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The concept of stability is central to the study and sustainability of vital ecosystem goods and services as disturbances increase globally. While ecosystem ecologists, including carbon (C) cycling scientists, have long‐considered multiple dimensions of disturbance response, our discipline lacks an agreed‐upon analytical framework for characterizing multidimensional stability. Here, we advocate for the broader adoption of a standardized and normalized multidimensional stability framework for analyzing disturbance response. This framework includes four dimensions of stability: the degree of initial change in C fluxes (i.e., resistance); rate (i.e., resilience) and variability (i.e., temporal stability) of return to pre‐disturbance C fluxes; and the extent of return to pre‐disturbance C fluxes (i.e., recovery). Using this framework, we highlight findings not readily seen from analysis of absolute fluxes, including trade‐offs between initial and long‐term C flux responses to disturbance; different overall stability profiles among fluxes; and, using a pilot dataset, similar relative stability of net primary production following fire and insect disturbances. We conclude that ecosystem ecologists' embrace of a unifying multidimensional stability framework as a complement to approaches focused on absolute C fluxes could advance global change research by aiding in the novel interpretation, comprehensive synthesis, and improved forecasting of ecosystems' response to an increasing array of disturbances. [ABSTRACT FROM AUTHOR]

Many secondary deciduous forests of eastern North America are approaching a transition in which mature early‐successional trees are declining, resulting in an uncertain future for this century‐long carbon (C) sink. We initiated the Forest Accelerated Succession Experiment (FASET) at the University of Michigan Biological Station to examine the patterns and mechanisms underlying forest C cycling following the stem girdling‐induced mortality of >6,700 early‐successional Populus spp. (aspen) and Betula papyrifera (paper birch). Meteorological flux tower‐based C cycling observations from the 33‐ha treatment forest have been paired with those from a nearby unmanipulated forest since 2008. Following over a decade of observations, we revisit our core hypothesis: that net ecosystem production (NEP) would increase following the transition to mid‐late‐successional species dominance due to increased canopy structural complexity. Supporting our hypothesis, NEP was stable, briefly declined, and then increased relative to the control in the decade following disturbance; however, increasing NEP was not associated with rising structural complexity but rather with a rapid 1‐yr recovery of total leaf area index as mid‐late‐successional Acer, Quercus, and Pinus assumed canopy dominance. The transition to mid‐late‐successional species dominance improved carbon‐use efficiency (CUE = NEP/gross primary production) as ecosystem respiration declined. Similar soil respiration rates in control and treatment forests, along with species differences in leaf physiology and the rising relative growth rates of mid‐late‐successional species in the treatment forest, suggest changes in aboveground plant respiration and growth were primarily responsible for increases in NEP. We conclude that deciduous forests transitioning from early to middle succession are capable of sustained or increased NEP, even when experiencing extensive tree mortality. This adds to mounting evidence that aging deciduous forests in the region will function as C sinks for decades to come. [ABSTRACT FROM AUTHOR]

The study of vegetation community and structural change has been central to ecology for over acentury, yet the ways in which disturbances reshape the physical structure of forest canopies remain relatively unknown. Moderate severity disturbances affect different canopy strata and plant species, resulting in variable structural outcomes and ecological consequences. Terrestrial lidar (light detection and ranging) offers an unprecedented view of the interior arrangement and distribution of canopy elements, permitting the derivation of multidimensional measures of canopy structure that describe several canopy structural traits (CSTs)with known links to ecosystem function. We used lidar-derived CSTs within a machine learning framework to detect and describe the structural changes that result from various disturbance agents, including moderate severity fire, ice storm damage, age-related senescence, hemlock woolly adelgid, beech bark disease, and chronic acidification. We found that fire and ice storms primarily affected the amount and position of vegetation within canopies, while acidification, senescence, pathogen, and insect infestation altered canopy arrangement and complexity. Only two of the six disturbance agents significantly reduced leaf area, counter to common assumptions regarding many moderate severity disturbances. While findings are limited in their generalizability due to lack of replication among disturbances, they do suggest that the current limitations of standard disturbance detection methods—such as optical-based remote sensing platforms, which are often above-canopy perspectives—limit our ability to understand the full ecological and structural impacts of disturbance, and to evaluate the consistency of structural patterns within and among disturbance agents. A more broadly inclusive definition of ecological disturbance that incorporates multiple aspects of canopy structural change may potentially improve the modeling, detection, and prediction of functional implications of moderate severity disturbance as well as broaden our understanding of the ecological impacts of disturbance. [ABSTRACT FROM AUTHOR]

