Stand-replacing Disturbance Research Articles

Linking instream wood recruitment to adjacent forest development in landscapes driven by stand-replacing disturbances: a conceptual model to inform riparian and stream management

Instream wood plays an important role in stream morphology and creation of fish habitat in conifer forests throughout the temperate zone. In some regions, such as the US Pacific Northwest, many streams currently have reduced amounts of instream wood due to past management activities (timber harvest, wood removal, etc.). These reductions exist against a backdrop of naturally dynamic amounts and distributions of instream wood, which likely fluctuate over time based in part on the stage of development (disturbance and succession) in adjacent riparian forests. Despite many studies on both forest development and instream wood accumulation, the linkages between these processes have not been fully described, particularly as they relate to stream restoration needs. In this paper, we combine literature on forest development, disturbance, and processes that drive instream wood recruitment to more explicitly connect the temporal dynamics of stream wood inputs with the dynamics of adjacent riparian forests. We use moist forests of the Pacific Northwest as an exemplary system, from which to draw broadly applicable patterns for landscapes influenced by stand-replacing disturbance regimes. This conceptual model highlights a U-shaped pattern of instream wood recruitment, in which instream wood is highest after a stand-replacing disturbance and during the old-growth stage, and lowest through the middle stages of forest development (currently the most abundant stages in many landscapes as a result of past forest management practices). This mid-successional period of scarce wood is likely exacerbated in streams with a history of wood removal. The U-shaped pattern suggests that, without higher-than-average levels of disturbance, many streams in landscapes dominated by mid-successional second-growth forests (∼30–80 yr old) will be deficient of instream wood until forest stands are over 200 years old. As such, the balance between the predominant riparian conservation strategy of passive restoration (e.g., unharvested riparian reserves) and the alternative of active restoration (e.g., wood additions and (or) riparian stand treatments) should be carefully considered, depending on management objectives, site context, and potential tradeoffs over time.

Peak plant diversity during early forest development in the western United States

Complex early-seral forests support biological legacies and unique understory vegetation (grasses, forbs, shrubs, juvenile trees) important to wildlife and ecosystem functioning. Intensive management in early-seral forest and other stages could reduce understory plant diversity and homogenize community compositions. Furthermore, the relative effects of management regime may vary at different stages of forest development. To what extent do taxonomic, functional and phylogenetic diversity vary by seral stage and/or ownership (a proxy for management regime) after accounting for climatic variation? For 2224 plant species sampled systematically at 16,330 sites in sixteen Plant Association Zones (PAZs) of Washington, Oregon and California, we estimated taxonomic, functional and phylogenetic diversity (both alpha = richness and beta = heterogeneity measures). Seral stage was inferred using forest structural metrics. Permutational multivariate analyses tested the relative effects of seral stage and ownership type. Across PAZs, a common motif emerged: low diversity in sparse, untreed stages indicated an accumulation period after stand-replacing disturbances, followed immediately by peak diversity in early sapling stages, after which gradually declining diversity coincided with the loss of shade-intolerant species during stand development and canopy closure. Departures from this motif tended to be in climatically less-permissive locations. More of the variation in diversity was explained by seral stage than ownership type (and their interactions were seldom significant). Discriminating the nuanced effects of different management regimes will require context-dependent proxies other than ownership. Identifying generalities in vegetation dynamics—and departures from those generalities—can help balance ecological and social objectives in rapidly changing forest landscapes.

