Long-Term Steady-State Dry Boreal Forest in the Face of Disturbance

We used bioproxies from paleosoils buried within two aeolian dunes to test hypotheses concerning the origin of dry sandy boreal forests in Canada. These forests are dominated today by Pinus banksiana Lamb. One hypothesis is that too frequent Holocene stand-replacing fires would have transformed the original vegetation through extirpation of susceptible species to fire in water stress habitat. Alternatively, the ecosystem would have not changed since the dunes stabilized enough to support forest establishment. The vegetation composition and richness were determined by identification of charcoal and macroremains and radiocarbon dating for the chronology. Both sites revealed a similar history covering 6400 years. Half of the charcoal layers were less than 2500 years old in both sites, indicating that they had been subjected to the same fire history. Data indicated a stable plant composition and richness, although the percentage of Pinus decreased slightly over 4000 years (decreasing rate 1% per century). The fungus Cenococcum geophilum was consistently present, with a stochastic abundance. The vegetation grew under natural fire conditions and soil dryness since 6000 years. The ecosystem was probably not stressed by late-Holocene fires or climate changes, as the multi-millennial steady state reveals a resistant and resilient ecosystem.

A global database of Holocene paleotemperature records

A comprehensive database of paleoclimate records is needed to place recent warming into the longer-term context of natural climate variability. We present a global compilation of quality-controlled, published, temperature-sensitive proxy records extending back 12,000 years through the Holocene. Data were compiled from 679 sites where time series cover at least 4000 years, are resolved at sub-millennial scale (median spacing of 400 years or finer) and have at least one age control point every 3000 years, with cut-off values slackened in data-sparse regions. The data derive from lake sediment (51%), marine sediment (31%), peat (11%), glacier ice (3%), and other natural archives. The database contains 1319 records, including 157 from the Southern Hemisphere. The multi-proxy database comprises paleotemperature time series based on ecological assemblages, as well as biophysical and geochemical indicators that reflect mean annual or seasonal temperatures, as encoded in the database. This database can be used to reconstruct the spatiotemporal evolution of Holocene temperature at global to regional scales, and is publicly available in Linked Paleo Data (LiPD) format.

Control of the multimillennial wildfire size in boreal North America by spring climatic conditions

Wildfire activity in North American boreal forests increased during the last decades of the 20th century, partly owing to ongoing human-caused climatic changes. How these changes affect regional fire regimes (annual area burned, seasonality, and number, size, and severity of fires) remains uncertain as data available to explore fire-climate-vegetation interactions have limited temporal depth. Here we present a Holocene reconstruction of fire regime, combining lacustrine charcoal analyses with past drought and fire-season length simulations to elucidate the mechanisms linking long-term fire regime and climatic changes. We decomposed fire regime into fire frequency (FF) and biomass burned (BB) and recombined these into a new index to assess fire size (FS) fluctuations. Results indicated that an earlier termination of the fire season, due to decreasing summer radiative insolation and increasing precipitation over the last 7.0 ky, induced a sharp decrease in FF and BB ca. 3.0 kyBP toward the present. In contrast, a progressive increase of FS was recorded, which is most likely related to a gradual increase in temperatures during the spring fire season. Continuing climatic warming could lead to a change in the fire regime toward larger spring wildfires in eastern boreal North America.

Genetic consequences of fragmentation in "arbor vitae," eastern white cedar (Thuja occidentalis L.), toward the northern limit of its distribution range

