Frost Disturbance Research Articles

A landscape assessment of frost damage in the northmost Neotropical dry forest

Extreme climatic and weather events are inducing widespread tree mortality in many forest ecosystems worldwide. A severe frost related to the historic cold wave that affected much of North America in early February 2011 triggered extensive tree mortality in the lowland tropical dry forest in northwestern Mexico. In this landscape, we assessed frost-induced vegetation damage using anomalies of the widely used NDVI spectral index calculated with a 12-year pre-disturbance reference period. In addition, NDVI anomalies were related to ground-based measures of frost-disturbance severity (density of frost-killed trees) and forest recovery (post-frost recruits and resprouting of surviving trees) obtained in 48 1-ha study plots distributed across the landscape. We found negative NDVI anomalies in 84% of our study area, indicating widespread frost-induced vegetation damage. The relationship between NDVI anomalies and density of frost-killed trees confirmed the level of frost damage across the lowland vegetation, suggesting that even small deviations from long-term mean NDVI values may hint to severe disturbance on forest structure and function. Higher frost-disturbance severity stimulated more new recruitment and less tree resprouting. Our results demonstrate the reliability of spectral indices for rapid assessment of climate-induced vegetation change, the high vulnerability of tropical dry forests to extreme frost and the predominant mechanism of vegetation recovery after severe frost disturbance.

Evolving the structure: climatic and developmental constraints on the evolution of plant architecture. A case study in Euphorbia

Plant architecture strongly influences ecological performance, yet its role in plant evolution has not been explored in depth. By testing both phylogenetic and environmental signals, it is possible to separate architectural traits into four categories: development constraints (phylogenetic signal only); convergences (environmental dependency only); key confluences to the environmental driver (both); unknown (neither). We analysed the evolutionary history of the genus Euphorbia, a model clade with both high architectural diversity and a wide environmental range. We conducted comparative analyses of 193 Euphorbia species world-wide using 73 architectural traits, a dated phylogeny, and climate data. We identified 14 architectural types in Euphorbia based on trait combinations. We found 22 traits and three types representing convergences under climate groups, 21 traits and four types showing phylogenetic signal but no relation to climate, and 16 traits and five types with both climate and phylogenetic signals. Major drivers of architectural trait evolution likely include water stress in deserts (selected for succulence, continuous branching), frost disturbance in temperate systems (selected for simple, prostrate, short-lived shoots) and light competition (selected for arborescence). Simple architectures allowed resilience to disturbance, and frequent transitions into new forms. Complex architectures with functional specialisation developed under stable climates but have low evolvability.

Growth of Grimmia mosses on volcanic tephra: Geoecological processes of biocrust development in Haleakalā crater (Maui, Hawai′i)

This research examines high-elevation biocrusts on volcanic tephra in Haleakalā Crater, Maui, Hawai′i; geomorphic, ecological, and pedological processes are discussed, in order to provide an integrated geoecological view of linkages that have influenced biocrust genesis. The study considers four spatial scales: (i) the landscape scale; (ii) the site scale; (iii) the miniature scale and, (iv) the microscopic scale. (i) Biocrusts grow at ~ 2335 m on a hydrologic basin circumscribed by four cinder cones which contributed tephra materials during eruptions between 11000 and 5000 BP, ~2700 BP, and 970 ± 50 BP. Basin landforms provide multiple clues about past geomorphic processes that have affected crust evolution, including intermittent runoff, which decreased the length of cryptobiosis periods, allowing poikilohydric mosses to gradually expand into dense biocrusts; and widespread frost activity, which caused tephra sorting and extensive slope creep on cinder cones. Ubiquitous signs of current frost disturbance are also found on basin soils. (ii) The study site, located amidst bare volcanic tephra—ash and lapilli—includes mainly two organisms: an acrocarpous moss, Grimmia torquata, and a foliose lichen, Peltigera rufescens. Biocrusts attained 40.23% groundcover;G. torquata was found on 64 undisturbed specimens, and showed 78.3% frequency on 46 sampling plots, whereas P. rufescens grew on 23.4% of specimens. Biocrusts also supported 13 vascular species. The most common were grasses, with 5.6% cover and 67.4% total frequency, including Trisetum glomeratum (47.8%) and Deschampsia nubigena (19.6%); other vascular plants reached just 0.85% combined cover. Tests show 72% of vascular-plant cover was statistically correlated with presence of biocrusts, suggesting these have beneficial—presumably pedological—effects on vascular plant growth. (iii) Biocrust microtopography was characterized by intermingling areas of rolling, 15–35 mm-high, protrusions, and 40–75 mm-high pinnacles. (iv) Soil micromorphology indicated crust growth was probably influenced by a host of genetic processes, including wind abrasion and erosion; aeolian dust deposition; soil desiccation during seasonal wet-dry cycles; rainwash and raindrop impact; diurnal temperature cycles; and bioturbation by burrowing fauna. The interactions among these agents are integrated into an idealized crust developmental sequence during the past few millennia.

