Ontogenetic niche shifts in a locally endangered tree species (Olea europaea subsp. cuspidata) in a disturbed forest in Northern Ethiopia: Implications for conservation.

Understanding the responses of different ontogenetic stages to environmental and human disturbance factors is essential for developing efficient conservation strategies for endangered plant species. We examined how three ontogenetic stages of a locally endangered tree species, Olea europaea subsp. cuspidata, responded to environmental factors and human disturbance in Hugumburda dry Afromontane forest in Ethiopia. We counted individual seedlings, saplings and adults of O. europaea in 70 20 × 20 m quadrats over ca. 2.8 ha, and measured biotic (woody species richness, canopy cover, aboveground tree biomass, herbaceous cover), abiotic (soil and topographic variables), and human disturbance factors (logging and tracks). To detect ontogenetic niche shifts, we compared observed vs. simulated locations of trees in the three life stages and how they related to the environmental and human disturbance factors. We found that the population structure of O. europaea showed generally low recruitment, with few seedlings per hectare compared with the abundance of saplings and adults. The probability of finding O. europaea individuals was influenced by biotic (woody species richness) and abiotic (soil depth, slope) environmental conditions and human disturbance (logging intensity), but the direction, strength and shape of the relationships differed between seedling, sapling and adult life stages, indicating ontogenetic niche shifts. All life stages showed a positive relationship with elevation. The observed environmental niches of the different lifestages of O. europaea, and their association with human disturbance levels, should be considered when conservation strategies are developed for this species. Human disturbance in terms of logging decreases the abundance of saplings, but may facilitate emerging seedlings through creation of gaps with improved light conditions. Recruitment is, however, very low in the study area, and seedlings should be protected from browsing to enhance survival. Woody species richness in general should be conserved to optimize conditions also for O. europaea saplings.

Experimental warming differentially affects vegetative and reproductive phenology of tundra plants.

Rapid climate warming is altering Arctic and alpine tundra ecosystem structure and function, including shifts in plant phenology. While the advancement of green up and flowering are well-documented, it remains unclear whether all phenophases, particularly those later in the season, will shift in unison or respond divergently to warming. Here, we present the largest synthesis to our knowledge of experimental warming effects on tundra plant phenology from the International Tundra Experiment. We examine the effect of warming on a suite of season-wide plant phenophases. Results challenge the expectation that all phenophases will advance in unison to warming. Instead, we find that experimental warming caused: (1) larger phenological shifts in reproductive versus vegetative phenophases and (2) advanced reproductive phenophases and green up but delayed leaf senescence which translated to a lengthening of the growing season by approximately 3%. Patterns were consistent across sites, plant species and over time. The advancement of reproductive seasons and lengthening of growing seasons may have significant consequences for trophic interactions and ecosystem function across the tundra.

Enhancing pollination is more effective than increased conventional agriculture inputs for improving watermelon yields.

Agricultural practices to improve yields in small-scale farms in Africa usually focus on improving growing conditions for the crops by applying fertilizers, irrigation, and/or pesticides. This may, however, have limited effect on yield if the availability of effective pollinators is too low. In this study, we established an experiment to test whether soil fertility, soil moisture, and/or pollination was limiting watermelon (Citrullus lanatus) yields in Northern Tanzania. We subjected the experimental field to common farming practices while we treated selected plants with extrafertilizer applications, increased irrigation and/or extra pollination in a three-way factorial experiment. One week before harvest, we assessed yield from each plant, quantified as the number of mature fruits and their weights. We also assessed fruit shape since this may affect the market price. For the first fruit ripening on each plant, we also assessed sugar content (brix) and flesh color as measures of fruit quality for human consumption. Extra pollination significantly increased the probability of a plant producing a second fruit of a size the farmer could sell at the market, and also the fruit sugar content, whereas additional fertilizer applications or increased irrigation did not improve yields. In addition, we did not find significant effects of increased fertilizer or watering on fruit sugar, weight, or color. We concluded that, insufficient pollination is limiting watermelon yields in our experiment and we suggest that this may be a common situation in sub-Saharan Africa. It is therefore critically important that small-scale farmers understand the role of pollinators and understand their importance for agricultural production. Agricultural policies to improve yields in developing countries should therefore also include measures to improve pollination services by giving education and advisory services to farmers on how to develop pollinator-friendly habitats in agricultural landscapes.

