Seed survival on experimental dishes in a central European old-growth mixed-species forest - effects of predator guilds, tree masting and small mammal population dynamics

Offspring may succeed well next to their relatives, but it needs particular traits

BACKGROUND AND AIMS

There is ongoing debate about whether offspring perform best next to phylogenetically distantly related adult neighbours (due to the scarcity of enemies and competitors) or next to closely related adults (due to the abundance of mutualists). Here we hypothesize that relatedness of adult neighbours affects which traits confer performance rather than performance itself.

METHODS

We studied seed removal, seed germination and sapling growth in sessile oaks (Quercus petraea and hybrids) and how they depend on size, shape and other traits, under both closely and distantly related canopies, manipulating offspring density, presence of insects and fungi, and spatial proximity to oaks.

KEY RESULTS

We found that phylogenetic distance of adult neighbours affects only little the performance of offspring but strongly affects which traits confer performance on offspring, in particular the size and shape of seeds and saplings. Differences in trait-performance relationships mostly disappear once insects or conspecific competitors are excluded (albeit exclusion of fungi reinforced these differences). Effects of phylogenetic distance of neighbours were not replaceable by the percentage of gymnosperms among neighbours or by the environmental conditions considered.

CONCLUSIONS

We suggest that, by responding to a biotic micro-mosaic of selection pressures, sessile oak flexibly succeeds in diverse neighbourhoods. Sessile oak might maintain the potential for both convergence with and divergence from phylogenetically distantly related species, thereby reinforcing or eroding phylogenetic signal of niches.

Tracking individual seed fate confirms mainly antagonistic interactions between rodents and European beech

Food-hoarding granivores act as both predators and dispersers of plant seeds, resulting in facultative species interactions along a mutualism-antagonism continuum. The position along this continuum is determined by the positive and negative interactions that vary with the ratio between seed availability and animal abundance, particularly for mast-seeding species with interannual variation and spatial synchrony of seed production. Empirical data on the entire fate of seeds up to germination and the influence of rodents on seed survival is rare, resulting in a lack of consensus on their position along the mutualism-antagonism continuum. Here, we quantified annual seed rain and rodent abundance in an old-growth European beech forest and tracked 639 beechnuts to the seedling stage with 84% of seeds successfully located. Over 4 study years that covered the range of seed-to-rodent ratios, not a single seed successfully germinated after dispersal, illustrating a predominantly antagonistic interaction between rodents and seeds of European beech. Therefore, our findings do not support the predator dispersal hypothesis and partially contradict the predator satiation hypothesis, as the highest number of germinants and intact seeds were found in situ after an intermediate seed crop, not a bumper crop. Our results underline the necessity to track seeds up to germination.

High Frequency of Apodemus Mice Boosts Inverse Activity Pattern of Bank Voles, Clethrionomys glareolus, through Non-Aggressive Intraguild Competition

Sympatric animals with similar requirements can separate their ecological niches along the microhabitat, food and time axes. There may be alternative reasons for an interspecific different activity pattern, such as intraspecific social constraints, predator avoidance or physical conditions such as temperature, precipitation and illumination. We investigated the importance of intraguild competition in a 2-year study in an inner-alpine mixed forest, using small forest rodents as our model species. Apodemus mice were the physically superior, and bank voles, Clethrionomys glareolus, the inferior competitor. We predicted that bank voles would exhibit increased diurnal activity when frequencies of the almost exclusively nocturnal Apodemus mice were high during a seed mast year. To investigate this, we recorded 19,138 1 min videos. Controlling for confounding variables, bank vole diurnal activity was significantly related to the frequency of Apodemus mice. We assume that at high densities of Apodemus mice, a purely nocturnal separation in the niche dimensions of time, habitat and microhabitat is no longer sufficient, and therefore an inverse activity pattern by the bank voles is reinforced. Our videos showed, however, that this does not require persistent aggressive meetings and we explain this by the long co-evolution of the taxa under study.

