Reduced Tree Density Research Articles

Impact of the extreme 2015-16 El Niño climate event on forest and savanna tree species of the Amazonia-Cerrado transition

Extreme drought events, driven by the El Niño Southern Oscillation (ENSO), are linked to increased tree mortality and alterations in vegetation structure, dynamics, and floristic composition in tropical forests. Existing analyses, primarily focusing on Africa, Central America, and Amazonia, overlook the floristic impacts on biome transitions. This study evaluates the profound effects of the severe 2015/2016 ENSO event on tree density and floristic composition in the critical transition zone between Amazonia and Cerrado, South America's largest biomes. Our findings not only document significant biodiversity loss but also offer insights into species resilience, guiding conservation strategies under changing climate conditions. We inventoried long-term plots before and after the extreme drought event, sampling 12,465 individuals from 526 species, 224 genera, and 65 families, in Open Ombrophilous Forest (OF), Seasonal Forest (SF), Cerradão (CD), and Typical Cerrado (TC). We document the disappearance from our plots of 97 species after the ENSO, with only 61 new species being recorded. The total loss of individuals across the transition zone was almost 10 %. The SF and CD forest plots showed the greatest replacements, species losses, and reductions in tree density. Their markedly seasonal baseline climate probably drove these changes. In most phytophysiognomies, there was an increase in pioneer species and drier environment habitat specialist species, indicating that although many species are vulnerable to extreme climate events, others benefit, especially those with a short life cycle. We found that the vegetation of the Amazonia-Cerrado transition overall is vulnerable to climate anomalies, with widespread loss of tree density and change in floristic composition. Our study also provides a species-by-species list of the most vulnerable and resistant trees which helps point to overall climate change vulnerabilities and assist with initiatives to recover degraded areas.

Ecological partnership: Restoring Pinyon woodlands on the Navajo Nation, USA

Pinyon woodlands in the semi-arid southwestern United States developed in response to environmental factors and historical human interactions, providing sustenance and traditional ecological benefits to Native Nations, such as the Navajo (Diné). Historically, Diné communities practiced land stewardship through seasonal movements and resource management, ensuring ecosystem resilience. However, the constraints imposed by European colonization led to transformative changes in their livelihoods and environments. Encompassing 71,000 km2, pinyon-juniper woodlands comprise 89 % of forested area on the Navajo Nation reservation, providing crucial ecosystem services and cultural resources. Since reservation establishment in 1868, increased tree density has diminished understory plant communities and elevated woodland susceptibility to wildfires, compromising overall forest health. Recognizing the imperative for ecological restoration, a collaborative effort between Diné College and Northern Arizona University (NAU) was established in 2018 to develop a pinyon woodland research site on the Diné College campus in Tsaile, Arizona, on the Navajo Nation reservation. We assessed the effectiveness of restoration thinning treatments in reinstating historical woodland structure, modifying fuels for fire mitigation, and engaging students in ecological practices. Thinning resulted in a 62 % reduction in tree density and a 25 % reduction in basal area, altering the woodland structure to reflect historical conditions more accurately. This alteration mitigated the risk of high-intensity wildfires while preserving the age structure of older trees (oldest dating back to 1435 C.E.). Forest floor fuels were low throughout the study, as thinned wood was removed, and activity fuels were immediately treated with chipping. The active participation of Diné College and NAU students, interns, and faculty throughout the study contributed to a cohesive approach in advancing our goals. The comprehensive training empowered students to contribute meaningfully to fieldwork and data collection, aligning with Diné College's mission to enhance post-secondary student learning and development for the benefit of the Diné. By integrating contemporary and culturally sensitive perspectives, our study sheds light on the effectiveness of restoration thinning treatments, emphasizing the delicate balance between ecological resilience and cultural preservation for the Navajo Nation.

Effects of tree species and planting density on the total yield of an agroforestry system in the Brazilian Cerrado

ABSTRACT This study aimed to evaluate the effects of spacing and population density of three tree species on the primary yield of an agroforestry system in the Cerrado. The experiment was conducted in the experimental area of the State University of Goiás, São Luís de Montes Belos, Brazil. The experiment was designed to assess the effects of tree species, density, and spacing in a factorial scheme of 3 × 2 × 3, totaling 18 treatments with two tree rows each. The tree species assessed were Corymbia citriodora (seedlings), Eucalyptus cloeziana (seedlings), and an Eucalyptus urophylla clone. The trees were planted in single and triple rows (Trees planted in single and triple rows, varying density). In the first year of the experiment, maize (Zea mays) and Crotalaria juncea were sown, maize and Marandu palisadegrass (Uroclhoa brizantha cv. Marandu) in the second year, and a monoculture of Marandu palisadegrass during the third and fourth years. Due to the larger tree-covered area, the arrangement in triple rows reduced maize yield by 36% in the first year, silage yield by 40% in the second year, and pasture yield by approximately 30%. Reducing tree density is crucial to ensure the functionality of the agroforestry system.

