High Seedling Densities Research Articles

Optimal intensity and age of management in young Aleppo pine stands for post-fire resilience

During the last few decades, the number of forest fires has been increasing in the Mediterranean Basin due to several factors, among others global warming. Large burned areas are promoting a higher number of naturally regenerated stands which are being poorly managed in general, causing the risk of new wildfires to increase. The most important conifer in dry areas is Pinus halepensis Mill., a well-adapted species to fire-prone habitats due to its high post-fire seeding capacity. High seedling density in the regenerated areas produces high inter-specific competition, which could induce low cone production related to an insufficient amount of seeds stored in the canopy for post-fire recolonization (immaturity risk); adequate young pine tree stand management is therefore required, mainly based on thinning and pruning. In this study different thinning intensities, with and without pruning, were tested in two Aleppo pine post-fire regenerated areas at 5 and 10 years after a fire event. Significant relationships between seed and cone characteristics to site, treatment, cone age and cone type were found. Also the relatively little importance of Aleppo pine seeds in the soil seed bank was checked. The optimal management strategies for Aleppo pine forests in good site quality 5 years after fire was a final density of 1600 trees ha −1 (plus pruning after 5 years) or 800 trees ha −1 as well as a final density of 800 trees ha −1 plus pruning after 10 years. In poor site quality, the best method was no treatment or thinning to 1600 trees ha −1 5 years after fire plus pruning after another 5 years. This management policy can produce benefits in the reproductive processes of Aleppo pine after fire regeneration, as it shortens the juvenile phase and promotes a higher amount of closed cones stored in the canopy, thus increasing the canopy seed bank and reducing the immaturity risk.

Density- and distance-dependent seedling survival in a ballistically dispersed subtropical tree speciesPhilenoptera sutherlandii

Abstract:We examine the density- and distance-dependent seedling survival ofPhilenoptera sutherlandii, a common pod-bearing and dehiscent legume (Fabaceae) in Ongoye Forest, South Africa. Short-range ballistic dispersal causes seed to fall beneath the parent tree, where density- or distance-dependent mortality effects are expected to be concentrated. One hundred and eighty marked seedlings were monitored in a 0.5-ha plot containing 30 adults. Our survival data do not support the escape hypothesis. Predation levels declined with increasing seedling density (positive density-dependent survival), but seedling survival after 15 mo was not distance-dependent. Nevertheless, a unimodal (hump-shaped) recruitment curve, typically associated with decreasing seedling density and increasing seedling survival with distance, was observed. In the context of ballistic dispersal, this recruitment curve may indicate a hump-shaped dispersal kernel with predator satiation at high seedling densities near a parent tree. This recruitment curve likely arises because generalized insect seedling predators while attracted to the adult trees also tend to forage farther away. Short dispersal distances, in turn generate the high densities needed to satiate seed and seedling predators. Predator satiation results in long-term survival rates inP. sutherlandiisimilar to more widely dispersed and less common tree species.

Structure of Isolated Populations of Populus tremuloides (Quaking Aspen) in the Davis Mountains of Far-West Texas

Abstract Populus tremuloides is one of the most widespread woody species in North America, occurring across Canada, the northern United States, and at higher elevations in the southwestern United States and parts of Mexico. In Texas, it is found only at the highest elevations in a few disjunct populations in the Guadalupe, Davis, and Chisos Mountains. We examined the structure of six isolated P. tremuloides stands in the Davis Mountains. Stand age ranged from 25 to 65 years. The density of tree-sized P. tremuloides was 429 to 15,520 stems/ha, average basal area was 8.2 to 305.9 cm2/plant, and total basal area 5.1 to 22.9 m2/ha. As stand age increased, density of P. tremuloides decreased and average basal area and total basal area increased. The highest density of seedlings and saplings was in the younger stands. Seventeen woody species were found, with four to 10 woody species in any one stand. While these stands were dominated by P. tremuloides, two stands had Quercus gambelii relative basal area of 34 a...

