Spatial patterns of tree seedling establishment and their relationship to environmental variables in a cold-temperate deciduous forest of eastern North America

Patterns of seedling recruitment may have persistent effects on population and community processes. Assuming seed availability is not limiting, the environmental sieve (i.e., the suite of factors influencing seed germination and seedling emergence and survival) determines how many seedlings establish and, most importantly, where they do so. In this study, we identify the spatial structure of some resources and abiotic conditions known to be significant for tree seedling emergence and survival and determine how these environmental factors influence the establishment of Fagus grandifolia, Acer saccharum, Fraxinus americana, and Ostrya virginiana in a deciduous forest of southern Québec (Canada). We expect an increase from Fagus, through Acer and Fraxinus, to Ostrya in the control of environmental variables on seedling emergence and survival, because of differences in the seed size of these species. Density of newly-emerged seedlings of all four species showed positive spatial autocorrelation at distances of up to ca. 10 m. Environmental variables were also structured at the same spatial scale, except for soil moisture. Acer seedling emergence pattern was positively correlated to photosynthetic photon flux density (PPFD), and the pattern of Fraxinus to soil N and moisture. Seedling survival was not spatially autocorrelated for any of the four species, although it was positively density-dependent in Acer and Fagus. In only Ostrya was seedling survival correlated (positively) to one of the environmental variables studied, i.e., PPFD. Overall, environmental variables were spatially less heterogeneous than seedling emergence and survival. Either seed availability was not saturating or factors not considered here, such as competition and predation (the intensity of which often varies with resources and/or abiotic conditions), modified the influence that the physical environment had on patterns of seedling establishment. Our prediction of a greater environmental control on seedling emergence and survival in small-seed species was not totally confirmed.     

Herbivory and seedling performance in a fragmented temperate forest of Chile

Forest fragmentation alters plant-animal interactions, including herbivory. Relying manipulative experiments, we test if the reduction in insect herbivory associated with forest fragmentation translates into increased seedling growth and survival of three tree common species (Aristotelia chilensis, Cryptocarya alba and Persea lingue) in forest fragments and continuous forests in coastal Maulino forest, central Chile. Furthermore, we test if after protecting seedlings from herbivorous insects, plant performance is increased regardless of forest fragmentation. Nursery grown seedlings were transplanted into four forest fragments and a continuous forest during 2002. Insects, important herbivores in this forest, were excluded from half the seedlings by repeated applications of insecticides. Compared to continuous forests, in forest fragments, herbivory was reduced in all three species, seedling growth was greater in A. chilensis and C. alba but not in P. lingue, and survivorship was unaffected by herbivory or fragmentation in all three species. Protecting seedlings from insects reduced herbivory in the continuous forest to similar levels attained in the forest fragments. No change in herbivory results from by protecting seedlings in forest fragments. These results confirm that insects are important herbivores in the Maulino forest and also support the hypothesis that fragmentation can have strong indirect effects on plant communities as mediated through trophic interactions.     

Facilitation and competition influence succession in a subtropical old field

The effects of neighboring vegetation and soil fertility on the establishment, survival and growth of tree species were studied in a subtropical old-field area in south Brazil. Seed damage, germination and seedling establishment of four tree species plus growth and survival of two transplanted tree species were monitored under factorial combinations of the following treatments: (1) pioneer vegetation (presence and absence); (2) soil fertility (addition of NPK and control). Facilitation was the main process affecting plant performance. The presence of pioneer vegetation significantly improved germination, establishment, growth and survival of most study species. Around 90% of sown seeds were damaged and the removal of pioneer vegetation significantly increased seed damage for all species studied, decreased germination in three out of four species, and decreased establishment in one species. Moreover a significantly higher seedling growth rate of Inga virescens was found after the first year of the experiment in plots where vegetation was present. The presence of vegetation significantly increased seedling survival of I. virescens by protecting seedlings from leaf loss due to winter frosts. Competition was detected by the second year when a higher growth of transplanted seedlings of the species Araucaria angustifolia occurred in plots where vegetation was present and fertilizer were applied. A lower growth rate was detected in plots where vegetation was present but fertilizer was not applied. These results indicate a balance between competition for soil nutrients and protection by neighbor vegetation. Damage of seedlings by leaf cutter ants was an important barrier for plant survival. Damage occurred in 80% of the A. angustifolia seedlings and 58% of these damaged seedlings died. The presence of neighboring vegetation tended to protect seedlings from ant damage. Although competition occurred, facilitation seems to be the main process driving early successional changes in this subtropical old field. This was mainly due to the improvement of local microclimatic conditions and protection against herbivores by neighboring vegetation. Facilitation occurred during establishment and growth phases in a subtropical area that is considered a productive, low stress environment. Our results indicate that facilitation may be more frequent in productive environments than previously thought.     

