Recruitment Of Saplings Research Articles

Genetic diversity and differentiation of the frankincense tree (Boswellia papyrifera (Del.) Hochst) across Ethiopia and implications for its conservation

Boswellia papyrifera is used to produce frankincense, a bark resin that has been a commodity of domestic and international trade since ancient times. It is harvested from natural forests. The tropical dry forest (Terminalia–Combretum) woodland ecosystems in which B. papyrifera is one of the dominant species, are facing anthropogenic threats. In Ethiopia B. papyrifera populations have decreased tremendously to smaller and isolated remnant patches, and many forests in the North-western and North-eastern parts of Ethiopia completely lack recruitment of saplings. This regeneration bottleneck, in combination with adult mortality, threatens the persistence of the species. Devising an effective strategy to conserve wild genetic resources needs information on the genetic diversity and the pattern of genetic differentiation across the species area. In the present study we analysed adult trees sampled in twelve populations across the growing area of the species in Ethiopia for genetic diversity and spatial genetic differentiation using 10 polymorphic microsatellite loci. The mean level of observed and expected heterozygosity were 0.669 and 0.681 respectively, and these levels were similar for trees from larger populations and those from degraded populations. A moderate level of among populations genetic differentiation (FST=0.084) was detected. Genetic distance between populations was correlated with geographic distance (r=0.663, p<0.05). STRUCTURE analysis distinguished four distinct genetic clusters corresponding to regions with different environmental conditions. In the Western populations we detected recruitment of many seedlings and saplings, which is a significant novel finding as most of the other populations are completely devoid of saplings. We conclude that currently a high level of genetic variation is still maintained in B. papyrifera adult trees across the species’ range in Ethiopia including the highly degraded remnant B. papyrifera population patches scattered on farm and pasture lands. An effective conservation strategy for the species has to take into account the geographic distribution of source populations.

Change in Mesoherbivore Browsing Is Mediated by Elephant and Hillslope Position.

Elephant are considered major drivers of ecosystems, but their effects within small-scale landscape features and on other herbivores still remain unclear. Elephant impact on vegetation has been widely studied in areas where elephant have been present for many years. We therefore examined the combined effect of short-term elephant presence (< 4 years) and hillslope position on tree species assemblages, resource availability, browsing intensity and soil properties. Short-term elephant presence did not affect woody species assemblages, but did affect height distribution, with greater sapling densities in elephant access areas. Overall tree and stem densities were also not affected by elephant. By contrast, slope position affected woody species assemblages, but not height distributions and densities. Variation in species assemblages was statistically best explained by levels of total cations, Zinc, sand and clay. Although elephant and mesoherbivore browsing intensities were unaffected by slope position, we found lower mesoherbivore browsing intensity on crests with high elephant browsing intensity. Thus, elephant appear to indirectly facilitate the survival of saplings, via the displacement of mesoherbivores, providing a window of opportunity for saplings to grow into taller trees. In the short-term, effects of elephant can be minor and in the opposite direction of expectation. In addition, such behavioural displacement promotes recruitment of saplings into larger height classes. The interaction between slope position and elephant effect found here is in contrast with other studies, and illustrates the importance of examining ecosystem complexity as a function of variation in species presence and topography. The absence of a direct effect of elephant on vegetation, but the presence of an effect on mesoherbivore browsing, is relevant for conservation areas especially where both herbivore groups are actively managed.

