Post-disturbance reorganization of forest ecosystems in a changing world.

Forest ecosystems are strongly impacted by continuing climate change and increasing disturbance activity, but how forest dynamics will respond remains highly uncertain. Here, we argue that a short time window after disturbance (i.e., a discrete event that disrupts prevailing ecosystem structure and composition and releases resources) is pivotal for future forest development. Trees that establish during this reorganization phase can shape forest structure and composition for centuries, providing operational early indications of forest change. While forest change has been fruitfully studied through a lens of resilience, profound ecological changes can be masked by a resilience versus regime shift dichotomy. We present a framework for characterizing the full spectrum of change after disturbance, analyzing forest reorganization along dimensions of forest structure (number, size, and spatial arrangement of trees) and composition (identity and diversity of tree species). We propose four major pathways through which forest cover can persist but reorganize following disturbance: resilience (no change in structure and composition), restructuring (structure changes but composition does not), reassembly (composition changes but structure does not), and replacement (structure and composition both change). Regime shifts occur when vegetation structure and composition are altered so profoundly that the emerging trajectory leads to nonforest. We identify fundamental processes underpinning forest reorganization which, if disrupted, deflect ecosystems away from resilience. To understand and predict forest reorganization, assessing these processes and the traits modulating them is crucial. A new wave of experiments, measurements, and models emphasizing the reorganization phase will further the capacity to anticipate future forest dynamics.

Patterns of early post-disturbance reorganization in Central European forests.

Disturbances catalyse change in forest ecosystems, and a climate-driven increase in disturbance activity could accelerate forest reorganization. Here, we studied post-disturbance forests after the biggest pulse of tree mortality in Central Europe in at least 170 years, caused by drought and bark beetle (Scolytinae) outbreaks in 2018-2020. Our objectives were to characterize the early state of tree regeneration after mortality, quantify patterns of reorganization relative to undisturbed reference conditions and assess how management and patch size affect forest reorganization after disturbance. We surveyed 1244 plots in 120 patches under managed (salvage-logged, often planted) and unmanaged (deadwood remaining on site, no planting) conditions in Germany. We found that regeneration density on disturbed sites was high (median 11 897 stems ha-1), resulting from a cohort of advance regeneration. Disturbances were strong drivers of change, with indications for resilience on only 36.3% of patches. Reassembly (i.e. a change in species composition) was the dominant pattern of reorganization (61.5%), and Picea abies forests changed most strongly. Post-disturbance management facilitated forest change, particularly promoting a change in species composition. The strength of reorganization increased with patch size. We conclude that the recent wave of tree mortality will likely accelerate forest change in Central Europe.

Increasing fire and the decline of fire adapted black spruce in the boreal forest.

Intensifying wildfire activity and climate change can drive rapid forest compositional shifts. In boreal North America, black spruce shapes forest flammability and depends on fire for regeneration. This relationship has helped black spruce maintain its dominance through much of the Holocene. However, with climate change and more frequent and severe fires, shifts away from black spruce dominance to broadleaf or pine species are emerging, with implications for ecosystem functions including carbon sequestration, water and energy fluxes, and wildlife habitat. Here, we predict that such reductions in black spruce after fire may already be widespread given current trends in climate and fire. To test this, we synthesize data from 1,538 field sites across boreal North America to evaluate compositional changes in tree species following 58 recent fires (1989 to 2014). While black spruce was resilient following most fires (62%), loss of resilience was common, and spruce regeneration failed completely in 18% of 1,140 black spruce sites. In contrast, postfire regeneration never failed in forests dominated by jack pine, which also possesses an aerial seed bank, or broad-leaved trees. More complete combustion of the soil organic layer, which often occurs in better-drained landscape positions and in dryer duff, promoted compositional changes throughout boreal North America. Forests in western North America, however, were more vulnerable to change due to greater long-term climate moisture deficits. While we find considerable remaining resilience in black spruce forests, predicted increases in climate moisture deficits and fire activity will erode this resilience, pushing the system toward a tipping point that has not been crossed in several thousand years.

Structure of Ground Vegetation and Natural Regeneration of Tree Species in 12- to 15-Year-Old Bilberry Pine Forest-Clear-cut Complex of Middle Taiga Subzone.

