Response of oriental beech (Fagus orientalis Lipsky) seedlings to canopy gap size

The response of nursery seedlings and wildings of oriental beech(Fagus orientalis Lipsky) to canopy gap size was studied ina climax beech forest in northern Iran with respect to seedlingsurvival, shoot growth and vitality. These parameters were followedfor 2 years after planting in gaps of 50, 200 and 600 m², aswell as in the open field. Foliage coloration was used as acriterion for vitality. The survival rate of nursery seedlingsand wildings dropped rapidly with increasing gap size from 84per cent in 50-m² canopy openings to     

Overstorey density influence on the height of Picea abies regeneration in northern Sweden

The study aimed specifically at investigating if canopy opennesswas a better predictor of the height growth of Norway spruce(Picea abies (L.) Karst.) advance regeneration than overstoreybasal area or overstorey standing volume. In 1990, a field experimentwith 3 x 2 factorial design and two replications (blocks) wasestablished in an uneven-aged Norway spruce forest. Plots hada net plot area of 30 x 30 m, each with a 10-m-wide treatedbuffer zone. Three overstorey density levels retained approximately15, 40 and 70 per cent of the pre-harvest overstorey standingvolume and were allotted to the plots. Two types of thinningthat harvested smaller trees or harvested larger trees wererandomly allocated to each pair of overstorey density plots.In mid-June 2000, canopy openness was estimated from hemisphericalphotographs taken at five marked points in the centre of eachof the plots at 0.9 m from ground to the top of the ‘fish-eye’camera lens. Regression results showed that canopy opennesswas a better predictor of height increments of spruce seedlings(0.1< height < 0.5 m), saplings (0.5 height < 2.0m), and small trees (height 2.0 m, diameter at 1.3 m height< 5 cm) than with overstorey basal area (m² ha^–1) oroverstorey standing volume (m³ ha^–1). The height incrementof the spruce advance regeneration was not significantly correlatedto stand basal area or to standing volume. Overstorey basalarea in the net plots was significantly negative (P 0.05) withmean canopy openness estimates, and the r² value was 0.40. Resultsindicated that basal area was not linearly related to canopyopenness as it increased, which might explain the lack of predictivepower of retained basal area on spruce regeneration height indense stands in boreal Sweden.     

Thirty years of gap dynamics in a central european beech forest reserve

Investigation of gap characteristics and tree regeneration patternsis central for our understanding of forest dynamics. By integratingaerial photograph analyses and ground surveys, we provide astudy of long-term canopy gap dynamics and tree regenerationpatterns in a Hungarian beech forest reserve. We found (1) thatin spite of the overall increase of gap area during the investigated30-years (from 2.5 to 7.7 per cent), total gap area and averagegap size (40–93 m²) were remarkably similar to those foundin other temperate and tropical forests, (2) if the fate ofindividual gaps was followed, two to three times more intensivecanopy dynamics (gap creation, closure and expansion) couldbe recognized than simple change of gap area indicated, and(3) average seedling density was considered to be sufficientfor natural regeneration. However, it was apparent that recentincreased deer browsing had prevented establishment of youngertrees of 1–2 m in height, as taller saplings were recordedonly in old gaps. Our results not only provide useful informationon forest dynamics but can also contribute to understandingthe potential roles that small forest reserves can play in providingessential reference data for nature-based forest managementof this forest type.     

Characteristics of lowland broadleaved woodland being restocked by natural regeneration

A survey of 78 sites in southern England with approved managementplans for restocking by natural regeneration was made duringthe summers of 1993 and 1994. The following features were recorded:species, stocking, canopy cover and seed-bearing potential oftrees present in the overstorey; species and canopy cover ofthe understorey; ground cover; species, browsing damage, numberand heights of tree seedlings. In general, sites were poorlystocked with overstorey trees having an average of 135 stemsha^–1 and a mean canopy cover of 36 per cent. Similarly,the understoreys were poorly developed with an average coverof only 23 per cent. Twenty-nine species of tree were foundin the overstorey, Quercus spp. and Fraxinus excelsior werethe most common. Many of the stems present had poor seed-bearingpotential. Cover of the ground flora often exceeded 75 per cent.Seedlings were present on most sites, with F. excelsior andBetula pendula being most abundant with mean seedling densitiesexceeding 10000 ha^–1. Most seedlings were >20 cm tall,few exceeded 120 cm. About 30 per cent of seedlings were browsed.The results are related to current guidance and the future prospectsfor use of natural regeneration.     