• We examined the effects of beech bark disease (BBD) on aboveground wood production. • BBD did not reduce production in middle or late successional forest stands. • At the patch-scale, the degree of tree mortality drove changes in production. • Initial responses to BBD sometimes predicted longer-term changes in production. • BBD management should consider scale and the timing of disease progression. The distribution of pests and pathogens is increasing in many forested regions, producing uncertainty for ecological functions, including aboveground wood net primary production (NPP). In North American deciduous forests, beech bark disease (BBD) is restructuring and modifying the composition of forest stands, producing gradients of Fagus grandifolia mortality at finer patch scales. We investigated the multi-decadal effects of BBD on the aboveground wood NPP of a moderately productive middle-successional stand positioned on a glacial outwash plain and a relatively high productivity late-successional stand located on a moraine. Despite average stand-scale basal area losses of ∼ 21% from BBD, aboveground wood NPP increased over time in both the middle- and late- successional stands. At the patch scale, the initial magnitude of change in aboveground wood NPP following BBD infestation correlated with the extent of recovery in the late, but not middle, successional stand, suggesting early responses to disturbance sometimes – but not always – predict long-term production patterns. Patch-scale aboveground wood NPP during different stages of BBD infestation was associated with vegetation quantity and production efficiency, with the latter generally increasing in later stages of the BBD progression. We conclude that the aboveground wood NPP of two forest stands increased through late stages of BBD, despite differences in stand productivity, structure, and age, while patch-scale aboveground wood NPP responses were more variable. [ABSTRACT FROM AUTHOR]

Contents Summary 1188 I. Introduction 1188 II. Forest aging and carbon storage 1189 III. Successional trends of NEP in northern deciduous forests 1190 IV. Mechanisms sustaining NEP in aging deciduous forests 1191 Acknowledgements 1192 References 1192 Summary: Large areas of forestland in temperate North America, as well as in other parts of the world, are growing older and will soon transition into middle and then late successional stages exceeding 100 yr in age. These ecosystems have been important regional carbon sinks as they recovered from prior anthropogenic and natural disturbance, but their future sink strength, or annual rate of carbon storage, is in question. Ecosystem development theory predicts a steady decline in annual carbon storage as forests age, but newly available, direct measurements of forest net CO2 exchange challenge that prediction. In temperate deciduous forests, where moderate severity disturbance regimes now often prevail, there is little evidence for any marked decline in carbon storage rate during mid‐succession. Rather, an increase in physical and biological complexity under these disturbance regimes may drive increases in resource‐use efficiency and resource availability that help to maintain significant carbon storage in these forests well past the century mark. Conservation of aging deciduous forests may therefore sustain the terrestrial carbon sink, whilst providing other goods and services afforded by these biologically and structurally complex ecosystems. [ABSTRACT FROM AUTHOR]

Moderate severity disturbances, which only kill a subset of canopy trees (e.g., via insects, pathogens, and windthrow), are increasingly widespread in North America, and can alter forest structure and production. Whether the net primary production (NPP) of forest stands differing in pre-disturbance site quality and composition respond similarly to moderate severity disturbance, however, is unknown, but critical to understanding the disturbance response dynamics of patchy landscapes. We experimentally disturbed three, 2-ha stands varying in pre-disturbance primary production and community composition, temporarily reducing live stand basal area by 38% to 66% through the stem girdling of all mature early successional aspen (Populus tremuloides Michx. and Populus grandidentata Michx.) and birch (Betula papyrifera Marshall). Disturbance significantly altered stand-scale physical and biological structure and prompted a similar decade-long pattern of wood NPP decline and recovery. All stands exhibited an initial reduction in wood NPP, followed by a recovery period and eventual return to pre-disturbance levels within eight years, with the most productive stand exhibiting an increase in primary production following recovery. Following wood NPP recovery, more biologically diverse forest canopies with higher leaf area indexes intercepted more light, and, consequently, had higher rates of wood NPP.We conclude that, despite substantial pre-disturbance differences in productivity and community composition, relative wood NPP recovery patterns can be similar, though long-term post-recovery primary production may trend higher in more productive and compositionally diverse stands. We suggest that improved mechanistic understanding of different forest ecosystems' responses to disturbances remains critical to informing management decisions across diverse landscape mosaics. [ABSTRACT FROM AUTHOR]

