Multi-stemmed Individuals Research Articles

Novel Encapsulated Herbicide Delivery Mechanism: Its Efficacy in Mimosa Bush (Vachellia farnesiana) Control.

Mimosa bush (Vachellia farnesiana) is an invasive woody weed widely distributed in Australia. While it can be controlled using several mechanical and chemical techniques, this study evaluated a novel herbicide delivery mechanism that minimizes the risk of spray drift and potential non-target damage. This method, developed by Bioherbicides Australia, involves the implantation of encapsulated granular herbicides into the stem of intact plants or into the stump after cutting off plants close to ground level (cut stumps). Trials were implemented near Moree (New South Wales, Australia) on intact (two experimental runs) plants and cut stumped (two experimental runs) plants. For each trial, an untreated control plus the conventional basal bark application of a liquid formulation of triclopyr + picloram mixed with diesel was included for comparison. Encapsulated glyphosate, aminopyralid + metsulfuron-methyl, hexazinone and clopyralid were also tested in all trials. In addition, encapsulated triclopyr + picloram, and metsulfuron-methyl were included in one of the intact plant trials. Aminopyralid + metsulfuron-methyl was consistently most effective on cut stump and intact plants, whilst clopyralid provided highest mortality when applied to cut stumps and single-stemmed intact plants. Particularly for multi-stemmed intact plants, clopyralid should be applied to each stem. Overall, the highest efficacy was achieved on single stemmed plants, but with further refinement of the technique, it should be possible to achieve similar results for multi-stemmed individuals. This method resulted in a reduction in the use of herbicide and environmental contamination while significantly improving speed of treatment.

OLD-GROWTH CHARACTERISTICS ON A COAST REDWOOD (SEQUOIA SEMPERVIRENS (D. DON) ENDL.) PYGMY FOREST ECOTONE

Old-growth Sequoia sempervirens (D. Don) Endl. (Cupressaceae) forests are commonly assumed to be populated by massive trees, but those growing on nutrient poor soils can be stunted and display unusual growth-forms. We characterized soils, stand structure, growth form, and crown complexity using a combination of random and targeted stratified sampling across an old-growth redwood/pygmy forest ecotone. ANOVA indicated significant variation in stand structure metrics and soil characteristics [pH, cation exchange capacity (CEC), and nutrient availability] among strata, with tree height, basal area, and the relative dominance of redwood-associated species declining under the influence of pygmy forest soils; while shrub cover and the dominance of pygmy associated species increased. Pearson's coefficients indicated positive correlations between tree height and diameter with CEC, P, K, Ca, and Na. Stepwise linear regression suggested that P, K, and CEC were the strongest predictors of tree height and diameter. Growth form and crown structure of S. sempervirens also varied considerably across the ecotone, with stunted multi-stemmed individuals and clusters of small diameter trees observed where pygmy soil conditions were most extreme, and larger, but still stunted ancient redwoods with complex crown structures, resident epiphytes, and large fire hollows, existing on the edge of the pygmy forest. The ancient redwoods sampled within the old-growth/ pygmy ecotone were remarkably small in stature, yet contained many of the same structural characteristics associated with the largest and oldest S. sempervirens.

Population structure and regeneration strategy of relict deciduous broadleaved trees on Mount Tianmu, Zhejiang Province, China.

The population structure, regeneration ways and spatial relationships of six relict deci-duous broadleaved trees were examined based on community investigation on Mount Tianmu, East China. The results showed that relict deciduous broadleaved trees had a strong ability of vegetative reproduction and their population structure was generally belonged to intermittent type. Some relict species such as Cyclocarya paliurus and Liquidamb aracalycina regenerated discontinuously at appropriate sites via long-distance diffusion. Some species such as Emmenoptery henryi and Euptelea pleiospermum regenerated discontinuously around mature trees due to habitat limitation and frequent disturbance. After occupying forest gaps by seedling regeneration, C. paliurus, L. acalycina, Nyssasinensis and Liquidambar formosana could recruit and form multi-stemmed individuals by their inherent sprouting ability at relatively stable sites such as crest slope and side slope. At riverbed, valley slope and head hollow, recognized as unstable habitats, Emmenopterys henryi, E. pleiospermum, and L. formosana could also recruit by strong sprouting ability due to sparse seedlings and individual loss incurred by disturbance. Therefore, the relict deciduous broadleaved trees could be classified into two ecological species groups (repairment species for forest gap and supplement species for special habitat) based on spatial distribution and strategy of regeneration and competition. We suggested that the protection of special habitats of their communities should be strengthened.

