0–0.25 μm) connections. 4. Synthesis. These results confirm that net carbon transfer occurs through mycorrhizal pathways; however, the amount transferred was very small. Mycorrhizal pathways were facilitating net transfer of carbon to large, vigorous natural seedlings in disturbed soils, whereas smaller planted seedlings received more net carbon gain in undisturbed soils. The size variation within these planted seedlings was great enough to elicit a positive relationship between net carbon gain and seedling size and growth rate. These findings are relevant to regeneration of forests characterized by mixed severity disturbance regimes, which leave a suite of environmental conditions that may result in a greater magnitude of transfer.

The foundational role of mycorrhizal networks in self-organization of interior Douglas-fir forests

Mycorrhizal fungal networks occur where mycorrhizal fungal mycelia link the roots of multiple plants, including those of different species, sometimes facilitating interplant transfer of carbon, nutrients or water. In this paper, I review recent research on the structure and function of mycorrhizal networks, and how they may serve a foundational role in the self-organization and stability of interior Douglas-fir ( Pseudotsuga menziesii var. glauca) forests through their influences on forest regeneration, stand dynamics and spatial patterns. The stabilizing influence of mycorrhizal networks is particularly important in these forests because they experience an unpredictable and stressful climate; a mixed severity disturbance regime; and episodic seed dispersal with multiple constraints on recruitment. After seed dispersal following gap disturbance, we have found that interior Douglas-fir seedlings are rapidly colonized by ectomycorrhizal (ECM) fungal spores or by the ECM fungal mycelial network formed among residual trees or shrubs, helping the species to regenerate in a patchy pattern. This occurs whether disturbance severity is low or high. In small, shaded gaps of uneven-aged forests, new regeneration also establishes within the extensive mycorrhizal networks of old trees. In open gaps created by variable retention harvesting, establishment of new regeneration is substantially enhanced where seedlings are integrated within the existing network of residual trees, and this is associated with resource transfer through mycorrhizal pathways. As the young trees grow, belowground resource transfer fluctuates with changing source–sink gradients, until the trees eventually gain dominance in stands and become nutrients sources to a new generation of seedlings growing beneath them. Although preliminary, our studies suggest that the degree to which mycorrhizal networks facilitate regeneration establishment increases with disturbance or drought stress, in keeping with the stress-gradient hypothesis of facilitation. The patchy regeneration pattern of Douglas-fir across a climatic gradient can be partly explained by scale-dependent feedbacks involving local facilitation by networks and longer-range competition for resources. The regeneration pattern influenced by mycorrhizal networks may also interact with other organisms, including small mammals. Based on this, I propose that interacting networks function as a hierarchical “meta-network”, where each falls under a network guild and influences other networks to stabilize the forest. Hub trees for mycorrhizal networks are “foundational” because they even out resource availability and create favorable local conditions for tree establishment, which is fundamental to structuring of the whole forest community. Conserving hub trees and mycorrhizal networks during forest management therefore appears important to the conservation, regeneration and restoration of interior Douglas-fir forests.

Dry forests in the Southern Interior of British Columbia: Historic disturbances and implications for restoration and management

We critically examine the hypothesis that dry forests in southern British Columbia evolved in the context of a low-severity fire-dominated disturbance regime, that fire suppression has led to ecological conditions which are radically different from the past, and that “restoration” initiatives are required to re-establish former ecological conditions. Four sources of information were used to infer historic disturbance regimes and forest condition and to quantify the nature of disturbance since the early 1900s: (1) patterns of annual and seasonal weather and lightning strikes, (2) topographic variability, (3) records of wildfire, insect attack, and timber harvesting practices, and (4) early systematic forest surveys. Our analyses consistently indicate that historic natural disturbances were likely diverse and episodic at multiple spatial and temporal scales. High seasonal and annual variability in weather and the number of lightning strikes in complex topography suggest that a widespread low-severity fire regime is very unlikely, with a mixed-severity disturbance regime more consistent with our analyses. Although the nature of disturbance has changed from one largely dominated by fire and insect attack historically to harvesting and insect attack since 1950, the area disturbed annually has not diminished. Several interacting factors including climate, extensive fires coincident with European settlement, harvesting, fire suppression and insect attack have been key drivers in creating the conditions observed today. A complex, mixed-severity disturbance regime creates uncertainty about what represents “natural” forest conditions, or what the target conditions for restoration activities are if the objective is to “restore natural conditions”. We conclude that dry forest ecosystems in British Columbia typically experienced mixed-severity disturbance regimes that included fire, bark beetles and defoliators. Trying to “restore” these forests with applications of frequent, low-severity fire is not an ecologically sound objective over large areas. Landscape management should focus on maintaining forest heterogeneity that would have existed historically under a mixed-severity disturbance regime.