Century-old forests in the U.S. upper Midwest and Northeast power much of North America's terrestrial carbon (C) sink, but these forests' production and C sequestration capacity are expected to soon decline as fast-growing early successional species die and are replaced by slower growing late successional species. But will this really happen? Here we marshal empirical data and ecological theory to argue that substantial declines in net ecosystem production ( NEP) owing to reduced forest growth, or net primary production ( NPP), are not imminent in regrown temperate deciduous forests over the next several decades. Forest age and production data for temperate deciduous forests, synthesized from published literature, suggest slight declines in NEP and increasing or stable NPP during middle successional stages. We revisit long-held hypotheses by EP Odum and others that suggest low-severity, high-frequency disturbances occurring in the region's aging forests will, against intuition, maintain NEP at higher-than-expected rates by increasing ecosystem complexity, sustaining or enhancing NPP to a level that largely offsets rising C losses as heterotrophic respiration increases. This theoretical model is also supported by biological evidence and observations from the Forest Accelerated Succession Experiment in Michigan, USA. Ecosystems that experience high-severity disturbances that simplify ecosystem complexity can exhibit substantial declines in production during middle stages of succession. However, observations from these ecosystems have exerted a disproportionate influence on assumptions regarding the trajectory and magnitude of age-related declines in forest production. We conclude that there is a wide ecological space for forests to maintain NPP and, in doing so, lessens the declines in NEP, with significant implications for the future of the North American carbon sink. Our intellectual frameworks for understanding forest C cycle dynamics and resilience need to catch up to our more complex and nuanced understanding of ecological succession. [ABSTRACT FROM AUTHOR]

Forested landscapes are shaped by disturbances varying in severity and source. Moderate disturbance from weather, pathogens, insects, and age-related senescence that kills only a subset of canopy trees may increase standing woody debris and alter the contribution of coarse woody debris (CWD) to total ecosystem respiration ( R E ). However, woody debris carbon (C) dynamics are rarely examined following moderate disturbances that increase standing dead wood pools. We used an experimental manipulation of moderate disturbance in an upper Great Lakes forest to: (1) examine multi-year changes in CWD mass through a moderate disturbance; (2) quantify in situ CWD respiration during different stages of decay for downed and standing woody debris and; (3) estimate the annual contribution of CWD respiration to the ecosystem C balance through comparison with R E and net ecosystem production (NEP). Six years following disturbance, we found that the standing dead wood mass of 24.5 Mg C ha −1 was an order of magnitude greater than downed woody debris mass and a large source of ecosystem C flux. Instantaneous in situ respiration rates from standing and minimally decayed downed woody debris were not significantly different from one another. Separate estimates of ecosystem CWD respiration of 1.1–2.1 Mg C ha −1 yr −1 six years following disturbance were comparable in magnitude to NEP and 12.5–23.8% of R E , representing a substantial increase relative to pre-disturbance levels. Ecosystem respiration and NEP were stable following moderate disturbance even though ecosystem CWD respiration increased substantially, suggesting a reduction in the respiratory C contribution from other sources. We conclude that standing and downed CWD can be essential components of the ecosystem C balance following moderate severity disturbance. [ABSTRACT FROM AUTHOR]

The global carbon (C) balance is vulnerable to disturbances that alter terrestrial C storage. Disturbances to forests occur along a continuum of severity, from low-intensity disturbance causing the mortality or defoliation of only a subset of trees to severe standreplacing disturbance that kills all trees; yet considerable uncertainty remains in how forest production changes across gradients of disturbance intensity. We used a gradient of tree mortality in an upper Great Lakes forest ecosystem to: (1) quantify how aboveground wood net primary production (ANPPw) responds to a range of disturbance severities; and (2) identify mechanisms supporting ANPPw resistance or resilience following moderate disturbance. We found that ANPPw declined nonlinearly with rising disturbance severity, remaining stable until .60% of the total tree basal area senesced. As upper canopy openness increased from disturbance, greater light availability to the subcanopy enhanced the leaf-level photosynthesis and growth of this formerly light-limited canopy stratum, compensating for upper canopy production losses and a reduction in total leaf area index (LAI). As a result, whole-ecosystem production efficiency (ANPPw/LAI) increased with rising disturbance severity, except in plots beyond the disturbance threshold. These findings provide a mechanistic explanation for a nonlinear relationship between ANPPw and disturbance severity, in which the physiological and growth enhancement of undisturbed vegetation is proportional to the level of disturbance until a threshold is exceeded. Our results have important ecological and management implications, demonstrating that in some ecosystems moderate levels of disturbance minimally alter forest production. [ABSTRACT FROM AUTHOR]