Logging in mature stands where part of the forest is harvested in one or several cuts and part is retained (clearcutting and alternate strip cutting) results in the formation of an ecotone complex (EC), which includes a forest (F) zone, a forest edge (FE) as a transition from the forest to the clear-cut under the canopy, a clear-cut edge (CE) as a transition from the forest to the clear-cut outside of the canopy, and the clear-cut proper (C). The composition and structure of ground vegetation and natural regeneration of woody species (Pinus sylvestris L., Picea abies (L.) H. Karst., Betula sp., Populus tremula L., Sorbus aucuparia L., and Juniperus communis L.) were studied in a bilberry pine forest-clear-cut ecotone complex 12-15 years after stand removal. Specific structural features of ground vegetation and undergrowth (including tree regeneration) were observed in each of the four zones of the ecotone complex formed after logging of the mature forest. A typical forest habitat (zone F) showed a minimum number of young regeneration of Pinus sylvestris, Picea abies, Betula sp., Populus tremula, and Sorbus aucuparia and the highest abundance of the lingonberry V. vitis-idaea L. and bilberry Vaccinium myrtillus L. with a maximum height and a maximum yield of bilberry plants. The amount of tree regeneration in the FE zone was much the same as in the F zone. The projective cover, maximum shoot height, and yield of bilberry and the maximum shoot height of lingonberry in the FE zone were significantly lower than in the F zone. The transitional zone on the clear-cut side (CE) and the clear-cut proper (C) strikingly differed from the forest (F and FE) zones of the ecotone complex by a greater number of deciduous and pine regeneration and a low abundance of dwarf shrubs. The clear-cut proper (C) differed from the CE zone by a higher abundance of grasses and forbs and an established tree regeneration layer composed of pine, birch, and aspen.

Clinal variations in seedling traits and responses to water availability correspond to seed-source environmental gradients in a foundational dryland tree species.

BACKGROUND AND AIMS: In dryland ecosystems, conifer species are threatened by more frequent and severe droughts, which can push species beyond their physiological limits. Adequate seedling establishment will be critical for future resilience to global change. We used a common garden glasshouse experiment to determine how seedling functional trait expression and plasticity varied among seed sources in response to a gradient of water availability, focusing on a foundational dryland tree species of the western USA, Pinus monophylla. We hypothesized that the expression of growth-related seedling traits would show patterns consistent with local adaptation, given clinal variation among seed source environments. METHODS: We collected P. monophylla seeds from 23 sites distributed across rangewide gradients of aridity and seasonal moisture availability. A total of 3320 seedlings were propagated with four watering treatments representing progressively decreasing water availability. Above- and below-ground growth-related traits of first-year seedlings were measured. Trait values and trait plasticity, here representing the degree of variation among watering treatments, were modelled as a function of watering treatment and environmental conditions at the seed source locations (i.e. water availability, precipitation seasonality). KEY RESULTS: We found that, under all treatments, seedlings from more arid climates had larger above- and below-ground biomass compared to seedlings from sites experiencing lower growing-season water limitation, even after accounting for differences in seed size. Additionally, trait plasticity in response to watering treatments was greatest for seedlings from summer-wet sites that experience periodic monsoonal rain events. CONCLUSIONS: Our results show that P. monophylla seedlings respond to drought through plasticity in multiple traits, but variation in trait responses suggests that different populations are likely to respond uniquely to changes in local climate. Such trait diversity will probably influence the potential for future seedling recruitment in woodlands that are projected to experience extensive drought-related tree mortality.

Early γGT and bilirubin levels as biomarkers for regeneration and outcomes in damaged bile ducts after liver transplantation.

BACKGROUND: Early patient and allograft survival after liver transplantation (LT) depend primarily on parenchymal function, but long-term allograft success relies often on biliary-tree function. We examined parameters related to cholangiocyte damage that predict poor long-term LT outcomes after donation after brain death (DBD). METHODS: Sixty bile ducts (BD) were assessed by a BD damage-score and divided into groups with "major" BD-damage (n = 33) and "no relevant" damage (n = 27) during static cold storage. Patients with "major" BD damage were further investigated by measuring biliary excretion parameters in the first 14 days post-LT (followed-up for 60-months). RESULTS: Patients who received LT showing "major" BD damage had significantly worse long-term patient survival, versus grafts with "no relevant" damage (p = .03). When "major" BD damage developed, low bilirubin levels (p = .012) and high gamma-glutamyl transferase (GGT)/bilirubin ratio (p = .0003) were evident in the early post-LT phase (7-14 days) in patients who survived (> 60 months), compared to those who did not. "High risk" patients with bile duct damage and low GGT/bilirubin ratio had significantly shorter overall survival (p < .0001). CONCLUSIONS: Once "major" BD damage occurs, a high GGT/bilirubin ratio in the early post-operative phase is likely indicator of liver and cholangiocyte regeneration, and thus a harbinger of good overall outcomes. "Major" BD damage without markers of regeneration identifies LT patients that could benefit from future repair therapies.

