Another perspective on altitudinal limits of alpine timberlines

Recent hypotheses of timberline causation include the possibility that limitations to growth processes may be more limiting than restrictions on photosynthetic carbon gain, and that cold soil is a primary limiting factor at high altitude. However, almost all of the supporting data for timberline causation have come from studies on older trees, with little focus on the mechanisms of seedling establishment and the growth of saplings away from the forest edge into the treeline ecotone. We describe a conceptual model of timberline migration that invokes a strong dependence on ecological facilitation, beginning with seed germination and continuing through seedling establishment and sapling growth to the stage where trees with forest-like stature form new subalpine forest at a higher altitude. In addition to protection from severe mechanical damage, facilitation of photosynthetic carbon gain and carbon processing is enhanced by plasticity in plant form and microsite preference, enabling seedling survival and sapling growth inside and through the often severe boundary layer just above the ground cover. Several forms of facilitation (inanimate, interspecific, intraspecific and structural) result in substantial increases in photosynthetic carbon gain throughout the summer growth period, leading to enhanced root growth, subsequent amelioration of drought stress, and increased seedling survival. Avoidance of low temperatures and low-temperature photoinhibition of photosynthesis may be major benefits of the facilitation, enhancing photosynthetic carbon gain and respiratory-driven growth processes. We propose that the growth of vertical stems (flagged tree forms) from krummholz mats is analogous functionally to the facilitated growth of a seedling/sapling in and away from ground cover. Increasing abundance and growth of newly established trees in the treeline ecotone generates a structural and microsite facilitation characteristic of the subalpine forest below. This is followed by the formation of new subalpine forest with forest-like trees, and a new timberline at higher altitude.     

Status and Regeneration of Himalayan Maple in the Himachal Pradesh: Honing Red List of Plants

The population status and regeneration of a data-deficient tree species were examined in both protected and unprotected areas. The target species was found in six habitat types, seven aspects between 2,100- to 3,300-m altitude, and sites dominated by different species. The species was best represented in a shady moist habitat, north and northwest aspects, altitudinal range >2,700 m, and sites dominated by Acer caesium in both protected and unprotected sites; but the generalized linear model indicated nonsignificant effect of sites, habitat, altitude, and dominance except for aspects. Difference in sapling and seedling density among aspects and sites only was significant. Probability of use and resource use index indicated poor preference and less pressure as fuel. It can be concluded that the species is not under any significant threat in the study area. But, further studies on population status, regeneration, and relationship with environmental factors in a wider geographic region are suggested to evaluate species and place under suitable priority category.     

Effects of anthropogenic disturbances on natural regeneration and population structure of gum arabic tree(Acacia senegal)in the woodlands of Lake Baringo ecosystem,Kenya

Despite the ecological and economic importance of Acacia senegal, little is known about the effects of anthropogenic disturbances on its natural regeneration patterns and population structure. We investigated the effects of these factors within the Lake Baringo woodland ecosystem. Data was collected from 60 plots of 20 m 9 20 m systematically distributed in four A. senegal-dominated populations within the Lake Baringo woodland. Sample populations spanned a degradation gradient measured by a population disturbance index(PDI). Trees were measured for diameter at breast height(DBH) and categorized by growth stages: seedling, sapling and adult tree. Higher seedling and sapling densities were recorded in lightly than heavily disturbed populations, but only sapling density was significantly different between the two disturbance levels(P = 0.02). Lightly disturbed populations revealed a reversed J-shape size-class distribution(SCD) indicative of stable structure unlike the heavily disturbed populations.The quotient and permutation indices indicated unstable populations with episodic recruitment and mortality.Our study reveals that natural regeneration and population structure of A. senegal were affected majorly by selective harvesting and heavy browsing. Suitable management strategies to control livestock grazing and illegal tree harvesting within the woodland is required to promote conservation of the species genetic resources     

Spatial variation in tree seedling density after the site preparation for planting in a cleared coniferous plantation in Hokkaido,northern Japan

To study spatial variation in natural tree seedling density and the relationship between variation in seedling density and seed dispersal mode at a cleared site, we surveyed natural tree seedlings after the site preparation for planting in a coniferous plantation cleared by a typhoon disturbance in 2004. The site was located near Sikotsuko Lake, Hokkaido, northern Japan. Twenty-five tree seedling species were found and the mean seedling density was 9.8 seedlings/m². Seedlings of non-animal-dispersed species (7.2 seedlings/m²) were approximately five times more abundant than those of animal-dispersed species (1.4 seedlings/m²), and 87% of all seedlings were current. The seedling density of non-animal-dispersed species (almost all wind-dispersed) decreased with distance from a natural forest; however, this trend did not hold for animal-dispersed species (almost all bird-dispersed). The spatial variation in seedling density was lower in animal-dispersed species than in non-animal-dispersed species. Seed dispersal patterns and the viability duration of buried seeds likely affected the variation in seedling density by seed dispersal mode.     

