Effect Of Canopy Openness Research Articles

Effects of subtle variation in forest canopy openness on cache pilferage and its implications for forest regeneration.

Scatter-hoarding rodents play important roles in plant regeneration and species coexistence in many forest ecosystems. Cache pilferage, the behavior of rodents seeking or relocating seeds cached by other individuals, is ubiquitous during the scatter-hoarding process. The effects of canopy openness on cache pilferage have received considerable attention, most of which have focused on the comparison between full canopy cover and completely open areas, such as forest gaps. However, little attention has been given to whether the subtle variation in forest canopy openness affects cache pilferage, although subtle variation in light environments exists in many forests, especially tropical and subtropical forests, where the overall canopy is large and the forest window is relatively small. Here, we directly tested these questions by simulating 400 artificial caches, each containing one seed from four selected tree species, in a subtropical forest in southwestern China. The overall canopy openness of the forest was relatively small (with a mean value of 11.1%), but subtle spatial variation still existed (ranging from 5.7% to 19.5%). Overall, caches with lower canopy openness were more likely to be pilfered and removed faster, although not all species showed the same pattern. Our study highlights that subtle variation in forest canopy openness, even in a closed primary forest, has significant effects on cache pilferage by rodents, which may influence the following seed germination and forest regeneration processes. Additionally, seedling species composition may further be altered because the canopy effects on cache pilferage are species-specific.

Microclimatic buffering in forest, agricultural, and urban landscapes through the lens of a grass‐feeding insect

AbstractFor small ectotherm species, dealing with microclimatic variation during sensitive stages of their development is a critical component of their ecological interactions within and among different landscape settings. However, it is often unknown whether the use of habitats in different landscape settings may affect microclimatic conditions at the level of microhabitats (e.g., at host plants for herbivorous insects). As opposed to standard ecological monitoring experiments, we aim to identify the microclimatic variables that accurately represent the environment in which a small arthropod spends most of its life. As a case study, we selected grassy sites that corresponded to the microhabitat of grass‐feeding insects in general, and larvae of the butterfly Pararge aegeria in particular, as this insect recently expanded its habitat use from forest (edges) to agricultural and urban environments. We tested the extent to which local microclimates and microclimatic buffering capacity differed between tufts of grass in forest and in two anthropogenic (i.e., agricultural and urban) landscape settings by measuring microclimatic variables with sensors at the level of the grasses. We compared temperature, relative humidity, and vapor pressure deficit (VPD) during an exceptionally warm and dry summer period with parallel data from weather stations and tested for differences between microclimatic profiles among the three landscape settings. Microclimatic conditions correspond only partly to variables based on weather station data. Differences are particularly significant for relative humidity and VPD, but not for day and night temperatures. Effects of canopy openness on the variability of microclimatic variables are landscape‐specific. Host plants in agricultural settings experience strong microclimatic fluctuations and low short‐term variability in VPD. In urban settings, urban heat island effects increase night temperature but also show microclimatic buffering effects similar to the ones in forest settings. Our findings stress the functional implications of landscape‐specific microclimatic profiles at the appropriate organism‐centered scale. We discuss the significance of such microclimatic profiles for ecological studies dealing with ectotherms under climate and land use change.

Long‐term responses of gastropods to simulated hurricanes in a tropical montane rainforest

