Masting Events Research Articles

Reconstructing 120 years of climate change impacts on Joshua tree flowering.

Quantifying how global change impacts wild populations remains challenging, especially for species poorly represented by systematic datasets. Here, we infer climate change effects on masting by Joshua trees (Yucca brevifolia and Y. jaegeriana), keystone perennials of the Mojave Desert, from 15 years of crowdsourced observations. We annotated phenophase in 10,212 geo-referenced images of Joshua trees on the iNaturalist crowdsourcing platform, and used them to train machine learning models predicting flowering from annual weather records. Hindcasting to 1900 with a trained model successfully recovers flowering events in independent historical records and reveals a slightly rising frequency of conditions supporting flowering since the early 20th Century. This reflects increased variation in annual precipitation, which drives masting events in wet years-but also increasing temperatures and drought stress, which may have net negative impacts on recruitment. Our findings reaffirm the value of crowdsourcing for understanding climate change impacts on biodiversity.

Resource availability alters breeding strategies in a small mammal community.

Following a resource pulse, animals may finance reproduction by consuming concurrently available resources (income breeding) or by storing resources for future reproduction (capital breeding). Understanding how these reproductive strategies are used is important for determining the ecological mechanisms that structure timing of reproduction and that drive interannual population fluctuations in animals. We gathered a reproductive dataset for five small mammal species over a 12-year period in Northeastern USA during which six masting events of American beech (Fagus grandifolia) and eastern hemlock (Tsuga canadensis) occurred. Masting created alternate years where seeds were either available late (masting year) or early (cached from the previous year) in the breeding season. The small mammal species differed in reliance on seeds and overwintering strategies. We quantified the diet using stable isotopes and recorded reproduction timing, proportion breeding and litter size in females and testes size in males. Timing of seed availability minimally affected litter size but strongly affected proportion breeding and timing of reproduction. During masting years (late seed availability), a higher proportion of females reproduced, with breeding taking place later in the season (lactation timed with peak seed availability), although the delay was restricted in Napaeozapus insignis, an obligate hibernator. After a fall mast, cached seeds were used as capital in the following spring (early seed availability) to support a litter that, depending on the species, occurred 24-79 days sooner than a mast year. No late-season reproduction occurred in years with early seed availability except for Myodes gapperi which produced a second litter, likely financed by fungal consumption. Males also showed strong responses to seed availability, mirroring female reproduction with testes size staying constant in years with late seed availability and sharply decreasing over the breeding season in years with early seed availability. Our results highlight that although photoperiod and temperature broadly set bounds of the breeding season in temperate environments, resource availability influences the reproductive strategies that species use, which in turn alters reproductive timing and can drive large inter-annual population fluctuations. Differences in overwintering strategies and diet may further modulate reproductive timing and output relative to resource pulses.

Geographic variation in growth and reproduction trade‐offs: Implications for future tree performance

AbstractForests play a crucial role providing ecosystem services to humans, yet many aspects of forest dynamics remain unknown. One key area is how climate change might impact reproduction of tree species. While most studies have focused on predicting tree growth, understanding how reproduction may change will be vital to forecasting future forest communities. Of particular interest is the relationship between annual growth and reproductive output, which has often been hypothesized as a trade‐off between allocating resources to growth or to reproduction. Two proposed pathways of this trade‐off, resource accumulation, that is, storage of resources over time, and resource allocation, that is, same year allocation of resources to reproduction, have been widely explored in relation to masting events. It has also been proposed that there is no internal trade‐off between the two functions, but rather there exists one or more climate variables that are intrinsically linked to both, that is, the weather hypothesis. In this study, we use 15 years of dendrochronological data and seed rain collections from forest stands at two latitudes to determine whether one or more of these strategies are taking place in two commonly occurring tree species: red maple, Acer rubrum; and sugar maple, Acer saccharum. We found evidence of a trade‐off in both species. We also found a combination of strategies was the norm, and there appeared to be evidence to also support the weather hypothesis. However, in both species, the strategy which dictated the trade‐off switched between the northern and southern regions, indicating a degree of plasticity that could be beneficial under changing environmental conditions. By identifying the ways in which growth and reproduction are connected and how these connections vary between different populations, we can gain insights into how trees allocate resources in response to changing conditions.

