Plant species diversity on logged versus burned sites in central Alaska

Natural fires and logging are two of the main disturbances affecting upland boreal forest in Alaska. The objectives of this study were to determine whether logged sites differ from burned sites in (1) overall plant species richness, (2) successional trajectories, and (3) species diversity at particular stand structural development stages. We compared plant species diversity on sites burned in natural fires to sites that were logged and not subsequently burned in central Alaska. We sampled 12 logged and 12 burned former upland white spruce (Picea glauca (Moench) Voss) forests in four stand development stages representing stand initiation (stage A), early stem exclusion (stage B), understory reinitiation (stage C), and mature hardwood (stage D) stages. In this study the dates of disturbance varied from 1990 to 1994 in stage A, 1978 to 1983 in stage B, 1957 to 1965 in stage C, and 1900 to 1920 in stage D plots. All sites were similar in slope, aspect, and soil type. Vascular plants were identified to the species level (except for certain willows) and bryophytes and lichens were identified to the level of presumptive (usually unknown) species within family groups. Organic layer thickness was significantly greater on logged sites compared to burned sites overall and at each stage. Burned sites (all stages combined) supported more species (146) than logged sites (111), and more species at each stand development stage. Burned plots in stages A and B supported abundant cover of a few apparent fire specialist species (Ceratodon purpureus (Hedw.) Brid., Marchantia polymorpha L. and Leptobryum pyriforme (Hedw.) Wils.) that were present in only minor amounts on logged sites. Burned plots exhibited higher species turnover from stage to stage and among all stages than logged plots. Species dominant in burned stage A plots were nearly absent in burned stage C and D plots, while logged stage A dominants, which were common mature forest species, increased in each subsequent stage. We compared floristic similarity between our disturbance plots and mature upland white spruce stands in Bonanza Creek Long-Term Ecological Research (LTER) site. Only five species found in the LTER dataset were not also present in this study, which suggests that nearly all species compositional change in our study area occurs during the first century after disturbance. Logged sites appear to begin and continue succession with a greater share of the original mature forest understory plants, while burned sites initiate succession with more distinctive and specialized plant species.     

Epiphytic lichen diversity in old-growth and managed Picea abies stands in Alpine spruce forests

In the last decades, a large body of literature has grown to evaluate the impact of forest management on epiphytic lichens in boreal coniferous forests. However, information is still lacking on coniferous forests of the Alps. This study compares lichen diversity between spruce forest stands of four successional stages: (1) young, (2) intermediate, (3) mature forests managed for timber production with a rotation cycle of 120–180 years, and (4) old-growth protected forests. The emphasis was placed on the occurrence of nationally rare and calicioid species (lichens and fungi traditionally referred to as Caliciales, known to be indicative of forest age and continuity). For each forest successional stage, four plots were selected. In each plot, 7 spruce individuals were surveyed for epiphytic lichens according to a standardised sampling method. Species richness increased from young to mature stands, while no difference was detected between mature and old-growth stands. This pattern was also confirmed for rare and calicioid species which are, however, more frequent in old-growth stands. Differences in species composition were also found between the different forest successional stages. Mature and old-growth plots slightly overlap, indicating that to some extent comparable lichen assemblages could be found in these stands. A nested pattern of species assemblages was found, old-growth stands hosting most of the species which were also found in stands belonging to the previous forest successional stages. Our results support the hypothesis that the management regime applied to spruce forests of the Italian Alps renders mature stands managed for timber production somewhat similar to old-growth stands as lichen habitat. However, we found a higher complexity in old-growth forests, and many species of conservation concern clearly preferred old-growth stands. In this perspective, a further prolongation of the normal cycle it is likely to be a most favourable conservation-oriented management to be recommended at least within protected areas and Natura 2000 sites, where conservation purposes should receive a high priority.     