Regeneration dynamics in an uneven-aged virgin Norway spruce forest in northern Sweden

The study was conducted at Kirjesålandet, a multistorey primeval Norway spruce [Picea abies (L.) Karst.] forest in northern Sweden. The objectives were: (1) to establish whether the presence of dead trees was associated with higher seedling density or growth; (2) to establish whether the influence of overstorey standing volume on height increment of seedlings differed between small and large seedlings; (3) to determine the time needed for seedlings to reach 1.3 m height; and (4) to estimate levels of recruitment, mortality and ingrowth into the tree layer, with equations using the relation between the height distribution and height increment of the seedling population. Seedlings (0.1–1.3 m height) and trees (diameter at breast height ≥5.0 cm) were measured on 46 circular plots. Presence of dead trees did not affect seedling density or growth. Height increment of small seedlings (0.1–0.5 m height) was unaffected by overstorey standing volume (range 13—333 m3 ha−1), whereas height increment of large seedlings (0.5–1.3 m height) decreased with increasing overstorey standing volume. Time to grow through the size interval 0.1–1.3 m was estimated as 58 years using height increment, and 50 years using analysis of annual rings. Seedling recruitment, annual mortality and ingrowth into the tree layer were estimated as 46 seedlings ha−1 year−1, 1.8% year−1 and 13 stems ha−1 year−1, respectively.

A rapid and robust method of identifying transformed Arabidopsis thaliana seedlings following floral dip transformation

BackgroundThe floral dip method of transformation by immersion of inflorescences in a suspension of Agrobacterium is the method of choice for Arabidopsis transformation. The presence of a marker, usually antibiotic- or herbicide-resistance, allows identification of transformed seedlings from untransformed seedlings. Seedling selection is a lengthy process which does not always lead to easily identifiable transformants. Selection for kanamycin-, phosphinothricin- and hygromycin B-resistance commonly takes 7–10 d and high seedling density and fungal contamination may result in failure to recover transformants.ResultsA method for identifying transformed seedlings in as little as 3.25 d has been developed. Arabidopsis T1 seeds obtained after floral dip transformation are plated on 1% agar containing MS medium and kanamycin, phosphinothricin or hygromycin B, as appropriate. After a 2-d stratification period, seeds are subjected to a regime of 4–6 h light, 48 h dark and 24 h light (3.25 d). Kanamycin-resistant and phosphinothricin-resistant seedlings are easily distinguished from non-resistant seedlings by green expanded cotyledons whereas non-resistant seedlings have pale unexpanded cotyledons. Seedlings grown on hygromycin B differ from those grown on kanamycin and phosphinothricin as both resistant and non-resistant seedlings are green. However, hygromycin B-resistant seedlings are easily identified as they have long hypocotyls (0.8–1.0 cm) whereas non-resistant seedlings have short hypocotyls (0.2–0.4 cm).ConclusionThe method presented here is an improvement on current selection methods as it allows quicker identification of transformed seedlings: transformed seedlings are easily discernable from non-transformants in as little as 3.25 d in comparison to the 7–10 d required for selection using current protocols.

Balanites wilsoniana: Regeneration with and without elephants

Tropical forest trees that produce large fruits and/or large seeds are believed to be dependant on a few and often rare large vertebrates for dispersal. However, little is known about the population dynamics of such trees when they lose their specialised dispersers. This study examines the juvenile spatial distribution of Balanites wilsoniana, Dawe & Spraque, which is believed to be dependant solely on elephants for dispersal, in forests with and without elephants. Using camera traps, elephants were confirmed to be the only frugivores feeding and thus dispersing Balanites wilsoniana fruits. There was a high density of seedlings beneath adult trees in one forest without elephants, which was attributed to low seed and seedling predation in this forest. Nevertheless, it was only in the forest with elephants that juveniles were established away from adult trees. Analysis of the spatial distribution of these juveniles by size revealed that saplings and poles are more likely to be found away from adult trees thus providing evidence that dispersal enhances juvenile survival. This study underlines the importance of seed dispersal for trees that cannot maintain their populations through seedlings germinating and surviving beneath adult conspecifics. The study also demonstrates that loss of vertebrates has ramifications for tree species dynamics above and beyond loss of seed dispersers.

