Sapling Establishment Research Articles

Trichoderma harzianum Rifai 1295-22 Mediates Growth Promotion of Crack Willow (Salix fragilis) Saplings in Both Clean and Metal-Contaminated Soil

We investigated if the plant growth promoting fungus Trichoderma harzianum Rifai 1295-22 (also known as "T22") could be used to enhance the establishment and growth of crack willow (Salix fragilis) in a soil containing no organic or metal pollutants and in a metal-contaminated soil by comparing this fungus with noninoculated controls and an ectomycorrhizal formulation commercially used to enhance the establishment of tree saplings. Crack willow saplings were grown in a temperature-controlled growth room over a period of 5 weeks' in a garden center topsoil and over 12 weeks in a soil which had been used for disposal of building materials and sewage sludge containing elevated levels of heavy metals including cadmium (30 mg kg(-1)), lead (350 mg kg(-1)), manganese (210 mg kg(-1)), nickel (210 mg kg(-1)), and zinc (1,100 mg kg(-1)). After 5 weeks' growth in clean soil, saplings grown with T. harzianum T22 produced shoots and roots that were 40% longer than those of the controls and shoots that were 20% longer than those of saplings grown with ectomycorrhiza (ECM). T. harzianum T22 saplings produced more than double the dry biomass of controls and more than 50% extra biomass than the ECM-treated saplings. After 12 weeks' growth, saplings grown with T. harzianum T22 in the metal-contaminated soil produced 39% more dry weight biomass and were 16% taller than the noninoculated controls. This is the first report of tree growth stimulation by application of Trichoderma to roots, and is especially important as willow is a major source of wood fuel in the quest for renewable energy. These results also suggest willow trees inoculated with T. harzianum T22 could be used to increase the rate of revegetation and phytostabilization of metal-contaminated sites, a property of the fungus never previously demonstrated.

Potential lateral expansion of Polylepis forest fragments in central Ecuador

High-Andean forests formed by Polylepis are among the most endangered forest types in South America and effective strategies are urgently required to facilitate their conservation. The remaining forest stands are presently distributed in discontinuous patches which usually form abrupt boundaries with the adjacent grassland vegetation. Mechanisms of natural regeneration in this transition zone between forest and grass paramo are still poorly understood. The present study investigates the establishment of seedlings, saplings and vegetative shoots along edges of Polylepis incana and P. pauta forests in the Páramo de Papallacta, central Ecuador. For each species, we analysed 18 transects of 20 m in length, which traversed the edge zone, and sampled 10 subplots of 2 m × 2 m each. Differences between subplot positions were analysed using a repeated measures ANOVA. In addition, we determined correlations of altitude and grazing parameters of the P. incana transects with regeneration data. Seedling numbers of both species were significantly higher in the forest interior than in the exterior, presumably due to less extreme climatic conditions and a more continuous seed rain. Sapling and ramet numbers, however, showed a clear maximum at the forest boundary, which indicates that favourable light conditions are required for sapling and ramet establishment. The importance of vegetative reproduction increased towards P. incana stands at higher altitudes, because the overall number of seedlings and saplings decreased significantly with increasing altitude. Sexual regeneration is limited at the upper distribution limit of the species, which we ascribe to extreme climatic conditions. In P. pauta stands, in contrast, there was no effect of altitude. Grazing by ungulates did not affect the recruitment of saplings or vegetative shoots in the investigated Polylepis stands, but trampling resulted in a significant increase in the abundance of P. incana seedlings. Our results show that boundaries of Polylepis stands are zones of high natural recruitment. Fire is expected to be the most relevant factor for the formation of edges at lower altitudes, as the use of fire strongly affects the reproductive boundary zone of the remaining Polylepis forests, whereas grazing at moderate animal loads does not lead to any severe consequences for these forest communities. We conclude that stands below the upper treeline would expand continuously towards the exterior if human impact ceased, and that the present patchy distribution of Polylepis is as a result of anthropogenic influence and not of natural site conditions.

