Jeffrey Pine Seeds Research Articles

Seed harvest by scatter-hoarding yellow pine chipmunks (Tamias amoenus)

Seed-caching rodents store large quantities of seeds during the summer and autumn but the number of stored seeds has rarely been estimated. Understanding how many seeds each individual stores has implications for energetics during the period of food scarcity, and the number of seeds scatter hoarded has implications for how well animals disperse the seed of plants and thereby effect ecological processes. The number of seeds of antelope bitterbrush (Purshia tridentata) and Jeffrey pine (Pinus jeffreyi) scatter hoarded by yellow pine chipmunks (Tamias amoenus) was estimated in a year of good seed production. Both of these plant species are adapted for seed dispersal by scatter-hoarding animals. Individual chipmunks scatter hoarded an estimated 29,629 bitterbrush seeds and 5,945 Jeffrey pine seeds each year. These estimates are likely underestimates and do not include the many thousands of seeds eaten during the harvest period. Combined, the number of seeds of these two species that chipmunks scatter hoard in soil provided 2.8 times more energy than required to survive the autumn and winter. Populations of yellow pine chipmunks have the potential to scatter hoard nearly a million seeds per hectare in good seed years.

Forest rodents provide directed dispersal of Jeffrey pine seeds

Some species of animals provide directed dispersal of plant seeds by transporting them nonrandomly to microsites where their chances of producing healthy seedlings are enhanced. We investigated whether this mutualistic interaction occurs between granivorous rodents and Jeffrey pine (Pinus jeffreyi) in the eastern Sierra Nevada by comparing the effectiveness of random abiotic seed dispersal with the dispersal performed by four species of rodents: deer mice (Peromyscus maniculatus), yellow-pine and long-eared chipmunks (Tamias amoenus and T. quadrimaculatus), and golden-mantled ground squirrels (Spermophilus lateralis). We conducted two caching studies using radio-labeled seeds, the first with individual animals in field enclosures and the second with a community of rodents in open forest. We used artificial caches to compare the fates of seeds placed at the range of microsites and depths used by animals with the fates of seeds dispersed abiotically. Finally, we examined the distribution and survival of naturally establishing seedlings over an eight-year period. Several lines of evidence suggested that this community of rodents provided directed dispersal. Animals preferred to cache seeds in microsites that were favorable for emergence or survival of seedlings and avoided caching in microsites in which seedlings fared worst. Seeds buried at depths typical of animal caches (5-25 mm) produced at least five times more seedlings than did seeds on the forest floor. The four species of rodents differed in the quality of dispersal they provided. Small, shallow caches made by deer mice most resembled seeds dispersed by abiotic processes, whereas many of the large caches made by ground squirrels were buried too deeply for successful emergence of seedlings. Chipmunks made the greatest number of caches within the range of depths and microsites favorable for establishment of pine seedlings. Directed dispersal is an important element of the population dynamics of Jeffrey pine, a dominant tree species in the eastern Sierra Nevada. Quantifying the occurrence and dynamics of directed dispersal in this and other cases will contribute to better understanding of mutualistic coevolution of plants and animals and to more effective management of ecosystems in which directed dispersal is a keystone process.

Linking summer foraging to winter survival in yellow pine chipmunks (Tamias amoenus)

This study links summer foraging and scatter-hoarding to winter larder-hoarding and winter survival in yellow pine chipmunks (Tamias amoenus) by comparing patterns of time allocation and winter larder contents in 2 years with very different levels of resource availability. In 2003, seed production and the number of trees and shrubs producing seeds were high. In 2004 seed crops were small. Chipmunks allocated more time to foraging when food resources were scarce (66% in 2004) compared to when they were abundant (39% in 2003). Increased time allocated to foraging in 2004 corresponded to significant decreases in time allocated to vigilance, resting, and social interactions. When seeds were scarce (i.e., in autumn 2004), chipmunks spent more time searching for cached food items than gathering seeds from plants or the ground surface. Despite the increase in foraging effort, the edible mass and caloric contents of larders were significantly smaller in 2004. In the year with low seed production, the diversity of seed species found in larders increased, and many of these seeds were of species that ripened in summer. When autumnal seed production by Jeffrey pine seeds was high, Jeffrey pine seeds were nearly the exclusive food item found in larders. Larder contents would have provisioned chipmunks for an estimated 116-257 days in 2003 and but only 6-111 days in 2004. It is likely that all chipmunks would have survived the winter of 2003 (duration approximately 110-120 days). However, none of the larders recovered in 2004 contained enough food to have provisioned the inhabitant for the approximately148-158 days of winter.