Boom and bust: the effects of masting on seed predator range dynamics and trophic cascades

AbstractAimSpatiotemporal variation in resource availability is a strong driver of animal distributions. In the northern hardwood and boreal forests of the northeastern United States, tree mast events provide resource pulses that drive the population dynamics of small mammals, including the American red squirrel (Tamiasciurus hudsonicus), a primary songbird nest predator. This study sought to determine whether mast availability ameliorates their abiotic limits, enabling red squirrel elevational distributions to temporarily expand and negatively impact high‐elevation songbirds.LocationNortheastern United States.MethodsWe used two independent datasets to evaluate our hypotheses. First, we fit a dynamic occupancy model using data from camera trap surveys to evaluate red squirrel distributional responses to pulses in the tree mast. We also assessed population responses using systematic auditory surveys analysed with an open‐population binomial mixture model. Further, we used modelled red squirrel abundance in nest‐survival models to evaluate whether their abundance is correlated with the daily nest survival of three songbird species.ResultsThe tree mast provided a critical resource pulse that resulted in a two‐fold increase in the annual elevational distribution of red squirrels. The elevational distribution of red squirrels ranged from a minimum of ~450 m (range: 663–1145 m asl) following two consecutive years without a masting event to a maximum of over 1000 m (range: 443–1545 m asl) after a large mast event. The daily nest survival of three songbird species tended to decline with an increase in the abundance of red squirrels.Main ConclusionsTree mast is a central biological phenomenon in many temperate and boreal forests. This study reveals how this resource pulse results in range changes in a small mammal that is both a seed and bird predator, as well as prey for many carnivores. Thus, understanding this phenomenon can inform the conservation and management of northern forests, including breeding songbirds.

Nonlinear dynamics of multiple Alfvén modes driven by trapped energetic ions in tokamaks: A triplet paradigm

Non-linear dynamics of multiple infernal Alfvén eigenmodes—a subset of global Alfvén eigenmodes in tokamak plasmas with extended low-shear central core [Marchenko et al., Phys. Plasmas 16, 092502 (2009)]—is studied. The analysis is carried out for a mode triplet with toroidal mode-numbers n=1, 2, 3. It was assumed that the n = 1 mode was linearly unstable due to precession resonance with trapped fast ions, whereas the other modes were linearly damped. The modes were coupled due to a non-linearity in a bounce-averaged drift kinetic equation for the distribution function of fast ions. Nonlinear equations for the mode amplitudes and phases are derived and solved numerically. It is found that the temporal evolution of the amplitudes and the phase (responsible for the frequency chirping) of the modes exhibit Hopf bifurcations to stable limit cycles. This can explain a synchronous cyclic destabilization of multiple modes in Alfvén avalanches (sudden growth of amplitudes of the mode cluster with different n and approximately equal frequency spacing) in NSTX and bursting modes in MAST—events, which resulted in enhanced loss of fast ions.

Through eagle eyes—the potential of satellite-derived LAI time series to estimate masting events and tree-ring width of European beech

Masting is a well-known phenomenon in forest science in which trees recurrently produce exceptional high seed amounts throughout their lifespan. Occurring simultaneously across wide regions, masting may impact carbon sequestration and carbon cycling of forest ecosystems. To identify masting-induced changes in tree canopies, remote sensing methods may offer a suitable tool, though so far only few studies have used RS methods to investigate the relationship between masting and tree-ring width (TRW). Therefore, we employed a regional network of 19 European beech sites along a precipitation gradient in north-eastern Germany, providing time series of TRW (1961–2017) and categorical masting data (1992–2020). Combined with leaf area index (LAI) from the medium-resolution satellite sensors SPOT-VGT/PROBA-V and MODIS, we assessed the relationship between all three parameters by correlation analysis. Furthermore, we identified common climatic drivers and spatial masting patterns along gradients of water availability. We found negative relationships for both TRW and LAI with masting and a positive correlation for TRW with LAI for SPOT-VGT/PROBA-V data. Contrary, MODIS LAI provided no significant relationships. Water availability did not impact masting occurrence or intensity. The climate correlations showed that masting and TRW are commonly driven by summer climate in June and July, while we found no correlations between climate and LAI. We conclude that medium-resolved SPOT-VGT/PROBA-V LAI is suitable to observe masting but not to estimate TRW. Nonetheless, the results should be thoroughly interpreted in the context of uncertainties of RS measurements and complex interactions of different environmental drivers.