Forest fragmentation modifies the composition of bumblebee communities and modulates their trophic and competitive interactions for pollination.

Understanding the effects of landscape fragmentation on global bumblebee declines requires going beyond estimates of abundance and richness and evaluating changes in community composition and trophic and competitive interactions. We studied the effects of forest fragmentation in a Scandinavian landscape that combines temperate forests and croplands. For that, we evaluated how forest fragmentation features (patch size, isolation and shape complexity, percentage of forest in the surroundings) as well as local flowering communities influenced bumblebee abundance, richness and community composition in 24 forest patches along a fragmentation gradient. In addition, we assessed the effect of fragmentation on bumblebee-plant network specialization (H2'), and potential inter- and intraspecific competition via shared plants. Patch isolation was associated with lower bumblebee abundance, whereas flower density was positively related to both bumblebee abundance and richness. Overall, forest fragmentation reduced the abundance of forest-specialists while increasing the abundance of open-habitat species. Patches with complex shapes and few flowers showed more generalized bumblebee-plant networks (i.e., fewer specific interactions). Patch shape complexity and the percentage of forest also modified inter- and intraspecific competitive interactions, with habitat generalists outcompeting forest specialists in fragmented areas. Understanding these mechanisms is necessary to anticipate to the impact of forest fragmentation on bumblebee decline.

Disentangling direct and indirect effects of habitat fragmentation on wild plants' pollinator visits and seed production.

Habitat fragmentation threatens plant and pollinator communities, as well as their interactions. However, the effects of landscape fragmentation on the pollination of wild plant species are not well understood yet, partly because there are many correlated features in fragmented landscapes (e.g., decreased patch size, increased isolation, and patch complexity) whose influences are difficult to disentangle. Using a structural equation modeling approach, we assessed the direct and indirect effects of landscape fragmentation (patch size, isolation and complexity, percentage of surrounding land in forest) on the abundance, functional-group richness, and evenness of pollinators of 24 habitat fragments within an agricultural landscape in Southern Norway. In addition, we studied how these variables affected visitation rates (visits per flower) and seed production (seed set, seed mass) in the four most abundant plant species in the area. Flower abundance was higher in larger and complex patches and decreased with the percentage of forest in the surroundings, while flower richness increased with patch complexity. We found a direct negative relationship between patch complexity and the overall number of pollinator visits that the habitat fragments received. Apart from this direct landscape effect, pollinator visits were mostly affected by the floral communities, with overall flower abundance and richness increasing both total number of pollinator visits and pollinator-group richness, and flower richness having an additional negative influence on pollinator-group evenness. Interestingly, we did not find any direct link between visitation rates and reproductive success for any of the study plant species. Instead, several landscape variables directly affected species seed production, although the effects of landscape on seed production were highly species specific. Patch complexity had a negative effect on seed production in two of the four focal species, while other components of the landscape had species-specific effects. Increasing fragmentation of agricultural landscapes affects pollination interactions at the community level and the reproduction of wild plants. However, understanding the effects of fragmentation on seed production requires going beyond estimating visitation rates, since landscape effects on plant reproduction are not always related to overall interaction frequencies.

Author Correction: Warming shortens flowering seasons of tundra plant communities.

In the version of this Article originally published, the following sentence was missing from the Acknowledgements: "This work was supported by the Norwegian Research Council SnoEco project, grant number 230970". This text has now been added.

Warming shortens flowering seasons of tundra plant communities.

Advancing phenology is one of the most visible effects of climate change on plant communities, and has been especially pronounced in temperature-limited tundra ecosystems. However, phenological responses have been shown to differ greatly between species, with some species shifting phenology more than others. We analysed a database of 42,689 tundra plant phenological observations to show that warmer temperatures are leading to a contraction of community-level flowering seasons in tundra ecosystems due to a greater advancement in the flowering times of late-flowering species than early-flowering species. Shorter flowering seasons with a changing climate have the potential to alter trophic interactions in tundra ecosystems. Interestingly, these findings differ from those of warmer ecosystems, where early-flowering species have been found to be more sensitive to temperature change, suggesting that community-level phenological responses to warming can vary greatly between biomes.

