Acorn Crop Research Articles

SEASONAL BODY WEIGHT VARIATION IN FIVE SPECIES OF WOODPECKERS

We investigated patterns of seasonal variation in body weight in six populations of five resident species of temperate-zone woodpeckers: Acorn Woodpecker ( formicivorus), Red-bellied Woodpecker (M. carolinus), Red-cockaded Woodpecker (Picoi- des borealis), Downy Woodpecker (P. pubescens), and Great Spotted Woodpecker (Den- drocopos major). After controlling for time of day and overall body size, annual variation in body weight was small and generally not statistically significant. However, analysis re- vealed evidence of significant ''winter fattening,'' comparable in magnitude to other tem- perate-zone resident species, in three of the species. The degree of winter fattening did not correlate with either the size of the acorn crop (for the Acorn Woodpecker) or latitude, two variables potentially related to predictability of food resources. However, the smaller species exhibited significantly greater winter fattening than the larger species, as predicted by the hypothesis that energy storage should be more important for small-bodied species. Further- more, the food-storing Acorn Woodpecker exhibited considerably less winter fattening than the nonfood-storing species, supporting the hypothesis that food storage provides an eco- logical alternative to winter fattening.

Does the shelterwood method to regenerate oak forests affect acorn production and predation?

The shelterwood system is one of the primary methods currently used to encourage regeneration of oak forests; yet, little is known about its influence on acorn production and predation. We compared acorn production, and predation by insects and mammals in stands of red oak ( Quercus rubra L.) that were regenerated by the shelterwood method (50% canopy removal) to that of uncut (control) stands in the first year of production after the harvest treatment. In each plot, we measured stand and tree characteristics and estimated acorn production by using both acorn traps and a visual crown survey to place trees into productivity classes. Acorns collected in traps were examined to record the external and internal conditions, percentage of cotyledon damaged and the presence of insects. Exclusion cages were used to quantify acorn predation by mammals in harvested and control plots. Oaks in the shelterwood plots produced more acorns than oaks in uncut stands if the acorn crop was rated by the crown survey method, but not when production was measured with acorn traps. We found no evidence that the shelterwood method influenced acorn predation by insects or mammals. Visual examination of the exterior of the pericarp indicated that insects attacked 44% and 47% of the acorns in harvested and control plots, respectively. Most of the damage produced by insects was attributed to pip galls (cynipid wasps) and acorn weevils ( Curculio sp.), which in most cases damaged more than 75% of the cotyledon. Squirrels and chipmunks were the primary mammals responsible for removing 50% of the acorns in the fall-spring whereas mice took 33% of the acorns. Use of the shelterwood method in conjunction with leaving the best acorn producers can be used to create the desired stand structure while maintaining or increasing acorn production and oak regeneration potential in the stand.

The effect of climate on the phenology, acorn crop and radial increment of pedunculate oak (Quercus robur) in the middle Volga region, Tatarstan, Russia

Our data, collected in the extreme east of Europe, show that a significant biological effect of climate change has been experienced even in territories where temperature increase has been the lowest. This study documents the climatic response of pedunculate oak (Quercus robur) growing near its north-eastern limits in Europe. It demonstrates the potential of oak trees in old-growth forest to act as proxy climate indicators. Many factors may influence the temporal stability of the growth-climate, acorn crop-climate and first leafing-climate relationships. Climate data, climatic fluctuations, reproduction, genetics and tree-age may relate to this instability. Our results stress that an increase in climate variability or climatic warming resulting from warmer winters or summers could affect the oak population in eastern Europe in a similar way to that in western Europe. These findings, from remnants of oak forest in the middle Volga region of Russia, allow a further understanding of how species could be affected by future climates.

