Patterns Of Seedling Establishment Research Articles

Microhabitats shift rank in suitability for seedling establishment depending on habitat type and climate

Summary Spatial patterns of seedling establishment have been shown to be largely affected by differences at the microhabitat scale. However, it is far from clear whether such differences are consistent both in space and time, and how they influence the spatial structure of plant populations. We analyse the spatiotemporal variability in microhabitat quality for seedling emergence and survival of the Mediterranean tree Acer opalus ssp. granatense, examining seedling dynamics at the microhabitat scale in two maple populations intermingled in pine forests and two established on stony slopes during three consecutive years with different climatic conditions. Seedling establishment varied markedly between microhabitats. In general, seedlings benefited from the presence of pre‐established vegetation, with lower emergence and survival of maple seedlings in open interspaces than under adult maples, non‐conspecific trees and shrubs. However, the quality of most microhabitats depended on both habitat type and year. Emergence and survival under adult maples, heterospecific trees and in open interspaces was higher in forest than in stony‐slope populations, and increased significantly with summer rainfall. Shrubs, on the contrary, showed relatively high values of seedling emergence and survival at all sites and in both dry and wet years. Consequently, the benefits of living under shrubs were comparatively greater at lower precipitation levels. The different microhabitats found in heterogeneous landscapes such as Mediterranean forests vary, over relatively large spatiotemporal scales, in quality for seedling establishment. Shrubs showed the highest consistency in patterns of both seedling emergence and survival. This finding, together with the predominance of dry years in Mediterranean areas, may explain why Acer opalus ssp. granatense has a stable spatial distribution at a geographical scale with most saplings associated with shrubs. Spatiotemporal patterns of seedling dynamics at the microhabitat scale may cascade through later demographic stages, eventually impacting the spatial structure of plant populations.

Microsite patterns of conifer seedling establishment and growth in a mixed stand in the southern Alps

In many forests of the Alps, permanent forest cover and, therefore, its continuous renewal is the main silvicultural goal. Regenerating these forests must be based on a sound understanding of the ecology of the tree species in question. The regeneration of silver fir ( Abies alba Mill.) and Norway spruce ( Picea abies (L.) Karst.) in the upper montane zone of the southern central Alps has so far received little attention. The aim of this study was, therefore, to characterize the ecological niche for the establishment and growth of silver fir and Norway spruce seedlings in this zone. The study was conducted as a case study on a plot of 1.5 ha in the upper montane zone of southern Switzerland, at an altitude of 1380 m. The basal area of the mixed Norway spruce–silver fir stand was 43.7 m 2 ha −1. Seedlings were censused on 375 plots of 1.0 m 2 area. Seedling density was 1.30 ± 4.25 m −2 (mean ± standard deviation) for Norway spruce and 0.87 ± 1.50 m −2 for silver fir. Logistic regression models were used to test the effect of microsite characteristics on seedling occurrence, and general linear models for effects on seedling height growth and biomass increment. Most seedlings received less than 10% light (photosynthetic photon flux density) as compared to values in the open. Silver fir occurrence was positively related to microsites at the edge of canopy trees, but unrelated to ground cover type, light and micro-relief. Norway spruce occurrence was only, and positively, related to the presence of mosses. Height growth and biomass increment of seedlings of both species were only loosely correlated with microsite conditions. Seedlings without canopy cover grew faster than those under canopy cover, probably as a result of light and moisture limitations under the canopy of adult trees. Diffuse radiation was positively correlated with average annual biomass increment of silver fir, but not of Norway spruce seedlings. In general, the results suggest that silver fir seedlings have less specific microsite requirements than Norway spruce seedlings in terms of ground cover. They are also more shade tolerant, and therefore, grow faster than Norway spruce in low-light environments of the upper montane zone of the southern Alps. While the microsite concept can be helpful in designing silvicultural operations, it has limitations when only patch characteristics that are easy to assess are used, and others neglected. Finally, our study suggests that counting the number of green shoots is a promising method to quickly and non-destructively estimate the biomass of a great number of small seedlings.