Seed Dispersal by Multiple Frugivores

We investigated the advantages of directed dispersal by multiple frugivores to plant recruitment. We used probability recruitment models of a long-lived rain forest tree in Madagascar, to compare recruitment success with dispersal by multiple frugivorous lemurs, with no dispersal, and random dispersal. Our results show that frugivore-mediated seed dispersal was nonrandom with respect to canopy cover and increased modeled per-seed sapling recruitment fourfold compared to no dispersal. Seeds dispersed by one frugivore, Eulemur rubriventer, had higher modeled recruitment probability than seeds dispersed randomly. However, as a group, our models suggest that seeds dispersed by lemurs would have lower recruitment than if dispersal were random. The rain forest of Ranomafana National Park, Madagascar. Photo credit: Onja H. Razafindratsima. Canopy opening in Ranomafana National Park, Madagascar. Recruitment of Cryptocarya crassifolia saplings was higher in more open canopy cover. Photo credit: Onja H. Razafindratsima. A southern black and white ruffed lemur (Varecia variegata editorum), feeding on fruits in Ranomafana National Park, Madagascar. Photo credit: Mamy-Fy Rakotondrainibe. These photographs illustrate the article “Assessing the impacts of nonrandom seed dispersal by multiple frugivore partners on plant recruitment” by Onja H. Razafindratsima and Amy E. Dunham, published in Ecology 96(1):24–30, January 2015. http://dx.doi.org/10.1890/14-0684.1

Does fire limit tree biomass in Australian savannas?

Processes allowing coexistence of trees and grasses in tropical savannas have long intrigued ecologists. Early theories focused on climatic controls, but a conceptual model has emerged suggesting that savanna trees are subject to a fire-mediated recruitment bottleneck, with frequent fires preventing recruitment of saplings into the tree layer and maintaining biomass well below its climate-determined upper bound. We propose that this conceptual model has been overemphasised in northern Australia, where tree abundance is more strongly controlled by water availability. The dominant trees, eucalypts, have a remarkable capacity to grow through the ‘fire trap’ to reach fire-resistant sizes. This fire tolerance makes eucalypts relatively unresponsive to management-imposed reductions in fire frequency and intensity. Other trees in these savannas are typically more fire sensitive and respond positively to such management. There are suggestions that savanna fire management could lead to increases in woody biomass, but we contend that if tree biomass is strongly limited by water availability, then potential increases in tree biomass are relatively limited, at least in relation to the dominant eucalypt component. There is potential to increase the biomass of the more fire-sensitive non-eucalypts, but the upper bound of non-eucalypt tree biomass in these eucalypt-dominated systems remains poorly understood.

Recent tree mortality and recruitment in mature and old-growth forests in western Washington

Tree mortality is a fundamental driver of forest ecosystems and occurs both in catastrophic events and as a chronic process. Persistent changes in the rate of chronic or background mortality can dramatically alter the structure and composition of forests stands. Tree recruitment is the complement to tree mortality, combining with mortality to determine changes in tree density and species composition. The purpose of this study is to ascertain trends of mortality and recruitment in mature and old-growth forests in western Washington state, U.S.A. We used a set of permanent plots established in units of the National Park System spanning the environmental gradient of forests in these parks (from near sea-level to 1800m elevation). Duration of observation was five years for most plots and two or three years for a small number of plots. Principal tree species on the plots were Picea sitchensis, Tsuga heterophylla, Pseudotsuga menziesii, Thuja plicata, Abies lasiocarpa, and Abiesamabilis. Tree mortality was evaluated annually; recruitment was recorded at the end of the study. For small (dbh 12.7–76.1cm) and large (dbh >76.1cm) trees, annual mortality pooled across all elevations, parks, plots, and years was <1% (mean and upper 95% confidence intervals). For saplings (dbh 2.5–12.6cm), mean mortality was <1% though the upper limit of the confidence interval was slightly greater than 1%. Recruitment outweighed mortality (and growth into the small-tree class) for saplings. Recruitment and losses of stems balanced one another for small trees; both processes were negligible for large trees. Recruitment of saplings increased representation of shade-tolerant species (e.g., T. heterophylla, and A. amabilis). Biotic factors predominated as the proximate cause of death for all size-classes of trees (i.e., trees died standing), though particular agents of mortality were not identified in most cases. Our results contrasted with a recent report of annual tree mortality >1% for unmanipulated, old-growth forests in the Pacific Northwest.

