Sexual reproduction of clonal dwarf shrubs in a forest–tundra ecotone

Successful sexual reproduction may be more important for regeneration of clonal species in high-latitude and -altitude areas than has been previously suggested. We investigated the potential of Vaccinium myrtillus, V. vitis-idaea and Empetrum nigrum ssp. hermaphroditum (E. hermaphroditum) for sexual reproduction at three sites in the forest–tundra ecotone in Finnish Lapland. We studied whether the potential differs between plant communities, whether disturbance enhances germination, and whether seedling emergence is limited by seed availability. We established a field experiment with disturbance and sowing treatments, and monitored seed and seedling numbers and survival rates for two years. The number of mature seeds of V. myrtillus was higher in plants from the tundra heath than in those from the coniferous and mountain birch forests. The number of mature seeds and seedlings emerging from the seed bank of E. hermaphroditum tended also to be higher in the tundra heath. Disturbance marginally increased the seedling emergence of V. myrtillus and E. hermaphroditum, whereas sowing generally increased the seedling numbers. The seedling number of V. myrtillus was lower in the tundra heath and that of E. hermaphroditum was lower in the coniferous forest than at the other sites. Seedling survival was equal for all plant species at all sites. We conclude that the capacity for sexual reproduction varies among plant communities, and seed availability is a stronger constraint than microsite availability for the studied species. Once the crucial early phase of seedling establishment is overcome, seedling survival enables successful recruitment of V. myrtillus, V. vitis-idaea and E. hermaphroditum in the subarctic area.     

Tree spatial patterns and environmental relationships in the forest–alpine tundra ecotone at Niwot Ridge,Colorado, USA

Forest–alpine tundra ecotones (FTEs) are dynamic transition zones between forest and alpine tundra ecosystems that play an important role in regulating ecological processes, which are in turn directly influenced by the spatial patterns of trees and environmental constraints such as topography and climate. Our objectives were to characterize the spatial patterns of tree species and size classes, determine whether spatial patterns of trees differed among three FTE types, and examine FTE- and tree-environmental relationships in our study area on Niwot Ridge, CO, USA. Overall, spatial aggregation was more extensive for seedlings than saplings or trees. Distributions were largely random in limber pine but were highly aggregated in Engelmann spruce and especially subalpine fir, reflecting these species’ relative shade tolerance and expected sequence of establishment following disturbance. Fragmented and patchy tree distributions were observed in the FTE with the most heterogeneous topography, characterized by high relief and associated physical disturbances. The least patchy distributions were associated with the FTE containing a relative absence of disturbance. Intermediate levels of tree aggregation were associated with low topographic relief and presence of meadows and wetlands. Our results emphasize the importance of spatial structure as an initial controlling factor of vegetation pattern in FTEs occurring in the same landscape.     

The effects of reindeer grazing on the composition and species richness of vegetation in forest–tundra ecotone

We conducted an 8-year exclosure experiment (1999–2006) in a forest–tundra ecotonal area in northwestern Finnish Lapland to study the effects of reindeer grazing on vegetation in habitats of variable productivity and microhabitat structure. The experimental sites included tundra heath, frost heath and riparian habitats, and the two latter habitats were characterized by hummock-hollow ground forms. The total cover of vegetation, cover of willow (Salix spp.), dwarf birch (Betula nana), dwarf shrubs, forbs and grasses (Poaceae spp.) increased in exclosures in all habitats. The increase in the total cover of vegetation and in the covers of willow and dwarf birch tended to be greatest in the least productive tundra heath. Opposing to the increase in the dominant vascular plant groups, the cover and species number of bryophytes decreased in exclosures. We conclude that the effects of reindeer grazing on vegetation composition depend on environmental heterogeneity and the responses vary among plant groups. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Soil nutrient’s spatial variability in forest–tundra ecotones on the Kola Peninsula, Russia

This article addresses spatial variability in soil nutrients in altitudinal and latitudinal forest–tundra ecotones in the Kola Peninsula, Russia. Higher total carbon, nitrogen, and bio-available nutrients were found in the organic horizons of latitudinal ecotone against the background of lower nutrients in soil-forming rock. This is supposedly related to higher contribution of nutrient-rich plants in latitudinal ecotone and intense downward within-profile matter migration in the altitudinal ecotone. Elevated bio-available soil nutrients in spruce forests compared with birch forests and tundra sites, and in birch forests compared with tundra against the background of different trends in the soil-forming rock were attributed to the effects of predominant plants. The effects of predominant plants on soil nutrients were distinctly pronounced at the level of site patches. Soil-forming rock effects on soil nutrients were clear at the level of zones/belts and the whole ecotones. Strong negative correlations between the soil nutrients and altitudes were explained by replacement of vascular plants by low-ash lichens at higher elevations.     