AbstractClimate‐induced disturbances such as hurricanes affect the structure and functioning of ecosystems, especially those in the Caribbean Basin, where high‐energy storms have long affected ecosystem dynamics. Because climate change will likely continue to alter the frequency and intensity of hurricanes in the Caribbean, it is increasingly important to understand the mechanistic bases for ecosystem responses. Although long‐term surveys and nonmanipulative “natural experiments” provide considerable insight, manipulative experiments are required to decouple confounded factors associated with high‐intensity storms. To address this, we exploited a replicated factorial design to experimentally isolate the long‐term effects of canopy opening and debris deposition on population‐ and community‐level characteristics of gastropods in the Luquillo Experimental Forest of Puerto Rico. The canopy trimming experiment included four treatments: (1) the reference treatment received no manipulations; (2) canopy trimmed, but debris from trimming not deposited on the forest floor (trim only); (3) canopy not trimmed, but debris deposited on the forest floor (debris only); and (4) canopy trimmed and debris deposited on the forest floor (trim and debris). After 10 years, the trim and debris treatment was repeated to simulate reoccurring hurricane events, whereas the trim‐only and debris‐only treatments were not subject to additional manipulation at this time. We evaluated responses to treatment and time for gastropod populations (abundance) and communities (biodiversity and composition). Population‐level responses were species specific. Three species and total gastropod abundance exhibited consistent responses to treatments regardless of time, four species exhibited consistent temporal trends regardless of treatment, and five species exhibited an interaction in which the effects of time depended on treatment. In general, point‐, α‐, and γ‐biodiversity decreased through time, whereas β‐biodiversity increased through time. Gastropod populations and communities were resistant and resilient to the simulated disturbances, exhibiting quick recovery from any short‐term changes in abundance or biodiversity. From an evolutionary perspective, long‐term exposure to hurricane‐induced disturbances likely leads to species‐specific adaptations that enhance resistance and resilience.

Are Juglans neotropica Plantations Useful as a Refuge of Bryophytes Diversity in Tropical Areas?

Neotropical montane forests are considered biodiversity hotspots, where epiphytic bryophytes are an important component of the diversity, biomass and functioning of these ecosystems. We evaluated the richness and composition of bryophytes in secondary successional forests and mixed plantations of Juglans neotropica. In each forest type, the presence and cover of epiphytic bryophytes was registered in 400 quadrats of 20 cm × 30 cm. We analyzed the effects of canopy openness, diameter at breast height (DBH) and forest type on bryophyte richness, using a generalized linear model (GLM), as well as the changes in species composition using multivariate analysis. Fifty-five bryophyte species were recorded, of which 42 species were in secondary forests and 40 were in mixed plantations. Bryophyte richness did not change at forest level; however, at tree level, richness was higher in the mixed plantation of J. neotropica compared to the secondary forests, due to the presence of species adapted to high light conditions. On the other hand, bryophyte communities were negatively affected by the more open canopy in the mixed plantation of J. neotropica, species adapted to more humid conditions being less abundant. We conclude that species with narrow microclimatic niches are threatened by deforestation, and J. neotropica plantations do not act as refuge for drought-sensitive forest species present in secondary forests.

Effect of canopy openness and meteorological factors on spatial variability of throughfall isotopic composition in a Japanese cypress plantation

AbstractSpatial variability of throughfall (TF) isotopic composition, used as tracer input, influences isotope hydrological applications in forested watersheds. Notwithstanding, identification of the dominant canopy factors and processes that affect the patterns of TF isotopic variability remains ambiguous. Here, we examined the spatio‐temporal variability of TF isotopic composition in a Japanese cypress plantation, in which intensive strip thinning was performed and investigated whether canopy structure at a fine resolution of canopy effect analysis is related to TF isotopic composition and how this is affected by meteorological factors. Canopy openness, as an index of canopy structure, was calculated from hemispherical photographs at different zenith angles. TF samples were collected in a 10 × 10 m experimental plot in both pre‐thinning (from July to November 2010) and post‐thinning (from May 2012 to March 2013) periods. Our results show that thinning resulted in a smaller alteration of input δ18O of gross precipitation, whereas the changes in deuterium excess varied in both directions. Despite the temporal stability of spatial patterns in TF amount, the spatial variability of TF isotopic composition was not temporally stable in both pre‐ and post‐thinning periods. Additionally, after thinning, the isotopic composition of TF was best related to canopy openness calculated at the zenith angle of 7°, exhibiting three different relationships, that is, significantly negative, significantly positive, and nonsignificant. Changes in meteorological factors (wind speed, rainfall intensity, and temperature) were found to affect the relationships between TF δ18O and canopy openness. The observed shifts in the relationships reveal different dominant factors (partial evaporation and the selection), and canopy water flowpaths control such differences. This study provides useful insights into the spatial variability of TF isotopic composition and improves our understanding of the physical processes of interception through canopy passage.

