Rodent acorn selection in a Mediterranean oak landscape

Quercus suber, Quercus ilex and Quercus coccifera (Cork, Holm and Kermes oaks, respectively) are common evergreen oak species that coexist in the landscapes of the western part of the Mediterranean basin. Rodents are the main acorn predators and thus one of the main factors for understanding recruitment patterns in oaks. In this paper we analyse to what extent mice prefer acorns from one oak species over another in three oak species studied using acorn removal experiments and video tape recordings. Twenty labelled acorns from each of the three Quercus species (60 acorns) were placed in 40 cm×40 cm quadrats on each plot. Because selection might vary as a result of the vegetation context, we performed the trials in the five main vegetation types within the study area (four replicates in each vegetation type) in order to control for habitat influences on rodent acorn preferences (a total of 20 plots). The removal of 1,200 acorns occurred within 68 days. Mice removed 98.7% of the acorns. Q. ilex acorns were preferred over Q. suber and Q. coccifera in all vegetation types except in pine forest, where no acorn preferences were detected. Acorn removal rates differed with vegetation type, correlating positively with shrub cover. The distance at which acorns were displaced by rodents (mean =4.6 m±5.1 SD) did not differ between acorn species, but varied among vegetation types. Bigger acorns of Q. coccifera were selected only after Q. ilex and Q. suber acorns were depleted, while no size selection was detected for the latter two species. Thus, we conclude that rodents show preference for some oak acorns and that landscape context contributes significantly to rodent activities and decisions.     

Acorn crop size and pre-dispersal predation determine inter-specific differences in the recruitment of co-occurring oaks

The contribution of pre-dispersal seed predation to inter-specific differences in recruitment remains elusive. In species with no resistance mechanisms, differences in pre-dispersal predation may arise from differences in seed abundance (plant satiation) or in the ability of seeds to survive insect infestation (seed satiation). This study aimed to analyse the impact of pre-dispersal acorn predation by weevils in two co-occurring Mediterranean oaks (Quercus ilex and Quercus humilis) and to compare its relevance with other processes involved in recruitment. We monitored the patterns of acorn production and acorn infestation by weevils and we conducted experimental tests of acorn germination after weevil infestation, post-dispersal predation and seedling establishment in mixed forests. Monitoring and experimental data were integrated in a simulation model to test for the effects of pre-dispersal predation in recruitment. In both oaks pre-dispersal acorn infestation decreased with increasing acorn crop size (plant satiation). This benefited Q. ilex which exhibited stronger masting behaviour than Q. humilis, with almost a single and outstanding reproductive event in 6 years. Acorn infestation was more than twice as high in Q. humilis (47.0%) as in Q. ilex (20.0%) irrespective of the number of seeds produced by each species. Although germination of infested acorns (seed satiation) was higher in Q. humilis (60%) than in Q. ilex (21%), this could barely mitigate the higher infestation rate in the former species, to reduce seed loss. Conversely to pre-dispersal predation, no inter-specific differences were observed either in post-dispersal predation or seedling establishment. Our results indicate that pre-dispersal predation may contribute to differences in seed supply, and ultimately in recruitment, between co-existing oaks. Moreover, they suggest that seed satiation can barely offset differences in seed infestation rates. This serves as a warning against overemphasising seed satiation as a mechanism to overcome seed predation by insects. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of acorn masting on population dynamics of three forest-dwelling rodent species in Hokkaido,Japan

The effects of the abundance of acorns of the oak, Quercus crispula, on the population dynamics of three rodent species (Apodemus speciosus, A. argenteus, and Clethrionomys rufocanus) were analyzed using time series data (1992–2006). The data were obtained in a forest in northern Hokkaido, Japan, by live trapping rodents and directly counting acorns on the ground. Apodemus speciosus generally increased in abundance following acorn masting. However, the clear effect of acorn abundance was not detected for the other two rodent species. Acorns of Q. crispula contain tannins, which potentially have detrimental effects on herbivores. Apodemus speciosus may reduce the damage caused by acorn tannins with tannin-binding salivary proteins and tannase-producing bacteria, whereas such physiological tolerance to tannins is not known in the other two rodent species. The differences in the effects of acorns between the three species may be due to differences in their physiological tolerance to tannins.     