Architectural and morphological variation of Hintonia latiflora (Rubiaceae) in relation to bark harvest and environmental factors.

Hintonia latiflora is a rare ecologically and economically important species from the Tropical Deciduous Forest of the Balsas Basin, Mexico, whose bark is traded as medicinal. Debarking practices have modified the shape of plants, their architecture and morphometry; but it is unknown if some topographic and edaphic factors may also influence in these attributes. Here we propose that the ecological conditions of this species distribution and harvesting areas, may determine the morphometric characteristics and the individuals variation in their architecture. To test this hypothesis we assessed the relationship of topographic and edaphic factors on the morphological characteristics of saplings (n = 143) and adults (n = 117), in eight harvesting areas of H. latiflora . The harvesting areas, with a surface of 0.6 ha, were selected randomly with the support of participatory mapping elaborated by gatherers of the study site. Data were analyzed using principal components (PCA) and canonical correspondence (CCA) analyses. The first three PC explained 92.1 % of the morphological variation; height, basal diameter, and coverage, explained morphometric differences in both, saplings and adults, in all eight harvesting zones. The terrain slope, slope orientation, soil depth and stoniness, were ecological factors statistically related with morphometric differences. A lower terrain slope, little stoniness, soil depth, slope orientation E-W in saplings (azimuth 90˚-270˚) and N-S in adults (azimuth 0˚-180˚) were the factors associated to height, number of basal branches, basal diameter and coverage of H. latiflora . Areas with higher harvesting intensity were composed by monopodic trees and some reiterated basitone individuals; the adults of these areas also had more fruits. At these sites, plants of H.latiflora were more abundant and produced more bark, but the high-intensity of harvesting, changes plant architecture, forming shrubby, multi-stemmed individuals, that originally were mesotone trees. Areas with lower harvesting intensity were characterized by steep slopes, shallow and rocky soils, and were dominated by basitone trees of smaller size, thin and scarce in the area. These sites are the least recommended for harvesting, because they produce less bark per tree, and serve to preserve the species at the site. Based on these results, we can develop restoration programs in areas affected by commercial harvesting, establish forestry plantations in key sites to ensure the establishment and development of individuals with desirable characteristics for bark harvesting and contribute to in situ conservation of H. latiflora in the Alto Balsas Basin.

Variación arquitectónica y morfológica de Hintonia latiflora (Rubiaceae) en relación a la cosecha de corteza y factores ambientales

Hintonia latiflora is a rare ecologically and economically important species from the Tropical Deciduous Forest of the Balsas Basin, Mexico, whose bark is traded as medicinal. Debarking practices have modified the shape of plants, their architecture and morphometry; but it is unknown if some topographic and edaphic factors may also influence in these attributes. Here we propose that the ecological conditions of this species distribution and harvesting areas, may determine the morphometric characteristics and the individuals variation in their architecture. To test this hypothesis we assessed the relationship of topographic and edaphic factors on the morphological characteristics of saplings (n = 143) and adults (n = 117), in eight harvesting areas of H. latiflora. The harvesting areas, with a surface of 0.6 ha, were selected randomly with the support of participatory mapping elaborated by gatherers of the study site. Data were analyzed using principal components (PCA) and canonical correspondence (CCA) analyses. The first three PC explained 92.1 % of the morphological variation; height, basal diameter, and coverage, explained morphometric differences in both, saplings and adults, in all eight harvesting zones. The terrain slope, slope orientation, soil depth and stoniness, were ecological factors statistically related with morphometric differences. A lower terrain slope, little stoniness, soil depth, slope orientation E-W in saplings (azimuth 90˚-270˚) and N-S in adults (azimuth 0˚-180˚) were the factors associated to height, number of basal branches, basal diameter and coverage of H. latiflora. Areas with higher harvesting intensity were composed by monopodic trees and some reiterated basitone individuals; the adults of these areas also had more fruits. At these sites, plants of H.latiflora were more abundant and produced more bark, but the high-intensity of harvesting, changes plant architecture, forming shrubby, multi-stemmed individuals, that originally were mesotone trees. Areas with lower harvesting intensity were characterized by steep slopes, shallow and rocky soils, and were dominated by basitone trees of smaller size, thin and scarce in the area. These sites are the least recommended for harvesting, because they produce less bark per tree, and serve to preserve the species at the site. Based on these results, we can develop restoration programs in areas affected by commercial harvesting, establish forestry plantations in key sites to ensure the establishment and development of individuals with desirable characteristics for bark harvesting and contribute to in situ conservation of H. latiflora in the Alto Balsas Basin.

