Birch Decline Research Articles

The post-fire shift of temperate white pine-birch forest to boreal balsam fir forest in eastern Canada: climate-fire implications

Extensive 14C dating and botanical identification of charcoal fragments located in the organic surface soil layer and buried in the mineral podzolic solum were used to reconstruct the successional pathways of a balsam fir forest site. The studied forest site developed in a context of continuous fire disturbance over the last 9000 years with at least 26 fires occurring at a mean interval of 330 years. Tree vegetation of the site followed a four-step trajectory consisting of an early-Holocene spruce forest and a late-Holocene mixedwood balsam fir forest. Boreal-like spruce-birch and temperate-like white pine-birch forests dominated the site between 7900 and 5900 cal. B.P. and 5600 and 1275 cal. B.P., respectively. Because all forest types developed repeatedly after fire since early deglaciation, changes in forest composition, in particular the shift of white pine forest to balsam fir forest, and concurrent decline of birch (yellow birch and/or paper birch) and pine populations were most likely related to progressive cooler and wetter conditions from mid- to late Holocene. Fire disturbance on this part of the southern boreal biome has been a continuous, positive regenerative process over the Holocene, allowing the successional turnover of boreal and temperate trees under the influence of climatic change.

Millennial land use explains modern high‐elevation vegetation in the submediterranean mountains of Southern Europe

AbstractAimPinewood decline and scrubland expansion are major features of Late Holocene vegetation history in the Cantabrian Range. However, the drivers of this remarkable vegetation shift remain to be investigated. Here, we aim at disentangling the role of past land use and climate in shaping the high‐elevation Cantabrian landscape during the past two millennia.LocationCantabrian Range (northern Iberia).TaxaPinus sylvestris, Betula, Ericaceae, Juniperus, Poaceae.MethodsWe conducted high‐resolution multiproxy palaeoecological analyses (pollen, plant macrofossils, microscopic charcoal and dung fungi) on lake sediments from Lago del Ausente to reconstruct vegetation, fire occurrence and grazing through time. The chronology is based on 14C (terrestrial plant macrofossils) and 210Pb dating, and Bayesian age‐depth modelling (‘rbacon’). We carried out cross‐correlation analysis to quantify vegetation responses to fire.ResultsBetween 250 and 900 CE, the vegetation above 1700 m a.s.l. consisted of subalpine scrubland and scattered P. sylvestris trees/stands. Pinewoods with Betula were widespread at slightly lower elevation. This vegetation was resilient to moderate fire disturbance associated with limited pastoral activities. In contrast, enhanced fire occurrence alongside heavier pastoralism led to the demise of pinewoods and their replacement with Betula stands, subalpine scrublands, and meadows between 900 and 1100 CE. Later, the subalpine scrubland‐birch tree line did not respond to Little Ice Age cooling. However, further intensification of transhumant herding between 1300 and 1860 CE (‘La Mesta’) triggered birch decline and the establishment of the modern treeless landscape.Main conclusionsThe extant high‐elevation vegetation of the Cantabrian Range is largely the legacy of intensive land use starting more than one millennium ago. Recurrent and severe fires to promote pasturelands led to the regional extirpation of the previously widespread Pinus sylvestris. Future management should aim at preserving the valuable cultural open landscape of mountain scrubland and meadows and also at restoring patches of ancient pine‐birch woodlands.

Long term variation of leaf abundance in a northern hardwood forest

Leaf abundance of trees plays a dominant role in energy, water and nutrient flux of forest ecosystems, in defining the habitat structure of entrained biota and in mediating interspecific competition among tree species. We quantified leaf abundance of three dominant tree species (Acer saccharum Marsh.; Fagus grandifolia Ehrh.; Betula alleghaniensis Britt.) for 27 years in mature northern hardwood forest at the Hubbard Brook Experimental Forest, New Hampshire, USA using annual counts of leaves of each species collected in 117 litter traps (0.1 m2, each). We hypothesized that variation in leaf abundance would reflect known disturbance events and canopy dieback episodes. No significant trend in leaf abundance of the three species over 27 years (1993–2019) was observed in seven reference stands despite known canopy dieback of sugar maple and American beech in these stands, indicating that 1) maple decline preceded our sampling interval and 2) expansion of beech understory compensated for canopy dieback caused by beech bark disease. Leaf abundance temporarily declined significantly in response to known disturbances (ice storm, 34 to 47%; late-spring frost, 13 to 16%). Increased leaf abundance, especially for sugar maple (30%), on a watershed where calcium addition restored acidified soils, was probably associated with increased soil pH, Ca availability and decreased Al. The temporal sequence of decline of paper birch at higher elevations following a severe drought in 2002 also was apparent. Leaf abundance based on leaf litterfall in deciduous forests provides a useful indicator of forest ecosystem function and forest health.