Mariculture of Hypnea musciformis (Wulfen) Lamouroux in South east coast of India

Exploitation of agar, algin and carrageenophytic seaweeds in south east coast of India has increased significantly over the past few decades. Hypnea musciformis is one of the species exploited and the harvesting pressure could have negative consequences on the sustainability of this resource. Although demand for raw material of H. musciformis continues to increase, basic knowledge on requirements to cultivate this indigenous carrageenophyte is lacking. In this study, experiments were conducted along the south-east coast of India for 21 months from June 2001 to February 2003 on the seasonality of growth of H. musciformis, growth on polypropylene and coir ropes, the effect of seedling density, depth of water and duration of culture period to optimise its field cultivation. Biomass yield (g fw m − 1 ) and Daily Growth Rate (DGR) (%) were determined. Results showed that the biomass yield was lowest in January (8.0 ± 1.5 g fw m − 1 ) and significantly different ( P < 0.001) than the highest biomass yield in August (403.0 ± 10.0 g fw m − 1 ). Biomass yield increased with increase in seedling density (30 g fw m − 1 and above). Higher DGR (7.6–10.9%) were determined for 10 g fw m − 1 initial seedling density and it differed significantly ( P < 0.001) from the DGR of all other seedling densities. The depth of water had a considerable effect; biomass yield (130.0 ± 15.0 g fw m − 1 ) and DGR (7.0 ± 0.6%) were maximum at the surface (0 cm depth) and significantly different ( P < 0.001) at depths of 30, 60, 90 and 120 cm. Duration of cultivation period was not a limiting factor in H. musciformis. The higher biomass yield was obtained in 75 and 150 days after plantation. Biomass yield (35.0 ± 4.5 g fw m − 1 ) and growth rate (3.8 ± 0.2%) were lowest in minimum duration (25 days) and significantly different to 150 days after plantation ( P < 0.001) during all culture periods. Biomass yield (50.0–305.0 g fw m − 1 ) and DGR (3.8–9.6%) were consistently higher on coir rope and significantly different ( P < 0.001) when compared to polypropylene rope during all culture months. The observed seasonality of response to culture H. musciformis; higher biomass yield in higher seedling densities; higher biomass yield and DGR at the surface with little impact of duration of culture period and higher DGR and biomass yield on coir rope suggest cultivation is possible throughout the year along the southeast Indian coast. The findings of the present study suggest that the cultivation of H. musciformis on coir rope is viable and fisher folk can take up such activities as a part-time income generating activity in southeast Indian coast.

Conflicting Selective Forces Underlying Seed Dispersal in the Endangered Plant Silene diclinis

Selective pressures underlying the distance of seed dispersal in Silene diclinis were evaluated in natural populations. We expected that high seed predation and high seedling density around mother plants would favor relatively long‐distance dispersal and that microhabitat conditions might favor short‐distance dispersal. We experimentally altered seed density and seed dispersal distance and found that seed predation by ants increased with density but not with distance. Observations of naturally occurring seedlings showed that their survival probability was higher at lower density and when closer to female plants. Overall, calculated cumulative survival probabilities (including both seed predation and seedling survival) favored limited dispersal. Observations of naturally occurring seedlings and adults showed a strongly clumped spatial population structure. Selection for reduced dispersal and the resulting reduction of colonization ability make S. diclinis populations extremely sensitive to environmental st...