Fire History Reconstruction in the Black Oak (Quercus velutina) Savanna of High Park, Toronto

ABSTRACT: We employed tree-ring analysis to reconstruct the fire history of High Park, Toronto, Canada, in one of the largest remnants of black oak (Quercus velutina) savanna in Ontario. This heavily urbanized area has a long history of fire suppression, which has degraded the native savanna. Efforts to reintroduce fire would benefit from a fire history record. Ring width patterns were quantified for 38 black oak chronologies on 14 cross-sections and 10 cores using dendrochronology software that employs scanned (digital) ring sequences. Scar and sealing patterns and rapid growth responses following scarring were dated to indicate the timing of fire events. These dates were compared with the timing of regeneration pulses and the establishment of multistemmed individuals that would have sprouted in response to fire disturbance. Our records suggest that most of the mature black oak stems established synchronously, around 1865, following an apparently extensive fire event or set of events. We have evidence of...

Size dependency of post-disturbance recovery of multi-stemmed resprouting trees.

In frequently burned ecosystems, many plants persist by repeated resprouting from basal or belowground buds. This strategy requires that plants reach a balance between biomass loss and recovery, which depends on the shape of the relationship between pre- and post-fire size. Previous analyses of this relationship, however, have focused on the size of the largest stem, which ignores the importance of the multi-stem growth habit that is common in pyrogenic ecosystems. We hypothesized that the presence of multiple stems causes a substantial shift in the relationship between pre- and post-fire size and in the relationship between pre-fire size and size recovery. We measured the height and basal diameter, then calculated volume and biomass, of all stems of six tree species before and nine months after complete removal of aboveground biomass via coppicing. The number of resprouts was correlated with the original number of stems for four species. For all species, the relationship between pre-coppicing and resprout size fit a positive curvilinear function, and the shape of this curve did not differ for maximum and total stem size. Smaller individuals recovered a larger proportion of their pre-coppicing size than larger individuals, but the shape of the size recovery curves were the same regardless of whether the analysis was performed with all stems or only the largest stem. Our results indicate that measuring only the largest stem of multi-stemmed individuals is sufficient to assess the ability of individuals to recover after complete loss of aboveground biomass and persist under frequent burning.

Local-scale drivers of multi-stemmed tree formation in Acer, in a temperate forest of Northeast China

Acer is an important genus in temperate forests in Northeast China. Individual Acer trees can re-sprout from the root collar and can occur in clonal units, either as a single-stemmed or multi-stemmed tree. However, the factors that induce multiple-stems in Acer remain only partly understood. In this study, we determined the relative importance of abiotic and biotic variables in driving the production of multiple-stems in this genus, within a 25-hm2 experimental forest dynamics plot in Changbaishan (CBS) temperate forest. We used generalized linear mixed models to perform analyses at two levels (community- and species-level). We found seven Acer species in total within the plot, where they form a key part of the forest community. Our results show that abiotic factors play a more important role in producing multi-stemmed trees at the community level in CBS. At the species level, the relative importance of different factors varied among species. Shrub species tended to have a higher frequency of multi-stemmed trees under stressful conditions, whereas tree species tended to have more multi-stemmed trees in more suitable habitat. Our results indicate that the relative importance of different factors influencing the frequency of multi-stemmed individuals in Acer differs at the community and species level in the temperate forest.