Vertical distribution of 137Cs in the contaminated soil of a spruce forest in Southern Germany 12 years after regeneration cutting

Abstract The Chernobyl fallout of 1986 contaminated soils in Southern Germany. Levels of 137Cs activity still remain high in forest soils. The contamination of forest food products still poses a serious problem for consumer health. Silvicultural interventions may offer an option to reduce the bioavailability of 137Cs. The objective of this study is to investigate a possible effect of canopy opening and clearcutting as well as the selection of conifers and broadleaves for stand regeneration on the vertical soil profile of 137Cs. The investigation uses a long-term silvicultural experiment on the artificial regeneration of a spruce stand 30 km southwest of the town Augsburg, Germany. 12 years after silvicultural intervention soil samples were taken horizon-wise and analysed to compare among the treatments. A suite of soil physical and chemical properties was determined per horizon including 137Cs radioactivity, total carbon, texture, and cation exchange capacity. The results show that (i) the 137Cs contamination at this site is high compared to other sites in Central Europe, (ii) regeneration cuttings did not markedly change the vertical 137Cs profile in general within one decade, (iii) clear cutting modified the boundary between the organic soil layer and the mineral soil with expectable effects on the future 137Cs bioavailability.

Species Richness of Epiphytic Lichens in Coniferous Forests: the Effect of Canopy Openness

Epiphytic lichen communities were studied in old coniferous forests across Estonia to find out the effect of site openness on the lichen species richness. All lichen species were recorded on the basal 2 m of 105 Picea abies and 105 Pinus sylvetsris trunks. The canopy openness readings were taken with a spherical densiometer around every sampled tree. The number of species on the trunks of both tree species increased significantly with increasing canopy openness. Nine lichen species, e.g. Cladonia cenotea, Lecidea nylanderi, Platismatia glauca, were significantly favoured by higher light availability, and only one species, Coenogonium pineti, by lower light availability. The results demonstrate that in the coniferous forests higher light availability supports higher species richness of epiphytic lichens on the lower trunk of trees.

Effect of canopy openness on the pressure of predatory arthropods and birds on epigeic insects

Abstract As canopy structure produces spatial heterogeneity of litter microclimatic conditions and thus is a crucial factor affecting ground insects, we hypothesized that low canopy openness has a positive effect on the activity of ground insect predators in forest and non-forest habitats. Blowfly larvae were used as bait along the canopy openness gradient (forest interior, forest edge, base of a solitary tree and meadow) and the attack rate was assessed after 30 min of exposure. Although the predation rate has a varying pattern throughout the year in different habitats, in contrast to previous studies, we observed a significant positive trend in predation rates toward the forest interior. A significant trend in predation rate was not observed in non-forest areas. We found that the trend was strongly influenced by ants as the most active taxon of predators (65%) attacking our baits, whereas ground beetles, the second-most active predators (21%), showed the opposite trend along the canopy openness gradient.

Environmental factors influencing adult sex ratio in Trinidadian guppies

Sex ratios can influence mating behaviour, population dynamics and evolutionary trajectories; yet the causes of natural sex ratio variation are often uncertain. Although secondary (birth) sex ratios in guppies (Poecilia reticulata) are typically 1:1, we recorded female-biased tertiary (adult) sex ratios in about half of our 48 samples and male-biased sex ratios in none of them. This pattern implies that some populations experience male-biased mortality, perhaps owing to variation in predation or resource limitation. We assessed the effects of predation and/or inter-specific resource competition (intraguild predation) by measuring the local catch-per-unit-effort (CPUE) of species (Rivulus killifish and Macrobrachium prawns) that may differentially prey on male guppies. We assessed the effects of resource levels by measuring canopy openness and algal biomass (chlorophyll a concentration). We found that guppy sex ratios were increasingly female-biased with increasing CPUE of Macrobrachium, and perhaps also Rivulus, and with decreasing canopy openness. We also found an interaction between predators and resource levels in that the effect of canopy openness was greatest when Macrobrachium CPUE was highest. Our study thus also reveals the value of simultaneously testing multiple environmental factors that may drive tertiary sex ratio variation.