Where can managers effectively resist climate-driven ecological transformation in pinyon-juniper woodlands of the US Southwest?

Pinyon-juniper (PJ) woodlands are an important component of dryland ecosystems across the US West and are potentially susceptible to ecological transformation. However, predicting woodland futures is complicated by species-specific strategies for persisting and reproducing under drought conditions, uncertainty in future climate, and limitations to inferring demographic rates from forest inventory data. Here, we leverage new demographic models to quantify how climate change is expected to alter population demographics in five PJ tree species in the US West and place our results in the context of a climate adaptation framework to resist, accept, or direct ecological transformation. Two of five study species, Pinus edulis and Juniperus monosperma, are projected to experience population declines, driven by both rising mortality and decreasing recruitment rates. These declines are reasonably consistent across various climate futures, and the magnitude of uncertainty in population growth due to future climate is less than uncertainty due to how demographic rates will respond to changing climate. We assess the effectiveness of management to reduce tree density and mitigate competition, and use the results to classify southwest woodlands into areas where transformation is (a) unlikely and can be passively resisted, (b) likely but may be resisted by active management, and (c) likely unavoidable, requiring managers to accept or direct the trajectory. Population declines are projected to promote ecological transformation in the warmer and drier PJ communities of the southwest, encompassing 37.1%-81.1% of our sites, depending on future climate scenarios. Less than 20% of sites expected to transform away from PJ have potential to retain existing tree composition by density reduction. Our results inform where this adaptation strategy could successfully resist ecological transformation in coming decades and allow for a portfolio design approach across the geographic range of PJ woodlands.

Modelling the influence of thinning intensity and frequency on the future provision of ecosystem services in Mediterranean mountain pine forests

Thinnings are silvicultural operations that repetitively reduce tree density to improve the vigour of the remaining trees and the economic benefits of the stands. Thinning can also enhance the provision of various ecosystem services throughout the rotation period. In this study, we use a modelling approach to evaluate stand dynamics and the provision of ecosystem services (mushroom production, wood for timber, carbon storage, blue water, and habitat for biodiversity) in Mediterranean mountain pine forests. We simulated thirteen thinning regimes—defined by intensity and frequency—under two different climate change scenarios. We implemented the thinning regimes using SORTIE-ND, an individual-based model of forest dynamics, and then we used models developed for the study area to calculate the provision of services. We used as a case study Pinus sylvestris and Pinus nigra stands, and we evaluated the effect of the thinning regime, climate change, and forest type over 100 years. Our simulations suggest that the production of timber and carbon decreased with increasing intensity and shortening frequency of thinnings, while the provision of mushrooms and blue water generally increased under those conditions. Large timber was better supported by a thinning regime with heavy intensity and shorter frequencies, which also favoured the rapid presence of large dead trees (dbh > 30 cm) in the stand. We found synergies between the production of timber and carbon, while the provision of mushrooms and particularly blue water were in a trade-off relationship with these services. Our simulations show that climate change will lower the production of ecosystem services under the most severe climate predictions and alter the effect of different regimes on the provision of services. We conclude that our modelling approach is a useful and efficient tool for answering questions that would otherwise require long-term studies, and that it can provide useful information to guide management efforts to adapt forest management to the challenges of climate change.