C-exchange and balance following clear-cutting in hemiboreal forest ecosystem under summer drought

Growth of CO2 concentration level has strong interactions with forests. Forests are able to sequester carbon (C) through photosynthesis and can help to mitigate the effects of climate warming, as well as to reduce the CO2 concentration in the atmosphere. Drought and other extreme weather conditions play a key role in ecosystem functioning and the C-cycle. The eddy covariance (EC) method can be used to better understand forest ecosystems CO2 exchange by directly measuring net carbon and water fluxes. In our study, EC results for measurement of fluxes between the atmosphere and forest canopy are reported for the study period from May to August 2018 in Järvselja, Estonia. Stand-replacing disturbance (clear-cutting) took place in April 2013. The young forest stand is dominated by Norway spruce (Picea abies) and birch (Betula spp.). Findings so far include (1) a C-budget for the study period that showed a slight C-sink status; (2) net ecosystem exchange (NEE) was −0.0084 µmol m−2 s−1 indicating C-uptake during the measurement period; (3) in May, June, July and August, NEE was −0.027, −0.015, 0.001 and 0.006 µmol m−2 s−1, respectively; (4) NEE fluxes are lower in drought conditions and are affected by temperature that averaged 15 °C.

Predicting oak regeneration success at the stem exclusion stage of stand development in upland hardwood forests

Oaks (Quercus spp.) are becoming less abundant in most of the Central Hardwood Region of the eastern United States, and this is creating shifts in forest composition that will likely have important economic and ecological consequences. In large measure, these changes originate with deficiencies in the oak regeneration cohort preceding stand-replacement disturbances such as timber harvest. To sustain the oak resource, managers need better information on the connection between pre-harvest and early stand conditions and regeneration outcomes. In this study, we used direct observations of oak seedling dominance in the stem exclusion stage of stand development (mean age = 17.4 years) to model the probability of successful regeneration during stand initiation as a function of stand conditions before and after harvest (ages −1, 1, 4, and 7 years). For pre-harvest conditions, the most predictive model was based solely on the aggregate height of advance regeneration oak seedlings >15 cm in sample plots. As expected, post-harvest models were more predictive, and increasingly more predictive with the passage of time, and they were optimized by contrasting the height of the plot-dominant oak seedling with the heights of competing tree species. The predictive power of post-harvest models increased most between ages 1 and 4 years and only slightly between ages 4 and 7, indicating that age 4 is an optimal time to evaluate opportunities to favor oak regeneration with early silvicultural interventions. Of the two most common competitors, black birch (Betula lenta) had the more inhibitory effect on the success of oak regeneration when it was present. However, red maple (Acer rubrum) was the more important competitor because of its very high frequency of occurrence in plots occupied by oak seedlings.

Successional dynamics of an Appalachian pin oak (Quercus palustris Münchh.) swamp forest

Bottomland forests provide ecosystem services, such as nutrient exchange and pollution entrapment, and generally improve aquatic and riparian habitats. In West Virginia, little is known about the development and resiliency of isolated bottomland sites dominated by Quercus L. species, which have declined in area primarily from human impacts (e.g., draining and development). In this study, we used vegetation surveys and tree-ring analysis to investigate successional dynamics and the timing and drivers of recent overstory mortality in a remnant stand located on the floodplain of the Meadow River. Our results indicate a rapid change in vegetation cover at the site between 2011 and 2017, with a significant decrease in tree canopy cover and an increase in light-demanding, flood-tolerant, herbaceous species cover. Historical trends in tree establishment indicate an absence of Quercus palustris Münchh. recruitment after the 1960s and an increase in the recruitment of Acer rubrum L. and other shade-tolerant species. Furthermore, temporal trends in hydroclimate-tree growth relationships suggest that an increasingly temperate climate and higher water table benefited A. rubrum at the expense of Q. palustris beginning in the 1990s. Taken together, these findings suggest a gradual, multidecade process of plant community reorganization beginning in the mid-20th century. This process might have continued unabated, but flooding and persistent high water in 2003 amplified the wetting trend and initiated the progression of widespread overstory mortality. Our results suggest that Q. palustris forests in the Meadow River wetlands are maintained by infrequent stand-replacing disturbances (i.e., prolonged inundation), but the current long-term wetting trend could create site conditions unfavorable to tree growth. Furthermore, these findings highlight the need for additional analysis of bottomland vegetation dynamics in the Appalachian Mountains, where increased hydroclimate variability, including extreme flooding, is predicted.

