Responses to natural climatic variation and experimental warming in two tundra plant species with contrasting life forms: Cassiope tetragona and Ranunculus nivalis

Two circumpolar tundra plant species, the evergreen dwarfshrub Cassiope tetragona and the perennial herb Ranunculus nivalis , were studied at Latnjajaure in northern Swedish Lapland during three consecutive growing seasons (1993–95) as a contribution to the ITEX programme. Open-top chambers (OTCs) were used in a passive heating experiment, and the performance of the plants in unmanipulated controls was correlated with climatic fluctuations among the years. Phenological, vegetative, and reproductive variables were measured. In both species phenological responses were controlled mainly by ambient air temperature. In the evergreen C. tetragona vegetative growth was controlled mainly by the influx of global solar radiation and was not temperature-dependent, whereas the opposite applied in the herbaceous R. nivalis . Vegetative growth in C. tetragona was rather stable among years as well as between treatments, whereas it was strongly influenced by annual climate in R. nivalis . Both species increased their reproductive success with increasing temperature, but R. nivalis was also radiation-dependent in this case, probably because of its green, photosynthetic nutlets. Ovule number in R. nivalis increased steadily in the experimentally heated plots during the study in response to the constant temperature amelioration above the ambient. At the community level, evergreen C. tetragona seems to have low competitive ability under warmer conditions. The situation for vernal low-growing herbs like R. nivalis is more complex; despite a strong positive response to increased temperature, they may exhibit decreased reproductive success if overgrown by a vigorous graminoid canopy.     

Effects of simulated climate change on phenology and life history traits in Carex bigelowii

Climate change may have large effects on plants, especially in the Arctic. At two different sites, we studied the effects of enhanced temperature by using open-top chambers (OTCs) on the clonal sedge Carex bigelowii , a common plant in arctic and alpine tundra. At the subarctic-alpine site Latnjajaure, northern Sweden, overall flowering phenology was accelerated by open-top chambers (OTCs) during the five years of treatment. For this protogynous population, male flower phenology accelerated more than female flower phenology, which resulted in an increased gender phase overlap. Sexual reproductive effort at the ramet-level increased, both in male and female functions. Smut fungus incidence was not different among treatments, but a Dipteran seed predator attacked more ramets in the OTCs in one of five years. However, neither the fungus nor the seed predators affected plant growth or seed set measurably. Ramet-level growth increased in the OTCs at Latnjajaure, but decreased in the OTCs at the maritime subarctic site Thingvellir, Iceland after three years of treatment. At Latnjajaure, the initial ramet- level responses were still evident after five years, while responses at the ramet population level disappeared: there was no difference in flowering frequency of ramets in the fifth year of the warming treatment. This is interpreted as either meristem limitation or internal resource depletion. At Thingvellir flowering frequency was unaffected by warming treatment, while ramet production decreased. Ramet production was unaffected by the treatment at Latnjajaure. These site differences in responses could be the consequences of differences in climate or site specific conditions. The trade-off (negative correlation) between the number and size of vegetative offspring at Latnjajaure disappeared under enhanced temperatures (within the OTCs). This trade-off was not apparent under the warmer conditions at Thingvellir.     

Carbohydrate reserves in the facilitator cushion plant <Emphasis Type="Italic">Laretia acaulis</Emphasis> suggest carbon limitation at high elevation and no negative effects of beneficiary plants

The elevational range of the alpine cushion plant Laretia acaulis (Apiaceae) comprises a cold upper extreme and a dry lower extreme. For this species, we predict reduced growth and increased non-structural carbohydrate (NSC) concentrations (i.e. carbon sink limitation) at both elevational extremes. In a facilitative interaction, these cushions harbor other plant species (beneficiaries). Such interactions appear to reduce reproduction in other cushion species, but not in L. acaulis. However, vegetative effects may be more important in this long-lived species and may be stronger under marginal conditions. We studied growth and NSC concentrations in leaves and stems of L. acaulis collected from cushions along its full elevational range in the Andes of Central Chile. NSC concentrations were lowest and cushions were smaller and much less abundant at the highest elevation. At the lowest elevation, NSC concentrations and cushion sizes were similar to those of intermediate elevations but cushions were somewhat less abundant. NSC concentrations and growth did not change with beneficiary cover at any elevation. Lower NSC concentrations at the upper extreme contradict the sink-limitation hypothesis and may indicate that a lack of warmth is not limiting growth at high-elevation. At the lower extreme, carbon gain and growth do not appear more limiting than at intermediate elevations. The lower population density at both extremes suggests that the regeneration niche exerts important limitations to this species’ distribution. The lack of an effect of beneficiaries on reproduction and vegetative performance suggests that the interaction between L. acaulis and its beneficiaries is probably commensalistic.     