Carbon dioxide emissions from periglacial patterned ground under changing permafrost conditions and shrub encroachment in an alpine landscape, Jotunheimen, Norway

AbstractWhether Arctic and alpine ecosystems will act as a future net sink or source of carbon remains uncertain. The present study investigates ways in which ecosystem (soil and vegetation) and geomorphological (cryogenic disturbance) factors may control or affect the future release of carbon in an alpine permafrost landscape. Rates of ecosystem respiration (Re) were examined using a portable gas analyzer across an altitudinal transect ranging from mid‐ to high‐alpine vegetation zones underlain by discontinuous to continuous permafrost on Galdhøpiggen (Norway). Measurements were made of Re during the peak growing season on active and relict sorted circles exhibiting varying levels of frost disturbance and shrub encroachment. Re was found to be controlled more strongly by soil microclimate and plant growth forms than by geomorphic indicators of cryoturbation in thawing permafrost or by atmospheric conditions. The results indicate that increasing shrub cover leads to elevated Re, while an increase in surface disturbance has the potential to lower Re. We conclude that vegetation is likely to colonize frost‐disturbed surfaces at progressively higher altitudes as freeze–thaw processes slow down or cease, and this will result in increased Re.

Long-term growth rates of two caulescent rosette species, Coespeletia timotensis (Cuatrec.) Cuatrec., and Espeletia schultzii Wedd., in an Andean páramo

Growth rates of two caulescent rosette species, Coespeletia timotensis and Espeletia schultzii, were estimated in Piedras Blancas, an Andean páramo, on 4 sites at 4270–4525 m, near their upper elevation limits. Stem and crown height were measured at the start and end of the study periods, and differences in plant stature divided by number of years to obtain annual growth rates. An isolated C. timotensis on a granite outcrop at 4525 m grew 1.08 cm/yr average for 19.5 yrs, but its elongation rate dropped gradually with time. 60 C. timotensis on a dense valley-floor stand at ≤4285 m showed an average rate of 1.34 ± 0.59 cm/yr for >10 yrs; growth varied between −0.2 and 3.16 cm/yr and was not associated with initial plant height. 31 E. schultzii rosettes on a basal slope stand at ≤4300 m grew 1.15 ± 0.51 cm/yr during >10 yrs; growth ranged from 0.44 to 2.37 cm/yr and appeared to be inversely correlated with plant stature. Each stand exhibited fairly uniform aspect, gradient, slope position, and groundcover, but microsite differences, including soil nutrient and moisture content, proximity to rocks, and competitive plant interactions, may have affected rosette development. Mortality was similar for both species (18.4–18.9%) but was higher for C. timotensis seedlings and taller plants, which tilted and toppled, presumably due to ground frost disturbance. Mortality was also concentrated on larger E. schultzii plants, but these remained standing and did not topple. Due to extreme growth variation, use of average rates to calculate ages of individual rosettes is not recommended.

Effects of a large-scale late spring frost on a beech (Fagus sylvatica L.) dominated Mediterranean mountain forest derived from the spatio-temporal variations of NDVI

A remote sensing-based approach was implemented to detect the effect of a late spring frost on beech forests in the Mediterranean mountain region. The analysis of spatio-temporal variability of frost effects on normalised difference vegetation index (NDVI) highlighted the distribution of the canopy damage across the forest according to geomorphic factors such as slope, aspect, and altitude. Increased intensity and frequency of extreme temperatures such as late spring frosts and heat waves represent the main drivers affecting forest ecosystem structure and composition in the Mediterranean region. The main objective of this study was to evaluate the effects of a late spring frost disturbance, which occurred during spring 2016 in southern Italy, through the assessment of the spatial pattern of the damage to the beech forest canopy associated with the peak decrease in normalised difference vegetation index (NDVI), and the analysis of the NDVI temporal recovery after this frost disturbance. The forest areas affected by frost were detected through the NDVI differencing technique based on Landsat 8 (OLI/TIRS) imagery time series. The influence of local geomorphic factors (i.e., aspect, elevation, and slope) on forest NDVI patterns was assessed by means of a generalized additive model (GAM). A rather counterintuitive NDVI patterns emerged according to the forest exposition, with NDVI significantly higher on the north facing areas than the southerly ones. The main canopy damage occurred at about 1250 m and reached up to 1500 m asl, representing the altitudinal range affected by the frost disturbance. Finally, the full canopy recovery occurred within 3 months of the frost event. The analysis of seasonal Landsat 8 image time series related to local geomorphic factors, such as aspect, slope, and altitude, and plant phenology on a frost event date, contributed to highlight the NDVI spatio-temporal variation and canopy recovery of a Mediterranean mountain beech forest.

