An increase in the upper tree-limit of silver fir (Abiesalba Mill.) in the Alps since the mid-20th century: A land-use change phenomenon

Global environmental changes observed during recent decades are likely to have had an impact on the distribution of species. Currently, silver fir (Abies alba) is becoming established in the subalpine forests of the west central Alps at elevations higher than 2000 m a.s.l.; prior to the 1970s its upper altitudinal limit was 2000 m. Several hypotheses could explain this recent expansion of the upper tree-limit. Silver fir regeneration could be linked (1) to land-use changes or (2) to current climatic warming. Using dendrochronology, the age structure of 31 forest plots containing at least one silver fir was examined in order to elucidate the population dynamics of subalpine communities. This allowed us to decipher the timing of fir regeneration in relation to the mean age of the stands examined and of the other tree-canopy species present. The majority of the firs germinated sporadically since 1950, before the regional temperature increase. The pattern of fir recruitment did not appear to relate to altitude, but followed a pattern characteristic of secondary succession. The age structures identified showed an exponential increase in tree-density during the 20th century; the stands were first dominated by Larix decidua during the 18th and 19th centuries, and then by Pinus cembra during the 20th century. In most stands, fir regeneration occurred after Larix and before P. cembra dominated, following a similar pattern to Picea abies regeneration. The number of local inhabitants and temperature both exhibited a negative relationship with fir tree recruitment, thus supporting the land-use change hypothesis. There has been a significant upward shift of the altitudinal range of fir, amounting to an increase of about 300 m since 1950. This followed the abandonment of low-productivity land. This trend is likely to continue during the 21st century, because of new agricultural and forestry practices which involve limited intervention in low-productivity areas and may be because of the effects of global warming.     

Gap-, stand-, and landscape-scale factors contribute to poor sugar maple regeneration after timber harvest

Natural regeneration in canopy gaps is a key process affecting long-term dynamics of many forests, including northern hardwood forests. The density and composition of regenerating trees are often highly variable, reflecting sensitivity to a suite of driving factors operating at different scales (e.g., harvest gap to regional landscape), including production of seeds, physical characteristics of gaps and stands, competition with non-tree vegetation, and browsing by animals. Multivariate analyses over broad geographic areas provide insights into the relative effects of these factors and permit exploration of spatial patterns in regeneration. We examined the effects of gap-, stand-, and landscape-scale factors on densities of tree seedlings (<1 m tall) and saplings (1-2 m tall) in 59 selection-harvested northern hardwood stands located across a 4500 km² region of Michigan's Upper Peninsula. We used Bayesian multilevel modeling to account for the hierarchical structure of the data and assess uncertainty in parameter estimates. Sugar maple (Acer saccharum) saplings were absent from 61% of 154 m² plots centered in harvest gaps (n = 347) despite its high shade tolerance and overstory dominance, but densities were high in other gaps. Densities of sugar maple seedlings and/or saplings were negatively associated with a combination of greater stand-scale densities of white-tailed deer (Odocoileus virginianus), greater gap-scale cover of non-tree vegetation, and lower gap-scale light availability, with deer density having the greatest effect. Densities of unpalatable and commercially less valuable ironwood (Ostrya virginiana), the second most common regeneration species, were positively related to gap-scale seed-production potential but were unrelated to factors affecting sugar maple. Ironwood tended to replace sugar maple saplings in areas with high deer density. At the landscape scale, densities of sugar maple seedlings and saplings decreased with decreasing latitude and snow depth and increasing winter deer densities. These inverse spatial patterns suggest that deer herbivory can lead to landscape-scale variation in regeneration success. However, the spatial distribution of habitat types (a proxy for soil moisture and nutrient conditions) confound this observation, with higher densities of sugar maple generally located on stands with less nutrient-rich habitat types. Results demonstrate that combinations of factors operating at different scales, and with different relative magnitudes of impact, contribute to high variation in regeneration composition and density following timber harvest. Selection silvicultural practices, as currently applied, do not ensure regeneration of desirable species; practices might require modifications in general (e.g., increasing gap size) and to match them to regionally varying factors like deer density.     