Multiscale Controls on Natural Regeneration Dynamics after Partial Overstory Removal in Douglas-Fir Forests in Western Oregon, USA

We examined natural regeneration following operational-scale variable density retention treatments in 40 – 60-year-old Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) forests at seven sites for a decade following treatment. Treatments included residual overstory densities of 300, 200, and 100 trees/ha, with leave islands and gaps of three sizes (0.1, 0.2, and 0.4 ha) and an untreated control (600 trees/ha). Natural regeneration was influenced by factors from multiple spatial scales including broad-scale differences in overstory composition among sites, mesoscale variability in topographic position, and fine-scale variability in overstory and understory competition. High local basal area (BA) decreased the probability of seedling establishment, though some seedlings established even under high BA, particularly shade-tolerant western hemlock. In contrast, recruitment of saplings (> 1.37 m height) required lower residual overstory density (i.e., 100 trees/ha), especially for shade-intolerant Douglas-fir. Understory vegetation had little effect on saplings but was negatively related to seedling densities, particularly when overstory density was low. Variable density prescriptions can take advantage of the importance of fine-scale variability to promote regeneration of desired species mixtures, though other factors such as site overstory species composition and variation in topographic position will also influence regeneration dynamics. Including heavy overstory removal or gap creation could facilitate rapid recruitment of saplings, especially for shade-intolerant Douglas-fir.

Sapling recruitment and mortality dynamics following partial harvesting in aspen-dominated mixedwoods in eastern Canada

Boreal mixedwood management has shifted from a relatively narrow focus on commercial wood supply to greater consideration of the natural dynamics and multiple ecological services. This recognition has generated interest in ecosystem management approaches that include diversifying and adapting silvicultural practices, including partial harvesting. The effects of partial harvesting on stand dynamics was assessed over a 12-year period in trembling aspen (Populus tremuloides Michx.) dominated stands in northwestern Quebec, Canada. Four treatments were tested: clearcuts (100% basal area (BA) removal); 1/3 partial cut (1/3 PC, 33% BA removal using low thin); 2/3 partial cut (2/3 PC, 61% BA removal using high thin) and controls (0% removal). Aspen sapling recruitment was directly affected by harvesting intensity with 1/3 and 2/3 partial cuts generating 5% and 56%, respectively, of aspen sapling densities in clearcuts. Aspen sapling recruitment increased continuously following clearcut and partial cut treatments with no significant mortality in the sapling layer over the 12-year period. Recruitment of conifer saplings also increased with time and was significantly higher in the two partial cuts than in the clearcut treatment. Twelve years after treatments, mortality of residual aspen stems (⩾10cm DBH) reached 250stemsha−1 12yr−1 in controls, compared to 106, and 170stemsha−1 12yr−1 in 1/3 PC, and 2/3 PC stands, respectively. Initially (1–3years after treatments), higher overstory aspen mortality was associated with the 2/3 PC treatment. Aspen mortality was strongly associated with small-sized merchantable stems (10–19.9cm DBH) regardless of treatment. Both partial harvesting treatments had the effect of maintaining mountain maple (Acer spicatum Lamb.), a shade-tolerant, high woody shrub, at densities similar to those in control stands whereas recruitment of mountain maple saplings was negligible in clearcuts due to high aspen recruitment. Our results indicate that (i) heavy-high partial harvesting promotes sapling recruitment of both aspen and conifers when advance regeneration of the latter is present, (ii) because aspen sucker response can be controlled by varying harvesting intensities and stem selection, it is possible to create a range of mixedwood conditions, depending on whether mixed, structurally complex or more regular aspen-dominated stands are desired, and (iii) on rich mixedwood sites, tall woody shrubs could hinder desirable partial harvesting outcomes.