Frequency and factors of earlywood frost ring formation in jack pine (Pinus banksiana) across northern lower Michigan

Late spring frost disturbances have significant ecological and physiological impacts on forests. Frost-induced cambial damage that occurs when cells are actively dividing can result in the formation of frost rings, abnormal modifications to wood anatomy within the annual growth rings of an injured tree. Frost rings are indicators of growing season frost damage to the cambium and therefore have potential to be used both as a proxy in the reconstruction of extreme climatic events and to identify frost-prone environmental conditions. In this study, we measured the occurrence of earlywood frost rings across cambial age and diameter class in 11 jack pine (Pinus banksiana) populations of northern lower Michigan. Earlywood frost ring formation was greater in younger trees and in trees with smaller diameters. Biotic (cambial age, diameter, and ring width) and abiotic (elevation, initial site-related growth rate, and minimum temperature) factors demonstrated significant influence on the probability of earlywood frost ring formation. When using frost rings as a proxy of historical climate, susceptibility to abrupt freezing temperatures during the growing season and thus the ability of an individual tree to record a frost disturbance should be considered.

Do freeze events create a demographic bottleneck for Colophospermum mopane?

Frost disturbance is often mentioned in southern African savanna literature, but it is seldom discussed or investigated further. However, it can represent an above-ground disturbance as effective as fire or browsing, depending on the resistance capacity of the effected plants. A severely freeze-damaged stand of Colophospermum mopane along a slope in the Venetia Limpopo Nature Reserve provided an opportunity to investigate the nature of freeze-damage impacts on C. mopane. Is this disturbance a possible demographic limitation of C. mopane preventing its southwards spread? Freeze-damage of individual trees was assessed according to tree height and landscape position — with lower elevations representing the most severe freeze zones and higher elevations representing the least severe. Lower elevation trees were relatively small (2.24m) and coppicing, whilst higher elevation trees were taller (3.65m) with no coppice present. No freeze-damage was observed on tree canopies above 4m in height. Trees<4m in height that had experienced 100% freeze-damage, failed to regrow to their original heights of the previous season. This is a possible driver of the pre-freeze height differences seen across the slope; with trees at low elevations having to recover from freeze events and subsequent topkill more frequently, resulting in a net decrease in tree height for that growing season. It appears that C. mopane has limited resistance to freeze events, and this may be linked to the absence of this species at colder latitudes in the Southern Hemisphere.

Effects of Recreational Traffic on Alpine Plant Communities in the Northern Canadian Rockies

Recreational activities in alpine areas have been increasing in recent decades, creating the need to improve our understanding of the impacts of these activities and how they are best managed. We explored impacts of recreational trail use on dry alpine meadows in the northern Canadian Rockies of Alberta. Data collected in 142 plots (0.5 m × 1 m) were used to compare plant community metrics among (1) a recreational trail, (2) intact tundra meadows (undisturbed), and (3) sparsely vegetated gravel steps formed by frost disturbance (naturally disturbed). As compared to undisturbed tundra, trails had substantially lower cover of vascular plants (4% vs. 35%), lichen (0% vs. 10%), and cryptogamic crust (0% vs. 4%); trails also had lower species richness (7 vs. 11 species per plot), but greater soil compaction (2.75 vs. 1.25 kg cm-2). Trails differed from natural gravel steps, which had three times more biotic cover and different composition. This highlights the difference in effects of human and natural disturbance. Positive feedback effects of trampling in tundra ecosystems may lead to altered environmental conditions, including decreased infiltration capacity and nutrient cycles in soils, and more extreme temperatures at the soil surface. These feedbacks could inhibit regeneration of abandoned trails.