Secondary production of Gammarus pulex Linnaeus in small temperate streams that differ in riparian canopy cover

Variation in the amount of riparian canopy cover affects bioenergetic processes in streams. Effects of canopy opening on environmental parameters including water temperature and the quality and quantity of food resources (leaf litter and/or associated biofilm) are likely to influence detritivore production. The objective of this study was to quantify population dynamics of the common shredder Gammarus pulex in three temperate lowland stream reaches that differ in riparian canopy cover. Benthic samples were taken monthly from October 2001 until October 2002, and secondary production was estimated using the size-frequency method. Populations differed in size/age structure, length-weight ratio, mean and maximum amphipod size, which were related to the amount of riparian canopy cover. Population density and secondary production were less clearly associated with canopy cover, which was probably related to a difference in fish predation between sites. Canopy opening was associated with differences in the range of variation in annual thermal conditions and most likely in the nutritional quality (microflora) of the food source, whereas the autumnal pulse of leaf fall lead to oxygen depletion at the heavily shaded woodland reach. Our study demonstrated that moderate differences in riparian canopy cover affect shredder-detritivore population dynamics and suggests that even detrital-based and shredder-dominated food webs can be influenced by the indirect effects of light. The temporal differences in temperature appear biologically less important unlike the effect of increased algal production. Whereas the array of interacting factors (temperature, radiation, leaf processing and microbial colonization) emphasize the complexity of effects associated with canopy opening.

Redução de esforço amostral vs. retenção de informação em inventários de pteridófitas na Amazônia Central

A extensa área coberta pelo domínio amazônico e os recursos financeiros limitados para estudos da biodiversidade exigem programas de pesquisa que obtenham o máximo de informação com os menores custos. Com o objetivo de avaliar as conseqüências da redução do esforço amostral sobre a retenção da informação ecológica, testou-se como diferentes larguras de parcela afetam os resultados da relação entre variação ambiental e composição da comunidade de pteridófitas da Amazônia Central. Foram feitas medidas da inclinação do terreno, abertura do dossel e do teor de argila do solo, em 37 parcelas de floresta de terra-firme. Todos os indivíduos de pteridófitas foram identificados e mapeados com relação a três faixas de amostragem. Foram testados os efeitos das variáveis ambientais sobre a composição de pteridófitas em parcelas de 250 x 2,5 m e em sub-amostras com a largura da parcela reduzida. A redução de 2,5 para 1 m na largura da parcela (60% de redução no esforço amostral) correspondeu a uma redução de 24% no custo do projeto, considerado os custos das excursões à campo, e a uma redução no número de espécies amostradas de 52 para 44. A redução 2,5 para 2 m na largura da parcela causou redução de apenas duas (4%) nas espécies amostradas e 8% no custo do projeto. Para todas as larguras testadas, a composição da comunidade esteve associada ao teor de argila e não esteve associada à inclinação do terreno. O efeito de abertura de dossel sobre a composição de espécies de pteridófitas não foi coerente entre os diferentes tamanhos de amostra. Esta incoerência deve estar relacionada à importância relativamente menor da luz sobre a estruturação da comunidade. Concluímos que as amostras com menor tamanho foram suficientemente informativas para detectar os principais gradientes de composição e sua associação aos fatores ambientais. Isso permitiria a redução nos custos totais ou alocação de mais parcelas pelo mesmo custo, o que poderia aumentar o poder dos testes estatísticos, reduzir o intervalo de confiança e aumentar a probabilidade de detecção de mais espécies.