Shade, nutrients, and grass competition are important for tree sapling establishment in a humid savanna

The structure and function of savannas is contingent upon tree sapling establishment into the ecosystem. Frequent rainfall, canopy shade and nutrient availability are the main factors influencing recruitment of tree saplings. Grass competition has potential to change the tree establishment dynamics by interacting with these factors. The factors affecting tree sapling establishment have been well studied and tested in arid and semi‐arid environments (approx. 0.350–0.750 m mean annual precipitation [MAP]) compared to humid (&gt;1.000 m MAP) savanna ecosystems. We studied the effects of water (frequent irrigation vs. rainfall), shade (presence vs. absence), nutrients (addition vs. no addition) and grass competition (presence vs. absence) on sapling survival and functional traits related to growth and size of four dominant humid savanna species and four dominant mesic savanna species (approx. 0.750 m MAP), under controlled field conditions in a humid South African savanna. None of the treatments had an effect on sapling survival. Growth of the saplings did not depend on the frequency of watering, probably due to high rainfall received in humid savannas. Shade had a strong negative effect on sapling growth, suppressing the beneficial effects of nutrient availability and absence of grass competition. Net positive effects of nutrient availability had less influence over sapling growth than the negative effects exerted by grass competition. Overall shoot growth increased with nutrient addition while investment in leaves drastically declined with grass competition. We conclude that canopy shade is a strong potential driver of structure and function of humid savanna ecosystems, although we recognize that nutrient limitation and grass competition additionally influence tree sapling establishment.

Herbivores equalize the seedling height growth of three dominant tree species in an African tropical rain forest

Determining the impact of insect herbivores on forest tree seedlings and saplings is difficult without experimentation in the field. Moreover, this impact may be heterogeneous in time and space because of seasonal rainfall and canopy disturbances, or ‘gaps’, which can influence both insect abundance and plant performance. In this study we used fine netting to individually protect seedlings of Microberlinia bisulcata, Tetraberlinia bifoliolata and Tetraberlinia korupensis trees (Fabaceae=Leguminosae) from insects in 41 paired gap-understorey locations across 80ha of primary rain forest (Korup, Cameroon).For all species, growth in height and leaf numbers was negligible in the understorey, where M. bisulcata had the lowest survival after c. 2years. In gaps, however, all species responded positively with pronounced above-ground growth across seasons. When exposed to herbivores their seedling height growth was similar, but in the absence of herbivores, M. bisulcata significantly outgrew both Tetraberlinia species and matched their leaf numbers. This result suggests that insect herbivores might play an important role in maintaining species coexistence by mitigating sapling abundance of the more palatable M. bisulcata, which in gaps was eaten the most severely. The higher ratio in static leaf damage of control-to-caged M. bisulcata seedlings in gaps than understorey locations was consistent with the Plant Vigour Hypothesis. This result, however, did not apply to either Tetraberlinia species. For M. bisulcata and T. korupensis, but not T. bifoliolata (the most shade-tolerant species), caging improved relative seedling survival in the understory locations compared to gaps, providing restricted support for the Limiting Resource Model. Approximately 2.25years after treatments were removed, the caged seedlings were taller and had more leaves than controls in all three species, and the effect remained strongest for M. bisulcata.We conclude that in this community the impact of leaf herbivory on seedling growth in gaps is strong for the dominant M. bisulcata, which coupled to a very low shade-tolerance contributes to limiting its regeneration. However, because gaps are common to most forests, insect herbivores may be having impacts upon functionally similar tree species that are also characterized by low sapling recruitment much more widely than currently appreciated. An implication for the restoration and management of M. bisulcata populations in forests outside of Korup is that physical protection from herbivores of new seedlings where the canopy is opened by gaps, or by harvesting, should substantially increase its subcanopy regeneration, and thus, too, its opportunities for adult recruitment.