Tree regeneration in gap-understory mosaics in a subtropical Shorea robusta(Sal) forest

Spatial variation in tree-regeneration density is attributed to the specialization of tree species to light availability for germination and growth. Light availability,in turn, varies across the gap-understorey mosaic. Canopy gaps provide an important habitat for the regeneration of tree species that would otherwise be suppressed in the understory. In subtropical forests, there is still a knowledge-gap relating to how canopy disturbances influence tree-regeneration patterns at local scale, and if they disproportionately favor regeneration of certain species. We aim to analyze whether canopy gaps promote tree regeneration, and tree species are specialized to gaps or understory for germination and growth. We sampled vegetation in 128 plots(0.01 ha), equally distributed in gaps and below canopy, in two subtropical Shorea robusta Gaertn.(Sal) forests in Nepal, recording the number of tree seedlings and saplings in each plot. We compared the regeneration density of seedlings and saplings separately between gaps and the understorey. The mean densities of seedlings and saplings were higher in the gaps at both sites;although there was no difference in the seedling density of the majority of the species between the habitats. No species were confined to either gap or understorey at the seedling stage. We conclude that gaps are not critical for the germination of tree species in Sal forests but these are an important habitat for enabling seedlings to survive into saplings. The classification of trees into regeneration guilds mainly based on germination does not apply to the majority of tree species in subtropical Sal forests. Our results reaffirm that gap creation promotes tree regeneration by favouring seedling survival and growth and can influence forest management for conservation, as well as for plantations.     

The effects of selective logging on stand structure and the regeneration of subboreal forests in Hokkaido, northern Japan

We investigated the effects of selective logging on stand structure and regeneration in selectively logged subboreal forests in Taisetsuzan National Park in Hokkaido in northern Japan. The basal area decreased and the size structure of trees altered in the stands studied due to repeated, intense selective logging, in which larger trees were cut down as a priority. Sapling density in the stands was much lower than that in primary forests. In the simple and multiple regression analyses that were used to estimate the effects of selective logging on sapling density, sapling density had a significant positive correlation with tree density and had little correlation with the density of logged stumps or the height ofSasa (dwarf bamboo) growing on the forest floor. These results suggest that the establishment sites around canopy trees influenced the establishment of saplings, rather than the gaps caused by selective logging. However, both the coefficient of determination and the standardized partial regression coefficient of multiple regression analysis were higher for the stand with a dense cover ofSasa than for the stand with a sparse cover ofSasa. Thus, the success of regenerating forests with selective logging depends on both the site of advanced regeneration and the light conditions that regulate growth.     

Effects of anthropogenic disturbances on natural regeneration and population structure of gum arabic tree (<Emphasis Type="Italic">Acacia senegal</Emphasis>) in the woodlands of Lake Baringo ecosystem,Kenya

Despite the ecological and economic importance of Acacia senegal, little is known about the effects of anthropogenic disturbances on its natural regeneration patterns and population structure. We investigated the effects of these factors within the Lake Baringo woodland ecosystem. Data was collected from 60 plots of 20 m × 20 m systematically distributed in four A. senegal-dominated populations within the Lake Baringo woodland. Sample populations spanned a degradation gradient measured by a population disturbance index (PDI). Trees were measured for diameter at breast height (DBH) and categorized by growth stages: seedling, sapling and adult tree. Higher seedling and sapling densities were recorded in lightly than heavily disturbed populations, but only sapling density was significantly different between the two disturbance levels (P = 0.02). Lightly disturbed populations revealed a reversed J-shape size-class distribution (SCD) indicative of stable structure unlike the heavily disturbed populations. The quotient and permutation indices indicated unstable populations with episodic recruitment and mortality. Our study reveals that natural regeneration and population structure of A. senegal were affected majorly by selective harvesting and heavy browsing. Suitable management strategies to control livestock grazing and illegal tree harvesting within the woodland is required to promote conservation of the species genetic resources     