Vegetation establishment, succession and microsite frost disturbance on glacier forelands within patterned ground chronosequences

AbstractAim The impact of microscale frost disturbance on vegetation colonization and successionary trends was examined within patterned ground features of Little Ice Age chronosequences. The goal was to investigate and compare vegetation response to micro‐site frost disturbance with that of previous studies done at a coarser landscape scale.Location The study sites occur on Little Ice Age glacier forelands within Jotunheimen, Norway (61°–62° N). The forelands of the glaciers Slettmarkbreen, Styggedalsbreen and Vestre Memurubreen have been well studied providing chronological controls for landscape studies. Sorted patterned ground features are found within the chronosequences, typically declining with frost intensity and disturbance with increasing terrain age.Methods Micro‐plots (8.3 × 8.3 cm) were placed at the inner borders and centres of patterned ground features. Species were identified and per cent species cover and per cent cover of life‐form category were noted. Nonparametric Kruskal–Wallis and Mann–Whitney U‐tests were used to test for differences between percent cover of life‐form categories within patterned ground features as well as to identify thresholds of successional change across the chronosequences.Results Significant relationships between life‐from groups and patterned ground positions of varying ages were deduced using nonparametric statistics. Findings were then used to discuss trends of succession within patterned ground features and across the chronosequences. Vegetation establishment occurs at the border positions of young (&lt; 30 years) patterned ground features. With time and distance from the ice margin, vegetation encroaches inwards toward the disturbed centres. Succession within patterned ground exhibits several stages: (1) bryophytes/crusts and lichens, (2) grasses/sedges and (3) woody shrubs. The occurrence of forbs was sporadic and generally non‐significant.Main conclusions Frost disturbance in patterned ground appears to delay successional trends of vegetation communities when compared with previous studies on ‘stable’ terrain, producing micro‐site lag effects. These small patches of disturbed ground are therefore important regarding vegetation assemblages across the landscape and are unlikely to be detected at the landscape scale.

Formation of patterned ground and fine-scale soil development within two late Holocene glacial chronosequences: Jotunheimen, Norway

Chronosequences of ∼250 to ∼130 years were established on two late Holocene glacier forelands in the Jotunheimen region of southern Norway. Patterned features occurring within chronologically established time units were studied. Young patterned features, forming ∼10–20 years after deglaciation, are frost active. Vegetation cover is minimal within the young patterned features, consisting of bryophytes/organic crusts. Soil development within patterned ground is also minimal/absent because frost action retards horizonization. With time and distance from the glaciated ice margin, frost activity declines within the patterned features, suggesting that a thin, active “periglacial zone” exists near the ice margin. Initially, frost activity decreases at the borders of the features with the centers stabilizing later in time. This results in fine-scale soil heterogeneity and variations of soil development. Fine-scale pedogenic development is first encountered at the borders of patterned ground that has developed on terrain exposed since the ∼1930s, yet soil development is predominantly absent at the centers of patterned ground. With time and distance from the ice margin, frost activity declines and allows patterned features to homogenize from border to center positions in regard to soil characteristics. Across the chronosequences, soils within patterned features pedogenically follow previous soil chronosequence studies, evolving from USDA classifications of Entisols into Inceptisols. Frost disturbance within patterned ground, however, produces a lag effect, that results in longer periods of time for pedogenesis to occur and thinner soils than that of the surrounding terrain.

La flore vasculaire alpine du plateau Big Level, au parc national du Gros-Morne, Terre-Neuve

La flore alpine du nord-est de l'Amérique du Nord est bien documentée à l'exception de celle des monts Long Range, à Terre-Neuve. Le plateau gneissique de Big Level est l'un des plus grands plateaux de haute altitude des monts Long Range. Parmi les habitats alpins qui s'y trouvent, les nombreuses combes à neige contribuent le plus à la biodiversité végétale avec 73 des 92 espèces observées à cet étage du plateau. Les 21 espèces rares observées sur le plateau appartiennent toutes à des habitats associés aux combes à neige. Ces espèces représen- tent 20 % des plantes rares connues dans le parc national du Gros-Morne, dont trois constituent des extensions méridionales de leur aire depuis l'extrémité nord de l'île : Carex lachenalii, Salix argyrocarpa et Veronica wormskjoldii . La composition biogéographique des combes à neige de la région du golfe du Saint-Laurent - Nouvelle-Angleterre, avec ses éléments circumpolaires, cordillériens, du nord-est de l'Amérique du Nord et amphi-atlantiques, contribue au caractère floristique exceptionnel du plateau. Le broutage intense des gazons périphériques par les importants troupeaux de caribous du plateau constitue une caractérisitique écologique intéressante de ces combes alpines. Également exceptionnelle est la présence de vastes landes à buttes dominées par la camarine et le calamagrostide ( Empetrum-Calamagrostis pickeringii ), les buttes étant causées par la cryoturbation sur les pentes douces exposées de la partie est du plateau. À notre connaissance, ce type de terrain réticulé, connu en Arctique, n'a pas été observé pour la zone boréale de l'est de l'Amérique du Nord ; c'est un témoin de la rigueur des hivers sur le plateau. Aucun autre plateau alpin gneissique de l'est de l'Amérique du Nord boréale n'a une telle étendue ni un tel nombre de combes à neige.