High Acorn Predation Prevents the Regeneration of Quercus liaotungensis in the Dongling Mountain Region of North China

Abstract The mature oak (Quercus liaotungensis Koidz) forests in the Dongling mountains of northern China have become degraded in recent years because regeneration has been limited. To determine whether or not seedling establishment of the oak is seed limited, microsite limited, or predator limited and to determine whether seedling establishment is affected by ground cover, we conducted field experiments during a mast year and investigated the fate of seeds and the soil seed bank dynamics of the oak. A large acorn crop (128.8 acorns/m2) was observed in the study period, and the peak density of acorns on the forest floor reached 46.5 acorns/m2, suggesting that tree recruitment was not seed limited. Acorns in the soil seed bank were mainly lost through decay (principally after fungal attack), consumption in situ, and removal by animals. Predation (including consumption in situ and removal) accounted for 86.4% of acorn loss and was therefore likely to have been the most important factor influencing seed dynamics. More than 70% of acorns were found to have germinated, but no established seedling was observed on the forest floor. Using cages to exclude predators, it was estimated that 87% of acorns germinated and 49% became established as seedlings, indicating that the acorns on the forest floor could emerge and grow in the absence of predators. We conclude that the regeneration of the tree population is limited by predators rather than by the availability of microsites. The presence of ground cover increased the germination rate and increased the chance of seed survival in the early stage of the experiment, but at the end of the investigation, no established seedling was found in the quadrats both with and without ground cover, possibly because of high density of animal predators. On the basis of these results, we suggest that selective tree felling will increase the coverage of the herbaceous layer, which can further decrease the population density of the rodents, and thereby improve the regeneration of oak trees.

PEROMYSCUS LEUCOPUS ABUNDANCE AND ACORN MAST: POPULATION FLUCTUATION PATTERNS OVER 20 YEARS

Summer abundance of white-footed mice (Peromyscus leucopus) fluctuated periodically during 20 years (1983‐ 2002) at the Holt Research Forest, Maine, USA, a pine-oak forest within 125 km of the northern edge of this species’ range. The oscillation period of the series was 4.0 years, with fluctuations greater than in P. leucopus populations in Indiana, Virginia, Ohio, and Pennsylvania. Abundance always increased after a low population phase coupled with a large acorn (Quercus) crop, but in 3 summers population declines followed population peaks despite large acorn crops. We used linear autoregression to identify periodicity after accounting for the relationship between mice and acorn mast. Among 3 candidate models, a 2nd-order lagged abundance (AR[2]) model fit the data better and had greater predictive value (total r 2 ¼ 0.69) than either a 1st-order (AR[1]) or an acorns-only model. The AR(2) term could represent unknown limiting factors, such as response of predators.

Biennial acorn maturation and its relationship with flowering phenology in Iberian populations of Quercus suber

Since the XIX century, there is a controversy about the existence of biennial maturation of the acorns in Quercus suber L. While some authors recognised biennial cycles as an adaptation to habitats with short vegetative periods, other authors discarded the biennial pattern. Successive flowering events from spring to autumn and annual acorn ripening are proposed as an explanation of the multiple acorn crops typical of Iberian forests. To clarify this discussion, the presence of annual and biennial acorns was assessed in seven cork oak stands, covering a wide range of environmental conditions. In each stand, 100 individuals were sampled once in spring and once in autumn. Biennial acorns were observed with variable frequencies in all populations. There was a significant and positive relationship between latitude and the percentage of trees with biennial acorns within northern and central populations. On the contrary, this trend was not significant among southern populations. The hypothesis that the presence of biennial acorns in Quercus suber is related to individual female flowering phenology was confirmed in four populations located in the southwestern Iberian Peninsula. Unregarding local differences in the distribution of phenological stages anticipated trees bore significantly less biennial acorns than delayed individuals of the same stand. This result is coherent with the idea that the length of the vegetative period plays a crucial role in the frequency of annual and biennial acorn ripening patterns. The relationship between annual and biennial ripening cycles and the multiple acorn crops is discussed.