Costs of resprouting are traded off against reproduction in subtropical coastal dune forest trees

Resprouting can be an important means of regeneration for forest tree species resulting in multi-stemmed architecture, especially at less productive or frequently disturbed sites. However, the cost of resprouting may be traded off against growth or reproduction. In subtropical coastal forest in South Africa, trees grow on steep, sandy dunes with unstable soils and low to moderate nutrient availability. These coastal forests experience seasonally strong anticyclonic winds from August through October. We examined the hypothesis that basal resprouting resulting in multiple stems causes lower rates of sexual reproduction and recruitment by individuals. We examined whether trees traded off resprouting against seed output, seed size, seedling abundance and recruitment by seedlings. Species were designated as good and poor resprouters based on their frequency of multi-stemmed individuals at Cape Vidal. Good resprouters had more stems, produced less seed and had lower seed mass than poor resprouters, and had lower seedling abundance and fewer individuals in small diameter classes than large diameter classes. Seedling abundance in good resprouters was not influenced by the availability of understorey gaps. Good resprouters were most abundant on dune crests and seaward slopes that were exposed to sea winds. Persistence of established individuals by producing multiple stems from basal resprouts is important where a chronic disturbance regime potentially reduces the survivorship of single-stemmed individuals and thereby their opportunities for reproduction. Good resprouters appear to trade-off recruitment of new individuals for multiple stems that increase the persistence of established ones against disturbance. We conclude that multi-stemming arising from basal resprouts has evolved to promote individual persistence under low to moderate intensity but pervasive wind stress.

Structure, diversity, and spatial patterns in a permanent plot of a high Restinga forest in Southeastern Brazil

We assessed the structure, diversity and distribution of tree species in a 10.24-ha permanent plot of high Restinga forest (HRF) in southeastern Brazil. We sampled 15,040 individuals belonging to 45 families, 87 genera and 116 species (density= 1,468 trees ha-1, and basal area= 28.0 m² ha-1). Mean richness was lower than other types of tropical forests and other HRFs that have been less intensively sampled, which is probably due to the younger age and hydromorphy of the soils in the study site. Tree density, basal area, species composition, and diversity all varied considerably depending on the size class considered. The studied plot had a density similar to other tropical forests but with a smaller basal area (fewer individuals > 60 cm in diameter), and higher density of smaller trees. Multi-stemmed individuals were not frequent (11% of the trees), though they were widespread among species (74% of the species). Spatial distribution was predominantly clumped, a pattern common to other types of tropical forests. Besides growing over white-sand soils, the PEIC plot and other white-sand forests of the world also had very similar patterns of community structure, richness and diversity. Therefore, we consider the HRF as a recognizable subtype of white-sand forest.

Wood productivity of Prosopis flexuosa D.C. woodlands in the central Monte: Influence of population structure and tree-growth habit

A balance between forest production and protection is hard to achieve in arid zones due to their low potential for wood production. Prosopis flexuosa woodlands are the major woody formations in the Monte desert and are currently in a degraded state due to intense use. The main degradation factors in the study area are overgrazing and firewood extraction. We developed allometric models to estimate the aerial biomass of P. flexuosa, compared annual growth rates of one- and multi-stemmed individuals through dendrochronological methods, and estimated the productivity of four structurally different woodlands in the central Monte. Total dry weight was best estimated by power equations. Annual increments in basal area and dry weight were initially larger for multi- than one-stemmed individuals. However, whereas multi-stemmed individuals rapidly decreased their growth rates after 60 years of age, one-stemmed trees maintained steady growth rates during the first 100 years. Depending on woodland density and tree size, total woodland biomass varied between 4000 and 15 000 kg ha −1. Wood productivity was similar in all four woodlands studied (121.6–173.7 kg ha −1 year −1). Our results reveal the importance of tree growth habit to productivity, and suggest that regulated extraction of firewood and poles from old multi-stemmed individuals could optimize wood productivity and contribute to the sustainable use and conservation of these woodlands.