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.     

Factors influencing tree species diversity and Betula alleghaniensis establishment in silvicultural openings

We examined the short-term effects of group-selection harvestingwith seed tree retention on the diversity, abundance and establishmentof tree seedlings in a northern hardwood forest in the UpperPeninsula of Michigan (49 openings, 20 closed canopy referencesites). Three opening sizes were examined – opening radius0.5 x canopy height (267 ± 62 m², n = 16), 0.75 x canopyheight (642 ± 85 m², n = 17) and 1.0 x canopy height(1192 ± 155 m², n = 16) (canopy height = 22 m). A singleyellow birch (Betula alleghaniensis Britt.) seed tree was retainedin the centre of each opening. Tree seedling density was significantlyhigher in the largest group-selection opening than at the closedcanopy reference sites (P < 0.05), the main factor for thiswas the increased proportion of yellow birch, red maple (Acerrubrum L.) and other minor species. Nevertheless, yellow birchwas still a minor component of the developing gap cohort, comprising5.9 per cent of the seedlings and 1.1 per cent of the saplings.Within openings, microsite variables, such as per cent coversof bare soil and coarse woody debris, were the best predictorsof yellow birch occurrence and density. Our results suggestthat microsite limitations and competing vegetation may greatlyreduce the efficacy of openings for ensuring the maintenanceof mid-tolerants.     

Canopy recruitment in the beech (<Emphasis Type="Italic">Fagus engleriana</Emphasis>) forest of Mt. Shennongjia,Central China

A dendrochronological technique was used to investigate canopy recruitment of beech (Fagus engleriana Seem.) trees in western Hubei Province of Central China by identifying growth releases through gap formation. The results indicated that 83% and 17% of the canopy trees regenerate in gaps and the understory, respectively. Radial growth analyses demonstrated that there are three modes for beech regeneration and canopy recruitment: (1) gap origin without suppression; (2) gap origin with period(s) of suppression; and (3) understory origin. Multiple periods of release and suppression imply that beech regeneration and canopy recruitment are very sensitive to gap formation.     

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.

     

Photosynthesis and biomass allocation of beech (Fagus crenata) and dwarf-bamboo (Sasa kurilensis) in response to contrasting light regimes in a Japan Sea-Type beech forest

Regeneration of beech (Fagus crenata) forests depends on the formation of canopy gaps. However, in Japan Sea-type beech forests, a dwarf bamboo (Sasa kurilensis) conspicuously occupies sunny gaps. Therefore,F. crenata seedlings must escape the severe interference ofS. kurilensis in the gaps and persist beneath a closed canopy of the beech forest. We hypothesized that the growth ofF. crenata seedlings in the understory would be favored by their being more plastic thanS. kurilensis in photosynthetic and morphological traits, which would support the matter production ofF. crenata seedlings in a wide range of light availabilities. To examine this hypothesis, the photosynthetic-light response of individual leaves and the biomass allocation in aboveground parts (i.e., the culm/foliage ratio) were surveyed at sites with contrasting light availabilities in a Japan Sea-type beech forest in central Japan. InF. crenata, photosynthetic light utilization efficiency at relatively low light was greater, and the dark respiration rate was smaller in the leaves of seedlings (10 cm in height) beneath the closed canopy than in the leaves of saplings at the sunny forest edge. The culm/foliage (C/F) ratio of theF. crenata seedlings at the shady site was small, suggesting effective matter-production beneath the beech canopy. On the other hand,S. kurilensis both in the gap and beneath the beech canopy showed low plasticity in photosynthesis and the culm/foliage ratio. Because the shoot density ofS. kurilensis was smaller beneath the beech canopy than in the gap, the light availability at the bottom of theS. kurilensis layer was greater beneath the beech canopy. These results suggest that the photosynthetic productivity of theF. crenata seedlings would be enough for the seedlings to survive in the understory with a low density ofS. kurilensis shoots beneath the closed beech canopy.     