Overabundant deer and invasive plants drive widespread regeneration debt in eastern United States national parks.

Advanced regeneration, in the form of tree seedlings and saplings, is critical for ensuring the long-term viability and resilience of forest ecosystems in the eastern United States. Lack of regeneration and/or compositional mismatch between regeneration and canopy layers, called regeneration debt, can lead to shifts in forest composition, structure, and, in extreme cases, forest loss. In this study, we examined status and trends in regeneration across 39 national parks from Virginia to Maine, spanning 12 years to apply the regeneration debt concept. We further refined the concept by adding new metrics and classifying results into easily interpreted categories adapted from the literature: imminent failure, probable failure, insecure, and secure. We then used model selection to determine the potential drivers most influencing patterns of regeneration debt. Status and trends indicated widespread regeneration debt in eastern national parks, with 27 of 39 parks classified as imminent or probable failure. Deer browse impact was consistently the strongest predictor of regeneration abundance. The most pervasive component of regeneration debt observed across parks was a sapling bottleneck, characterized by critically low sapling density of native canopy species and significant declines in native canopy sapling basal area or density for most parks. Regeneration mismatches also threaten forest resilience in many parks, where native canopy seedlings and saplings were outnumbered by native subcanopy species, particularly species that are less palatable deer browse. The devastating impact of emerald ash borer eliminating ash as a native canopy tree also drove regeneration mismatches in many parks that contain abundant ash regeneration, demonstrating the vulnerability of forests that lack diverse understories to invasive pests and pathogens. These findings underscore the critical importance of an integrated forest management approach that promotes an abundant and diverse regeneration layer. In most cases, this can only be achieved through long-term (i.e., multidecadal) management of white-tailed deer and invasive plants. Small-scale disturbances that increase structural complexity may also promote regeneration where stress from deer and invasive plants is minimal. Without immediate and sustained management intervention, the forest loss we are already observing may become a widespread pattern in eastern national parks and the broader region.

Anatomical acclimation of mature leaves to increased irradiance in sycamore maple (Acer pseudoplatanus L.).

Trees regenerating in the understory respond to increased availability of light caused by gap formation by undergoing a range of morphological and physiological adjustments. These adjustments include the production of thick, sun-type leaves containing thicker mesophyll and longer palisade cells than in shade-type leaves. We asked whether in the shade-regenerating tree Acer pseudoplatanus, the increase in leaf thickness and expansion of leaf tissues are possible also in leaves that are already fully formed, a response reported so far only for a handful of species. We acclimated potted seedlings to eight levels (from 1 to 100%) of solar irradiance and, in late summer, transferred a subset of them to full sunlight. Within 30 days, the pre-shaded leaves increased leaf mass per area and became thicker mostly due to the elongation of palisade cells, except for the most shaded individuals which suffered irreversible photo-oxidative damage. This anatomical acclimation was accompanied by a transient decline in photosynthetic efficiency of PSII (Fv/FM), the magnitude of which was related to the degree of pre-shading. The Fv/FM recovered substantially within the re-acclimation period. However, leaves of transferred plants were shed earlier in the fall, indicating that the acclimation was not fully effective. These results show that A. pseudoplatanus is one of the few known species in which mature leaves may re-acclimate anatomically to increased irradiance. This may be an important mechanism enhancing utilization of gaps created during the growing season.

Anatomical adjustment of mature leaves of sycamore maple (Acer pseudoplatanus L.) to increased irradiance.