Density‐dependent reproductive success among sympatric dipterocarps during a major mast fruiting event

AbstractAlthough masting in Southeast Asia is characterized by the interspecific synchronization of reproduction, little is known about the variation in regeneration strategies among sympatric tree species during major masting events. Herein, we examined the hypothesis that non‐abundant species achieve greater seed survival per seed production at the pre‐ and post‐dispersal stages by synchronizing reproduction with abundant species. During the masting event from May to September 2010, we installed seed traps in a primary forest plot of the Deramakot Forest Reserve, Borneo. To identify the possible causes of post‐dispersal seed mortality, we conducted a seed‐sowing experiment from September 2010 to July 2011 in a primary forest plot with high community‐level dipterocarp seed density and two surrounding secondary forests with low seed densities. An abundant species (Shorea multiflora) produced more seeds than other species and exhibited a lower survival rate during the pre‐dispersal stage. The ratio of aborted seeds was greater in species with greater seed production, while the ratio of seeds predated by insects was not, suggesting that resource limitations and/or greater inbreeding frequency may explain inefficient seed production. Interspecific variation was rarely observed for post‐dispersal seed survival rates. Our study highlights the density‐dependent variation in reproductive success between abundant and non‐abundant dipterocarp species at the pre‐dispersal stage. Non‐abundant species achieved greater reproductive success by synchronizing reproduction with the masting of abundant species, which might be an important mechanism for sustainability of rare species populations.

Toward a novel multi-century archive of tree mast using pollen from lake sediments

Mast seeding – the pronounced inter-annual variability and synchrony in seed production – can have profound local ecological impacts. We explore the potential for generating long (centuries to millennia) proxy records of tree mast seeding, from pollen deposited in lake sediments. This proxy record uses pollen recovered from annually-banded (varved) sediments from a core collected from Lake Ohau, South Island, New Zealand to reconstruct masting events for two genera of Southern Beech, Fuscospora spp. and Lophozonia spp. We find average mast frequencies inferred from Fuscospora pollen from a 43-year sediment core collected from Lake Ohau comparable to seed fall expected for the catchment using a differential-temperature (ΔT) statistical model for the period 1974–2016. In contrast, Lophozonia pollen mast frequency in the 1974–2016 timeseries was consistently lower than that predicted by the ΔT model, although the patterns of variability were broadly similar. We explore this approach in a second 32-year pollen timeseries from Lake Ohau, spanning the pre-instrumental period 1833–1864. During this interval, average air temperature was ~1°C cooler than the late 20th century, and interannual variability of air temperature was subdued, such that mast frequency predicted by the Δ T model is the lowest in 200 years. We find mast frequency in our pollen records reflects this pattern for Fuscospora, with a minima of mast frequency from 1850, compared to the 1974–2016 record, but not for Lophozonia. This paper demonstrates that a centuries-long pollen record from the Lake Ohau sedimentary sequence has the potential to form a valuable proxy for Fuscospora masting that would supplement existing seedfall records. Long records of this type could significantly enhance our understanding of the environmental drivers of mast seeding.

Demographic consequences of heterogeneity in conspecific density dependence among mast-fruiting tropical trees.

The role of conspecific density dependence (CDD) in the maintenance of species richness is a central focus of tropical forest ecology. However, tests of CDD often ignore the integrated effects of CDD over multiple life stages and their long-term impacts on population demography. We combined a 10-year time series of seed production, seedling recruitment and sapling and tree demography of three dominant Southeast Asian tree species that adopt a mast-fruiting phenology. We used these data to construct individual-based models that examine the effects of CDD on population growth rates (λ) across life-history stages. Recruitment was driven by positive CDD for all species, supporting the predator satiation hypothesis, while negative CDD affected seedling and sapling growth of two species, significantly reducing λ. This negative CDD on juvenile growth overshadowed the positive CDD of recruitment, suggesting the cumulative effects of CDD during seedling and sapling development has greater importance than the positive CDD during infrequent masting events. Overall, CDD varied among positive, neutral and negative effects across life-history stages for all species, suggesting that assessments of CDD on transitions between just two stages (e.g. seeds seedlings or juveniles mature trees) probably misrepresent the importance of CDD on population growth and stability.