Successional process of Picea crassifolia forest after logging disturbance in semiarid mountains: A case study in the Qilian Mountains,northwestern China

Selective logging is the most widely employed method of commercial timber production in Asia, and its impact on forest structure, composition, and regeneration dynamics is considerable. However, the successional processes in forest communities after logging in semiarid mountains are poorly understood. To provide more information on these processes, we used data from tree rings, direct and indirect age determinations, and field measurements of stand structure to reconstruct the historical disturbance regime, stand development patterns, and successional processes in a natural Picea crassifolia forest community in the Qilian Mountains of northwestern China. The results showed that the density of P. crassifolia forest increased significantly after logging. The densities of second growth forests 30 and 70 years after logging disturbance had increased to 2874% and 294% of primary forest's density, respectively. Logging disturbance did not alter tree species composition of logged stands. However, the diversity of understory species changed significantly among the successional phases. Logging disturbance decreased the spatial heterogeneity of second growth forest. The spatial distributions of recruitment were affected by the location of the remaining trees. There was less recruitment near the remaining trees than near forest that had been cut. In addition, logging disturbance also induced a growth release for the trees on the sites sampled. Our results imply that the succession and regeneration of P. crassifolia forest may be improved if the remaining trees could be retained relative uniform distribution pattern, thinning or selective logging could be performed to height density, exotic shrubs could be removed or the shrubs cover could be reduced during the earlier successional stages.     

Influence of fire intensity on structure and composition of jack pine stands in the boreal forest of Quebec: Live trees,understory vegetation and dead wood dynamics

North American jack pine (Pinus banksiana Lamb.) stands are generally characterized by an even-aged structure resulting from high intensity fires (HIF). However, non-lethal fires of moderate intensity (MIF), which leave behind surviving trees, have also been reported. The objectives of this study were two-fold: (1) assess the concurrent dynamics of live trees, understory vegetation and different types of coarse woody debris (CWD) during succession after HIF; and (2) document how MIF affects stand structure component dynamics compared to HIF. Stands affected by both HIF and MIF were selected. Tree characteristics and age structure, understory biomass, and CWD volume were assessed. Our results suggest that the structural succession of jack pine stands following HIF comprises three stages: young stands (<48 years), premature and mature stands (58–100 years) and old stands (>118 years). Canopy openness and jack pine density significantly decreased with time since HIF, while black spruce density and CWD volume significantly increased. The highest structural diversity was measured in the premature and mature stands. Compared to HIF, MIF increased mean jack pine basal area, decreased average stand density, delayed the replacement of jack pine by black spruce replacement in the canopy, decreased CWD volume, and significantly increased bryophytes mass. MIF increased the diversity of live trees and generally decreased CWD structural diversity. The study confirms the diversity of natural disturbance magnitude and successional processes thereby initiated. Thereafter, it appeared to be relevant for adjustment of disturbance emulating forest-management systems.     

Storm legacies shaping post-windthrow forest regeneration: learnings from spatial indices in unmanaged Norway spruce stands

The anticipated increase in extreme disturbance events due to climate change is likely to expose Norway spruce (Picea abies (L.) Karst.) dominated forests in northern Europe to new conditions. Empirical data on the resilience of such natural (unmanaged) forests to disturbance and the long-term patterns of regeneration in its aftermath are currently scarce. We performed a quantitative assessment of natural forest stands in north–western Latvia to identify and characterise the patterns of stand structure 44 years after a stand-replacing disturbance and investigated the effects of legacies on regeneration. The spatial distribution of tree species and their dimensions were assessed in 71 circular sample plots (500 m² each) in natural forest areas, where Norway spruce dominated prior to the windthrow and salvage logging was not carried out. Spatial indices (species mingling index, size differentiation index, and aggregation index) were used to characterise stand structure and diversity. The different initial states (age and coverage of surviving trees) of stands affected eventual tree species dominance, size differentiation, degree of mingling and aggregation. Our results demonstrate a close relationship between disturbance legacies and spatial indices. The pre-storm understory and canopy survivors decreased species mingling, whereas survivors increased size differentiation. The size differentiation increased also with a higher degree of species mingling. Leaving differential post-storm legacies untouched promotes a higher structural and species diversity and therefore supports the management approach of preserving canopy survivors.