Structural variability and species diversity of a dwarf Caribbean dry forest

Low stature woody vegetation of the south-west coast of Puerto Rico grows on a rocky calcareous substrate where plants can only root in holes, cracks, and crevices accumulating water and sediments that allow seed germination and seedling development. Being in a coastal location these communities are influenced by steady onshore winds, high solar radiation, and salt spray. We studied dwarfed forest communities located at the southern limit of the Guánica State Forest, Puerto Rico, that have a well known floristic composition, but with little information on their organization and heterogeneity. We quantified the species composition of vegetation plots located along the southern coastline and compared their structure and diversity with those of dry forests on neighboring hills. The hypothesis was that forest structural development was negatively associated with proximity to the coast line and altitude above sea level. Species richness of the coastal dwarf forest area was similar to that of inland semi deciduous forest plots with which it shares at least 15 woody species. In addition, it contains a number of species resistant to salt spray and possibly brackish water, such asConocarpus erectus, Strumpfia maritima, and Coccoloba uvifera. The average canopy height of the coastal vegetation increases from 0.4 to 2.3 m between 0 to 150 m from the coastline and 7 to 19 m elevation above sea level. Within this spatial range the predominant bearing of the canopy changes from a SE-NW direction to a SW-NE direction revealing the influence of onshore winds in combination with salt spray. The dominant woody species occur as multistemmed individuals, a characteristic probably associated to occasional heavy winds and recurrent drought that impairs shoot apical dominance. Floristic and structure comparisons with mature forest plots located between 25 and 150 m altitude showed that the coastal dwarf vegetation has a more even species dominance distribution associated with the discontinuity of substrate available for establishment.

La régénération de cinq espèces de Combretaceae le long d’un gradient latitudinal en zone sahélo-soudanienne du Burkina Faso

• Seedling density and the regeneration mechanisms of five tree species, Anogeissus leiocarpa, Combretum aculeatum, Combretum micranthum, Combretum nigricans, and Pteleopsis suberosa were investigated in relation to latitudinal gradient across the Sahelo-Sudanian zone of West Africa. • Data were collected on 461 quadrats (2 m × 5 m) laid out every 30 m on transect lines through Combretaceae communities at four latitudinal positions. Regeneration mechanisms were determined by excavating the below ground root system and assessing basal and aerial sprouts. • The results showed a significant species × latitudinal position effect on the total density of seedling populations, and the density of single- and multi-stemmed individuals (p < 0.001). C. aculeatum and C. micranthum were abundant in the North-Sahelian sector, C. nigricans and P. suberosa in the Sudanian sector and A. leiocarpa across a wide range from the South-Sahelian to South-Sudanian sectors. In general, 58% of the seedlings were regenerated asexually (as coppice, water sprout, layer, and root sucker) while 42% were sexual recruits (as true seedling and seedling sprouts). The proportion of vegetatively propagated seedlings increased with increasing latitude for all species except C. micranthum, for which a clear decreasing trend was observed. The relative importance of the different regeneration mechanisms varied among species: seedling sprouts were important for A. leiocarpa, C. aculeatum and C. nigricans, coppice for C. micranthum and sucker for P. suberosa • The significant interaction observed between species and latitudinal position highlights the importance of accurate species-site matching to ensure successful restoration of degraded areas in the Sahelo-Sudanian zone. Inter-species differences in regeneration mechanism could be related to their biology and ecological adaptation to the site-specific biotic and abiotic factors.

Impact of disturbance from roadworks on Pteleopsis suberosa regeneration in roadside environments in Burkina Faso, West Africa

The seedling population structure of Pteleopsis suberosa and their regeneration mechanisms were investigated in four roadside environments (graded, adjacent, intermediate and ungraded areas) along paved and unpaved roads in West Africa. A total of 203 quadrats of 2 m × 5 m in size were surveyed and placed along transects parallel to the roads. Within each quadrat, the total number of seedlings and the number of living shoots per seedling base were recorded. Regeneration mechanisms were determined by assessing basal and aerial sprouts and excavating the root systems below ground level. The results show that the total seedling density and the densities of single- and multi-stemmed individuals varied significantly (p < 0.05) among the four roadside environments. However, all seedlings were produced asexually; root suckers were predominant (98%) compared to water sprout (1%) and coppices (less than 1%). This study demonstrates that an intermediate level of soil disturbance from grading along paved and unpaved roads may stimulate P. suberosa regeneration by root suckering. Road type (paved and unpaved) did not affect seedling density, but was a highly significant variable in relation to the coppicing ability of P. suberosa populations in roadside sites. In conclusion, P. suberosa is a disturbance-tolerant species which can proliferate mainly by root suckering after roadwork disturbance.