We tested the hypothesis that marginal fragmented populations of eastern white cedar (EWC) are genetically isolated due to reduced pollen and gene flow. In accordance with the central-marginal model, we predicted a decrease in population genetic diversity and an increase in differentiation along the latitudinal gradient from the boreal mixed-wood to northern coniferous forest. A total of 24 eastern white cedar populations were sampled along the north-south latitudinal gradient for microsatellite genotyping analysis. Positive F_is values and heterozygote deficiency were observed in populations from the marginal (F_is = 0.244; P_HW = 0.0042) and discontinuous zones (F_is = 0.166; P_HW = 0.0042). However, populations from the continuous zone were in HW equilibrium (F_is = −0.007; P_HW = 0.3625). There were no significant latitudinal effects on gene diversity (H_s), allelic richness (AR), or population differentiation (F_st). Bayesian and NJT (neighbor-joining tree) analyses demonstrated the presence of a population structure that was partly consistent with the geographic origins of the populations. The impact of population fragmentation on the genetic structure of EWC is to create a positive inbreeding coefficient, which was two to three times higher on average than that of a population from the continuous zone. This result indicated a higher occurrence of selfing within fragmented EWC populations coupled with a higher degree of gene exchange among near-neighbor relatives, thereby leading to significant inbreeding. Increased population isolation was apparently not correlated with a detectable effect on genetic diversity. Overall, the fragmented populations of EWC appear well-buffered against effects of inbreeding on genetic erosion.

Le couvert végétal du Québec-Labrador il y a 6000 ans BP : essai

Une carte du couvert végétal du Québec-Labrador est proposée pour l'époque des 6000 ans BP, à partir des données paléopalynologiques et paléobotaniques. Une Toundra arbustive dense s'interposait entre les mers de Tyrrell ou d'Iberville et les glaces résiduelles qui occupaient alors plus de 100 000 km2 en Ungava (Nunavik) et jusque dans la région de Schefferville. Étroite au sud des glaciers, cette Toundra s'étalait à l'est des glaces sur plus de 400 km jusqu'à la mer du Labrador, au nord du 54° de latitude. La limite des arbres s'établissait donc alors à peu près au centre de l'actuelle zone de Taïga (Forêt boréale ouverte), mais la limite nord de la Forêt fermée correspondait déjà à peu près à la position moderne. Comme la Toundra, la Taïga de l'époque était arbustive, plus dense et plus diversifiée qu'actuellement, sans doute en raison d'un climat favorable, mais aussi d'une plus grande richesse des sols en éléments nutritifs. Les domaines forestiers méridionaux des Pessières, des Sapinières et des Érablières atteignaient dans l'ensemble les limites modernes de leur aire de peuplement, mais l'abondance de certaines essences compagnes différait le plus souvent de l'actuelle. Ces différences sont attribuables 1) à une migration tardive de certaines essences, telles le pin gris (Pinus banksiana) dans les Pessières occidentales ou le hêtre à grande feuilles (Fagus grandifolia) dans les Érablières, 2) à une incidence généralement plus faible des feux naturels, 3) à un entourbement moindre des stations hydriques, et 4) à un climat dans l'ensemble plus clément. L'analyse régionale permet d'identifier des gradients et des champs phytogéographiques plus nuancés éventuellement influencés par le climat, notamment en zones montagnardes et subalpines.

Late Quaternary Vegetation History of Northern North America Based on Pollen, Macrofossil, and Faunal Remains*

Biome maps spanning the interval from the last glacial maximum to modern times are presented. The biome distributions at 18 ka BP were probably as nearly in equilibrium with climate as are the modern distributions, but deglacial biomes were probably in disequilibrium. Ice sheet configuration was a strong control of climate until 7 ka BP. Regional climate trends can be inferred from changing biome distributions, but during periods of disequilibrium, biome distributions under-represent summer warming. Because of summer cooling by 2-4 °C during the Holocene, largely in the last 3-5 ka, middle and certain early Holocene biome distributions and species compositions are reasonable analogues of future equilibrium displacements due to equivalent warming, at least in areas that were long-since deglaciated. Past biome migration rates in response to rapid regional warming during deglaciation were mainly in the range of 100-200 m per year. If these rates pertain in the future, biomes may shift 10-20 km in most regions over the next century. A major impediment to using former Holocene conditions as a guide to future conditions is that warmer Holocene summers were accompanied by colder winters, whereas warmer future summers will be accompanied by warmer winters.