Glen M. MacDonald

Geographical Variation of Lodgepole Pine in Relation to Population History

Relation des caracteristiques genetiques et morphologiques des populations de Pinus contorta ssp. latifolia depuis leur origine

Rapid response of treeline vegetation and lakes to past climate warming

FUTURE greenhouse warming is expected to be particularly pronounced in boreal regions1, and consequent changes in vegetation in these regions may in turn affect global climate2–4. It is therefore important to establish how boreal ecosystems might respond to rapid changes in climate. Here we present palaeoecological evidence for changes in terrestrial vegetation and lake characteristics during an episode of climate warming that occurred between 5,000 and 4,000 years ago at the boreal treeline in central Canada. The initial transformation — from tundra to forest-tundra on land, which coincided with increases in lake productivity, pH and ratio of inflow to evaporation — took only 150 years, which is roughly equivalent to the time period often used in modelling the response of boreal forests to climate warming5,6. The timing of the treeline advance did not coincide with the maximum in high-latitude summer insolation predicted by Milankovitch theory7, suggesting that northern Canada experienced regionally asynchronous middle-to-late Holocene shifts in the summer position of the Arctic front. Such Holocene climate events may provide a better analogue for the impact of future global change on northern ecosystems than the transition from glacial to nonglacial conditions.

The reconstruction of boreal forest fire history from lake sediments: A comparison of charcoal, pollen, sedimentological, and geochemical indices

Abstract Fossil charcoal, fossil pollen, sedimentological and geochemical analyses of lake sediments have been used previously to reconstruct a history of local fires and resulting vegetation change. The rationale behind these approaches is described and the usefulness of each technique for reconstructing fire history in the boreal forest is assessed empirically. Historical and dendrochronological records provide regional and local fire histories for a site in Wood Buffalo National Park, Alberta, Canada. The local and regional history of fires is compared with the microscopic charcoal content, macroscopic charcoal content, elemental carbon content, fossil pollen content, sedimentology, and geochemistry of annually laminated sediments from a small lake. There is no significant correlation between the abundance of microscopic charcoal, macroscopic charcoal and total elemental carbon content of the sediments. Automated measures of microscopic charcoal abundance made with an image analysis system are correlated significantly with optical counts of microscopic charcoal. None of the charcoal measures provide unequivocal records of local fire activity and the abundance of microscopic charcoal appears to be influenced by variations in regional fire activity. However, the highest depositional rate of macrofossil charcoal occurred during the time of a fire that burned within the drainage basin. Variations in sedimentological measures and geochemistry do not correlate with local fire activity. Fossil pollen percentages and accumulation rates display a pattern of variation that is consistent with observed vegetation responses to fire in the boreal forest. What is likely apparent in the pollen record are the results of a series of burns of sufficient extent and intensity to kill most of the above-ground biomass of the vegetation in an area at least as great as the drainage basin. The inability of evidence from lake sediments to provide detailed histories of past fire activity is not surprising given the wide range of variation in the spatial extent, proximity, intensity and impact of individual fires.

A 1,200-year perspective of 21st century drought in southwestern North America

A key feature of anticipated 21st century droughts in Southwest North America is the concurrence of elevated temperatures and increased aridity. Instrumental records and paleoclimatic evidence for past prolonged drought in the Southwest that coincide with elevated temperatures can be assessed to provide insights on temperature-drought relations and to develop worst-case scenarios for the future. In particular, during the medieval period, ∼AD 900–1300, the Northern Hemisphere experienced temperatures warmer than all but the most recent decades. Paleoclimatic and model data indicate increased temperatures in western North America of approximately 1 °C over the long-term mean. This was a period of extensive and persistent aridity over western North America. Paleoclimatic evidence suggests drought in the mid-12th century far exceeded the severity, duration, and extent of subsequent droughts. The driest decade of this drought was anomalously warm, though not as warm as the late 20th and early 21st centuries. The convergence of prolonged warming and arid conditions suggests the mid-12th century may serve as a conservative analogue for severe droughts that might occur in the future. The severity, extent, and persistence of the 12th century drought that occurred under natural climate variability, have important implications for water resource management. The causes of past and future drought will not be identical but warm droughts, inferred from paleoclimatic records, demonstrate the plausibility of extensive, severe droughts, provide a long-term perspective on the ongoing drought conditions in the Southwest, and suggest the need for regional sustainability planning for the future.

Recent white spruce dynamics at the subarctic alpine treeline of north-western Canada

1 Dendroecological techniques are used to reconstruct the recent population dynamics of white spruce (Picea glauca) at the alpine treeline of north-western Canada in order to test the following hypotheses: (i) the recruitment/survival and mortality patterns of white spruce at treeline are episodic, controlled by climatic variations; and (ii) the response of treeline position to climate change has been limited by a degree of inertia inherent in marginal white spruce populations. 2 The oldest white spruce at all sites, including those at treeline, established at or before AD1800. Rates of successful establishment were low in the early to mid19th century, but have since increased substantially. Mathematical modelling of age distributions and comparison with proxy climate data suggest that summer temperatures both at time of germination and for up to 50 years after germination are important in determining recruitment and survival. 3 Dendrochronological dating of dead spruce remains indicates that a mass mortality of white spruce occurred at a hilltop site during the culmination of the Little Ice Age in the mid-19th century. Subsequent re-establishment at this site has not taken place despite a long-term warming trend. 4 The correlation of recruitment/survival records and mortality events with proxy climate data support the first hypothesis. The second hypothesis is supported by (i) the lack of any substantial increases in treeline altitude during the past 150 years; and (ii) the survival of white spruce throughout a long period of fluctuating climatic conditions at a site currently unsuitable for establishment of spruce. During the past 100-150 years the general trend within these forest-tundra stands has therefore been one of increasing population density, accompanied by only very minor changes in the upper limit of trees.