• Disturbance effects on forest canopy structural complexity are poorly understood. • We synthesized the results of three forest disturbance manipulations. • Disturbance had variable effects on the temporal dynamics of structural complexity. • Moderate severity disturbance sometimes increased structural complexity. • Disturbance severity, source, and scale constrain temporal changes in complexity. The temporal dynamics of forest canopy structure are influenced by disturbances that alter vegetation quantity and distribution. While canopy structural indicators such as leaf area index (LAI), canopy cover, and canopy height have been widely studied in the context of disturbance, the post-disturbance temporal dynamics of structural complexity, which summarizes the heterogeneity of vegetation arrangement, are poorly understood. With the goal of advancing conceptual and empirical understanding of the temporal dynamics of structural complexity following disturbance, we synthesized results from three large-scale disturbance manipulation experiments at the University of Michigan Biological Station (UMBS): the 4-year Forest Resilience Threshold Experiment (FoRTE) manipulating levels of disturbance severity; the decade-long Forest Accelerated Succession Experiment (FASET), in which all early successional tree species were stem-girdled within 39 ha in the same landscape; and forest chronosequences established following clear-cut harvesting. We found that the temporal dynamics of canopy structure following disturbance were dependent upon three factors: (1) the source and severity of disturbance; (2) the spatial and temporal scales of analysis; and (3) the measure of structure assessed. Unlike vegetation area index and canopy cover, which initially decreased in response to disturbance, structural complexity measures such as canopy and top rugosity did not consistently respond to moderate levels of disturbance severity. Over multi-decadal timescales, structural complexity increased to a maximum, regardless of whether fire occurred at the time of stand establishment, but intervening low-to-moderate severity disturbance in regrown century-old forests altered trajectories of canopy rugosity. We conclude that structural complexity indicators display a more nuanced temporal and directional response to disturbance than conventional leaf area and cover indexes. Predicting what disturbance conditions modify trajectories of structural complexity remains critical to disturbance characterization and the inference of ecosystem functioning. [ABSTRACT FROM AUTHOR]

Canopy structure affects forest function by determining light availability and distribution. Many forests throughout the upper Great Lakes region are dominated by mature, even-aged, early successional aspen and birch, which comprise 35%-40% of canopy leaf area, and which are senescing at accelerating rates. In 2008 at the University of Michigan Biological Station, we initiated the Forest Accelerated Succession Experiment (FASET) by stem girdling all aspen and birch in replicated stands to induce mortality. Our objective was to understand type and rate of canopy structural changes imposed by rapid but diffuse disturbance consisting of mortality of a single age-species cohort. We characterized changes in canopy structural features in 2008-2011 using ground-based Portable Canopy Lidar (PCL) in paired treated and control stands. As aspen and birch in treated plots died, gap fraction of the upper canopy increased, average leaf height decreased, total canopy height declined, and openness of the whole-canopy increased. All of these trends became more pronounced with time. Our findings suggest that as forests throughout the region pass through the impending successional transition prompted by widespread mortality of canopy-dominant early successional aspen and birch species, the canopy will undergo significant structural reorganization with consequences for forest carbon assimilation. [ABSTRACT FROM AUTHOR]

Forest harvesting and wildfire were widespread in the upper Great Lakes region of North America during the early 20th century. We examined how long this legacy of disturbance constrains forest carbon (C) storage rates by quantifying C pools and fluxes after harvest and fire in a mixed deciduous forest chronosequence in northern lower Michigan, USA. Study plots ranged in age from 6 to 68 years and were created following experimental clear-cut harvesting and fire disturbance. Annual C storage was estimated biometrically from measurements of wood, leaf, fine root, and woody debris mass, mass losses to herbivory, soil C content, and soil respiration. Maximum annual C storage in stands that were disturbed by harvest and fire twice was 26% less than a reference stand receiving the same disturbance only once. The mechanism for this reduction in annual C storage was a long-lasting decrease in site quality that endured over the 62-year timeframe examined. However, during regrowth the harvested and burned forest rapidly became a net C sink, storing 0.53 Mg C ha−1 yr−1 after 6 years. Maximum net ecosystem production (1.35 Mg C ha−1 yr−1) and annual C increment (0.95 Mg C ha−1 yr−1) were recorded in the 24- and 50-year-old stands, respectively. Net primary production averaged 5.19 Mg C ha−1 yr−1 in experimental stands, increasing by < 10% from 6 to 50 years. Soil heterotrophic respiration was more variable across stand ages, ranging from 3.85 Mg C ha−1 yr−1 in the 6-year-old stand to 4.56 Mg C ha−1 yr−1 in the 68-year-old stand. These results suggest that harvesting and fire disturbances broadly distributed across the region decades ago caused changes in site quality and successional status that continue to limit forest C storage rates. [ABSTRACT FROM AUTHOR]

