Spatial Distribution Of Seedlings Research Articles

Effects of El Niño drought on seedling dynamics in a seasonally dry tropical forest in Northern Thailand.

As El Niño is predicted to become stronger and more frequent in the future, it is crucial to understand how El Niño-induced droughts will affect tropical forests. Although many studies have focused on tropical rainforests, there is a paucity of studies on seasonally dry tropical forests (SDTFs), particularly in Asia, and few studies have focused on seedling dynamics, which are expected to be strongly affected by drought. Seedlings in SDTFs are generally more drought-tolerant than those in the rainforests, and the effects of El Niño-induced droughts may differ between SDTF and tropical rainforests. In this study, we explored the impact of El Niño-induced drought at an SDTF in northern Thailand by monitoring the seedling dynamics at monthly intervals for 7 years, including a period of strong El Niño. The effects were compared between two forest types in an SDTF: a deciduous dipterocarp forest (DDF), dominated by deciduous species, and an adjacent lower montane forest (LMF) with more evergreen species. El Niño-induced drought increased seedling mortality in both the forest types. The effect of drought was stronger in evergreen than in the deciduous species, resulting in higher mortality in the LMF during El Niño. However, El Niño increased seedling recruitment only in the DDF, mainly because of the massive recruitment of the deciduous oak, Quercus brandisiana (Fagaceae), which compensated for the mortality of seedlings in the DDF. As a result, El Niño increased seedling density in the DDF and decreased it in the LMF. This is the first long-term study to identify the differences in the impacts of El Niño on seedlings between the two forest types, and two leaf habits, evergreen and deciduous, in Southeast Asia. Our findings suggest that future climate change may alter the species composition and spatial distribution of seedlings in Asian SDTFs.

Spatial patterns of a savanna palm treeBorassus aethiopumand its temporal variability

AbstractThe spatial distributions and associations of plant species in a stand can provide essential information about their dynamics. However, since tree spatial distribution and association depend on factors that operate at different scales, disentangling the effects of environmental heterogeneity and plant–plant interactions requires the choice of a suitable null model for spatial analysis. We analyzed the spatial distributions of the same savanna palm tree populations that were studied, the underlying hypothesis being that these distributions have changed in 20 years. To remove the effect of large-scale environmental heterogeneity, the inhomogeneous L-function under the heterogeneous Poisson null model was used. We showed: (i) unlike 20 years ago, adults had a regular distribution instead of an aggregated distribution in the grass savanna; (ii) although the spatial distribution of seedlings and juveniles was always aggregated, we observed a decrease in the size of the aggregates (intensities); (iii) except for juveniles, no other stages were associated with nutrient-rich patches, which was also different 20 years ago; (iv) we did not observe any particular difference in structure between two study sites, only that female palm trees were spatially associated with entire-leafed seedlings in the reserve while they were spatially independent in the rural area. Our study supports the hypothesis that the spatial distributions have partially changed, and that the management of spatial heterogeneity has improved and given more precision in the results.

Aerial surveys reveal biotic drivers of mangrove expansion along a Thai salt flat ecotone

Rapid sea level rise (SLR) could overwhelm the adaptive capacity of many mangrove forests. The conservation of functional mangrove ecosystems will rely on their expansion into upper intertidal areas, including salt marshes and salt flats, in advance of SLR. The development of new mangrove habitats could be accelerated by amplifying positive biotic interactions among herbaceous plants, macrofauna, and mangrove trees, such as trapping of hydrochorous propagules or site amelioration. It was investigated how biotic and abiotic factors influenced the expansion of mixed‐species mangrove stands along a mangrove‐salt flat ecotone. Multi‐temporal unmanned aerial vehicle (UAV) imagery (< 1 cm/pixel) was used to monitor seedlings of Avicennia alba, Bruguiera cylindrica, and Lumnitzera racemosa in three sample plots (0.7–0.9 ha) over 2 years. The aerial images were used to map mangrove crowns, sedges (Fimbristylis tetragona), substrate elevation, and microtopography. The spatial distribution of seedlings was modeled using point pattern processes; mixed‐effects models were used to analyze how environmental factors influenced seedling survival and growth. The spatio‐temporal patterns of mangrove regeneration revealed that sedges and macrofauna facilitated the regeneration of mangroves into higher‐elevated areas of low tree density. Sedges not only trapped seeds but also increased the likelihood of survival among seedlings of all species. Large‐sized propagules established in undulating microtopography created through the bioturbation of macrofauna. Herbaceous vegetation expanded the range of macrofauna in the absence of trees. Thus, the migration of mangroves into hypersaline retreat areas may be accelerated by enhancing the growth of herbaceous vegetation and promoting burrowing macrofauna.