A simple mechanistic model of seed dispersal, predation and plant establishment: Janzen‐Connell and beyond

Summary Although, in nature, seed dispersal usually declines with distance from the source, seedling establishment patterns are highly variable. An increase in seed survival can lead to either hump‐shaped (Janzen‐Connell (J‐C) pattern) or declining (Hubbell pattern) establishment with distance from seed source, but declining establishment can also be generated if survival decreases with distance (McCanny pattern). Pathogens and seed predators are considered to be major mortality agents structuring recruitment patterns, but it is unclear how well predation alone can explain variation in these patterns. We introduce a simple mechanistic model showing that distance and density‐dependent seed predation can generate all of the observed recruitment patterns. Our approach provides the first mathematical reconstruction of conceptual models previously considered to be based on contrasting underlying mechanisms. Three easily measurable quantities (the proportion of seeds escaping predation at the source, and the mean distance from the source of dispersed seeds and of predators’ activity) can be used to test for consistency with the J‐C pattern. The association between recruitment patterns and plant (dispersal) and animal (predation) characteristics is robust with respect to parameter values and various functional forms. The model shows that the J‐C pattern can occur only if the mean distance over which predators are active is lower than that over which seeds are dispersed, corresponding to a system with host‐specific, or immobile, seed predators (often invertebrates) that are restricted to areas of high seed density near adult plants, and therefore selecting for longer dispersal distances of seeds. The Hubbell pattern is generated by the model when dispersal and predation distances are of equivalent magnitudes. The McCanny pattern emerges if more generalized, or more mobile, seed predators (often vertebrates) are attracted to the adult trees but also tend to forage farther away, thereby selecting for short dispersal distances that generate high densities needed to satiate seed predators. The model also predicts that the total number of seeds surviving predation is lowest at intermediate distances, suggesting that distance‐dependent predation promotes either short or long dispersal distances, or both (dimorphism).

Mycorrhizal networks: a review of their extent, function, and importance

It is well known from laboratory studies that a single mycorrhizal fungal isolate can colonize different plant species, form interplant linkages, and provide a conduit for interplant transfer of isotopic carbon, nitrogen, phosphorus, or water. There is increasing laboratory and field evidence that the magnitude and direction of transfer is influenced by physiological source–sink gradients between plants. There is also evidence that mycorrhizal fungi play a role in regulating transfer through their own source–sink patterns, frequency of links, and mycorrhizal dependency. Although it is plausible that connections are extensive in nature, field studies have been hampered by our inability to observe them in situ and by belowground complexity. In future, isotopic tracers, morphological observations, microsatellite techniques, and fluorescent dyes will be useful in the study of networks in nature. Mycorrhizal networks have the potential to influence patterns of seedling establishment, interplant competition, plant diversity, and plant community dynamics, but studies in this area are just beginning. Future plant community studies would benefit from concurrent experimental use of fungal network controls, isotopic labeling, direct observation of interplant linkages, and long-term observation in the field. In this paper, we review recent literature on mycorrhizal networks and interplant carbon transfer, suggest future research directions, and highlight promising scientific approaches.Key words: common mycorrhizal network, carbon transfer, source–sink, establishment, competition, diversity.

Temporal patterns in seedling establishment on pocket gopher disturbances.