Degradation of White Birch Shelterbelts by the Attack of White-Spotted Longicorn Beetles in Central Hokkaido, Northern Japan

A widespread decline of white birch (Betula platyphylla var. japonica) shelterbelts was observed in central Hokkaido, Japan. Many exit holes bored by white-spotted longicorn beetles (Anoplophora malasiaca) were found at the base of the trunks of trees in these stands. The present study aims to evaluate the effects of infestation on the degradation, and demonstrates whether the number of exit holes (Nholes) can be used as an index of the decline of trees. We selected 35 healthy appearing stands and 16 degraded stands in the study area. A generalized linear mixed model with zero inflation revealed that Nholes of standing dead trees tended to be greater than that of living trees, and the tree vigor decreased with increasing Nholes. These results implied that the degradation of the shelterbelts was caused by the beetle. We also found size-dependent mortality, i.e., only a few larvae can cause the death of smaller trees, but not larger trees. Furthermore, evaluation of the degradation at the stand level (Nholes) using a logistic regression analysis revealed that the degradation at the stand level could be predicted by Nholes. Our findings can be used as a useful index marker for diagnosing white birch shelterbelts.

The Toledo Mountains: A Resilient Landscape and a Landscape for Resilience? Hazards and Strategies in a Mid-Elevation Mountain Region in Central Spain

The Toledo Mountains are a mid-elevation mountain range that separates the Tagus and Guadiana basins in the central area of the Iberian Peninsula. The location of these mountains allows the development of typical Mediterranean vegetation with some Atlantic influence. Consequently, typical broadleaved evergreen Mediterranean vegetation currently dominates the regional landscape, with the remarkable presence of more mesophilous species in sheltered and more humid microsites such as gorges (e.g., Prunus lusitanica, Taxus baccata, Ilex aquifolium) and mires/bogs (e.g., Betula pendula susbp. fontqueri, Erica tetralix, Myrica gale). Palaeoecological studies in these mountains are essential to understand the long-term ecology and original distribution of these valuable communities and are key to assess their resilience. Understanding the hazards and opportunities faced in the past by the plant communities of the Toledo Mountains is necessary to enhance the management and protection of those species currently threatened. This study focuses on El Perro mire, a peatland on the southern Toledo Mountains (central Spain) where climatic variability has played a major role in landscape dynamics at multi-decadal to millennial timescales. Climatic events such as the 4.2 ka cal. Before Present (BP) or the Little Ice Age triggered relevant landscape changes such as the spread and latter decline of birch and hazel forests. Human communities also seemed to be affected by these events, as their resilience was apparently jeopardized by the new climatic conditions and they were forced to find new strategies to cope with the new scenarios.

Long-term succession of closed boreal forests at their range limit in eastern North America shows resilience to fire and climate disturbances

The distribution of vegetation zones in northeastern North America forms a gradual transition from closed to open forests, and to tundra. Closed forests near the open forest/tundra boundary generally correspond to residual patches of a once larger forest community. Whether these forest patches have survived as untouched, fire-free communities or as resilient communities during the Holocene remains unknown. To answer to this question, we reconstructed the successional history of two black spruce closed-crown forests and one jack pine forest near the northern limit of closed forest ecosystems. Stand-scale postfire succession was analysed based on charcoal macrofossils. Extensive 14C dating and botanical identification of charcoal fragments located at the organic soil surface and buried in the mineral podzolic solum were used to reconstruct the successional pathways of the sites. The forest sites developed in a context of continuous fire disturbance, except for a short interval of several centuries. The fire history of the three sites is similar with a fire frequency of 34–37 fires over the last 5000–5800 years, which corresponds to a mean fire interval of 150–180 years. Based on parametric and non-parametric estimators, the regional fire regime of the Laforge area (combined fire data of two sites located about 7 km from each other) was most likely characterized by 38 fires over the last 5000–5800 years, i.e., at an interval of about 150–160 years. Birch (Betula papyrifera) was present in the spruce sites between 5800 and 3000 cal. years BP. Jack pine arrived at the pine site about 3000 cal. years BP, at the time when birch was extirpated from the spruce forests. The closed-crown forests near the open forest/tundra boundary are resilient postfire ecosystems which regenerated successfully since mid-Holocene. Compositional change of these forests, which led to the decline of birch but maintained the dominance of conifers, appears related to cooler and wetter conditions of late Holocene. Climatic conditions also slowed the eastward migration of jack pine, which explains the late arrival of this species at its northeastern range. Our data suggest that fire disturbance in these sites of the boreal biome has been a continuous and positive regenerative process since mid-Holocene despite the influence of climatic change on the tree flora.