Effects of Biological Soil Crusts on Seed Bank, Germination and Establishment of Two Annual Plant Species in the Tengger Desert (N China)

The presence of biological soil crusts can affect the germination and survival of vascular plants, but the reasons are not well investigated. We have conducted a field investigation and greenhouse experiments to test the effect of crusts on two desert annual plants, which occur on the stabilized dunes of the Tengger Desert in N China. The results showed that biological soil crusts negatively influenced the seed bank of Eragrostis poaeoides and Bassia dasyphylla. The important effect of biological soil crusts on seed germination and establishment were performed indirectly through reducing the amount of germinating seeds. Field investigation and experimental results with regard to the seed bank indicated that higher seedling density was found in disturbed crust soil and bare soil surface than in intact crust soils. Greenhouse experiments showed that the effects of biological soil crusts on germination and establishment of the two plants are not obvious in moist condition, while disturbed crusts are more favorable to seed germination in dry treatment. Significant differences in biomass were found between disturbed crust soil and bare soil. Survival and growth of the two annual plants were enhanced in both algal and moss crusts during the season of rainfall or in moist environment, but crusts did not affect seedling survival in the dry period. The small seeded E. poaeoides has higher germination than larger-seeded B. dasyphylla in crust soils, but B. dasyphylla has a relatively higher survival rate than E. poaeoides in crust soils.

Formulating a general statistical model for Betula spp. invasion of lowland heath ecosystems

Summary Numerous studies describe thresholds at which transitions between alternate ecosystem states occur but few quantitatively delimit these conditions and present them in simple frameworks that are of use in ecosystem management. Previous research on heathland ecosystems has provided a site‐specific statistical model that describes the determinants of the threshold point in heath–scrub vegetation transition, but the wider applicability, and thus utility, of this model was unknown. Multi‐site experimental manipulations were conducted to assess whether a consistent set of factors limited the recruitment of Betula species into heathland vegetation. Data were pooled and used to fit a single general statistical model. The applicability of this model to a wider range of environments was tested using two independent data sets. The identity of the factors controlling Betula colonization, which include Betula seed bank density, phosphorus (P) availability and disturbance effects, were broadly similar between sites, but the strength of their effect varied widely. The general model (explained deviance 59·8%) described Betula seedling densities as a function of biomass and necromass density, vegetation height, Betula seed bank density, P availability and soil water content. These relationships were complex, with numerous interaction and polynomial terms. Although the model was derived from data from a single community type it was reasonably accurate in the prediction of seedling densities over a wider range of heath conditions. However, data capable of validating predictions of high seedling densities were not available. Synthesis and applications. The apparent success of the general statistical model suggests that an approach incorporating multi‐site experiments and statistical modelling can facilitate our understanding of ecosystem state transitions and inform the management of invasive species. By describing Betula invasion as a function of variables that are simple, general descriptions of the environment, the model can potentially inform management in a wide range of conditions. The results suggest that heaths close to seed sources and in the degenerate state of the dwarf shrub cycle are the most vulnerable to invasion, and management should target such sites as a priority. At regional scales, these conditions are probably most common in high soil phosphorus sorption capacity areas, where management should be prioritized.

Marginal holly ( Ilex aquifolium L.) populations in Mediterranean central Spain are constrained by a low-seedling recruitment

Seedling density and survival was studied in holly woodland located in the mountains of the Sistema Central, Madrid (Spain) in 1998. Ten permanent quadrats of 50×50 cm were randomly placed in each of the following habitats: under closed holly canopy, in canopy gaps, at the edge of the holly woodland, in the adjacent open grassland, and under fleshy- and dry-fruited shrubs located in this grassland. Emerged seedlings were periodically monitored and their survival was checked during the following 2 years. Additionally, a set of 20 seedlings were planted in each of the cited habitats and their survival was followed in parallel with the naturally emerged seedlings. The influence of light on holly survival was tested by experimental planting of holly seedlings under six different controlled light intensities. Seedling emergence was strongly habitat dependent, with the highest seedling densities in the closed holly woodland and in canopy gaps, and a complete absence in the grassland. Survival to the first year was nil. No naturally emerged seedling reached 1 year of age, independent of the habitat studied. Only one of the planted seedlings in the field survived to the first year. Despite this, mean seedling lifespan was different between habitats, with canopy gaps, woodland edge, and under fleshy-fruited shrubs being the habitats with highest average lifespan. Seedling response to the distinct light intensities is largely similar to that seen in the different field habitats, but other factors in addition to light, mainly summer drought and disturbances are proposed to explain the 100% mortality observed. Seedling incorporation is the recruitment dynamic bottleneck of this holly population, being more severe here than in other places studied further north. This may explain the increasing rarity of the species in the southern half of the Iberian Peninsula, where Ilex is close to its southern limit of its distribution.