Disturbances often facilitate seedling establishment, and can change the species composition of a community by increasing recruitment of disturbance-adapted species. To understand the effects of pocket gopher disturbances on alpine seedling dynamics, we examined the gopher disturbances' effects on seedling emergence and survival on gopher disturbances 0 to 5 years old. In contrast to results from most other ecosystems, these recently created gopher mounds had lower seedling emergence and survival rates than undisturbed areas. A lack of correlation between species' abundances on gopher mounds and undisturbed sites in one of the two communities studied suggested that a suite of disturbance-adapted species recruited onto the mounds. To explain low seedling emergence on recent gopher mounds, we quantified gopher mound seed banks and studied recruitment in a site with mounds that ranged from 0 to >20 years old. Seed numbers in first-year gopher mound soils were extremely low relative to undisturbed soils, and this pattern was mirrored in seedling establishment patterns over the long term. Gopher disturbance depressed seedling emergence density for the first 5 years. Subsequently, emergence density increased until at least 20 years following the disturbance. Emergence on disturbances more than 20 years old was higher than on undisturbed sites. Therefore, gopher disturbances probably facilitate seedling establishment in alpine dry and moist meadow; however, this process takes place over decades.

Microsite availability and establishment of native species on degraded and reclaimed sites

Summary Restoration of native plant communities on previously disturbed land requires the identification and modification of environmental factors that impede or restrict ecosystem succession or development. One of the key factors in successful reclamation is to ensure colonization and persistence of native species within an area. Therefore, the identification of microsite types that favour colonization by native species should improve our ability to successfully reclaim degraded areas. The objectives of this study were to (i) identify and describe microsite types that were conducive to the establishment of five native plant species (Euphrasia frigida, Galium normanii, Thymus praecox, Cerastium spp. and Rumex acetosa) on degraded and reclaimed sites in Iceland; and (ii) to determine the effects of reclamation activities that included the application of manure and inorganic fertilizer on the distribution of microsite types and seedling establishment. Reclamation sites of different ages were examined on an eroded area where the target community was a grass or shrub heath. The spatial distribution of plant establishment was not random but varied among microsite types and study sites. More seedlings than expected were found in microsite types characterized by small rocks and biological soil crusts. These may have entrapped seed and provided more soil stability and moisture than other microsite types. The cover of these microsite types varied among the study sites but the pattern of seedling establishment among the five native forbs was similar. Synthesis and applications. Practical reclamation methods are improved by identifying those factors that promote successful seedling establishment by native species. Application of fertilizer without additional seeding proved to be a simple reclamation approach that increased the availability of one favourable microsite type, enhanced the establishment of native species and subsequently allowed vegetation cover to expand. Seed of native plants may be difficult to obtain commercially, but this approach can be used on degraded land where propagules are available but recovery is slowed by soil instability or nutrient limitations.

Effects of forest fragmentation on Heliconia acuminata seedling recruitment in central Amazonia.

I present the results of a 2-year experiment comparing seed predation, seed germination, and seedling survivorship patterns of the Amazonian understory herb Heliconia acuminata in forest fragments and continuous forest. These empirical results were compared with natural patterns of recruitment in permanent 5,000m2 demographic plots adjacent to experimental areas. The number of naturally occurring seedlings established in demographic plots was 1.5-6 times greater in continuous forest than it was in 1-ha or 10-ha fragments. This result mirrors the pattern of seedling establishment in experimental transects, in which seeds in fragments were 3-7 times less likely to become established than those in continuous forest. Predation of experimentally sown seeds was extremely low at all sites, and is therefore not responsible for the observed pattern. Instead, reductions in seedling abundance in forest fragments are probably due to lower levels of seed germination. Forest fragments have higher air and soil temperatures, lower relative humidity, and increased leaf-litter accumulation, all of which can alter the cues used to initiate germination. While the growth of seedlings was similar in forest fragments and continuous forest, seedling survivorship in fragments was highly variable. These results suggest that altered environmental conditions may exacerbate reductions in plant recruitment resulting from modified plant-animal interactions. Strategies aimed at reducing the intensity of abiotic edge effects should therefore be incorporated into plant conservation efforts.