Density and spatial distribution of seedlings and saplings in different gap sizes of a spruce fir mixed stand in Changbai Mountains, China

In August 2019, we investigated natural regeneration (seedlings height between 0.2 m and 1 m; saplings ≥1 m in height and <5 cm in DBH) inside canopy gaps (n=48) in a plot (0.36 hm2) established in a typical mixed spruce-fir conifer broadleaved stand. To examine the short-term effects of gap size (small <20 m2, medium 20-50 m2, large 50-120 m2, and extra large >120 m2) on the regeneration density and growth (height and ground diameter) of Korean pine (Pinus koraiensis), Ezo spruce (Picea jezoensis) and Khingan fir (Abies nephrolepis), the kernel density estimation was used to examine their spatial distribution within gaps. The results showed that spruce and fir regeneration density generally decreased with the increases of gap size (significant effect only on saplings). The density of spruce and fir saplings in small gaps was 0.34 and 1.74 trees·m-2, respectively. In contrast, the density of Korean pine was not affected by gap size. The effects of gap size on seedling and sapling growth were strongest in Khingan fir and weakest in Korean pine, with greater height and ground diameter in larger gaps. Within a given canopy gap, the Korean pine and Ezo spruce saplings in small, medium, and large gaps were taller and had larger DBH in the northeastern corner of the expanded gap than in other sections, whereas those in extra large gaps had the highest growth in the northwestern part of the core gap. Small gaps favoring seed germination and seedling establishment could be created through selective removal of Khingan fir, which should be expanded later to larger sizes (>50 m2) to enhance sapling growth. Further monitoring would be required to understand the long-term effects of gap size on natural regeneration of spruce-fir forest.

Spatial distribution of seedlings on the forest floor of a coastal forest composed of broad-leaved trees and relationship with mature trees

近年のマツ材線虫病による松枯れ等により，海岸林の構成樹種に広葉樹が選択される場合がある。本研究では，主に広葉樹により構成される海岸林が長期的且つ安定して維持されるかを検討するため，沿海域に成立する広葉樹林を対象に，実生と成木の空間分布の比較を行った。結果，沿海域に成立する広葉樹林は天然更新により長期的に維持される可能性が示唆された。ヒメユズリハ，マサキ，ホルトノキ，バリバリノキは更新によって今後も現在の成木と近い林分を維持する可能性が考えられる。一方でグミ類，ヤブニッケイ，タブノキは実生と成木の空間分布に差異がみられ，今後は空間内での分布範囲や優占度に変化が生じる可能性が考えられる。

Soilborne fungi have host affinity and host-specific effects on seed germination and survival in a lowland tropical forest

The Janzen-Connell (JC) hypothesis provides a conceptual framework for explaining the maintenance of tree diversity in tropical forests. Its central tenet-that recruits experience high mortality near conspecifics and at high densities-assumes a degree of host specialization in interactions between plants and natural enemies. Studies confirming JC effects have focused primarily on spatial distributions of seedlings and saplings, leaving major knowledge gaps regarding the fate of seeds in soil and the specificity of the soilborne fungi that are their most important antagonists. Here we use a common garden experiment in a lowland tropical forest in Panama to show that communities of seed-infecting fungi are structured predominantly by plant species, with only minor influences of factors such as local soil type, forest characteristics, or time in soil (1-12 months). Inoculation experiments confirmed that fungi affected seed viability and germination in a host-specific manner and that effects on seed viability preceded seedling emergence. Seeds are critical components of reproduction for tropical trees, and the factors influencing their persistence, survival, and germination shape the populations of seedlings and saplings on which current perspectives regarding forest dynamics are based. Together these findings bring seed dynamics to light in the context of the JC hypothesis, implicating them directly in the processes that have emerged as critical for diversity maintenance in species-rich tropical forests.