Disturbances and Climate Drive Structure, Stability, and Growth in Mixed Temperate Old-growth Rainforests in the Caucasus

The Colchic rainforest of the Western Caucasus is one of the few temperate rainforests dominated by broadleaf deciduous trees. Understanding natural dynamics of broadleaf-dominated temperate rainforests is essential for their conservation and management. Here, we investigate for the first time the structure, natural disturbance, and recruitment dynamics of a mixed Colchic old-growth rainforest, dominated by Fagus orientalis and Picea orientalis. We used forest inventories and dendrochronological analysis of tree growth in five 30-m-radius plots to quantify forest structure, growth, and disturbances. For the last 400 years, the forest experienced a mixed disturbance regime dominated by frequent small gaps superimposed onto medium disturbances with about a 25-year recurrence period, with no evidences of stand-replacing disturbances. This disturbance regime favored the dominance of shade-tolerant, late successional species with slow tree canopy access through multiple growth releases. These dynamics impose low growth rates and continuous recruitment of spruce and beech, and contributed to a high heterogeneity of tree ages and sizes that result in stable forest structure, as suggested by the low stand slenderness. Spruces were the oldest (up to 427 years) and fastest growing trees in the forest, suggesting that their low presence in the forest is due to low disturbance rates that limit their recruitment. Spring climate conditions that promoted beech growth were detrimental for spruce growth, suggesting that interspecies interactions may condition the effect of climate on forest growth and development. The dynamic equilibrium state we reconstructed in this old-growth forest could likely be disrupted by anthropogenic disturbances or management.

Effects on understory biomass and forage 8–10 years after precommercial thinning of Sitka spruce – western hemlock stands in southeast Alaska

In southeast Alaska, United States, multiple-use forest management objectives include both timber production and wildlife habitat. Following stand-replacing disturbances such as clear-cutting, Sitka spruce (Picea sitchensis (Bong.) Carrière) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) naturally regenerate and competitively dominate resources, excluding understory biomass and biodiversity. Thinning may mitigate the effects of canopy closure and permit understory development, but evidence of the effect on understories 8–10 years after thinning is lacking. We report results 4–5 and 8–10 years after thinning experiments on the Tongass National Forest to demonstrate the effects of precommercial thinning (thinned versus control), stand age (15–25, 25–35, and 35–50 years), and weather on understory dynamics and Sitka black-tailed deer (Odocoileus hemionus sitkensis Merriam, 1898) forage availability. Stand density negatively affected understory biomass, whereas temperature and precipitation positively interacted to increase biomass. Thinning had an enduring effect on understories, with biomass at least twice as great in thinned versus unthinned stands through year 10. We identified compositional differences from thinning as stand age class increased. Deer forage responded similarly to biomass, but thinning-induced differences faded with increased winter snowfall scenarios, especially in older stands. This study aids the understanding of stand overstory and understory development following silvicultural treatments in the coastal temperate rain forest of Alaska and suggests management implications and applications for balancing objectives throughout the forest type.

Reconstruction of the disturbance history of a temperate coniferous forest through stand-level analysis of airborne LiDAR data

AbstractSpatially explicit information about stand-level Time Since the last stand-replacing Disturbance (TSD) is fundamental for modelling many forest ecosystem processes, but most of the current satellite remote sensing mapping approaches are based on change detection and time series analysis, and can detect only disturbances that have occurred since the start of the optical satellite data record. The spatial legacy of stand-replacing disturbances can however persist on the landscape for several decades to centuries, in the form of distinct horizontal and vertical stand structure features. We propose a new approach to reconstruct the long-term disturbance history of a forest, estimating TSD through stand-level analysis of LiDAR data, which are highly sensitive to the three-dimensional forest canopy structure. The study area is in the Nez Perce-Clearwater National Forest in north-central Idaho, where airborne LiDAR covering about 52,000 ha and ancillary TSD reference data for a period of more than 140 years were available. The root mean square difference (RSMD) between predicted and reference TSD was 17.5 years with a BIAS of 0.8 years; and on 72.8% of the stands the predicted TSD was less than 10 years apart from the reference TSD (78.2% of the stands when considering only disturbances occurred in the last 100 years). The results demonstrate that airborne LiDAR-derived data have enough explanatory power to reconstruct the long-term, stand-replacing disturbance history of temperate forested areas at regional scales.