Edge effects in a tropical montane forest mosaic: experimental tests of post-dispersal acorn removal

Forest edges created by scattered-patch clear-cutting have become common in tropical montane cloud forests in the highlands of Chiapas, Mexico. It was hypothesised that forest edges may influence regeneration of oak species, which are canopy dominants in these forests, by affecting the activities of small mammal species. Acorns of different oak species varying in germination timing were offered to predators and/or dispersers at different positions along replicated forest edges during 2 consecutive years. We investigated the effects of (1) edge type (hard and soft), (2) distance from the edge (0, 15, 30, 45 and 60 m inside forest fragments) and (3) oak species, on the rate of acorn removal mainly by small mammals. During a non-masting year, acorn removal was affected by the interaction of edge type and distance from the edge (P<0.05), with acorn removal being highest near hard edges compared to adjacent forest interiors. As predicted, acorn removal was greater along soft (100%) than along hard edges (82%), but this pattern was recorded only during the non-masting year. This study partly supports previous studies of rodents preferentially consuming acorns with early germination rather than acorns exhibiting dormancy, however these patterns may change with variation in acorn abundance. These results suggest that patch clear-cutting affects regeneration processes within forest fragments by influencing the activities of small mammals, but the nature of this effect also depends on acorn abundance and the characteristics of the forest edge created.     

Re-evaluation of the relationship between rodent populations and acorn masting: a review from the aspect of nutrients and defensive chemicals in acorns

The responses of rodent populations to acorn masting were examined by reviewing 17 studies from the aspect of acorn nutrients and defensive chemicals. Oak species were grouped into three types based on their acorn nutritional characteristics by cluster analysis: Type 1 acorns (two North American red oaks, subgenus Erythrobalanus) were high in tannins and high in fat and proteins (and consequently rich in metabolizable energy); Type 2 acorns (two Japanese evergreen oaks, Cyclobalanopsis; three Japanese deciduous oaks, Lepidobalanus; one North American white oak, Lepidobalanus) were high in tannins but low in fat and proteins; and Type 3 acorns (one Cyclobalanopsis species; seven Lepidobalanus species) were low in tannins and had intermediate levels of fat and proteins. These three acorn groups were nutritionally, and thereby ecologically, not equivalent. Rodents, in general, responded differently to acorn masting depending on their feeding habits and the nutritional characteristics of acorns. Granivorous rodents showed positive responses to masting of Type 1 and 3 acorns, whereas rodents with feeding habits intermediate between granivory and herbivory showed positive responses to masting of Type 3 acorns. In addition, for herbivorous rodents, the responses to masting of any types of acorns have not been reported. The present findings emphasize that the relationship between rodents and acorn masting should not easily be generalized, because there are large variations in characteristics of both acorns and rodents. The viewpoint presented in this review could offer more convincing interpretations to the contradictory observations, found in the studies reviewed, on the response of rodent populations to acorn masting.     

Impacts of elevated nitrogen inputs on oak reproductive and seed ecology

The effects of increased anthropogenic inputs of reactive nitrogen (N) have been studied at the Harvard Forest Chronic N Experiment, where NH₄NO₃ has been applied experimentally since 1988 to increase atmospheric deposition rates ～6‐ and ～18‐fold above ambient. This paper asks whether conditions favorable to primary production also resulted in plastic increases to flower, fruit, and seedling traits, and focuses primarily on the oaks that dominate the hardwood stands of the Harvard Forest experiment. Litterfall samples collected between 1996 and 2001 revealed that flowers and fruits were significantly more abundant in N‐treated plots, and an analysis of oak tree abundance found significant variation both among and within plots. Acorn samples collected during 2003 and 2004 (a mast and a postmast year) were therefore analyzed using ancova models that included an estimate of oak tree abundance. This tree abundance estimate was the only significant driver of increased acorn production during the mast year, and in both years it was a significant factor on plots receiving the highest levels of N. In the postmast year, acorn production was increased in direct response to N‐related factors other than tree abundance. Our comparisons of control and N‐treated plots for acorn quality traits (e.g. rates of acorn damage, germination percentage, seedling growth) revealed negligible or only transient differences. Shifts in overall acorn abundance – particularly disproportionate N‐mediated increases during nonmast years – could have a wide range of ecological consequences beyond the more frequently examined impacts of N deposition on primary production and carbon sequestration.     