Regeneration dynamics of Sitka spruce in artificially created forest gaps

This study examined the variation in the development of naturally regenerated and planted seedlings of Sitka spruce (Picea sitchensis (Bong.) Carr.) within gaps cut in a 32-year-old stand of the same species. The circular gaps were 20 m in diameter and designed to allow sunlight into only half of the gap floor at midsummer given the latitude of 56°45′N. Eight plots (8 m × 3 m) were laid out along a north–south transect through each gap (four within the gap and two each under the closed canopy north and south of the gap). Each plot was sub-divided and seedlings were planted into one part and the other part was left to naturally regenerate. In subsequent seasons, plots were further subdivided into ‘weed free’ and ‘vegetation left untouched’. Results showed that while the two central plots within the gaps had the highest value of canopy openness, the highest accumulated temperature and lowest soil moisture were recorded in plots that received direct sunlight. However, level of germination was significantly higher in the shaded area of the gap than in the part that received direct sunshine suggesting that higher moisture levels in shaded areas are important to successful germination. Minimal germination was recorded in the plots beneath the canopy. Seedling survival was significantly influenced by the influx of competing vegetation, but only in the part of the gaps that received direct sunlight. The success of Sitka spruce regeneration within gaps appears to depend on sufficient moisture and light to support regeneration and early growth, but not too much light to encourage the development of competing vegetation. The permanently shaded areas of the gaps appeared to offer ground conditions with sufficient moisture and light to ensure successful germination and early growth of seedlings, but without excessive competition from other vegetation.     

Growth and mortality in underplanted tree seedlings in response to variations in canopy closure of Norway spruce stands

Six broadleaved tree species and Picea abies (L.) Karst. wereplanted under spruce plantations of varying densities, in Sweden.Treatments included control (994 stems ha^–1), dense (538stems ha^–1), sparse (294 stems ha^–1) and gap (0stems ha^–1) overstory treatments. There was an increasein height and diameter growth from control to sparse overstorytreatment of all underplanted tree species except for ash (Fraxinusexcelsior L.) and Norway maple (Acer platanoides L.). Site conditionsmay have hampered the growth of these species, as well as wildcherry (Prunus avium L.). Both oak (Quercus robur L.) and sprucehad greater growth in the gap treatment, relative to the othertreatments. Insecticide application did not influence seedlinggrowth or survival. The performance of beech (Fagus sylvaticaL.), lime (Tilia cordata Mill.), spruce and oak was consistentwith shade tolerance ranking. Beech and lime had a very highsurvival rate, even under the densest canopy. The growth andmortality of ash, maple and wild cherry differed significantlyfrom what was expected. This experiment demonstrated significantvariation in interspecific growth and mortality between sevenplanted tree species in relation to canopy density. Correctsite and species selection is crucial when underplanting inshelterwood systems.     

Forest regeneration in artificial gaps twelve years after canopy opening in Acre State Western Amazon

The main objectives were to study the effect of gap size and canopy openness on the natural regeneration dynamics considering the parameters of sapling growth, recruitment, mortality, density, species composition and above-ground biomass accumulation. The study was carried out in 32 artificial gaps with sizes varying from 100 to 1200 m² and canopy openness from 10 to 45%, from the second to the twelfth year after gap creation. The gap size was measured using the vertical projection of the tree crowns on the ground (Brokaw's definition), and the canopy openness measurement by hemispherical photography. In the first five years, mean sapling growth (0.54 cm year⁻¹), mortality (3.9% year⁻¹) and AGB (26.2 Mg ha⁻¹ or 8.7 Mg ha⁻¹ year⁻¹) were significantly higher in the gaps than in the forest understorey (0.17 cm year⁻¹, 1.5% year⁻¹ and −0.59 Mg ha⁻¹ year⁻¹ respectively) and positively correlated with gap size and canopy openness. In the same period, recruitment was also significantly higher in the gaps (5.8% year⁻¹) than in the forest understorey (0.4% year⁻¹) but decreased with gap size and negatively correlated with canopy openness. In the first five years, the relative density of pioneer species was higher in the gaps but not significantly correlated with gap size or canopy openness. AGB increased linearly since canopy opening, and twelve years after gap creation it was still higher in larger (121.2 Mg ha⁻¹ or 10.1 Mg ha⁻¹ year⁻¹) rather than smaller (62.5 ha⁻¹ or 5.2 ha⁻¹ year⁻¹) gaps. Twelve years after gap creation there were no significant differences in the parameters of sapling growth, recruitment, and mortality which could be attributed to the original gap size and canopy openness.     