     

Composition and structure of natural mixed-oak stands in northern and central Portugal

Stand composition and structure of natural mixed-oak stands of common-oak (Quercus robur L.) and pyrenean-oak (Quercus pyrenaica Willd.) were studied. Diverse compositional and structural elements in early and late successional stand stages were analysed. The study was conducted in north and central Portugal where different natural mixed oak forests types are located. The following mixed-oak forest types involving common-oak and pyrenean-oak were studied: common-oak & other hardwoods; common-oak & cork-oak (Quercus suber L.); ash (Fraxinus angustifolia Vahl) & pyrenean-oak; and pyrenean-oak & madrone (Arbutus unedo L.). Measurements were made in early and late successional stand stages on the different mixed oak forest types. Different stand characteristics and indices were used to describe and compare stand structure and composition. The study showed changes in species diversity and stand structure. Most tree species in mature stands are present in early stages but with higher abundance. Shannon diversity index may change between 0.798 and 1.915. Significant differences on species diversity and abundance were found depending on the forest type and successional stage. Mature mixed-oak forests have high species diversity with an abundance of small to medium tree size species. Species distribution and diameter differentiation indices range from 0.30 to 0.70 and 0.52 to 0.82, respectively, revealing significant structural complexity. The average number of standing and downed dead trees was 265 and 83 trees ha⁻¹ for early and late stage, respectively, with 6.9 and 65.4 m³ ha⁻¹. Higher values of stand diversity index were 41 and 53 in more complex and developed forests. Later stand stages have complex structure, with a wider range of tree diameter distribution and higher degree of irregularity.     

Gap dynamics in the Western Carpathian mixed beech old-growth forests affected by spruce bark beetle outbreak

Understanding how species-specific disturbances affect the dynamics of mixed forests is becoming increasingly important due to rapidly changing disturbance regimes. This study estimated the effect of Norway spruce (Picea abies (L.) Karst.) mortality on the disturbance processes in two mixed beech stands of the Western Carpathians that were affected by a bark beetle outbreak. We evaluated the size distribution, fraction of canopy and expanded gaps, the characteristics of gapmakers and the contribution of different species to gap size. The measured canopy gap fraction was <5%, and most canopy gaps were small (<100 m²). Spruce was the most abundant gapmaker, and its share among gapmakers was 3–6 times higher than its share in the canopy. We found that the increase in spruce mortality due to the outbreak resulted in a fine-scale mortality pattern. However, spruce gapmakers did not contribute much to gap area size, as shown by a weak correlation between the number of spruce gapmakers and the area of expanded gaps. Diameter distribution of living versus recently dead trees showed that beech mortality occurred disproportionately in large size classes. However, dead spruce trees were equally frequent in all diameter classes, which means beetles did not exclusively attack larger trees in these stands during the outbreak. We conclude that spruce mortality may have influenced successional processes by giving a competitive advantage to two other species that were not affected by the outbreak, provided that a high deer browsing intensity does not hinder the regeneration of new seedlings.

     

Assessing stand structure in successional stages of dark coniferous forests in western Sichuan,China

Subalpine dark coniferous forests in the western Sichuan Province of China play an important role in the hydrological processes in the upper reaches of the Yangtze River. Second-growth forests, with different stand successional stages, have developed as a result of logging over the past 50 years. Forest cover and stand structure changed greatly with concomitant degradation of forest ecosystem functions. To understand how the stand structures of the second-growth forests change during the stand succession process, we analyzed stand structure characteristics and an old-growth state index of the bamboo and moss-forest types. We found that stand structure at the young successional stage featured one-third of the structure characteristics of the old-growth dark coniferous forests,while the structure of the medium-aged stage had reached half the structure of the old-growth state. The two forest types were similar in the rate of development at the young successional stage but differed at the medium-aged stage;the moss-forest type had more advanced development than the bamboo-forest type at the medium-aged successional stage.     