Linking frequencies of acorn masting in temperate forests to long‐term population growth rates in a songbird: the veery (Catharus fuscescens)

Many terrestrial ecosystems are characterized by intermittent production of abundant resources for consumers, termed pulsed resources. The impact of resource pulses on populations downwind of the initial pulse are only beginning to be characterized, while the relationship between the frequencies of pulses and the long‐term growth rate of affected species is unknown. I monitored the reproductive success of veeries (Catharus fuscescens) breeding in oak‐dominated forest in southeastern New York State from 1998 to 2002. During this time veeries experienced high interannual variability in growth rates as a consequence of trophic cascades stemming from pulsed production of acorns. Rodent populations that benefited from acorns also depredated veery nests, while raptors that increased in response to rodent outbreaks are major predators on adult and juvenile birds. Veeries may recoup losses following low to moderate acorn crops that lead to rodent population declines. Thus, veeries fluctuate between years of positive and negative growth rate, however, long‐term population trends, and thus true source‐sink designation, cannot be made until the frequency of various year types is characterized. I simulated long‐term growth rates using reproductive parameters estimated from field studies and survivorship data from the literature. Simulations suggest that variability in the frequency of masting events in oaks can lead to ∼10% fluctuation in long‐term growth rates in veeries. These studies suggest that temporal variability in masting dynamics has the potential to substantially influence songbird population trends. Furthermore, spatial variability in masting characteristics (e.g. the frequency of masting events and/or the size of seed crops) may greatly contribute to regional differences in songbird population trends. Because even less is known about the relationship between sizes of acorn crops and songbird populations, the influence of pulses in seed production on songbird population dynamics is likely to be underestimated.

LONG-TERM PATTERNS OF ACORN PRODUCTION FOR FIVE OAK SPECIES IN XERIC FLORIDA UPLANDS

The number of seeds produced by a population of woody plants can vary markedly from year to year. Unfortunately, knowledge of the patterns and causes of crop-size variation is limited, and most studies have examined only single species in a single mesic environment. We examined long-term patterns of acorn crop sizes for five species of shrubby oaks in three xeric upland vegetative associations of south-central peninsular Florida for evidence of regular fruiting cycles and in relation to winter temperature and precipitation. Counts of acorns on two white oak species (Quercus chapmanii and Q. geminata) and three red oak species (Q. inopina, Q. laevis, and Q. myrtifolia) were conducted annually from 1969 to 1996 (except in 1991) on grids in southern ridge sandhill, sand pine scrub, and scrubby flatwoods associations. As the result of the dissimilar timing of reproductive events in species of red and white oaks and individual species responses to climatic variables, annual acorn production of red and white oak species in a given vegetation association were not synchronized, which dampened the variability of combined annual acorn production. Consequently, acorns were produced with reasonable abundance every year by at least one species. Oaks of nearly closed-canopy scrub produced fewer acorns than similar-sized oaks of the same species in the more open-canopied sandhill and scrubby flatwoods, suggesting light limitation. We identified regular cycles of acorn production that ranged from 2 to 2.4 yr for white oak species and from 3.6 to 5.5 yr for red oak species and found evidence that annual acorn production is affected by the interactions of precipitation, which is highly variable seasonally and annually in peninsular Florida, with endogenous reproductive patterns. Multiple regression models using precipitation variables accounted for as much as 74% of crop-size variation in Q. inopina, 65% in Q. laevis, 60% in Q. chapmanii, and 44% in Q. geminata, but only 29% for Q. myrtifolia. In contrast, acorn production showed no significant association with minimum winter temperatures. Of the various hypotheses offered to explain supra-annual variation in seed crops, our data are most compatible with the nonadaptive resource-matching hypothesis.