Climate induced birch mortality in Trans-Baikal lake region, Siberia

The Trans-Baikal (or Zabailkal’e) region includes the forest-steppe ecotones south and east of Lake Baikal in Russia and has experienced drought for several years. The decline and mortality of birch (Betula pendula) stands within the forest-steppe ecotone Trans-Baikal region was studied based on a temporal series of satellite data, ground measurements, and tree ring analysis. During the first decade of the 21st century birch stands decline and mortality were observed on about 5% of the total area of stands within our 1250km2 study area. Birch forest decline and mortality occurs mainly at the margins of stands, within the forest-steppe ecotone on slopes with direct insolation. During the first decade of the 21st century summer (June–August) precipitation was about 25% below normal. Soil water content measurements were lowest within dead stands and highest within healthy stands and intermediate within damaged stands. Drought impact on stands was amplified by an increase in summer air temperatures (+0.9°C) in comparison with the previous decade. Tree ring data of “surviving” and “dead” tree groups showed a positive correlation with summer/annual precipitation and negative correlation with summer air temperatures. Temperature and precipitation extreme anomalies tend to occur in the region with a period of about 27years. The observed anomaly was the most severe since the beginning of meteorological observations in the year 1900. Data for the other sites showed a positive climate impact on the growth and expansion of Siberian forests. That is, the same species (B. pendula) showed considerable increase (1.4 times both in height and stem volume) during 20th–21st centuries as temperature increased but precipitation remained at adequate levels.

The younger dryas cooling in northeast Germany: summer temperature and environmental changes in the Friedländer Groβe Wiese region

AbstractA lake sediment record from the Friedländer Groβe Wiese in northeast Germany was studied to reconstruct summer temperature changes associated with changes in vegetation development during the Weichselian Lateglacial. The record was analysed for pollen, chironomids, and oxygen and carbon isotopes of lake marl. The combination of radiocarbon dates, the presence of the Laacher See Tephra and correlation of lithological and palynological changes with other records from the region indicated that the record encompassed the Allerød to the early Holocene. Pollen assemblages reflect development of birch and later pine‐dominated forests during the Allerød, comparable to other sites in the region. Chironomid‐inferred mean July air temperatures (C‐IT) for this period range between ∼14.0 and 14.8°C. A temporary decrease in C‐IT of ∼1°C, a negative shift in the isotope records, and a minor decline of birch may correspond to Greenland Interstadial 1b. Even though the transition to the Younger Dryas appears to be affected by reworking and redeposition processes, a drop in C‐IT to ∼11.1°C is reconstructed for the later part of the Younger Dryas, while it appears that pine locally persisted in the region. Comparison with a nearby pollen record further indicates a local expansion of wetland grasses during this period. At the transition to the Holocene, C‐IT increased to ∼15.7°C, while birch and pine forests re‐expanded. Copyright © 2012 John Wiley & Sons, Ltd.