The spatial relationship between post-crop remaining trees and the establishment of saplings in Pinus sylvestris stands in Spain

The spatial relationship between post-crop remaining trees and the establishment of saplings in Pinus sylvestris stands in Spain

Spatial patterns of regeneration in managed uneven-aged ponderosa pine/Douglas-fir forests of Western Montana, USA

In uneven-aged ponderosa pine ( Pinus ponderosa Dougl.) and Douglas-fir ( Pseudotsuga menziesii var. glauca (Mirbel) Franco) stands managed by individual tree selection, the recruitment of new sapling cohorts necessary to sustain structure and composition is generally assumed to occur within canopy openings created by harvest. However, few studies have examined the spatial patterns of new regeneration in relation to reserve trees in the overstory in this forest type. In this study, we analyzed the spatial distributions and associations between saplings and overstory trees using univariate and bivariate transformations of Ripley's L-functions. Furthermore, we introduced a ratio-based index to quantify the strength of bivariate spatial associations, which allowed us to further evaluate potential causes for the observed patterns. We found that saplings of both ponderosa pine and Douglas-fir established predominantly in clumps, and that saplings of the two species generally had a positive association with one another at short scale (up to 5 m). We found no evidence to suggest that saplings of ponderosa pine and Douglas-fir occupied separate spatial locations in these stands. We also found mixed results for bivariate associations between saplings of both species and overstory trees, where both positive and negative spatial associations occurred depending on the putative moisture level of plots indicated by habitat type group. The strength of positive spatial association between saplings and medium-sized overstory trees appeared to decrease as plot moisture-level increased, suggesting that sapling establishment and survival might be facilitated by shading on moisture-limited sites.

Getting trees on farms the easy way? Lessons from a model of eucalypt regeneration on pastures

Substantial revegetation is required across much of Australian agricultural landscapes. Managers need to minimise the risks of failure and costs associated with revegetation. Whereas ‘natural regeneration’ of eucalypts is relatively cheap compared with direct seeding or planting tubes, natural regeneration in grazed lands occurs infrequently and under limited circumstances. Management needs to understand the situations in which natural regeneration is most likely to occur and what actions can improve the chances of regeneration. We used a rule- and stage-based model of eucalypt regeneration focusing on events between seed supply and sapling escape to synthesise current knowledge and learn how to improve the success rate of natural regeneration of eucalypts. The model used deterministic rules but with stochastic rainfall, and fire was applied stochastically as well as deterministically. Results from simulations suggested that low-productivity pastures have greater likelihood of supporting saplings than do high-productivity pastures. Fire and grazing can increase the chances of subsequent germination and early seedling survival, particularly in high-productivity pastures. As a result, management actions, such as fire and strategic grazing, can improve the probability of sapling establishment in a high-productivity pasture; however, frequency and timing of management actions are important. The following three sources of uncertainty appear crucial: first, variability in rainfall; second, uncertainty about rainfall effects on stage transitions; and third, variability in seed supply. These uncertainties can overwhelm improvements to the chances of regeneration owing to management interventions. Because rainfall and seed supply have overwhelming effects on the process of regeneration, they limit the ability of managers to influence the outcomes and this makes investment in natural regeneration inherently risky. Thus, better data are needed on the effect of rainfall on stage transitions and on spatial and temporal variation in seed supply. This would aid managers to estimate the likelihood of success of regeneration, and make decisions about if, when and where to invest in natural regeneration and what actions to implement.

Regeneration niches of Norway spruce (Picea abies [L.] Karst.) saplings in small canopy gaps in mixed mountain forests of the Bavarian Limestone Alps