Assessing the effectiveness of landscape-scale forest adaptation actions to improve resilience under projected climate change

Climate change will increase disturbance pressures on forested ecosystems worldwide. In many areas, longer, hotter summers will lead to more wildfire and more insect activity which will substantially increase overall forest mortality. Forest treatments reduce tree density and fuel loads, which in turn reduces fire and insect severity, but implementation has been limited compared to the area needing treatment. Ensuring that forests remain near their reference conditions will require a significant increase in the pace and scale of forest management. In order to assess what pace and scale may be required for a landscape at risk, we simulated forest and disturbance dynamics for the central Sierra Nevada, USA. Our modeling framework included forest growth and succession, wildfire, insect mortality and locally relevant management actions. Our simulations accounted for climate change (five unique global change models on a business-as-usual emissions pathway) and a wide range of plausible forest management scenarios (six total, ranging from less than 1% of area receiving management treatments per year to 6% per year). The climate projections we considered all led to an increasing climatic water deficit, which in turn led to widespread insect caused mortality across the landscape. The level of insect mortality limited the amount of carbon stored and sequestered while leading to significant composition changes, however, only one climate change projection resulted in increased fire over contemporary conditions. While increased pace and scale of treatments led to offsets in fire related tree mortality, managing toward historic reference conditions was not sufficient to reduce insect-caused forest mortality. As such, new management intensities and other adaptation actions may be necessary to maintain forest resilience under an uncertain future climate.

The Interplay of the Tree and Stand-Level Processes Mediate Drought-Induced Forest Dieback: Evidence from Complementary Remote Sensing and Tree-Ring Approaches

Drought-induced forest dieback can lead to a tipping point in community dominance, but the coupled response at the tree and stand-level response has not been properly addressed. New spatially and temporally integrated monitoring approaches that target different biological organization levels are needed. Here, we compared the temporal responses of dendrochronological and spectral indices from 1984 to 2020 at both tree and stand levels, respectively, of a drought-prone Mediterranean Pinus pinea forest currently suffering strong dieback. We test the influence of climate on temporal patterns of tree radial growth, greenness and wetness spectral indices; and we address the influence of major drought episodes on resilience metrics. Tree-ring data and spectral indices followed different spatio-temporal patterns over the study period (1984–2020). Combined information from tree growth and spectral trajectories suggests that a reduction in tree density during the mid-1990s could have promoted tree growth and reduced dieback risk. Additionally, over the last decade, extreme and recurrent droughts have resulted in crown defoliation greater than 40% in most plots since 2019. We found that tree growth and the greenness spectral index were positively related to annual precipitation, while the wetness index was positively related to mean annual temperature. The response to drought, however, was stronger for tree growth than for spectral indices. Our study demonstrates the value of long-term retrospective multiscale analyses including tree and stand-level scales to disentangle mechanisms triggering and driving forest dieback.

Dry forest decline is driven by both declining recruitment and increasing mortality in response to warm, dry conditions

AbstractAimAnticipating when and where changes in species' demographic rates will lead to range shifts in response to changing climate remains a major challenge. Despite evidence of increasing mortality in dry forests across the globe in response to drought and warming temperatures, the overall impacts on the distribution of dry forests are largely unknown because we lack comparable large‐scale data on tree recruitment rates. Here, our aim was to develop range‐wide population models for dry forest tree species (pinyon pine and juniper), quantifying both mortality and recruitment, to better understand where and under what conditions species range contractions are occurring.LocationWestern United States.Major taxa studiedTwo pinyon pine (Pinus spp.) and three juniper (Juniperus spp.) species.MethodsWe developed range‐wide demographic models for five species using forest inventory data from across the western United States and estimated population trends and climate vulnerability.ResultsWe find that four of the five species are declining in parts of their range, with Pinus edulis having the largest proportion of populations declining (24%). Population vulnerability increases with aridity and temperature, with up to ~50% of populations declining in the warmest and driest conditions. Mortality and recruitment were both essential to explaining where populations are declining.Main conclusionsOur results suggest that dry forest species are undergoing an active range shift driven by both changing recruitment and mortality, and that increasing temperatures and drought threaten the long‐term viability of many of these species in their current range. While four of the five species examined were experiencing some declines, P. edulis is currently most vulnerable. Management actions such as reducing tree density may be able to mitigate some of these impacts. The framework we present to estimate range‐wide demographic rates can be applied to other species to determine where range contractions are most likely.