Influence of local stand basal area on density and growth of regeneration in uneven‐aged Picea abies stands

In single‐tree selection, trees removed by harvest or lost through mortality are replaced by ingrowth from the seedling/sapling bank. Because the level of ingrowth is governed not only by the recruitment rate of new seedlings, but also by mortality and growth rates within the seedling/sapling stratum, knowledge of how these processes are related to the tree stratum is important for successful application of single‐tree selection. Therefore, Norway spruce (Picea abies (L.) Karst.) regeneration (0.1 m ≤ height ≤ 2.0 m) was measured on seventy 100 m² circular subplots at each of two sites in central and northern Sweden. Both sites had previously been selectively logged, but the time elapsed since the last harvest was at least 30 yrs. Basal area of trees of at least 2 m height within three different radii from the subplot centres was measured. Measurements on regeneration included height and leader length. Influence of local stand basal area on density and growth of regeneration was analysed by ordinary least‐squares linear regression for each of four height intervals. Regeneration was significantly aggregated at both sites, but number of Norway spruce seedlings and saplings per circular subplot was not correlated to local stand basal area. Out of 36 regressions, only eight showed a significantly (p < 0.05) negative correlation between seedling and sapling height growth and local stand basal area. In five of these cases, removal of one point (subplot) made p > 0.05. The results suggest that density and growth of Norway spruce regeneration in selectively logged uneven‐aged Norway spruce stands is affected more by ground conditions than by local stand basal area.     

浙江建德青冈常绿阔叶林种群结构和动态的研究

青冈林是我国东部亚热带常绿阔叶林的最主要的类型之一。本文应用植物种群生态学的研究方法,对浙江建德山区青冈林中主要种群的大小结构和分布格局进行了研究,探讨了其动态过程及其机制。结果表明谝我青冈林中的优势种青冈,石栎的种群结构呈现不规则的倒Ｊ字型,为间歇型种群。     

Factors influencing germination and establishment of Eucalyptus pauciflora near the alpine tree line

Factors affecting seed germination, seedling establishment and growth to reproductive maturity in Eucalyptus pauciflora Sieb. ex Spreng. at the tree line in southeastern Australia were investigated. Timing of germination was determined principally by temperature but could be strongly modified by soil water. Mortality of germinants was heaviest during the first growing season (dependent on soil water) and the first winter (dependent on the depth and duration of snow cover). Neither plant nor soil treatments with insecticide or fungicide had an overriding or consistent effect on germination or seedling establishment. The probability of reaching the sapling stage was greater for planted seedlings than for plants germinated on site, indicating the dependence of survival on accumulated reserves. There was no clear indication of a general decrease in survival with increasing altitude, because of an interplay between elevation and site factors including competing vegetation, snow depth and period of snow cover. Five-year-old saplings planted up to 200 m above the tree line produced reproductive structures and viable seed between the ages of 13 and 18 years. The relevance of the results to an understanding of the location of the present tree line is discussed.     

Abiotic factors limiting photosynthesis in Abies lasiocarpa and Picea engelmannii seedlings below and above the alpine timberline

Most research on the occurrence and stability of alpine timberlines has focused on correlations between adult tree growth and mean temperatures rather than on specific mechanisms. Timberline migration to higher altitude is dependent on new seedling establishment in the tree-line ecotone; however, reductions in photosynthetic carbon gain in establishing seedlings have previously been interpreted solely in terms of decreased seedling survival. Our objective was to evaluate the impact of abiotic factors (temperature, light and water) on photosynthetic carbon gain in young seedlings of the two dominant conifer tree species occurring naturally above (tree-line ecotone site, TS) and below (forest site, FS) a Rocky Mountain timberline in southeastern Wyoming, USA. Coincidentally, measurements were made during an unusually dry summer. Mean daily photosynthesis in seedlings of both Abies lasiocarpa (Hook.) Nutt. (subalpine fir) and Picea engelmannii Parry ex Engelm. (Engelmann spruce) was less at TS than at FS (19 and 29%, respectively). Minimum nighttime temperatures below 2 degrees C were more frequent at TS than at FS and were associated with reduced maximum photosynthesis the following day. Low midday water potentials were associated with a reduction in carbon gain at both sites early in the season, prior to snowmelt, as well as late in the season when soils began to dry. However, the lower photosynthetic rates at TS than at FS appeared to be unrelated to seedling water status because seedlings at both sites had similar xylem pressure potentials. Solar irradiance was highly variable at both sites as a result of uneven shading by neighboring trees, although this variation was substantially reduced on cloudy days (44% of all days observed). Compared with sunny days, cloudy days resulted in greater integrated daily carbon gain at both sites (41% increase at TS and 69% increase at FS), based on a simulated photosynthesis model. Photosynthetic responses to temperature, sunlight and water suggest that variable solar irradiance and nighttime temperatures were major abiotic factors limiting photosynthetic carbon acquisition in these young seedlings, especially for seedlings growing in the tree-line ecotone.     