SONGBIRD POPULATIONS IN FLUCTUATING ENVIRONMENTS: PREDATOR RESPONSES TO PULSED RESOURCES

In response to mast production of acorns, populations of small mammals, such as the white-footed mouse and eastern chipmunk, greatly increase and, in turn, initiate cascades of direct and indirect effects that permeate throughout forest communities. To examine the strength of the effects stemming from acorn crops, we conducted studies in oak-dominated deciduous forest of the Hudson Valley, New York, documenting the connections between predator abundance (small mammals and raptors) and the abundance and reproductive success of five woodland songbirds. Nest predation rates increased at high rodent densities for Veeries, Red-eyed Vireos, and Wood Thrushes. Analysis of Breeding Bird Survey (BBS) data from surrounding counties demonstrated a unimodal relationship between rodent density (year t) and songbird abundance (year t + 1), rather than a monotonic decline. Regional Christmas Bird Count data for Sharp-shinned and Cooper's Hawks showed a positive correlation between rodent density and accipiter abundance the following winter, suggesting that hawks exhibit a numerical response to rodents. Taken together, we suggest that predation by accipiters when rodents are scarce, in combination with high nest predation when rodents are abundant, may explain the unimodal relationship between rodents and songbirds. Nonlinear relationships between predator and prey abundance and indirect interactions appear to characterize this system. Corresponding Editor: J. R. Walters.

Hoarding of acorns by granivorous mice and its role in the population processes of Pasania edulis (Makino) Makino

In order to evaluate the role of granivorous mice, Apodemus speciosus Temminck and Apodemus argenteus Temminck, in the regeneration of Pasania edulis (Makino) Makino, an evergreen tree, we conducted field studies examining acorn crops, the population dynamics and hoarding behavior of mice and pre‐ and postdispersal acorn predation in an evergreen broad‐leaved forest dominated by P. edulis in Kagoshima, southern Japan. The study was conducted from 1994 to 2000. Apodemus mice selected sound acorns and hoarded them in the soil after transporting them for distances ranging from 5.4 to 19.9 m. This behavior appeared to be beneficial to the establishment of seedlings of P. edulis. However, the positive effects were negated by a high rate of recovery of the acorns hoarded by the mice. A life‐table analysis of five cohorts of P. edulis indicated that postdispersal predation of acorns by Apodemus mice was a major contributor to annual variation in the number of new seedlings. A considerable number of acorns germinated only in years with a good acorn crop and a low level of resident mice. These results suggest that Apodemus mice are serious acorn predators rather than important dispersers. After germination, acorns were attacked by the acorn borer at a high rate. From these results, we can conclude that acorn production probably makes a smaller contribution to the regeneration of P. edulis than coppice shoot production at the study site.

Acorn production by Kashiwa oak in a coastal forest under fluctuating weather conditions

We modeled the annual acorn crops of Kashiwa oak (Quercus dentata Thunb.) in a coastal forest in Hokkaido, northern Japan. Shoots of Kashiwa oak withered away during winter because of sea breeze and cold, and succeeding production of female inflorescences were strongly affected by mortality of buds. Thus, strong sea breeze and cold winters reduced the survival ratio of buds and further resulted in reduction of female flowering. Number of female flowers was related with current acorn crops, however, survival of female flowers after pollination was strongly influenced by warmth in the flowering period. Regression analysis of the annual acorn crop versus weather conditions suggests that acorn crop was decreased by cool conditions in the flowering period. A model equation was constructed to estimate the annual acorn crops by three weather variables: cumulative velocity of sea breeze and mean monthly temperature in winter (December–March) and maximum monthly temperature in current flowering period (June). This model equation explained 89.2% of observed acorn crops of Kashiwa oak in the coastal forest.

Pollen-limited reproduction in blue oak: implications for wind pollination in fragmented populations.