Potential role of soil calcium in recovery of paper birch following ice storm injury in Vermont, USA

In recent years, an increased number of mature paper birch ( Betula papyrifera Marsh.) and heart-leafed paper birch ( B. papyrifera var. cordifolia (Regel) Fern.) in northeastern United States forests have exhibited decline symptoms including foliar loss, reduced fine branching, and tree mortality. We assessed crown health, radial growth, and available soil cations in 2006 as a preliminary assessment of factors that may be influencing paper birch decline. Tree rings began to decrease in width in 1998—the year of a severe region-wide ice storm. All trees (regardless of their current decline status) experienced reduced growth starting in 1998 and tree growth continued to decrease for two years following the ice storm. After this generalized growth decrease, trees that now have vigorous crowns showed a marked increase in growth, whereas trees that now have low crown vigor did not rebound in growth, but instead exhibited a significantly higher incidence of locally absent annual rings. Extractable soil-aluminum (Al), a phytotoxic element mobilized by acid deposition, was significantly higher in soils associated with declining trees compared to those adjacent to vigorous trees. Higher soil calcium (Ca) availability was associated with both vigorous crowns and increased radial growth following the 1998 ice storm. Furthermore, increased soil Ca availability was negatively correlated to the percentage of declining trees, and positively related to increased radial growth, whereas elevation was not significantly associated with either parameter. Although previously overlooked as a factor influencing paper birch recovery from injury, we found that available soil Ca was linked to crown vigor and rebounds in growth following an inciting event.

Wind-Conditioned 20th Century Decline of Birch Treeline Vegetation in the Swedish Scandes

The study focused on a frequent, although not dominant, mode of treeline change in the Swedish Scandes over the past century. Monitoring of stand density decline in a wind-exposed subalpine birch (Betula pubescens ssp. tortuosa) population was carried out over the past 30 years. The overall result included substantial and unbalanced individual mortality and a drastic reduction in stature of surviving birches. Radiocarbon dating of in situ subfossil birch-tree remains in an adjoining but windier site revealed complete stand demise over the past century or so. It is inferred that the extant and extinct birch populations represent different phases in an unbroken process of stand-level demise, hypothetically initiated in response to reindeer disturbance during the final phase of the Little Ice Age. This process was subsequently communicated more or less autogenically in a leeward direction as a consequence of the exposed and drought-prone nature of the site. Increasing westerly wind circulation and decreasing soil moisture (earlier snowmelt) during the past century may have contributed in this respect, although these aspects require further study. Within strongly wind-exposed terrain, a century of substantial warming has not sufficed to offset the birch decline. The results highlight the conclusion that under certain circumstances, wind and associated ecological and physiological effects are the overriding determinants of treeline position and structure. These are fundamental insights for generating realistic landscape-scale models of treeline change in a hypothetically warmer future.

Environmental impacts of the Norse settlement: palaeoenvironmental data from Myvatnssveit, northern Iceland

Lawson, I. T., Gathorne‐Hardy, F. J., Church, M. J., Newton, A. J., Edwards, K. J., Dugmore, A. J. & Einarsson, Á. 2007 (January): Environmental impacts of the Norse settlement: palaeoenvironmental data from Mývatnssveit, northern Iceland. Boreas, Vol. 36, pp. 1–19. Oslo. ISSN 0300–9483.The first stratigraphically continuous pollen profile spanning the Norse and Medieval periods from the archaeologically‐rich Mvatnssveit region of northern Iceland is presented. Detailed analyses were made of the tephra, sediment characteristics, pollen and chironomids of a 3 kyr sediment sequence from Helluvastjörn, a small, shallow lake. The pollen data show a steady decline in the percentage abundance of tree birch (Betula pubescens) pollen between the Norse settlement (landnám, c. AD 870) and c. AD 1300, a pattern that contrasts with the abrupt fall in birch pollen percentages immediately following the Norse colonization at almost all previously studied sites in Iceland. Some lines of evidence suggest that the gradual birch decline could be a result of reworking of soil pollen, but independent evidence suggests that this may not necessarily be the case. The pollen record indicates that birch woodland was replaced by acidophilic taxa (notably Empetrum nigrum and Sphagnum), again contrasting with the more usual pattern of Poaceae expansion seen in post‐landnám pollen diagrams from mires close to farm sites. Chironomid and Pediastrum accumulation data show that the limnic environment became more productive immediately after landnám, probably because of anthropogenic disturbance. An increase in sedimentation rate after landnám appears initially to have been caused by increased lake productivity, while reworked inorganic soil materials became a significant contributor to the sediments after c. AD 1200. The data suggest that the impact of settlement on terrestrial vegetation may have been more variable than previously thought, while freshwater ecosystems experienced significant and rapid change.

