Ann Delwaide

Shift of Conifer Boreal Forest to Lichen–Heath Parkland Caused by Successive Stand Disturbances

In the southern boreal forest (Québec, Canada), tree harvesting is a major disturbance affecting the dominant black spruce (Picea mariana) stands already suffering from naturally recurrent insect and fire disturbances. Although recovery of the spruce forest after an insect infestation or a fire is possible under current site conditions, it is less likely when both types of disturbance occur during a short period of time. The addition of yet another disturbance, such as tree harvesting, can thus have catastrophic consequences. We analyzed the impact of three successive disturbances—tree harvesting, insect infestation, and fire—on the regeneration of boreal spruce–moss forests within a period of approximately 50 years. The spruce forests were harvested in the 1940s and the 1950s. Recovery from the logging consisted of advance regeneration (spruce layers less than 1 m high that were left intact during clear-cuts), which was burned in 1991. The vegetation cover (mostly heath and lichen species) and soil conditions (acidic, nutrient-poor podzolic soils developed from coarse materials) of the postfire sites that we studied were similar. Stand structure and tree regeneration were documented from large quadrats (0.25 ha) using age, size, and tree ring data from postlogged and postfire spruce. At an early stage of development, the growing advance regeneration was damaged by insect defoliators in the late 1970s and the mid-1980s, and several trees died a few years before the 1991 fire. The successive disturbances considerably reduced the number of seed-bearers, leading to the collapse of postfire regeneration and a shift to parkland. Through a successional trajectory far from the expected trend for boreal forests influenced by single disturbance, the shift resulted in the formation of divergent plant communities. The development of divergent communities at the landscape scale is generally overlooked due to their small size. They indicate, however, the weak resilience of boreal forests faced with cascading perturbations, which are likely to increase in intensively logged areas.

Spatially explicit fire-climate history of the boreal forest-tundra (Eastern Canada) over the last 2000 years

Across the boreal forest, fire is the main disturbance factor and driver of ecosystem changes. In this study, we reconstructed a long-term, spatially explicit fire history of a forest-tundra region in northeastern Canada. We hypothesized that current occupation of similar topographic and edaphic sites by tundra and forest was the consequence of cumulative regression with time of forest cover due to compounding fire and climate disturbances. All fires were mapped and dated per 100 year intervals over the last 2000 years using several fire dating techniques. Past fire occurrences and post-fire regeneration at the northern forest limit indicate 70% reduction of forest cover since 1800 yr BP and nearly complete cessation of forest regeneration since 900 yr BP. Regression of forest cover was particularly important between 1500s–1700s and possibly since 900 yr BP. Although fire frequency was very low over the last 100 years, each fire event was followed by drastic removal of spruce cover. Contrary to widespread belief of northward boreal forest expansion due to recent warming, lack of post-fire recovery during the last centuries, in comparison with active tree regeneration more than 1000 years ago, indicates that the current climate does not favour such expansion.

DYNAMICS OF SUBARCTIC WETLAND FORESTS OVER THE PAST 1500 YEARS

Boreal forests at high latitudes are climate-sensitive ecosystems that respond directly to environmental forcing by changing their position according to latitude or by changing their abundance at local and regional scales. South of the arctic treeline, external forcing (warming, cooling, drought, fire) necessarily results in the changing abundance of the impacted forests; in particular, the deforestation of well-drained sites through fire is the most important factor. In this study, we examined the changing abundance of wetland forests located at the arctic treeline (northern Quebec, Canada) during the last 1500 years, a period of known contrasting climatic conditions. Black spruce (Picea mariana) trees submerged in small lakes and peatland ponds and soil-peat stratigraphy were used con- currently to reconstruct the millennial-long developmental sequence of wetland stands as- sociated with moisture changes and fire disturbance. Changing lake levels from AD 300 to the present were identified based on radiocarbon-dated submerged paleosols and tree- ring cross-dating of submerged trees distributed in three wetlands from the same watershed. Dead and living trees in a standing position below and above present water level of a small lake (LE Lake) showed direct evidence of past water levels from the 12th century to the present day. Submerged subfossil trees from another lake (LB Lake) and two peatland ponds (PB Peatland) also responded synchronously to changes in soil moisture during the last 1500 years. Regional-scale catastrophic flooding around AD 1150, inferred from paleosol and subfossil tree data, eliminated riparian peat and wetland trees growing at least since AD 300. Also, the coincidence of events such as the mass mortality of wetland spruce and post-fire deforestation of a small hill surrounding LE Lake during the late 1500s suggests the impact of local-scale flooding, probably attributable to greater snow transportation and accumulation on the lake surface after fire disturbance. Massive tree mortality climaxed at ca. 1750, when all wetland trees at LB Lake and PB Peatland died because of permafrost disturbance and soil upthrusting. Lower water levels from AD 300 to 1750 were associated with drier conditions, possibly caused by greater evaporation and/or reduced snow accu- mulation. Permafrost development in shallow waters occurred during the Little Ice Age, after 1600. It is concluded that the climate at the eastern Canadian treeline was warmer and drier from AD 300 to the onset of the Little Ice Age and promoted tree establishment. The highest water levels were recorded recently (19th and 20th centuries), causing lake and peatland expansion. Any future moisture changes at these subarctic latitudes will result in important spatial rearrangements of wetland ecosystems.