Multi-centennial phase-locking between reproduction of a South American conifer and large-scale drivers of climate.

Climate forcings determine the episodic occurrence of local climate anomalies that trigger the occurrence of masting events (massive, synchronized and intermittent seed production by perennial plants). This suggests some kind of phase-locking of the reproductive cycles of individual plants to the climatological cycle, thus further reinforcing reproductive synchrony and the Moran effect. We propose a dendrochronological approach to filter out the long-term direct effects of climate on tree radial growth and temporal reproductive effort by sex by using actual trees as climatic controls to reconstruct masting events in Araucaria araucana, a long-lived dioecious masting conifer. In this way, we developed a multi-century-long tree masting reconstruction for South America using female-male radial growth determined by differences in timing and magnitude of the reproductive effort between sexes. We provide evidence for a regional synchronizing mechanism of masting which is drought induced by strong cold La Niña phases of El Niño/Southern Oscillation (ENSO) amplified by the positive phases of the Southern Annular Mode (SAM) that activate both female and male cone bud formation during year -2 before seed fall; that is, a long-term phase-locking between the ENSO cycle and the reproductive cycle modulated by the strength of SAM. In addition, our regional index of masting frequency showed its maximum during the late twentieth century relative to the previous centuries, suggesting that the species is currently at its maximum masting frequency concurrent with a period of enhanced temperature and drought conditions in Patagonia, probably driven by the positive phase of the SAM.

ForeMast: an R package for predicting beech (Fagus sylvatica L.) masting events in European countries

Forecasting annual seed production will improve the management of forests across Europe. The foreMast R package we developed predicts current year masting probability in beech (Fagus sylvatica L.) using climate data easily accessible by any stakeholder. Modelling and predicting forest masting is one of the most challenging tasks in forest management, as it is a strategy shared by several species, very important for tree dispersion and forest regeneration, mainly related to climate and ecological processes. As many studies focus on European beech (Fagus sylvatica L.) masting without simple practical implementations, we developed a tool capable of predicting beech masting years. The tool is an R package (foreMast) made by three functions, which relies mainly on climate data. The algorithm performance is compared with the records of the MASTREE database, which gather several beech seed production series for various sites across European countries. Overall, the results show a tight correlation with the compared sites (ρ = 0.50 to 0.61, p-value < 0.0001, respectively), especially when temperatures weigh three times more than precipitation. Nevertheless, in some sites, seed production seems to be more related to precipitation dynamics than to temperatures. foreMast can be used both for studying changes in mast events in relation to climate changes and in operative forest management and planning. It is flexible and thus amenable to future implementation of additional predicting variables or target species.

Masting effect on canopy greenness and climate response on seed production of Fagus grandifolia subsp. mexicana across the Sierra Madre Oriental, Mexico

Specific proximate mechanisms as climate spatial fluctuations modify the pattern of synchronic reproduction (masting) or seed production at population level in the Fagus genus worldwide. However, phenological processes as mass flowering, beechnut development and seed rain have attracted attention to understand the masting behavior in relict-endangered tree species. The normalized difference vegetation index (NDVI), temperature and precipitation could act as proxy signals resembling the onset of mast-seeding events. We studied a masting event in 2017 in ten relict-endangered Mexican beech forests (Fagus grandifolia subsp. mexicana) in eastern Mexican montane cloud forests. Our aims were 1) test if NDVI shows a particular pattern before and after masting event in 2017 at 10 sites; 2) explore how monthly temperature and precipitation change two years before and during the masting across the Mexican beech forests; and 3) assess how the beechnut amount and quality was affected by spatial and temporal specific climatic variables in the recorded masting event among the beech forests. We found that the masting was related to the canopy greenness variation (2015–2018) in the beech forests. Nut production variability among beech forests was related to temperature and precipitation fluctuations. We found that maximum and mean temperature and high precipitation during May to August were decisive to trigger the synchronic Mexican beech masting spatial behavior.