Population and Community Responses of Small Mammals to Single-tree Selection Harvest in Laurentian Hardwood Forests

Small mammals are ubiquitous members of vertebrate communities that are sensitive to habitat change. In the Great Lakes region of North America, small mammal communities have changed rapidly, but experimental tests of potential mechanisms are lacking. Using a before-after, control-treatment design, we quantified the response of small mammals to single-tree selection harvest in Laurentian hardwood forests of Wisconsin, United States. We documented changes in forest structure and small mammal abundance, species diversity, and community similarity from silvicultural treatment. Treatment reduced tree density and canopy cover and increased mean tree diameter, woody stem density, variation in woody stem density, and volume of coarse woody debris. Peromyscus and northern short-tailed shrews (Blarina brevicauda) were dominant community members across treatments and years. White-footed mice (P. leucopus) outnumbered woodland deer mice (P. maniculatus gracilis) before treatment, but declined by almost fifty percent after treatment; deer mice and total rodent (i.e., Rodentia) abundances were unchanged. Small mammal species diversity increased twofold following treatment. Our experiment identified species-specific responses within Peromyscus to timber harvest: white-footed mice, the numerically dominant and generalist species, were most sensitive to habitat change, and their response produced cascading effects to small mammal community structure. Future experiments should assess these small mammal responses in a multi-year framework and quantify their effects on the broader vertebrate community.

The response of sub‐adult savanna trees to six successive annual fires: An experimental field study on the role of fire season

Abstract In mesic savannas worldwide, trees experience frequent fires, almost all set by humans. Management fires are set to reduce or enhance tree cover. Success depends greatly on responses of sub‐adult trees to such fires. To date, the number of successive years that sub‐adult trees can resprout nor the number of years that they must resist being top‐killed by successive fires, nor the requisite height, have been reported. In a 6‐year experimental field study in Guinean savannas of West Africa, we monitored annually the heights and responses of 1,765 permanently tagged sub‐adult trees under annual fires set in three different periods of the long dry season: early‐dry season (EDS), mid‐dry season (MDS) and late‐dry season (LDS). Annual MDS fires are the common local management protocols of Guinean savannas, although EDS fires are common in some of the savannas. Results showed that overall, the proportion of sub‐adults that resisted being top‐killed differed across fire seasons. Furthermore, resisting one fire gave a better chance of resisting the next. Only sub‐adults that were able to resist direct damage for three successive EDS and MDS fires reached sufficient height to be recruited to the adult stage. Resistance height (avoiding topkill) was ∼1 m for EDS and ∼2 m for both MDS and LDS fires. Recruitment height (threshold for transition to adult stage) was ∼3 m for EDS and ∼ 3.3 m for MDS fires. No height was great enough for sub‐adult trees to be recruited to adult stages in LDS fire. Synthesis and applications. The results of this novel field study showed clearly that successive early‐ and mi‐dry season fires can enhance tree density and that successive late‐dry season fires alone reduce tree density in Guinean savannas due to the effects of successive fires on sub‐adult trees. The results suggest that a planned regime of these seasons of fire could be used to maintain the desired tree density in Guinean savannas and may inform fire management in other mesic savannas where goals are to increase or decrease tree densities. It also provides relevant information for comparative studies on the mechanisms of recruitment of sub‐adult trees to an adult stage in all mesic savannas, a process that ultimately determines savanna physiognomy.

Tree growth responses to extreme drought after mechanical thinning and prescribed fire in a Sierra Nevada mixed-conifer forest, USA

An estimated 128 M trees died during the 2012–2016 California drought, largely in the southern Sierra Nevada Range. Prescribed burning and mechanical thinning are widely used to reduce fuels and restore ecosystem properties, but it is unclear if these treatments improve tree growth and vigor during extreme drought. This study examined tree growth responses after thinning, prescribed burning, and extreme drought at the Teakettle Experimental Forest, a historically frequent fire mixed-conifer forest in the southern Sierra Nevada of California, USA. Mechanical thinning (no thin, understory thin, and overstory thin) and prescribed burning (unburned, fall burning) were implemented in 2000–2001. Using annual growth data from increment cores, over 10,000 mapped and measured trees, and lidar-derived metrics of solar radiation and topographic wetness, we had two primary questions. First, what were the growth responses to thinning and prescribed burning treatments, and did these responses persist during the 2012–2016 drought? Second, what tree-level attributes and environmental conditions influenced growth responses to treatments and drought?Thinning increased residual tree growth and that response persisted through extreme drought 10–15 years after treatments. Growth responses were higher in overstory versus understory thinning, with differences between thinning types more pronounced during drought. Species-specific growth responses were strongest with overstory thinning, with sugar pine (Pinus lambertiana) and incense-cedar (Calocedrus decurrens) having higher growth responses compared to white fir (Abies concolor) and Jeffery pine (Pinus jeffreyi). For individual trees, factors associated with higher growth responses were declining pretreatment growth trend, smaller tree size, and post-treatment low neighborhood basal area. Growth responses were initially not influenced by topography, but topographic wetness became important during extreme drought. Mechanical thinning resulted in durable increases in residual tree growth rates during extreme drought over a decade after thinning occurred, indicating treatment longevity in mitigating drought stress. In contrast, tree growth did not improve after prescribed burning, likely due to fire effects that reduced surface fuels, but had little effect on reducing tree density. Thinning treatments promoted durable growth responses, but focusing on stand-level metrics may ignore important tree-level attributes such as localized competition and topography associated with higher water availability. Mechanical thinning was effective at improving growth in trees that had been experiencing declining growth trends, but was less effective in improving growth responses in large old trees of higher ecological importance.