The hare, the tortoise and the crocodile: the ecology of angiosperm dominance, conifer persistence and fern filtering

SummaryAngiosperm trees often dominate forests growing in resource‐rich habitats, whereas conifers are generally restricted to less productive habitats. It has been suggested that conifers may be displaced by angiosperms except where competition is less intense, because conifer seedlings are inherently slow growing, and are outpaced by faster‐growing angiosperm species. Here we investigate whether competition with ferns and deeply shading trees also contributes to a failure of conifers to regenerate in resource‐rich habitats.We examined how changes in soil nutrient availability and drainage affected vegetation along the retrogressive stages of a soil chronosequence in southern New Zealand. Vegetation composition shifted from angiosperm‐tree dominance on ‘recent’ alluvial terraces (&lt; 24 ky), via coniferous‐tree dominance on older marine terraces (79–121 ky), to coniferous‐shrub dominance on the oldest marine terraces (291 ky). Soil drainage deteriorated along the sequence, and N : Pleavesand N : Psoilindicate increasing P‐limitation. Conifer species appear to be adapted to persistence on infertile and poorly drained soils.The floor of the relatively fertile alluvial forests was deeply shaded (∼1% light transmission) by dense groves of tree‐ferns and ground‐ferns, and by large‐leaved subcanopy trees. Few seedlings of any type were found on the forest floor, even in tree‐fall gaps, and establishment was restricted to rotting logs and tree‐fern trunks. Angiosperms were particularly successful at colonizing these raised surfaces.Less shade was cast by the conifer‐dominated forests on infertile marine terraces (∼5% light transmission), which lacked tall ferns. There were many opportunities for conifer establishment, with high seedling densities recorded on the forest floor and on logs. By contrast, angiosperm seedlings were mainly restricted to logs.Our results suggest that several mechanisms act in concert to reduce regeneration opportunities for conifers in productive habitats. In particular, we suggest that tall ferns and deep shade are responsible for a restriction of regeneration opportunities in relatively productive forests in New Zealand, diminishing the opportunities for conifers to escape the competitive effects of fast‐growing angiosperms. Thus ‘crocodiles’ may alter the outcome of the race between ‘hares’ and ‘tortoises’.

Pair-wise competition-trials amongst seedlings of ten dipterocarp species; the role of initial height, growth rate and leaf attributes