Factors affecting the regeneration of northern white cedar in lowland forests of the Upper Great Lakes region, USA

Regeneration of northern white cedar ( Thuja occidentalis) has been poor in the Upper Great Lakes region for decades. To understand why, we used a spatially extensive approach to examine which factors limit regeneration in lowland swamp forests at a regional scale. We investigated patterns of seedling establishment and sapling recruitment in 1990–1991 and 1996 among 77 lowland stands distributed across northern Wisconsin and the eastern Upper Peninsula of Michigan. These stands differed in ownership, local site conditions, and ambient deer densities, allowing us to evaluate several factors potentially limiting regeneration. Lowland cedar seedlings in this region typically require 10 years to grow to 30 cm and 30 years to attain 3 m. Regression and path analyses demonstrate that initial seedling establishment increases in areas with greater seed input (as inferred from cedar basal area) and (in 1996) higher light levels. Subsequent recruitment to saplings +30 cm tall, however, depends far more on demographic inertia (the number of smaller seedlings) and escape from deer browsing. Surprisingly, estimated deer browse was as significant a factor as demographic inertia, reflecting cedar’s palatability, slow growth, and chronic high browsing pressure. Efforts to regenerate this species seem unlikely to succeed without significant and sustained reductions in deer density or extensive efforts to mechanically protect saplings from browsing.

Distribution of plant life history types in California chaparral: the role of topographically‐determined drought severity

Abstract. Spatial patterns of shrub life history and Ceanothus distribution are examined in relation to topographically‐mediated differences in drought severity within 3 watersheds on the coastal and inland flank of the Santa Ynez Mountains, California. Spatially distributed fields of drought severity are simulated for the studied watersheds using high‐resolution digital terrain data and daily climate data in combination with a process‐based hydro‐ecological model (RHESSys). Field samples of species composition are spatially integrated with the distributed drought data for analysis of ecological relationships. Patterns of seedling recruitment type correspond to topographic variability in drought severity in ways that are consistent with concepts presented in the literature. Species that depend on fire for recruitment are increasingly represented with increasing drought severity, the converse also applies. Sites that experience moderate drought severity permit co‐dominance of species from both recruitment modes. Residual analysis suggests that some of the unexplained variability is related to substrate. Analyses also indicate that the distribution of 5 Ceanothus shrubs reflect differences in drought severity in ways that are consistent with their resistance to water stress‐induced xylem dysfunction. Species from the subgenus Cerastes sort in accordance with moisture availability and have unique spatial distributions. Results are evaluated and discussed with respect to studies on plant morphology, resource use and seedling establishment patterns.

Distribution of plant life history types in California chaparral: the role of topographically‐determined drought severity

Spatial patterns of shrub life history and Ceanothus distribution are examined in relation to topographically-medi- ated differences in drought severity within 3 watersheds on the coastal and inland flank of the Santa Ynez Mountains, Califor- nia. Spatially distributed fields of drought severity are simu- lated for the studied watersheds using high-resolution digital terrain data and daily climate data in combination with a process-based hydro-ecological model (RHESSys). Field sam- ples of species composition are spatially integrated with the distributed drought data for analysis of ecological relation- ships. Patterns of seedling recruitment type correspond to topographic variability in drought severity in ways that are consistent with concepts presented in the literature. Species that depend on fire for recruitment are increasingly repre- sented with increasing drought severity, the converse also applies. Sites that experience moderate drought severity per- mit co-dominance of species from both recruitment modes. Residual analysis suggests that some of the unexplained vari- ability is related to substrate. Analyses also indicate that the distribution of 5 Ceanothus shrubs reflect differences in drought severity in ways that are consistent with their resistance to water stress-induced xylem dysfunction. Species from the subgenus Cerastes sort in accordance with moisture availabil- ity and have unique spatial distributions. Results are evaluated and discussed with respect to studies on plant morphology, resource use and seedling establishment patterns.