Seven Decades of Spontaneous Forest Regeneration after Large-Scale Clear-Cutting in Białowieża Forest do not Ensure the Complete Recovery of Collembolan Assemblages

The long-term effects of large-scale disturbance on forest ecosystem processes and structure are poorly understood. To assess the effects of large-scale clear-cutting on the taxonomic and functional structure of collembolan assemblages, 18 plots were established in the Polish part of Białowieża Forest. All plots, situated in a mixed Tilio-Carpinetum broad-leaved forest, had eutrophic Cambisol developed on rich glacial deposits. The Collembola assemblages in the stands that had naturally regenerated on large-scale clear-cuts performed at the beginning of the 20th century were compared to those in old-growth forests (i.e., the endpoint of stand development following stand-replacing disturbance). Collembolans, one of the most numerous soil microarthropods, are successfully used to assess the consequences of forest management and ecosystem restoration. Our study tested whether seven decades of spontaneous forest development after large-scale anthropogenic disturbance ensures the complete recovery of the soil Collembola. Using complementary taxonomic and life-form approaches, we provide evidence that the collembolan assemblages associated with the tree stands that had spontaneously developed in large harvesting plots distinctly differed from those in old-growth deciduous forests in this region despite seven decades of regenerative forest succession. The species diversity of the assemblages in the naturally regenerated tree stands was significantly lower, and their life-form structure was noticeably different from those in the reference forests. Moreover, the shift in the functional group structure of the collembolan assemblages in the stands that had regenerated after clear-cutting indicates that their activity seven decades after disturbance is concentrated mainly on the decomposition of the litter in the upper layers, whereas the processes controlled by these organisms in the deeper soil layers are not fully restored.

Combining allometry and landsat-derived disturbance history to estimate tree biomass in subtropical planted forests

Planted forests are a key component of sustainable forest management, as their fast-growing biomass can provide many important ecosystem goods and services essential to human well-being. Accurate estimation of forest biomass is important for global carbon accounting and afforestation policy making, yet saturated reflectance signals over vegetation and a shortage of cloud-free images during the growing season largely limit the utility of optical remote sensing for biomass estimation of planted forests in subtropical and tropical regions. Stand age is a crucial factor in determining forest growth and stand development but hasn't been widely used in remote sensing-based biomass estimation approaches. The main objective of this study was to investigate the applicability of an allometric analysis approach combined with Landsat-derived forest age structure to estimate stand biomass of fast-growing forest plantations in a subtropical landscape of China. We used the Vegetation Change Tracker (VCT) approach and Landsat time series data from 1986 to 2016 to detect annual stand-replacing disturbance and estimate forest stand age. Allometric equations and relative growth rate functions were combined to establish the linkage between tree biomass and forest stand age. We also modeled the spatial distribution of tree biomass using a conventional remote sensing (CRS) method that only utilizes spectral and textural variables and an extended CRS method that incorporates stand age (CRS_EX). We then compared the CRS and CRS_EX methods with the allometric analysis method to highlight the importance of stand age and post-disturbance forest regrowth for biomass estimation. The Landsat-derived disturbance history was significantly correlated with stand age (R2 = 0.82, RMSE = 3.7 year, p < 0.01). The CRS_EX and allometric analysis methods substantially improved tree biomass estimates (R2CRS_EX = 0.77, RMSECRS_EX = 37.7 t/ha; R2Allometric = 0.70, RMSEAllometric = 43.5 t/ha) compared to the CRS method (R2CRS = 0.53, RMSE CRS = 54.38 t/ha). Tree biomass estimated from the allometric analysis method aligned better with the allometric growth curves, while CRS_EX and CRS methods both exhibited notable overestimation for young and mid-aged forests. This study suggests that allometric models have the potential to be applied in subtropical forest plantations through the use of the existing forest stand development knowledge and increasingly open access remote sensing data.