Interacting effects of grass height and herbivores on the establishment of an encroaching savanna shrub

Shrub encroachment is a widely observed problem in Southern African savannas. Although the effects of herbivory and grass height on woody species recruitment have been studied individually, little information exists about how these factors interact. In this study seeds and seedlings of the encroaching shrub Dichrostachys cinerea were planted in clipped and unclipped grass plots, with and without large herbivores present. Seed germination, seedling survival and seedling predation were monitored for 8 months. Germination started earlier in plots where herbivores were excluded. Overall, the earlier the seeds germinated, the longer the seedlings survived. Clipping positively affected the number of germinated seeds, seedling growth and survival but effects varied among herbivore exclusion treatments and sites. Invertebrates caused the majority of the seedling damage. We conclude the recruitment of D. cinerea is influenced by the interplay of grass height and herbivory. In this study, the presence of large herbivores early in the wet season, and the absence of simulated grazing later on, affected the regeneration of D. cinerea negatively. However, differences in effects among sites suggest that the mechanisms found here may work differently in other habitats.      

Efficiency of endozoochorous seed dispersal in six dry-fruited species (Cistaceae): from seed ingestion to early seedling establishment

We combined laboratory and nursery experiments to analyse the effectiveness of sheep as endozoochorous seed dispersers of six native shrubby Cistaceae species collected in SE Spain (Helianthemum apenninum (L.) Mill., H. violaceum (Cav.) Pers., Fumana ericoides (Cav.) Grand., F. thymifolia (L.) Spach, Cistus monspeliensis L. and C. laurifolius L.), considering the main stages after seed ingestion, i.e. seed recovery, seed germination, seedling emergence and early seedling establishment. Seed recovery after gut passage was high (around 40%) for all the species, except F. thymifolia (12%). Most seeds (ca. 90%) were recovered within 48 h after ingestion for all the species, although seeds were still recovered up to 96 h after ingestion. Gut passage increased germination up to seven-fold compared to non-ingested seeds. Furthermore, seedling emergence from seeds contained in pellets was overall similar (intact pellets) to or higher (crumbled pellets) than emergence from seeds without dung. Survival of emerged seedlings and mass of seedlings after 20 days were not reduced by dung. Sheep act therefore as effective dispersers of these Cistaceae species by scattering seeds and promoting germination, while faeces do not hamper seedling establishment. We conclude that the interaction between herbivorous ungulates and these dry-fruited species may be considered a mutualism qualitatively similar to the mutualism between frugivorous vertebrates and fleshy-fruited plants.     

Recruitment Limitation, Seedling Performance and Persistence of Exotic Tree Monocultures

Many native plant communities are replaced by exotic monocultures that may be successional stages or persistent community types. We surveyed a stand of Sapium sebiferum (Chinese Tallow Tree) that replaced tallgrass prairie in Texas and performed experiments with seeds and seedlings to determine the contributions of recruitment limitation and natural enemy release to allowing such a forest type to persist or to allowing native species to reduce Sapium dominance. The stand was dominated by Sapium, especially for mature trees (>99) and annual seed input (97) but less so for saplings (80). Field sown Sapium seeds had lower germination and survival rates than Celtis seeds. Together with the extreme dominance of Sapium in seed rain this suggests that native species are currently recruitment limited in this stand by seed supply but not by germination, early growth or survival. To investigate whether Sapium may benefit from low herbivory or diseases, we transplanted Sapium and Celtis seedlings into the forest and manipulated foliar fungal diseases and insect herbivores with sprays. As predicted, insect herbivores caused greater damage to Celtis seedlings than to Sapiumseedlings. However, suppression of insect herbivores caused significantly greater increases in survivorship of Sapium seedlings compared to Celtis seedlings. This suggests that herbivores in the understory of this Sapiumforest may significantly reduce Sapiumseedling success. Such a pattern of strong herbivore impact on seedlings growing near adult conspecifics was unexpected for this invasive species. However, even with insects and fungi suppressed, Sapium seedling performance was poor in this forest. Our results point towards Sapium as a successional species in a forest that will eventually be dominated by native trees that are currently recruitment limited but outperform Sapium in the understory.     