Wood thrush nest success and post-fledging survival across a temporal pulse of small mammal abundance in an oak forest

1. Synchronized mass production of seed crops, such as acorns, produces a resource pulse that may have far-reaching consequences for songbird populations through its effects on avian predators. Seed production in these forests represents only the first of several pulsed events. Secondary pulses emerge as mast-consuming rodents numerically respond to seed production and tertiary pulses emerge as generalist predators, such as raptors, numerically respond to rodents. In turn, these two groups reduce nest productivity and juvenile survivorship 1 and 2 years, respectively, after the initial pulse in seed production. 2. At our study site in south-eastern New York, USA, autumn acorn abundance (primary pulse) largely determines rodent abundance (secondary pulse) the following spring. We tested the hypotheses that the population dynamics of a shrub-nesting passerine (wood thrush Hylocichla mustelina), is influenced by rodents through the: (a) direct effect of predation by rodents; (b) indirect effect of rodents on the abundance of raptors (tertiary pulse); and (c) indirect effect of rodent abundance on raptor diet. The latter specifically hypothesizes that a crash in the rodent population in the wake of region-wide failure of acorn production leads to an extreme diet shift in raptors that increases post-fledging mortality in birds. 3. We conducted a 3-year study to examine variation in wood thrush nest success and fledgling survival, using radio telemetry, across a pulse of rodent abundance (i.e. low, medium and high). We also updated and reanalysed regional wood thrush population growth rates as a function of the annual variation in rodent abundance. 4. Fledgling survivorship, but not nest success, varied in relation to annual rodent abundance. Raptors and eastern chipmunks Tamias striatus were the most commonly identified predators on fledglings. Fledgling survivorship was greatest at intermediate rodent abundance consistent with a shift in raptor diet. Regional rate of wood thrush population growth showed a unimodal relationship with rodent abundance, peaking during years with intermediate rodent abundance. This unimodal pattern was due to wood thrush population growth rates near or below zero during rodent population crashes. 5. The telemetry study, pattern of regional abundance and synchronized population dynamics of coexisting thrushes suggest a common mechanism of behavioural changes in raptors in response to declines in rodent prey, which in turn affects thrush population dynamics.     

Main determinants of rodent population fluctuations in managed Central European temperate lowland forests

Whilst studies have shown that climatic (North Atlantic Oscillation (NAO)) and biotic (acorn production) factors influence rodent populations, mechanisms driving temporal and spatial fluctuation of rodent populations are understudied. This study evaluates relationships between the influence of environmental factors (biotic and abiotic) and phenotypic characteristics across two rodent feeding guilds (granivorous and non-granivorous species) represented by four species of rodents in Central Europe. We hypothesise that the relationship between acorn density and population growth rate are indirectly affected by climatic factors (winter NAO) and that these effects differ amongst herbivorous and granivorous species. In addition, we also tested whether effects of weather and competition on individual phenotype characteristic vary amongst mast and non-mast years. Rodent populations were estimated by catching individuals in snap traps during the growing season (from March to November) over a period of 9 years at three sites. The results of the generalised linear model provide evidence that acorn production best explained the population fluctuations. We therefore conclude that the between-year population fluctuations in rodent abundance were governed by density dependence and initiated primarily by acorn mast years. Auto-regressive models also revealed direct density dependence in combination with the direct effects of mast years. Therefore, strong intraspecific competition for food is likely in years following mast years. Our results also showed that abundance of non-granivorous species is mainly influenced by local weather conditions which could regulate food quality and abundance. On the other hand, population dynamics of granivorous species are caused directly by acorn density and indirectly by climatic condition influencing acorn production.     