Key perspectives on early successional forests subject to stand-replacing disturbances

In forests subject to stand-replacing disturbances, early successional stands can provide important habitats for a range of species not typically present in long-undisturbed areas. Compared to old-growth forests, the habitat values of – and key ecological processes in – early successional forests have been less studied, perhaps due to a perception that early successional forests revert to a homogenous “clean slate” following stand-replacing disturbances. In this paper, we draw on 36 years of long-term research in the Mountain Ash (Eucalyptus regnans) and Alpine Ash (Eucalyptus delegatensis) forests of south-eastern Australia, together with examples from elsewhere around the world, to show that not all kinds of early successional forests are created equal. We argue that the ecological values of early successional forests can be profoundly affected by six inter-related factors: (1) The evolutionary context and environmental domain of a given ecosystem. (2) Successional stage and condition of a forest stand prior to disturbance. (3) Disturbance intensity, severity and type (e.g. wildfire versus clearcutting). (4) Post-disturbance conditions including climate and weather. (5) Post-disturbance management (e.g. salvage logging) which can have significant impacts on biological legacies. And, (6) The relative spatial extent and spatial arrangement of early and late successional forest across a landscape. These factors can influence ecological values directly, or through effects on the types, amount and spatial patterns of biological legacies present in early successional forest. We present a conceptual model highlighting the inter-relationships between these factors and illustrate its use through a detailed case study.Strategies to improve the management of early successional forests include: (1) Identifying the species associated with post-disturbance environments and the reasons why they occur in such environments. (2) Understanding the types, numbers, and spatial patterns of biological legacies that remain after natural disturbance. (3) Identifying critical areas that should be excluded from logging or other human disturbance. (4) Limiting the extent of post-disturbance activities like salvage logging that undermine the ecological values of, and ecosystem processes in, early successional forests. And, (5) Balancing the relative amounts of early successional versus late successional forest in a given landscape or region to ensure that a variety of forest types are present at any given time, and that critical biological legacies are retained. Paradoxically, ensuring that landscapes support extensive areas of late successional forest is critical so that future early successional forests are not devoid of the biological legacies necessary for ecosystem function and recovery.

Prevalence of multiple forest disturbances and impact on vegetation regrowth from interannual Landsat time series (1985–2015)

Given long time series of satellite imagery, multiple disturbances can be detected for a particular location at different points in time. We assessed multiple disturbances for the 650 Mha of Canada's forested ecosystems using annual change information derived from Landsat time series imagery (1985–2015). Changes were typed by agent (fire, harvest, and non-stand replacing). Spectral change rate and time between successive disturbances were used to characterize disturbance-type combination differences. Short-term spectral recovery following the last disturbance was compared to a reference sample of pixels disturbed only once. Results indicated that of the 97.6 Mha disturbed, 13.5 Mha have had two or more disturbances, with low magnitude non-stand replacing disturbances involved in the majority of occurrences (77.2%). The total area disturbed represents 18.27% of forest ecosystems, with 2.53% having multiple disturbances and 0.54% having multiple stand-replacing disturbances. Systematic time series-based investigation of multiple disturbance events and agents provides insights on forest disturbance dynamics and recovery processes.