Remote Sensing of Forest Regeneration in Highland Tropical Forests

Extensive post-agriculture forest regeneration has been observed in Celaque National Park (CNP), Honduras. Landsat TM images were useful in classifying these highland tropical forests into four regeneration stages and one degraded class. Regeneration stages were best separated in a plot of Landsat bands TM5 and TM4. TM5 had the best correlation with regeneration stage followed by TM2. NDVI could not discriminate forests past the early regrowth stage due to the rapid saturation of the index. A clear successional path was observed in the Tasseled Cap feature space. Forest development was characterized by a continuous decrease in Brightness, a continuous increase in Wetness, and a rapid initial increase in Greenness followed by a gradual decrease. Regenerating forests followed spectral paths more similar to lowland tropical than to temperate forests. Early rapid increases in NIR reflectance and the Tasseled Cap Greenness index were in line with those of tropical lowland rainforests and unlike those found in temperate forests. Temperate oldgrowth forests may exhibit higher levels of Greenness and lower levels of Wetness than equivalent highland tropical forests due to the effects of canopy opening and senescence that tend to occur in aging temperate forests. Highland tropical forests are expected to regenerate more slowly than lowland tropical forests but this was not the case in CNP.

Effect of canopy openness on growth, specific leaf area, and survival of tree seedlings in a temperate rainforest of Chiloé Island, Chile

Tree species of intermediate‐shade tolerance may regenerate in canopy gaps as well as in the forest understory. However, seedling performance may vary widely between shaded and open habitats. In this study, we assessed the differences in relative growth rate (RGR), specific leaf area (SLA), and survival of seedlings of the tree species Drimys winteri (Winteraceae) and Gevuina avellana (Proteaceae) planted in three forest habitats differing in light availability: (1) forest interior, (2) forest edge, and (3) tree‐fall gap (1%, 6%, and 12% of canopy openness, respectively). A total of 60 3‐month‐old seedlings of each species were monitored over 16 months (December 2001‐April 2003) in the three habitats of a Valdivian rainforest fragment on Chiloé Island, southern Chile. RGR was greatest in the tree‐fall gap in both species. Allocation to leaves was proportionally higher in the forest interior. SLA tended to be lower in the tree‐fall gap, intermediate in the forest edge, and greater in the forest interior in both species. In addition, SLA of D. winteri seedlings was always greater than that of G. avellana in all three habitats. Survival of both species was greatest under the canopy gap and lowest in the forest interior. Survival of seedlings of the larger‐seeded species, G. avellana, was greatest during the first few months of the experiment in the forest interior, but then dropped significantly in the period during which seedlings of the smaller‐seeded species, D. winteri, survived in greater numbers. Our comparative study of two intermediate‐shade‐tolerant tree species documented wide variation in growth and survival of seedlings among habitats differing in canopy openness.

Quercus michauxii regeneration in and around aging canopy gaps

Floodplain forests are subject to frequent windstorms, which create canopy gaps and microtopographic heterogeneity. Forest regeneration may be enhanced when light and microtopographic conditions are both favorable, but slower growing canopy species may still require multiple disturbance events to reach the canopy. In 2001, we revisited a cohort of Quercus michauxii Nutt. seedlings planted in 1995 on pit–mound microsites that were constructed in and around canopy gaps to determine patterns of seedling persistence and investigate the effects of canopy openness and microtopography on seedling survival and growth. After 7 years, canopy openness in gap centers had decreased to levels that did not differ from levels in forest canopy. Seedling height and maximum root depth were greatest in gap centers, where light was initially greater but seedling growth rates declined over time. Soil moisture was greater in pits, where establishment and survival were very low. Roots of some seedlings reached from mound surfaces to depths and moisture levels comparable to those of adjacent pits, which might facilitate survival in both floods and droughts. Quercus michauxii can persist on elevated sites in aging gaps, and positive feedback in sites favorable for recruitment can enhance seedling growth; ascent into the canopy will likely require additional canopy-opening events.