Population fluctuations, reproduction and survival in the Striped fieldmouse Rhabdomys pumilio on the Cape Flats, South Africa

A wild population of Striped fieldmice was studied for a continuous period of five years in an area of alien Acacio bush about 24 km from Cape Town, South Africa. Mice were live-trapped, marked by toe-clipping and released on a 60-station grid, and also kill-trapped in a separate area. Marked annual fluctuations in the population size were correlated with a 6–8 month summer breeding season. There were also marked inter-annual differences in peak population size. The age of sexual maturity of females was determined from the first pregnancy which occurred at 6–7 weeks old; and of males from the presence of visible spermatozoa in the vas deferens which occurred at about 11 weeks old. Litter size was affected by both the age and the body mass of the female. It appeared that mean survival from birth was only approximately 1.5 months and that survival from first capture was about 1.9–2.5 months. This high mortality appeared to be the major reason for the sharp seasonal fluctuations in population size and may also have been responsible for the interannual differences since marked changes in survival were recorded between some years.     

Tree population dynamics in relation to climate and forest history in the Oswegatchie Plains,northern New York

This study was conducted to describe the age structure and variation in annual radial growth in populations of Pinus strobus, Larix laricina, and Prunus serotina as a means of characterizing tree establishment in the Oswegatchie Plains, a shrub barren surrounded by forest in the Adirondack Mountains of northern New York. Population age structure was determined in 10 m wide transects established perpendicular to the plain-forest border. Age of individual stems was determined by counts of internodes for pine seedlings, counts of growth rings on cut larch seedlings, and counts of growth rings in increment cores extracted from larger stems of all three species. Variation in tree annual radial growth was described by measuring the width of individual growth rings on increment cores and analyzing the data in relation to climatic and historical records of the site. The populations of P. strobus and L. laricina were about 100 years old. The P. serotina population in the open and at the forest margin was only 50 years old. P. strobus and L. laricina had a discontinuous age structure marked by an age gap of about 30 years which occurred between the 1910's and the 1940's in the two populations. Seedlings of P. strobus and L. laricina were more abundant and older at the forest edge than under the forest canopy. Pattern of age distribution in seedlings of P. strobus was described equally well with a negative exponential model as with a power function model. In all three species tree age was poorly related to the tree diameter at breast height. Variations in annual radial growth throughout population life-history were more marked in L. laricina than in P. strobus or P. serotina. Monthly rainfall and monthly mean temperature and snowfall explained 94% of variation in radial growth of P. serotina, 67 to 78% of variation in radial growth of P. strobus, and 54 to 55% of variation in radial growth of L. laricina. P. serotina growth was best correlated with weather conditions during the first half of the growing season. In contrast, P. strobus growth was better correlated with weather conditions toward the end of the season. Major variation in L. laricina growth was more closely related to past outbreaks of the larch sawfly, Pristophora erichsonii, than to climatic conditions. Tree esta blishment through time has been episodic in all three species, with little or no establishment during the intervening time periods. Age structure data do not support the hypothesis that a significant advance of the forest edge has occurred over the past several decades. Successful establishment of trees may occur at times but because of the severity of site conditions and the vulnerability of forest stands to natural disturbances, the vegetational characteristics of the Plains will probably remain very different from the surrounding forest.     