Natural regeneration measurements are the main silvicultural objective in overaged protective forests of the Bavarian Limestone Alps. While manifold problems with these stands, especially the impact of browsing, are widely recognised, the regeneration niches of Norway spruce (Picea abies [L.] Karst.) are insufficiently known. The purpose of this study was to determine favourable combinations of site factors for the development of spruce in small, unfenced canopy gaps, located on Aposerido-Fagetum caricetosum albae forest sites. We recorded the occurrence of spruce saplings (as dependent variable) and of six site factors (as independent variables) on 480 0.5 m2-subplots. In addition, we estimated the coverage of six acid adapted plant species to determine correlations with the humus depth. A binary logistic regression analysis was used to predict the probability of the occurrence of a spruce sapling in dependency of the different site factors. Supported by other studies, we assumed that the supply of solar radiation was adequate for the sufficient regeneration of spruce within the canopy gaps. Other site factors significantly determined the regeneration niches of spruce saplings. More spruce saplings were found near hindrances and on rough surfaces than would be expected from a random occurrence of saplings. These microsite types may have characteristics, especially protection against snow gliding that promotes spruce establishment. A calculated “hindrance index”, which accounted for the number, and the distance of surrounding hindrances might be a good specific value to describe the influence of hindrances on steep slopes. The sapling establishment decreased on thin humus layers. Our assumption for the sites was that thick organic layers might represent a good seedbed for spruce. Decayed dead wood was scarce, but was exceedingly favoured by spruce saplings. Results obtained suggest that the natural regeneration establishment of spruce on steep slopes can be successfully influenced by site factors which inhibit the influence of snow gliding. According to a “positive microsite” concept, we recommend for artificial regeneration measurements with spruce, microsites close to hindrances (e.g. stumps, downed trees) and Vaccinium myrtillus as a predictor for thick, acid humus layers.

Stem distribution and clonal structure of Chamaecyparis pisifera growing in an old‐growth beech‐conifer forest

To elucidate sapling patch formation and the mode of regeneration occurring in Chamaecyparis pisifera, we investigated stem distribution and clonal structure of this species in an old‐growth beech‐conifer forest in central Japan. The study was carried out in a 1‐ha plot using small and large sapling (d.b.h. <5 cm, 0.5 m ≤ height < 2 m and d.b.h. < 5 cm, height ≥ 2 m, respectively) and adult (d.b.h. ≥5 cm) stems of Chamaecyparis pisifera. The numbers of small and large sapling stems were 677 and 149 in the plot, respectively, and they were strongly aggregated. Adult stems occurred at much lower density (90 stems in the plot) and were weakly clustered. Fifty multilocus genotypes at nine allozyme loci were detected among sampled stems. Small and large sapling stems with identical genotypes showed aggregated distribution, while some neighboring adult stems had the same genotypes. Spatial autocorrelation of alleles in small and large sapling stems revealed a strongly positive association among stems within a distance of 10 m; but a negative association as distance increased. In adult stems, a strongly positive value was found only in the shortest distance class. These results indicate that clonal growth by layering occurred at the sapling stage and contributed to patch formation. Our results also suggest that the clonal growth by layering is ecologically significant throughout the life history of Chamaecyparis pisifera, particularly as an important strategy for the sapling establishment in the early stage of regeneration process.

Seedling survival and seed size: a synthesis of the literature

Summary Large‐seeded species have long been known to have higher survivorship during establishment than small‐seeded species. Here, we assessed the size of this advantage by compiling published data on survival through seedling emergence, seedling establishment and sapling establishment. We found no relationship between seed mass and survival through the transition from viable seed in or on the soil to newly emerged seedlings (P = 0.47, n = 33 species). Synthesis of data from experimental studies on the advantages of large‐seeded species establishing under particular hazards (such as shade, drought or herbivory) confirmed that seedlings of large‐seeded species perform better than those of small‐seeded species in most situations. However, the magnitude of this advantage was not sufficient to counterbalance the greater number of seeds produced by small‐seeded species m−2 of canopy outline year−1. Synthesis of data from field studies of populations under natural conditions also showed that large‐seeded species have higher survival through early seedling establishment than small‐seeded species (P = 0.006, n = 112 species). However, the magnitude of this advantage would only be sufficient to counterbalance the greater number of seeds produced by small‐seeded species m−2 of canopy outline year−1 if mortality continued at the same rate for some time. The time required for a species with 10‐fold larger seeds to recoup the advantage gained by a smaller‐seeded species during seed production ranged from 8.8 weeks for the smallest seeded species in the data set, up to an implausible 4.2 years for the largest‐seeded species. Thus, while large‐seeded species do have a survival advantage over small‐seeded species during seedling establishment, the available evidence suggests that advantages must also accrue during other stages in the life cycle. One possibility is that the greater seed production of small‐seeded species (m−2 of canopy outline year−1) is partly offset by larger canopies and longer reproductive life spans in large‐seeded species.