Spatial extent of winter thaw events in eastern North America: historical weather records in relation to yellow birch decline

An algorithm (Weather Reader) was developed and used to analyze daily weather records from all existing Canadian and American weather stations of eastern North America (in excess of 2100 stations), from 1930 through 2000. Specifically, the Weather Reader was used to compile daily minimum, mean, and maximum air temperatures for weather stations with at least 30 years of data, and was used to calculate accumulated degree days for winter thaw–freeze events relevant to yellow birch (Betula alleghaniensis Britt.) from beginning to end. A thaw–freeze event relevant to yellow birch was considered to take place when (i) the station daily maximum temperature reached or exceeded +4°C after being below freezing for at least 2 months of the winter, (ii) sufficient growing degree days accumulated (>50 growing degree days) to cause the affected yellow birch trees to prematurely deharden, and (iii) the daily minimum temperature dropped below −4°C causing roots and/or shoots of dehardened trees to experience freeze-induced injury and possibly dieback. The threshold temperature of +4°C represents the daily temperature above which biological activity occurs in yellow birch. The station growing degree day summaries were subsequently spatially interpolated with the Kriging function in GS+™ and mapped in ArcView™ GIS in order to display the geographic extent of the most severe thaw–freeze events. The ArcView™ maps were then compared with the extent of historically observed yellow birch decline. It was found that the years 1936, 1944, and 1945 were particularly uncharacteristic in terms of region-wide winter thaw–freeze extremes, and also in terms of observed birch decline events during 1930–1960. An overlay of suspected accumulated birch decline based on thaw–freeze mapping and observed decline maps prepared by Braathe (1995), Auclair (1987), and Auclair et al. (1997) for 1930–1960 demonstrated similar geographic patterns. The thaw–freeze projection for 1930–1960 was shown to coincide with 83% of the birch decline map appearing in Braathe (1995) and 55% of the geographic range of yellow birch in eastern North America. Thaw–freeze mapping was also applied to two significant events in 1981. Greatest impact was recorded to occur mostly in southern Quebec and Ontario, and several American Great Lake States, specifically in northern Michigan and New York, where the greatest growing degree day accumulation prior to refreeze in late February (February 28th) was projected to have occurred; and in southern Quebec, most of Atlantic Canada, and Maine, prior to a late spring frost in mid-April (April 17).

A decade of forest tree monitoring in Canada: evidence of air pollution effects

In Canada, the eastern tolerant hardwood forest is potentially at risk from air pollution. Surveys indicate that, overall, the condition of hardwood forests has either not changed or has marginally improved. However, stands growing on shallow, poorly buffered soils have a higher frequency and severity of decline symptoms, and some of these stands have continued to decline, in contrast to the general trend. In Ontario on the most sensitive sites (<6 mequiv. exchangeable bases/100 g), decline symptoms increased coinciding with a decrease in B horizon pH and base saturation and with an exchangeable Al increase. On these sites the chemical analysis of foliage, root, xylem, and soils indicate that nutrient deficiencies, nutrient imbalances, or metal/nutrient antagonism are related to stand decline. There is a consistent spatial relationship between white birch decline in the Bay of Fundy area and acidic sea fog. Acid fog injures and leaches foliage and hastens soil acidification, leaving the stands weakened and making them more sensitive to stress, particularly drought. Some recent studies suggest that forest growth rates are declining and that the relationship with climate has deteriorated, implying that the forests are responding to a regional, nonclimatic, nonpathogenic stress. This could be a response to altered forest nutrient cycling associated with acidic deposition and chronic O3exposure. A study of sugar maple in Ontario estimated that after the mid-1960s growth has declined between 0.66 and 0.96 m3·ha-1·year-1, and that the decline was greatest on poorly buffered soils. Without enhanced forest management and more stringent air pollution regulations, the sustainability of forest productivity in Eastern Canada is likely to be difficult because of air pollution and soil fertility losses.Key words: dendroecology, acid deposition, ozone, forest decline, sugar maple, climate change.