Factors influencing vascular plant diversity on 22 islands off the coast of eastern North America

Aim The influence of physiographic and historical factors on species richness of native and non‐native vascular plants on 22 coastal islands was examined. Location Islands off the coast of north‐eastern USA and south‐eastern Canada between 41° and 45° N latitude were studied. Island size ranges from 3 to 26,668 ha. All islands were deglaciated between 15,000 and 11,000 yr bp ; all but the four New Brunswick islands were attached to the mainland until rising sea level isolated them between 14,000 and 3800 yr bp . Methods Island species richness was determined from floras compiled or revised since 1969. Simple and multiple regression and rank correlation analysis were employed to assess the relative influence of independent variables on species richness. Potential predictors included island area, latitude, elevation, distance from the mainland, distance from the nearest larger island, number of soil types, years since isolation, years since deglaciation, and human population density. Results Native vascular plant species richness for the 22 islands in this study is influenced most strongly by island area, latitude, and distance from the nearest larger island; richness increases with island area, but decreases with latitude and distance from the nearest larger island as hypothesized. That a similar model employing distance from the mainland does not meet the critical value of P confirms the importance of the stepping‐stone effect. Habitat diversity as measured by number of soil types is also an important predictor of native plant species richness, but at least half of its influence can be attributed to island area, with which it is correlated. Two historical factors, years since deglaciation and years since isolation, also appear to be highly correlated with native species richness, but their influence cannot be separated from that of latitude for the present sample size. Non‐native vascular plant species richness is influenced primarily by island area and present‐day human population density, although human population density may be a surrogate for the cumulative effect of several centuries of anthropogenic impacts related to agriculture, hunting, fishing, whaling, tourism, and residential development. Very high densities of ground‐nesting pelagic birds may account for the high percentage of non‐native species on several small northern islands. Main conclusions Many of the principles of island biogeography that have been applied to oceanic islands apply equally to the 22 islands in this study. Native vascular plant species richness for these islands is strongly influenced by physiographic factors. Influence of two historical factors, years since deglaciation and years since isolation, cannot be assessed with the present sample size. Non‐native vascular plant species richness is influenced by island area as well as by human population density; human population density may be a surrogate for other anthropogenic impacts.     

Survival rate, abundance, and residency of long-finned pilot whales in the Strait of Gibraltar

Long-finned pilot whales in the Strait of Gibraltar are distributed over the main shipping routes. This exposes them to risks of collisions and probable acoustic and physical disturbance. This species is also the target of whale-watching operations. The aim of this study was to estimate the annual population size, survival rate, and population growth rate of pilot whales occurring in the Strait and their inter-annual variation using photo-identification. A robust design was used to estimate all three parameters. A total of 10,784 individual pilot whale fins were photographed and analyzed. The population size estimation in summer ranged from a low of 147 individuals in 1999 to a high of 265 individuals in 2003. The annual population growth rate was estimated from mark recapture models to be 5.5%. The survival rate of adults was estimated at 0.982 (95% CI: 0.955–0.993). The same individuals have been observed between years. This suggests that this population is resident in the Strait, at least during summer. This study provides baseline knowledge prior to a predicted increase in shipping traffic throughout the main foraging area due to the opening in 2007 of a major shipping harbor along the Moroccan coast of the Strait.     

Age and radial growth pattern of four tree species in a subtropical forest of China

Subtropical forests are usually composed of many tree species. Knowledge of the age and radial growth variation of the dominant tree species is useful for understanding forest dynamics and community structure and function. The aims of this study are to explore whether there are identifiable annual growth rings in the main tree species and to examine the growth characteristics within and among the species in Mount Gutian subtropical forest of China. The results showed that four out of eight tree species from which samples were collected had visible and cross-datable rings. There were no stable relationships between the age and diameter for these subtropical trees. Significant differences existed in radial growth rate within and among the four species, suggesting a high spatial heterogeneity in the mixed-species subtropical forest. The common pattern in age distribution of multiple species suggests a stand-wide disturbance occurring around the 1960s. It is interesting to note that the growth rate at the same age intervals was different for trees younger than 40 years of age and older than 40 years of age, suggesting a change in climate or forest structure in the two time periods. The results obtained from this study help understand the growth dynamics in other subtropical forests having these tree species.     