Calculating long-term fire frequency at the stand scale from charcoal data

Fire frequency is a statistical metric used to evaluate long-term fire activity at stand and landscape scales. Fire frequency is defined as the number of fires occurring per unit time in a given area. In this study a method to calculate fire frequency at the stand scale is described, based on direct fire evidence of radiocarbon-dated macrocharcoal fragments (>2 mm diameter) at the soil surface and buried in the mineral soil. A jack pine (Pinus banksiana Lamb.) stand was used as a model site to calculate the long-term fire frequency. The number of fires recorded at the soil surface is a function of fire activity and residence time of charcoal, the fewer fires occurring in the site the longer the residence time of charcoal. The residence time of charcoal at the surface of the study site totals 1710 calibrated years (calibrated age in years before 2010). Fourteen fire events occurred over the last 1000 years, i.e., an average fire interval of 75 years, a situation facilitating the long-term maintenance of ja...

Recent Permafrost Dynamics in a Subarctic Floodplain Associated with Changing Water Levels, Quebec, Canada

We have reconstructed from tree-rings the dynamics of mineral frost mounds on the floodplain of a subarctic river (Riviere Boniface, northern Quebec) likely associated with snow precipitation and temperature changes during the past centuries. Due to their peculiar location in the river bed, we have postulated that the inception and decay of frost mounds (thermokarst ponds) were associated with snow-controlled water levels of the river. The periods of establishment, growth, and mortality of spruce around and in two thermokarst ponds on the shore zone were identified. The oldest tree-ring dates show that permafrost mounds formed during a period of water lowering in the 17th century, likely in a long-enduring sequence of low waters initiated around cal AD 1150 (910 yr BP), which persisted for at least 200 yr until the beginning of the 20th century. Two main periods of spruce mortality were identified, in the late 19th century and early 20th century and in the 1950s-1960s, which corresponded to a rising river level probably due to greater snow precipitation. The patterns of spruce establishment and mortality were strikingly similar in the studied sites. The first spruce to establish were all located along the ponds edges and they were also the first ones to die; most of the youngest spruce established later in the central part of each feature, i.e., in the early to mid-1850s to 1910, and died during the 1950s-1960s. The sequence of events reported here suggests an important lowering of the Riviere Boniface during the first part of the Little Ice Age (end of the 16th century-17th century) which was drier and cold. During the second part of the Little Ice Age (mainly 19th century), greater precipitation occurred and climaxed in the 20th century when climate warmed and the river stage reached its maximum level. Changes in snow precipitation were probably more instrumental than temperature changes in the rise and fall of permafrost landforms because of the snowpacks direct influence on the soil thermal regime and river level during snowmelt.

Stem analysis of a long-lived black spruce clone at treeline

Because of its ability to layer and to produce different phenotypes, black spruce (Picea mariana [Mill.] BSP.) develops a complex clonal structure ensuring its survival and longevity. Here we report tree stem development and demise of a black spruce clone at treeline over the last 500 yr. Since the 16th century, the apical meristems of the clonal spruce experienced three periods of stem development associated with brief warmings and two periods of stem decline corresponding to known cold spells of the Little Ice Age. Ortet development was particularly vigorous in the 16th century, while the two layered stems slowly developed in the late 17th century and in the 20th century, respectively. Stem decline appeared as a progressive process lasting for several decades in the form of a basipetal death-gradient along the bole amplified by the above/below snow-pack position. Stem elongation was possibly facilitated by lesser winter-snow abrasion and/or thicker snowpack. Clonal stem development may have important implications for spruce spread in the arctic tundra in a warmer world. 26 refs. 2 figs.

Insect-induced tree dieback and mortality gaps in high-altitude balsam fir forests of northern New England and adjacent areas

ABSTRACT In recent decades, high-altitude balsam fir (Abies balsamea) forests of northern New England and adjacent areas have been fragmented by canopy openings associated with several stand disturbances. To document the causes of extensive tree dieback and mortality and the source of canopy gaps, we studied the tree growth patterns of live balsam fir from five stands in four sites in the northeastern United States (NUS): Whiteface Mtn. in New York state and Mt. Moosilauke (two stands), Mt. Blue, and Mt. Lafayette in New Hampshire. Additionally, tree-ring patterns of dead fir were documented at two sites, Whiteface Mtn. and Mt. Mégantic (southern Québec). Organic horizons from four of the five NUS stands were analyzed for identification of insect remains and plant macrofossils in 11 and seven soil cores, respectively. Four of the five balsam fir tree-ring chronologies from the NUS stands showed a major growth decline in the 1970s, which was also apparent in the two chronologies from dead trees at Whiteface Mtn. and Mt. Mégantic. At the NUS stands, spruce budworm (Choristoneura fumiferana) head capsules were found in all 11 soil cores from forest and open stands. In spite of large variations in the number of head capsules among levels (range: 1–34; mean: 9.5 ± 8.8 for 71 levels analyzed), their occurrence at all the levels analyzed indicates that the insect had sustained activity through time. Macrofossil remains from the organic horizons included plant assemblages similar to the aboveground forest vegetation. Rubus idaeus, an intolerant species, indicated that gap conditions prevailed sometime in the past. Our tree-ring and macrofossil data along with insect surveys from northern New England provide evidence for the major role played by spruce budworm in tree dieback and mortality during infestations and subsequent gap formation in high-altitude fir forests. The impact of other stand disturbance factors is discussed.