Persistent composition legacy and rapid structural change following successive fires in Sierra Nevada mixed conifer forests

Mixed conifer forests in the Sierra Nevada, California, USA, face threats from frequent high-severity reburns caused by fuel accumulation and increasingly dry, hot conditions associated with climate change. Stand replacing fire and establishment of expansive shrub patches may result in positive feedbacks causing landscapes to convert to persistent shrubland or grassland. Our objective was to examine changes in mixed conifer forest density and species composition after successive fires across a range of initial and reburn severities. We resampled 134 field plots 5–6 years after the Chips Fire reburned stands initially burned by the Storrie and Rich fires on the Lassen and Plumas National Forests. Generalized Linear Models and post-hoc margins analyses were used to examine the interaction effects of initial and reburn fire severities on changes in conifer tree and seedling densities, percent cover of shrubs, and changes in species composition evaluated using fire tolerance index. We found that successive fires with severities less than high-severity significantly reduced tree densities to values within or close to the natural range of variation for mixed conifer stands. Despite these changes in density, species composition did not shift toward increasing dominance of fire tolerant species; mature trees and seedlings of true firs and other fire intolerant species remained dominant after successive low and moderate severity fires. Seedling establishment pulses were associated with sampling years with abundant precipitation, and unburned, unchanged, and low-severity plots in the reburn sample had significant increases in seedling densities across the range of initial burn severities. Shrub cover increased significantly with only a few reburn severity interactions, but declined significantly with repeated high-severity fire. Although two low- to moderate-severity fires may reduce tree densities to within the natural range of variation, they may not shift species composition towards historical patterns. Selective thinning in areas that burned at low- to moderate-severity may be necessary to restore historical species composition in Sierra Nevada mixed conifer forests.

Relative Inference for Paired Data: More than Meets the Eye

Relative Inference for Paired Data: More than Meets the Eye

Southwestern ponderosa pine forest patterns following wildland fires managed for resource benefit differ from reference landscapes

ContextManagers aiming to utilize wildland fire to restore southwestern ponderosa pine landscapes require better understanding of forest cover patterns produced at multiple scales. Restoration effectiveness of wildland fires managed for resource benefit can be evaluated against natural ranges of variation.ObjectivesWe describe landscape patterns within reference landscapes, including restored and functioning ponderosa pine forests of northern Arizona, and compare them to wildland fires managed for resource benefit. We make comparisons along a gradient of extents and assess the effects of scale on landscape differences.MethodsUsing Sentinel-2 imagery, we classified ponderosa pine forest cover and calculated landscape metrics across a gradient of landscape extent within reference and managed landscapes. We used non-parametric tests to assess differences. We used random forest models to assess and explore which landscape metrics were most importance in differentiating landscape patterns.ResultsRestored forests exhibited landscapes patterns consistent with those of ecologically intact forest landscapes. Managed wildfire landscape patterns differed significantly when compared to reference landscape patterns among nearly all landscape metrics considered and became increasingly different with increasing landscape extent (15–840 ha), tending towards both denser and larger patch areas.ConclusionsLandscape patterns from wildland fires managed for resource benefit we examined differ from those of reference landscapes. Differences become more pronounced with increasing landscape size. Landscape patterns among large managed forest landscapes suggest that the predominately single-entry, low-severity disturbance regime from these managed fires is failing to reduce tree densities and break up large contiguous areas of canopy cover.