Sensitivity of Bolivian seasonally-dry tropical forest to precipitation and temperature changes over glacial–interglacial timescales

We used fossil pollen to investigate the response of the eastern Chiquitano seasonally-dry tropical forest (SDTF), lowland Bolivia, to high-amplitude climate change associated with glacial–interglacial cycles. Changes in the structure, composition and diversity of the past vegetation are compared with palaeoclimate data previously reconstructed from the same record, and these results shed light on the biogeographic history of today’s highly disjunct blocks of SDTF across South America. We demonstrate that lower glacial temperatures limited tropical forest in the Chiquitanía region, and suggest that SDTF was absent or restricted at latitudes below 17°S, the proposed location of the majority of the hypothesized ‘Pleistocene dry forest arc’ (PDFA). At 19500 yrs b.p., warming supported the establishment of a floristically-distinct SDTF, which showed little change throughout the glacial–Holocene transition, despite a shift to significantly wetter conditions beginning ca. 12500–12200 yrs b.p. Anadenanthera colubrina, a key SDTF taxon, arrived at 10000 yrs b.p., which coincides with the onset of drought associated with an extended dry season. Lasting until 3000 yrs b.p., Holocene drought caused a floristic shift to more drought-tolerant taxa and a reduction in α-diversity (shown by declining palynological richness), but closed-canopy forest was maintained throughout. In contrast to the PDFA, the modern distribution of SDTF most likely represents the greatest spatial coverage of these forests in southern South America since glacial times. We find that temperature is a key climatic control upon the distribution of lowland South American SDTF over glacial-interglacial timescales, and seasonality of rainfall exerts a strong control on their floristic composition.     

The forest–alpine ecotone: a multi-scale approach to spatial and temporal dynamics of treeline change at Niwot Ridge

Background: Current understanding of treeline or forest-alpine ecotone (FAE) dynamics does not fully explain past and present FAE patterns and their underlying processes, nor allow prediction of their response to climate change.

Aims: We address the overarching hypothesis that the FAE is a mosaic of distinct landscape units of vegetation and landforms that result in differential responses to climate change. We focus on climate-related, landscape and vegetation characteristics, but also consider the effect of landscape heterogeneity on biogeochemistry and overall resilience of the FAE to climate change.

Results: There are three distinct FAE land units at Niwot Ridge, generated by different interactions of climate with vegetation, landforms and topography. Within these FAEs, a process of self-organisation takes place from organism to patch to landscape scales, and is modulated by positive and negative feedback loops along an elevation gradient. The underlying controls cannot be attributed solely to temperature, but to a combination of interactions along a physical/biotic gradient.

Conclusions: FAE dynamics result from interactions among mechanisms and processes at the microsite, patch and landscape scales: (1) tree persistence; (2) forest patch establishment; (3) drivers of patch forest configurations and (4) resilience, increasing along a gradient of biotic control.     

Do elevations in temperature,CO2, and nutrient availability modify belowground carbon gain and root morphology in artificially defoliated silver birch seedlings?

Climate warming increases the risk of insect defoliation in boreal forests. Losses in photosynthetically active surfaces cause reduction in net primary productivity and often compromise carbon reserves of trees. The concurrent effects of climate change and removal of foliage on root growth responses and carbohydrate dynamics are poorly understood, especially in tree seedlings. We investigated if exposures to different combinations of elevated temperature, CO₂, and nutrient availability modify belowground carbon gain and root morphology in artificially defoliated 1‐year‐old silver birches (Betula pendula). We quantified nonstructural carbohydrates (insoluble starch as a storage compound; soluble sucrose, fructose, and glucose) singly and in combination in fine roots of plants under winter dormancy. Also the total mass, fine root proportion, water content, and length of roots were defined. We hypothesized that the measured properties are lower in defoliated birch seedlings that grow with ample resources than with scarce resources. On average, fertilization markedly decreased both the proportion and the carbohydrate concentrations of fine roots in all seedlings, whereas the effect of fertilization on root water content and dry mass was the opposite. However, defoliation mitigated the effect of fertilization on the root water content, as well as on the proportion of fine roots and their carbohydrate concentrations by reversing the outcomes. Elevation in temperature decreased and elevation in CO₂ increased the absolute contents of total nonstructural carbohydrates, whereas fertilization alleviated both these effects. Also the root length and mass increased by CO₂ elevation. This confirms that surplus carbon in birch tissues is used as a substrate for storage compounds and for cell wall synthesis. To conclude, our results indicate that some, but not all elements of climate change alter belowground carbon gain and root morphology in defoliated silver birch seedlings.     