Salinity and temperature significantly influence seed germination, seedling establishment, and seedling growth of eelgrass Zostera marina L.

Globally, seagrass beds have been recognized as critical yet declining coastal habitats. To mitigate seagrass losses, seagrass restorations have been conducted in worldwide over the past two decades. Seed utilization is considered to be an important approach in seagrass restoration efforts. In this study, we investigated the effects of salinity and temperature on seed germination, seedling establishment, and seedling growth of eelgrass Zostera marina L. (Swan Lake, northern China). We initially tested the effects of salinity (0, 5, 10, 15, 20, 25, 30, 35, and 40 ppt) and water temperature (5, 10, 15, and 20 °C) on seed germination to identify optimal levels. To identify levels of salinity that could potentially limit survival and growth, and, consequently, the spatial distribution of seedlings in temperate estuaries, we then examined the effect of freshwater and other salinity levels (10, 20, and 30 ppt) on seedling growth and establishment to confirm suitable conditions for seedling development. Finally, we examined the effect of transferring germinated seeds from freshwater or low salinity levels (1, 5, and 15 ppt) to natural seawater (32 ppt) on seedling establishment rate (SER) at 15 °C. In our research, we found that: (1) Mature seeds had a considerably lower moisture content than immature seeds; therefore, moisture content may be a potential indicator of Z. marina seed maturity; (2) Seed germination significantly increased at low salinity (p < 0.001) and high temperature (p < 0.001). Salinity had a much stronger influence on seed germination than temperature. Maximum seed germination (88.67 ± 5.77%) was recorded in freshwater at 15 °C; (3) Freshwater and low salinity levels (< 20 ppt) increased germination but had a strong negative effect on seedling morphology (number of leaves per seedling reduced from 2 to 0, and maximum seedling leaf length reduced from 4.48 to 0 cm) and growth (seedling biomass reduced by 46.15–66.67% and maximum seedling length reduced by 21.16–69.50%). However, Z. marina performed almost equally well at salinities of 20 and 30 ppt. Very few germinated seeds completed leaf differentiation and seedling establishment in freshwater or at low salinity, implying that freshwater and low salinity may potentially limit the distribution of this species in coastal and estuarine waters. Therefore, the optimum salinity for Z. marina seedling establishment and colonization appears to be above 20 ppt in natural beds; (4) Seeds germinated in freshwater or at low salinity levels could be transferred to natural seawater to accomplish seedling establishment and colonization. This may be the optimal method for the adoption of seed utilization in seagrass restoration. We also identified seven stages of seed germination and seedling metamorphosis in order to characterize growth and developmental characteristics. Our results may serve as useful information for Z. marina habitat establishment and restoration programs.

The fencing paradigm in woodland conservation: consequences for recruitment of a semi‐arid tree

AbstractQuestionHow does long‐term fencing against large domestic herbivores affect regeneration of the dominant tree, Prosopis flexuosa, and hence the structure of semi‐arid woodlands?LocationWoodlands in the Central Monte Desert biome of Argentina, Ñacuñán Man and the Biosphere Reserve area (34° 20′ S, 67° 58′ W) and surrounding cattle‐grazed ranches.MethodsWe compared seedling emergence and survival, the spatial distribution of seedlings and saplings, and the population stage‐based structure of P. flexuosa between paired sites inside and outside the Reserve of Ñacuñán (Argentina), which has been fenced to exclude domestic cattle for ca. 40 yr.ResultsReserve sites had lower tree recruitment and seedling emergence, in spite of having greater seed production and seedling survival. Outside the reserve, survival was higher for seedlings in high‐density clumps than for isolated seedlings. Seedling clumps occurred mostly near adult individuals, where cattle dung was abundant, suggesting an effect of cattle on seed dispersal. The balance between the effects of cattle exclusion on seedling emergence and on seedling survival was reflected in the stage structure of P. flexuosa woodlands, as populations at fenced sites were dominated by adult individuals, whereas those at cattle‐grazed areas presented greater proportions of seedlings and saplings.ConclusionFencing is a common practice used worldwide to exclude conservation areas from human disturbance. For example, it is assumed that disturbed woodlands may recover after fencing, thanks to increased tree recruitment after the exclusion of large herbivores. However, the actual effectiveness of fencing as a tool for forest conservation in overgrazed environments could be controversial because sequential effects of herbivores during the tree regeneration cycle may hamper predictions of the overall balance of the recruitment process. Our results suggest that dispersal by cattle influences the regeneration of P. flexuosa woodlands, and that the positive effects of cattle on seedling emergence compensate for the negative effects on seed production and seedling and sapling survival. Future management of P. flexuosa woodlands in the Central Monte Desert should consider that tree recruitment is closely related to land use, and that cattle exclusion does not necessarily guarantee woodland recovery in the long term.