Effects of canopy conditions on the regeneration of major tree species in an old-growth Chamaecyparis obtusa forest in central Japan

The regeneration mode of major tree species ( Chamaecyparis obtusa (Sieb. et Zucc.) Endl., Thujopsis dolabrata Sieb. et Zucc., Chamaecyparis pisifera (Sieb. et Zucc.) Endl., Quercus mongolica Fischer ex Turcz., Magnolia obovata Thunb. and Betula grossa Sieb. et Zucc.) in relation to four canopy conditions (closed evergreen conifer canopy, closed evergreen conifer canopy adjacent to canopy gap (gap-adjacent), canopy gap, and closed deciduous broad-leaved tree (DBL) species canopy) was studied from 1988 to 1998 in a 4-ha permanent plot in the Akasawa Forest Reserve, central Japan. The vertical distribution of stems and the major species composition in four height (canopy; ≥25 m tall overtopping neighboring trees, subcanopy; ≥25 m tall, middle; ≥10 m tall, and understory; <10 m tall) classes varied with canopy condition. The understory and middle layers were generally dominated by T. dolabrata, but this species in the subcanopy layer existed only under gap-adjacent. Canopy gaps were usually filled by DBL tree species, especially M. obovata and less often by T. dolabrata, while the probability that they were filled by Chamaecyparis sp. increased in larger gaps. C. obtusa and C. pisifera occurred more often under closed DBL canopy, T. dolabrata under gap-adjacent canopy, and Q. mongolica, M. obovata and B. grossa under canopy gaps. The mortality and recruitment rates of understory stems of major tree species also varied with canopy condition. Most mortality resulted from suppression with standing-dead and natural disturbances with broken trunks, although the relative importance varied with canopy condition and species. Generally, evergreen conifers died as a result of natural disturbances, especially in the 1998 frost disturbance under canopy conditions other than closed evergreen conifer canopy. In conclusion, canopy conditions affect the vertical population structure, demographic parameters, and causes of tree mortality of major tree species. Of the favored regeneration sites in this old-growth C. obtusa forest, C. obtusa and C. pisifera generally may regenerate under closed DBL canopy, T. dolabrata under gap-adjacent canopy, and Q. mongolica, M. obovata, and B. grossa under canopy gaps. Consequently, this semi-natural old-growth C. obtusa forest will gradually change into a more diverse forest.

Stratigraphy, age and formation of peaty earth hummocks (pounus), Finnish Lapland

Detailed investigations of peaty earth hummocks called pounus were carried out in northemmost Finnish Lapland in the vicinity of the Kevo Subarctic Research Station. Pounus are complex, often vegetated, cryogenic mounds formed partly of peat. Some have a core of mineral soil or several large stones, or just one frost-heaved boulder covered by peat. They belong to the same group of earth hummocks as thufur and non-sorted circles. The typology of pounus is described and compared with other types of cryogenic earth hummocks. Stratigraphical study of pounus shows how cryoturbation has deformed the previous soil horizons of typical Arctic podzols and has been followed by intense peat growth and later frost reactivation. Microstructures in pounus confirm the frost activity and several phases of peat growth during their formation. Alternating warmer and dryer conditions with water-table rise and renewed peat formation could be inferred. The present stage of development is frost reactivation and peat growth. Pounus are relatively young features. The maximum age for pounu development corresponds with the age of peat formation. Peat of a boulder pounu yielded a data of 330 + 70 BP and a minerogenic pounu gave an age of 710 + 50 BP. The Sphagnum peat formation on peat pounus started intensively about 1000 BP. They are random surficial forms with variable control by frost heave and vegetation growth, independent of the presence or absence of permafrost. They are related to a dynamic snowcover in areas of open birch stands or tundra and their development is polygenetic with alternating frost disturbance and phases of humification or podzolization. Pounus act thermally as a biological refuge in the autumn and spring. At present, perennial frozen cores seem to be common, especially in large peat pounus. Pounu development, as with other earth hummocks, occurs after the main phase of interglacial pedogenesis and can be considered as an indicator of a general cooling trend, developing mostly during the second half of the Holocene. Pounus' relation to climatic change is discussed. Pounus can be considered as an indication of a general cooling trend associated with progressive depletion of summer insolation. Hummock extent is coherent and parallel with trends in permafrost growth, but not directly linked to it.