Continuously missing outer rings in woody plants at their distributional margins

Woody plants growing in seasonal climates normally form one growth ring each year. However, under severe stress conditions they may not complete annual wood production all the way down to the root collar resulting in continuously missing outer rings at lower stem sections (CMORs). Here we test whether CMORs occur at different distributional margins of woody plant species, where stress levels are expected to be high. We tested 13 species (202 individuals) of trees and shrubs growing at elevational and latitudinal shrub- and tree-lines, under conditions that are normally associated with a reduction of radial growth such as drought, cold or nutrient deficiency. Samples were collected from Alaska, north western Russia, Central Europe, Scandinavia and south eastern Spain. Annual radial growth was measured along several disks or cores of each individual applying the serial sectioning technique. Individuals of nine species showed CMORs. The proportion of individuals with CMORs within a sampled species and site ranged from 0% to 80%. The number of CMORs within an individual increased with decreasing sampling height on the stem. Significant correlations existed between the amount of CMORs and (i) cambial age, (ii) stem length, (iii) stem proportion/length below peat surface, and (iv) herbivore-caused defoliation intensity depending on the species and sampling location. Our results suggest that CMORs can be associated with a cambium that may be inactive, yet functional, for up to 18 years. We conclude that CMORs can occur in a wide variety of species, growth forms and biomes, suggesting a general strategy of woody plant growth. To avoid miss-dating in dendrochronological studies, we further recommend the use of the serial sectioning technique for woody plants growing in unfavorable environments, above all, when sample size is limited and when no established chronology for cross-dating is available.     

Variation in the relationship between numbers of breeding pairs and woodland area for passerines in fragmented habitat

Species may differ in the relationship between the numbers of breeding pairs present and woodland area, because the proportion of a wood that forms suitable habitat will vary with woodland size. In this paper, we examine the pattern of variation in abundance with woodland area for eight breeding bird species, and also show how this pattern varied between years. During 1990-1997, we made annual censuses of 53–160 woods, of up to 10 ha in size, and fitted a power function to describe the relationships between numbers of breeding pairs and woodland area. Seven of the eight species, blackbird Turdus merula , dunnock Prunella modularis , wren Troglodytes troglodytes , great tit Parus major , chaffinch Fringilla coelebs , robin Erithacus rubecula and blue tit Parus caeruleus showed a pattern of proportionally higher numbers in smaller woods. Only long-tailed tit Aegithalos caudatus occurred in proportionally higher numbers in larger woods. Blackbird and dunnock showed a trend towards lower numbers in large woods during years with low regional population levels; for these species large woods may provide sub-optimal habitat. Great tit, blue tit, chaffinch and robin showed the opposite trend, towards lower numbers in small woods during years with low regional population levels; for these species small woods may provide sub-optimal habitat. Wren and long-tailed tit, which also showed large annual population fluctuations, showed no change in distribution with regional population level. In great tit and chaffinch, the distribution of pairs in any one year may have been influenced by site fidelity producing a lag in the response associated with regional population levels.     

Presence of cushion plants increases community diversity in the high equatorial Andes

Cushion plants are a common growth form in the equatorial páramo vegetation and their surfaces are often colonized by other plants. This paper analyzes the effect of the cushion plants on the community diversity at 4650 m on the eastern slope of the Iliniza volcano in Ecuador. Ninety sample plots of 1 m² size were located in the study area and were divided into 25 subplots in which presence and abundance of plant species was recorded. The community diversity was expressed as species richness, Simpson's diversity index, and evenness. Correlation between the cushion species and the composition of the colonists was measured with the CCA ordination analysis, correlation between the cushion cover and community diversity was measured by means of correlation analysis. Randomized species–area curves were used to compare richness of plant communities with and without the cushions. A total of 32 species were found including five cushion plants. Most species preferred to grow on the cushion surface whereas only a few species were able to colonize open ground. Species richness and Simpson's index were significantly correlated to the cushion area but no correlation was found for evenness. The cushions were usually composed of more than one species which hampered the examination of the cushion–colonist specific relationships. Nevertheless, cushions of Azorella and Arenaria seemed to provide more favorable habitat for colonization than the other cushion species. Comparison with an earlier study made on Iliniza indicates that the presence of the cushions significantly increases the richness of the plant community.     