Herbivores differentially limit the seedling growth and sapling recruitment of two dominant rain forest trees

Resource heterogeneity may influence how plants are attacked and respond to consumers in multiple ways. Perhaps a better understanding of how this interaction might limit sapling recruitment in tree populations may be achieved by examining species' functional responses to herbivores on a continuum of resource availability. Here, we experimentally reduced herbivore pressure on newly established seedlings of two dominant masting trees in 40 canopy gaps, across c. 80 ha of tropical rain forest in central Africa (Korup, Cameroon). Mesh cages were built to protect individual seedlings, and their leaf production and changes in height were followed for 22 months. With more light, herbivores increasingly prevented the less shade-tolerant Microberlinia bisulcata from growing as tall as it could and producing more leaves, indicating an undercompensation. The more shade-tolerant Tetraberlinia bifoliolata was much less affected by herbivores, showing instead near to full compensation for leaf numbers, and a negligible to weak impact of herbivores on its height growth. A stage-matrix model that compared control and caged populations lent evidence for a stronger impact of herbivores on the long-term population dynamics of M. bisulcata than T. bifoliolata. Our results suggest that insect herbivores can contribute to the local coexistence of two abundant tree species at Korup by disproportionately suppressing sapling recruitment of the faster-growing dominant via undercompensation across the light gradient created by canopy disturbances. The functional patterns we have documented here are consistent with current theory, and, because gap formations are integral to forest regeneration, they may be more widely applicable in other tropical forest communities. If so, the interaction between life-history and herbivore impact across light gradients may play a substantial role in tree species coexistence.

Do isolated gallery-forest trees facilitate recruitment of forest seedlings and saplings in savannna?

Facilitation is an ecological process that allows some species to establish in environments they can hardly afford in the absence of the process. This study investigated if the subcanopy of gallery-forest trees isolated in savanna is suitable for the early recruitment of forest woody species. We measured tree crown area as well as the density of seedlings and saplings of gallery-forest tree species beneath isolated trees and in the savanna matrix along 50 transects of 5-km long and 600 m wide located along four gallery forests. We then tested the nurse-plant effect and Janzen-Connell hypothesis beneath isolated trees. We also examined the relationships between the crown area and the density of seedlings and saplings. Among the eight identified tree species isolated in savanna, only Daniellia oliveri and Khaya senegalensis showed nurse-plant effect and promoted a significant, yet low early recruitment with a seedling-to-sapling survival of 0.044 and 0.578, respectively. The suitability of the subcanopy of isolated trees decreased with the recruitment progression and Janzen-Connell effects were absent. Seedlings had neutral association with the crown area of isolated trees which shifted to positive at the sapling stage. The species of the isolated tree and the crown area explained less than 20% of total variance, indicating that other predictive factors are important in explaining the nurse-plant effect observed in this study.

Regional and temporal patterns of natural recruitment in a California endemic oak and a possible ‘research reserve effect’

AbstractAimThe perceived absence of young cohorts in many long‐lived trees, particularly Quercus species, has raised concerns about their long‐term viability. While there is a robust body of literature on valley oak (Quercus lobata) planting experiments, conducted mainly on research reserves, an assessment of natural recruitment across a range of climate and land use types has been lacking. At a regional scale over time, we explored patterns of natural recruitment in this endemic California oak reported to be experiencing persistent recruitment failure.LocationCalifornia.MethodsWe conducted a regional‐scale study, including dendroecology and historical resurveys, field surveys and published planting experiments to elucidate the prevalence, timing, distribution and relation to environmental drivers of valley oak sapling recruitment.ResultsWe detected a substantial increase in the presence of recruitment over time at 10 sites originally surveyed between 20 and 40 years ago (sites with sapling recruitment increased from 10% to 70%), potentially related to a corresponding period of relatively wetter years. We found saplings recruiting from a range of years in a variety of land management regimes, and surprisingly, sites designated as research reserves had lower recruitment than other sites. Sapling recruitment did not appear to be synchronous or episodic.Main conclusionsOur results indicate that sapling recruitment failure is not a persistent condition across our sites, and long‐term conservation prospects for the species may be better than previously reported. Research reserves had lower natural sapling recruitment and lower seedling survival in experimental controls than other sites, and our findings highlight the importance of field data from sites with a representative range of climate and land use regimes.