Modeling flight activity and population dynamics of the pine engraver, Ips pini, in the Great Lakes region: effects of weather and predators over short time scales

Ascertaining the relative effects of factors such as weather and predation on population dynamics, and determining the time scales on which they operate, is important to our understanding of basic ecology and pest management. In this study, we sampled the pine engraver Ips pini (Say) (Coleoptera: Scolytidae) and its predominant predators Thanasimus dubius (F.) (Coleoptera: Cleridae) and Platysoma cylindrica (Paykull) (Coleoptera: Histeridae) in red pine plantations in Wisconsin, USA, over 2 years. We sampled both the prey and predators using flight traps baited with the synthetic aggregation pheromone of I. pini. Flight models were constructed using weather variables (temperature and precipitation), counts of bark beetles and their predators, and temporal variables to incorporate possible effects of seasonality. The number of I. pini per weekly collection period was temperature dependent and decreased with the number of predators, specifically T. dubius in 2001 and P. cylindrica in 2002. The number of predators captured each week was also weather dependent. The predators had similar seasonal phenologies, and the number of each predator species was positively correlated with the other. Including a term for the number of prey did not improve the model fits for either predator for either year. Our results suggest that exogenous weather factors strongly affect the flight activity of I. pini, but that its abundance is also affected by direct density-dependent processes acting over weekly time scales. Adult predation during both colonization and dispersal are likely processes yielding these dynamics.     

A glimpse at future forests: predicting the effects of <Emphasis Type="Italic">Phytophthora ramorum</Emphasis> on oak forests of southern Appalachia

The highly pathogenic Phytophthora ramorum, causal organism of sudden oak death (SOD), is established in forests of the Pacific Northwest (USA) and is threatening invasion of other regions. Given the breadth of its host range, with dozens of asymptomatic ornamental hosts and with oaks, Quercus spp., in the red oak (Erythrobalanus) subgenus particularly susceptible, we investigated the consequences of its invasion and establishment in oak-dominated deciduous forests of the eastern USA. We evaluated the nature and extent of pathogen invasion using vegetation assessments coupled with growth simulations. The woody plant community was assessed in three strata (upper, mid- and lower) and was used to characterize forest composition and structure. Using the Forest Vegetation Simulator (FVS), we then projected woody vegetation growth 50 years into the future with and without the effects of SOD. In forest simulations lacking pathogen invasion, little change in composition or structure is forecasted. Both red oaks and white oaks (subgenus Leucobalanus) increase slightly but significantly over the length of the simulation. In contrast, in SOD-affected forests our projections predict a significant loss of red oaks within 10 years of pathogen invasion. Basal area of white oaks and non-oaks is expected to increase more so in the absence of red oaks. The loss of red oaks to pathogen infection will result in greater increases in red maple, Acer rubrum, and yellow poplar, Liriodendron tulipifera, than in forests free of SOD. Loss of red oak represents a significant loss of hard mast, with potentially devastating consequences for wildlife. Red oak loss will also affect decomposition rates, nutrient cycling, forest structure, and timber values, with consequences for forest health and sustainability.     

Effects of acorn abundance on density dependence in a Japanese wood mouse (Apodemus speciosus) population

We analysed the effects of Quercus crispula acorn abundance on the density dependence of the large Japanese wood mouse Apodemus speciosus using time series data (1992–2007). The data were obtained in a forest in northern Hokkaido, Japan, by live-trapping rodents and directly counting acorns on the ground. Acorn abundance in one year clearly influenced the abundance of wood mice in the following year in all models examined based on the Gompertz and Ricker model; in addition, the abundance of wood mice had effects on the population. Acorn abundance influenced the strength of density dependence (intraspecific competition) of the wood mouse population. When the abundance of acorns was high, density dependence was relaxed, and as a result the equilibrium density at which the population growth rate decreased to zero became higher. Those effects of acorn abundance were regarded as a nonlinear perturbation effect (sensu Royama 1992). The nonlinearity of density dependence was also detected; higher densities had stronger effects on population growth rates. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Different population responses of three sympatric rodent species to acorn masting—the role of tannin tolerance

Rodent population dynamics are predicted to respond positively to the masting of acorns, but diverging results have been published. This study tested the hypothesis that population responses to acorn masting vary depending on differences in the tolerance to tannins of different rodent species. The effects of acorn abundance on the rodent population densities were analyzed using a dataset obtained in Hokkaido, Japan. Specifically, population fluctuations of three rodent species (Apodemus speciosus, A. argenteus, and Myodes rufocanus) and the abundance of Quercus crispula acorns have been monitored since 1992. Acorn production in previous years had a positive effect on the annual population growth rates of A. speciosus; however, this trend was not clear in the other two species. Tannin tolerance, assessed by body weight changes in an acorn feeding experiment, exhibited clear differences among the rodent species; namely, the body weight of A. speciosus increased, whereas that of the other two species decreased. The observed responses to acorn masting of populations of the three sympatric rodent species reflected tannin tolerance. It suggests that only populations of species with high tannin tolerance positively respond to acorn masting. Previous studies tend to overlook species-specific abilities of coping with tannins in acorns. Our results emphasize the necessity of evaluating the tannin tolerance of rodents and the tannin content of acorns to understand how rodent population dynamics and acorn production are related.     