The relative importance of vertical soil nutrient heterogeneity, and mean and depth-specific soil nutrient availabilities for tree species richness in tropical forests and woodlands.

The relative importance of resource heterogeneity and quantity on plant diversity is an ongoing debate among ecologists, but we have limited knowledge on relationships between tree diversity and heterogeneity in soil nutrient availability in tropical forests. We expected tree species richness to be: (1) positively related to vertical soil nutrient heterogeneity; (2) negatively related to mean soil nutrient availability; and (3) more influenced by nutrient availability in the upper than lower soil horizons. Using a data set from 60, 20 × 40-m plots in a moist forest, and 126 plots in miombo woodlands in Tanzania, we regressed tree species richness against vertical soil nutrient heterogeneity, both depth-specific (0-15, 15-30, and 30-60 cm) and mean soil nutrient availability, and soil physical properties, with elevation and measures of anthropogenic disturbance as co-variables. Overall, vertical soil nutrient heterogeneity was the best predictor of tree species richness in miombo but, contrary to our prediction, the relationships between tree species richness and soil nutrient heterogeneity were negative. In the moist forest, mean soil nutrient availability explained considerable variations in tree species richness, and in line with our expectations, these relationships were mainly negative. Soil nutrient availability in the top soil layer explained more of the variation in tree species richness than that in the middle and lower layers in both vegetation types. Our study shows that vertical soil nutrient heterogeneity and mean availability can influence tree species richness at different magnitudes in intensively utilized tropical vegetation types.

Experimental simulation of pollinator decline causes community-wide reductions in seedling diversity and abundance.

Pollinator decline can disrupt the mutualistic interactions between plants and pollinators and potentially affect the maintenance of plant populations. However, there is still little knowledge on how changes in pollinator abundance can affect seedling recruitment, which is essential for population persistence. We experimentally simulated a community-wide reduction in pollinator availability during four years to examine its effects on seedling recruitment in 10 perennial herbs in a Norwegian hay meadow. Our experimental reduction in pollinator availability significantly reduced community-wide seedling diversity. Overall seedling abundance was also consistently lower under reduced pollinator availability, although this effect was only significant when the most abundant plant species in the community was excluded from the analysis. Despite an overall negative effect on seedling abundance, the experimental reduction in pollinator availability had contrasting effects on individual plant species. This tended to cause a larger change in seedling species composition in the experimental than in the control plots after the four study years. Our study demonstrates for the first time a direct causal link between reduced pollinator availability and reduced plant diversity and abundance.

The effects of habitat management on the species, phylogenetic and functional diversity of bees are modified by the environmental context.

Anthropogenic landscape elements, such as roadsides, hedgerows, field edges, and power line clearings, can be managed to provide important habitats for wild bees. However, the effects of habitat improvement schemes in power line clearings on components of diversity are poorly studied. We conducted a large-scale experiment to test the effects of different management practices on the species, phylogenetic, and functional diversity of wild bees in power line clearings (n = 19 sites across southeastern Norway) and explored whether any treatment effects were modified by the environmental context. At each site, we conducted the following treatments: (1) Cut: all trees cut and left to decay in the clearing; (2) Cut + Remove: all trees cut and removed from the plot; and (3) Uncut: uncleared. The site-specific environmental context (i.e., elevation and floral diversity) influenced the species, phylogenetic, and functional diversity within bee species assemblages. The largest number of species was found in the Cut + Remove treatment in plots with a high forb species richness, indicating that the outcome of management practices depends on the environmental context. Clearing of treatment plots with many forb species also appeared to alter the phylogenetic composition of bee species assemblages, that is, more closely related species were found in the Cut and the Cut + Remove plots than in the Uncut plots. Synthesis and applications: Our experimental simulation of management practices in power line clearings influenced the species, phylogenetic, and functional diversity of bee species assemblages. Frequent clearing and removal of the woody debris at low elevations with a high forb species richness can increase the value of power line clearings for solitary bees. It is therefore important for managers to consider the environmental context when designing habitat improvement schemes for solitary bees.