Flowering, seed dispersal, seed predation and seedling recruitment in two pyrogenic flowering resprouters

A few resprouting plants in fire-prone environments have no local seed bank (soil or canopy) when a fire occurs. These species rely on post-fire flowering and the production of non-dormant seeds to exploit favourable post-fire establishment and growth conditions. For two such pyrogenic flowering species (Doryanthes excelsa Correa and Telopea speciosissima (Smith) R.Br.), we examined the timing of seed release, patterns of fruit production, seed dispersal, seed predation and seedling establishment following a fire in the Sydney region of south-eastern Australia. Both species took some 19 months after the fire to flower and the first seeds were released 2 years after the fire. D. excelsa flowered and fruited only once after the fire. For T. speciosissima, plants also flowered at least once more in the subsequent 5 years, but produced seed in only the first three post-fire flowering years. Fruit production differed between species, with fruiting individuals of D. excelsa producing fewer infructescences, similar numbers of follicles, but many more seeds per follicle than fruiting individuals of T. speciosissima. Ultimately, D. excelsa produced approximately six times as many seeds per m2 and four times as many seeds per adult in one flowering season than T. speciosissima did after four flowering (three successful fruiting) seasons. Seeds were passively dispersed from fruits borne 3–4 m (D. excelsa) or 1–2 m (T. speciosissima) above the ground. Most seeds were found within 5 m (D. excelsa) or 3 m (T. speciosissima) of parent plants. The primary seed shadow of both species was a poor predictor of the distribution of seedlings, with more seedlings occurring further from the adults than expected from the distribution of seeds. Seed loss to predators was high in both species in exclusion experiments where mammals had access to clumps of seeds (77–88%). It was variable and generally lower (8–65%) in experiments where seeds were not locally clumped. However, for T. speciosissima, at one site, some 65% of seeds were lost to mammals and invertebrates in these latter experiments. At this site, these losses appeared to influence subsequent recruitment levels, as very low seedling densities were observed. For both species, germination of seedlings first occurred some 2.5–3 years after the passage of the fire. The percentage of seeds produced to seedlings successfully established was low in D. excelsa (2–3%) and more variable across sites and years in T. speciosissima (0–18%). Resultant post-fire seedling densities of D. excelsa (two sites) and T. speciosissima at one site were similar, but they were much lower at the T. speciosissima site that had high levels of seed predation. Both D. excelsa and T. speciosissima are amongst the slowest woody resprouting species to recruit seedlings after fire in south-eastern Australia and lag years behind species with soil or canopy seed banks.

Seed Size, Seed Germination, and Seedling Survival of Brazilian Tropical Tree Species Differing in Successional Status1

ABSTRACTSeed germination and seedling establishment patterns have been used to classify species as shade tolerant or intolerant. The main objective of this research was to investigate, under controlled conditions, seed germination of species from different successional positions as well as to follow seed germination and seedling survival under natural shade in the field. The species studied were Solarium granuloso‐leprosum, Trema micrantha, Cecropia pachystachya, Croton piptocalyx, Bauhinia forficata subsp. pruinosa. Senna macranthera, Schizolobium parahyba, Piptadenia gonoacantha, Chorisia speciosa, Pseudobombax grandiflorum, Ficus guaranitica, Esenbeckia leiocarpa, Pachystroma longifolium, Myroxylon peruiferum, and Hymenaea courbaril. Field trials were carried out at Santa Genebra Municipal Reserve, Campinas, SP, Brazil, at the forest edge and in the understory. No significant correlations were detected between successional status and seed size or seed water content. Light‐regulated germination was present only in small‐seeded species. In field experiments, most species, including the light‐sensitive ones, were able to germinate under the canopy, where a low red/far‐red ratio predominates. Most species, mainly those of early‐ and intermediate successional positions, presented low seedling survival rates under shade. Myroxylon peruiferum was the most shade tolerant species, while 5. granuloso‐leprosum, C. speciosa, P. gonoacantha, F. guaranitica, T. micrantha, and 5. parahyba were the most shade intolerant. These latter species showed little or no survival under the shade conditions.