Herbivory and competition slow down invasion of a tall grass along a productivity gradient

Competition models including competition for light predict that small plant species preferred by herbivores will be outshaded by taller unpreferred plant species with increasing productivity. When the tall plant species is little grazed by the herbivores, it can easily invade and dominate short vegetation. The tall-growing grass Elymus athericus dominates the highly productive stages of a salt-marsh succession in Schiermonnikoog and is not preferred by the herbivores which occur there, hares and geese. We studied how interspecific competition and herbivory affected performance during early establishment of this species with increasing productivity. Seedlings were planted in the field in a full factorial design, manipulating both interspecific competition and herbivory. The experiment was replicated along a natural productivity gradient. Competition reduced aboveground biomass production and decreased the number of ramets that were produced but did not affect survival of seedlings. The negative effects of competition on seedling performance increased with increasing productivity. In contrast to our expectations, herbivory strongly reduced seedling survival, especially at the unproductive sites and had only small effects on seedling growth. The present study shows that unpreferred tall-growing species cannot easily invade vegetation composed of short preferred species. Grazing by (intermediate-sized) herbivores can prevent establishment at unproductive sites, and increased competition can prevent a rapid invasion of highly productive sites. Herbivores can have a long-lasting impact on vegetation succession by preventing the establishment of tall-growing species, such as E. athericus, in a window of opportunity at young unproductive successional stages.Plant nomenclature follows Van der Meijden et al. (1990)     

The effects of neighbors on the growth and survival of shrub seedlings following fire

Where plant species vie for limited resources, disturbances might preclude competition by releasing a flush of nutrients, or by reducing biomass and thereby diminishing the consumption of resources. However, if new seedlings colonize in clumps, they may still deplete resources within the local aggregations, which may then reduce their growth and survivorship. We investigated competition among seedlings in a burned area by examining the relationship between the performance of newly established shrub seedlings of Ceanothus impressus and (1) the proximity and (2) the identity of their near neighbors. We also investigated the relationship between neighbor proximity and the availability of water. Both survivorship and growth of C. impressus were positively associated with increasing distance to near neighbors, in a manner consistent with resource competition. The availability of water (as determined by pre-dawn xylem pressure potentials) tended to be greater when neighbors were farther away, providing evidence that water was a resource for which plants were competing. This conclusion is reinforced by the finding that the effects of neighbors were stronger in drought years, suggesting that yearly variation in the availability of an important resource (water) can affect the strength of competitive interactions. This suggests that after disturbances, when some resources are apparently abundant on a large scale, competition may be important in determining the small scale patterns of seedling growth and survival.     

Competitive effects of the invasive shrub,Lonicera maackii (Rupr.) Herder (Caprifoliaceae), on the growth and survival of native tree seedlings