Regeneration of Polylepis australis Bitt. in the mountains of central Argentina

Polylepis australis woodlands range from northern to central Argentina. They support a high number of endemisms (both plants and animals) and constitute the main cover type in watersheds contributing to retain soil and catch water. Natural regeneration of P. australis in the Córdoba mountains, Argentina, was studied in two communities dominated by P. australis: shrublands and woodlands. The main objectives of the study were: (1) to characterize shrublands and woodlands in terms of physical (physical and chemical soil properties) and biological ( Polylepis’ growth form, fern and moss cover) characteristics; (2) to describe P. australis’ soil seed bank and compare seedling and sapling establishment between both communities. The presence of seedlings as well as saplings was quantified, and soil samples were taken so as to characterize the seed bank. Results showed significant differences in habitat conditions between shrublands and woodlands. Seedlings’ density was higher in the shrublands, while no differences appeared regarding the sapling numbers. A negative correlation was found between seedling numbers and total community cover as well as mosses and ferns cover. Seed density in the soil bank was higher in the woodlands, but overall seed viability was less than 5%. In the shrublands, the most critical stage of the species regeneration cycle was the transition from seedling to sapling, while in the woodlands the main “filter” occurred during seed germination. The low number of saplings observed in both units suggests a stagnation of P. australis regeneration, which could be translated into an unlikely expansion of the species throughout the Córdoba mountains.

State of fallen logs and the occurrence of conifer seedlings and saplings in boreal and subalpine old‐growth forests in Japan

Fallen logs on the forest floor play an important role in tree seedling establishment in boreal and subalpine old‐growth forests. We examined the abundance and state (degree of decay, moss coverage on their surfaces) of fallen logs on the forest floor and compared the occurrence of conifer seedlings and saplings on soil and fallen logs in old‐growth evergreen boreal (Taisetsu) and subalpine (Yatsugatake) coniferous forest stands in Japan. The forest floors of both stands were covered mostly by soil, and fallen logs covered only a small proportion (4.5% in Taisetsu and 2.5% in Yatsugatake) of the forest floor. In both stands, no seedlings were found on freshly fallen logs. Both moss coverage and the occurrence of seedlings or saplings on fallen logs increased with decay in both stands. In Taisetsu, the occurrence of all conifer seedlings and saplings was, in general, restricted to fallen logs. In contrast, in Yatsugatake, Picea and Tsuga seedlings occurred much more frequently on fallen logs than on soil, whereas Abies seedlings and saplings became established on both soil and fallen logs. Seedlings were taller on decayed fallen logs, but taller saplings were rare on fallen logs in Yatsugatake. Results indicate that species’ characteristics, the condition of the fallen logs and environmental factors determine the preponderance of seedling and sapling establishment on fallen logs. The condition of fallen logs varies with the degree of decay, and fallen logs that function as seedling establishment sites on the forest floor are thought to be limited by time and space.

Recent Forest Encroachment into Subalpine Grasslands near Mount Hotham, Victoria, Australia

Establishment of forest trees into subalpine grasslands near Mt. Hotham, southeast Australia, was quantified in 1998 across long-established forest-grassland boundaries in belt transects at four sites. Although the grasslands varied in their dominant species and groundlayer biomass, tree encroachment (principally by Eucalyptus pauciflora) occurred at all sites. Tree encroachment is a recent and synchronous event; all invading saplings were ≤31-yr-old and the majority (54%) established between 1991 and 1995. Most sapling establishment (66%) occurred within 5 m of the forest-grassland boundary where the number of plants present was positively associated with the amount of overhanging tree cover at three of the four sites. No correlation between encroachment and groundcover type or biomass, however, was found at any site. Some of the recently established plants are now small trees (1–8 m in height) and have become reproductive, indicating that establishment in grasslands is successful, making ecotone shifts possible. Any changes in boundary position, however, will be slow given the limited distance that trees established from the forest edge. Tree encroachment near Mt. Hotham is likely the outcome of small-scale (e.g., regeneration microsite) and landscape-scale (e.g., climate, grazing) processes that require further clarification.