Effects of overstorey retention on ash regeneration and bramble growth during conversion of a pine plantation to native broadleaved woodland

Conversion of monoculture plantations to mixed stands with greater diversity is an important aim of sustainable forest management, and in Britain, this is concentrated on the restoration of native broadleaved woodland on ancient woodland sites that were planted with conifers. Current British guidelines for this restoration have rarely been examined, and this study is the first to test their value for natural regeneration. The survival and growth of naturally regenerating ash (Fraxinus excelsior L.) seedlings was observed in thinned, selectively felled and clearfelled plots within a c. 40-year-old stand of Corsican pine (Pinus nigra Arnold ssp. laricio Maire). After felling in 2001, the ground flora in all treatments became dominated by bramble (Rubus fruticosus L. agg.) and its cover 5 years after felling was 75–95?% at a height of 0.9–1.2?m. During the study, seedling numbers declined 80–90?%. Although bramble had some adverse effects, seedling survival was unaffected by felling treatment. Seedling growth varied between treatments with those in the clearfell being tallest. Mean height of seedlings was always less than that of bramble, but by 2006 65?% of quadrats in the clearfelled plots had well-established individuals taller than the bramble compared with 35?% in the thinned plots. Models that described the relationships between seedling growth, basal area and bramble were complex, but results clearly indicate that maintaining canopy cover during restoration is probably an ineffective method of suppressing bramble on sites where it grows well. Results do not support the current British advice on conversion, but this may reflect the characteristics of pine canopies and the maintenance of canopy cover during conversion may be appropriate in stands of species, which cast more shade.     

Tree recruitment in gaps of various size,clearcuts and undisturbed mixed forest of interior British Columbia,Canada

Tree seedling recruitment was monitored after various types of logging in mixed conifer and deciduous forests of northern British Columbia, Canada. Predicting tree seedling recruitment after disturbance is fundamental to understanding forest dynamics and succession and is vital for forest management purposes. Seedling recruitment success in multi-species northern latitude forests varied as a function of mature tree canopy cover, gap size and position in a gap. Recruitment was abundant within canopy gaps across a wide range of gap sizes (20–5000 m²), but recruit numbers dropped off rapidly under the closed forest canopy and in the open conditions of clearcuts. Inside canopy gaps, recruitment was similar by gap position in small gaps (<300 m²) but, in these northern latitude forests, exhibited a trend of increasing density from the sunny north to shady south end of larger gaps. This was true for all tree species regardless of their shade tolerance ranking. There was no evidence of gap partitioning by any of the tree species during the regeneration phase suggesting that adaptation to the subtleties of gap size during early recruitment are not well developed in these tree species. Favorable locations for emergence and early establishment of germinants were less favorable for growth and survival of established seedlings, i.e. the regeneration niches in these forests were discordant. Tree abundance and species diversity appears to be controlled more by differentiation among growth and survival niches than by the regeneration niches. From the perspective of forest management, abundant natural regeneration of all the dominant tree species of these mixed-species forests can be obtained after partial cutting.     

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.     

Development of Quercus robur advance regeneration following canopy reduction in an oak woodland

Observations to follow the development of 2-year-old oak seedlingstook place over a 6-year period at a fenced site within a maturestand of oak in southern England. Bramble and bracken were abundant,but the cover of other species was low. Although seedlings doubledin height, only 13 per cent survived and few were taller thaneither bramble or bracken. The initial number of seedlings wasinversely related to both canopy cover and distance to the nearestparent tree. The final number of seedlings could be predictedfrom the initial percentage covers of bramble and bracken, andeither the overstorey canopy cover or the initial number ofseedlings. The average annual rate of seedling mortality, whichwas 25–50 per cent, was significantly influenced by thevegetation present. Grass was positively related to survivalwhereas there was an inverse relationship with both brambleand bracken cover. Models estimated 80 per cent annual seedlingsurvival at 25 per cent bramble cover but only 50 per cent at60 per cent cover. Although grasses can be competitive species,results indicate that oak seedling establishment may be betterin areas with a ground flora comprising grass.     

Response to canopy opening does not act as a filter to Fagus sylvatica and Acer sp. advance regeneration in a mixed temperate forest

? In mixed-species forest stands, large losses in tree species diversity often occur during the regeneration phase. In a former coppice-with-standards, we investigated whether the limiting stage in the recruitment process of advance regeneration is the immediate seedling response to canopy release. Experimental canopy gaps were opened and the survival and growth of advance seedlings (Fagus sylvatica, Acer pseudoplatanus, Acer campestre, Acer platanoides) growing in the gaps or under closed canopy were monitored for three years.