Effects of seed size of wind‐dispersed pines (Pinus) on secondary seed dispersal and the caching behavior of rodents

Seeds and nuts dispersed by scatter‐hoarding animals are relatively large compared to propagules dispersed by other means. Possible selective forces in the evolution of large seed size include the selectivity of foraging animals and the ways that food‐storing animals treat seeds and nuts after harvest. Treatment by rodents, primarily yellow pine chipmunks (Tamias amoenus), of four species of pine seeds that vary in size was studied in the Carson Range of western Nevada. The pines, lodgepole pine (Pinus contorta, 8.7 mg), ponderosa pine (P. ponderosa, 55 mg), Jeffrey pine (P. jeffreyi, 157 mg), and sugar pine (P. lambertiana, 213 mg), produce winged seeds that are initially wind‐dispersed but are gathered by rodents and cached in the soil. Radioactive scandium‐46 was used to follow the fates of seeds of all fours species placed around three source trees during autumn 1998 to 2000. Rodents gathered the seeds of all four species, but they took fewer of the lodgepole pine seeds and only six lodgepole seed caches (n=2106 total caches) were found during the three years. Among the other three species, number of seeds per cache decreased with increasing seed mass. However, the product of number of seeds per cache and seed mass was similar for all species. Sugar pine seeds were cached slightly deeper than ponderosa and Jeffrey pine seeds. For the species examined, seed size appeared to have had little effect on several other attributes, including mean dispersal distance, substrate choice, and microhabitat choice. Large size decreases wind dispersibility of pine seeds, but secondary dispersal by scatter‐hoarding rodents compensates for poor wind dispersal so that total dispersibility of large‐seeded pines is not compromised.

The Effects of Seed Value on the Caching Behavior of Yellow Pine Chipmunks

Yellow pine chipmunks scatter-hoard a variety of seed types, and these seeds vary considerably in their inherent value to the chipmunks. Models of optimal scatter hoarding behavior predict that animals should space items of greater value farther apart than items of lesser value to discourage theft from cache robbers. This prediction was tested by comparing how yellow pine chipmunks Tamias amoenus distribute Jeffrey pine Pinus jeffreyi and antelope bitterbrush Purshia tridentata seeds to cache sites. Chipmunks prefer Jeffrey pine seeds over bitterbrush seeds ; the pine seeds provide energy 3.8 times faster and contain a higher proportion of fat than bitterbrush seeds. The prediction of the model was upheld on several counts : chipmunks carry Jeffrey pine seeds farther from source plants to cache sites, they space Jeffrey pine seed caches farther apart, and they place fewer Jeffrey pine seeds than bitterbrush seeds in individual caches. Yellow pine chipmunks appear to be sensitive to the qualities of the food items they store and modify their caching behavior to reduce loss of more valuable food items.

Removal of Wind-Dispersed Pine Seeds by Ground-Foraging Vertebrates

Nine treatments involving three species of pine (Jeffrey pine, Pinus jeffreyi ponderosa pine, P. ponderosa; and lodgepole pine, P. contorta) seeds were used to test five hypotheses concerning the determinants of seed removal by animals in the field. Each treatment comprised 100 seeds. Winged seeds were tethered so they could not be blown away. Seeds were checked daily for up to 16 d. Animals (primarily chipmunks) removed most seeds within a few days. Winged Jeffrey pine seeds (large) disappeared significantly faster than ponderosa pine seeds (medium) which disappeared faster than lodgepole pine seeds (small) (...)