Stand development of Norway spruce dominated subalpine forests of the Swiss Alps

In densely populated regions, forests can help protect communities and infrastructures from natural hazards such as avalanches and rockfall. To promote the protective function, substantial efforts are made to actively manage forest stands. In 2009 alone the Swiss government invested more than 60 million sfr for the maintenance of protection forests. However, to date there has been no comprehensive evaluation of how the structural development of actively managed stands differs from that of passively managed stands in the Alps. Over the past century the structure of Norway-spruce dominated subalpine forests of the Swiss Alps has been changing and it is not clear how these changes affect the potential protective function of these forests, as well as other forest functions such as wildlife habitat. Furthermore, it is not clear how stand dynamics and structural changes differ between stands that are actively managed and those that are passively managed, and thus to what degree active efforts of forest management are contributing to stands that actually have a greater protective function than passively managed forests.In this study, we analyzed the dynamics of subalpine forests using time series of forest inventory data and examined the influence of active vs. passive management, exposition, distance to treeline, elevation and slope steepness on stand structure and dynamics. We analyzed data from 395 plots in dense, subalpine, spruce-dominated forests in the Swiss Alps. Data were collected during three distinct periods (1983-1985, 1993-1995 and 2004-2006) as part of the Swiss National Forest Inventory (NFI). Using regression trees and multivariate statistics, we investigated which factors have the most important influence on tree growth and stand development.Overall, forest density increased significantly over the last 20 years and the predominance of dense forests increased at higher elevations. However, forest density has not increased in actively managed forests over the past 10 years. In passively managed stands, density was higher on south-facing slopes than on north-facing slopes. The volume and density of dead wood has increased over the last 20 years in both actively managed and passively managed forests. Active management over the last 20 years has maintained forest conditions that adequately maintain stands’ protection functions in the Swiss Alps. However, stand development, especially increasing density, in the passively managed stands of the Swiss Alps suggests that the majority of passively managed stands also provide adequate protective functions against rock and avalanche hazards without the high costs of active management.     

Twentieth-century decline of large-diameter trees in Yosemite National Park,California, USA

Studies of forest change in western North America often focus on increased densities of small-diameter trees rather than on changes in the large tree component. Large trees generally have lower rates of mortality than small trees and are more resilient to climate change, but these assumptions have rarely been examined in long-term studies. We combined data from 655 historical (1932–1936) and 210 modern (1988–1999) vegetation plots to examine changes in density of large-diameter trees in Yosemite National Park (3027 km²). We tested the assumption of stability for large-diameter trees, as both individual species and communities of large-diameter trees. Between the 1930s and 1990s, large-diameter tree density in Yosemite declined 24%. Although the decrease was apparent in all forest types, declines were greatest in subalpine and upper montane forests (57.0% of park area), and least in lower montane forests (15.3% of park area). Large-diameter tree densities of 11 species declined while only 3 species increased. Four general patterns emerged: (1) Pinus albicaulis, Quercus chrysolepis, and Quercus kelloggii had increases in density of large-diameter trees occur throughout their ranges; (2) Pinus jeffreyi, Pinus lambertiana, and Pinus ponderosa, had disproportionately larger decreases in large-diameter tree densities in lower-elevation portions of their ranges; (3) Abies concolor and Pinus contorta, had approximately uniform decreases in large-diameter trees throughout their elevational ranges; and (4) Abies magnifica, Calocedrus decurrens, Juniperus occidentalis, Pinus monticola, Pseudotsuga menziesii, and Tsuga mertensiana displayed little or no change in large-diameter tree densities. In Pinus ponderosa–Calocedrus decurrens forests, modern large-diameter tree densities were equivalent whether or not plots had burned since 1936. However, in unburned plots, the large-diameter trees were predominantly A. concolor, C. decurrens, and Q. chrysolepis, whereas P. ponderosa dominated the large-diameter component of burned plots. Densities of large-diameter P. ponderosa were 8.1 trees ha⁻¹ in plots that had experienced fire, but only 0.5 trees ha⁻¹ in plots that remained unburned.     