Introduction of the European minnow into a subalpine lake: habitat use and long-term changes in population dynamics

Minnows Phoxinus phoxinus , studied 30 years after the first record of the species in the subalpine Lake Øvre Heimdalsvatn, Norway, ≥55 mm L _T, were estimated to have densities of c . 4.7 kg ha⁻¹ (120 000 fish) in June 1999 and 2.1 kg ha⁻¹ (63 000 fish) in June 2000. The population was characterized by low individual growth, delayed age of maturity and lived longer when compared to values reported in a previous study in the early phase of its establishment, and other values reported in the literature. Most minnows reached sexual maturity at 4–5 years and >55 mm L _T. Although the estimated annual survival of minnows >55 mm was low ( S =0.2), ages up to 13 years were recorded. Despite a moderate increase in the population size during the last 20 years, the present reduction in individual growth, followed by delayed age of maturity, suggested the existence of density-dependent effects on the population dynamics of the minnows. The minnows were restricted to the littoral zone and near bottom areas. A vertical or horizontal expansion in habitat use was probably prevented by the presence of piscivorous brown trout Salmo trutta .     

Longleaf pine cone–radial growth relationships in the southeastern U.S.A.

Longleaf pine (Pinus palustris Mill.) cones have been counted annually by the United States Forest Service (USFS) at eleven locations throughout the species’ range since 1958. These data have been useful for understanding spatiotemporal patterns in longleaf pine cone production, and are beneficial in timing regeneration efforts. Variations in annual mast (i.e. seed crop) are known to influence ring widths in numerous tree species, yet this relationship is poorly understood for longleaf pine. This research examines the relationship between longleaf pine cone data and tree-ring growth from trees sampled in the multi-decadal USFS cone-crop study. We examined cone–radial growth relationships using individual tree-ring data and proprietary cone data for each tree from six sites in four locations in the southeastern USA. We found that longleaf pine cones were correlated with basal area increment growth (BAI) over the three-year cone-development cycle. Low BAI years were more frequently associated with above-average cone crop and BAI during years that coincided with the largest cone-crop class (bumper, > 100 cones per tree) were statistically less than any other cone class. We prepared linear models that predicted radial growth using PDSI and cones as predictors, and found that including cones in the models did not improve adjusted R² values. We conclude that while cone production is inversely related to radial growth, the combination of infrequent bumper years and the concentration of cone production by a few trees per stand, creates an environment where radial-growth chronologies assembled from longleaf pine for dendroclimatic purposes are unlikely to be significantly influenced by reproductive strain.     

Recent cooling and recession of Norway spruce (Picea abies (L.) Karst.) in the forest—alpine tundra ecotone of the Swedish Scandes

Abstract. Repeated crown condition surveys, 1974–94, of subalpine clonal groups of Norway spruce ( Picea abies (L.) Karst.) were carried out in the Swedish Scandes. Complementary analyses concerned radial and vertical growth, sexual regeneration and range limit responses of other plant species. Significant defoliation of spruce progressed linearly over the period of study, reaching cumulative values of about 85%. It is inferred that defoliation was preconditioned by decreasing radial growth since the thermal climax in 1937 and was proximately initiated by the extremely cold winter of 1965/66 and paralleled by consistently declining radial growth and staggering vertical increase. It appears that severe and prolonged ground freezing invoked winter desiccation (xylem cavitation), extensive needle loss and reduced radial growth. Hypothetically, from circumstantial evidence, these processes are interrelated in a positive feedback system, implying increasing sensitivity to climatic stress and decreasing ability to take advantage of positive climatic anomalies. Thus, the total demise of the supranival stems is cautiously predicted, by linear regression of the 20-yr defoliation pattern, to be less than a decade ahead. The recession of P. abies , clearly relevant in a landscape perspective, conforms with analogous responses of Pinus sylvestris L. and Betula pubescens Ehrh. ssp. tortuosa (Ledeb.) Nyman and a significant altitudinal range-limit retraction of certain silvine field-layer species. The structural development examined in this study concurs with long-term climate cooling and cold events and strongly contrasts with simulations of the performance of this system in response to a putative enhanced'greenhouse'effect.     