Recent Forest Encroachment into Subalpine Grasslands near Mount Hotham, Victoria, Australia

Establishment of forest trees into subalpine grasslands near Mt. Hotham, southeast Australia, was quantified in 1998 across long-established forest-grassland boundaries in belt transects at four sites. Although the grasslands varied in their dominant species and groundlayer biomass, tree encroachment (principally by Eucalyptus pauciflora) occurred at all sites. Tree encroachment is a recent and synchronous event; all invading saplings were ≤31-yr-old and the majority (54%) established between 1991 and 1995. Most sapling establishment (66%) occurred within 5 m of the forest-grassland boundary where the number of plants present was positively associated with the amount of overhanging tree cover at three of the four sites. No correlation between encroachment and groundcover type or biomass, however, was found at any site. Some of the recently established plants are now small trees (1–8 m in height) and have become reproductive, indicating that establishment in grasslands is successful, making ecotone shifts possible. Any changes in boundary position, however, will be slow given the limited distance that trees established from the forest edge. Tree encroachment near Mt. Hotham is likely the outcome of small-scale (e.g., regeneration microsite) and landscape-scale (e.g., climate, grazing) processes that require further clarification.

Similar Susceptibility to Excess Irradiance in Sun and Shade Acclimated Saplings of Norway Spruce [Picea Abies (L.) Karst.] and Stone Pine (Pinus Cembra L.)

We compared the responses of sun and shade acclimated saplings of Picea abies and Pinus cembra to excess photosynthetic photon flux density (PPFD) equivalently exceeding the level for saturating net photosynthetic rate (PN). Exposure for 2 h up to 2000 µmol(photon) m-2 s-1 did not affect radiant energy saturated PN. Photoinhibition of photosynthesis was indicated by a small (10 %) reduction of the potential efficiency of photosystem 2 as derived from measurements of chlorophyll fluorescence (FV/FM). However, the extent of FV/FM reduction and half-time for recovery were similar in sun and shade acclimated saplings of both species. Furthermore, the effect on FV/FM was not stronger when the plants were exposed to excess PPFD at 5 °C instead of 15 °C. Frost-hardening of plants increased slightly their resistance to excess PPFD. Establishment of these conifer saplings usually acclimated to shade in their natural habitat may hardly be endangered by a sudden increase of PPFD, e.g., by gap formation.

99/03319 Overlapping of the devolatilization and char combustion stages in the burning of coal particles

The Aspidosperma quebracho-blanco is an important sclerophyllous evergreen species of the Argentine Chaco. There is scarce information about demographic patterns of A. quebracho blanco in the Arid Chaco. This paper focused on describing consequences of the process of degradation of the vegetation on the regeneration and size structure of A. quebracho-blanco population, compared to the contrasting scenarios of traditional management (cutting, grazing and fire) and a long undisturbed situation. Seedling and sapling establishment of A. quebracho-blanco has seldom been observed in Córdoba's Arid Chaco. A. quebracho blanco did not have significant seedling and sapling recruitment in any of the stands submitted to the effects of traditional management (extensive grazing and uncontrolled cutting). It had substantial seedling and sapling recruitment in the Chancanı́ Forest Reserve, which was different to the other sites. In protected areas, the adult plant densities were higher than in areas of low grazing and cutting. Disturbed habitats will be often considered to be suboptimal, whereas more stable undisturbed habitats have higher success of regeneration. Though, the general patterns of size distributions are similar, the disturbing effect promotes changes in their distribution parameters.

Lemurs and the Regeneration of Dry Deciduous Forest in Madagascar

Abstract: We sought to assess the role of lemurs for seed dispersal in the dry deciduous forest of western Madagascar and the possible consequences of the demise of lemurs for forest regeneration. Forest regeneration was studied in eight plots in two large blocks of primary forest and in seven fragments of primary forest (1 plot per fragment). In 4 of the 15 study plots, the abundance of saplings was negatively and significantly correlated (p < 0.05) with the abundance of mature individuals of the same tree species. In another 10 study plots there were negative correlations, although these were not significant on the community level. Second‐order statistics were significant with p < 0.001 and indicated that seed dispersal away from the parent trees was important for successful establishment of saplings. Apart from possibly the bush pig (Potamochoerus larvatus), only one vertebrate species of the dry forest, the brown lemur (Eulemur fulvus), ingested seeds> 11 mm long and passed them through its digestive tract unharmed. These results for lemurs were based on direct observations and fecal analyses. To evaluate the role of E. fulvus, we compared regeneration in forest plots with and without E. fulvus. In forest fragments without E. fulvus, fewer lemur‐dispersed tree species regenerated than would be expected based on the presence of mature tree species that are lemur‐dispersed (p < 0.05). No such effect was seen in primary forests with E. fulvus or for trees whose seeds can also be dispersed by other vertebrates. Thus, regeneration of the dry deciduous forest of western Madagascar with the complete set of primary forest tree species seems to depend upon the presence of E. fulvus.