Species Interactions in the Dynamics of Even- and Uneven-Aged Boreal Forests

Many boreal tree stands are neither clearly even-aged nor clearly uneven-aged. The stands may undergo a series of stages, during which an even-aged stand is transformed into two-storied mixed stand, and finally to multistoried or uneven-aged stand structure. The species composition often changes during the succession of stand stages. This study developed models for stand dynamics that can be used in different stand structures and species compositions. The model set consists of species-specific individual-tree diameter increment and survival models, and models for ingrowth. Separate models were developed for Scots pine, Norway spruce, and hardwood species. The models were used in a growth simulator, to give illustrative examples on species influences and stand dynamics. Methods to simulate residual variation around diameter increment and ingrowth models are also presented. The results suggest that mixed stands are more productive than one-species stands. Spruce in particular benefits from an admixture of other species. Mixed species improve diameter increment, decrease mortality, and increase ingrowth. Pine is a more beneficial admixture than birch. Simulations showed that uneven-aged management of spruce forests is sustainable and productive, and even-aged conifer stands growing on medium sites can be converted into uneven-aged mixed stands by a series of strong high thinnings.     

Mycorrhizas and secondary succession in aspen-conifer forests: Light limitation differentially affects a dominant early and late successional species

Plant succession and mycorrhizal fungi both play crucial roles in shaping the development of forest ecosystems. However, despite the strong potential for interactions between them, few studies have examined how patterns of forest succession affect mycorrhizal associations that a majority of plant species depend on to alleviate soil resource constraints. Fire suppression in subalpine forests over the last century has changed successional patterns in ways that may have important implications for mycorrhizal associations of forest tree species. To better understand these relationships we conducted a field and greenhouse study in which we examined mycorrhizal infection along gradients of light intensity and soil nutrient availability that develop as aspen becomes seral to conifers under longer fire return intervals. We examined whether ectomycorrhizal associations of quaking aspen (Populus tremuloides), a shade intolerant, early succession species, were more sensitive to light and soil resource limitations than subalpine fir (Abies lasiocarpa), a shade tolerant, late succession species. In the field study, ectomycorrhizal infection of aspen roots was reduced by 50% in conifer dominated stands relative to aspen stands. In contrast, subalpine fir maintained its EM associations regardless of the successional status of the stand. The greenhouse results were consistent with field results and indicated that light limitation was the driving force behind reductions in EM infection of aspen roots in later stages of succession. These results suggest that nutrient limitations constraining early successional species may be exacerbated by losses in EM associations via light limitations created by late successional species. This is one potential mechanism by which climax forest species create a competitive advantage over early successional species and these results suggest that it is likely exacerbated by longer fire return intervals.     

Stand and landscape level effects of a major outbreak of spruce beetles on forest vegetation in the Copper River Basin,Alaska

From 1989 to 2003, a widespread outbreak of spruce beetles (Dendroctonus rufipennis) in the Copper River Basin, Alaska, infested over 275,000 ha of forests in the region. During 1997 and 1998, we measured forest vegetation structure and composition on one hundred and thirty-six 20-m × 20-m plots to assess both the immediate stand and landscape level effects of the spruce beetle infestation. A photo-interpreted vegetation and infestation map was produced using color-infrared aerial photography at a scale of 1:40,000. We used linear regression to quantify the effects of the outbreak on forest structure and composition. White spruce (Picea glauca) canopy cover and basal area of medium-to-large trees [≥15 cm diameter-at-breast height (1.3 m, dbh)] were reduced linearly as the number of trees attacked by spruce beetles increased. Black spruce (Picea mariana) and small diameter white spruce (<15 cm dbh) were infrequently attacked and killed by spruce beetles. This selective attack of mature white spruce reduced structural complexity of stands to earlier stages of succession and caused mixed tree species stands to lose their white spruce and become more homogeneous in overstory composition. Using the resulting regressions, we developed a transition matrix to describe changes in vegetation types under varying levels of spruce beetle infestations, and applied the model to the vegetation map. Prior to the outbreak, our study area was composed primarily of stands of mixed white and black spruce (29% of area) and pure white spruce (25%). However, the selective attack on white spruce caused many of these stands to transition to black spruce dominated stands (73% increase in area) or shrublands (26% increase in area). The post-infestation landscape was thereby composed of more even distributions of shrubland and white, black, and mixed spruce communities (17–22% of study area). Changes in the cover and composition of understory vegetation were less evident in this study. However, stands with the highest mortality due to spruce beetles had the lowest densities of white spruce seedlings suggesting a longer forest regeneration time without an increase in seedling germination, growth, or survival.     