Structural development following stand-replacing disturbance in a boreal mixedwood forest

Knowledge of forest stand successional pathways, and responses to disturbances, is crucial for improved sustainable management of the resource. Forest structure, one attribute of which is the size and height distribution of trees in a stand (known as the stem size distribution; SSD), is highly sensitive to changes in stand dynamics. However, monitoring changes in SSD can be difficult and costly, especially across large spatial extents and by different disturbance types and species groups. As a result, forest managers increasingly utilise three-dimensional remote sensing such as airborne laser scanning (ALS) as a means of modeling forest structure in a cost-effective and spatially consistent manner. In this study, approximately 7000 ALS-derived SSDs representing a chronosequence of over 50 years of forest disturbance in Alberta, Canada were used to evaluate differences in structural development by species groups and disturbance types. Stands were clustered into three structural development phases based on ALS-derived SSDs. Results showed no difference in SSDs for early development phases, but significantly higher proportions of trees in medium-sized diameter classes (up to 25 cm diameter at breast height) in previously burned stands than in previously harvested stands. For each structural development phase, in both burned and harvested stands, there was no significant difference in modeled SSD among species groups. No difference in SSD for each phase was also seen when comparing previously burned to harvested stands for mixed species and white spruce stands. However, aspen and pine-dominated stands had significantly higher proportions of larger trees in burned stands than harvested stands in the second and third phase of structural development. Results from this study provide valuable insights to the current state of the forest, as well as an improved understanding of the general patterns of structural development following disturbance.

Important role of forest disturbances in the global biomass turnover and carbon sinks.

Forest disturbances leading to replacement of whole tree stands are a cornerstone of forest dynamics, with drivers including fire, wind-throw, biotic outbreaks and harvest. The frequency of disturbances may change over the next century, impacting the age, composition and biomass of forests. However, the variation in disturbance return time, i.e. the mean interval between disturbance events, across the world’s forested biomes remains poorly characterised, hindering quantification of their role in the global carbon cycle. Here we present the global distribution of stand-replacing disturbance return time inferred from satellite-based observations of forest loss. Prescribing this distribution within a vegetation model with a detailed representation of stand structure, we quantify the importance of stand-replacing disturbances for biomass carbon turnover globally over 2001-2014. Return time varied from less than 50 years in heavily-managed temperate ecosystems to over 1000 years in tropical evergreen forests. Stand-replacing disturbances accounted for 12.3% (95% confidence interval, 11.4-13.7%) of annual biomass carbon turnover due to tree mortality globally, and in 44% of forested area biomass stocks are strongly sensitive to changes in disturbance return time. Relatively small shifts in disturbance regimes in these areas would substantially influence the forest carbon sink, that currently limits climate change by offsetting emissions.

Annual continuous fields of woody vegetation structure in the Lower Mekong region from 2000‐2017 Landsat time-series

Spatially and temporally consistent vegetation structure time-series have great potential to improve the capacity for national land cover monitoring, to reduce latency and cost of international reporting, and to harmonize regional land cover characterizations. Here we present a semi-automatic, operational algorithm for mapping and monitoring of woody vegetation canopy cover and height at a regional scale using freely available Landsat time-series data. The presented algorithm employs automatic data processing and mapping using a set of lidar-based vegetation structure prediction models. Changes in vegetation cover are detected separately and integrated into the structure time-series. Sample-based validation and inter-comparison with existing datasets demonstrates the spatial and temporal consistency of our regional data time-series. The dataset reliably reflects changes in tree cover (tree cover loss user's accuracy of 0.84 and producer's accuracy of 0.75) and can serve as a tool to map annual forest extent (user's accuracy of 0.98 and producer's accuracy of 0.81 for 10% canopy cover threshold to define the forest class). The tree height estimates are consistent with a GLAS-based global map (mean average error of 3.7 m, the correlation coefficient of 0.92 and the R2 of 0.85). The algorithm was prototyped within the Lower Mekong region where it revealed an intensive woody vegetation dynamic. Of the year 2000 forest area (defined using canopy cover threshold of 10% and tree height threshold of 5 m), 9.4% was deforested by the year 2017, and 16.6% was affected by stand-replacement disturbance followed by reforestation. The average annual area of stand-level forest disturbance within the region was 2.34 Mha, and increased by 34% from 2001 (1.85 Mha) to 2017 (2.48 Mha). Total forest area decreased by 6.2% within the region, and 11.1% of year 2000 primary forest area was lost by 2017. At the national level, Cambodia demonstrated the highest rate of deforestation, with a net forest area loss of 22.5%. We estimated that 21.3% of 2017 forest cover had an age of 17 years or less, illustrating the intensive forest land uses within the region. The time-series product is suitable for mapping annual land cover and inter-annual land cover change using customized class definitions. The regionally-consistent data are publicly available for download (https://glad.umd.edu/), and online analysis (https://rlcms-servir.adpc.net/en/forest-monitor/), and serve as an input to the SERVIR-Mekong Regional Land Cover Monitoring System.