Seedling establishment on small cutting areas with or without site preparation in a drained spruce mire – a case study in northern Finland

A large proportion of drained spruce mire stands is currently approaching regeneration maturity in Finland. We studied the effect of cutting – small canopy openings (78, 177, and 314 m) and small clear-cuts (0.25–0.37 ha) – with or without site preparation (scalping) on the establishment of natural Norway spruce seedlings in one experimental drained spruce mire stand in northern Finland. The cuttings were made in winter 2004–2005 and site preparation with scalping in early June 2005. The experimental design was composed of four blocks with altogether four clear-cuts and 33 canopy openings. The seedling establishment was surveyed annually (2006, 2008–2010) from five circular sample plots (one 10 m and four 5 m plots in size) located within the canopy openings and from 18 circular 5 m sample plots systematically located in the scalped and untreated halves of the clear-cuts. Site preparation was found unnecessary, because it resulted in a clearly lower number of seedlings in the openings. A slight negative effect was also found in the clear-cuts. In the two years following the cuttings, the number of seedlings increased quickly in the canopy openings, but more gradually in the clear-cut areas. In 2010, on average 15 500 new seedlings were observed in the canopy openings and 6700 in the clear-cut areas, of which 5050 and 1200, respectively, were &gt;0.1 m tall spruces. The proportion of birch increased in the last two years, being ca. 22% in the openings and 45% in the clear-cuts in 2010. The spatial distribution of seedlings was more uneven in the clear-cuts than in the openings, with 41% and 20% of survey plots empty, respectively.2222

PARTITIONING THE FACTORS OF SPATIAL VARIATION IN REGENERATION DENSITY OF SHADE-TOLERANT TREE SPECIES

Understanding coexistence of highly shade-tolerant tree species is a longstanding challenge for forest ecologists. A conceptual model for the coexistence of sugar maple (Acer saccharum) and American beech (Fagus grandibfolia) has been proposed, based on a low-light survival/high-light growth trade-off, which interacts with soil fertility and small-scale spatiotemporal variation in the environment. In this study, we first tested whether the spatial distribution of seedlings and saplings can be predicted by the spatiotemporal variability of light availability and soil fertility, and second, the manner in which the process of environmental filtering changes with regeneration size. We evaluate the support for this hypothesis relative to the one for a neutral model, i.e., for seed rain density predicted from the distribution of adult trees. To do so, we performed intensive sampling over 86 quadrats (5 x 5 m) in a 0.24-ha plot in a mature maple-beech community in Quebec, Canada. Maple and beech abundance, soil characteristics, light availability, and growth history (used as a proxy for spatiotemporal variation in light availability) were finely measured to model variation in sapling composition across different size classes. Results indicate that the variables selected to model species distribution do effectively change with size, but not as predicted by the conceptual model. Our results show that variability in the environment is not sufficient to differentiate these species' distributions in space. Although species differ in their spatial distribution in the small size classes, they tend to correlate at the larger size class in which recruitment occurs. Overall, the results are not supportive of a model of coexistence based on small-scale variations in the environment. We propose that, at the scale of a local stand, the lack of fit of the model could result from the high similarity of species in the range of environmental conditions encountered, and we suggest that coexistence would be stable only at larger spatial scales at which variability in the environment is greater.