Cactus height increases the modularity of a plant–frugivore network in the Caatinga dry forest

Cacti fruits are key resources to many frugivorous animals in Neotropical arid and semiarid regions. However, most studies have focused on a particular animal group or cacti species, but few have explored the overall interactions of such species at the community level. Here we monitored frugivory on five cacti species using camera traps that sampled diurnal and nocturnal interactions. We investigated the structure of interactions with bird, mammal, and reptile frugivores in the Brazilian Caatinga dry forest. We hypothesized that the height of cacti limit interactions with different types of frugivores, which would result in highly structured and modular interaction networks. In 2929 camera-days, we recorded 23 vertebrate species feeding on cacti fruits, including seven new records, all determined to be primary seed dispersers. As predicted, the cacti-frugivore network was modular and non-nested, with the two shortest cacti species grouped in a module dominated by interactions with reptiles and non-flying mammals. The tallest cacti species were dominated by frugivory interactions with birds and had comparatively less interaction diversity than shorter cacti species. Our results support the contention that cacti are keystone species in semiarid ecosystems where they produce small-seeded fleshy fruits year-round.     

Seed–seedling conflict in conifers as a result of plant–plant interactions at the forest-tundra ecotone

Background: As the climate warms, plant interactions between shrubs and conifer seedlings may affect migration of boreal trees into alpine areas; however these interactions have not been widely tested across conifer life stages.

Aims: Determine the role of shading by Betula glandulosa shrubs on early Picea mariana recruitment in the forest-tundra ecotone of the Mealy Mountains, Labrador (Canada).

Methods: Four shrub treatments were established with varying degrees of shade (unaltered, trimmed, cleared, shaded). Emergence and growth of P. mariana in treatment plots were compared to control plots without shrubs.

Results: Seedling emergence was facilitated; seedlings beneath shrubs emerged at greater levels (16% ± 5.1%) than in the control (0.5% ± 0.3%); however, evidence of facilitation post-emergence was weak, likely due to stronger interactions with seedbed species at this stage. Vertical growth of young seedlings was greatest in control and shaded plots, but needle production significantly decreased with increased shade suggesting that seedlings may compete with shrubs for light early in life.

Conclusions: Although facilitation has been previously implicated as an important mechanism in stressful environments, our study indicates that the early life stages of some conifers at the tree line are not strongly facilitated by shrubs as has been previously suggested for adults.     

Mycorrhizal colonizations of ferns and lycophytes on the island of La Réunion in relation to nutrient availability

Very little is known about the degree and ecological correlates of mycorrhization of lycophytes and ferns, although mycorrhizae are believed to be crucial for the majority of land plants. We screened the degree of mycorrhizal colonization for all 75 fern species recorded in nine survey plots across a wide range of soil conditions on the island of La Réunion, including five plots on lava flows of different age. Overall, 66% of all samples had mycorrhizae, but ground-dwelling species had significantly higher colonization levels (78%) than epiphytes (58%). Among ground-dwelling species, 98% of the individuals belonged to species with mycorrhizae, which was significantly higher than at the species level. Most samples had glomeromycetes as fungal partners, but septate endophytes were found in 41% of the colonized samples. While we found no significant relationship between the percentage of fern species with mycorrhizae per plot and seven ecological parameters, the percentage of fern individuals with mycorrhizal colonizations per plot significantly increased on shallow soils with high pH values and high base-saturation. This supports the idea that mycorrhizal colonizations confer an ecological advantage to colonized individuals, and that this advantage is more pronounced on nutrient-deficient sites. Our study thus provides evidence for an ecological advantage of mycorrhizae for ferns, but raises the question why, despite this advantage, species level mycorrhization among ferns (68%) is so much lower than the average for land plants (85%).     