Shrub facilitation of tree establishment varies with ontogenetic stage across environmental gradients.

Plant-plant interactions are important drivers of ecosystem structure and function, yet predicting interaction outcomes across environmental gradients remains challenging. Understanding how interactions are affected by ontogenetic shifts in plant characteristics can provide insight into the drivers of interactions and improve our ability to anticipate ecosystem responses to environmental change. We developed a conceptual framework of nurse shrub facilitation of tree establishment. We used a combination of field experiments and environmental measurements to test the framework with a shrub (Artemisia tridentata) and a tree (Pinus monophylla), two foundation species in a semiarid environment. Shrub microsites allowed trees to overcome an early population bottleneck and successfully establish in areas without tree cover. Shrubs facilitated trees at multiple ontogenetic stages, but the net outcome of the interaction shifted from strongly positive to neutral after the transition of P.monophylla from juvenile to adult foliage. Microhabitat conditions varied across a broad elevational gradient, but interaction outcomes were not strongly related to elevation. Favorable microsites provided by A.tridentata cover are crucial for P.monophylla recovery after stand-replacing disturbance. Models of vegetation response to rapid global environmental change should incorporate the critically important role of nurse shrub interactions for ameliorating population bottlenecks in tree establishment.

The tortoise and the hare: A race between native tree species and the invasive Chinese tallow

Species-specific growth rate and its response to interspecific competition can determine the winners and losers in forest stand development following disturbance. In the southeastern US, Chinese tallow [Triadica sebifera (L.) Small], a non-native, fast-growing, invasive tree readily displaces native species. However, its rapid early height growth may not compensate for its shorter ultimate stature and earlier senescence when competing with fast growing native tree species of larger stature and longer lifespans. In this study, we compared the growth and competitiveness of Chinese tallow to two native species, slash pine (Pinus elliottii Englem.) and sweetgum (Liquidambar styraciflua L.), using two datasets representing different spatial scales. Plot data from Parris Island, South Carolina obtained by conducting stem analyses, were used to determine height and diameter growth patterns in relation to age and competition. Landscape-scale data from the U.S. Forest Inventory and Analysis (FIA) program were used to determine the relationship of relative importance value (rIV) and periodic annual DBH increment along a competition gradient. We found that Chinese tallow displayed faster diameter and height growth initially, but slowed down considerably after eight years, compared to slash pine. Slash pine was the least tolerant of competition among the three species, and competition had less effect on the growth of Chinese tallow and sweetgum. Our findings suggest that stand-replacing disturbance favors the rapid growth of Chinese tallow for the first decade, even under intense competition. Further, the establishment of native tree species would require effective control of Chinese tallow immediately following disturbance. Efforts to manage Chinese tallow while promoting the growth of native trees may include site preparation to reduce initial abundance of this invasive species, artificial regeneration of native species to provide them ‘head-start’ during the first few years of growth, and release treatments early in stand development to reduce competition for native species.