First evidence of a massive recruitment event in Posidonia oceanica: Spatial variation in first-year seedling abundance on a heterogeneous substrate

A recruitment event of Posidonia oceanica was recorded in the Gulf of Ventilegne (Corsica, Western Mediterranean) in June 2005. One-year seedlings (alive and dead) were found at shallow depth (2–3 m) on a mix of rock and sand substrates. Pattern in the distribution of seedlings was examined over spatial scales ranging from centimetres to hundreds of metres. We also examined (a) whether establishment was preferentially associated with a specific substrate type; (b) the relation between proportional mortality and the density of established seedlings; and (c) the growth characteristics of seedlings. The average density of seedlings varied from 19 to 189 seedlings m −2, with a maximum of 480 seedlings m −2. While significant variation was detected at the scale of area (i.e., tens of metres), no difference was found in abundance of seedlings at the scale of sites (i.e., hundreds of metres). Most of the overall variability was, however, at on the smallest scale examined (i.e., among sampling units tens of centimetres apart). No differences were also observed in the establishment probability and seedling abundance between sand and rock substrates. The fine scale analysis of spatial patterns on rock and sand substrates showed a clumped distribution of seedlings. These results indicated that the spatial distribution of seedlings was not random, but organized at small spatial scales within the study sites. Hydrodynamics and micro-scale bottom features appeared to account for such patterning. Proportional seedling mortality was highly variable (ranging from 0.3 to 0.8), and tended to increase with increasing settlement density. A total of 128,000 seedlings were recoded over the 1600 m 2 surface area investigated. Such event reflected conspicuous seed supply and particularly favourable local conditions. This finding contradicts the current statement of low recruitment ability of P. oceanica and indicates that the species can recruit both on consolidated and unconsolidated substrates, at least in the study area.

Modelling seed dispersal to predict seedling recruitment: Recolonization dynamics in a plantation forest

The extend to which seed dispersal influences seedling recruitment is of major importance for forest dynamics. In non-equilibrium situations, seed dispersal might be of even further importance as it is a major determinant of the rate and composition of secondary succession. We modelled primary seed dispersal in a planted pine–spruce stand intensively recolonized by Abies alba and Fagus sylvatica, an ecological situation commonly found throughout southern Europe. We then evaluated the role of primary seed dispersal in seedling emergence and recruited seedling density. Using a seed trap experiment and inverse modelling methods, we calibrated short- and medium-distance seed dispersal functions. The relationship between the density of dispersed seed, the density of emerging seedlings and microsite characteristics were assessed using generalized linear models. The cumulative distribution function of seedling-to-nearest-adult distances made it possible to test the concordance between seed rain patterns and seedling spatial distributions. Seed production was highly variable between years for A. alba and F. sylvatica. Seed dispersal was shown to be locally restricted (median dispersal distance of 13.2–19.2 m for A. alba depending on the year and 6.49 m for F. sylvatica). Model prediction was considerably increased when seed production was directly assessed (and not indirectly estimated using diameter, for example). The number of A. alba seedlings that emerged in 1999 was positively correlated with the number of seeds dispersed in 1998 and with the local density of Picea abies adults, and negatively correlated with grass cover. The spatial distribution of seedlings was less aggregated around adult trees than expected from seed dispersal models, but significantly different from random beyond 6 m to the nearest adult for F. sylvatica and 26 m for A. alba. Thus, seed rain patterns are only partially responsible for recruitment dynamics in our model forest. Our study demonstrates that inverse modelling methods are well suited for the study of seed dispersal at the local scale, especially when a direct count of seed production can be made, and are therefore of particularly high interest in forests where several successional stages are present.

Regeneration patterns in boreal Scots pine glades linked to cold-induced photoinhibition

Regeneration patterns of Pinus sylvestris L. juveniles in central Siberian glades were studied in relation to cold-induced photoinhibition. Spatial distribution of seedlings in different height classes revealed higher seedling densities beneath the canopy than beyond the canopy, and significantly higher densities of seedlings < 50 cm tall on the north side of the trees. These patterns coincided with differences in light conditions. Compared with plants on the north side of canopy trees (north-exposed), photosynthetic photon flux (PPF) received by plants on the south side of canopy trees (south-exposed) was always higher, making south-exposed plants more susceptible to photoinhibition, especially on cool mornings. Chlorophyll fluorescence data revealed lower photochemical efficiency and increased non-photochemical quenching of small (20-50 cm in height), south-exposed seedlings from spring to early autumn, indicating increased excitation pressure on photosynthesis. Maximum rate of oxygen evolution was less in south-exposed plants than in north-exposed plants. Increased pools of xanthophyll cycle pigments and formation of the photoprotective zeaxanthin provided further evidence for the higher susceptibility to photoinhibition of south-exposed seedlings. A linear mixed model analysis explained many of the physiological differences observed in seedlings according to height class and aspect with early morning temperature and PPF as predictors. The link between photoinhibition and differential distribution of seedlings by height class suggests that photoinhibition, together with other environmental stresses, decreases the survival of small, south-exposed P. sylvestris seedlings, thereby significantly affecting the regeneration pattern of central Siberian pine glades.