Carbon (C) cycling processes are particularly dynamic following disturbance, with initial responses often indicative of longer-term change. In northern Michigan, USA, we initiated the Forest Resilience Threshold Experiment (FoRTE) to identify the processes that sustain or lead to the decline of C cycling rates across multiple levels (0, 45, 65 and 85% targeted gross leaf area index loss) of disturbance severity and, in response, to separate disturbance types preferentially targeting large or small diameter trees. Simulating the effects of boring insects, we stem girdled > 3600 trees below diameter at breast height (DBH), immediately and permanently disrupting the phloem. Weekly DBH measurements of girdled and otherwise healthy trees (n > 700) revealed small but significant increases in daily aboveground wood net primary production (ANPPw) in the 65 and 85% disturbance severity treatments that emerged six weeks after girdling. However, we observed minimal change in end-of-season leaf area index and no significant differences in annual ANPPw among disturbance severities or between disturbance types, suggesting continued C fixation by girdled trees sustained stand-scale wood production in the first growing season after disturbance. We hypothesized higher disturbance severities would favor the growth of early successional species but observed no significant difference between early and middle to late successional species' contributions to ANPPw across the disturbance severity gradient. We conclude that ANPPw stability immediately following phloem disruption is dependent on the continued, but inevitably temporary, growth of phloem-disrupted trees. Our findings provide insight into the tree-to-ecosystem mechanisms supporting stand-scale wood production stability in the first growing season following a phloem-disrupting disturbance. [ABSTRACT FROM AUTHOR]

• Upper Great Lakes forests exhibit high overall production temporal stability. • Production temporal stability varied by >2-fold among stands. • More severely disturbed forests displayed greater temporal production stability. • Stand structure and leaf traits were inconsistent predictors of temporal stability. • Properties conferring greater production rates and temporal stability may differ. Sustaining the terrestrial carbon (C) sink requires knowledge of the forest properties supporting stable production under increasingly variable climate conditions. We examined how stand disturbance history and age, structural complexity and species diversity, and leaf properties relate to the 10-yr stability of above-ground wood net primary production (NPPw) in northern temperate forests of Michigan, USA. Our investigation centered on separate deciduous, evergreen, and mixed late successional stands initiated over a century ago and free of recent disturbance, a "Cut Only" chronosequence established following clearcut harvesting, and a "Cut and Burn" chronosequence that regenerated following experimental clearcut harvesting and fire. The temporal stability of stand production was calculated from the 10-yr coefficient of variation (CV) of annual NPPw estimated from tree cores; canopy rugosity, a measure of structural complexity, was estimated using terrestrial LiDAR; and >1500 subcanopy leaves were sampled for leaf mass area and chlorophyll fluorescence intensity. The temporal stability of stands differed by >2-fold, from 5% to 11% CV of NPPw. Counter to expectations, we found that NPPw stability was greatest in the more severely disturbed Cut and Burn stands and lowest in late successional stands. Despite similar successional patterns of species diversity and structural complexity, NPPw stability increased in Cut Only stands and declined in Cut and Burn stands as age, diversity and canopy rugosity increased. The NPPw of more diverse, late successional deciduous forests was more temporally stable than that of evergreen forests. We conclude that management for maximal rates of production may not confer temporal stability, indicating future studies are needed to elucidate the stand and canopy properties that support both high production rates and stability. [ABSTRACT FROM AUTHOR]

Deforestation and forest degradation cause the deterioration of resources and ecosystem services. However, there are still no operational indicators to measure forest status, especially for forest degradation. In the present study, we analysed the thermal response number (TRN, calculated by daily total net radiation divided by daily temperature range) of 163 sites including mature forest, disturbed forest, planted forest, shrubland, grassland, savanna vegetation and cropland. TRN generally increased with latitude, however the regression of TRN against latitude differed among vegetation types. Mature forests are superior as thermal buffers, and had significantly higher TRN than disturbed and planted forests. There was a clear boundary between TRN of forest and non-forest vegetation (i.e. grassland and savanna) with the exception of shrubland, whose TRN overlapped with that of forest vegetation. We propose to use the TRN of local mature forest as the optimal TRN (TRN opt ). A forest with lower than 75% of TRN opt was identified as subjected to significant disturbance, and forests with 66% of TRN opt was the threshold for deforestation within the absolute latitude from 30° to 55°. Our results emphasized the irreplaceable thermal buffer capacity of mature forest. TRN can be used for early warning of deforestation and degradation risk. It is therefore a valuable tool in the effort to protect forests and prevent deforestation. [ABSTRACT FROM AUTHOR]