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Over the last decade, the field of plant ecology has significantly developed and expanded, especially in research concerning the herb layer and ground vegetation of forests. This revised second edition accounts for that growth, presenting research that approaches the ecology of the herb layer of forests from a variety of disciplines and perspectives. The book synthesizes the research of top ecologists and biologists on herbaceous layer structure, composition, and dynamics of a variety of forest ecosystem types in eastern North America. The 2003 first edition of this book was praised for containing the most extensive listing of herb-layer literature in existence. This second edition brings this material up to date, revised to include current research and data. The book incorporates quantitative data to support analyses that was previously unavailable during the publication of the first edition. Also featured are six entirely new chapters, focused on the response of the herbaceous layer to a wide variety of natural and anthropogenic disturbances.

• Premise of the study: Near-future climate changes are likely to elicit major vegetation changes. Disequilibrium dynamics, which occur when vegetation comes out of equilibrium with climate, are potentially a key facet of these. Understanding these dynamics is crucial for making accurate predictions, informing conservation planning, and understanding likely changes in ecosystem function on time scales relevant to society. However, many predictive studies have instead focused on equilibrium end-points with little consideration of the transient trajectories.• Methods: We review what we should expect in terms of disequilibrium vegetation dynamics over the next 50–200 yr, covering a broad range of research fields including paleoecology, macroecology, landscape ecology, vegetation science, plant ecology, invasion biology, global change biology, and ecosystem ecology.• Key results: The expected climate changes are likely to induce marked vegetation disequilibrium with climate at both leading and trailing edges, with leading-edge disequilibrium dynamics due to lags in migration at continental to landscape scales, in local population build-up and succession, in local evolutionary responses, and in ecosystem development, and trailing-edge disequilibrium dynamics involving delayed local extinctions and slow losses of ecosystem structural components. Interactions with habitat loss and invasive pests and pathogens are likely to further contribute to disequilibrium dynamics. Predictive modeling and climate-change experiments are increasingly representing disequilibrium dynamics, but with scope for improvement.• Conclusions: The likely pervasiveness and complexity of vegetation disequilibrium is a major challenge for forecasting ecological dynamics and, combined with the high ecological importance of vegetation, also constitutes a major challenge for future nature conservation.

Aim: To review and synthesize multiple lines of evidence that describe the spatial patterns of land use associated with prehistoric and early historical Native American societies in eastern North America in order to better characterize the type, spatial extent and temporal persistence of past land use. Location: Temperate forests of eastern North America, and the Eastern Woodlands cultural region. Methods: Ethnohistorical accounts, archaeological data, historical land surveys and palaeoecological records describing indigenous forms of silviculture and agriculture were evaluated across scales ranging from local (10⁰ km) to regional (10² km) to produce a synthetic description of land-use characteristics. Results: Indigenous land-use practices created patches of distinct ecological conditions within a heterogeneous mosaic of ecosystem types. At all scales, patch location was dynamic, and patches underwent recurrent periods of expansion, contraction and abandonment. Land-use patches varied in their extent and persistence, and are broadly categorized as silvicultural (management of undomesticated woodland taxa) or agricultural (cultivation of domesticated taxa). Silvicultural patches persisted for centuries and extended kilometres to tens of kilometres around settlements and travel corridors. The dynamics of agricultural patches varied among groups, with persistence ranging from decades to centuries and extent ranging from less than a kilometre to tens of kilometres around settlements. Beyond patch boundaries, human impacts on ecosystems become indistinguishable from other drivers of environmental heterogeneity. These characteristics of patches are evident across scales and multiple lines of evidence. Main conclusions: Our findings challenge the view that prehistoric human impacts on vegetation were widespread and ubiquitous, and build on previous work showing these impacts to be more localized and heterogeneous by providing quantitative descriptions of land-use patch characteristics. Collaborative efforts that combine multiple data sources are needed to refine these descriptions and generate more precise measures of land-use pattern to further investigate the history of human impacts on the Earth system.