THE ROLE OF ANTS IN SEED DISPERSAL OF GLOBBA LANCANGENSIS AND THE SPATIAL DISTRIBUTION OF ITS SEEDLINGS

Seed dispersal is a key process determining the spatial pattern of plant populations. Myrmecochory is a term referring to a plant_ant mutualism in which ants disperse plant seeds while using the nutritionally valuable elaiosomes as food. In this study, our goal was to understand how ants affect seed dispersal and seedling distribution patterns of a myrmecochorous plant, Globba lancangensis. G. lancangensis is a small perennial herb of Zingiberaceae mainly distributed in southwest Yunnan Province. It propagates primarily by seeds but also by a small number of bulbils. There are 31.2±2.13 (Mean±SE, n =32) seeds in each fruit. The seeds are 2-3 mm in length and bear a white_colored elaiosome. We conducted an observational study on the seed dispersal of G. lancangensis in October 2002 in its natural habitat in Xishuangbanna, Yunnan Province of China (21°59′ N, 100°16′ E; 1 180 m asl; annual mean temperature, 18.3 ℃; annual mean rainfall, 1 339 mm). A total of ten species of ants were identified that transported the seeds of G. lancangensis and their activities were recorded. The mean dispersal distance of seeds was (0.47±0.03) m ( n =216), ranging from 0.01 m to 3.35 m. Odontoponera transversa , Pheidole sp. and Pachycondyla luteipes were the most important seed dispersing ant species, occurring 61%, 50% and 28% of the time, respectively, with mean dispersal distances of (0.60±0.09) m, (0.20±0.01) m and (0.32±0.05) m, respectively. As a whole, ants promoted a decrease in the degree of aggregation of G. lancangensis seeds. Ants of Pheidole sp. collected a total of 43% of the seeds of G. lancangensis but consumed most of the seeds collected, suggesting that Pheidole sp. is a seed harvester ant O. transversa did not remove seeds with artificially removed elaiosomes and appeared to be the most sensitive ant species to seed elaiosomes. Overall, O. transversa made the most significant contribution to seed dispersal, as demonstrated by the highest visiting frequency (61%) and the greatest mean dispersal distance (0.60±0.09) m of all ants. These results imply that a tight mutualism exists between O. transversa and G. lancangensis. To understand whether ants influence the spatial pattern of seedlings, we compared the spatial distribution of seedlings of G. lancangensis in its natural habitat with that of two other species in the Globba genus, G. barthei and G. racemosa; the latter two species propagate mainly by bulbils which are dispersed by abiotic means. Nine 25 m×25 m plots, three plots for each species, were established and the location of every Globba seedling inside the plots was recorded. The mean distance to the nearest neighbor of G. lancangensis was significantly higher than both G. barthei ((36.8±1.45) cm vs. (29.8±2.70) cm, Mean±SE; t 73,33 = 2.11, p =0.037) and G. racemosa ((36.8±1.45) cm vs. (28.7±3.16) cm, t 73,31 =2.33, p = 0.022 ). We used Z _values to describe the degree of aggregation of the populations of the three species. The Z_value of G. lancangensis (-1.70±0.19, Mean±SE) was significantly larger than the Z_value of both G. barthei (-2.58±0.37, t 73,33 =2.36, p =0.020) and G. racemosa (-3.28±0.53, t 73,31 =3.54, p = 0.001), indicating that G. lancangensis was significantly less aggregated. These results suggest that ants make a significant contribution to dispersing G. lancangensis seedlings by increasing the distance to its nearest neighbor and decreasing the degree of aggregation in its natural habitat.