? All species responded positively and rapidly to canopy release. Survival was not affected by gap opening. Diameter increment after gap opening was similar across species, and height increment was greater for Acer platanoides and for Acer pseudoplatanus. Post-release diameter and height growth were mainly determined by pre-release seedling size. Competition from neighbouring seedlings did not affect growth in the three years following canopy opening.

? In the recruitment process of F. sylvatica and Acer sp. advance regeneration, the recovery from canopy release did not appear as a limiting step that would filter against some species. Pre-release size was the main factor accounting for post-release growth and is probably a major determinant of long-term seedling dominance.

     

Correlations between canopy gaps and species diversity in broad-leaved and Korean pine mixed forests

Regeneration of tree species associated with canopy gaps in broad-leaved Korean pine forests was investigated. Species diversity in gaps and under closed canopy was compared, the relationship between biodiversity and gap structure was analyzed. Results indicate that there were significant differences between tree species diversity in gaps and that under canopy (p<0.01). In terms of Shannon-Wiener index, evenness index, and abundance index, the biodiversity in gap community were higher than those under forest canopy in regeneration layer. In terms of Simpson’s dominance index, the dominance of certain species in the regeneration layer increased from gaps to closed canopy (p<0.01). In contrast, trends of biodiversity changes of succession layer in gaps and under closed canopy were opposite. Tree species diversity of different layers reacted directly to the change of gap size class. For example, Shannon-Wiener index and abundance index is higher and Simpson’s dominance index is the lowest in succession layer of medium-size gap (100–250 m²) in the broad-leaved Korean pine forest of Changbai Mountains. Shannon-Wiener index reached the highest in a size of ≥250 m² and <100 m², reached the lowest in a size of 200–250 m² in the regeneration layer. Simpson’s dominance index reached its maximum when the gap size was between 200 and 250 m². Generally, species of different layers reacted differently to the changes of gap size classes. The gap size class with more seedlings did not correspond to size class containing more medium-size trees. Tree species diversity indices in the two layers behaved reciprocally during the development process of forest gaps. __________ Translated from Chinese Journal of Applied Ecology, 2005, 16(12): 2,236–2,240 [译自: 应用生态学报, 2005, 16(12): 2,236–2,240]     

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.     

Interacting effects of canopy gap, understory vegetation and leaf litter on tree seedling recruitment and composition in tropical secondary forests

We experimentally investigated interacting effects of canopy gaps, understory vegetation and leaf litter on recruitment and mortality of tree seedlings at the community level in a 20-year-old lowland forest in Costa Rica, and tested several predictions based on results of previous studies. We predicted that experimental canopy gaps would greatly enhance tree seedling recruitment, and that leaf litter removal would further enhance recruitment of small-seeded, shade-intolerant seedlings in gaps. We created a large (320–540 m²) gap in the center of 5 out of 10 40 m × 40 m experimental plots, and applied the following treatments bimonthly over a 14-month-period in a factorial, split–split plot design: clipping of understory vegetation (cut, uncut), and leaf litter manipulations (removal, addition, control). As expected, experimental gaps dramatically increased tree seedling recruitment, but gap effects varied among litter treatments. Litter addition reduced recruitment in gaps, but enhanced recruitment under intact canopy. Species composition of recruits also differed markedly between gap treatments: several small-seeded pioneer and long-lived pioneer species recruited almost exclusively in gaps. In contrast, a few medium-to-large-seeded shade-tolerant species recruited predominantly under intact canopy. Leaf litter represents a major barrier for seedling emergence and establishment of small-seeded, shade-intolerant species, but enhances emergence and establishment of large-seeded, shade-tolerant species, possibly through increased humidity and reduced detection by predators. Periodic clipping of the understory vegetation marginally reduced tree seedling mortality, but only in experimental gaps, where understory vegetation cover was greatly enhanced compared to intact canopy conditions. Successful regeneration of commercially valuable long-lived pioneer trees that dominate the forest canopy may require clear-cutting, as well as weeding and site preparation (litter removal) treatments in felling clearings. Management systems that mimic natural canopy gaps (reduced-impact selective logging) could favor the regeneration of shade-tolerant tree species, potentially accelerating convergence to old-growth forest composition. In contrast, systems that produce large canopy openings (clear-cutting) may re-initiate succession, potentially leading to less diverse but perhaps more easily managed “natural plantations” of long-lived pioneer tree species.