Seed Size, Seed Germination, and Seedling Survival of Brazilian Tropical Tree Species Differing in Successional Status1

Abstract Seed germination and seedling establishment patterns have been used to classify species as shade tolerant or intolerant. The main objective of this research was to investigate, under controlled conditions, seed germination of species from different successional positions as well as to follow seed germination and seedling survival under natural shade in the field. The species studied were Solanum granuloso-leprosum, Trema micrantha, Cecropia pachystachya, Croton piptocalyx, Bauhinia forficata subsp. pruinosa, Senna macranthera, Schizolobium parahyba, Piptadenia gonoacantha, Chorisia speciosa, Pseudobombax grandiflorum, Ficus guaranitica,Esenbeckia leiocarpa, Pachystroma longifolium, Myroxylon peruiferum, and Hymenaea courbaril. Field trials were carried out at Santa Genebra Municipal Reserve, Campinas, SP, Brazil, at the forest edge and in the understory. No significant correlations were detected between successional status and seed size or seed water content. Light-regulated germination was prese...

Limiting mechanisms in the regeneration of the Chilean matorral – Experiments on seedling establishment in burned and cleared mesic sites

The Chilean matorral is characterized by multispecific shrub clumps in dry areas but has a continuous canopy in wetter sites. It has been hypothesized that this difference is due to easier recolonization of open patches by shrub seedlings under more mesic conditions. Within the mesic range of the matorral we designed a field experiment to compare shrub seedling emergence, growth, and survival under the closed canopy of a secondary forest versus three types of open patches: burned, cleared of shrubs but with a herbaceous layer present, and clear without a herbaceous layer. After the first summer, survival of Quillaja saponaria seedlings was 75% in the burned site, 30% in the cleared patch without herbaceous vegetation, and 15% with herbaceous vegetation present, whereas there was 0% survival under the secondary forest canopy. After eight years, the percentages had dropped to: 22%, 12% and 3%, respectively. These results contrast strongly with the seedling establishment patterns in drier areas of the matorral where early seedling survival is higher under the shade of large shrubs. In the experimental mesic sites, seedlings did best on the burned site, not only in terms of survival, but also in terms of growth. After one year, seedling mean height was 10.2 cm in the burned site, whereas 3.8 cm and 5.3 cm in the cleared patches without and with herbaceous respectively. After eight years, mean height differences between treatments had increased further: 147.7 cm in the burned site, 40.3 cm in the cleared patch without herbaceous cover and 13 cm in the cleared patch with herbs. Our results indicate that the facilitative effect of nurse shrubs on seedling establishment found in dry ranges of the matorral is less important in more mesic sites. This difference may explain the continuous shrub cover in relatively mesic areas as opposed to the characteristic patchy structure of the matorral in its drier range.

Post‐fire seedling establishment in Florida sand pine scrub

Abstract. This study deals with a quantification of pre‐ and post‐fire seedling establishment and microsite characteristics in two Florida sand pine scrub sites burned in May 1993. In addition, life history characteristics related to seedling establishment are described for five perennial species –Calamintha ashei, Chapmannia floridana, Eriogonum floridanum, Garberia heterophylla and Palafoxia feayi. Post‐fire seedling establishment in sand pine scrub was sparse (median = 1, 12 seedling/m2), with 17 of 35 species establishing seedlings. Chapmannia, Eriogonum, Garberia and Palafoxia resprouted and flowered after fire; Eriogonum and Garberia had strong post‐fire seedling establishment responses within 19 months post‐fire. Calamintha individuals were killed by fire, but this species had a strong post‐fire seedling establishment response, presumably from seeds in a soil seed bank. Eriogonum and Calamintha seedlings established preferentially in plots centered on conspecific adults. For these species with poor seed dispersal, spatial patterns of seedling establishment may be influenced more by pre‐fire adult plant location than by post‐fire microsite conditions. Post‐fire seedling density in sand pine scrub was much lower than in California chaparral and South African sand plain lowland fynbos.