POPULATION REGULATION OF A DOMINANT RAIN FOREST TREE BY A MAJOR SEED PREDATOR

We take advantage of a fortuitous local extinction and recolonization of white-lipped peccaries (WLPs) at the Cocha Cashu Biological Station in southeastern Peru to assess the impact of this high-biomass seed predator on the recruitment of a dominant member of the tree community, the palm Astrocaryum murumuru. WLPs were common at Cocha Cashu in the mid-1970s. In 1978, the species vanished from the entire region and did not reappear until 1990. To assess the impacts of the presence-absence of WLPs on Astrocaryum recruitment, we conducted transect counts of both the number and spatial distribution of palm seedlings in 1978 (when WLPs were present), in 1990 (after a 12-yr absence of WLPs), and in 1999 (after 10 yr of recovery). Other factors affecting recruitment, such as climate variability, differences in tree fecundity, and fluctuations of vertebrate and invertebrate seed predators were also examined as alternative hypotheses for any changes in seedling spatial distribution and abundance. In the absence of WLPs, the density of Astrocaryum seedlings increased 1.7-fold, and the spatial distribution of seedlings with respect to safe sites was significantly altered. After the return of WLPs to the study area, seedling density dropped to its former level, and the spatial distribution of seedlings with respect to safe sites returned to the same pattern found 21 yr earlier in 1978. None of the other factors investigated varied systematically with the census intervals. These results demonstrate that the absence of a single, albeit important, member of a diverse seed predator guild can have a major impact on the demography of a common tree species. We infer that individual seed predators can play central roles in regulating the demography of tropical trees, as envisioned many years ago by Janzen and Connell, and that their extirpation or extinction can have cascading effects in tropical ecosystems.

Consequences of frugivore diversity for seed dispersal, seedling establishment and the spatial pattern of seedlings and trees.

Many plants depend on frugivorous animals for the dispersal of their seeds. However, it is only poorly known whether regional differences in frugivore diversity have consequences for seed dispersal, seedling establishment, and the spatial distribution of seedlings and trees. This comparative study of seed dispersal investigated the consequences of regional differences in frugivore diversity for two tree species of the genus Commiphora. C. harveyi was studied in South Africa where avian frugivore diversity is high, C. guillaumini was studied in Madagascar where the avian frugivore community is depauperate. At both study sites, the percentages of handled and dispersed seeds in Commiphora trees were quantified by fruit traps, and visitation rates, seed handling rates and dispersal rates were quantified for each animal species for two consecutive years. Seedlings were mapped and the spatial distribution of trees quantified. At both study sites, fruits were mainly eaten by birds. The total percentage of dispersed seeds in South Africa was significantly higher than in Madagascar (70.8% vs. 7.9%) because there was a lack of effective dispersers that swallowed seeds in Madagascar. Seed dispersal benefit, i.e. the increase in the probability of becoming established as a seedling away from parent trees due to dispersal was much higher in Madagascar (80times higher probability) compared to South Africa (6times higher). Corresponding with the different dispersal percentages, seedlings in South Africa were found at relatively large distances from the nearest Commiphora tree (median distance=21.0m), whereas in Madagascar seedlings were found mostly under and close to the nearest Commiphora tree (median distance=0.9m). Finally, Commiphora trees in the Malagasy study site were clumped, but were more randomly distributed in the South African study site. These results suggest that regional differences in frugivore diversity and behaviour strongly affect seed dispersal of trees, seedling establishment and the spatial distribution of seedlings and trees.

La régénération, un processus multi-étape au résultat imprévisible : l’exemple d’une Sapotaceae en forêt de Guyane française