To investigate whether seedlings of ten dipterocarp species differed significantly in terms of growth and mortality or whether species were not significantly different and could be considered ecologically similar, seedlings were grown, two per pot, in two experiments: (1) where the two seedlings were of equal height (30 cm); and (2) where one seedling was 10 cm shorter than the other. Seedlings were grown in a shade house with 15% above-canopy light in a 50:50 forest soil–sand mixture and were watered frequently; pots were placed so that seedling density was 130 seedlings m−2 of ground. In the first experiment there were 45 pairwise combinations of species when seedlings were 30 cm tall (AB, AC, AD,…. BC, BD…IJ; where A, B, C…J signify different species); each combination was replicated 10 times so there were 450 pots with 900 seedlings. In the second experiment there were 100 pairwise combinations of species and size e.g. Aa (30 cm A with 20 cm a), Ab (30 cm A with 20 cm b), each combination was replicated 10 times hence there were 1000 pots with 2000 seedlings. After 22 mo 79% of the initial 2900 seedlings survived; on average they had grown 42 cm (i.e. to 72 cm tall from their initial 30 cm). The most frequent outcome of competition-trials between different sized individuals (784 of 1000 trials) was that the initially taller seedling of each pair ‘won’ (it was the taller or surviving seedling). When 900 of these trials (setting aside, Aa, Bb, Cc etc.) were analysed as 45 comparisons between species with different sized individuals (Ab and aB are one interspecific comparison for these purposes), initial height determined the outcome in 23 cases (even in some competitions between light hardwood species and heavy hardwood species); in 6 cases a species (mostly light hardwoods) behaved as a ‘dominant’ – they usually won even if they were smaller initially. We found few significant differences between species in: initial seedling heights; leaf nitrogen concentrations; and specific leaf areas when they were grown in similar conditions, and these attributes were not correlated with growth rates. The similarity of seedlings of different species meant that often a height difference of just 10 cm was enough to determine the outcome of a pairwise competition-trial in high seedling densities and light equivalent to that in forest gaps.

Seedling dynamics of Acer mono and Fagus crenata: an environmental filter limiting their adult distributions

The difference in distribution of tree populations between riparian sites and terraces might be explained by differential limitation of cohort establishment in the early stages of a tree’s life history. To test this hypothesis, the early population dynamics of the riparian specialist Acer mono Maxim. var. marmoratum Hara f. dissectum Rehder and the generalist Fagus crenata Blume were compared at a riparian site and an adjacent terrace in a temperate forest in northern Japan. Survival and growth of seedling cohorts were analyzed for seven years (1994–2000) by studying the topography, light conditions, and initial density of conspecific seedlings. At one year after germination, A. mono cohorts survived best at the riparian site with a lower seedling density (40%), but were less successful at the terrace (18%) and at the riparian site with a higher seedling density (25%). Survival of F. crenata cohorts was not affected by environmental factors (ca. 14% one year after germination). The critical difference in A. mono survival occurred during the first growing season; cohorts at the riparian site experienced higher mortality (0.02 day–1) before July, whereas those at the terrace decreased rapidly in August (0.03 day–1) when there was little rainfall. The height of seedlings of both species increased with age (becoming ≥ 20 cm during the study period), except for A. mono seedlings at the terrace which remained at the same height as at emergence for seven years (ca. 6 cm). Interspecific comparison revealed that A. mono survived better than F. crenata at the riparian site although their growth was equivalent, and that F. crenata grew better than A. mono on the terrace although their survival rates were equivalent. The seedling performance of the two species corresponds well with the adult distribution in the forest.

Regeneration in experimental gaps of subalpine Picea abies forest in the Slovenian Alps

In 1998, we analysed regeneration success in four 20 year old medium-sized (600–800 m2) and four small experimental gaps (100–200 m2) in subalpine Norway spruce forest in Triglav National Park, Slovenia. We assessed the influence of site (depressions, slopes), gap size, and position within gap (centre, gap south, and north edge) on regeneration success and interactions between regeneration density, height, height increment, direct and diffuse light, ground vegetation, soil depth, thickness of humus horizons, presence of woody debris, and microrelief. Seedling density was higher in depressions, where more seedlings developed in central positions of gaps in both gap sizes, and in microsites with deeper soils, less competition from ground vegetation and convex microsites. On slopes, a higher seedling density was found in small gaps, and at the northern edge of gaps regardless of gap size. Here, seedling density was negatively influenced by diffuse radiation and positively by soil depth. The results suggest that regeneration should be initiated from medium to large gaps in depressions and small gaps on slopes. Development of established seedlings was enhanced by higher radiation levels on both sites, therefore favourable extension of the gaps would be lateral to northwards.