Identifying the scales of variation in forest structures and the underlying processes are fundamental for understanding forest dynamics. Here, we studied these scale-dependencies in forest structure in naturally dynamic boreal forests on two continents. We identified the spatial scales at which forest structures varied, and analyzed how the scales of variation and the underlying drivers differed among the regions and at particular scales. We studied three 2 km × 2 km landscapes in northeastern Finland and two in eastern Canada. We estimated canopy cover in contiguous 0.1-ha cells from aerial photographs and used scale-derivative analysis to identify characteristic scales of variation in the canopy cover data. We analyzed the patterns of variation at these scales using Bayesian scale space analysis. We identified structural variation at three spatial scales in each landscape. Among landscapes, the largest scale of variation showed the greatest variability (20.1–321.4 ha), related to topography, soil variability, and long-term disturbance history. Superimposed on this large-scale variation, forest structure varied at similar scales (1.3–2.8 ha) in all landscapes. This variation correlated with recent disturbances, soil variability, and topographic position. We also detected intense variation at the smallest scale analyzed (0.1 ha, grain of our data), partly driven by recent disturbances. The distinct scales of variation indicated hierarchical structure in the landscapes studied. Except for the large-scale variation, these scales were remarkably similar among the landscapes. This suggests that boreal forests may display characteristic scales of variation that occur somewhat independent of the tree species characteristics or the disturbance regime.

The southernmost spruce-lichen woodlands in the Parc des Grands-Jardins, Québec, Canada, are situated 500 km south of their usual range in the northern lichen woodland zone. Their co-occurrence within a spruce-moss forest matrix suggests the existence of alternative stable states. We investigate the possibility of these spruce-lichen woodlands as an alternative stable state along with the factors contributing to their origin and spatiotemporal distribution. Analysis of plant macrofossils, charcoal, head capsules of defoliating insects, and pollen were used along with vegetation surveys to reconstruct the past and present disturbance dynamics along an east-west transect, corresponding to a precipitation and fire frequency gradient. At each site, spruce budworm head capsules were found preceding the charcoal layer delineating the shift to spruce-lichen woodland, demonstrating the compound disturbance (insect-fire) origin of the lichen woodlands. Moss forests previously occupied all lichen woodland sites, with the oldest record starting ca. 8300-9400 yr BP. A change to a higher fire frequency around 2500 yr BP was evident in the lichen woodland zone of the park. A lower fire frequency west of the lichen woodland zone likely is a result of orographic precipitation. While the spruce budworm affects the entire region, lichen woodlands are found exclusively within an increased fire frequency zone. Thus, it is the superimposition of these two disturbance factors that is responsible both for their creation and spatial distribution. Lichen woodland inception dates range between 580 and 1440 yr BP, demonstrating that these lichen woodlands have maintained their open structure with time and have not transformed into closed forests. Their persistence, along with their previous moss forest histories and current occurrence adjacent to closed moss forests, indicate that they are an alternative stable state to the spruce-moss forests and not a successional stage. In contrast to other examples of reported alternative stable states, this one is a result of natural disturbances inherent to the system and not anthropogenic impacts.

AIM: Comparisons of how different species respond to changing climatic conditions offer insight into future community composition and the potential formation of novel communities. This study investigated changes at a subarctic forest–tundra ecotone, or ‘tree line’. Our objectives were: (1) to explore species‐specific growth forms; (2) to identify temporal patterns of establishment and stand density; and (3) to explore relationships between climate and recruitment/survival amongst co‐dominant tree species, with the expectation that climate change will affect species differentially. LOCATION: The Mealy Mountains in the High Subarctic Tundra ecoregion in central Labrador, Canada. METHODS: We examined tree line dynamics for four tree species over the past two centuries. Using ecological and age‐structure data, we compared diameter/height relationships across the tree line and generated static age structures from which changes in stand density through time were compared. In addition, model residuals were used to quantify relationships between multi‐decadal windows of temperature/palaeotemperature/Palmer Drought Severity Index and decadal tree recruitment. RESULTS: Trees were more stunted as elevation increased, except for white spruce (Picea glauca) for which tree islands became the dominant growth form. The only tree seedlings found at the tree line were of larch (Larix laricina) and to a lesser extent black spruce (Picea mariana). From the age structure of trees (height > 2.0 m), only black spruce showed evidence of an advancing tree line. Larch and balsam fir (Abies balsamea) have become established at the tree line most recently and have undergone greater increases in density over the past few decades. Variability in recruitment increased with elevation: larch recruitment was positively correlated with temperature and negatively correlated with drought at low elevations but negatively correlated with temperature and positively correlated with drought at high elevations, whereas black spruce recruitment was consistently positively correlated with temperature and drought. MAIN CONCLUSIONS: The multispecies approach provides evidence that species are responding differentially to climate. With continued climate change, we expect density increases and advances of larch and black spruce, giving rise to novel tree line communities.