Vegetation structure controls on snow and soil moisture in restored ponderosa pine forests

AbstractThinning of semi‐arid forests to reduce wildfire risk is believed to improve forest health by increasing soil moisture. Increased snowpack, reduced transpiration and reduced rainfall interception are frequently cited mechanisms by which reduced canopy density may increase soil moisture. However, the relative importance of these factors has not been rigorously evaluated in field studies. We measured snow depth, snow water equivalent (SWE) and the spatial and temporal variation in soil moisture at four experimental paired treatment‐control thinning sites in high elevation ponderosa pine forest northern Arizona, USA. We compared snow and soil moisture measurements with forest structure metrics derived from aerial imagery and 3‐dimensional lidar data to determine the relationship between vegetation structure, snow and soil moisture throughout the annual hydrologic cycle. Soil moisture was consistently and significantly higher in thinned forest plots, even though the treatments were performed 8–11 years before this study. However, we did not find evidence that SWE was higher in thinned forests across a range of snow conditions. Regression tree analysis of soil moisture and vegetation structure data provided some evidence that localized differences in transpiration and interception of precipitation influence the spatial pattern of soil moisture at points in the annual hydrologic cycle when the system is becoming increasingly water limited. However, vegetation structure explained a relatively low amount of the spatial variance (R2 < 0.23) in soil moisture. Continuous measurements of soil moisture in depth profiles showed stronger attenuation of soil moisture peaks in thinned sites, suggesting differences in infiltration dynamics may explain the difference in soil moisture between treatments as opposed to overlying vegetation alone. Our results show limited support for commonly cited relationships between vegetation structure, snow and soil moisture and indicate that future research is needed to understand how reduction in tree density alters soil hydraulic properties.

Mechanical thinning without prescribed fire moderates wildfire behavior in an Eastern Oregon, USA ponderosa pine forest

Reducing fuels to better manage risk of high severity wildfire in seasonally dry, fire-prone forests of the western U.S. is an important goal of forest managers, including private landowners, non-governmental organizations, tribal, state, and local governments, and federal agencies. Managing fire risk is a critical objective of the U.S. Forest Service, which emphasizes the use of thinning to reduce tree density and ladder fuels followed by prescribed fire to reduce surface fuel. But the area of Forest Service land treated with thinning and prescribed fire is lagging far behind the area treated only with mechanical thinning due to regulatory and logistical challenges in prescribed fire implementation. Determining if mechanical thinning alone (without prescribed fire) can achieve adequate fire risk reduction has important implications for addressing the fire and fuel management goals set by Congress and the Administration, as well as the management objectives set by non-federal actors. In this study, we report on the effects of mechanical thinning and standard post-thinning fuels management but without prescribed fire on modeled fire behavior and changes in fuel loading over time in a ponderosa pine forest in Eastern Oregon. Thinning without prescribed fire significantly reduced potential crown fire immediately following thinning and also moderated surface modeled fire behavior beginning 2–3 years following thinning. Although small (<7.6 cm diameter) woody surface fuel loading increased following thinning, other ground and surface fuels (i.e., litter and duff) declined substantially, which we attribute to surface disturbance from ground-based logging, decreased deposition of litter, and increased decomposition. These results suggest that fuel reduction and fire risk management objectives can be met with mechanical thinning alone for a number of years. Prescribed fire is likely necessary to extend the effectiveness of mechanical thinning after significant tree or shrub regeneration. Continued monitoring will allow managers to use prescribed fire most efficiently to achieve fire and fuel management objectives.

Contrasting genetic signal of recolonization after rainforest fragmentation in African trees with different dispersal abilities

Although today the forest cover is continuous in Central Africa, this may have not always been the case, as the scarce fossil record in this region suggests that arid conditions might have significantly reduced tree density during the ice ages. Our aim was to investigate whether the dry ice age periods left a genetic signature on tree species that can be used to infer the date of the past fragmentation of the rainforest. We sequenced reduced representation libraries of 182 samples representing five widespread legume trees and seven outgroups. Phylogenetic analyses identified an early divergent lineage for all species in West Africa (Upper Guinea) and two clades in Central Africa: Lower Guinea-North and Lower Guinea-South. As the structure separating the Northern and Southern clades-congruent across species-cannot be explained by geographic barriers, we tested other hypotheses with demographic model testing using δαδι. The best estimates indicate that the two clades split between the Upper Pliocene and the Pleistocene, a date compatible with forest fragmentation driven by ice age climatic oscillations. Furthermore, we found remarkably older split dates for the shade-tolerant tree species with nonassisted seed dispersal than for light-demanding species with long-distance wind-dispersed seeds. Different recolonization abilities after recurrent cycles of forest fragmentation seem to explain why species with long-distance dispersal show more recent genetic admixture between the two clades than species with limited seed dispersal. Despite their old history, our results depict the African rainforests as a dynamic biome where tree species have expanded relatively recently after the last glaciation.