Invasive plants are often associated with reduced cover of native plants, but rarely has competition between invasives and natives been assessed experimentally. The shrub Lonicera maackii, native to northeastern Asia, has invaded forests and old fields in numerous parts of eastern North America, and is associated with reduced tree seedling density in Ohio forests. A field experiment was conducted to test the effects of established L. maackii on the survival and growth of transplanted native tree species. The experiment examined above-ground competition (by removing L. maackii shoots) and below-ground competition (by trenching around transplanted seedlings). The effects of above-ground competition with L. maackii were generally more important than below-ground competition, though both were detected. Shoot treatment was the key determinant for the survival of all species except P. serotina, whereas trenching only enhanced survival for A. saccharum caged and P. serotina, and only in the shoot removal treatment. For the surviving seedlings, L. maackii shoot removal increased growth of A. saccharum seedlings protected with cages, but actually reduced the growth of unprotected Q. rubra and A. saccharum seedlings, indicating that L. maackii shoots confer some protection from deer browsing. Significant interactions between root and shoot treatment on Q. rubra growth parameters, specifically greatest growth in the shoot present & trenched treatment, is attributed to protection from deer browsing combined with release from below-ground competition. Despite this protective function of L. maackii shoots, the overall effect of this invasive shrub is increased mortality of native tree seedlings, suggesting it impacts the natural regeneration of secondary forests.     

Invasive Plants can Inhibit Native Tree Seedlings: Testing Potential Allelopathic Mechanisms

The mechanisms by which invasive species affect native communities are not well resolved. For example, invasive plants may influence other species through competition, altered ecosystem processes, or other pathways. We investigated one potential mechanism by which invasive plants may harm native species, allelopathy. Specifically, we explored whether native tree species respond differently to potential allelopathic effects of two invasive plant species. We assessed the separate effects of Lolium arundinaceam (tall fescue) and Elaeagnus umbellata (autumn olive) on three common successional tree species: Acer saccharinum (silver maple), Populus deltoides (eastern cottonwood), and Platanus occidentalis (sycamore). Tall fescue and autumn olive are widely planted and highly invasive or persistent throughout North America where they often grow in forest edges, old fields, and other sites colonized by pioneering tree species. In an exploratory greenhouse experiment, we applied aqueous extracts derived from soil, leaf litter, or live leaves to native trees. We compared these treatments to a sterile water control and also to minced leaves leached in water, a common, but potentially less realistic method of testing for allelopathy. For all tree species, minced leaves from tall fescue reduced the probability that seedlings emerged, and minced leaves of autumn olive reduced the number of days to emergence. During other demographic stages, the three native tree species diverged in their responses to the invasive plants. Platanus occidentalis exhibited the widest range of responses, with reduced root biomass due to minced tissue from both invasive species, reduced days to emergence and marginally reduced survival from minced tall fescue, and reduced leaf biomass from tall fescue leaf litter. Populus deltoides appeared insensitive to most extracts, although survival was marginally increased with application of minced or fresh leaf extracts from autumn olive. In addition, minced tall fescue shortened the time to seedling emergence for Acer saccharinum, potentially a positive effect. Overall, results suggest that allelopathy may be one mechanism underlying the negative impacts of tall fescue and autumn olive on other plant species, but that effects can depend strongly upon the source of allelochemicals and the tree species examined.     

Comparative demography of co-occurring introduced and native tussock grasses: persistence and potential expansion

Summary Demographic characteristics associated with the maintenance and growth of populations, such as seed dynamics, seedling emergence, survival, and tiller dynamics were examined for two tussock grasses, the native Agropyron spicatum and the introduced Agropyron desertorum in a 30-month field study. The introduced grass was aerially sown onto a native grassland site. Seed production of the introduced grass was greater than the native grass in both above- and below-average precipitation years. Seeds of A. spicatum were dispersed when they mature, while A. desertorum retained some seeds in inflorescences, and dispersed them slowly throughout the year. This seed retention allowed some seeds of the introduced grass to escape peak periods of seed predation during the summer and allowed seeds to be deposited constantly into the seed bank. Carryover of seeds in the seed bank beyond one year occurred in the introduced grass but not in the native species. For both species, seedling emergence occurred in both autumn or spring. Survival rates for A. desertorum were higher than A. spicatum when seedlings emerged between November and March. Survival rates of cohorts emerging before November favored A. spicatum whereas survival rates did not differ between species for cohorts emerging after March. Individuals of both species emerging after April were unable to survive the summer drought. Demographic factors associated with seeds of A. desertorum seemed to favor the maintenance and spread of this introduced grass into native stands formerly dominated by A. spicatum.     