Regeneration and structure of Aspidosperma quebracho-blanco Schl. in the Arid Chaco (Córdoba, Argentina)

The Aspidosperma quebracho-blanco is an important sclerophyllous evergreen species of the Argentine Chaco. There is scarce information about demographic patterns of A. quebracho blanco in the Arid Chaco. This paper focused on describing consequences of the process of degradation of the vegetation on the regeneration and size structure of A. quebracho-blanco population, compared to the contrasting scenarios of traditional management (cutting, grazing and fire) and a long undisturbed situation. Seedling and sapling establishment of A. quebracho-blanco has seldom been observed in Córdoba's Arid Chaco. A. quebracho blanco did not have significant seedling and sapling recruitment in any of the stands submitted to the effects of traditional management (extensive grazing and uncontrolled cutting). It had substantial seedling and sapling recruitment in the Chancanı́ Forest Reserve, which was different to the other sites. In protected areas, the adult plant densities were higher than in areas of low grazing and cutting. Disturbed habitats will be often considered to be suboptimal, whereas more stable undisturbed habitats have higher success of regeneration. Though, the general patterns of size distributions are similar, the disturbing effect promotes changes in their distribution parameters.

The Effects of Selective Logging on Stand Structure and the Regeneration of Subboreal Forests in Hokkaido, Northern Japan

We investigated the effects of selective logging on stand structure and regeneration in selectively logged subboreal forests in Taisetsuzan National Park in Hokkaido in northern Japan. The basal area decreased and the size structure of trees altered in the stands studied due to repeated, intense selective logging, in which larger trees were cut down as a priority. Sapling density in the stands was much lower than that in primary forests. In the simple and multiple regression analyses that were used to estimate the effects of selective logging on sapling density, sapling density had a significant positive correlation with tree density and had little correlation with the density of logged stumps or the height ofSasa (dwarf bamboo) growing on the forest floor. These results suggest that the establishment sites around canopy trees influenced the establishment of saplings, rather than the gaps caused by selective logging. However, both the coefficient of determination and the standardized partial regression coefficient of multiple regression analysis were higher for the stand with a dense cover ofSasa than for the stand with a sparse cover ofSasa. Thus, the success of regenerating forests with selective logging depends on both the site of advanced regeneration and the light conditions that regulate growth.

Effect of saline irrigation and its schedules on growth, biomass production and water use byAcacia niloticaandDalbergia sissooin a highly calcareous soil

Afforestation programmes in arid areas suffer mainly from low rainfall, high evapo-transpirational demands, high salt content of ground-waters and impediment bykankar(calcareous) subsoil layers of root proliferation. Two of the most preferred multi-purpose tree (MPT) species, Kikar (Acacia nilotica(L.) Del.) and Shisham (Dalbergia sissooRoxb. ex DC) were furrow planted in August 1991 under such an environment in north-western parts of India and the effects of saline irrigation (ECw10·5 dS m−1, sodium adsorption ratio 20 mmol l−1) and various irrigation schedules were evaluated for 45 months. Saline irrigation reduced biomass by 16% inA. niloticacompared to canal water irrigation, while the reduction was 57% inD. sissoo. Irrigation water supplies equalling 10% of the open pan evaporation (OPE) values met water requirements during the initial 2 years after transplanting. However, trees receiving high quantities of water (20% of OPE) during the third year had improved biomass yields by 13% and 21% inD. sissooandA. nilotica, respectively. Water-use efficiency (WUE) ofA. nilotica(10·7 mg.ha.m−1) was much higher compared toD. sissoo(1·7 mg.ha.m−1). Most of the salts added through saline irrigation accumulated below channels and were leached both downward and to lateral regions with monsoon rains. Thus furrow planting and irrigation technique, in addition to reducing the applied water quantities, resulted in favourable salt and water regimes for the better establishment of tree saplings with saline water.