Tree regeneration and future stand development after bark beetle infestation and harvesting in Colorado lodgepole pine stands

In the southern Rocky Mountains, current mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreaks and associated harvesting have set millions of hectares of lodgepole pine (Pinus contorta var. latifolia Engelm. ex Wats.) forest onto new stand development trajectories. Information about immediate, post-disturbance tree regeneration will provide insight on dynamics of future stand composition and structure. We compared tree regeneration in eight paired harvested and untreated lodgepole pine stands in the Fraser Experimental Forest that experienced more than 70% overstory mortality due to beetles. New seedlings colonized both harvested and untreated stands in the first years after the beetle outbreak. In harvested areas the density of new seedlings, predominantly lodgepole pine and aspen, was four times higher than in untreated stands. Annual height growth of pine and fir advance regeneration (e.g., trees established prior to the onset of the outbreak) has doubled following overstory mortality in untreated stands. Growth simulations based on our regeneration data suggest that stand basal area and stem density will return to pre-beetle levels in untreated and harvested stands within 80-105 years. Furthermore, lodgepole pine will remain the dominant species in harvested stands over the next century, but subalpine fir will become the most abundant species in untreated areas. Owing to terrain, economic and administrative limitations, active management will treat a small fraction (<15%) of the forests killed by pine beetle. Our findings suggest that the long-term consequences of the outbreak will be most dramatic in untreated forests where the shift in tree species composition will influence timber and water production, wildfire behavior, wildlife habitat and other forest attributes.     

Early seedling establishment of two tropical montane cloud forest tree species: The role of native and exotic grasses

Tropical montane cloud forest has been undergoing a drastic reduction because of its widespread conversion to pastures. Once these forests have been cleared exotic grasses are deliberately introduced for forage production. Exotic grass species commonly form monodominant stands and produce more biomass than native grass species, resulting in the inhibition of secondary succession and tree regeneration. The purpose of this study was to assess the effect of native vs. exotic grass species on the early establishment of two native tree seedlings (Mexican alder, Alnus acuminata and Jalapa oak, Quercus xalapensis) on an abandoned farm in central Veracruz, Mexico. Seedling survival and growth were monitored (over 46 weeks) in relation to grass cover and height, and available photosynthetic active radiation (PAR). More seedlings survived in the presence of the native grass Panicum glutinosum than those growing with the exotic grass Cynodon plectostachyus (92% vs. 48%). The causes of seedling mortality varied between species; Q. xalapensis was affected by herbivory by voles but mainly in the exotic grass-dominated stands, whereas A. acuminata seedlings died due to competition with the exotic grass. A. acuminata seedlings increased more in height in the exotic grass-dominated stands (102 ± 7.8 cm) compared to native grass-dominated stands (51 ± 4.7 cm). Grass layer height, cover and available PAR were correlated (Pearson; p < 0.05). In the exotic grass dominated plots, grass layer height was correlated with the relative height growth rates of Q. xalapensis (Pearson; p < 0.05). These results indicate that the exotic grass may be affecting tree regeneration directly (grass competition) and indirectly (higher herbivory). Passive restoration may occur once P. glutinosum dominated pastures are abandoned. However, when C. plectostachyus dominates, introduction of early and mid successional tree seedlings protected against vole damage is needed.     

Establishment and growth of oak (Quercus alba, Quercus prinus) seedlings in burned and fire-excluded upland forests on the Cumberland Plateau

Recurrent problems with regeneration of oaks (Quercus spp.) have been documented across a wide range of ecosystems. In oak-dominated forests of the central and Appalachian hardwood regions of the United States, a lack of competitive oak regeneration has been tied, in part, to fire suppression in these landscapes, and managers throughout the region are using prescribed fire to address this concern. To examine fire effects on oak regeneration, researchers have generally relied on inventories or population studies of existing seedlings. These studies are valuable but do not permit examination of the role of fire in enhancing the establishment and growth of new oak seedlings stemming from oak mast events. In this study, white (Quercus alba) and chestnut oak (Quercus prinus) acorn mast crops serendipitously occurred in year three (fall 2005) of a landscape-scale prescribed fire experiment. We examined establishment, survival, height and diameter of new seedlings on sites on the Cumberland Plateau in eastern Kentucky. Treatments were fire exclusion, a single prescribed fire (1x-burn; 2003), and repeated prescribed fire (3x-burn; 2003, 2004, and after acorn drop in 2006), all conducted in late spring. Initial densities of newly established chestnut and white oak seedlings were statistically similar across treatments (P = 0.42), despite fires on the 3x-burn site having occurred after acorns were on the ground. Oak seedling density was significantly predicted by oak basal area on all sites (R² = 0.12–0.46), except for chestnut oak on fire-excluded sites (R² = 0.04). Litter depth was less on 3x-burn sites compared to 1x-burn and fire-excluded sites, whereas canopy openness was greater on both burn treatments compared to fire-excluded sites. Seedling mortality was generally higher on fire-excluded sites compared to burn sites, especially for white oak. Oak seedling mortality in the first two growing seasons was significantly predicted by initial litter depth and open sky, with greater litter depth and lower percent open sky leading to higher mortality. In the third growing season none of the measured variables predicted chestnut oak seedling survival; for white oak, percent open sky remained a significant predictor of mortality. Initially, seedlings on the fire-excluded sites had similar height but smaller diameter; after three growing seasons there were few differences in seedling height or diameter among treatments. Our findings suggest a potential role for prescribed fire in establishing forest floor and light conditions that may enhance the success of new oak germinants, although different responses among species may suggest the need to target management for individual oak species.     