A trophic cascade in a macrophyte-based food web at the land–water ecotone

Trophic cascades may purportedly be more common in aquatic than terrestrial food webs, but herbivory on freshwater vascular plants has historically been considered low. Water lilies are an exception, suffering severe grazing damage by leaf beetles. To test whether a central prediction of cascade models—that predator effects propagate downwards to plants—operates in a macrophyte-based food web, we experimentally manipulated predation pressure on a key herbivore of water lilies in the littoral zone of a lake in Michigan, USA. Field experiments comprised combinations of caging treatments to alter the number of predators (larvae of the ladybird beetle Coleomegilla maculata) that hunt the grazers of the macrophytes (larvae of the leaf beetles Galerucella nymphaeae) on the leaves of the water lily Nuphar advena. Predatory larvae of the ladybird beetles significantly reduced grazing damage to water-lily leaves by 35–43%. The predators reduced plant damage chiefly via density-mediated effects, when lower densities of grazers translated to significant declines in plant damage. Plant damage caused by the surviving herbivores was less than predicted from individual grazing rates under predator-free conditions. This suggests that trait-mediated effects may possibly also operate in this cascade. The observed strong effect of predators on a non-adjacent trophic level concurs with an essential component of the trophic cascade model, and the cascade occurred at the ecotone between aquatic and terrestrial habitats: Nuphar is an aquatic macrophyte with emergent and floating leaves, whereas both beetle species are semi-terrestrial and use the dry, emergent and floating leaves of the water lily as habitat. Also, the cascade is underpinned by freshwater macrophytes—a group for which trophic processes have often been underappreciated in the past.     

Influence of submerged macrophytes,temperature, and nutrient loading on the development of redox potential around the sediment–water interface in lakes

Redox potential is a significant factor in aquatic systems to regulate the availability of nutrients and some metals. To assess the driving variables regulating redox potential, background parameters (dissolved oxygen, pH, temperature, chlorophyll-a, soluble reactive and total phosphorus content of water, coverage and height of submerged macrophytes) and redox potential profiles around the sediment–water interface (SWI) were measured in simulated shallow lake ecosystems. There were two nutrient regimes (enriched and non-enriched) and three temperature scenarios (unheated; +3.5°C; +5°C) installed in the experimental setups, which were constructed to study the effects of global climate change. Temperature did not have any detectable effect on redox potentials, and we presume that nutrient addition had only indirect positive effects through triggering phytoplankton dominance which causes macrophyte absence. When submerged macrophytes were present in high density (80–100% coverage), redox potentials at the SWI varied between 60–215 mV and the mean redox potential was 133 ± 34 mV (mean ± 1 SD). In contrast to this, when phytoplankton dominance was coupled to low macrophyte density (0–20% coverage), the range of redox potentials at the SWI was 160–290 mV and the mean redox potential was 218 ± 34 mV. The results revealed the primary importance of submersed macrophytes; macrophyte coverage determined alone the redox potential of the sediment–water interface by 81%. This study suggests that possible positive effects of macrophytes on redox potential can be suppressed by their negative effects in case of 80–100% coverage and total inhabitation of the water column.     

Airborne pollen characteristics and the influence of temperature and precipitation in Raleigh,North Carolina,USA (1999–2012)

The incidence of allergic diseases has been increasing in recent decades, in part due to increased exposure to aeroallergens, particularly pollen. Allergic diseases have a major burden on the health care system, with annual costs in the USA alone exceeding $30 billion. There is evidence that the production of aeroallergens, including pollen, is increasing in response to environmental and climatic change, which has important implications for the treatment of allergy sufferers. In this study, pollen data from a Rotorod sampler in Raleigh, North Carolina, was used to characterize and examine trends in the atmospheric pollen seasons for trees, grasses, and weeds over the period 1999–2012. The influence of mean monthly antecedent and concurrent temperature and precipitation on the timing, duration, and severity of the pollen seasons was assessed using Pearson’s product-moment correlation coefficients and multiple linear regression models. An increasing trend was noted in seasonal tree pollen concentrations, while seasonal and peak weed pollen concentrations declined over time. The atmospheric pollen seasons for grasses and weeds trended toward earlier start dates and longer durations, while the tree pollen season trended toward an earlier end date. Peak daily tree pollen concentrations were strongly associated with antecedent temperature and precipitation, while peak daily grass pollen concentrations were strongly associated with concurrent precipitation. The strongest relationships between climate and weed pollen were associated with the timing and duration of the pollen season, with drier antecedent and warmer concurrent conditions tied to longer weed pollen seasons.     

Northward migrating trees establish in treefall gaps at the northern limit of the temperate–boreal ecotone, Ontario, Canada

Climate change is expected to promote migration of species. In ecotones, areas of ecological tension, disturbances may provide opportunities for some migrating species to establish in otherwise competitive environments. The size of and time since disturbance may determine the establishment ability of these species. We investigated gap dynamics of an old-growth red pine (Pinus resinosa Sol. ex Aiton) forest in the Great Lakes–St. Lawrence forest in northern Ontario, Canada, a transition zone between temperate and boreal forest. We investigated the effects of gaps of different sizes and ages on tree species abundance and basal area. Our results show that tree species from the temperate forest further south, such as red maple (Acer rubrum L.), red oak (Quercus rubra L.), and white pine (Pinus strobus L.), establish more often in large, old gaps; however, tree species that have more northern distributions, such as black spruce (Picea mariana Mill.), paper birch (Betula papyrifera Marsh.), and red pine show no difference in establishment ability with gap size or age. These differences in composition could not be attributed to autogenic succession. We conclude that treefall gaps in this forest facilitate the establishment of northward migrating species, potentially providing a pathway for future forest migration in response to recent changes in climate.     