Trees regenerating in the understory respond to increased availability of light caused by gap formation by undergoing a range of morphological and physiological adjustments. These adjustments include the production of thick, sun-type leaves containing thicker mesophyll and longer palisade cells than in shade-type leaves. We asked whether in the shade-regenerating tree Acer pseudoplatanus, the increase in leaf thickness and expansion of leaf tissues are possible also in leaves that had been fully formed prior to the increase in irradiance, a response reported so far only for a handful of species. We acclimated potted seedlings to eight levels (from 1 to 100%) of solar irradiance and, in late summer, transferred a subset of them to full sunlight. Within 30 days, the shaded leaves increased leaf mass per area and became thicker mostly due to elongation of palisade cells, except for the most shaded individuals which suffered irreversible photo-oxidative damage. This anatomical acclimation was accompanied by partial degradation of chlorophyll and a transient decline in photosynthetic efficiency of PSII (Fv/FM). These effects were related to the degree of pre-shading. The Fv/FM recovered substantially within the re-acclimation period. However, leaves of transferred plants were shed significantly earlier in the fall, indicating that the acclimation was not fully effective. These results show that A. pseudoplatanus is one of the few known species in which mature leaves may re-acclimate anatomically to increased irradiance. This may be a potentially important mechanism enhancing utilization of gaps created during the growing season.

Level and spatial pattern of overstory retention impose trade-offs for regenerating and retained trees.

Variable retention (VR) has been adopted globally as an alternative to more intensive forms of regeneration harvest. By retaining live trees within harvest units, VR seeks balance among the commodity, ecological, and aesthetic values of managed forests. Achieving these multiple, often competing objectives requires an understanding of how level and spatial pattern of retention shape the abundance, growth, and mortality of regenerating and retained trees. Using long-term (18-19 yr) data from a regional-scale VR experiment, we explore the individual and interactive effects of retention level (15% vs. 40% of initial basal area) and pattern (dispersed vs. aggregated) on the post-harvest dynamics of forests of differing structure and seral composition. Level and pattern of retention imposed trade-offs for the density and growth of regenerating trees (≥0.1 m tall, <5 cm dbh) and ingrowth (trees attaining 5 cm during the study). Greater retention led to greater density of late-seral regeneration, but lower density of early-seral ingrowth, and slower growth of late-seral ingrowth. Dispersed retention enhanced the density of early- and late-seral regeneration (compared to aggregated treatments), but reduced the growth of early-seral ingrowth. We also observed trade-offs for retained trees. Lower retention enhanced the growth of smaller trees (<25 cm dbh), particularly in dispersed settings, but reduced the survival of larger trees, which were more susceptible to windthrow. Greater retention reduced the growth, but enhanced the survival of smaller trees. Pattern imposed similar trade-offs, with dispersed retention enhancing growth, but reducing survival of small trees. Finally, level and pattern resulted in trade-offs for productivity of regenerating vs. retained-tree cohorts. Ingrowth productivity was greater at lower retention and in aggregated treatments; retained-tree productivity was greater at higher retention and in dispersed treatments. Our results provide a unique, long-term perspective on the sensitivity of tree regeneration, growth, and mortality to key structural elements of VR systems. Strong responses to level and pattern of retention produce trade-offs for different ecological or resource objectives. Balancing these objectives may require the combined use of aggregates, dispersed retention, and clearings, to mimic the spatial heterogeneity of habitats, physical structures, and resource conditions that are produced by natural disturbances.

Wildfire-Driven Forest Conversion in Western North American Landscapes.

Changing disturbance regimes and climate can overcome forest ecosystem resilience. Following high-severity fire, forest recovery may be compromised by lack of tree seed sources, warmer and drier postfire climate, or short-interval reburning. A potential outcome of the loss of resilience is the conversion of the prefire forest to a different forest type or nonforest vegetation. Conversion implies major, extensive, and enduring changes in dominant species, life forms, or functions, with impacts on ecosystem services. In the present article, we synthesize a growing body of evidence of fire-driven conversion and our understanding of its causes across western North America. We assess our capacity to predict conversion and highlight important uncertainties. Increasing forest vulnerability to changing fire activity and climate compels shifts in management approaches, and we propose key themes for applied research coproduced by scientists and managers to support decision-making in an era when the prefire forest may not return.

The functional role of temperate forest understorey vegetation in a changing world.

Temperate forests cover 16% of the global forest area. Within these forests, the understorey is an important biodiversity reservoir that can influence ecosystem processes and functions in multiple ways. However, we still lack a thorough understanding of the relative importance of the understorey for temperate forest functioning. As a result, understoreys are often ignored during assessments of forest functioning and changes thereof under global change. We here compiled studies that quantify the relative importance of the understorey for temperate forest functioning, focussing on litter production, nutrient cycling, evapotranspiration, tree regeneration, pollination and pathogen dynamics. We describe the mechanisms driving understorey functioning and develop a conceptual framework synthesizing possible effects of multiple global change drivers on understorey-mediated forest ecosystem functioning. Our review illustrates that the understorey's contribution to temperate forest functioning is significant but varies depending on the ecosystem function and the environmental context, and more importantly, the characteristics of the overstorey. To predict changes in understorey functioning and its relative importance for temperate forest functioning under global change, we argue that a simultaneous investigation of both overstorey and understorey functional responses to global change will be crucial. Our review shows that such studies are still very scarce, only available for a limited set of ecosystem functions and limited to quantification, providing little data to forecast functional responses to global change.