Floristic diversity, regeneration status, and vegetation structure of woodlands in Metema Area, Amhara National Regional State, Northwestern Ethiopia

We studied woodland vegetation in broad-leaved deciduous woodlands of Metema in northwestern Amhara regional state, Ethiopia Our objective was to describe plant species composition, diversity, regeneration status, and population structure by a selective approach with a systematic sampling design. A total of 74 quadrats (each for 25 m × 25 m, spaced at intervals of 150–200 m) were sampled along established transect lines following the homogeneity of the vegetation. Vegetation data including cover-abundance, height, diameter at breast height (DBH), and numbers of seedlings and saplings of woody species were analyzed using Excel spreadsheet, Shannon Weiner diversity index, and PAST version 1.62. A total of 87 vascular plant species of 74 genera and 36 families were recorded. The dominant family was Fabaceae represented by 16 (18.39 %) species of 13 genera. Shannon Weiner diversity and evenness were 3.67 and 0.82, respectively, which showed that the area was endowed with rich floral diversity evenly distributed. The vegetation structure, as quantified by cumulative diameter class frequency distribution, plotted as an interrupted inverted-J-shape pattern with a sharp decrease in the 2nd diameter class. This indicated poor vegetation structure. The diameter classes frequency distributions of selected species plotted in four general patterns i.e., interrupted Inverted-J-shape, J-shape, Bell-shape and Irregular-shape. In conclusion, although the area showed high floral diversity and evenness, woody species including Sterculea setigera, Boswellia papyrifera, and Pterocarpus lucens showed lowest recruitment of seedlings and saplings.

Tree recruitment of European tree species at their current upper elevational limits in the Swiss Alps

AbstractAim The physical and physiological mechanisms that determine tree‐line position are reasonably well understood, but explanations for tree species‐specific upper elevational limits below the tree line are still lacking. In addition, once these uppermost positions have been identified, questions arise over whether they reflect past expansion events or active ongoing recruitment or even upslope migration. The aims of this study were: (1) to assess current tree recruitment near the cold‐temperature limit of 10 major European tree species in the Swiss Alps, and (2) to rank species by the extent that their seedlings and saplings exceed the elevational limit of adult trees, possibly reflecting effects of the recent climate warming.Location Western and eastern Alps of Switzerland.Methods For each species, occurrences were recorded along six elevational transects according to three size classes from seedlings to adult trees in 25‐m‐elevation steps above and below their regional upper elevational limit. Two methods were used to compare upper elevational limits between seedlings, saplings and adults within species. First, we focused on the uppermost occurrence observed in each life stage for a given species within each studied region; and second, we predicted their upper distribution range using polynomial models fitted to presence/absence data.Results Species exhibited a clear ranking in their elevational limit. Regional differences in species limits (western versus eastern Swiss Alps) could largely be attributed to regional differences in temperature. Observed and predicted limits of each life stage showed that all species were represented by young individuals in the vicinity of the limit of adult trees. Moreover, tree recruitment of both seedlings and saplings was detected and predicted significantly beyond adult tree limits in most of the species. Across regions, seedlings and saplings were on average found at elevations 73 m higher than adult trees.Main conclusions Under current conditions, neither seed dispersal nor seedling establishment constitutes a serious limitation of recruitment at the upper elevational limits of major European trees. The recruits found beyond the adult limits demonstrate the potential for an upward migration of trees in the Alps in response to ongoing climate warming.