The net result of the multi-stage recruitment from seed production to seedling establishment was assessed along 3 consecutive years in Chrysophyllum lucentifolium (Sapotaceae) in a mature rain forest of French Guiana. According to year, between 37 and 42 % of seed production was primarily dispersed away from fruiting trees without damage, especially by primates. Seeds dispersed in howler monkeys’ (Mouatta seniculus) defecation were serendipitously buried by dung beetles (14%), eaten by terrestrial vertebrates such as rodents, or destroyed by insects or pathogens. The proportion of seeds secondarily dispersed by scatterhoarding rodents varied from 3 to 17 % according to year. The survival rate of unremoved and uneaten seeds after 20 days, varied from 2 to 56 % according to year and pattern of spatial distribution generated by primary dispersal (clump or scattered). The empirical model based on the seed and seedling fate diagram showed that, according to years, from 6 to 15 % of produced seeds lead to seedling establishment away from parent tree. Variations in patterns of seed removal and predation by rodents were the most crucial factors governing the effectiveness of primary and secondary dispersers and the spatial distribution of seedlings. Between-year variations of recruitment were important and stochastic because the fluctuations of seed production and seed dispersal/predation varied independently. The proposed empirical model has thus no predictive value. However it can be useful for instance to test the effects of seed size and of some faunal perturbations.

Spatial pattern and size distribution of the animal-dispersed tree Quercus robur in two spruce-dominated forests

We investigated the degree to which the spatial distribution of oaks (Quercus robur L.) was related to habitat conditions, as reflected by vegetation type and structural features presumed to attract animal dispersers (trails, community borders). We hypothesized that the distribution pattern of oaks, with their potential to establish in many habitats, depends on the behaviour of the dispersing animals to a greater extent than micro-habitat conditions. One 100 m ¥ 100 m plot was surveyed in each of two coniferous forests in east-central Sweden. No adult oak trees grew in the forests; all oaks were considered as dispersed into the plots by animals. We tested whether oak distribution was clumped with spatial autocorrelation analyses and whether oak distribution was related to vegetation type, species composition, tree cover, distance to nearest fertile oak tree, or distance to animal trails. Our study showed that oak trees were also spatially aggregated in a small-scale context. The spatial distribution of seedlings and older trees were associated with species richness and tree cover but not with any specific vegetation type, even though fewer oaks than expected grew in spruce forest habitats. Furthermore, we found that oak trees were associated with trails. There were differences in oak distribution between the two study sites in total number of oaks, the number of first-year seedlings, caches, and oak occurrence in relation to species richness and distance to nearest fertile oak. Seed-dispersing animals seem to be of importance for oak distribution even though animal activities seem to differ between sites.

Impacts of secondary seed dispersal and herbivory on seedling survival in Aesculus turbinata

Abstract. Aesculus turbinata is a tree species with large seeds (6.2 g mean dry weight). We studied the demography of its seeds and seedlings in a temperate deciduous forest in northern Japan to elucidate the ecological significance of large seeds with special reference to herbivory and secondary dispersal. Both seed and seedling stages suffered greatly from herbivores. Seedling herbivory was important judged from experiments with shoot clipping and hypogeal cotyledon removal. However, some seedlings survived through re‐sprouting after herbivory. Survival rate and percentage resprouting seedlings were lower than those with remaining cotyledons, though seedling size was not affected. This suggests that stored resources in hypogeal cotyledons are working as a kind of ‘risk hedge’ against severe aboveground shoot clipping experienced by A. turbinata. The spatial distribution of seedlings was expanded via seed scatter‐hoarding by rodents. Seedling survival rate was higher within canopy gaps than under closed canopy, indicating that canopy gaps are safe sites for establishment, and was negatively correlated with seedling density. Therefore, secondary seed dispersal in this species seems to be effective in ‘finding’ safe sites and in ‘escaping’ density‐dependent mortality. The large seeds and seedlings of A. turbinata are attractive to herbivores, but the high resistance of seedlings to herbivory due to large reserves and the effective secondary dispersal appear to mitigate these disadvantages.

Ecological relationships between seedling emergence and litter cover in the earliest stage of plant succession on sandy soil

AbstractThe relationships between seedling emergence and litter cover were studied in the earliest successional stage of a plant community. During a period of 3 years, changes in vegetation cover and species composition were assessed on three permanent quadrats with virgin sandy soil substrates A successional pattern from summer annual grasses to winter annual herbacoeus plants to biennial and perennial plants was confirmed, and invasion and replacement of the component species were conspicuous. The spatial distributions of seedlings and litter were heterogeneous in winter, and some patches consisting of both seedlings and litter were observed. There was a strong positive correlation between distribution patterns of seedlings and litter; seedling density increased with increasing litter cover. The results suggest that the litter cover of the previous stage plays an imporrant role in promoting successional changes in the early stage of plant succession.