Human activities are fragmenting forests and woodlands worldwide, but the impact of reduced tree population densities on pollen transfer in wind-pollinated trees is poorly understood. In a 4-year study, we evaluated relationships among stand density, pollen availability, and seed production in a thinned and fragmented population of blue oak (Quercus douglasii). Geographic coordinates were established and flowering interval determined for 100 contiguous trees. The number of neighboring trees within 60m that released pollen during each tree's flowering period was calculated and relationships with acorn production explored using multiple regression. We evaluated the effects of female flower production, average temperature, and relative humidity during the pollination period, and number of pollen-producing neighbors on individual trees' acorn production. All factors except temperature were significant in at least one of the years of our study, but the combination of factors influencing acorn production varied among years. In 1996, a year of large acorn crop size, acorn production was significantly positively associated with number of neighboring pollen producers and density of female flowers. In 1997, 1998, and 1999, many trees produced few or no acorns, and significant associations between number of pollen-producing neighbors and acorn production were only apparent among moderately to highly reproductive trees. Acorn production by these reproductive trees in 1997 was significantly positively associated with number of neighboring pollen producers and significantly negatively associated with average relative humidity during the pollination period. In 1998, no analysis was possible, because too few trees produced a moderate to large acorn crop. Only density of female flowers was significantly associated with acorn production of moderately to highly reproductive trees in 1999. The effect of spatial scale was also investigated by conducting analyses with pollen producers counted in radii ranging from 30m to 80m. The association between number of pollen-producing neighbors and acorn production was strongest when neighborhood sizes of 60m or larger were considered. Our results suggest that fragmentation and thinning of blue oak woodlands may reduce pollen availability and limit reproduction in this wind-pollinated species.

Nestling sex ratio variation in the cooperatively breeding acorn woodpecker ( Melanerpes formicivorus )

We examined sex ratio variation in the cooperatively breeding acorn woodpecker (Melanerpes formicivorus) over a 27-year period in central coastal California. The sex ratio of eggs is even or possibly biased toward females. However, hatching asynchrony and brood reduction differentially favor the slightly larger males such that the sex ratio when young become self-sufficient is 54.0% males, slightly but significantly different from 50:50. This bias, or at least the differential mortality that produces it, does not appear to be explainable by any of the major hypotheses for an overall sex bias. These include the repayment model, local resource competition, and sexual size dimorphism, all of which predict a female bias or a male bias significantly smaller than that observed. On an individual level, we found no evidence for facultative sex ratio manipulation related to annual differences in the acorn crop, apparent seasonal declines in food availability, or differences in territory quality. We also found no evidence for a non-binomial distribution of sexes within broods despite the potential fitness benefits of creating large coalitions of same-sexed siblings. Males inherit their natal territories more frequently than do females, and we speculate that birds may invest more in sons either to increase the probability that they will be able to pass along their territory to their descendants or to facilitate their sons' ability to compete as future cobreeders. Alternatively, acorn woodpeckers may be unable to fully compensate for the ancillary bias caused by differential juvenile survivorship, leading to unequal investment in the sexes contrary to the prediction of Fisher. Opposite to what has been found in other birds and mammals, the faster growth and larger size of male nestlings appears to confer a survival benefit rather than a cost.

Male Harvest in Relation to Female Removals in a Black-Tailed Deer Population

The role of density dependence in management of ungulate populations is a debated topic. I tested the effect of female removals on the yield of males in annual public hunting seasons for black-tailed deer (Odocoileus hemionus columbianus) in a 19-year study at Hopland Research and Extension Center, California. Separate treatment and control areas were not available, so pre- and posttreatment control periods were employed to examine for non-treatment effects (i.e., 6 yrs pretreatment, 7 yrs treatment, and 6 yrs posttreatment). During the treatment period, about 20 adult females/year for 3 years and 30/year for 4 years were removed. There was no difference in male harvests between pretreatment and posttreatment control periods (x = 28.5 pre- and 27.3 posttreatment, P = 0.44), so the controls were pooled for testing treatment effects. Male harvest during the treatment period (x = 34.9) was greater than the combined control periods (x = 27.9, P = 0.02). Increased male harvest occurred despite the treatment period coinciding with a 6-year drought. Neither rainfall nor acorn (Quercus spp.) crop was related to male harvest. At the end of the treatment period, combined male harvest and female removal comprised about 21 % of the estimated pre-hunt population. Males taken during the treatment period were younger and weighed less than those taken during control periods. However, 2-year-old males harvested during the treatment period had larger antlers. These results demonstrated that female harvests increased male yield apparently through lowering population density.