Questions: What was the tree species composition of forests prior to European settlement at the northern hardwood range limit in eastern Québec, Canada? What role did human activities play in the changes in forest composition in this region? Location: Northern range limit of northern hardwoods in the Lower St. Lawrence region of eastern Québec, Canada. Methods: We used early land survey records (1846—1949) of public lands to reconstruct pre-settlement forest composition. The data consist of ranked tree species enumerations at points or for segments along surveyed lines, with enumerations of forest cover types and notes concerning disturbances. An original procedure was developed to weigh and combine these differing data types (line versus point observations; taxa versus cover enumerations). Change to present-day forest composition was evaluated by comparing survey records with forest decadal surveys conducted by the government of Québec over the last 30 years (1980—2009). Results: Pre-settlement dominance of conifers was strong and uniform across the study area, whereas dominance of maple and birches was patchy. Cedar and spruce were less likely to dominate with increasing altitude, whereas maple displayed the reverse trend. Frequency of disturbances, especially logging and fire, increased greatly after 1900. Comparison of survey records and modern plots showed general increases for maple (mentioned frequency increased by 39%), poplar (36%) and paper birch (31%). Considering only taxa ranked first by surveyors, cedar displayed the largest decrease (19%), whereas poplar (15%) and maple (9%) increased significantly. Conclusions: These changes in forest composition can be principally attributed to clear-cutting and colonization fire disturbances throughout the 20th century, and mostly reflected the propensity of taxa to expand (maples/aspen) or decline (cedar/spruce) with increased disturbance frequency. Québec's land survey archives provide an additional data source to reconstruct and validate our knowledge of North America's pre-settlement temperate and sub-boreal forests.

Forested peatlands are widespread in the boreal landscape, but their role as carbon (C) pools remains poorly documented. In this study, we investigated the long-term C sequestration function of boreal forested bogs in relation to fires in eastern Canada. Results show that the forested peatlands comprise substantial peat C mass reaching values similar to open peatlands. At the six studied peatland sites, the amount of C stored in peat (62–172 kg C m⁻²) exceeds substantially the aboveground tree biomass C (1.5–5.3 kg C m⁻²). The C locked up in live conifers on the peatlands corresponds only to a small fraction of the C stored in peat (1–6 %). In comparison, the shallow organic layer (≤ 30 cm) in the adjacent paludifying stands store 10.8 kg C m⁻² on average, which is about twice as much C as the live conifers. Long-term apparent C accumulation rates are relatively low in the studied forested bogs (mean: 15.9 g C m⁻² y⁻¹), suggesting that these ecosystems have lower C sequestration potential than non-forested bogs over millennia. The charcoal data suggest that past local fires reduced C sequestration rates, but these peatlands burn much less frequently than upland forests and are thus more efficient long-term C stores. This study highlights the importance of boreal forested peatlands as C reservoirs and helps understanding how fires, logging and climate change can affect their C sequestration function. These findings have important implications for ecosystem management that aims at maximizing C sequestration at the landscape level to mitigate climate change.

The Earth’s climate has been warming rapidly since the beginning of the industrial era, forcing terrestrial organisms to adapt. Migration constitutes one of the most effective processes for surviving and thriving, although the speed at which tree species migrate as a function of climate change is unknown. One way to predict latitudinal movement of trees under the climate of the twenty-first century is to examine past migration since the Last Glacial Maximum. In this study, radiocarbon-dated macrofossils were used to calculate the velocity of past migration of jack pine (Pinus banksiana) and black spruce (Picea mariana), two important fire-adapted conifers of the North American boreal forest. Jack pine migrated at a mean rate of 19 km per century (km-cent) from unglaciated sites in the central and southeastern United States to the northern limit of the species in subarctic Canada. However, the velocity increased between unglaciated and early deglaciated sites in southern Quebec and slowed from early to mid-Holocene in central and eastern Quebec. Migration was at its lowest speed in late-Holocene times, when it stopped about 3,000 y ago. Compared with jack pine, black spruce migrated at a faster mean rate of 25 km-cent from the ice border at the last interstadial (Bølling/Allerød) to the species tree limit. The modern range of both species was nearly occupied about 6,000 y ago. The factors modulating the changing velocity of jack pine migration were closely associated with the warm-dry climate of the late Pleistocene—Holocene transition and the more humid climate of the mid- and late-Holocene.