Evidence for declining forest resilience to wildfires under climate change.

Forest resilience to climate change is a global concern given the potential effects of increased disturbance activity, warming temperatures and increased moisture stress on plants. We used a multi-regional dataset of 1485 sites across 52 wildfires from the US Rocky Mountains to ask if and how changing climate over the last several decades impacted post-fire tree regeneration, a key indicator of forest resilience. Results highlight significant decreases in tree regeneration in the 21st century. Annual moisture deficits were significantly greater from 2000 to 2015 as compared to 1985-1999, suggesting increasingly unfavourable post-fire growing conditions, corresponding to significantly lower seedling densities and increased regeneration failure. Dry forests that already occur at the edge of their climatic tolerance are most prone to conversion to non-forests after wildfires. Major climate-induced reduction in forest density and extent has important consequences for a myriad of ecosystem services now and in the future.

Traditional cattle vs. introduced deer management in Chaco Serrano woodlands (Argentina): Analysis of environmental sustainability at increasing densities.

Wild ungulate populations have increased and expanded considerably in many regions, including austral woodlands and forests where deer (Cervus elaphus) have been introduced as an alternative management to traditional cattle grazing. In this study, we compared traditional cattle with introduced deer management at increasing deer densities in the "Chaco Serrano" woodlands of Argentina to assess their ecological sustainability. We used three ecological indicators (abundance of tree regeneration, woody plant diversity and browsing damage) as proxies for environmental sustainability in woody systems. Our results indicate that traditional cattle management, at stocking rates of ∼10 ind km-2, was the most ecologically sustainable management since it allowed greater tree regeneration abundance, higher richness of woody species and lower browsing damage. Importantly, cattle management and deer management at low densities (10 ind km-2) showed no significant differences in species richness and abundance of seedlings, although deer caused greater browsing damage on saplings and juveniles. However, management regimes involving high deer densities (∼35 deer km2) was highly unsustainable in comparison to low (∼10 deer km-2) and medium (∼20 deer km-2) densities, with 40% probability of unsustainable browsing as opposed to less than 5% probability at low and medium densities. In addition, high deer densities caused a strong reduction in tree regeneration, with a 19-30% reduction in the abundance of seedlings and young trees when compared to low deer densities. These results showed that the effect of increasing deer densities on woody plant conservation was not linear, with high deer densities causing a disproportional deleterious effect on tree regeneration and sustainable browsing. Our results suggest that traditional management at low densities or the use of introduced ungulates (deer breeding areas) at low-medium densities (<20 deer km-2) are compatible with woody vegetation conservation. However, further research is needed on plant palatability, animal habitat use (spatial heterogeneity) and species turnover and extinction (comparison to areas of low-null historical browsing) to better estimate environmental sustainability of Neotropical ungulate-dominated woodlands.

Two centuries of masting data for European beech and Norway spruce across the European continent.

Tree masting is one of the most intensively studied ecological processes. It affects nutrient fluxes of trees, regeneration dynamics in forests, animal population densities, and ultimately influences ecosystem services. Despite a large volume of research focused on masting, its evolutionary ecology, spatial and temporal variability, and environmental drivers are still matter of debate. Understanding the proximate and ultimate causes of masting at broad spatial and temporal scales will enable us to predict tree reproductive strategies and their response to changing environment. Here we provide broad spatial (distribution range-wide) and temporal (century) masting data for the two main masting tree species in Europe, European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) H. Karst.). We collected masting data from a total of 359 sources through an extensive literature review and from unpublished surveys. The data set has a total of 1,747 series and 18,348 yearly observations from 28 countries and covering a time span of years 1677-2016 and 1791-2016 for beech and spruce, respectively. For each record, the following information is available: identification code; species; year of observation; proxy of masting (flower, pollen, fruit, seed, dendrochronological reconstructions); statistical data type (ordinal, continuous); data value; unit of measurement (only in case of continuous data); geographical location (country, Nomenclature of Units for Territorial Statistics NUTS-1 level, municipality, coordinates); first and last record year and related length; type of data source (field survey, peer reviewed scientific literature, gray literature, personal observation); source identification code; date when data were added to the database; comments. To provide a ready-to-use masting index we harmonized ordinal data into five classes. Furthermore, we computed an additional field where continuous series with length >4 yr where converted into a five classes ordinal index. To our knowledge, this is the most comprehensive published database on species-specific masting behavior. It is useful to study spatial and temporal patterns of masting and its proximate and ultimate causes, to refine studies based on tree-ring chronologies, to understand dynamics of animal species and pests vectored by these animals affecting human health, and it may serve as calibration-validation data for dynamic forest models.