Sustainable use of non-timber forest products: Impact of fruit harvesting on Pentadesma butyracea regeneration and financial analysis of its products trade in Benin

Pentadesma butyracea Sabine (Clusiaceae) is a multi-purpose tree that provides non-timber forest products (NTFPs). In particular, fruit almonds can be transformed into butter for cooking and cosmetics. During the present study, the following hypotheses were tested: (i) diameter structure of P. butyracea populations is independent of its fruit gathering intensity; (ii) P. butyracea seedling and sapling density and origin are independent of its fruit gathering intensity; (iii) P. butyracea fruit gathering and processing of its almonds are profitable activities and (iv) P. butyracea fruit collectors and almond transformers are receiving the lowest marketing margins in the commercial channel. The class distribution in the low-intensity harvesting sites showed a typical inverse J-shaped curve whereas the high-intensity harvesting ones showed an almost bell curve (G² = 23.93, p = 0.0008). After data analysis, all hypotheses turned out to be wrong except the hypothesis (iii). In order to assess the effects of fruit harvesting on natural regeneration of P. butyracea, we compared seedling and sapling density of regeneration originating from seeds and roots suckers in plots that had been differentiated according to fruit harvesting intensity. These plots were laid out in riparian forests, which are the natural habitats for the species in Benin. Observed seedling and sapling density was high (13,872 ± 7886 seedlings and saplings/ha) in low-intensity harvesting sites but very low (4200 ± 3810 seedlings and saplings/ha) in high-intensity harvesting sites (F = 17.16; p = 0.0006). However, there was no significant difference between root sucker density in either type of harvesting site (F = 0.79; p = 0.3861). Collection of P. butyracea fruits and subsequent processing of its almonds into butter is an important source of income for women involved in these activities. Commercial margin analysis showed that these women involved in almonds and butter trade, far from being exploited by traders, recuperate between 49% and 80% of the price paid by the consumer, depending on the quality of the product and the length of commercial channel used.     

Dynamics and determinants of Quercus alba seedling success following savanna encroachment and restoration

The scattered tree layer that defines savannas is important for structuring the understory community and determining patterns of overstory recruitment. However, encroachment by woody plants has altered overstory tree densities and regeneration dynamics. We characterized seedling success of the savanna-forming species Quercus alba within Midwestern (USA) oak savannas that had been degraded by encroachment (control; n = 4) or experimentally restored by removal of encroaching woody vegetation (treatment; n = 4). In early 2004, 981 seedlings were transplanted along transects radiating from tree boles of overstory Q. alba trees to inter-canopy gaps and monitored for three growing seasons. Seedlings in restored sites had greater survival (>2×), height growth (by >50%), and basal diameter growth (by >20%). In general, seedling survival and growth parameters increased with distance from overstory trees and were greatest in inter-canopy gaps of restored sites. By the final growing season (2006), the seedling survival-by-distance from tree correlation was stronger in control (r² = 0.25) than treatment sites (r² = 0.18), due to relatively uniform (and greater) survival at all distances from trees in treatment sites. In 2006, growth parameters (seedling height, diameter, Δ height, Δ diameter, and # leaves) were significantly (and more strongly) positively correlated with distance from trees in treatment sites. However, seedling herbivory was also greater after treatment and increased with distance from overstory trees. To understand seedling/microenvironment relationships, we created logistic (survival) and linear regression models (Δ height, Δ basal diameter, # leaves in 2006). Control seedling models had consistently greater predictive power and included more variables, suggesting that savanna restoration may decouple seedlings from their microenvironments, potentially by decreasing competition for limiting resources. Encroachment of the savannas in this study is limiting regeneration of Q. alba, suggesting substantially altered regeneration dynamics from those under which these savannas originally formed. Initial responses from our test of restoration, however, were promising and mechanical encroachment removal may be a means to promote overstory regeneration of this species. Finally, the savannas in this study appear inherently unstable and a scattered canopy tree configuration is unlikely to persist without regular disturbance, even in the restoration sites. Repeated mechanical thinning treatments with selected retention of recruiting Q. alba individuals or reintroduction of understory fire or grazing animals may be potential mechanisms for promoting long-term persistence of savannas at these sites.     