Effects of a deposit-feeding invertebrate on the entrapment of Zostera marina L. seeds

Eelgrass, Zostera marina, relies upon seed dispersal for colonization of new habitats. The seeds are not readily transported in suspension; however, they have low erosion thresholds and are subject to horizontal transport as bedload at relatively low bottom shear stress. Field germination patterns suggest that seeds rarely travel far from the point of release and quickly become buried in the sediment, even in habitats where boundary-layer flows exceed those necessary to erode seeds. In many sedimentary habitats it is likely that the activities of benthic and demersal organisms will affect the horizontal movement and burial of seeds, thus providing an explanation for the patterns of seedling establishment in previously reported experiments. We investigated the effects of a common animal in estuarine sediments on the entrapment of Z. marina seeds. In a series of flume experiments we manipulated the densities of the subsurface deposit-feeding polychaete Clymenella torquata (Low: 96 worms m −2; Medium: 192 worms m −2; High: 288 worms m −2) and related trapping of seeds to worm density and bioturbation rates. The results suggest that modifications to the sediment surface (i.e. topographic relief) resulting from feeding and defecation activities of subsurface deposit feeders can act to trap seeds. Seeds were trapped in the medium and high density worm treatments in greater numbers than in the low density and no worm treatments. Our findings indicate that benthic invertebrates, through their modification of sediments may affect the horizontal (bedload) and, hence, vertical (burial) transport of Z. marina seeds.

Local and regional variation in hemlock seedling establishment in forests of the upper Great Lakes region, USA

Successful tree regeneration requires both successful seedling establishment and subsequent survival and growth sufficient to ensure recruitment. We examined patterns of initial seedling establishment in randomly selected stands of eastern hemlock ( Tsuga canadensis) distributed across northern Wisconsin and eastern Upper Michigan. In 1990, we counted all hemlock seedlings (defined as 4–29 cm tall) and sampled substrate conditions in two 7×7 m quadrats of 142 hemlock and hemlock–hardwood stands. In 1996, we resampled 109 of these stands more intensively using six 7×7 m quadrats. In 1997, we also intensively surveyed which substrate-supported hemlock seedlings in three 50 m 2 replicate areas from each of five stands. In 1990, we found no hemlock seedlings at 64 of the 142 stands sampled, and 46% of the stands had more than 100 hemlock seedlings ha −1 (mean density=480±90 ha −1). In contrast, only 14 (13%) of the 109 stands resampled in 1996 had no seedlings and 66% had over 100 seedlings ha −1 (mean density=840±200 ha −1). These results suggest that seedling establishment is patchy, causing the estimated frequency of successful establishment to depend strongly on the area sampled. Seedling abundance within sites was correlated between years, suggesting that site history may in part account for the abundance of seedlings. Analysis of variance to mean ratios reveals that seedlings are indeed strongly clumped at the 7×7 m scale or smaller. Such clumping probably results from the tendency for hemlock seedlings to occur most frequently on particular substrates (i.e. decaying wood). Hemlock seedlings occur less frequently in stands with high basal area of sugar maple but do not decline in stands with greater hemlock basal area, suggesting that maple litter may restrict local opportunities for seedling establishment. Seedling densities do not vary among habitat-types, and no other stand or regional variable significantly affected hemlock seedling density. These results suggest that ground surveys are needed to assess hemlock seedling abundance, and that such surveys should be based on quadrats of at least 100 m 2. Successful hemlock regeneration should capitalize on local patches of existing hemlock seedlings where interference by hardwoods is minimal. Even high initial seedling establishment, however, may not guarantee regeneration in areas where high deer densities preclude recruitment into larger size classes.