Structural and environmental characterization of old-growth temperate rainforests of northern Chiloé Island,Chile: Regional and global relevance

Old-growth forests are ecologically relevant reservoirs of biodiversity and provide valuable and unique ecosystem functions in the landscape. However, what constitutes an old-growth stand is confusing because the definition depends largely on the forest type under study. Despite the ecological importance of old-growth temperate rainforests in southern Chile in comparison to other global forests, no attempts have been made to characterize them as a way to assess their structural variability. Here, we characterized old-growth stands of Valdivian and North Patagonian rain forest types located in Chiloé Island (Chile, 42°30′S) using inventory data from 23 permanent plots (0.1 ha each) located in rural landscapes and protected areas of northern Chiloé Island. For each stand, its age (average age of the oldest trees present in each stand) and disturbance regimes (evidence of recent human impact, e.g. cuttings or fires, and tree growth rates) were used as defining old-growth criteria. We characterized the structure (tree species richness, size-density distributions, vertical stratification and presence of snags) and floristic composition of each stand. Environmental variables (i.e. temperature, distance to coastline and elevation) were related to stand structure using multivariate constrained correspondence analysis. Old-growth forests were commonly characterized by (a) tree basal areas >80 m²/ha; (b) density of shade-tolerant tree species in the emergent and dominant canopy layer >36%; (c) higher tree species richness (>7 tree species) than successional stands; (d) presence of large canopy emergents (>80 cm dbh, >25 m tall); (e) high vertical heterogeneity; and (f) minimum stand ages older than 200 years. Old-growth forests showed a distinctive structural variability and floristic diversity influenced both by stand age and disturbance history of the stands. Structural variability was also related to environmental differences among sites (e.g. air temperature, distance to coastline, soil types). Old-growth forest features described here can offer a baseline for managers interested in maintaining and restoring old-growth forest structure in southern temperate rain forests.     

Riparian forest structure and succession in second-growth stands of the central Cascade Mountains,Washington, USA

We examined the relationship between landform types and riparian forest structure and succession in second-growth stands along mid order streams in the Cascade Mountains, Washington, USA. We sampled tree, sapling, seedling, and shrub characteristics across a range of fluvial geomorphic surfaces, which were classified into four landform classes, including low floodplain, high floodplain, terrace and hillslope. Landform classification was based on topographic characteristics, position relative to the stream channel, and estimated flood frequency. Statistical analyses using generalized estimating equations (GEE) showed that landform exerted a strong influence on the distribution and abundance of conifer and deciduous species and of different tree life stages. The floodplain landforms were characterized by initial disturbance from timber harvest, and ongoing fluvial disturbance, which favored the establishment of deciduous communities dominated by red alder (Alnus rubra) and maintenance of early successional riparian stands. In contrast, the terrace and hillslope landforms were also subject to timber harvest as the stand initiating agent but were unaffected by fluvial disturbance. However, based on differences in species distribution, we infer that forest structure on these two landforms differed from one another as a result of differences in soil moisture levels. Terraces and hillslopes were found to have high conifer tree abundance, but frequency of younger conifers was higher on hillslopes. Deciduous tree reproduction was very low on terraces and hillslopes. Our results also suggest that conifer recruitment in these second-growth riparian forests may be more successful on soil substrates than on coarse woody debris. We propose that the interplay between the disturbance regime (including type, frequency and intensity) and soil moisture conditions played an important role in influencing the course of riparian succession, present stand structure, and future successional trajectories and these were the primary mechanisms driving vegetation differences among landforms.     