杭州西湖山区青冈种群结构和动态的研究

本文对杭州西湖山区不同生境中的青冈（Ｃｙｃｌｏｂａｌａｎｏｐｓｉｓ ｇｌａｕｃａ）种数的结构和动态进行了研究。结果表明：（１）各样地青冈种群呈现不规则的倒Ｊ型种群结构,存在明显的低龄缺损,为间歇型的种群,但种群仍将长期维持相对稳定;（２）种群分布格局呈一定的规律性,在发育初期为集群分布,而在中后期则转变为随机分布;（３）青冈种群可通过种子和无性系进行更新,在不同生境中,其更新的方式不同。     

Deadwood volume in strictly protected,natural, and primeval forests in Poland

Standing and downed deadwood at different stages of decay provides a crucial habitat for a wide range of organisms. It is particularly abundant in unmanaged forests, such as strictly protected areas of national parks and nature reserves. The present work used the available data for such sites in Poland, analyzing a total of 113 studies concerning 79 sites to determine the causes contributing to variation in deadwood volume based on the duration of conservation, changes in deadwood volume over time (for those sites which were examined multiple times), elevation above sea level, forest type, stage of forest development, input of dead trees from the years preceding deadwood measurements, live tree volume, and the proportion of downed to standing deadwood). Depending on species composition and site altitude, most tree stands fell into one of four categories: subalpine spruce forests, montane beech-fir forests, low altitude beech-fir forests, or oak-hornbeam and riparian forests. The mean deadwood volume for all forest types amounted to 172.0 m³/ha. The mean volume of deadwood in montane beech-fir forests (223.9 m³/ha) was statistically significantly greater than in the other three forest types, for which it ranged from 103.5 to 142.5 m³/ha. A direct effect of the duration of conservation on deadwood volume was not identified. Nevertheless, analysis of repeated measurements on the same sample plots at 10-year intervals showed a consistent rise in mean deadwood volume. A linear regression model for all the analyzed factors reported from montane beech-fir forests and subalpine spruce forests showed that in addition to site altitude, another statistically significant variable was the input of dead trees (R²?=?63.54%).     

Altitudinal differences in bark stripping by sika deer in the subalpine coniferous forest of Mt. Fuji

Deer expansion is a growing concern for forest ecosystem management. In Japan, upward expansion to subalpine and alpine areas has reached alarming proportions in recent years. We examined bark stripping by sika deer along an altitudinal gradient in the subalpine coniferous forest at three altitude ranges (1800-2000 m, 2000-2200 m, and 2200-2400 m) on the southern slope of Mt. Fuji. We tested differences in densities and diameter at breast height (DBH) of trees and those with bark-stripped stems of all tree species among the three altitude ranges. Then, we compared the relative densities of deer, based on pellet counts, to determine the impact of deer in relation to deer use and forest stand patterns across the altitudinal range studied. The results of the study show that differences in bark stripping by sika deer depended largely on the elevation and the species. Larger stems were sparsely distributed in the lowest elevation zone between 1800 and 2000 m. The relative density of deer was highest in the areas exhibiting high bark-stripping intensity on small regenerating trees of the dominant coniferous species, Abies veitchii, and on broadleaf species. In the highest elevation zone between 2200 and 2400 m, smaller stems were densely distributed, and the relative deer density was lowest where the bark-stripping intensity on small stems was lower for all three species studied. The damage to subalpine tree stems corresponded to the availability of palatable tree species with a small diameter, as reflected by the successional stage along the elevational gradient of subalpine forest on Mt. Fuji. These results suggest that the continuous impact of bark stripping on the dominant tree species might cause severe changes in forest succession.     