Effect of human disturbance on the structure and regeneration of forests in the Nevado de Toluca National Park, Mexico

Sample plots were established in the principal forest types in the the Nevado de Toluca National Park, Mexico including those dominated by Pinus hartwegii, Abies religiosa, Quercus laurina and Alnus jorullensis. The vertical structure was defined by three strata in the coniferous forests and two strata in the broadleaved forests. Timber harvesting in Abies religiosa and Quercus laurina forests and fires generated by humans in Pinus hartwegii forests impeded the recruitment of saplings. Mature trees were also heavily impacted by logging in Pinus hartwegii forests. On the contrary, Alnus jorullensis forests were increasing due to the disturbance of Pinus and Quercus forests, as well abandoned crop lands within the park. A combination of logging, uncontrolled fire, and grazing appears to be compromising the recruitment of important tree species in this national park. These factors, together with human settlements, have also increased the proportion of early successional species. Changes in forest structure from human disturbance indicate a need to control these activities if conservation goals are not to be compromised.

Limitations to sustainable frankincense production: blocked regeneration, high adult mortality and declining populations

Summary1. Resins are highly valued non‐timber forest products (NTFP). One of the most widely traded resins is frankincense, tapped from several Boswellia tree species (Burseraceae). Exploited Boswellia populations often show poor regeneration, but the demographic consequences of these bottlenecks are unknown. Here we report on the first large‐scale demographic study of frankincense‐producing trees.2. We studied 12 populations of Boswellia papyrifera in northern Ethiopia, varying in altitude and productivity. Six of these populations had been tapped before and were tapped during the study. Survival, growth and fecundity were determined for 4370 trees and 2228 seedlings, in 22.8 ha over a 2‐year period. We also studied a remote population where no grazing and tapping took place. Matrix models were used to project population growth and frankincense production under four restoration scenarios.3. Population structures of both tapped and untapped populations showed clear gaps. Small seedlings were abundant in all populations, but none developed into persistent saplings. Such saplings were only present in the remote population. Fire and grazing are the likely causes of this regeneration bottleneck.4. Adult mortality was high (6–7% per year) in both tapped and untapped populations, probably caused by beetle attacks and fire. Unexpectedly, tapped populations presented higher diameter growth rates and fecundity compared to untapped populations. These differences are probably caused by non‐random selection of exploited populations by tappers.5. Under the ‘business as usual’ scenario, population models projected a 90% decline in the size of tapped and untapped populations within 50 years and a 50% decline in frankincense yield within 15 years. Model simulations for restoration scenarios revealed that populations and frankincense production could only be sustained with intensive management leading to full sapling recruitment and a 50–75% reduction in adult mortality.6. Synthesis and applications. Regeneration bottlenecks and high adult mortality are causing rapid decline in frankincense‐producing tree populations in Ethiopia. This decline is unlikely to be a consequence of harvesting and is probably driven by fire, grazing and beetle attacks. Fire prevention and the establishment of non‐grazing areas are needed. Our results show that other factors than exploitation may seriously threaten populations yielding NTFP.

Landscape-scale relationships between the exotic invasive shrub Lantana camara and native plants in a tropical deciduous forest in southern India

Abstract:Understanding exotic–native plant relationships within the context of landscape-scale environmental factors such as rainfall, topography, disturbance and forest structure, is important for distinguishing the role of invasions in native plant community change. In this study, the relationship between Lantana camara invasion and native forest understorey vegetation was described after accounting for environmental influences. Rainfall, terrain slope, altitude and fire frequency were measured from GIS layers, and tree density, dry above-ground L. camara biomass, and native plant abundance and species richness were measured in nested plots (four 1 × 1-m plots for grass, tree seedlings and L. camara within one 5-m-radius plot for tree saplings and herbs and shrubs within one 10-m-radius plot for trees) at 80 locations distributed across Bandipur Tiger Reserve, a tropical deciduous forest in southern India. Relationships between environmental factors and L. camara abundance were described using a multiple regression. Further, the role of L. camara abundance in explaining residual variation in native vegetation, after accounting for environmental influences, was described using linear models. Lantana camara abundance decreased with increasing tree density, supporting the notion that it thrives in disturbed forest with fewer trees. Whereas native tree seedling and herb and shrub density showed no relationship to L. camara abundance, both tree sapling density and grass volume decreased under L. camara biomass exceeding 2 kg m−2. These data suggest that, by association with grass decline and decreased recruitment of tree saplings, L. camara may be linked to the gradual transition of these tropical deciduous forests into exotic-dominated shrubland, and overall native biodiversity loss.