Comparing height growth and biomass production of black spruce trees in logged and burned stands

The strategy of using advanced layering for regenerating logged black spruce stands has become a common practice. Compared with natural post-fire black spruce stands, this strategy may alter stand structure with a possible change in stand productivity. Using harvested tree data from sample plots established in burned and logged stands, 50 years after disturbance, and on similar soils, we compared the structure, height growth, and biomass allocation of both types of stands. Stem analysis revealed that black spruce trees in logged stands reached their maximum height growth later and at a concomitant lower level than black spruce trees in burned stands. Biomass production was comparable between stand types but was reduced when ericaceous shrubs were abundant. Compared to natural post-fire stands, logged stands present a shift of biomass allocation to branches and to leaves. These mechanisms are interdependent and represent the tree adjustment to the altered structure in logged stands characterised by the fragmented canopy with the increase of ericaceous shrubs cover and of organic layer thickness.     

Spatial point-pattern analysis for detecting density-dependent competition in a boreal chronosequence of Alberta

In the boreal forest of Alberta, fire and wind often open gaps in the canopy where late-successional species can establish and over time cause a shift in the species distribution from deciduous (e.g., trembling aspen) dominated to mixedwood, to shade-tolerant conifer (e.g., white spruce) dominated stands. This study attempted to understand the change of density-dependent competition in a boreal chronosequence and the role of tree competition in affecting stand structure and mortality. Four 1-ha stem-mapped plots were established to represent a chronosequence comprised of aspen dominated, mixedwood, and spruce dominated stands in Alberta. Second order spatial point-pattern analysis using Ripley's K(t) function showed that intraspecific competition is a prevailing force causing conspecific tree mortality and thus shaping the stand structure. The results of bivariate K(t) function analysis did not reveal sufficient evidence of interspecific competition. This suggested that competitive interaction among heterospecific trees was not strong enough to cause significant tree mortality, but the analysis of marked correlation function revealed that interspecific competition could have a negative impact on tree growth. This study highlights the importance of density-dependent competition in understanding stand dynamics of boreal forests over succession.     

Variation in post-wildfire regeneration of boreal mixedwood forests: underlying factors and implications for natural disturbance-based management

To test the direct regeneration hypothesis and support natural disturbance-based forest management we characterized the structure and composition of boreal mixedwood forests regenerating after large wildfires and examined the influence of pre-fire stand composition and post-fire competing vegetation. In stands which had been deciduous (Populus sp.)-dominated, conifer (white spruce)-dominated, or mixed pre-fire we measured regeneration stocking (presence in 10 m² plots), density and height 10–20 years post-burn in five wildfires in Alberta, Canada. Most plots regenerated to the deciduous or mixed stocking types; plots in the older fire and in stands that were pure conifer pre-fire had higher amounts of conifer regeneration. Surprisingly, regeneration in pre-fire ‘pure’ white spruce stands was most often to pine, although these had not been recorded in the pre-fire inventory. Pre-fire deciduous stands were the most resilient in that poplar species dominated their post-fire regeneration in terms of stocking, density and height. These stands also had the highest diversity of regenerating tree species and the most unstocked plots. High grass cover negatively affected regeneration density of both deciduous and conifer trees. Our results demonstrate the natural occurrence of regeneration gaps, pre- to post-fire changes in forest composition, and high variation in post-fire regeneration composition. These should be taken into consideration when developing goals for post-harvest regeneration mimicking natural disturbance.     