Interactions between Canopy Structure and Herbaceous Biomass along Environmental Gradients in Moist Forest and Dry Miombo Woodland of Tanzania.

We have limited understanding of how tropical canopy foliage varies along environmental gradients, and how this may in turn affect forest processes and functions. Here, we analyse the relationships between canopy leaf area index (LAI) and above ground herbaceous biomass (AGBH) along environmental gradients in a moist forest and miombo woodland in Tanzania. We recorded canopy structure and herbaceous biomass in 100 permanent vegetation plots (20 m × 40 m), stratified by elevation. We quantified tree species richness, evenness, Shannon diversity and predominant height as measures of structural variability, and disturbance (tree stumps), soil nutrients and elevation as indicators of environmental variability. Moist forest and miombo woodland differed substantially with respect to nearly all variables tested. Both structural and environmental variables were found to affect LAI and AGBH, the latter being additionally dependent on LAI in moist forest but not in miombo, where other factors are limiting. Combining structural and environmental predictors yielded the most powerful models. In moist forest, they explained 76% and 25% of deviance in LAI and AGBH, respectively. In miombo woodland, they explained 82% and 45% of deviance in LAI and AGBH. In moist forest, LAI increased non-linearly with predominant height and linearly with tree richness, and decreased with soil nitrogen except under high disturbance. Miombo woodland LAI increased linearly with stem density, soil phosphorous and nitrogen, and decreased linearly with tree species evenness. AGBH in moist forest decreased with LAI at lower elevations whilst increasing slightly at higher elevations. AGBH in miombo woodland increased linearly with soil nitrogen and soil pH. Overall, moist forest plots had denser canopies and lower AGBH compared with miombo plots. Further field studies are encouraged, to disentangle the direct influence of LAI on AGBH from complex interrelationships between stand structure, environmental gradients and disturbance in African forests and woodlands.

Spatial distribution of temporal dynamics in anthropogenic fires in miombo savanna woodlands of Tanzania.

BACKGROUND: Anthropogenic uses of fire play a key role in regulating fire regimes in African savannas. These fires contribute the highest proportion of the globally burned area, substantial biomass burning emissions and threaten maintenance and enhancement of carbon stocks. An understanding of fire regimes at local scales is required for the estimation and prediction of the contribution of these fires to the global carbon cycle and for fire management. We assessed the spatio-temporal distribution of fires in miombo woodlands of Tanzania, utilizing the MODIS active fire product and Landsat satellite images for the past ~40 years. RESULTS: Our results show that up to 50.6% of the woodland area is affected by fire each year. An early and a late dry season peak in wetter and drier miombo, respectively, characterize the annual fire season. Wetter miombo areas have higher fire activity within a shorter annual fire season and have shorter return intervals. The fire regime is characterized by small-sized fires, with a higher ratio of small than large burned areas in the frequency-size distribution (ß = 2.16 ± 0.04). Large-sized fires are rare, and occur more frequently in drier than in wetter miombo. Both fire prevalence and burned extents have decreased in the past decade. At a large scale, more than half of the woodland area has less than 2 years of fire return intervals, which prevent the occurrence of large intense fires. CONCLUSION: The sizes of fires, season of burning and spatial extent of occurrence are generally consistent across time, at the scale of the current analysis. Where traditional use of fire is restricted, a reassessment of fire management strategies may be required, if sustainability of tree cover is a priority. In such cases, there is a need to combine traditional and contemporary fire management practices.

How do pollinator visitation rate and seed set relate to species' floral traits and community context?

Differences among plant species in visitation rate and seed set within a community may be explained both by the species' floral traits and the community context. Additionally, the importance of species' floral traits vs. community context on visitation rate and seed set may vary among communities. In communities where the pollinator-to-flower ratio is low, floral traits may be more important than community context, as pollinators may have the opportunity to be choosier when visiting plant species. In this study we investigated whether species' floral traits (flower shape, size and number, and flowering duration) and community context (conspecific and heterospecific flower density, and pollinator abundance) could explain among-species variation in visitation rate and seed set. For this, we used data on 47 plant species from two Norwegian plant communities differing in pollinator-to-flower ratio. Differences among species in visitation rate and seed set within a community could be explained by similar variables as those explaining visitation rate and seed set within species. As expected, we found floral traits to be more important than community context in the community with a lower pollinator-to-flower ratio; whereas in the community with a higher pollinator-to-flower ratio, community context played a bigger role. Our study gives significant insights into the relative importance of floral traits on species' visitation rate and seed set, and contributes to our understanding of the role of the community context on the fitness of plant species.