Age structure and regeneration of subalpine fir (Abies fargesii) forests across an altitudinal range in the Qinling Mountains,China

Age structure and regeneration dynamics of subalpine fir (Abies fargesii) forest were studied across the altitudinal range in both the north and south aspects of the Qinling Mountains, China. Ages of individual fir trees were determined based on the number of rings counted from cores and the number of years to reach coring height estimated using age–height regression. Fir age structure and regeneration dynamics were similar in both the north and south aspects. A unimodal population age structure was found at the low- and mid-elevations in both aspects, indicating that environmental factors might play an important role in shaping A. fargesii age structure and regeneration at those sites. There was a recruitment pulse during the time period 1830–1890 at each altitudinal site, but no stem recruitment occurred at the low- and mid-elevations in the last century, which might be attributed to the intensive cover of understory bamboo. Fir trees were, however, persistently recruited at the upper limits during the last 150 years, and the fir tree density at the upper limits was significantly higher than that at the lower limits in both aspects. The fir population at the upper limits showed a significant increase in recruitment and stem density relative to the fir population at the low- and mid-elevations in the last century. We propose that the differences in recruitment might promote variations in stand structure and regeneration dynamics of the subalpine fir forests along the altitudinal gradient in the Qinling Mountains, China.     

Structure and regeneration status of Komto Afromontane moist forest, East Wollega Zone, west Ethiopia

We conducted a study in Komto Forest in East Wollega Zone, Oromia National Regional State, West Ethiopia for determining vegetation structure and regeneration status in this forest. We systematically sampled 53 quadrats (20 m × 20 m) along line transects radiating from the peak of Komto Mountain in eight directions. Vegetation parameters such as DBH, height, seedling and sapling density of woody species, and location and altitude of each quadrat were recorded. In total, 103 woody plant species of 87 genera and 45 families were identified. Analysis of selected tree species revealed different population structures. Generally, the forest was dominated by small trees and shrubs characteristic of secondary regeneration. Observations on the regeneration of the forest indicated that there are woody species that require urgent conservation measures. Based on the results of this study, we recommend detailed ecological studies of various environmental factors such as soil type and properties, and ethnobotanical studies to explore indigenous knowledge on uses of plants.     

天山云杉种群结构的研究

种群是构成群落的基本单位,种群结构不仅对群落结构有直接影响,而且能表征群落的发展和演变趋势[1].种群结构包括径级结构、高度结构、年龄结构等[2].年龄结构是种群的重要特征,但由于年龄的数据在许多情况下非常难于得到,因此许多学者在进行种群结构的研究过程中,都采用了大小结构分析法[3-5].无论是基于年龄结构还是大小结构的种群结构分析,都经常被用来重建森林发展的历史,提供有关潜在种群长期稳定性的信息,并且预测其演替趋势[6-10].森林植物种群结构在一定程度上是由自然干扰历史决定的.     

小陇山林区漆树群落结构的初步研究

采用典型抽样的方法,在秦岭西段小陇山林区天然漆树群落中设置22块样地,进行了漆树群落结构调查,以样地调查数据分析了小陇山林区漆树的群落学特征、高度级结构和径级结构及其动态变化;结果表明:秦岭西段小陇山林区天然漆树群落中,漆树的伴生树种较多,种群趋于正金字塔型,种群处于缓慢增长阶段;从高度级和大小看,群落内幼苗幼树较多,大径级个体较少,促进群落进一步稳定增长的措施应该是限制采漆,加强保护。     

Influences of Mechanized Thinning and Prescribed Fire on Natural Regeneration in an Uneven-Aged Jeffrey Pine Stand

Thinnings using cut-to-length or whole-tree harvesting systems followed by underburning were evaluated for their effects on seedling and sapling demography in a pure, uneven-aged Jeffrey pine (Pinus jeffreyi Grev. & Balf.) stand containing a minor component of California white fir (Abies concolor var. lowiana [Gord.] Lemm.). Depression of seedling counts due to forest floor disturbance associated with thinning was followed by a recovery largely confined to Jeffrey pine in the whole-tree treatment where final seedling counts exceeded those found initially. The postburn substrate was more favorable for establishment of Jeffrey pine than white fir seedlings, and the largest increase in seedling counts between the initial and final inventories occurred in the burned portion of the whole-tree treatment. Live sapling losses from thinning were greatest in the cut-to-length treatment, while underburning induced complete sapling mortality. Absent treatment, several stand and site variables influenced seedling and sapling abundance, prominent among them a propensity for mahala mat (Ceanothus prostratus Benth.) to elevate counts of white fir within both size classes. These results provide land managers insight into the impacts of six combinations of thinning and burning treatment on natural regeneration in eastern Sierra Nevada Jeffrey pine and similar dry site forest types.