Vulnerability to forest loss through altered postfire recovery dynamics in a warming climate in the Klamath Mountains.

In the context of ongoing climatic warming, certain landscapes could be near a tipping point where relatively small changes to their fire regimes or their postfire forest recovery dynamics could bring about extensive forest loss, with associated effects on biodiversity and carbon-cycle feedbacks to climate change. Such concerns are particularly valid in the Klamath Region of northern California and southwestern Oregon, where severe fire initially converts montane conifer forests to systems dominated by broadleaf trees and shrubs. Conifers eventually overtop the competing vegetation, but until they do, these systems could be perpetuated by a cycle of reburning. To assess the vulnerability of conifer forests to increased fire activity and altered forest recovery dynamics in a warmer, drier climate, we characterized vegetation dynamics following severe fire in nine fire years over the last three decades across the climatic aridity gradient of montane conifer forests. Postfire conifer recruitment was limited to a narrow window, with 89% of recruitment in the first 4 years, and height growth tended to decrease as the lag between the fire year and the recruitment year increased. Growth reductions at longer lags were more pronounced at drier sites, where conifers comprised a smaller portion of live woody biomass. An interaction between seed-source availability and climatic aridity drove substantial variation in the density of regenerating conifers. With increasing climatic water deficit, higher propagule pressure (i.e., smaller patch sizes for high-severity fire) was needed to support a given conifer seedling density, which implies that projected future increases in aridity could limit postfire regeneration across a growing portion of the landscape. Under a more severe prospective warming scenario, by the end of the century more than half of the area currently capable of supporting montane conifer forest could become subject to minimal conifer regeneration in even moderate-sized (10s of ha) high-severity patches.

Exotic weeds and fluctuating microclimate can constrain native plant regeneration in urban forest restoration.

Restoring forest structure and composition is an important component of urban land management, but we lack clear understanding of the mechanisms driving restoration success. Here we studied two indicators of restoration success in temperate rainforests: native tree regeneration and epiphyte colonization. We hypothesized that ecosystem properties such as forest canopy openness, abundance of exotic herbaceous weeds, and the microclimate directly affect the density and diversity of native tree seedlings and epiphytes. Relationships between environmental conditions and the plant community were investigated in 27 restored urban forests spanning 3-70 years in age and in unrestored and remnant urban forests. We used structural equation modelling to determine the direct and indirect drivers of native tree regeneration and epiphyte colonization in the restored forests. Compared to remnant forest, unrestored forest had fewer native canopy tree species, significantly more light reaching the forest floor annually, and higher exotic weed cover. Additionally, epiphyte density was lower and native tree regeneration density was marginally lower in the unrestored forests. In restored forests, light availability was reduced to levels found in remnant forests within 20 years of restoration planting, followed shortly thereafter by declines in herbaceous exotic weeds and reduced fluctuation of relative humidity and soil temperatures. Contrary to expectations, canopy openness was only an indirect driver of tree regeneration and epiphyte colonization, but it directly regulated weed cover and microclimatic fluctuations, both of which directly drove the density and richness of regeneration and epiphyte colonization. Epiphyte density and diversity were also positively related to forest basal area, as large trees provide physical habitat for colonization. These results imply that ecosystem properties change predictably after initial restoration plantings, and that reaching critical thresholds in some ecosystem properties makes conditions suitable for the regeneration of late successional species, which is vital for restoration success and long-term ecosystem sustainability. Abiotic and biotic conditions that promote tree regeneration and epiphyte colonization will likely be present in forests with a basal area ≥27 m2 /ha. We recommend that urban forest restoration plantings be designed to promote rapid canopy closure to reduce light availability, suppress herbaceous weeds, and stabilize the microclimate.