Salivary Water Loss to Seeds In Yellow Pine Chipmunks and Merriam's Kangaroo Rats

Seeds are known to serve as sources of preformed and metabolic water for desert rodents, but water loss to hygroscopic seeds has received little attention. Salivary water loss by yellow pine chipmunks (Tamias amoenus) and Merriam's kangaroo rats (Dipodomys merriami) to Jeffrey pine (Pinus jeffreyi) and antelope bitterbrush (Purshia tridentata) seeds was measured to determine its magnitude and potential importance. Chipmunks rapidly lost salivary water to individual seeds inserted into their cheek pouches. Jeffrey pine seeds (129 mg) absorbed 9.8 mg of water and bitterbrush seeds (36 mg) absorbed 4.6 mg of water when held in the cheek pouches for 15 min. Only 3—5% of this water can be reclaimed if the chipmunk eats the seed because most of the absorbed water resides in the inedible seed coat, which the animal discards. The kangaroo rats, which possess external, fur—lined cheek pouches, lost no water to seeds held in their cheek pouches for 15 min. Chipmunk salivary water loss to Jeffrey pine and bitterbrush seeds was equivalent to 19 and 59%, respectively, of the water (performed plus metabolic water) contained in those seeds. Salivary water loss is a major and previously overlooked component of the water budgets of granivorous rodents with internal cheek pouches. The presence of fur—lined cheek pouches in heteromyid rodents (Kangaroo rats, pocket mice, and kangaroo mice) may have contributed to the great success of these rodents in the deserts of North America by solving the problem of salivary water loss to the seeds they handle and store.

The Role of Animals in Dispersing a "Wind‐Dispersed" Pine

Jeffrey pine (Pinus jeffreyi) produces winged seeds typical of many wind—dispersed pines. However, seedlings often occur in clumps, suggesting that many seeds are scatter—hoarded by animals. An experiment is described that examines how the initially wind—dispersed seeds end up in animal caches. Two arrays of 1064 Jeffrey pine seeds were arranged around two "source" trees in a pattern predicted from models of wind dispersal. Seeds were labeled with radioactive scandium—46 and numbered 1—12 with indelible ink to represent the 12 1—m wide annuli centered on the source tree where they were deployed. Within 2 d, between 95 and 99% of the seeds had been removed by rodents. Fifty—four and 35% of the seeds were found stored at the two sites, most of these in shallow surface caches. Seed caches were located 1.3—54.2 m and 6.2—62.9 m from the "source" trees at sites one and two, respectively. Mean distance of the "wind—dispersed" seeds from the source tree was 4.6 m, and rodents moved seeds a mean of 13.0 m farther at site 1 and 24.7 m farther at site 2. Of 375 minimum dispersal measurements, seeds were carried away from the source tree 362 times. Four rodent species may have participated in caching the seeds, but yellow pine chipmunks (Tamias amoenus) appeared to be the species that cached most of the seeds. In a second experiment where rodents were excluded, seeds placed on the soil surface in fall rarely became buried during the winter, and if not buried, they would not germinate. Jeffrey pine seeds appear to be dispersed in two phases; seeds are initially scattered by the wind, and subsequently carried farther and cached in soil by animals.

Effect of Location of Seed upon Germination

The influence of parent trees upon the size and germination of Jeffrey pine seeds has been shown in a previous paper.' The marked results obtained from that work resulted in the present study, in which it has been sought to determine the value of seeds from different parts of the pine cone; and to decide what relation, if any, exists between the position of the seed and germination. The cones used were collected from Pinus Jeffreyi trees on the eastern slope of the Sierras in Lassen County, California, in September i919. No attempt was made to choose the trees from which the cones were taken, except that the trees were young and growing thriftily, considering the site upon which they stood. The cones were grouped according to size in three divisions, based on the gross characteristics of length, breadth, and weight. They were dried slowly in a room at air temperature, and as they opened the seeds were extracted. The cones were divided into three sections of approximately equal size, to be known as the upper, middle, and lower portions. The seeds were carefully collected and graded into three classes, large, medium, and small, using ocular means of determining the size and comparing one seed with another. Inasmuch as a number of individuals helped to determine the size of the seed grains themselves, the individual variation from this source was very largely eliminated. The seeds were cleaned, counted, and weighed, each lot kept separately, and sufficient seeds to carry out the test taken at random from each lot. To determine the germination, a number of each lot of seeds were sown in cans containing a uniform depth of soil and covered by an approximately equal depth of sand in each case. As previous work has shown that for Jeffrey pine a soil moisture content of about I5 per cent gives the best results, frequent weighings were made to keep the moisture content of the samples a constant at this figure. The result of this study is presented in tables I-VII.