The influence of gap creation on the regeneration of Pinus tabuliformis planted forest and its role in the near-natural cultivation strategy for planted forest management

It remains unclear whether or not creating gaps in planted forests can increase the plant species composition, structure, and biodiversity, and also whether it can be helpful for restoring planted forests (to a more natural state). Based on a comparison of species composition and structure among forest patches, small gaps (4-25 m²), medium gaps (25-150 m²) and large gaps (150-450 m²), we found that (1) creating gaps enhanced vascular plant diversity. Both the species richness and Shannon diversity indices of small, medium and large gaps were significantly higher than in the understory. The pattern of increasing diversity of vascular plants with gap creation could be partly attributed to the emergence of novel shade intolerant species in gaps. (2) Creating gaps favored the colonization and regeneration of native species. Gap size influenced not only the emergence and density of individuals of different species, but also the emergence of different life form types. Small gaps promoted the regeneration of some shrub species, such as Ostryopsis davidiana, Rosa hugonis, and Forsythia suspense, leading to these species becoming canopy dominants early on in succession. The medium and large gaps favored the growth of tree species, such as Populus davidiana and Betula platyphylla (early successional stage), and Quercus liaotungensis and Pinus tabulaeformis (later successional phase). (3) The canonical correspondence analysis showed that plant species composition and distribution were mainly influenced by gap size and slope aspect, and that the recorded plant species could be divided into three life forms (trees, shrubs and herbs) on the biplot diagram. (4) Finally, creating gaps provided opportunities not only for native pioneer species in the early successional stage, but also for climatic climax species to grow to canopy dominants in later successional phases, suggesting that a more natural forest will develop with plant succession. Gap size plays an important role in plant regeneration, and it could be used to produce desired successional communities in near natural management for planted forests.     

Physiological performance of three pine species provides evidence for gap partitioning

Gradients of light and moisture availability peak at different positions within canopy gaps in northern latitudes providing the opportunity for niche partitioning in and around gaps based on differences in individual species’ life history attributes. This gap partitioning offers potential for increasing diversity in forests impacted by gap-creating disturbances. We examined resource availability and the physiological performance of three Pinus species with varying tolerances for shade and moisture stress across large (0.3 ha) canopy gaps to investigate relationships between gap position and species performance. Light availability was lowest in southern gap edges, while water availability was lowest in northern edges, and higher at gap interior positions than edges. Pinus banksiana seedlings had higher light-saturated CO₂ assimilation rates than P. resinosa or P. strobus seedlings at interior gap positions, and outperformed P. strobus at northern gap edges, but there were no differences between species at southern edges. Both transpiration and stomatal conductance were greatest for P. banksiana in gap centers, but showed few differences between species at edges. Foliar nitrogen concentrations were highest for P. banksiana, suggesting the dominance of this species in central gap locations may be due to a combination of high photosynthetic capacity and tight stomatal control to regulate moisture stress at drier gap positions. Our results suggest P. banksiana seedlings may be competitively superior in gap positions with high light and moisture availability, but P. resinosa and P. strobus become competitive under the drier conditions and moderate shade near gap edges. These findings support the concept of gap partitioning, and suggest silvicultural systems that incorporate patch cuttings could be used to promote diverse regeneration in northern pine forests.     