The impact of seed dispersal by black and gold howler monkeys on forest regeneration

AbstractIn Neotropical humid forest, the majority of tree species have seeds dispersed by vertebrates. Seed deposition by vertebrates is often spatially aggregated and a low per capita survival for seeds and seedlings is predicted. However, mortality factors could be saturated by high densities. I evaluated whether recruitment of saplings of species dispersed by black and gold howlers (Alouatta caraya) in latrines is higher than at control sites: (1) below parent trees, (2) in trees not used by monkeys to sleep, (3) randomly chosen sites within the forest, and determined whether howlers may influence current floristic composition of the Paraná River flooded forest. I recorded saplings several years old in the territories of five monkey groups. In total, I found four times more saplings in latrines than in the other areas, and results suggest that latrines are recruitment foci for most species, though larger samples would be required to assess this for every species. Frequency distribution of the diameter of tallest saplings of more abundant species reflected recruitment over time. I found saplings of more species growing in latrines than outside of them. Saplings higher than 1 m of two species of laurels (Ocotea diospyrifolia and Nectandra megapotamica) and one species of Myrtaceae (Eugenia punicifolia) had higher densities in latrines than below parent trees. Results suggest that mortality factors were saturated in latrines and that sapling may grow at a higher rate in latrines. In relation to the influence on floristic composition E. burkartiana, an uncommon species in the forest, could increase in abundance as consequence of seed dispersal by howlers.

The population dynamics and productivity of Acacia pennatula in the pasturelands of the Nature Reserve Mesas de Moropotente, Estelí, Nicaragua

Acacia pennatula trees are the most conspicuous woody species in the pasturelands of the Nature Reserve Mesas de Moropotente, Esteli, Nicaragua. Cattle ranchers keep A. pennatula because it produces fence posts, forage (pods) and firewood. A population projection matrix model was developed to: (1) estimate the sustainable harvest (H) of fence posts at different tree population densities, (2) explore the range of recruitment (R), and survival and growth of both saplings and small poles compatible with current population density, and (3) determine how much carbon is stored in the soil-pasture-tree system. Acacia pennatula trees take 40 years to reach H size (D30 ≥ 30 cm). Estimated sustainable H from current tree population density is 1.8l7 trees ha−1 year−1, yielding 2.8 large and 11.2 regular size fence posts ha−1 year−1. This annual output easily satisfies the needs of a typical 100 ha cattle ranch in the study area. Current population density is congruent with very low R (<100 saplings ha−1 year−1), very low survival rates (<0.30%) and/or retarded D30 growth of saplings and small poles. Total carbon in tree biomass was only 37 Mg ha−1. Cattle ranchers have learned to harness the invasive nature of the species to obtain valuable tree products for farm use or sale.

Simply the best: the transition of savanna saplings to trees

Tree cover in savannas is determined as much by disturbances from fire and herbivory as by rainfall and soil resources. Fire especially acts to limit tree cover via a demographic bottleneck, limiting the recruitment of tree saplings to adults. Because sapling growth rates determine rates of sapling to tree recruitment, predicting changes in tree cover requires data on sapling growth rates, commonly expressed as population means. Here, we discuss the variability in sapling growth rates in Acacia populations in a savanna in Hluhluwe iMfolozi Park in South Africa. Saplings growing at mean rates under typical fire regimes in African savannas would likely never escape the fire‐trap to become adults. Only the fastest growing saplings could grow above the flame zone between fires. We suggest that maximum growth rates are more ecologically relevant than mean growth rates in natural populations and experiments. Maximum growth rates are better than mean growth rates as predictors of sapling release within species, as shown here, and probably of which species are likely ‘winners’ in savanna tree communities.