Individual variation in acorn production by five species of southern Appalachian oaks

Acorns are an important wildlife food resource and seed source for oak regeneration. Most acorn production studies note wide and consistent differences in acorn productivity among individuals, but none clearly demonstrate determinants of productivity. Acorn production by black, northern red, scarlet, chestnut and white oak was measured from 1993 to 1997 in the southern Appalachians was measured and compared among species and individuals. To standardize comparisons among different sized trees and simplify for use by forest managers, the number of acorns per tree were converted to the number/m 2 BA (basal area). On average, white oak produced the most acorns and chestnut oak the fewest. Northern red and white oak produced higher green weight and dry biomass than the other three species. There was a significant positive relationship between tree basal area and the number of acorns produced per crown for all species ( r 2 between 0.10 and 0.27). However, this is because larger trees have greater crown areas for producing acorns, and not because they produce more acorns per unit area of crown. Alone, BA was significantly, positively correlated with the number of acorns/m 2 BA only in black, northern red ( p < 0.06) and white oak (not in scarlet or chestnut oak) but explained little of the variation in acorn production among individuals. Trees ≤25 cm DBH of most species produced significantly fewer acorns/m 2 BA than their larger counterparts. However, many small (<23 cm DBH) scarlet oaks originating from a 1967 clear-cut were prolific producers, whereas white oaks (<25 cm DBH) in the same stand were not. Frequency of acorn production ranged from never to yearly among individuals. Good producers (trees producing ≥5-year species mean) composed 20% (chestnut oak) to 46% (northern red oak) of sample populations but contributed disproportionately to the acorn crop in moderate and good crop years. Good producers produced acorns more frequently and had more acorns/m 2 BA on fruiting trees than did poor producers. However, in any given year good and poor producers were similarly represented in the fruiting population. Hence, good producers could not be easily identified by the presence of acorns during poor crop years, nor could poor producers be identified by an absence of acorns in good crop years.

THE INFLUENCE OF ACORN CROPS ON ANNUAL VARIATION IN RODENT AND BIRD POPULATIONS

I recorded mast production by oaks (Quercus sp.) at 12 forested sites in western Virginia for 6–12 yr and measured its impact on the abundance of small mammals, understory vegetation, and artificial-nest predation. White-tailed deer (Odocoileus virginianus) were excluded from half the 4-ha sites after at least one season of data collection. My hypothesis was that annual variation in acorn crops affected multiple species and that the strength of those interactions is mediated by white-tailed deer. The acorn crop was variable across sites and year, with some of the between-site variability explained by differences in elevation. All sites experienced at least one mast failure, and mast failure years were generally consistent across sites. White-footed mouse (Peromyscus leucopus), eastern chipmunk (Tamias striatus), and gray squirrel (Sciurus carolinensis) populations were significantly correlated with annual fluctuations in the acorn crop. The exclusion of deer had a significant impact on P. leucopus and T. striatus populations by increasing the number of animals captured following low acorn mast years. Annual fluctuations in the acorn crop, but not in rodent densities, were significantly correlated with the rates of predation on artificial nests the next summer. There was no significant interaction between predation rates and the exclusion of deer. An index from the Breeding Bird Survey (BBS) for Virginia was used to measure regional numbers for 11 common species captured at the sites. The index for two understory species was significantly negatively correlated with the mean acorn crop measured 2 yr previously. The effect of white-tailed deer on the forest community was not consistent across all conditions, as sites with large acorn crops were not strongly influenced by deer. These data are consistent with the hypothesis that mast crops from oaks serve as important determinants of community function within Appalachian forests.