Uncovering regional variability in disturbance trends between parks and greater park ecosystems across Canada (1985\u20132015)

We assess the protective function of Canada’s parks and protected areas (PPAs) by analyzing three decades of stand-replacing disturbance derived from Landsat time series data (1985–2015). Specifically, we compared rates of wildfire and harvest within 1,415 PPAs against rates of disturbance in surrounding greater park ecosystems (GPEs). We found that disturbance rates in GPEs were significantly higher (p < 0.05) than in corresponding PPAs in southern managed forests (six of Canada’s 12 forested ecozones). Higher disturbance rates in GPEs were attributed to harvesting activities, as the area impacted by wildfire was not significantly different between GPEs and PPAs in any ecozone. The area burned within PPAs and corresponding GPEs was highly correlated (r = 0.90), whereas the area harvested was weakly correlated (r = 0.19). The average area burned in PPAs/GPEs below 55° N was low (0.05% yr−1) largely due to fire suppression aimed at protecting communities, timber, and recreational values, while the average burn rate was higher in northern PPAs/GPEs where fire suppression is uncommon (0.40% yr−1 in PPAs/GPEs above 55° N). Assessing regional variability in disturbance patterns and the pressures faced by PPAs can better inform policy and protection goals across Canada and the globe.

Tree rings provide no evidence of a CO2 fertilization effect in old-growth subalpine forests of western Canada.

Atmospheric CO2 concentrations are now 1.7 times higher than the preindustrial values. Although photosynthetic rates are hypothesized to increase in response to rising atmospheric CO2 concentrations, results from in situ experiments are inconsistent in supporting a CO2 fertilization effect of tree growth. Tree-ring data provide a historical record of tree-level productivity that can be used to evaluate long-term responses of tree growth. We use tree-ring data from old-growth, subalpine forests of western Canada that have not had a stand-replacing disturbance for hundreds of years to determine if growth has increased over 19th and 20th centuries. Our sample consisted of 5,858 trees belonging to five species distributed over two sites in the coastal zone and two in the continental climate of the interior. We calculated annual increments in tree basal area, adjusted these increments for tree size and age, and tested whether there was a detectable temporal trend in tree growth over the 19th and 20th centuries. We found a similar pattern in 20th century growth trends among all species at all sites. Growth during the 19th century was mostly stable or increasing, with the exception of one of the coastal sites, where tree growth was slightly decreasing; whereas growth during the 20th century consistently decreased. The unexpected decrease in growth during the 20th century indicates that there was no CO2 fertilization effect on photosynthesis. We compared the growth trends from our four sites to the trends simulated by seven Earth System Models, and saw that most of the models did not predict these growth declines. Overall, our results indicate that these old-growth forests are unlikely to increase their carbon storage capacity in response to rising atmospheric CO2 , and thus are unlikely to contribute substantially to offsetting future carbon emissions.

Soil Nitrogen and Mercury Dynamics Seven Decades After a Fire Disturbance: a Case Study at Acadia National Park

Forest soils (mainly soil organic carbon) play an important role in the retention of nitrogen and mercury, and loss of the forest floor during wildfires can stimulate N and Hg losses. In this paper, we investigate long-term impacts of forest fire on soil N and Hg concentrations at Acadia National Park (ANP) in Maine. Acadia National Park experienced a severe fire in 1947. Within the national park, Hadlock Brook watershed was left unburned, whereas most of Cadillac Brook watershed was intensely burned, with substantial loss of the forest floor. Post-fire regeneration in Cadillac was mostly as hardwood species, whereas vegetation in Hadlock remained predominantly softwood. We sampled soils in both watersheds in 2015, approximately 70 years after the fire. The soils were analyzed for total carbon (TC), total nitrogen (TN), total mercury (THg), and methylmercury (MeHg) content. Compared to Hadlock, Cadillac soils had ~ 50% lower TC, ~ 40% lower TN, and ~ 50% lower THg content, reflecting the loss of forest floor 70 years ago. Methylmercury concentrations in Cadillac were approximately 2 times the concentrations in Hadlock, indicating that conditions were more conducive to methylation, potentially due to differences in forest type. Long-term comparisons of stream DOC, NO3−, and THg concentrations between the two watersheds demonstrated that concentrations were significantly lower in Cadillac Brook, reflecting greater retention in Cadillac and a legacy of lower atmospheric deposition in the hardwood as compared to softwood watershed. This study provides insights on the multi-decadal recovery from a stand-replacing disturbance and underscores the persistence of altered soil biogeochemistry.