Repeated wildfires alter forest recovery of mixed-conifer ecosystems.

Most models project warmer and drier climates that will contribute to larger and more frequent wildfires. However, it remains unknown how repeated wildfires alter post-fire successional patterns and forest structure. Here, we test the hypothesis that the number of wildfires, as well as the order and severity of wildfire events interact to alter forest structure and vegetation recovery and implications for vegetation management. In 2014, we examined forest structure, composition, and tree regeneration in stands that burned 1-18 yr before a subsequent 2007 wildfire. Three important findings emerged: (1) Repeatedly burned forests had 15% less woody surface fuels and 31% lower tree seedling densities compared with forests that only experienced one recent wildfire. These repeatedly burned areas are recovering differently than sites burned once, which may lead to alternative ecosystem structure. (2) Order of burn severity (high followed by low severity compared with low followed by high severity) did influence forest characteristics. When low burn severity followed high, forests had 60% lower canopy closure and total basal area with 92% fewer tree seedlings than when high burn severity followed low. (3) Time between fires had no effect on most variables measured following the second fire except large woody fuels, canopy closure and tree seedling density. We conclude that repeatedly burned areas meet many vegetation management objectives of reduced fuel loads and moderate tree seedling densities. These differences in forest structure, composition, and tree regeneration have implications not only for the trajectories of these forests, but may reduce fire intensity and burn severity of subsequent wildfires and may be used in conjunction with future fire suppression tactics.

Seedling ontogeny and environmental plasticity in two co-occurring shade-tolerant conifers and implications for environment-population interactions.

PREMISE OF THE STUDY: Seedling success is determined by evolved strategies of intrinsic genetic programming and plasticity that are regulated by extrinsic pathways. We tested the relative importance of these mechanisms in red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea Lin.), which share understory regeneration niches in northeastern North America. Although its reproductive effort is adequate, spruce has decreased in abundance, in relation to fir, in seedling and sapling populations, even in forests that have a predominance of spruce in the overstory. METHODS: To understand the factors that regulate this phenomenon and their implications for tree populations, we compared intrinsic and plastic regulation of first- and second-year seedlings under steady understory irradiance levels and in response to increases in light environment. KEY RESULTS: Both species exhibited interactions of ontogenetic patterns and plasticity in first- and second-year seedlings. Physiologically, spruce had higher photosynthetic capacity, allocation to photoprotective xanthophylls, and greater plasticity in response to light treatments. Although both species demonstrated an inability to plastically increase photosynthetic capacity in the short term, spruce benefited from greater allocation to foliage under increased irradiance. Fir showed a conservative strategy in root-shoot allocation that may better equip seedlings to withstand drought adaptations and attributes associated with greater shade tolerance. CONCLUSIONS: These attributes likely contribute to the relative success of fir seedlings in the current climate. By contrast, they indicate that spruce would be a superior competitor in cooler, moister climates, which suggests that future forest composition will be largely determined by an interaction of disturbance and moisture regimes.

Sapling recruitment as an indicator of carbon resiliency in forests of the northern USA.

Tree regeneration shapes forest carbon dynamics by determining long-term forest composition and structure, which suggests that threats to natural regeneration may diminish the capacity of forests to replace live tree carbon transferred to the atmosphere or other pools through tree mortality. Yet, the potential implications of tree regeneration patterns for future carbon dynamics have been sparsely studied. We used forest inventory plots to investigate whether the composition of existing tree regeneration is consistent with aboveground carbon stock loss, replacement, or gain for forests across the northeastern and midwestern USA, leveraging a recently developed method to predict the likelihood of sapling recruitment from seedling abundance tallied within six seedling height classes. A comparison of carbon stock predictions from tree and seedling composition suggested that 29% of plots were poised to lose carbon based on seedling composition, 55% were poised for replacement of carbon stocks (<5 Mg ha-1 difference) and 16% were poised to gain carbon. Forests predicted to lose carbon tended to be on steeper slopes, at lower latitudes, and in rolling upland environments. Although plots predicted to gain and lose carbon had similar stand ages, carbon loss plots had greater current carbon stocks. Synthesis and applications. Our results demonstrate the utility of considering tree regeneration through the lens of carbon replacement to develop effective management strategies to secure long-term carbon storage and resilience in the context of global change. Forests poised to lose C due to climate change and other stressors could be prioritized for regeneration strategies that enhance long-term carbon resilience and stewardship.