Age and growth rate determinations of an intertidal bivalve, Phacosoma japonicum, using internal shell increments

A venerid bivalve Phacosoma japonicum (Reeve) occurring commonly in the Japanese coastal area preserves periodic growth lines in the shell cross-section. Long-term shell growth patterns of this species have been traced for many individuals on the intertidal flat of the Seto Inland Sea, west Japan. Sclerochronological analysis of these individuals and specimens collected monthly shows that several growth cessation marks within their shells are formed during the winter of each year prior to spawning. Hence the marks were used for age and growth rate determinations. As large individuals showed little shell growth for more than two years after the formation of 7 or 8 annual increments, this species probably has a lifespan of more than ten years. Shell growth patterns of this species based on annual increments can be accurately approximated by a von Bertalanffy curve. The number of microgrowth increments formed during a year tends to decrease with age, although it varies markedly among specimens of the same age. Furthermore, even in summer during rapid shell growth, the microgrowth increments do not represent daily and/or sub-daily tidal rhythms in many specimens. The results of this study and those by several authors strongly suggest that the annual increments are the key for age and growth rate determinations of both living and fossil bivalve species.     

Attributes of the seed bank after a century of primary succession on islands in Lake Hjälmaren, Sweden

1 A large number of islands was created when the water table of Lake Hjälmaren, south central Sweden, was lowered between 1882 and 1886. We have complete lists of vascular plant species for 40 of these islands from 1886, 1892, 1903–04, 1927–28 and 1984–85.
2 We have investigated the seed bank on nine of these islands and compared species composition at different soil depths with the species lists from the islands in 1886–1985, and with the present vegetation in the area of seed bank sampling. We have also investigated the distribution in the soil of seeds from species with different ecological attributes, including seed longevity, successional status, seed weight, seed form and species longevity.
3 Seeds in soil samples were allowed to germinate over the course of two summers with an intermediate cold storage. We found 1944 seeds representing 65 taxa. The mean seed density was 84 seeds dm ^–2 .
4 The similarity between the surface soil (0–3 cm) seed bank and the vegetation at the different vegetation analyses increased from 1886 to 1993. The similarity between the present vegetation and the seed bank decreased with increasing soil depth, and the soil at 12–15 cm had no species in common with the present vegetation. Several species now absent from the vegetation were found in the seed bank.
5 Deeply buried seeds came from early successional, annual species with long-term persistent and low-weight seeds, as expected from seed bank theories, but were slightly elongated, which was in contrast to theories. Spherical seeds were associated with the surface soil, as were short-lived and high-weight seeds from late successional, perennial species.     

Moss cushions facilitate water and nutrient supply for plant species on bare limestone pavements

Dense moss cushions of different size are distributed across the bare limestone pavements on ?land, SE Sweden. Increasing cushion size is predicted to physically protect and improve performance and colonization by vascular plants. Therefore, we tested water balance, phosphorus supply, and species richness, and evaluated duration of plant activity during desiccation as a function of ground area, for a large collection of moss cushions. We found that lower evaporation and higher water storage contributed equally to extending the desiccation period with increasing cushion size. Evaporation rates declined by the -0.36 power of cushion diameter, and were not significantly different from -0.50 for the square root function previously predicted for the increasing thickness of the boundary layer, with greater linear dimensions for smooth flat objects at low wind velocities. Size dependence vanished under stagnant conditions. One moss species was added to the species pool for every nine-fold increase in cushion area. Vascular plants were absent from the smallest cushions, whereas one or two species, on average, appeared in 375- and 8,500-cm(2) cushions with water available for 6 and 10?days during desiccation. Phosphorus concentrations increased stepwise and four-fold from detritus to surface mosses and to vascular plants, and all three pools increased with cushion size. We conclude that cushion mosses and cushion size play a critical role in this resource-limited limestone environment by offering an oasis of improved water and nutrient supply to colonization and growth of plants.     