Rodents change acorn dispersal behaviour in response to ungulate presence

Small rodents are prominent seed predators, but they also favour plant recruitment as seed dispersers. The direct interactions of ungulates on plants are more one‐sided and negative, as they mainly reduce plant recruitment through predation on seeds and seedlings. The effects of small rodents and ungulates on plant recruitment have been considered and studied as independent episodes within plant regeneration cycles. However, ungulate–rodent interactions and their potential effects on plant regeneration have not been considered so far. A number of studies have recently documented ungulate effects on the abundance, diversity and spatial distribution of small rodents. Here, we hypothesize that ungulates may also affect rodent seed dispersal behaviour. We monitored acorn dispersal by small rodents (Mus spretus and Apodemus sylvaticus) in oak woodlands with and without exclosures for large ungulates, mainly red deer, Cervus elaphus, and wild boar, Sus scrofa. The study was carried out in a typical Mediterranean Holm oak, Quercus ilex, forest throughout the acorn fall season in 2003 and 2004. We found that, in both years, the proportion of acorns cached and not recovered in the short‐term was, on average, lower in the presence (1.4%) than in the absence (19.9%) of ungulates. Acorn dispersal distances were not affected by ungulate presence in either year. However, ungulates had an effect on the spatial distribution of dispersed seeds; rodents apparently avoided shrubs as caching sites in both years. This result was interpreted as a behavioural response to reduce the risk of cache pilferage by conspecifics, which are closely associated with shrubs in presence, but not in absence, of ungulates. Potential effects of different densities of rodents or predators were discarded, as none of them differed between the areas with and without ungulates. The present study found significant interactions between heterospecific seed and seedling consumers that had been considered as independent episodes within tree regeneration cycles. As a result of such interactions, ungulates may have negative indirect effects on oak recruitment by reducing (1) acorn caching frequency, and (2) the proportion of acorns cached under shrubs, key nurse‐plants for the establishment of Holm oak seedlings in Mediterranean areas.     

Not only size matters: Acorn selection by the European jay (Garrulus glandarius)

A strong selection for acorn characteristics is expected to have evolved in the mutualistic relationship between the European jay (Garrulus glandarius) and the oak (Quercus spp.). Bossema's pioneer work suggested that jays do not select acorns randomly, but rather they preferentially select some size and species. Preference for some seeds over others may have implications on plant community dynamics by conferring advantages (or disadvantages) on the selected (avoided) seed characteristics. In this paper we test to what extent jays select acorns by species and/or by size and the relation between these two traits in Mediterranean oak species. The experiments consist of a set of field tests in which acorns from four different coexisting Mediterranean oak species (Quercus ilex, Quercus faginea, Quercus suber, and Quercus coccifera) were placed in artificial feeders accessible to wild jays. The acorns were previously measured to control individual acorn characteristics. Using video-recording techniques, we followed jay activity and the fate of each acorn (sequence of acorn selection and method of transport). Q. ilex acorns were preferred over other acorns, and Q. coccifera acorns were avoided when other acorns were available. Preference for Q. faginea and Q. suber acorns was intermediate, that is, they were preferred over Q. coccifera acorns but not over Q. ilex acorns. Large acorns were also preferred although acorn species selection was stronger than size selection. Jays selected species and size both by visual means and by using acorn area as an indicator of size. Acorns wider than 17–19 mm were carried in the bill because of throat limitation. Our results confirm Bossema's study on temperate oaks and extend it to Mediterranean oak species, revealing implications on mixed oak forest dynamics.     