Seed and seedling predation by vertebrates mediates the effects of adult trees in two temperate tree species

Specialised natural enemies can locally suppress seeds and seedlings near conspecific adults more than far from them. Whilst this is thought to facilitate species coexistence, the relative contribution of multiple enemies to whether heterospecific seeds and seedlings rather than conspecifics perform better beneath a particular adult species remains less clear, especially in regions with spatially extensive monodominant stands. We designed a field exclusion experiment to separate the effects of fungi, insects and vertebrates on the seedling establishment and early survival of two temperate tree species, Fagus sylvatica and Picea abies, in the adult tree monocultures of these species. Our experiment demonstrates the key role of vertebrates in mediating the effects of adult trees on seeds and seedlings. Due to vertebrates and partly insects, Fagus sylvatica seedlings survived worse beneath conspecific than heterospecific adults and were also outperformed by Picea abies seedlings beneath their own adults. Picea abies seedling establishment was higher beneath conspecific than heterospecific adults, but Fagus sylvatica seedlings outperformed them beneath their own adults. The impact of enemies on Picea abies establishment beneath conspecific adults was less clear. Fungi did not influence seedling establishment and survival. Our findings highlight the need to compare enemy impacts on each seedling species beneath conspecific and heterospecific adults with their impacts on conspecific and heterospecific seedlings beneath a particular adult species. Such evaluations can shed more light on the role of enemies in tree communities by identifying the plant-enemy interactions that facilitate species coexistence and those that promote species monodominance.

The ecology and evolution of synchronized reproduction in long-lived plants

Populations of many long-lived plants exhibit spatially synchronized seed production that varies extensively over time, so that seed production in some years is much higher than on average, while in others, it is much lower or absent. This phenomenon termed masting or mast seeding has important consequences for plant reproductive success, ecosystem dynamics and plant-human interactions. Inspired by recent advances in the field, this special issue presents a series of articles that advance the current understanding of the ecology and evolution of masting. To provide a broad overview, we reflect on the state-of-the-art of masting research in terms of underlying proximate mechanisms, ontogeny, adaptations, phylogeny and applications to conservation. While the mechanistic drivers and fitness consequences of masting have received most attention, the evolutionary history, ontogenetic trajectory and applications to plant-human interactions are poorly understood. With increased availability of long-term datasets across broader geographical and taxonomic scales, as well as advances in molecular approaches, we expect that many mysteries of masting will be solved soon. The increased understanding of this global phenomenon will provide the foundation for predictive modelling of seed crops, which will improve our ability to manage forests and agricultural fruit and nut crops in the Anthropocene. This article is part of the theme issue 'The ecology and evolution of synchronized seed production in plants'.

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.

Seeds and seedlings of oaks suffer from mammals and molluscs close to phylogenetically isolated, old adults

BACKGROUND AND AIMS

Mammals and molluscs (MaM) are abundant herbivores of tree seeds and seedlings, but how the trees and their environment affect MaM herbivory has been little studied. MaM tend to move much larger distances during the feeding stage than the more frequently studied insect herbivores. We hypothesize that MaM (1) select and stay within the patches that promise to be relatively the richest in seeds and seedlings, i.e. patches around adult trees that are old and within a distantly related, less productive neighborhood; and (2) try to remain sheltered from predators while foraging, i.e. mammals remain close to adult trees or to cover by herbs while foraging, and might force their mollusc prey to show the opposite distribution.

METHODS

We exposed oak acorns and seedlings in a temperate forest along transects from adult conspecifics in different neighbourhoods. We followed acorn removal and leaf herbivory. We used exclusion experiments to separate acorn removal by ungulates vs. rodents and leaf herbivory by insects vs. molluscs. We measured the size of the closest conspecific adult tree, its phylogenetic isolation from the neighbourhood and the herbaceous ground cover.

KEY RESULTS

Consistent with our hypothesis, rodents removed seeds around adult trees surrounded by phylogenetically distant trees and by a dense herb cover. Molluscs grazed seedlings surrounding large conspecific adults and where herb cover is scarce. Contrary to our hypothesis, the impact of MaM did not change from 1 to 5 m distance from adult trees.

CONCLUSIONS

We suggest that foraging decisions of MaM repulse seedlings from old adults, and mediate the negative effects of herbaceous vegetation on tree recruitment. Also, an increase in mammalian seed predation might prevent trees from establishing in the niches of phylogenetically distantly related species, contrary to what is known from insect enemies.