Biomass conversion factors (density and carbon concentration) by decay classes for dead wood of Pinus sylvestris, Picea abies and Betula spp. in boreal forests of Sweden

For estimating the amount of carbon (C) in dead wood, conversion factors from raw volume per decay class to dry weight were developed using three different classification systems for the species Norway spruce (Picea abies L. Karst), Scots pine (Pinus sylvestris L.) and birch (Betula pendula Roth and B. pubescens Ehrh) in Sweden. Also the C concentration in dead wood (dry weight) was studied. About 2500 discs were collected from logs in managed forests located on 289 temporary National Forest Inventory (NFI) sample plots and in 11 strips located in preserved forests. The conversion factors were based on an extensive data compilation with a wide representation of different site-, stand-, species- and dead wood properties and were assumed to represent the population of fallen dead wood in Sweden. The density decreased significantly by decay class and the range in density for decay classes was widest for the NFI decay classification system, suggesting this to be the most suitable. The C concentration in dead wood biomass increased with increasing decay class and in average Norway spruce (P. abies) showed a lower C concentration than Scots pine (P. sylvestris). The average dead wood C store of Swedish forests was estimated to 0.85 Mg C/ha.     

Predicting tree regeneration in <Emphasis Type="Italic">Picea abies</Emphasis> snag stands

A bark beetle (Ips typographus) infestation caused the death of almost all Norway spruce (Picea abies) trees in a mountain forest in the Swiss Alps. We developed a tree regeneration model, ‘RegSnag’ (=REGeneration in a SNAG stand), to project the future amount and height of tree regeneration in these snag stands. The model combines a height-class structured tree module with a microsite-based module of snag decay and ground-vegetation succession. Microsite-specific rates of germination, mortality and height growth were modelled for four tree species (Picea abies, Sorbus aucuparia, Acer pseudoplatanus and Betula pendula) in eight height classes (from seedlings to saplings 5 m tall) and on 26 microsite types (e.g. moss, grass). Model tests with independent field data from 8 years after the Picea die-back demonstrated that microsites had a considerable effect on the development of tree regeneration on both the montane and the subalpine level. With microsite-specific parameters, the height and frequency of Picea in each microsite could be simulated more accurately than without considering microsite effects (e.g. bias of 8 vs. 119 saplings ha⁻¹ on the montane level). Results of simulations 40 years into the future suggest that about 330–930 Picea saplings per ha out of those that germinated in 1994 and 1996 will reach a height of 5 m within 30–35 years after Picea die-back. This is due to differences in seed inflow and browsing intensities. Picea and not Betula or Sorbus trees will replace the current herbaceous vegetation in these snag stands.     

Regeneration niches and functional traits of three common species in subtropical dune forest

South African coastal dune forests are young, highly disturbed subtropical communities where conventional models of forest dynamics may be challenged. We tested predictions from the gap-phase regeneration model by comparing seedlings of three common species representing contrasting regeneration strategies: Acacia karroo as a ruderal, Celtis africana as a coloniser of forest gaps, and Diospyros natalensis as a late-successional species. We grew seedlings under contrasting light and nitrogen levels in a greenhouse and in the field for 1 year to compare their growth and survival rates, allocation and photosynthetic traits. Species’ growth rates generally followed the expected order: Acacia > Celtis > Diospyros, but Acacia responded strongly to light and Celtis responded strongly to nitrogen, leading to cross-overs in growth rates. The plasticity of allocation and photosynthesis did not clearly differentiate the strategies, although it was greater in the light-demanding species. Acacia and Celtis tended to survive better in Acacia stands than in forest plots. Leaf-level light compensation points (LCPs) were similar for the three species in most conditions, but auxiliary data suggest Diospyros has a lower whole-plant LCP than Acacia. Growth rates and LCPs were lower than most of those reported for primary-forest species in the literature, suggesting an unusual degree of shade-tolerance in this habitat. We discuss reasons why variation in shade-tolerance may be less important here than in the prevailing model for forest regeneration and suggest other biotic factors that may help differentiate regeneration niches.     

Long-term dynamics of a mixed conifer stand in Slovenia managed with a farmer selection system