Resilience of stand structure and tree species diversity in subtropical forest degraded by clear logging

Subtropical forests in the Ryukyu Islands have been degraded by silvicultural practices, and thus their structural attributes are being shifted to other states dominated by a few tree species. This study clarified the mechanisms of the change, and examined the effect of clear logging on the resilience of a subtropical forest. Sprouting regeneration and typhoon disturbance were introduced into an individual-based model, SEIB-DGVM, for describing stand development and succession. The regeneration dynamics from young secondary to old-growth stands were reproduced fairly well with the model. Sprouting recruitment produced high stem density at the beginning of stand development, which caused a self-thinning trajectory following the −3/2 power law. In the late development stage after 70 years, tree species diversity fluctuated because of the regenerative response of sprouting species and the facilitatory effect of typhoon disturbance on the coexistence of subordinate species. The death of canopy trees because of typhoon disturbances reduced the dominance of Castanopsis sieboldii, and depressed its dominance in the understory. Consequently, the understory species could establish by virtue of fallen canopy trees, and tree species diversity increased at the stand level. Clear logging experiments in the model revealed that species diversity deteriorated, especially in the stand dominated by sprouting species. Resilience of subtropical forests was determined by initial species composition before clear logging. Our simulation results suggest that repeated logging drives subtropical forests with high species diversity to a stand monopolized by C. sieboldii.     

Breeding bird community composition in different successional vegetation in the montane coniferous forests zone of Taiwan

To examine the relationship between forest succession following fire and the composition of bird communities, we investigated the vegetation structure, bird population density, foraging behavior and guild structure in bamboo grasslands (11 years since the last fire), pine savanna (41 years), pine woodland (58 years), old-growth hemlock forest (never burned), and old-growth spruce forest (never burned) in the Tatachia area of central Taiwan. Canopy height, total foliage cover, tree density, total basal area of tree, total basal area of snags, foliage height diversity, and tree species richness all increased with successional age. However, shrub cover peaked in intermediate successional stages. The vertical profile of foliage cover was more diverse in later successional forests, which had more breeding bird species and ecological guilds. All the breeding bird species recorded in early and intermediate stages were also found distributed in the late successional forests. Because Taiwan has high precipitation and humidity, and most forest fires in Taiwan are caused by human activities, forest fires and large areas of early successional vegetation were probably rare in the mountain areas of Taiwan prior to the arrival of humans. Therefore, bird species have not had enough time to adapt to areas with early or intermediate successional vegetation. Moreover, late successional forests host all the major plant species found in the early and intermediate stages and have higher foliage height diversity index, which was positively correlated with the bird species richness and bird species diversity index in this study. As a result, all breeding bird species and guilds in the area can be found in late successional forests. Efforts for conserving avian diversity in Taiwan should focus on protecting the remaining native old-growth forests.     

Spatial distribution,advanced regeneration and stand structure of Nepalese Sal (Shorea robusta) forests subject to disturbances of different intensities

We investigated the spatial distribution, advanced regeneration and stand structure of five Shorea robusta-dominated forests in 25 1-ha plots subject to disturbances of different intensities. We aim to elucidate the relationships of advanced regeneration and spatial patterns of the tree species with degree of disturbance magnitude. Sixty-seven tree species were recorded in the forest plots; 41 species were found in the least disturbed forests, while only 10 species were found in the heavily disturbed forests. We found 5320 trees with >1.5 cm diameter at breast height, in total, and found that moderately disturbed forests contained the highest advanced regeneration (sapling)/pole densities. No significant differences were observed in stem basal area among forests. The overall stand density changed quadratically across the disturbance gradient. A strong inverse relationship was found between the overall stand density and diameter class in the least disturbed and moderately disturbed forests. Ten species showed variation in their dispersion patterns across the disturbance gradient. Most of the socio-economically important tree species analyzed showed little or no regeneration in the least and most heavily disturbed forests. Individual species showed different responses to disturbance ranging from ‘tolerant’ (Shorea robusta, Lagerstroemia parviflora and Symplocos spp.) to ‘sensitive’ (Trewia nudiflora, Adina cardifolia and Terminalia alata). We concluded that moderate disturbance intensity not only ensures high stand density, but also enhances the advanced regeneration of socio-economically important tree species and affects their dispersion patterns. Future management strategy must balance the consumptive needs of the local community with those of species conservation by allowing regulated access to the forests.