Substantial understory contribution to the C sink of a European temperate mountain forest landscape

CONTEXT

The contribution of forest understory to the temperate forest carbon sink is not well known, increasing the uncertainty in C cycling feedbacks on global climate as estimated by Earth System Models.

OBJECTIVES

We aimed at quantifying the effect of woody and non-woody understory vegetation on net ecosystem production (NEP) for a forested area of 158 km2 in the European Alps.

METHODS

We simulated C dynamics for the period 2000-2014, characterized by above-average temperatures, windstorms and a subsequent bark beetle outbreak for the area, using the regional ecosystem model LandscapeDNDC.

RESULTS

In the entire study area, woody and non-woody understory vegetation caused between 16 and 37% higher regional NEP as compared to a bare soil scenario over the 15-year period. The mean annual contribution of the understory to NEP was in the same order of magnitude as the average annual European (EU-25) forest C sink. After wind and bark beetle disturbances, the understory effect was more pronounced, leading to an increase in NEP between 35 and 67% compared to simulations not taking into account these components.

CONCLUSIONS

Our findings strongly support the importance of processes related to the understory in the context of the climate change mitigation potential of temperate forest ecosystems. The expected increases in stand replacing disturbances due to climate change call for a better representation of understory vegetation dynamics and its effect on the ecosystem C balance in regional assessments and Earth System Models.

The contrasting effects of short-term climate change on the early recruitment of tree species

Predictions of plant responses to climate change are frequently based on organisms' presence in warmer locations, which are then assumed to reflect future performance in cooler areas. However, as plant life stages may be affected differently by environmental changes, there is little empirical evidence that this approach provides reliable estimates of short-term responses to global warming. Under this premise, we analyzed 8 years of early recruitment data, seed production and seedling establishment and survival, collected for two tree species at two latitudes. We quantified recruitment to a wide range of environmental conditions, temperature, soil moisture and light, and simulated recruitment under two forecasted climatic scenarios. Annual demographic transitions were affected by the particular conditions taking place during their onset, but the effects of similar environmental shifts differed among the recruitment stages; seed production was higher in warmer years, while seedling establishment and survival peaked during cold years. Within a species, these effects also varied between latitudes; increasing temperatures at the southern location will have stronger detrimental effects on recruitment than similar changes at the northern locations. Our simulations illustrate that warmer temperatures may increase seed production, but they will have a negative effect on establishment and survival. When the three early recruitment processes were simultaneously considered, simulations showed little change in recruitment dynamics at the northern site and a slight decrease at the southern site. It is only when we considered these three stages that we were able to assess likely changes in early recruitment under the predicted conditions.

Remote Cameras Reveal Experimental Artifact in a Study of Seed Predation in a Semi-Arid Shrubland

Granivorous animals may prefer to predate or cache seed of certain plant species over others. Multiple studies have documented preference for larger, non-native seed by granivores. To accomplish this, researchers have traditionally used indirect inference by relating patterns of seed removal to the species composition of the granivorous animal community. To measure seed removal, researchers present seed to granivorous animals in the field using equipment intended to exclude certain animal taxa while permitting access to others. This approach allows researchers to differentiate patterns of seed removal among various taxa (e.g., birds, small mammals, and insects); however, it is unclear whether the animals of interest are freely using the exclusion devices, which may be a hindrance to discovering the seed dishes. We used video observation to perform a study of seed predation using a custom-built, infrared digital camera and recording system. We presented native and non-native seed mixtures in partitioned Petri dishes both within and outside of exclusion cages. The exclusion cages were intended to allow entrance by rodent taxa while preventing entrance by rabbits and birds. We documented all seed removal visits by granivorous animals, which we identified to the genus level. Genera exhibited varying seed removal patterns based on seed type (native vs. non-native) and dish type (open vs. enclosed). We documented avoidance of the enclosed dishes by all but one rodent taxa, even though these dishes were intended to be used freely by rodents. This suggests that preference for non-native seed occurs differentially among granivorous animals in this system; however, interpretation of these nuanced results would be difficult without the benefit of video observation. When feasible, video observation should accompany studies using in situ equipment to ensure incorrect assumptions do not lead to inappropriate interpretation of results.