Seed predation by mammals and forest dominance by Quercus oleoides,a tropical lowland oak

Summary Quercus oleoides Cham. and Schlecht is an unusual tree in several respects: it is an oak found in neotropical lowland forests, its distribution is not continuous but ratherdivided into many patches of various sizes, and it is a dominant in all the forests in which it occurs, attaining densities far higher than most species of tropical trees. This density pattern is related to the vulnerability of Q. oleoides acorns to predation by mammals. Observations of agoutis, deer, peccaries, squirrels, pocket mice and other seed consumers in Santa Rosa National Park, Costa Rica, showed that these mammals act only as predators, not dispersers, of Q. oleoides acorns. Experiments which involved placing acorns in deciduous forest where Q. oleoides does not occur, demonstrated that, due to high predation rates, the number of acorns produced by an isolated tree is far too low for adults to replace themselves.In oak forest, on the other hand, where the combined acorn crops of many oaks satiate the seed predators, acorn survivorship until germination is high enough to maintain the population. Furthermore, acorn survivorship in oak forest areas is inversely proportional to the apparent mammal density in those areas. Thus the pattern of forest dominance and patchy distribution is related to positively density-dependent acorn survivorship: where Q. oleoides is the forest dominant, it will survive, but if its density falls to the level typical of tropical trees, it will go locally extinct.     

Satiation of predispersal seed predators: the importance of considering both plant and seed levels

Plants can reduce the fitness costs of granivory by satiating seed predators. The most common satiation mechanism is the production of large crops, which ensures that a proportion of the seeds survive predation. Nevertheless, satiation of small granivores at the seed level may also exist. Larger seeds would satiate more efficiently, enhancing the probability of seed survival after having been attacked. However, a larger seed size could compromise the efficiency of satiation by means of large crops if there were a negative relationship between seed size and the number of seeds produced by an individual plant. We analyze both types of satiation in the interaction between the holm oak Quercus ilex and the chestnut weevil Curculio elephas. Both crop size and acorn size differed strongly in a sample of 32 trees. Larger crop sizes satiated weevils, and higher proportions of the seeds were not attacked as crop size increased. Larger seeds also satiated weevil larvae, as a larger acorn size increased the likelihood of embryo survival. Seedling size was strongly related to acorn size and was reduced by weevil attack, but seedlings coming from large weeviled acorns were still larger. The number and the size of the acorns produced by individual trees were negatively related. Larger proportions of the crop were infested in oaks producing less numerous crops of larger acorns. However, contrary to expectations, these trees did not satiate more effectively at the seed level either. Effective satiation by larger acorns was precluded by larger multi-infestation rates associated to smaller seed crops, in such a way that the proportion of attacked seeds that survived did not vary among trees with different acorn sizes. These results highlight the need of considering satiation by means of large crops and large seeds in studies of predispersal seed predation. Long-term monitoring on individual oaks will help to assess whether there is a trade-off between the number and the size of the acorns and, if it existed, how it could condition the fitness consequences of both types of satiation.     

Density Patterns of Piper Ant-Plants and Associated Arthropods: Top-Predator Trophic Cascades in a Terrestrial System?1

Top-predator (fourth-trophic-level) controlled trophic cascades are thought to be uncommon in terrestrial systems, but actual quantitative tests and comparisons of bottom-up and top-down forces in systems with more than three linear trophic levels are rare. Here, we describe the density patterns of the arthropod community associated with Piper ant-plants in Costa Rican wet forests. Consumers in this community comprise a complex, interacting web of herbivores, predaceous ants, and predators of ants. Although the hollow stems and petioles of the Piper plants provide some protection to resident ants from predation, specialized Dipoena spiders and Phyllobaenus beetles exploit Pheidole ants inhabiting Piper plants. We report abundance patterns of plants, ants and predators in four forests. These patterns of abundance are consistent with predictions of top-down cascades across four trophic levels when the top predators are effective (beetles). We discuss how top-down and bottom-up forces may interact in systems with less effective top predators (spiders).     