The influence of tree thinning on the reproduction dynamics of Colophospermum mopane

The reproduction dynamics of Colophospermum mopane, including flowering, fruit bearing, the germination potential of the seed and seedling establishment were studied over a period of three years on six plots where the trees were thinned to the approximate equivalents of 75%, 50%, 35%, 20% and 10% of the tree density of the control plot (2711 trees ha− 1). Thinning of C. mopane reduced inter-tree competition which resulted in marked increases in the flowering and fruit bearing of the remaining trees. Though the percentage reproductive trees were higher in the low tree density plots, the greater number of trees in the high tree density plots ensured that the total number of trees that flowered and produced fruit were of the same order of those in the low tree density plots. The fruit (seed with and without their pod covers) from trees within the low tree density plots were larger and heavier than those from the plots with a high tree density, but this did not have a marked effect on their potential to germinate. Since the greatest germination was achieved from fresh seeds, no dormancy appears to exist in C. mopane seeds. Seedling numbers were low in all plots and a specific pattern of seedling establishment was lacking. At high tree densities severe competition from the established C. mopane trees could account for the low rate of seedling establishment, while in the low tree density plots competition from newly established herbaceous plants, mainly grasses, could account for the tree seedling absence in these plots.

Seedling establishment patterns on the Pumice Plain, Mount St. Helens, Washington

Abstract. We examined the factors that control seedling establishment on barren substrates on the pyroclastic flows from Mount St. Helens. From June to September in 1993, we monitored seedling and microhabitat changes in 240 20 cm × 20 cm quadrats on the Pumice Plain. Seedlings emerged in 104 quadrats (43.3 %). The most abundant species were Anaphalis margaritacea, Hypochaeris radicata, Lupinus lepidus and Epilobium angustifolium. Measured site characteristics included topography, particle size distribution, ground surface movements, soil water content, organic matter, pH, and presence or absence of dead lupines. Quadrats with seedlings had higher cover of dead lupines, higher amount of rock and gravel substrate, and a greater cover of rills. More seedlings emerged where eroded material accumulated. Compared to coarse‐textured surfaces, silt surfaces had higher organic matter, held more water, and showed higher pH. However, seedlings became established more frequently on coarse‐textured surfaces. In greenhouse experiments, a higher percentage of Hypochaeris seeds germinated on silt than on sand or gravel. The germination of Anaphalis and Epilobium did not differ with soil texture, but was higher at higher moisture levels. Seedling colonization is more dependent on ground surface microtopography, particle size, and ground movement than on the chemical status of these volcanic deposits.

The Importance of Gap Processes in the Development and Maintenance of Oak Savannas and Dry Forests

1 We examined the dynamics of a Quercus-dominated dry forest-savanna complex in the Ozark Highlands of the central United States, focusing on regeneration patterns within gaps. The study area includes one of very few temperate Quercus ecosystems not degraded by fire suppression, logging or excessive grazing. 2 We compared patterns of tree seedling establishment, tree height growth and postfire survival in gaps between savanna, found only on south and west aspects in our study area, and dry forest, found only on north and east aspects, to determine which factors contributed to maintaining structural differences. 3 Canopy gaps constituted 42% of savanna area compared with 17% for dry forest. Mean canopy gap size in savanna was 316 m2 compared with 185 m2 for dry forest, but gap sizes were highly variable and the difference was not significant. Remnants of canopy trees were found in 37 of 38 gaps, indicating that gaps in both savanna and dry forest can usually support trees. 4 Savanna and dry forest gaps were generally favourable for seedlings and growth of small trees. Seedlings of most woody species were widespread in the understoreys of both savanna and dry forest but were more abundant in gaps than under closed canopies. Species had different height growth patterns depending on site, with growth rates ordered as follows: Quercus velutina in savanna > Q. velutina in forest > Q. stellata in forest > Q. stellata in savanna. Site differences were significant for Q. stellata but not for Q. velutina. 5 Tree survival in two surface fires was significantly lower in gaps in savanna compared with gaps in dry forest. Survival within individual gaps was inversely related to grass cover. In savanna, survival of small trees (2 5-7 cm d.b.h.) ranged from >80% for gaps with 90% grass cover. 6 Canopy gaps in the Ozarks are created primarily by windfall and drought-related dieback, but fire influences the timing and rate of gap infilling. Understanding the interactions among these disturbances may help clarify the dynamics of many temperate woodlands once dominated by surface fires.