A decade of forest tree monitoring in Canada: evidence of air pollution effects

In Canada, the eastern tolerant hardwood forest is potentially at risk from air pollution. Surveys indicate that, overall, the condition of hardwood forests has either not changed or has marginally improved. However, stands growing on shallow, poorly buffered soils have a higher frequency and severity of decline symptoms, and some of these stands have continued to decline, in contrast to the general trend. In Ontario on the most sensitive sites (<6 mequiv. exchangeable bases/100 g), decline symptoms increased coinciding with a decrease in B horizon pH and base saturation and with an exchangeable Al increase. On these sites the chemical analysis of foliage, root, xylem, and soils indicate that nutrient deficiencies, nutrient imbalances, or metal/nutrient antagonism are related to stand decline. There is a consistent spatial relationship between white birch decline in the Bay of Fundy area and acidic sea fog. Acid fog injures and leaches foliage and hastens soil acidification, leaving the stands weakened and making them more sensitive to stress, particularly drought. Some recent studies suggest that forest growth rates are declining and that the relationship with climate has deteriorated, implying that the forests are responding to a regional, nonclimatic, nonpathogenic stress. This could be a response to altered forest nutrient cycling associated with acidic deposition and chronic O3 exposure. A study of sugar maple in Ontario estimated that after the mid-1960s growth has declined between 0.66 and 0.96 m3·ha-1·year-1, and that the decline was greatest on poorly buffered soils. Without enhanced forest management and more stringent air pollution regulations, the sustainability of forest productivity in Eastern Canada is likely to be difficult because of air pollution and soil fertility losses.Key words: dendroecology, acid deposition, ozone, forest decline, sugar maple, climate change.

Effects of duration of a simulated winter thaw on dieback and xylem conductivity of Betula papyrifera.

Stems or roots + stems of potted, 2-year-old paper birch (Betula papyrifera L.) were subjected to simulated winter thaws of various durations in climate-controlled chambers. The simulated thaws induced dieback of shoots of the treated plants. Although the stem thaw treatment did not significantly increase dieback, there were significant (P < 0.05) correlations between growing degree days above 4 degrees C and both shoot dieback and percent reduction in conductive xylem. All trees that received > 60 growing degree days (GDD) > 4 degrees C died back to some extent. Plants in the root + stem thaw treatment that received more than 60 GDD > 4 degrees C showed a significant (P < 0.05) increase in dieback and a significant (P < 0.05) loss of conducting xylem after a period of growth and recovery in the greenhouse, especially in the xylem of 1-year-old stems. Furthermore, higher correlations between GDD > 4 degrees C during a thaw and both the extent of dieback and the loss in conductive xylem were found in trees subjected to the root + stem thaw treatments than in trees exposed only to the stem thaw treatments (P < 0.05). The root + stem thaw treatments also resulted in highly significant relationships (P < 0.05-0.001) between loss in conductive xylem and dieback. The occurrence of dieback in response to winter thaws, and its close correlation with irreversible losses of xylem conductivity due to embolisms, coupled with an inability to refill the xylem because of root damage, support the view that these processes may be key factors in initiating birch decline.

Late-glacial (Allerød – Younger Dryas) Coleoptera from central Cape Breton Island, Nova Scotia, Canada

Coleoptera from three late-glacial sites on Cape Breton Island, Nova Scotia, Canada, provide proxy climate data from 12 600 to 10 400 BP. Samples older than about 11 800 BP contain tundra to tree-line species. Between 11 800 and about 10 800 BP, beetle assemblages become typical of the modern boreal forest. Beetles younger than 10 800 BP responded to climatic deterioration during the Younger Dryas. Although coleopteran evidence for the Younger Dryas is not as strong as that presented by palynological and stratigraphic studies, the appearance of Olophrum boreale and other species found today in the northern boreal forest and on tundra or coastal tundra corresponds with the decline of shrub birch and the rise in willow and alder pollen. Beetles responded just as likely to changes in vegetation and ground cover as to changes in temperature. The extent to which Coleoptera were affected may lie somewhere between a moderated continental response seen in the Great Lakes region in North America and more extreme changes recorded in Europe. This response is consistent with shifts in the North Atlantic oceanic Polar Front in the Ruddiman and McIntyre model.