Sprouting by remobilization of above‐ground resources ensures persistence after disturbance of coastal dune forest trees

1. Resprouting in woody plants is a trait that facilitates persistence in disturbance-prone environments. Patterns of allocation of resources for resprouting may depend on the severity of disturbance to which the plants are adapted. Fire-adapted plants allocate more resources to below-ground structures for resprouting after destruction of above-ground structures. However, plants that resprout in response to disturbances where above-ground structures survive may remobilize above-ground resources for resprouting. 2. In coastal sand dunes, stem leaning and partial uprooting of trees result in high frequency of resprouting (38·9% of individuals; 90·6% of species). We tested whether 'good' and 'poor' resprouters differed in allocation to root biomass and root carbohydrate reserves. Species were assigned to categories of resprouting ability based on the frequency of multi-stemmed individuals in the local population. To control for phylogenetic effects, we contrasted poor and good resprouter species pairs from three families. 3. We tested whether plants stored more reserves in roots or stems and whether above-ground resources were remobilized for resprouting. The latter was measured from the number and dry mass of sprouts produced by trees cut to stump heights of 10 and 150 cm above-ground level. 4. Good resprouters had larger seedling root:shoot ratios and higher stem and root carbohydrate concentrations than poor resprouters. Both good and poor resprouters maintained higher carbohydrate concentrations in stems than in roots. 5. For both good and poor resprouters, 150-cm stumps produced more sprouts than 10-cm stumps. At each stump height, good resprouters produced more sprouts than poor resprouters. 6. Resource allocation in coastal dune trees appears to be a bet-hedging strategy. After low-severity disturbances, resprouting occurs by remobilization of above-ground resources. Below-ground resources may be more costly to remobilize but may allow recovery from occasional more severe disturbances.

Multi-stemmed trees in subtropical coastal dune forest: Survival strategy in response to chronic disturbance

Abstract Questions: Is multi-stemming a survival strategy in response to chronic disturbance? Is multi-stemming under phylogenetic control? What environmental factors are associated with multi-stemming? When do trees initiate growth of multiple stems? Do multi-stemmed trees have limited stature? Location: Subtropical Indian Ocean coastal dune forest at Cape Vidal in the Greater St. Lucia Wetland Park, northeastern South Africa. Methods: Tree physiognomy and environmental conditions were sampled in 20 transects that were 300 m long and 5 m wide. Results: 38.9 % of trees were multi-stemmed, with no correlation between multi-stemming and taxonomic grouping. The multi-stemming trait was most associated with stem decumbency and substrate erosion and multi-stemmed individuals were less common in protected dune slacks. Trees at Cape Vidal appear to facultatively produce multiple stems from an early stage. There was no trade-off between tree height and stem number. Conclusions: Coastal winds and the unstable dune...

Genetic Analysis of Swiss Stone Pine Populations (Pinus cembra L. subsp. cembra) from the Carpathians Using Chloroplast Microsatellites

The diversity of Swiss stone pine populations (Pinus cembra subsp. cembra) native to the Carpathians was studied with chloroplast microsatellite markers (cpSSR). Six loci were analyzed in four populations (altogether 148 individuals) and a total number of 22 size variants and 41 combinations referred to as haplotypes were detected. Diversity within populations was found to be high, whereas divergence between the populations was low (Fst= 0.02). The most variable population with the highest haplotype diversity (H=0.956) originated from the Retezat Mountains (South Carpathians). Multi-stemmed individuals were detected mainly in the Southern part of the Calimani Mountains. They were found to be genetically non-homogenous. It is assumed that these clusters of individuals are the result of plants emerging from seed caches by birds. The great haplotypic variation found in cpSSR loci makes all the populations a useful source for gene conservation purposes. Each population should be considered an important element of the local ecosystem diversity.