Climate change and the northern Russian treeline zone

The Russian treeline is a dynamic ecotone typified by steep gradients in summer temperature and regionally variable gradients in albedo and heat flux. The location of the treeline is largely controlled by summer temperatures and growing season length. Temperatures have responded strongly to twentieth-century global warming and will display a magnified response to future warming. Dendroecological studies indicate enhanced conifer recruitment during the twentieth century. However, conifers have not yet recolonized many areas where trees were present during the Medieval Warm period (ca AD 800–1300) or the Holocene Thermal Maximum (HTM; ca 10 000–3000 years ago). Reconstruction of tree distributions during the HTM suggests that the future position of the treeline due to global warming may approximate its former Holocene maximum position. An increased dominance of evergreen tree species in the northern Siberian forests may be an important difference between past and future conditions. Based on the slow rates of treeline expansion observed during the twentieth century, the presence of steep climatic gradients associated with the current Arctic coastline and the prevalence of organic soils, it is possible that rates of treeline expansion will be regionally variable and transient forest communities with species abundances different from todays may develop.

Water, climate change, and sustainability in the southwest

The current Southwest drought is exceptional for its high temperatures and arguably the most severe in history. Coincidentally, there has been an increase in forest and woodland mortality due to fires and pathogenic outbreaks. Although the high temperatures and aridity are consistent with projected impacts of greenhouse warming, it is unclear whether the drought can be attributed to increased greenhouse gasses or is a product of natural climatic variability. Climate models indicate that the 21st century will be increasingly arid and droughts more severe and prolonged. Forest and woodland mortality due to fires and pathogens will increase. Demography and food security dictate that water demand in the Southwest will remain appreciable. If projected population growth is twinned with suburb-centered development, domestic demands will intensify. Meeting domestic demands through transference from agriculture presents concerns for rural sustainability and food security. Environmental concerns will limit additional transference from rivers. It is unlikely that traditional supply-side solutions such as more dams will securely meet demands at current per-capita levels. Significant savings in domestic usage can be realized through decreased applications of potable water to landscaping, but this is a small fraction of total regional water use, which is dominated by agriculture. Technical innovations, policy measures, and market-based solutions that increase supply and decrease water demand are all needed. Meeting 21st-century sustainability challenges in the Southwest will also require planning, cooperation, and integration that surpass 20th-century efforts in terms of geographic scope, jurisdictional breadth, multisectoral engagement, and the length of planning timelines.

Peatlands of the Western Siberian lowlands: current knowledge on zonation, carbon content and Late Quaternary history

The Western Siberian lowlands (WSL) are the world’s largest high-latitude wetland, and possess over 900,000 km 2 of peatlands. The peatlands of the WSL are of major importance to high-latitude hydrology, carbon storage and environmental history. Analysis of the existing Russian data suggests that the mean depth of peat accumulation in the WSL is 256 cm and the total amount of carbon stored there may exceed 53,836 million metric tons. A synthesis of published and unpublished radiocarbon dates indicates that the peatlands first developed at the end of the Last Glacial, with a rapid phase of initiation between 11,000 and 10,000 cal yr BP. Initiation slowed after 8000 cal yr BP and reached a nadir at 4000 cal yr BP. There has been renewed initiation, particularly south of 621N, following 4000 cal yr BP. The initial development of peatlands in the WSL corresponds with the warming at the close of the Pleistocene. Cooling after 4000 Cal yr BP has likely led to increased permafrost and increased peatland development particularly in central and southern regions. Cold and dry conditions in the far north may have inhibited peatland formation in the late Holocene. r 2002 Elsevier Science Ltd. All rights reserved.

Response of the Central Canadian Treeline to Recent Climatic Changes

Dendrochronology and the analysis of historical records from eighteenth, nineteenth, and early twentieth-century expeditions are used to reconstruct the response of the central Canadian treeline to recent climatic variations. Cold summer temperatures during the early to mid-nineteenth century contributed to low rates of growth and poor recruitment of white and black spruce at treeline. Despite the harsh conditions, however, individuals of both species were able to persist at sites that define their northern range limits today. After 1880, Increases in regional and hemispheric temperature are associated with increased growth rates and recruitment by both white and black spruce. A drop in temperatures during the 1960s and 1970s corresponds with declines in growth rates and recruitment. Growth rates have responded positively to higher temperatures in the 1980s, yet despite this positive response to recent temperature increases, dendrochronological and historical evidence suggest that there has been no signif...

Lake morphometry, sediment mixing and the selection of sites for fine resolution palaeoecological studies

Abstract Palaeoecologists are increasingly using close interval or continuous sampling of lake sediments to construct fine resolution records of environmental change. The degree of temporal resolution sought in these studies is typically decades or years. The use of lake sediments for such fine resolution reconstructions assumes that temporal mixing of the sediments is negligible. In this paper we examine the physical and biological processes that cause sediment mixing in small lakes. We use physical models and empirical data to develop a set of heuristics relating lake depth and area to sediment mixing by slumping, waves, currents and bioturbation. We also develop a heuristic that allows information on lake depth and area to be used to determine if the sediments of a lake will likely be massive or annually laminated. In general, the degree of sediment mixing likely to occur in a small lake is directly related to the depth and surface area of the lake. However, each of the agents of mixing has a different critical relationship with lake depth and surface area. We present a series of equations and a summarizing figure that, based on lake surface area and maximum depth, allows the type of thermal stratification (polymictic, dimictic or meromictic) and the degree of sediment mixing present in the lake to be inferred.