Seeding date and polymer seed coating effects on plant establishment and yield of fall-seeded canola in the Northern Great Plains

The time interval for planting fall-seeded Brassica napus L. canola in the Northern Great Plains is narrow, since seeding must occur as close to soil freeze-up as possible to minimize the risk of winter kill. The Prairie ecoregion tends to have more stable fall and winter temperatures than the Rocky Mountain foothill region, where fall and winter chinooks may initiate germination and increase the risk of winter mortality. Our objectives were to determine an optimum seeding date interval for fall-seeded canola and whether a water-impervious polymer seed coating could advance the seeding date and reduce the risk of stand loss and corresponding yield reduction in two distinct agroecoregions. A range of fall seeding dates and a water-impervious polymer seed coating vs. a control treatment (water-soluble film) were assessed at 14 sites (five locations) from 1998 to 2001 in Alberta and Saskatchewan (Prairie sites) and at six sites (four locations 1999–2001) in Montana, USA (Foothill sites). Highest seedling densities and canola yield responses for the control treatment were attained in the first 2 wk of November in the Prairie sites, while responses to seed date were much more variable in the Foothill region. In the Prairie sites, the water-impervious polymer seed coating improved seedling density and seed yield as seeding date was progressively moved forward from the first week of November to the first week of October. The benefit of the water-impervious polymer seed coating became notable when soil temperatures were above 5°C. The water-impervious polymer coating provided only marginal improvement in plant stand and canola yield in the Foothill region. In the Foothill region and at Lethbridge, fall-seeded canola densities were on average 60% lower than spring-seeded canola densities and fall seeding increased canola yield in only one site-year when compared with spring-seeded canola. In the Prairie region, a water-impervious polymer coating can broaden the time interval for fall seeding; however, it does not ensure that recommended densities of spring seedlings will be attained. The optimum seeding date for fall seeding in the Foothill region could not be adequately defined for either the control or the water-impervious polymer coated seed due to the variable nature of the late fall and winter weather. Key words: Canola (Brassica napus L.), alternative seeding date, germination

Avian Use of Successional Cottonwood (Populus deltoides) Woodlands Along the Middle Missouri River

Cottonwood (Populus deltoides) woodlands are important habitats for birds. Yet, little is known of the relations between bird habitat and succession in these woodlands. We studied the bird community in cottonwood woodlands from early to late seral stages along the Missouri River in central South Dakota from 1990 to 1992 to describe quantitative relations between avifauna and ecological patterns of succession in cottonwood woodlands along the Missouri River. The vegetation in the early seral cottonwood was characterized by a high density of seedlings and saplings that were restricted to narrow bands along the rivers. Late seral cottonwoods were characterized by a few large old trees that extended across the flood plain. Seventy-nine percent of the bird species were woodland obligates. Birds that nest in trees or cavities were the most common, while shrub and ground nesting birds were relatively uncommon. Total bird abundance, species diversity, species richness, richness of woodland obligates, abundance in the tree-nesting guild, abundance in the cavity-nesting guild and abundance in the shrub-nesting guild were greater (P 0.10) in early or early intermediate seral cottonwood. Bell's vireos (Vireo belli), indigo buntings (Passerina cyanea) and brown thrashers (Toxostoma rufum) occurred predominantly in early or early intermediate seral stages, but no significant differences among seral stages were noted. Expanses of late seral cottonwood on flood plains will likely decline because controlled river flows reduce flooding that is necessary for cottonwood regeneration. Cottonwood regeneration was evident only in narrow bands along the river channels. Cavity nesting species will be the most negatively affected by loss of late seral cottonwood.