The single-tree selection system is an important option for management of Norway spruce (Picea abies (L.) Karst.) and silver fir (Abies alba Mill.) forests because it provides continuous cover, requires low investments for tending, and promotes natural regeneration as well as high stand resistance and elasticity. It is often regarded as a very conservative system that usually results in only minor spatiotemporal changes in forest structure and composition. We studied management history, structural changes, regeneration dynamics, and light climate of a traditional single-tree farmer selection silver fir-Norway spruce forest (site typology Bazzanio-Abietetum). Stand structure was analyzed on five 0.25 ha permanent plots in 1994, 2001, and 2008. Regeneration density and height growth, forest floor vegetation, and light climate were also assessed on 1.5 × 1.5 m regeneration subplots in 2001 and 2008. Tree cores extracted from dominant trees from both species in two plots were used for reconstructing stand history and age structure of the canopy layer. We documented the forest response to three types of selection management regimes: excessive, normal, and conservative. Excessive management with harvest intensity significantly above the increment was documented until the late 1950s, including two peaks of heavy fellings (diameter limit cut) in the 1880s and 1930s, which favoured establishment of Norway spruce and released regeneration. The period that followed was characterized by normal selection management, but was nevertheless marked by a decline of silver fir as a result of air pollution and several droughts. This led to sanitary fellings that were carried out from the late 1970s to the early 1990s. In the last two decades conservative management followed, which led to suppression and decline of regeneration, especially of Norway spruce, and loss of selection structure. Although we recorded lower regeneration potential of silver fir compared with Norway spruce within the seedling category, silver fir outcompeted Norway spruce within the small-sized tree category (1 cm < dbh ? 10 cm) because of its superior height growth in low light levels (diffuse light <6%) and occupied a greater share of the canopy. Nevertheless, we anticipate that over the long-term the low light regime will also cause regeneration decline of silver fir and broadleaves. Our research revealed significant structural changes in a single-tree farmer selection forest during the last 150 years. These were a result of variable management regime and environment. A farmer single-tree selection system could better mimic the natural disturbance regime if spatiotemporal combinations of diverse felling regimes would be used.     

Canopy gaps and regeneration in old-growth Oriental beech (Fagus orientalis Lipsky) stands, northern Iran

Virgin beech Fagus orientalis forests in northern Iran provide a unique opportunity to study the disturbance regimes of forest ecosystems without human influence. The aim of this research was to describe characteristics of natural canopy gaps and gap area fraction as an environmental influence on the success of beech seedling establishment in mature beech stands. All canopy gaps and related forest parameters were measured within three 25 ha areas within the Gorazbon compartment of the University of Tehran’s Kheyrud Experimental Forest. An average of 3 gaps/ha occurred in the forest and gap sizes ranged from 19 to 1250 m² in size. The most frequent (58%) canopy gaps were <200 m². In total, canopy gaps covered 9.3% of the forest area. Gaps <400 m² in size were irregular in shape, but larger gaps did not differ significantly in shape from a circle. Most gaps (41%) were formed by a single tree-fall event and beech made up 63% of gap makers and 93% of gap fillers. Frequency and diversity of tree seedlings were not significantly correlated with gap size. The minimum gap size that contained at least one beech gap-filling sapling (<1.3 m tall) was 23.7 m². The median gap size containing at least one beech gap-filling sapling was 206 m² and the maximum size was 1808 m². The management implications from our study suggest that the creation of small and medium sized gaps in mixed beech forest should mimic natural disturbance regimes and provide suitable conditions for successful beech regeneration.     

Enrichment of big-leaf mahogany (Swietenia macrophylla King) in logging gaps in Bolivia: The effects of planting method and silvicultural treatments on long-term seedling survival and growth

To insure adequate regeneration and future timber yields of mahogany (Swietenia macrophylla King), many logged forests will have to be restocked through enrichment planting and managed using silvicultural techniques that maintain this species’ long-term survival and growth. This study compared the effects of planting method and two silvicultural treatments on the survival and growth of mahogany seedlings in logging gaps in Bolivia. We tested the hypotheses that survival and growth will be higher among transplanted seedlings than seedlings established from sown seeds and higher in silvicultural treatments that reduce competing vegetation and increase light. The first silvicultural treatment consisted of gaps logged 6 months prior to planting, gaps logged just prior to planting, and gaps treated with herbicide prior to planting. The second treatment, applied 12 months after planting, consisted of manual vegetation cleaning around mahogany seedlings in half of the gaps. The first hypothesis was supported in terms of initial seedling growth but not survival, which was similar between planting methods during the 12-92 months after planting. Transplanted seedlings grew significantly faster than those established from sown seeds during the first year, but this growth advantage disappeared by the second year. Although transplants were 84 cm taller than seed-sown seedlings by the end of the study, this height gain was probably not worth the cost of growing and transplanting seedlings. The second hypothesis was supported in terms of both survival and growth. A significantly greater proportion of seedlings survived in herbicide (62%) compared to 6-month-old (46%) and recent gaps (18%) and in cleaned (51%) versus control gaps (39%). Seedlings initially grew faster in herbicide and recent gaps than in 6-month-old gaps. These differences among silvicultural treatments were largely explained by canopy cover, which, throughout the study, was at least 14% lower in herbicide gaps and 9% lower in cleaned gaps relative to their respective alternatives. By 64 months growth diminished to near zero and no longer differed among gap treatments, despite lower canopy cover in herbicide gaps. By 92 months, saplings in herbicide gaps were only 145 and 77 cm taller than those in recent and 6-month-old gaps, respectively. To maximize survival and growth of mahogany seedlings in logging gaps while minimizing costs, silvicultural strategies should focus on direct seed sowing and appropriately timed interventions (i.e. manual cleaning) to control competing vegetation.     