Plant functional traits mediate reproductive phenology and success in response to experimental warming and snow addition in Tibet.

Global climate change is predicted to have large impacts on the phenology and reproduction of alpine plants, which will have important implications for plant demography and community interactions, trophic dynamics, ecosystem energy balance, and human livelihoods. In this article we report results of a 3-year, fully factorial experimental study exploring how warming, snow addition, and their combination affect reproductive phenology, effort, and success of four alpine plant species belonging to three different life forms in a semiarid, alpine meadow ecosystem on the central Tibetan Plateau. Our results indicate that warming and snow addition change reproductive phenology and success, but responses are not uniform across species. Moreover, traits associated with resource acquisition, such as rooting depth and life history (early vs. late flowering), mediate plant phenology, and reproductive responses to changing climatic conditions. Specifically, we found that warming delayed the reproductive phenology and decreased number of inflorescences of Kobresia pygmaea C. B. Clarke, a shallow-rooted, early-flowering plant, which may be mainly constrained by upper-soil moisture availability. Because K. pygmaea is the dominant species in the alpine meadow ecosystem, these results may have important implications for ecosystem dynamics and for pastoralists and wildlife in the region.

Global assessment of experimental climate warming on tundra vegetation: heterogeneity over space and time.

Understanding the sensitivity of tundra vegetation to climate warming is critical to forecasting future biodiversity and vegetation feedbacks to climate. In situ warming experiments accelerate climate change on a small scale to forecast responses of local plant communities. Limitations of this approach include the apparent site-specificity of results and uncertainty about the power of short-term studies to anticipate longer term change. We address these issues with a synthesis of 61 experimental warming studies, of up to 20 years duration, in tundra sites worldwide. The response of plant groups to warming often differed with ambient summer temperature, soil moisture and experimental duration. Shrubs increased with warming only where ambient temperature was high, whereas graminoids increased primarily in the coldest study sites. Linear increases in effect size over time were frequently observed. There was little indication of saturating or accelerating effects, as would be predicted if negative or positive vegetation feedbacks were common. These results indicate that tundra vegetation exhibits strong regional variation in response to warming, and that in vulnerable regions, cumulative effects of long-term warming on tundra vegetation - and associated ecosystem consequences - have the potential to be much greater than we have observed to date.

Bamboo dominance reduces tree regeneration in a disturbed tropical forest.

Human disturbance may change dominance hierarchies of plant communities, and may cause substantial changes in biotic environmental conditions if the new dominant species have properties that differ from the previous dominant species. We examined the effects of bamboos (Bambusa tulda and Cephalostachyum pergracile) and their litter on the overall woody seedling abundance, species richness and diversity in a mixed deciduous forest in northeastern Thailand. These bamboo species are gaining dominance after human disturbance. Our results show that seedling abundance and species richness were reduced by bamboo canopies. Seedling abundance and species diversity under bamboo canopies were affected by bamboo litter, whereas seedling abundance and species diversity outside bamboo canopies did not respond to the mixed-tree litter manipulation. Removal of bamboo litter increased seedling abundance and species diversity. However, bamboo litter addition did not affect seedling abundance or species diversity compared to either control or litter removal. This may indicate that the effect of natural amount of bamboo litter is as high as for litter addition in preventing seedling establishment by woody species and hence in minimizing resource competition. We conclude that undergrowth bamboos and their litter affect tree seedling regeneration differently from mixed-tree litter, causing changes in plant community composition and species diversity. Increased human disturbance, causing a shift in dominance structure of these forests, may result in a concomitant reduction in their overall woody species abundance, richness and diversity. Thus, management of bamboos by controlling their distribution in areas of high bamboo density can be an important forest restoration method.