Acorn production prediction models for five common oak species of the eastern United States

Acorn production varies considerably among oak (Quercus) species, individual trees, years, and locations, which directly affects oak regeneration and populations of wildlife species that depend on acorns for food. Hard mast indices provide a relative ranking and basis for comparison of within- and between-year acorn crop size at a broad scale, but do not provide an estimate of actual acorn yield—the number of acorns that can potentially be produced on a given land area unit based on the species, number, and diameter at breast height (dbh) of oak trees present. We used 10 years of acorn production data from 475 oak trees to develop predictive models of potential average annual hard mast production by five common eastern oak species, based on tree diameter and estimated crown area. We found a weak (R² = 0.08–0.28) relationship between tree dbh and acorn production per unit crown area for most species. The relationship between tree dbh and acorn production per tree was stronger (R² = 0.33–0.57). However, this is because larger-dbh trees generally have larger crowns, not because they have a greater capacity to produce more acorns per unit crown area. Acorn production is highly variable among individual trees. We estimated that dbh of at least 60 dominant or codominant oak trees per species should be randomly sampled to obtain an adequate representation of the range of dbhs (≥12.7 cm dbh) in a given forest area, and achieve precise estimates when using these equations to predict potential acorn production. Our predictive models provide a tool for estimating potential acorn production that land managers and forest planners can apply to oak inventory data to tailor estimates of potential average annual acorn production to different forest management scenarios and multiple spatial scales. © 2011 The Wildlife Society.     

Environmental correlates of acorn production by four species of Minnesota oaks

We measured acorn production by four species of oaks in central Minnesota over a 17-year period with the goal of understanding the proximate drivers of masting behavior. All four species exhibited significant annual variation and within-population synchrony of acorn production, although masting behavior was more pronounced in the two species that require 1 year to develop acorns (‘1-year’ species) than the two species that require 2 years (‘2-year’ species). There was also strong synchrony between species that require the same number of years to mature acorns, but not between species requiring different numbers of years. Acorn production by three of the four species correlated with spring or summer conditions, while no significant environmental correlate of acorn production was detected for the fourth species. Acorn production by none of the four species correlated significantly with variables calculated from the differences in weather conditions from 1 year to the next. These results, combined with prior studies of oaks, suggest that environmental conditions during key periods of acorn development frequently correlate with acorn production, as expected if such factors bear a direct mechanistic relationship to seed production. On the other hand, the environmental factors involve vary greatly both among species and even among populations of the same species, a result consistent with the hypothesis that environmental correlates are simply cues used by plants to synchronize reproductive investment. In either case, our results do not support the recent proposition that variables based on differences in environmental conditions from 1 year to the next serve as a general cue for masting behavior.     

Summer-drought constrains the phenology and growth of two coexisting Mediterranean oaks with contrasting leaf habit: implications for their persistence and reproduction

This study analyses how coexisting evergreen and deciduous oaks adjust their phenology to cope with the stressful Mediterranean summer conditions. We test the hypothesis that the vegetative and reproductive growth of the winter deciduous (Quercus faginea Lam.) is more affected by summer drought than that of the evergreen [Quercus ilex L. subsp. ballota (Desf.) Samp.]. First, we assessed the complete aboveground phenology of both species during two consecutive years. Shoot and litter production and bud, acorn and secondary growth were monitored monthly. Second, we identified several parameters affected by summer conditions: apical bud size, individual leaf area (LA), leaf mass per area (LMA) and acorn yield in both species, and leaf-fall in Q. faginea; and analysed their variation over 10 years. Q. ilex performed up to 25% of shoot growth and most leaf development during summer, whereas Q. faginea completed most of both phenophases during spring. Secondary growth was arrested in summer under drought conditions. Approximately, 30–40% of bud and 40–50% of acorn growth was undertaken during summer in both species. Summer drought related to differences in LA, LMA and leaf senescence, but not to acorn yield. Both species had similar year-to-year patterns of acorn production, though yields were always lower in Q. faginea. Bud size decreased severely in both species during extremely dry years. In Q. ilex, bud size tended to alternate between years of large and small buds, and these patterns were followed by opposite trends in stem length. In Q. faginea, bud size was more stable through time. Q. ilex was more phenologically active during summer than Q. faginea, indicating a higher tolerance to drought. Furthermore, bud and fruit growth (the only two phenophases that both species performed during summer) were more severely affected by summer drought in Q. faginea than in the evergreen. The differential effects of summer drought on key phenophases for the persistence (bud growth) and colonization ability (fruit production) of both species may have consequences for their coexistence.