Canopy openness and exclusion of wild ungulates act synergistically to improve oak natural regeneration

The recruitment of forest trees is driven by both bottom-up processes (the acquisition of resources) and top-down processes (herbivory). To initiate stand regeneration, foresters commonly reduce tree density to increase light levels for seedlings and enhance primary productivity. These changes in vegetation dynamics, however, could also influence effects of ungulate browsing, resulting in unintended consequences for forest management. Here, we assessed how effects of ungulate exclusion and canopy opening interacted to affect the regeneration of two oak species: Quercus robur and Quercus petraea. We monitored the growth and survival of oak seedlings for two to three growth seasons in paired fenced and unfenced plots under contrasting conditions of canopy openness (8% to 52%) at five sites in southern Sweden and three sites in northeastern France. We scored browsing in the unfenced plots by the four cervids occurring in these areas (Alces alces, Capreolus capreolus, Cervus elaphus and Dama dama). Fencing increased the growth of (mostly taller) seedlings occurring in Sweden and the survival of (mostly smaller) seedlings in France. Both effects increased as canopies became more open. Browsing reduced oak seedling growth in both countries, independently of canopy openness. Canopy openness increased browsing levels in Sweden. Cervid densities did not appear to modify how fencing affected oak seedling growth and survival. In both contrasting forest environments, creating gaps tended to enhance ungulate damage on young forest stands as browsing frequency increased. We conclude that net forest regeneration reflects a subtle equilibrium between enhancing resource availability, boosting seedling growth, and limiting herbivory, which curtails seedling growth and survival.

Effects of disturbances by forest elephants on diversity of trees and insects in tropical rainforests on Mount Cameroon

Natural disturbances are essential for tropical forests biodiversity. In the Afrotropics, megaherbivores have played a key role before their recent decline. Contrastingly to savanna elephants, forest elephants’ impact on ecosystems remains poorly studied. Few decades ago, forests on Mount Cameroon were divided by lava flows, not being crossed by a local population of forest elephants until now. We assessed communities of trees, butterflies and two guilds of moths in the disturbed and undisturbed forests split by the longest lava flow. We surveyed 32 plots, recording 2025 trees of 97 species, and 7853 insects of 437 species. The disturbed forests differed in reduced tree density, height, and high canopy cover, and in increased DBH. Forest elephants’ selective browsing and foraging also decreased tree species richness and altered their composition. The elephant disturbance increased butterfly species richness and had various effects on species richness and composition of the insect groups. These changes were likely caused by disturbance-driven alterations of habitats and species composition of trees. Moreover, the abandonment of forests by elephants led to local declines of range-restricted butterflies. The recent declines of forest elephants across the Afrotropics probably caused similar changes in forest biodiversity and should be reflected by conservation actions.

Reducing tree density affects interactions between trees and atmospheric Tillandsia species (Bromeliaceae)

AbstractDisruptions in species interactions derived from forest fragmentation are a major driver of biodiversity declines. Atmospheric bromeliads, a group of extreme epiphytes in the Tillandsia genus, often thrive in anthropogenic landscapes, spreading beyond their usual distribution. Here, we investigate how distinct tree densities affect the interactions between tree traits and patterns of atmospheric bromeliad host selection, abundance, functional composition, and canopy distribution. We measured a total of 3130 atmospheric bromeliads hosted on 140 of the 384 trees sampled and found a greater contribution of tree traits on their host selection and abundance at low tree density. We observed that distinct tree traits were significantly correlated to atmospheric bromeliad traits under low and high tree densities, with most correlations positive under low tree density and negative under high tree density. Additionally, the canopy distribution of individuals under high tree density was less scattered and slightly restricted to outer canopy layers. Shading levels seem to be a strong environmental filter on atmospheric bromeliad fitness, limiting their spread in forests with high tree densities; however, major reductions in tree density may allow atmospheric bromeliads to spread to other canopy layers and increase their abundance, mainly boosted by traits related to tree size. Our results suggest that reductions in tree densities of forests may alter ecological interactions among plants, which could lead to the spread of opportunistic species.