Effects of adult neighbour and gap size on seedling emergence and early growth of <Emphasis Type="Italic">Bromus inermis</Emphasis> Leyss

A field study was conducted to determine the effects of neighbour root exclusion and gap size on the seedling emergence and early growth of Bromus inermis. Seeds of B. inermis were added to artificially created gaps in an improved shortgrass steppe in northern China. Neighbour root exclusion was accomplished using PVC tubes sunk in the soil of gaps. Emergence and survival of seedlings were greater in all gaps than in the control (0-cm diameter), but better growth performance (height of seedling, leaves, tillers and dry weight per seedling) was observed only in large gaps (20- and 40-cm diameter) with neighbour root present and gaps without neighbour root presence. Seedling growth performance was improved by reducing root and shoot competition. Neighbour root exclusion provided a favourable environment for seedling establishment. Differences between gaps in light levels and topsoil temperature can explain the patterns of germination. These results confirm that B. inermis is a gap-enhanced species. Our study strongly suggests that asymmetric competition by adult neighbour plants negatively influences the establishment of B. inermis.     

Capture and allocation of nitrogen byQuercus douglasii seedlings in competition with annual and perennial grasses

Summary The spatial overlap of woody plant root systems and that of annual or perennial grasses promotes competition for soil-derived resources. In this study we examined competition for soil nitrogen between blue oak seedlings and either the annual grassBromus mollis or the perennial grassStipa pulchra under controlled outdoor conditions. Short-term nitrogen competition was quantified by injecting¹⁵N at 30 cm depth in a plane horizontal to oak seedling roots and that of their neighbors, and calculating¹⁵N uptake rates, pool sizes and¹⁵N allocation patterns 24 h after labelling. Simultaneously, integrative nitrogen competition was quantified by examining total nitrogen capture, total nitrogen pools and total nitrogen allocation.Stipa neighbors reduced inorganic soil nitrogen content to a greater extent than didBromus plants. Blue oak seedlings responded to lower soil nitrogen content by allocating lower amounts of nitrogen per unit of biomass producing higher root length densities and reducing the nitrogen content of root tissue. In addition, blue oak seedlings growing with the perennial grass exhibited greater rates of¹⁵N uptake, on a root mass basis, compensating for higher soil nitrogen competition inStipa neighborhoods. Our findings suggest that while oak seedlings have lower rates of nitrogen capture than herbaceous neighbors, oak seedlings exhibit significant changes in nitrogen allocation and nitrogen uptake rates which may offset the competitive effect annual or perennial grasses have on soil nitrogen content.     

Inter-simple sequence repeat (ISSR) diversity within Monarda fistulosa var. brevis (Lamiaceae) and divergence between var. brevis and var. fistulosa in West Virginia

Inter-simple sequence repeat (ISSR) banding patterns were used to examine genetic diversity within and among populations ofMonarda fistulosa var.brevis, a rare taxon restricted to several populations in limestone glades and barrens in eastern West Virginia and Virginia. More than 34% of the total ISSR diversity in var.brevis occurred among populations, which is high when compared to the few other rare species that have been examined for ISSR variation. Prior studies demonstrated that var.brevis is morphologically distinct from the more widespread var.fistulosa, and that the differences are maintained when the two varieties are grown together in a uniform environment. The present study utilizing ISSR markers indicated that the two varieties are distinct, though quite similar genetically, and this is concordant with prior investigations documenting their morphological and habitat differences. However, the ISSR results suggest that the two varieties have diverged relatively recently and/or there is a low level of gene flow between them.     