Monodominance in the ectomycorrhizal Dicymbe corymbosa (Caesalpiniaceae) from Guyana

Dominance of primary rain forests by the ectomycorrhizal, leguminous canopy tree Dicymbe corymbosa (Caesalpiniaceae) was investigated in the Pakaraima Mountains of western Guyana. In five 1-ha forest inventory plots in the Upper Ireng and Upper Potaro River drainages basal areas of D. corymbosa ranged from 38.4-52.8 m2 ha-1 (63-85% of total) among all trees ≥10 cm diameter at breast height (dbh), values in the upper range for tropical moist forests worldwide. The high basal areas for D. corymbosa were due to the prevalence of large (&gt;150 cm dbh), multi-stemmed individuals. Stem densities in Dicymbe plots ranged from 276-433 ha-1, with D. corymbosa contributing 24.6-59.8%. In three 1-ha mixed forest plots adjacent to the Dicymbe plots, D. corymbosa was absent. In the mixed forests, stem densities were higher (480-585 ha-1), basal areas were lower (36.7-39.8 m2 ha-1), species diversity was higher, and canopy tree species were more equitably distributed than in the Dicymbe plots. Tree community composition was not qualitatively different between Dicymbe and mixed forests. In the Dicymbe plots, mean sapling and seedling densities of D. corymbosa were significantly higher than most other canopy species, indicating persistent monodominance. Edaphic variation did not account for variation in forest composition. Life history traits are discussed which may contribute to clumping in D. corymbosa, including coppicing and mast-fruiting, and the potential role of litter-trapping physiognomy and ectomycorrhiza-mediated nutrient dynamics in promoting monodominance is noted.

Diversity of the Sheanut tree (Vitellaria paradoxa C.F. Gaertn.) in Ghana

The Sheanut tree (Vitellaria paradoxa C.F. Gaertn.) is highly valued for oil obtained from its seeds and frequently maintained in the semi-arid parklands of sub-Saharan Africa from Senegal to Uganda. Although variation has been noted for V. paradoxa subsp. paradoxa, few studies have been undertaken on patterns of phenotypic or genotypic diversity. Results are presented from 294 accessions collected in Ghana, using easily quantifiable morphological parameters. Productive mature trees varied from shrubby multi-stemmed individuals (height <5 m) to 30-m straight-boled trees with high compact canopies. The leaf lamina (tree mean ±% coefficient of variation of total mean) varied from 9.2 to 22.5 cm (±15.0%); seed length from 1.74 to 3.74 cm (±11.7%); and fat content from 29.1 to 61.9% of dry kernel weight (±9.4%). Clinal trends significantly correlated with location parameters that in turn related to environmental variation. Population variation of seed characteristics increased to the Northeast, perpendicular to the annual movement of the inter-tropical convergence zone. It is proposed that this is a consequence of past climatic changes, coupled with a lack of methodical selection for seed type when trees are maintained on agricultural land. Isozyme analysis revealed moderate to high levels of heterozygosity (He 0.2142) and high geneflow (Fst = 0.0124, Nm = 19.9), supporting results obtained from morphological studies. Isozyme and multivariate morphology analyses showed similar but weak geographical separation patterns.

Estructura de un ecotono bosque Subalpino-pastos alpinos (Las Cutas, Ordesa, Pirineos Centrales)

We describe the structure of a subalpine forest-alpine pasture ecotone in the Central Pyrenees, that includes altitudinal timberline and treeline, and it is dominated by Pinus uncinata Ram . We measured for each P. uncinata 21individual located inside a rectangular plot parallel to the slope: the location and several structural and groivth form variables (number and type of stems per individual). Then, P. uncinata individuals were classified according to their size (adults, poles, saplings and seedlings) and growth form (krummholz —shrubby and multistemmed individuals—, krummholz with vertical stems). The ecotone structure was described through the study of groivth form and size changes of P. uncinata individuals across the ecotone and related to the area climate, that is characterized by maximum snow thickness in spring (April) and strong N-NW-W winds. Most of living individuals were krummholz, located above the timberline in the area where seedlings were abundant. Bigger, vertical and unistemmed individuals predominated in the forest. The structure or growth form changes were sharp across the ecotone. Crown injuries due to wind effects were evident in shrubby individuals and predominated in those directions that showed stronger winds during the year. The snow-wind interaction can explain partially the location of the studied timberline that could be considered a local phenomenon. The spatial location of the different classes of individuals, the spatial interaction between seedlings and krummholz individuals, and the changes of growth form (from arborescent to shrubby or vice versa) can modify the response of these P. uncinata populations to climate changes.