Forest soil paludification and mid-Holocene retreat of jack pine in easternmost North America: Evidence for a climatic shift from fire-prone to peat-prone conditions

Soil paludification is the main ecosystem process initiating the formation and development of most peatlands in the Northern Hemisphere. Sandy podzolic soils developed on coarse deposits are among a large variety of substrates overgrown by thick peat layers, particularly along the amphi-atlantic coasts of North America and Europe. Whether the podzolic soils beneath peat layers are remnants of former dry environments allowing the morphogenetic development of forest soils or the progressive outcome of natural succession towards full peatland growth is still debated. We have explored a part of this dual facet in documenting the interface between buried podzol profiles and basal peat. Two paludified sites located at the center and at the edge of an extensive plateau bog along the Gulf of St Lawrence were documented based on radiocarbon-dated tree and charcoal macrofossils. Paleosols beneath thick and relatively old (> 4000 cal. yr BP) peat were composed of slightly cemented, placic B horizons whereas those under thinner and younger (< 2500 cal. yr BP) peat were made of heavily cemented, ortstein B horizons. Forest soil paludification and peat growth at both sites commenced with the cessation of fire occurrence as evidenced by charcoal fragments in the paleosol matrix beneath Sphagnum peat layers devoid of charcoal fragments. Botanically identified charcoal fragments include several tree species, in particular jack pine (Pinus banksiana) presently absent from this part of the continent. The retreat of the species likely occurred after 5500 cal. BP with the cessation of fire occurrence, an indication of a shift in maritime Québec from dry to wetter conditions initiating peat growth and peatland expansion. It is concluded that the genesis and development of podzols with different degrees of soil cementation (placic and ortstein horizons) preceded the inception and development of the plateau bog which have been facilitated by wetter climatic conditions inimical to fire activity. The progressive lateral growth of large ombrotrophic peatlands during the Holocene is also an additional, possible factor influencing the natural occurrence and spread of fire.

A study of the composition, characteristics, and origin of modern driftwood on the western coast of Nunavik (Quebec, Canada)

Data concerning driftwood is of value to researchers in fields as diverse as oceanography, geomorphology, and human occupation. Yet studies on the subject in the Canadian Arctic have only recently been carried out, and the present study is the first in Nunavik (northeastern Canada). This paper documents the composition, characteristics, and origin of modern driftwood pieces on the beaches of the eastern coast of Hudson Bay. A total of 1057 samples from Ivujivik, Akulivik, Inukjuak, and Umiujaq were identified as belonging to four coniferous species (Picea sp., Larix sp., Abies sp. likely balsamea, and Thuja sp. likely occidentalis) and four deciduous species (Salix sp., Populus sp., Alnus sp., and Betula sp., likely papyrifera). Spruce largely predominate; white birch, white cedar, and fir are rare. The presence of the latter species proves that some of the wood originated from south-southeast of James Bay. Driftwood found in the southern area (Umiujaq) are more numerous, larger, and less degraded than driftwood in the north (Ivujivik). However, many large coniferous samples were found as far north as Akulivik, indicating that they likely traveled a great distance, unlike the smaller wood specimens (especially deciduous samples). All of the wood that we analyzed died relatively young, with an average age of 63 years for conifers and 23 years for deciduous. Measurements of ring widths and the cross-dating of samples with existing reference chronologies of living trees along Hudson Bay and James Bay revealed several possible correlations and origins for wood found in same areas.

Tree-Ring Evidence of Changes in the Subarctic Forest Cover Linked to Human Disturbance in Northern Labrador (Canada)

ABSTRACT We combined dendroecological analyses with historical and ethnographic information to document connections between forest use patterns since the 18th century and stand composition and structure in the Nain region of Labrador, Canada. The highest recruitment periods for both eastern larch and spruce, pulses in growth releases, and decades with greatest harvesting evidence (cut stumps) all occurred primarily between 1910 and 1970. The strongest disturbance signal occurred after 1940, resulting in the absence of old trees and increased larch recruitment. The 1910–1970 period coincides with significant shifts in human settlement and land use patterns. Most notably, the increased demand for fur in the 1920s and 1930s changed Inuit land use: the Inuit spent more time inland where fur-bearing animals and wood resources were available. Moreover, population growth in Nain, which was accelerated by the relocation of Inuit communities in northern Labrador between 1950 and 1960, increased local harvesting intensity. We argue that long-term land use needs to be accounted for as a driver of forest dynamics in this subarctic forest landscape.