Effect of floral orifice width and shape on hummingbird-flower interactions

Nectar guides are common among insect-pollinated plants, yet are thought to be rare or absent among hummingbird-pollinated plants. We hypothesize that the lower lips and trumpet-shaped orifices of many hummingbird flowers act as nectar guides to direct hummingbirds to the flowers' nectar and orient the birds for pollination. To test this hypothesis we conducted laboratory experiments using flowers of Monarda didyma (bee balm) and M. fistulosa (wild bergamot), which have orifice widths of about 4 mm and 2 mm, respectively, and latex flowers with orifice widths of 4 mm and 2 mm and three orifice shapes (trumpet, lipped, and lipless). Rubythroated hummingbirds (Archilochus colubris) made fewer errors during bill insertion and spent a smaller proportion of their feeding visit in error at M. didyma flowers than at M. fistulosa flowers, and at unaltered flowers of both species than at flowers with lower lips removed. Handling times were longer at both lipped and lipless flowers of M. didyma than at those of M. fistulosa, and at lipped than at lipless flowers of M. didyma. The average duration of contact between a hummingbird and a flower's anthers and stigma was longer at M. didyma than at M. fistulosa for both lipped and lipless flowers, and at lipped than at lipless M. didyma flowers. Hummingbirds missed the openings of latex flowers with their bills more frequently and spent a greater percentage of their total feeding visit in error at (i) 2-mm than at 4-mm flowers of all three shapes, (ii) lipless flowers than at trumpet or lipped flowers, and (iii) lipped flowers than at trumpet flowers of both widths. The duration of hummingbird/anther contact was longer at (i) 2-mm than at 4-mm flowers of all shapes, (ii) lipped than at trumpet or lipless flowers, and (iii) lipless than at trumpet flowers for both widths. No significant differences in handling times of hummingbirds were observed among any of the latex flower shapes or widths. Our results demonstrate that orifice shapes can act as guides by reducing the frequency of feeding errors by visiting hummingbirds, and that effects of orifice shape on pollination must be considered in conjunction with flower widths and locations of anthers and stigmas.     

What confines an annual plant to two separate zones along coastal topographic gradients?

We investigated the roles of flooding, salinity, and plant competition in creating a bimodal zonation pattern of the marsh dominant annual plant, Suaeda salsa, along coastal topographic gradients on the Pacific coast of northern China. In two consecutive years, we manipulated salinity and flooding, salinity, and competition for S. salsa seedlings that had been transplanted into the mudflat, the high marsh, and the upland, respectively. S. salsa plants that had been transplanted into the mudflat were completely eliminated in the non-elevated treatments whereas they performed much better in the 10 cm elevated treatments, regardless of salinity treatments. Although the performance of S. salsa transplanted into the high marsh did not differ between the fresh (watered) and the salt (control) treatments, S. salsa seedling emergence in the high marsh was nearly completely inhibited in the salt treatments. In contrast, a large number of S. salsa seedlings did emerge in the fresh treatments. S. salsa transplanted into the upland performed well when neighbors were removed, whereas it appeared to be strongly suppressed when neighbors were present. These data indicated that flooding, salinity, and competition all played a role in determining the zonation pattern of S. salsa. Furthermore, the importance of salinity was found to vary with life-history stage. Based on the results from these field manipulative experiments, we suggest that the marsh plant zonation paradigm may hold true for plant distributions along landscape-scale topographic gradients from mudflats to uplands in general. The relative importance of flooding, salinity, and competition, however, may vary at different elevations within a site and between sites. Handling editor: Pierluigi Viaroli     

Positive and negative effects of herbivory on the population dynamics of Senecio jacobaea L. and Cynoglossum officinale L.

Summary Herbivore effects were studied on populations of the biennial plant species Senecio jacobaea and Cynoglossum officinale. During a three year period (1985–1988) population characteristics (herbivory, number of seedlings, rosettes and flowering plants) were compared in-and outside exclosures, as well as parameters reflecting vegetation cover. In S. jacobaea, a strong negative effect of Tyria jacobaeae was found on seedling establishment, rosette growth and flowering. On the other hand, vertebrate herbivores (mainly rabbits) had an indirect positive effect by limiting the development of the surrounding vegetation (esp. grasses). The increasing vegetation cover in protected populations caused a reduction in germination, seedling- and rosette-growth. Herbivory on C. officinale was low (<10%), no direct effects of herbivores on plant populations were shown. Indirect effects of herbivory through an increasing vegetation were even more pronounced as in S. jacobaea. Therefore, although both plant species may first benefit from herbivore-exclusion, their populations are dependent on rabbits eating other plants (esp. grasses) and reducing competition.Publication of the Meijendel comité, New Series no. 108     