Mast seeding promotes evolution of scatter-hoarding.

Many plant species worldwide are dispersed by scatter-hoarding granivores: animals that hide seeds in numerous, small caches for future consumption. Yet, the evolution of scatter-hoarding is difficult to explain because undefended caches are at high risk of pilferage. Previous models have attempted to solve this problem by giving cache owners large advantages in cache recovery, by kin selection, or by introducing reciprocal pilferage of 'shared' seed resources. However, the role of environmental variability has been so far overlooked in this context. One important form of such variability is masting, which is displayed by many plant species dispersed by scatterhoarders. We use a mathematical model to investigate the influence of masting on the evolution of scatter-hoarding. The model accounts for periodically varying annual seed fall, caching and pilfering behaviour, and the demography of scatterhoarders. The parameter values are based mostly on research on European beech (Fagus sylvatica) and yellow-necked mice (Apodemus flavicollis). Starvation of scatterhoarders between mast years decreases the population density that enters masting events, which leads to reduced seed pilferage. Satiation of scatterhoarders during mast events lowers the reproductive cost of caching (i.e. the cost of caching for the future rather than using seeds for current reproduction). These reductions promote the evolution of scatter-hoarding behaviour especially when interannual variation in seed fall and the period between masting events are large. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

Assessing EDR and a novel deer repellent for reducing by-kill of white-tailed deer (Odocoileus virginianus), during aerial 1080 operations

ABSTRACT A 2014 carcass search survey confirmed that Tiakina Ngā Manu aerial 1080 operations, undertaken to protect resident mohua (Mohoua ochrocephala) from increases to predator populations following substantial beech (Nothofagus sp.) mast events, cause mortality for Wakatipu white-tailed deer. Non-target by-kill of deer generates significant public opposition to predator control operations. We assessed if a current deer repellent (EDR) and a repellent in development (Pestex-DR) were effective at repelling captive white-tailed deer from consumption of nontoxic baits, commonly used in Tiakina Ngā Manu operations. Both repellents were found to significantly decrease bait consumption, and we observed clear displays of aversion to repellent baits by captive deer. Ten GPS collared wild white-tailed deer were then monitored following a Tiakina Ngā Manu operation using EDR in 2016. One collared deer was poisoned following the operation, confirming that EDR is not 100% effective. We recommend the most effective available deer repellent be used for future Tiakina Ngā Manu operations over the Wakatipu white-tailed deer habitat to minimise public opposition to these predator control operations. In addition, our results suggest that differences in the boundary size and shape of operations may account for some of the variability in non-target by-kill observed between studies.

Differential spatial responses of rodents to masting on forest sites with differing disturbance history.

Mast seeding, the synchronized interannual variation in seed production of trees, is a well‐known bottom‐up driver for population densities of granivorous forest rodents. Such demographic effects also affect habitat preferences of the animals: After large seed production events, reduced habitat selectivity can lead to spillover from forest patches into adjacent alpine meadows or clear‐cuts, as has been reported for human‐impacted forests. In unmanaged, primeval forests, however, gaps created by natural disturbances are typical elements, yet it is unclear whether the same spillover dynamics occur under natural conditions. To determine whether annual variation in seed production drives spillover effects in naturally formed gaps, we used 14 years of small mammal trapping data combined with seed trap data to estimate population densities of Apodemus spp. mice and bank voles (Myodes glareolus) on 5 forest sites with differing disturbance history. The study sites, located in a forest dominated by European beech (Fagus sylvatica), Norway spruce (Picea abies), and silver fir (Abies alba), consisted of two primeval forest sites with small canopy gaps, two sites with larger gaps (after an avalanche event and a windthrow event), and a managed forest stand with closed canopy as a control. Hierarchical Bayesian N‐mixture models revealed a strong influence of seed rain on small rodent abundance, which were site‐specific for M. glareolus but not for Apodemus spp. Following years of moderate or low seed crop, M. glareolus avoided open habitat patches but colonized those habitats in large numbers after full mast events, suggesting that spillover events also occur in unmanaged forests, but not in all small rodents. The species‐ and site‐specific characteristics of local density responding to food availability have potentially long‐lasting effects on forest gap regeneration dynamics and should be addressed in future studies.