Population dependence in the interactions with neighbors for pollination: A field experiment with Taraxacum officinale.

PREMISE OF THE STUDY. The fitness of plants depends on their immediate biotic and abiotic environmental surroundings. The floral neighborhood of individual plants is part of this immediate environment and affects the frequency and behavior of their pollinators. However, the interactions among plants for pollination might differ among populations because populations differ in floral densities and pollinator assemblages. Despite that, manipulative experiments of the floral neighborhood in different populations with a specific focus on pollinator behavior are still rare. METHODS. We introduced mixtures of two species (Salvia farinacae and Tagetes bonanza) in two populations of Taraxacum officinale and examined their effect on pollinators' foraging behavior on Taraxacum. KEY RESULTS. The effects of the heterospecific neighborhood differed among pollinator groups and between the two populations. Only honeybees consistently preferred both the most diverse (containing three species) and completely pure patches of Taraxacum in both populations. We found a strong and positive effect of patch diversity on visitation to Taraxacum in one population, whereas in the other population either no effect or a negative effect of plant diversity occurred, which we attribute to differences between populations in the ratio of pollinators to inflorescences. Pollinator visitation consistently increased with local Taraxacum density in both populations. CONCLUSIONS. Our study shows that a similar local neighborhood can differentially affect the frequency and foraging behavior of pollinators, even in closely situated populations. Experimental studies conducted in several populations would contribute to determine which factors drive the variation in pollination interactions among populations.

Relationships between the floral neighborhood and individual pollen limitation in two self-incompatible herbs.

Local flower density can affect pollen limitation and plant reproductive success through changes in pollinator visitation and availability of compatible pollen. Many studies have investigated the relationship between conspecific density and pollen limitation among populations, but less is known about within-population relationships and the effect of heterospecific flower density. In addition, few studies have explicitly assessed how the spatial scales at which flowers are monitored affect relationships. We investigated the effect of floral neighborhood on pollen limitation at four spatial scales in the self-incompatible herbs Armeria maritima spp. maritima and Ranunculus acris spp. acris. Moreover, we measured pollen deposition in Armeria and pollinator visits to Ranunculus. There was substantial variation in pollen limitation among Armeria individuals, and 25% of this variation was explained by the density of compatible and heterospecific flowers within a 3 m circle. Deposition of compatible pollen was affected by the density of compatible and incompatible inflorescences within a 0.5 m circle, and deposition of heterospecific pollen was affected by the density of heterospecific flowers within a 2 m circle. In Ranunculus, the number of pollinator visits was affected by both conspecific and heterospecific flower densities. This did not, however, result in effects of the floral neighborhood on pollen limitation, probably due to an absence of pollen limitation at the population level. Our study shows that considerable variation in pollen limitation may occur among individuals of a population, and that this variation is partly explained by floral neighborhood density. Such individual-based measures provide an important link between pollen limitation theory, which predicts ecological and evolutionary causes and consequences for individual plants, and studies of the effects of landscape fragmentation on plant species persistence. Our study also highlights the importance of considering multiple spatial scales to understand the spatial extent of pollination processes within a population.

How does climate warming affect plant-pollinator interactions?

Climate warming affects the phenology, local abundance and large-scale distribution of plants and pollinators. Despite this, there is still limited knowledge of how elevated temperatures affect plant-pollinator mutualisms and how changed availability of mutualistic partners influences the persistence of interacting species. Here we review the evidence of climate warming effects on plants and pollinators and discuss how their interactions may be affected by increased temperatures. The onset of flowering in plants and first appearance dates of pollinators in several cases appear to advance linearly in response to recent temperature increases. Phenological responses to climate warming may therefore occur at parallel magnitudes in plants and pollinators, although considerable variation in responses across species should be expected. Despite the overall similarities in responses, a few studies have shown that climate warming may generate temporal mismatches among the mutualistic partners. Mismatches in pollination interactions are still rarely explored and their demographic consequences are largely unknown. Studies on multi-species plant-pollinator assemblages indicate that the overall structure of pollination networks probably are robust against perturbations caused by climate warming. We suggest potential ways of studying warming-caused mismatches and their consequences for plant-pollinator interactions, and highlight the strengths and limitations of such approaches.