Differential responses of three fungal species to environmental factors and their role in the mycorrhization of<Emphasis Type="Italic"> Pinus radiata</Emphasis> D. Don

Three ectomycorrhizal (ECM) isolates of Rhizopogon luteolus, R. roseolus and Scleroderma citrinum were found to differ markedly in their in vitro tolerance to adverse conditions limiting fungal growth, i.e. water availability, pH and heavy metal pollution. S. citrinum was the most sensitive, R. luteolus intermediate and R. roseolus the most tolerant species. Pinus radiata D. Don seedlings were inoculated in the laboratory and in a containerised seedling nursery with spore suspensions of the three ECM species. Colonisation percentage was considerably lower under nursery conditions, probably due to competition by native fungi. The effects of nursery ECM inoculation on seedling growth depended on the fungal species. Only R. roseolus-colonised plants showed a significantly higher shoot growth than non-mycorrhizal plants. All three fungi induced significantly higher root dry weights relative to control plants. Despite the low mycorrhizal colonisation, mycorrhization with all three species improved the physiological status of nursery-grown seedlings, e.g. enhanced root enzyme activity, shoot nutrient and pigment content, net photosynthesis rate and water use efficiency. Of the three fungal species, R. roseolus was the most effective; this species was also the most adaptable and showed the greatest range of tolerance to adverse environmental conditions in pure culture. It is, therefore, proposed as a promising fungal species for ECM inoculation of P. radiata in the nursery.     

Soil type, microsite, and herbivory influence growth and survival of Schinus molle (Peruvian pepper tree) invading semi-arid African savanna

Naturalization of Schinus molle (Anacardiaceae) has been observed in semi arid savanna of the Northern Cape Province of South Africa. However, with high dispersal ability, the species is expected to achieve greater densities and invade more widely. The study involved a field manipulation experiment over 14 months using a factorial block design to examine transplanted seedlings in different savanna environments. The experiments examine the effects of soil type (sandy and clay), microsite, and herbivores on seedling performance (establishment, growth and survival). Seedlings were grown in a greenhouse and individually transplanted into four treatment groups: in open grassland, under tree canopies, and with and without cages to exclude large herbivores (cattle and game). The same experiment was repeated in two different soil types: coarse sand and fine-textured clay soil. Results suggest that protection provided by canopies of large indigenous Acacia trees facilitates S. molle invasion into semi-arid savanna. In the field, S. molle seedlings performed considerably better beneath canopies of indigenous Acacia trees than in open areas regardless of soil type. Whether exposed or protected from large herbivores, no seedlings planted in open grassland survived the first winter. Although, seedlings grew better and had higher survival rates beneath tree canopies than in the open sites, exposure to large herbivores significantly decreased heights and canopy areas of seedlings compared with those protected from large herbivores. The effect was greater on clay soil than on sandy soil. The results suggest that low temperature (frost), and possibly inter-specific competition with grasses, may limit S. molle seedling establishment, survival and growth away from tree canopies in semi arid savannas. Low soil nutrient status and browsing may also delay growth and development of this species. The invasive potential of S. molle is thus greatest on fertile soils where sub-canopy microsites are present and browsing mammals are absent.     

Seedling recruitment ofChamaecyparis obtusa andSciadopitys verticillata in different microenvironments in an old-growthSciadopitys verticillata forest

Seedling recruitment ofChamecyparis obtusa andSciadopitys verticillata in different microenvironments in an old-growthS. verticillata forest was studied for five years. Exposed mineral soil beneath a gap, which was newly created by a typhoon, supported the best seedling emergence forC. obtusa. S. verticillata seemed to require no specific microenvironment for its seedling emergence. Mortality of seedlings was higher for both species beneath a closed canopy, where all new seedlings ofC. obtusa died within a year after their emergence. Beneath the gap, mortality of seedlings was generally lower for both species, regardless of the presence of litter on the ground. The occurrence of saplings was more frequent forC. obtusa beneath the gaps and forS. verticillata beneath a closed canopy. Difference of seedling recruitment behaviour could maintain the coexistence ofC. obtusa andS. verticillata in the canopy layer of this forest in a non-equilibrium state caused by occasional disturbance.