Tree hazards compounded by successive climate extremes after masting in a small endemic tree, Distylium lepidotum, on subtropical islands in Japan.

Ongoing global warming increases the frequency and severity of tropical typhoons and prolonged drought, leading to forest degradation. Simultaneous and/or successive masting events and climatic extremes may thus occur frequently in the near future. If these climatic extremes occur immediately after mass seed reproduction, their effects on individual trees are expected to be very severe because mass reproduction decreases carbohydrate reserves. While the effects of either a single climate extreme or masting alone on tree resilience/growth have received past research attention, understanding the cumulative effects of such multiple events remains challenging and is crucial for predicting future forest changes. Here, we report tree hazards compound by two successive climate extremes, a tropical typhoon and prolonged drought, after mass reproduction in an endemic tree species (Distylium lepidotum Nakai) on oceanic islands. Across individual trees, the starch stored within the sapwood of branchlets significantly decreased with reproductive efforts (fruit mass/shoot mass ratio). Typhoon damage significantly decreased not only the total leaf area of apical shoots but also the maximum photosynthetic rates. During the 5‐month period after the typhoon, the mortality of large branchlets (8–10‐mm diameter) increased with decreasing stored starch when the typhoon hit. During the prolonged summer drought in the next year, the recovery of total leaf area, stored starch, and hydraulic conductivity was negatively correlated with the stored starch at the typhoon. These data indicate that the level of stored starch within branchlets is the driving factor determining tree regrowth or dieback, and the restoration of carbohydrates after mass reproduction is synergistically delayed by such climate extremes. Stored carbohydrates are the major cumulative factor affecting individual tree resilience, resulting in their historical effects. Because of highly variable carbohydrate levels among individual trees, the resultant impacts of such successive events on forest dieback will be fundamentally different among trees.

Cycling and retention of nitrogen in European beech (&amp;lt;i&amp;gt;Fagus sylvatica&amp;lt;/i&amp;gt; L.) ecosystems under elevated fructification frequency

Abstract. Atmospheric deposition of nitrogen (N) has exceeded its demand for plant increment in forest ecosystems in Germany. High N inputs increased plant growth, the internal N cycling within the ecosystem, the retention of N in soil and plant compartments, and the N output by seepage water. But the processes involved are not fully understood, notably the effect of fructification in European beech (Fagus sylvatica L.) on N fluxes. The frequency of fructification has increased together with air temperature and N deposition, but its impact on N fluxes and the sequestration of carbon (C) and N in soils have been hardly studied. A field experiment using 15N-labeled leaf litter exchange was carried out over a 5.5-year period at seven long-term European beech (Fagus sylvatica L.) monitoring sites to study the impact of current mast frequency on N cycling. Mean annual leaf litterfall contained 35 kg N ha−1, but about one-half of that was recovered in the soil 5.5 years after the establishment of the leaf litter 15N exchange experiment. In these forests, fructification occurred commonly at intervals of 5 to 10 years, which has now changed to every 2 years as observed during this study period. Seed cupules contributed 51 % to the additional litterfall in mast years, which creates a high nutrient demand during their decomposition due to the very high ratios of C to N and C to phosphorus (P). Retention of leaf litter 15N in the soil was more closely related to the production of total litterfall than to the leaf litterfall, indicating the role of seed cupules in the amount of leaf N retained in the soil. Higher mast frequency increased the mass of mean annual litterfall by about 0.5 Mg ha−1 and of litterfall N by 8.7 kg ha−1. Mean net primary production (NPP) increased by about 4 %. Mean total N retention in soils calculated by input and output fluxes was unrelated to total litterfall, indicating that mast events were not the primary factor controlling total N retention in soils. Despite reduced N deposition since the 1990s, about 5.7 out of 20.7 kg N ha−1 deposited annually between 1994 and 2008 was retained in soils, notably at acid sites with high N/P and C/P ratios in the organic layers and mineral soils, indicating P limitation for litter decomposition. Trees retained twice as much N compared to soils by biomass increment, particularly in less acidic stands where the mineral soils had low C/N ratios. These results have major implications for our understanding of the C and N cycling and N retention in forest ecosystems. In particular the role of mast products in N retention needs more research in the future.