The Influence of Acorn Crops on Annual Variation in Rodent and Bird Populations

The Influence of Acorn Crops on Annual Variation in Rodent and Bird Populations

Relationships between Seed Fall of Three Tree Species and Peromyscus leucopus and Clethrionomys gapperi during 10 Years in an Oak-Pine Forest

We analyzed live-trapping data from 10 years (1983-1992) of a long-term study of forest ecosystems in southern Maine to investigate relationships between seed fall of red oak (Quercus rubra), white pine (Pinus strobus), and red maple (Acer rubrum), and populations of Peromyscus leucopus and Clethrionomys gapperi. Spring populations of Peromyscus were correlated positively with crops of both red oak and white pine in the previous autumn; summer populations were correlated only with the acorn crop of red oak in the previous year. Populations of Clethrionomys were not correlated with either red oak or white pine in spring or summer. The increase from spring to summer in Peromyscus was associated positively with the magnitude of seed fall from red maple in the current year. Following years of above average acorn production of red oak, adult male and female Peromyscus weighed more than in years following low acorn production. Following 2 years of high acorn production, captures of Peromyscus were associated positively with the number of live red oaks >10 cm diameter at breast height per 0.25-ha quadrat. Group-selection logging also was associated with increased summer captures of Peromyscus per quadrat for 2 of 4 years, probably due to an increase in both herbaceous cover and fruit production of shrubs.

Variation of red oak acorn production

Acorn production is characterized by extreme variation among years and among individual trees. The size of acorn crops affects many components of the ecosystem, and both annual and individual variation in acorn production influence the regeneration and management of oak forests. In order to assess the variation within a stand, we sampled the acorn production of 120 red oaks (Subgenus Erythrobalanus) for 11 years. Half of the trees were treated by thinning 2 years before the study began, and half were unthinned controls. Trees in the thinned part of the stand produced more acorns per tree than those in the unthinned area, but the treatment effect was small in comparison with individual and annual variation. Differences in acorn production between the most productive and least productive individual trees were about 11-fold for thinned and 28-fold for unthinned trees. The largest yearly acorn collection exceeded the smallest by 135 times for the thinned treatment and by 109 times for the unthinned treatment. The two best years (1991, 1993) accounted for 55% of the total 11-year acorn collection, while the five poorest years provided only 10% of the total collection, suggesting that efforts to enhance regeneration should coincide with mast years. Thinned and unthinned sample populations exhibited synchronous masting patterns, and good and poor producers within each population also exhibited year-to-year synchrony in acorn production. There was no evidence of cyclic acorn production by this population. Half of the total acorn collection was produced by about one-third of the trees. On average, we were able to identify 87% of the better-than-average acorn producers by monitoring individual tree production for any three consecutive years. This method of identifying superior acorn producers is effective and conceptually simple but it requires at least 3 years to implement.

Oaks, acorns, and the geographical ecology of acorn woodpeckers

SummaryWe investigated the geographical ecology of acorn woodpeckers (Melanerpes formicivorus) using 30 years of Audubon Christmas Bird Counts and data on the diversity and abundance of oaks. Spatial autocorrelation in acorn woodpecker population densities is not significantly greater than zero both in either the southwestern United States, where populations are often locally isolated, or along the Pacific Coast, where they are more evenly distributed. In both regions, the effective distributional limit of acorn woodpeckers is set not by the limits of oaks but by sites where oak diversity drops to a single species. This result is consistent with acorn production patterns in central coastal California demonstrating that variability in overall acorn production and the probability of acorn crop failure decline with increasing oak species number but drop most markedly when two, compared to one, species of oaks are present together. Along the Pacific Coast, acorn woodpecker densities increase and population variability decreases with increasing abundance and diversity of oaks; however, analyses indicate that overall population size in this region is primarily determined by resource abundance while population stability is determined by resource diversity. Comparable patterns are not obvious in the Southwest, where acorn woodpecker densities are much lower than along the Pacific Coast. This may be due to a combination of greater competition for resources and oak communities that differ both qualitatively and quantitatively in their productivity compared to those along the Pacific Coast.