Needle ice processes and sediment mobilization on river banks: the River Ilston, West Glamorgan, UK

This paper examines the role of needle ice processes in the bank erosion system of a site on the River Ilston, West Glamorgan, South Wales, UK. Bank retreat rates were monitored with erosion pins over a 2.25 year period. Concurrent detailed data were also obtained on river flow levels and ambient air and stream temperatures. Unique data on river bank thermal regimes are presented, along with rare information on the morphological properties and sediment yield characteristics of needle ice growths. Needle ice occurred even with mild freezing temperatures if sufficient bank moisture was present, with average growth rates of approximately 1 mm h −1. Bank material incorporation and mobilization within needle ice was significant: sediment yields of up to 4.02 kg m −2 of bank face were observed, and represented between 32% and 43% of total bank erosion recorded. Most sediment was incorporated within needle ice when growth became stressed (because of overrapid cooling and/or limited moisture supplies), which supports the models of Outcalt and Fukuda. Four mechanisms of direct needle ice sediment transport on river banks were identified: direct particle fall, sediment-laden rivulets, sliding failures, and toppling failures. However, the indirect, preparatory role of needle ice and frost disturbance of bank surfaces is also emphasized, and the enhanced fluvial erosion which takes place when bank materials have first been conditioned by frost-related processes is noted. Both indirect and direct effects are expressed in a simple conceptual model and a number of the important system linkages examined. Combinations of needle ice action and subsequent fluid entrainment of weakened bank material appear to be the dominant mechanism for bank erosion here. Pronounced and persistent erosional notches in the lower parts of the bank profile were observed, showing that, in winter, even single, low-magnitude, flow events can effect significant channel width adjustment here.

Distribution of littoral macrophytes in a North Swedish riverside lagoon in relation to bottom freezing

The relationships between the distribution of aquatic macrophytes and the winter habitat conditions of the littoral zone in a river in northern Sweden were studied during a 6-year period. Because the water level gradually decreases during the winter, the ice cover freezes solid to the shore substrate, i.e. bottom-freezing occurs. This is suggested to be a fundamental factor affecting the distribution of the macrophytes. The vertical frequency patterns of Isoetes setacea Lam., Eleocharis acicularis (L.) Roemer and Schultes and Subularia aquatica L. approached normal distribution around an intermediate level of frost disturbance. The vertical movements of these and other species followed the annual changes in the extent of bottom freezing ( P < 0.05). The various adaptations developed to obviate such disturbance by freezing are discussed.

Intermediate levels of soil disturbance maximize alpine plant diversity

The intermediate-disturbance hypothesis1–9 predicts that plant species diversity will be maximized at intermediate levels of vegetation disturbance (D). As yet, the only directly applicable data seem to be from Grime's1 study, which served more as a source of the conjecture than an independent test of it, and in which disturbance was not measured. Here I report an analysis of the relationship between soil frost disturbance and diversity of vascular plant species in Arctic–alpine fellfield vegetation in the White Mountains of interior Alaska. To study plant species diversity, a measure (SD) is calculated, which is affected by both the number of species (species richness, SR) and the evenness of their relative abundances. I show that intermediate levels of disturbance maximize SD by influencing the evenness of relative abundances, but not the overall number of species. Because disturbances caused by soil frost action and animals are frequent in many Arctic and alpine tundras, these results imply that disturbance could be an important factor in species diversity differences within and between tundra vegetation types. If theoretical predictions1–9 are confirmed by dynamic tests now under way, the theory could be applied to problems of tundra vegetation management such as revegetation and disturbance by trampling and vehicles.

ALPINE SOILS OF NORTH‐EAST SCOTLAND

SummaryMany features of the alpine soils of NE. Scotland are related to altitude. The deep contemporary humose A horizon has formed in Post‐Boreal to recent times. Its formation was preceded by a phase of profile development characterized by very scanty A horizon development. This phase was interrupted by a stage of frost disturbance, probably equated with the Highland Re‐advance. The earliest part of the pedogenetic cycle is represented only by cryogenic redistribution of the fine fraction, within the parent material column, which occurred between 12,000 and 28,000 B.P.