Masting by beech trees predicts the risk of Lyme disease

BackgroundThe incidence of Lyme borreliosis and other tick-borne diseases is increasing in Europe and North America. There is currently much interest in identifying the ecological factors that determine the density of infected ticks as this variable determines the risk of Lyme borreliosis to vertebrate hosts, including humans. Lyme borreliosis is caused by the bacterium Borrelia burgdorferi sensu lato (s.l.) and in western Europe, the hard tick Ixodes ricinus is the most important vector.MethodsOver a 15-year period (2004–2018), we monitored the monthly abundance of I. ricinus ticks (nymphs and adults) and their B. burgdorferi s.l. infection status at four different elevations on a mountain in western Switzerland. We collected climate variables in the field and from nearby weather stations. We obtained data on beech tree seed production (masting) from the literature, as the abundance of Ixodes nymphs can increase dramatically 2 years after a masting event. We used generalized linear mixed effects models and AIC-based model selection to identify the ecological factors that influence inter-annual variation in the nymphal infection prevalence (NIP) and the density of infected nymphs (DIN).ResultsWe found that the NIP decreased by 78% over the study period. Inter-annual variation in the NIP was explained by the mean precipitation in the present year, and the duration that the DNA extraction was stored in the freezer prior to pathogen detection. The DIN decreased over the study period at all four elevation sites, and the decrease was significant at the top elevation. Inter-annual variation in the DIN was best explained by elevation site, year, beech tree masting index 2 years prior and the mean relative humidity in the present year. This is the first study in Europe to demonstrate that seed production by deciduous trees influences the density of nymphs infected with B. burgdorferi s.l. and hence the risk of Lyme borreliosis.ConclusionsPublic health officials in Europe should be aware that masting by deciduous trees is an important predictor of the risk of Lyme borreliosis.Graphical

Molecular control of the floral transition in the mast seeding plant Celmisia lyallii (Asteraceae).

Mast flowering (or masting) is synchronous, highly variable flowering among years in populations of perennial plants. Despite having widespread consequences for seed consumers, endangered fauna and human health, masting is hard to predict. While observational studies show links to various weather patterns in different plant species, the mechanism(s) underpinning the regulation of masting is still not fully explained. We studied floral induction in Celmisia lyallii (Asteraceae), a mast flowering herbaceous alpine perennial, comparing gene expression in flowering and nonflowering plants. We performed translocation experiments to induce the floral transition in C. lyallii plants followed by both global and targeted expression analysis of flowering-pathway genes. Differential expression analysis showed elevated expression of ClSOC1 and ClmiR172 (promoters of flowering) in leaves of plants that subsequently flowered, in contrast to elevated expression of ClAFT and ClTOE1 (repressors of flowering) in leaves of plants that did not flower. The warm summer conditions that promoted flowering led to differential regulation of age and hormonal pathway genes, including ClmiR172 and ClGA20ox2, known to repress the expression of floral repressors and permit flowering. Upregulated expression of epigenetic modifiers of floral promoters also suggests that plants may maintain a novel "summer memory" across years to induce flowering. These results provide a basic mechanistic understanding of floral induction in masting plants and evidence of their ability to imprint various environmental cues to synchronize flowering, allowing us to better predict masting events under climate change.

Blacklegged tick population synchrony between oak forest and non‐oak forest

1. Fluctuations in abundance of blacklegged ticks in space and time are well‐documented, but the extent to which populations fluctuate synchronously across habitat types is poorly understood. In oak forests, blacklegged tick density depends on small mammal abundance, which is in turn driven by fluctuations in acorn production. It is currently unknown whether fluctuations in tick abundance in oak forest, long understood to depend largely on masting events, are shared with nearby non‐oak forest.2. In this study, we analysed 22 years of tick population data from nine forest plots in south‐eastern New York in order to compare fluctuations of nymphal and larval blacklegged tick populations in oak‐dominant forests and non‐oak forests.3. We found that population peak densities of nymphal ticks were strongly synchronous in oak and non‐oak forests among years and that larval population dynamics were weakly synchronous between these two forest types.4. Our results suggest that drivers of immature tick density in oak‐dominant forest, including climatic factors and mast‐driven host dynamics, may also influence tick population fluctuations in the surrounding landscape.