The phenology of Ophrys sphegodes (the early spider orchid): what annual censuses can miss

Ophrys sphegodes is a rare species in the United Kingdom. The largest extant population of the species, at Castle Hill National Nature Reserve in Sussex, has been monitored each year since 1975. At each annual census, which is conducted during the peak period of flowering (May), the co-ordinates of each plant are recorded, allowing the fate of individuals to be followed from one census to the next. In the present study the population was monitored throughout one complete year. The data collected allow the phenology of the species to be described and the effectiveness of the annual census to be assessed. The study shows that a substantial number of Ophrys sphegodes plants emerge above ground and re-enter the below ground phase before the census date. These results suggest that reliance on annual census records alone may result in the size of the population being underestimated and incorrect life-histories being ascribed to individual plants. True counts of the population may be obtained only if 'dormant' plants are retrospectively added to the population.     

Recovery of Tawa-dominated forest fragments in the Rotorua Basin, New Zealand, after cessation of livestock grazing

Summary   Tawa ( Beilschmiedia tawa )-dominated forest fragments on farms within the Rotorua Basin were surveyed to quantify the likely recovery processes following exclusion of domestic livestock grazing, using a space-for-time substitution approach. Vegetation structure, plant diversity and soil fertility were measured at 24 sites within 15 forest fragments on six farms, covering a range in time since exclusion from grazing of 1–53 years. The forest fragments were compared with a large area of ungrazed forest in the nearby Lake Okataina Scenic Reserve. As time since exclusion from grazing increased, indigenous plant species diversity increased (up to 30–35 years); ground fern and epiphyte abundance increased (up to 30–35 years); tree seedling and sapling numbers, and litter cover also increased (up to 10–15 years); and overall tree numbers increased, while average tree diameter at breast height and overall tree basal area did not differ significantly. The soil fertility status was highly variable, obscuring clear patterns, although Olsen P status decreased with time since grazing exclusion. Once grazing of forest fragments ceases, significant changes in their diversity, structure and soil characteristics can be expected, which indicate recovery of these plant communities towards the conditions observed in ungrazed forest.     

Response of tree-ring width to climate warming and selective logging in larch forests of the Mongolian Altai

Aims The Mongolian Altai is an old settlement area, which is populated by pastoral nomads since 2000–3000 years. Forests in this region (at ca. 2300 m a.s.l.) are highly fragmented and border on steppe and alpine grasslands, which are used for mobile livestock husbandry. The climate in Central Asia is warming to levels clearly above the global average, which affects the vegetation. Furthermore, the transition from planned to market economy and the decollectivization of livestock 20 years ago has strongly changed land use practices in Mongolia, especially resulting in an increase in recent logging activities. We were interested in the question how climate warming and selective logging influence the annual stem growth and the stand structure.Methods The impact of climate and land use by the pastoral nomads on the annual stem increment of more than 1800 trees of Siberian larch (Larix sibirica) was analyzed. Different groups of trees with divergent growth trends depending on the social position and stand history were identified by non-metric multidimensional scaling and analysis of similarities. Long-term trends in the annual stem increment were analyzed by establishing separate regional growth curves for trees of different age classes.Important findings Instrumental climate data substantiate an increase of temperature by 2.1°C since 1940 at constant precipitation. Trees benefit from the increased temperatures. Climate–response analysis revealed that radial stem increment was promoted by the temperature in early summer, but also high precipitation in spring and in the year before tree-ring formation. Forest dynamics is also strongly influenced by anthropogenic activities. In addition to the natural forest dynamics, logging resulted in divergent growth trends within given age classes and habitats (forest interior and forest edge); overall, 22 groups of trees with different characteristics in the annual radial stem increment were identified. A tree-ring series-based reconstruction of logging intensity since 1935 suggests that moderate selective logging occurred throughout the study period. However, selective logging was strongly intensified after 1990 as the result of the breakdown of the Communist regime in Mongolia and the transition from centrally planned to market economy. Because tree stump densities showed that the ratio of felled to live trees was 2:1 in the interior or even 0.9:1 at the edges of the forests and most logging occurred during the past 20 years, it must be concluded that the forests of the Mongolian Altai are presently exploited far beyond the level of sustainability. A close correlation of the ratio of felled to live trees with the density of summer camps of pastoral nomads in the vicinity suggests that trees are primarily felled by the local population.