Methodological falsification and the interpretation of palaeoecological records: the cause of the early Holocene birch decline in western Canada

The use of palynological data to test hypotheses concerning biological interactions, pedological development, or climatic change is often based upon speculative assumption about the causes of past vegetation change. In this study the approach of methodological falsification is used to investigate the causes of the decline of Betula in the western interior of Canada between 9000 and 7000 yr. BP. The approach consists of the following steps: (1) determining the timing and geographic extent of the Betula decline; (2) reviewing information on the modern ecology of Betula papyrifera and the modern environment and late Quaternary palaeoenvironments of western Canada to develop a series of parsimonious and testable hypotheses regarding causes for the birch decline; and (3) testing the hypotheses using internal evidence from the pollen record and independent palaeoenvironmental data. This approach leads to the conclusion that hypotheses linking the birch decline to a pathogen, interspecific competition or progressive soil development should be rejected. The surviving hypothesis is that the birch decline represents an individualistic response to climate change. However, the process of erecting and testing these hypotheses demonstrates that great uncertainty exists in extracting the causes of vegetation change from palaeoecological records.

Evaluating changes in forest condition potentially related to acidic deposition: an example using red spruce

Individual tree and stand responses to air pollutants, often inferred from changes in crown condition, mortality and growth rates, are difficult to correlate with each other because these data are often not measured on the same tree or plot. In evaluating changes in any of these measures it is necessary to define expectation for normal changes over time. Of the three measures, radial increment is the only one which has a temporal component to develop a historical expectation. Therefore, to develop an expectation of change in radial growth in the absence of high levels of acidic deposition, radial growth chronologies of red spruce ( Picea rubens Sarg.) from forest survey data in northern Maine were analyzed. Rather than concentrate on recent (post-1960) increases or decreases in radial increments, the entire radial growth chronology was analyzed for possible influences of disturbance events and stand development. As a result, two periods of large growth reductions are evident over the past 60 years. In the early 1930s, a trend of decreased radial growth was preceded by growth increases in the 1920s. The 1920s increase resulted from the release of understory trees and recovery of surviving trees, following a severe budworm epidemic. Likewise, a trend of decreased radial growth after 1960 was preceded by a growth increase in the 1940s. Release of the red spruce trees in the 1940s is believed in part to be the result of a regional birch decline that began in the mid-1930s. These increases and subsequent decreases in radial growth are expected given the disturbance events and natural dynamics of the stands. However, even if the growth trend departed from expectations (e.g. after accounting for stand disturbances), unless air pollutant levels are identified and measured for those forest survey sites, the strongest statement that could be made for this example or any similar situation is that a change in tree condition has been detected for which acidic deposition or air pollutants cannot be excluded as a cause.

The vegetational history of Glen Diomhan, North Arran, site of endemic whitebeams, Sorbus arranensis Hedl. and S. pseudofennica E.F. Warb.

A detailed pollen analysis was undertaken of sediment from a site in Glen Catacol on the Isle of Arran, Scotland, close to Glen Diomhan National Nature Reserve which has a large though possibly declining population of the endemic whitebeams, Sorbus arranensis Hedl. and S. pseudofennica E.F. Warb. Blanket peat formation began before 7000 years BP. The results provide evidence that while parts of the glen saw the development and subsequent decline of birch, oak, hazel and alder woodland a large proportion of the surrounding vegetation has remained open throughout the Flandrian with grass-heath similar to that which occurs over much of the glen in the present day.

EARLY‐AND MID‐FLANDRIAN ENVIRONMENTAL HISTORY OF THE BRECON BEACONS, SOUTH WALES

SUMMARYPollen‐stratigraphic data are described from two sites on the northern face of the Brecon Beacons escarpment in South Wales, U.K. Pollen percentage, pollen concentration and deteriorated pollen diagrams are presented which contain details of vegetational changes in this part of Britain from the time period immediately following the wastage of the Loch Lomond Advance glaciers around 10000 radiocarbon years ago, until the establishment of oak and alder woodland in the mid‐ Flandrian some 4000 years later. An initial phase of juniper scrub was followed by the expansion of birch woodland over much of the area, and these forests were subsequently invaded by large numbers of hazel. Quercus, Ulmus and Pinus were present throughout the mid‐Flandrian, but increasing climatic wetness, possibly beginning around 7000 radiocarbon years ago, led to the decline of elm, pine and birch as woodland components in this part of South Wales, and to the expansion of Quercus and particularly Alnus. These vegetational changes are discussed in terms of vegetational developments at other sites in upland Wales and western Britain.