Effects of ice storm canopy gaps on shoot architecture in young sugar maple (Acer saccharum)

:Crown damage to upper canopy trees during an ice storm in January 1998 in an eastern Ontario woodland increased mean canopy openness (percent open sky) to 23.6% from 8.8% in the previous growing season. The resulting effects of increased light penetration to understory vegetation were recorded for each of four size classes of young sugar maple (Acer saccharum) up to 200 cm tall. We used persistent terminal bud scale scars and leaf scars to identify leader growth, branch growth, and branching intensity (proportion of lateral axillary meristems that produced branches) in the two growing seasons prior to and following the January 1998 ice storm. Leader growth showed no differences between the two growing seasons prior to the ice storm for any of the size classes except the 25–50 cm size class, which had less growth in 1997 than in 1996. The magnitude of response in both leader growth and branching following canopy opening increased with increasing plant size, presumably reflecting the superior position of larger saplings to intercept light (compared with smaller saplings still shaded by the larger saplings). In larger saplings (100–200 cm tall), the effect of canopy opening involved a sequential two-stage response: increased leader growth only in the first growing season, with increased branching delayed until the subsequent year. This pattern of response to canopy gaps in sugar maple saplings may represent a strategy of meristem deployment that approaches an optimal compromise between racing vertically to the canopy and branching laterally to intercept light along the way.

Effects of canopy opening on the morphology and anatomy of naturally regenerated beech seedlings

Beech (Fagus sylvatica L.) seedlings growing under contrasting canopy closure conditions (gaps vs closed canopy) were collected in a naturally regenerating stand of northeastern France. Seedling morphology [total height, basal diameter, number of growth units (GUs), length of GUs] and anatomy (width of the pith and width of the annual rings at seedling base and at various levels along the stem) were described. Seedling ages, as estimated by counting the number of rings at seedling base and by counting the number of GUs along the stem, were compared. Seedling age ranged between 1 and 19 years. The best method for determining the age of seedlings with reduced growth appeared to be GU counting, whereas the best method for determining the age of actively growing seedlings was ring counting. No effect of seedling age on any parameter of seedling growth was found. The degree of canopy closure strongly affected seedling growth. Seedlings sampled in gaps were larger and allocated more biomass to secondary than to primary growth, compared to seedlings sampled under closed canopy. However, the increase in secondary growth relative to primary growth was only significant when analysing annual growth increment (GU length and ring width) and was not statistically significant when comparing seedling final size (total height and basal diameter).

Short‐term effects of canopy openness on insect herbivores in a rain forest in Guyana

Summary A before/after–control/impact experiment (BACI) was used to examine the short‐term effects of canopy openness on insect herbivores foraging on the seedlings of five rain forest species in an unlogged forest in Guyana. During the first and second study years, insects were collected monthly from 250 sites, representing 9750 seedlings spread over a plot of 1 km2. At the onset of the second collecting year, half of the parent trees at the sites were felled, creating an average canopy openness of 26%. Comparisons of the control and impact sites before and after the felling showed that overall insect abundance, particularly of psyllids, and species richness increased at the impact sites, whereas evenness decreased markedly. These responses were consistent with the prediction that intermediate disturbance promotes high diversity in tropical systems. Twenty‐nine per cent of insect species analysed showed a significant response to felling, with some populations doubling or halving at the impact sites after felling. The abundance of most species increased, although responses varied even between congeners. A canonical correspondence analysis at the impact sites after felling indicated that most species responded to an increase in leaf production by seedlings, not to canopy openness per se. Species declining at the impact sites were more likely to feed indiscriminately on young and mature foliage and probably reacted to changes in microclimate. The study illustrates the difficulty in identifying higher taxa, umbrella or indicator species that adequately integrate the responses of insect herbivores to forest disturbance. Because the integrity of insect communities in the understorey of this forest depends more on minimizing damage to plant resources than on canopy openness, we found some support for the conservation value of reduced impact logging in tropical wet forests.

Effects of canopy opening on height and diameter growth in naturally regenerated beech seedlings