Performance of established native seedlings in relation to invasive Lantana camara, rainfall and species’ habitat preferences in a seasonally dry tropical forest

Native species’ response to the presence of invasive species is context specific. This response cannot be studied in isolation from the prevailing environmental stresses in invaded habitats such as seasonal drought. We investigated the combined effects of an invasive shrub Lantana camara L. (lantana), seasonal rainfall and species’ microsite preferences on the growth and survival of 1,105 naturally established seedlings of native trees and shrubs in a seasonally dry tropical forest. Individuals were followed from April 2008 to February 2010, and growth and survival measured in relation to lantana density, seasonality of rainfall and species characteristics in a 50-ha permanent forest plot located in Mudumalai, southern India. We used a mixed effects modelling approach to examine seedling growth and generalized linear models to examine seedling survival. The overall relative height growth rate of established seedlings was found to be very low irrespective of the presence or absence of dense lantana. 22-month growth rate of dry forest species was lower under dense lantana while moist forest species were not affected by the presence of lantana thickets. 4-month growth rates of all species increased with increasing inter-census rainfall. Community results may be influenced by responses of the most abundant species, Catunaregam spinosa, whose growth rates were always lower under dense lantana. Overall seedling survival was high, increased with increasing rainfall and was higher for species with dry forest preference than for species with moist forest preference. The high survival rates of naturally established seedlings combined with their basal sprouting ability in this forest could enable the persistence of woody species in the face of invasive species.     

Effect of air temperature on forecasting the start of the Betula pollen season at two contrasting sites in the south of Europe (1995–2001)

In order to survive periods of adverse cold climatic conditions, plant requirements are satisfied by means of physiological adaptations to prevent cells from freezing. Thus, the growth of woody plants in temperate regions slows down and they enter into a physiological state called dormancy. In order to identify the chilling and heat requirements to overcome the dormancy period of Betula in the south of Europe, a comparative study was carried out with aerobiological pollen data of a 7-year (1995-2001) period in Vigo (Spain) and Perugia (Italy). To satisfy chilling requirements, base temperatures of 7 degrees C and 5.75 degrees C showed a lower standard variation coefficient: 3.94% and 2.36% in Perugia and Vigo respectively. In the case of heat accumulation, the sum of mean temperatures in Perugia and the sum of maximum temperatures in Vigo were the parameters that showed a minor coefficient of variation (11.13% and 14.51% respectively).     

Plant composition and diversity at edges in a semi-natural forest–grassland mosaic

Plant Ecology - As key components of landscapes, edges have received considerable scientific attention in anthropogenic ecosystems. However, edges in natural and semi-natural forest–grassland...     

Does rapid utilization of elevated nutrient availability allow eucalypts to dominate in the tropical savannas of Australia?

Northern Australia's savannas are among the most fire‐prone biomes on Earth and are dominated by eucalypts (Eucalyptus and Corymbia spp.). It is not clear what processes allow this group to dominate under such extreme fire frequencies and whether a superior ability to compete for nutrients and water might play a role. There is evidence that eucalypts are adapted to frequent fires; juvenile eucalypts escape the fire trap by growing rapidly in height between fires. However, non‐eucalypts are less able to escape the fire trap and tend to have stand structures strongly skewed toward suppressed juveniles. The mechanisms that drive these contrasting fire responses are not well understood. Here, we describe the results of a controlled glasshouse seedling experiment that evaluated the relative importance of nutrient and water availability in determining height growth and biomass growth of two eucalypt and one noneucalypt tree species, common in northern Australian savannas. We demonstrate that growth of eucalypt seedlings is particularly responsive to nutrient addition. Eucalypt seedlings are able to rapidly utilize soil nutrients and accumulate biomass at a much greater rate than noneucalypt seedlings. We suggest that a seasonal spike in nutrient availability creates a nutrient‐rich microsite that allows eucalypt seedlings to rapidly gain height and biomass, increasing their likelihood of establishing successfully and reaching a fire‐resistant size. Our results extend our understanding of how eucalypts dominate northern Australian savannas under extremely high fire frequencies.