Interaction of gap age and microsite type for the regeneration of Picea abies

To assess the influence of gap age and microsite type within a near-natural montane Norway spruce stand at the Harz National Park in Germany, we tested the following hypotheses: (1) The relationship between Picea abies regeneration and gap age is represented by an optimum curve. (2) Within gaps, tree regeneration mainly occurs on gap-induced microsites. (3) The contribution of specific microsites to regeneration changes with gap age.     

Dynamics of seedling establishment and survival in uneven-aged boreal forests

The aim of this study was to describe the dynamics of seedling establishment and development in spruce-dominated uneven-aged boreal forests. The study was based on empirical data from 15 stands with permanent plots, which had been intensively monitored for 10 years in southern Finland. All trees (height > 1.3 m) were measured every fifth year. Regeneration was measured on 64 permanent sample plots (4 m²) in each stand. The establishment of first-year seedlings was analysed on a yearly basis. The survival and development of older Norway spruce (Picea abies (L.) H. Karst.) seedlings were analysed based on observations made every five years. The establishment of spruce seedlings was closely correlated with the abundance of seed crops. Seedling cohorts originating from abundant seed crops were clearly detectable in the development of seedling height distributions over time. It took about 15 years for spruce seedlings to reach a height level of 15-30 cm. Local basal area had hardly any effect on the emergence or survival of small spruce seedlings, while the number of higher spruce seedlings decreased with increasing local basal area.     

Restoring longleaf pine (Pinus palustris Mill.) in loblolly pine (Pinus taeda L.) stands: Effects of restoration treatments on natural loblolly pine regeneration

Historical land use and management practices in the southeastern United States have resulted in the dominance of loblolly pine (Pinus taeda L.) on many upland sites that historically were occupied by longleaf pine (Pinus palustris Mill.). There is currently much interest in restoring high quality longleaf pine habitats to such areas, but managers may also desire the retention of some existing canopy trees to meet current conservation objectives. However, fast-growing natural loblolly pine regeneration may threaten the success of artificially regenerated longleaf pine seedlings. We evaluated the establishment and growth of natural loblolly pine regeneration following different levels of timber harvest using single-tree selection (Control (uncut, residual basal area ∼16 m²/ha), MedBA (residual basal area of ∼9 m²/ha), LowBA (residual basal area of ∼6 m²/ha), and Clearcut (complete canopy removal)) and to different positions within canopy gaps (approximately 2800 m²) created by patch cutting at two ecologically distinct sites within the longleaf pine range: Fort Benning, GA in the Middle Coastal Plain and Camp Lejeune, NC in the Lower Coastal Plain. The density of loblolly pine seedlings was much higher at Camp Lejeune than at Fort Benning at the end of the first growing season after harvesting. Following two growing seasons, there were no significant effects of canopy density or gap position on the density of loblolly pine seedlings at either site, but loblolly pine seedlings were taller on treatments with greater canopy removal. Prescribed fires applied following the second growing season killed 70.6% of loblolly pine seedlings at Fort Benning and 64.3% of seedlings at Camp Lejeune. Loblolly pine seedlings were generally less than 2 m tall, and completeness of the prescribed burns appeared more important for determining seedling survival than seedling size. Silvicultural treatments that include canopy removal, such as patch cutting or clearcuts, will increase loblolly pine seedling growth and shorten the window of opportunity for control with prescribed fire. Therefore, application of prescribed fire every 2-3 years will be critical for control of loblolly pine regeneration during restoration of longleaf pine in existing loblolly pine stands.