La dynamique de croissance de jeunes semis de hetre poussant dans les conditions de couvert contrastees a ete etudiee dans un peuplement a base de hetre dans lequel deux types d'ouverture du peuplement ont ete realises. En 1995, un simple releve de couvert a ete effectue dans l'ensemble du peuplement et des trouees ont ete ouvertes dans certaines parties. En 1998 et 1999, 113 semis naturels de hetre ont ete recoltes sous couvert ou dans les trouees. Les effets combines de l'ouverture du couvert et de l'âge des semis sur la croissance en hauteur et en diametre des semis ont ete analyses a l'aide de modeles lineaires mixtes. Les semis sous couvert presentaient un accroissement annuel en hauteur de 1,2 cm et un accroissement annuel en diametre de 0,18 mm. La croissance en diametre a augmente des la premiere annee apres l'ouverture du couvert et a ensuite montre de fortes variations inter-annuelles liees a des variations climatiques. En revanche, l'augmentation de la croissance en hauteur a la suite de l'ouverture du peuplement n'a pas ete immediate, et a continue de maniere progressive dans les quatre annees suivantes. Quatre ans apres l'ouverture du peuplement, les accroissements annuels en hauteur en en diametre etaient de 9,5 cm and 0,49 mm respectivement pour les semis dans les trouees et de 3,8 cm and 0,21 mm respectivement pour les semis sous releve de couvert. La croissance des semis n'est pas apparue liee a l'âge.

Effect of light on the germination of forest trees in Ghana

Summary 1 Seed germination in light and dark, and responses to irradiance and light quality, were tested in shadehouse experiments for 19 West African tropical forest tree species representing a wide range of ecological types. Germination in forest gaps of different size was tested for 11 species. 2 Percentage germination was reduced in the dark only for three small‐seeded species that are common in forest soil seed banks: Musanga cecropioides, Nauclea diderrichii and Milicia excelsa. Percentage germination of the other 16 species, including four widely regarded as ‘pioneers’, was unaffected. 3 Effects of different irradiances in shadehouses, where the seeds were watered, were significant for some species, but there was no consistent pattern. Irradiance effects in forest gaps, where the seeds received only natural wet season rainfall, were more widespread and substantial, and were most commonly shown as a depression of percentage germination at high irradiance. 4 Effects of light quality (neutral vs. green shade; red : far‐red = 0.43) were insignificant at 5% irradiance in shadehouses for all species except Nauclea diderrichii. In growth chamber experiments, the low energy response was only evident at 1.0 µmol m–2 s–1 (< 1% of unshaded forest irradiance) in Musanga and Nauclea. 5 The speed of germination was affected by irradiance in many species, but the effect was small compared with differences between species, in which time to complete germination varied between 3 weeks and over 6 months. 6 Seeds of Ceiba pentandra and Pericopsis elata planted in deep forest shade (2% irradiance) and in a small gap (30% irradiance) germinated well in both sites, showed exponential biomass growth in the gap but a linear decline in mean seedling biomass and subsequent death in deep shade. 7 Light‐mediated germination is relatively rare among these forest trees, even among pioneers, so that the working definition of a pioneer should be seen to depend more on a species’ ability to survive in forest shade. The effects of canopy opening on seed germination are small except in the largest openings, which severely depress germination in a number of species, including some species with strongly light‐demanding seedlings.

Effects of Canopy Opening on Recruitment in Clethra alnifolia L. (Clethraceae) Populations in Central New Jersey Wetland Forests

JORDAN, R. A. AND J. M. HARTMAN (Graduate Program in Ecology and Evolution, P.O. Box 231, Cook College, Rutgers University, New Brunswick, NJ 08903). Effects of canopy opening on recruitment in Clethra alnifolia populations in central New Jersey wetland forests. Bull. Torrey Bot. Club 123: 286-294. 1996.-We measured stem growth and recruitment in three populations of Clethra alnifolia established in wetland forests, in canopy gaps, and in a recently cleared powerline right-of-way (ROW) to assess the ways in which C. alnifolia responded to canopy disturbance. Seasonal growth and density of stems and flowering rates were greater in canopy gaps, although greater overall recruitment occurred in the closed canopy forest. Total stem density, extension growth, and flowering significantly increased in the ROW and were directly related to distance from the ROW edge. Seedling density and new stem recruitment also increased significantly in the disturbed area. Clethra alnifolia demonstrated a light flexible response to canopy disturbance, regenerating established populations in the forest primarily through vegetative reproduction and producing greater stem density and aerial cover under disturbed canopies, but requiring significant or long-term canopy disruption to initiate increased sexual reproduction.