Soil temperature and depth of legume germination during early and late dry season fires in a tropical eucalypt savanna of north‐eastern Australia

Abstract Temperatures that significantly increase seed germination of some tropical legumes (i.e. 80–100°C) were documented in the topsoil during the passage of early (May) and late (October) dry season fires in a tropical eucalypt savanna of north‐eastern Australia. Elevated temperatures penetrated at least 30 mm into the soil during the higher‐intensity, late dry season fires, but were only detected at 10 mm during the early dry season fires. The depth from which germination of two native legume forbs Galactia tenuiflora and Indigofera hirsuta occurred was positively related to the temperature elevation in the topsoil and was greater after late compared with early dry season fires. A broader range in germination depth, resulting in higher seedling densities, was recorded for I hirsuta after late dry season fires. These results suggest that seedling emergence of native leguminous forbs is likely to occur at a greater density after late rather than early dry season fires in tropical eucalypt savannas of north‐eastern Australia. Therefore, the season of burning, as a result of its relationship to fire intensity, can influence species composition through its effect on seed germination.

Establishment limitation reduces species recruitment and species richness as soil resources rise

Summary 1 At local spatial scales, species richness tends to fall as productivity rises. Most explanations have focused on increased extinction, but, instead, we test experimentally whether increased soil fertility reduces recruitment. Specifically, we test whether variation in recruitment is due to source limitation, germination limitation or establishment limitation, and how litter accumulation and seed predation contribute to these processes. 2 We established four crossed experimental treatments in a perennial-dominated early successional plant community over 3 years. We added seed of 30 species, manipulated access by selected seed predators, removed litter and added slow release fertilizer at four levels (0, 8, 16 and 32 g N m−2). 3 Species recruitment and richness both decreased with increasing fertility, but, counter to our expectations, we found that neither seed additions nor litter removal could counteract the negative effects of fertilizer. 4 Seed additions increased seedling density at all fertilizer levels, and seed predation appeared to have no influence on seedling densities. In spite of high seedling densities at all fertilizer levels, final stem density declined by 70% as fertilizer increased. A strong stem density–species richness relationship suggests that declines in final stem density caused more than half of the decline in species richness along this fertility gradient. 5 These results suggest that establishment limitation, i.e. the reduction of growth and survival from seedling to adult, controls species recruitment in highly fertile sites. 6 The high degree of recruitment limitation commonly observed in productive habitats suggests that high productivity causes establishment limitation, thereby isolating these communities from the regional species pool. We suggest that such isolation provides a mechanism to explain why the species composition of productive communities exhibits higher variability than the composition of less productive communities within the same regional source pool.

Factors influencing deer browsing damage to red spruce ( Picea rubens) seedlings in coastal red spruce–balsam fir stands of southwestern Nova Scotia

The impact of deer browsing on the distribution and growth of naturally regenerating red spruce seedlings established within recently harvested and mature, coastal red spruce–balsam fir stands was evaluated. More specifically, we examined browsing frequency and severity in relation to distance from forest cover, stand age, and seedling height. Relative height increment and condition of browsed (20% removal of total biomass) and non-browsed seedlings after one growing season were also compared. A significant distance×stand age interaction was observed for browsing frequency, largely the result of differences among recently harvested stands. Browsing damage within mature stands was negligible, but when occurring, was restricted to taller seedlings within 20 m of the forest edge. Eleven percent of all seedlings within harvested stands were browsed, with seedlings between 50 and 100 cm tall being preferred. Most of the seedlings (92%) sustained less than 30% biomass removal. Fifty-two percent of these seedlings experienced less than 10% biomass loss. Relative height increase of browsed seedlings was significantly reduced. Most seedlings were able to recover from clipping damage, regaining much of the initial height and biomass originally lost by the end of summer. Seedling condition was not detrimentally affected by the clipping rate, although overall biomass gain of affected seedlings was lower. If no further browsing damage is sustained, biomass increment may be delayed by approximately one growing season, relative to undamaged seedlings of equal size and established under similar micro-environmental conditions. Current population densities of deer appear to have minimal impacts on re-establishment of coastal red spruce–balsam fir stands in southwestern Nova Scotia. Low impacts may be due to high seedling densities, as more than adequate numbers of seedlings remained untouched throughout the harvested stands.