Saline-water irrigation for the establishment of furrow-planted trees in northwestern India

Appropriate tree species and planting methods can help rehabilitation of arid areas — that are characterized by low rainfall, high evapotranspiration demands, highly saline ground waters and calcareous subsoils. The growth of Acacia nilotica and Dalbergia sissoo saplings planted in irrigation furrows in such an arid zone of northwest India was not affected adversely by irrigation with saline water of EC W 10.5 dS m−1. The growth of A. nilotica, measured in terms of sapling survival, plant height and biomass yields, was better than that of D. sissoo. Increasing the period of irrigation from the recommended practice of irrigating only in the first year dry season (October–June) to second and third dry seasons (years) improved the sapling survival, growth and biomass (two-to-three-fold) and water use efficiency (two-to-four fold). Most of the salts added with saline irrigation were accumulated below the irrigation channels and were pushed laterally during the monsoon season. The results indicate that the furrow planting technique could be adopted as an afforestation practice in view of the creation of favourable water and salt regimes and their impact on the establishment of trees saplings. Irrigation water supplies for a minimum of intitially two years after transplanting seemed necessary.

Regeneración natural y diversidad de especies arbóreas en selvas húmedas

Uno de los grandes enigmas de la biología tropical es el de cómo en las selvas altas perennifolias se mantiene una enorme cantidad de especies de árboles a escalas locales tan pequeñas como una hectárea. Esta revisión explora la relación entre la dinámica de regeneración natural de estos bosques y los procesos y mecanismos (evolutivos y ecológicos) que facilitan el mantenimiento de tan extraordinaria diversidad. Como primer punto, se hace una breve revisión de las ideas sobre el origen de la diversidad de especies arbóreas en los trópicos. Se sugiere que a nivel local: i) fenómenos históricos (de índole evolutivo y biogegráfico) son determinantes de la riqueza de especies, y ii) que las comunidades de árboles de selvas altas perennifolias parecen no estar saturadas de especies, de modo que, una especie recién originada puede migrar libremente a través de una región. Como segundo punto, se dan detalles sobre cómo ocurre el proceso de la regeneración natural. La dinámica de formación de claros, promovida por la caída de ramas y árboles, es un componente fundamental en la renovación del dosel. Los claros pequeños facilitan el desarrollo de plántulas y árboles jóvenes mientras que los claros grandes permiten a los árboles alcanzar tallas reproductivas. La apertura de claros y el ingreso de nuevos árboles en el dosel son los extremos de un proceso dinámico de reemplazamiento de árboles y especies en el dosel de la selva. Como tercer y último punto, se relaciona el proceso de reemplazamiento de árboles con el mantenimiento de la diversidad de especies arbóreas a una escala local. Se argumenta que aquellos factores que promueven una elevada diversidad de especies en la comunidad de plántulas y árboles jóvenes que crece bajo los árboles maduros favorecen una frecuencia alta de reemplazos entre árboles heterospecíficos y por lo tanto, facilitan el mantenimiento de la diversidad de especies en el dosel. Entre estos factores, los animales frugívoros, que promueven diversidad en la lluvia de semillas, y los agentes bióticos de mortalidad de semillas, plántulas y árboles jóvenes que actúan principalmente sobre especies abundantes son facilitadores clave de la diversidad. La existencia de disyuntivas en los atributos de historia de vida (amplitud en la dispersión de semillas, sobrevivencia en sombra y crecimiento en los claros) de los árboles coadyuva al mantenimiento de la diversidad favoreciendo el reemplazamiento de árboles con diferentes historias de vida. La presente revisión no apoya la idea de que el mantenimiento de la diversidad de especies arbóreas en las selvas húmedas depende de procesos estocásticos, como algunos autores han propuesto. Por el contrario, se concluye que los fenómenos ecológicos tienen un papel preponderante en definir la posibilidad que tiene una especie de ganar una posición en estos bosques altamente diversos.