Ability of chestnut oak to tolerate acorn pruning by rodents: The role of the cotyledonary petiole

Substrate scent-induced disproportionate seed dispersal by rodents

Conspecific adults impose strong negative density-dependent effects on seed survival nearby parent trees, however, the underlying mechanisms are diversified and remain unclear. In this study, we presented consistent evidence that parent-scented forest floor masked seed odor, reduced cache recovery rate by scatter-hoarding animals, and then increased seed dispersal far away from mother trees. Our results showed that seed odors of Korean pine Pinus koraiensis match well with the volatile profile of their forest floor. Moreover, scatter-hoarding animals selectively transported P. koraiensis seeds toward the areas where seed odor was more contrasting against the background substrate, possibly due to the fact that accumulation of conspecific volatile compounds in caches hindered seed detection by scatter-hoarding animals. Our study provides insight into the role of leaf litter in directing seed dispersal process, representing a novel mechanism by which P. koraiensis increases selection for seed dispersal far away from the parent tree.

Plant hormones mediate the interaction between oak acorn germination and rodent hoarding behaviour

The interaction between animals and plants for seed dispersal and predation has received much attention; however, the underlying physiological mechanisms driving the responses of both seeds and animals remain unclear. We conducted a series of behaviour and physiology experiments to examine the role of plant hormones in regulating seed germination and rodent hoarding behaviour in the Quercus variabilis and Leopoldamys edwardsi systems. We found that acorns that were partially consumed by rodents had increased gibberellin (GA) levels and shortened germination time. Rodents preferred scatter-hoarded abscisic acid (ABA)-treated and intact acorns but consumed germinated and GA-treated acorns; such treatment differences disappeared for inactivated acorns by boiling water. Moreover, we found that seven potential compounds may be linked to seed germination and rodent hoarding behaviour. Our results indicate that acorns of oak showed rapid germination when facing predation risk, while rodents could identify the germination status of seeds for hoarding; GA and ABA may play an important role in regulating seed germination of oak and hoarding behaviour of rodents.

Behavioral adaptation of sympatric rodents to early germination of oak acorns: radicle pruning and embryo excision

The seed germination schedule is a key factor affecting the food-hoarding behavior of animals and the seedling regeneration of plants. However, little is known about the behavioral adaptation of rodents to the rapid germination of acorns. In this study, we provided Quercus variabilis acorns to several rodent species to investigate how food-hoarding animals respond to seed germination. We found that only Apodemus peninsulae adopted embryo excision behavior to counteract seed germination, which is the first report of embryo excision in nonsquirrel rodents. We speculated that this species may be at an early stage of the evolutionary response to seed perishability in rodents, given the low rate of embryo excision in this species. On the contrary, all rodent species preferred to prune the radicles of germinating acorns before caching, suggesting that radicle pruning is a stable and more general foraging behavior strategy for food-hoarding rodents. Furthermore, scatter-hoarding rodents preferred to scatter-hoard and prune more germinating acorns, whereas they consumed more nongerminating acorns. Acorns with embryos excised rather than radicles pruned were much less likely to germinate than intact acorns, suggesting a behavioral adaptation strategy by rodents to the rapid germination of recalcitrant seeds. This study provides insight into the impact of early seed germination on plant-animal interactions.

Nondormant Acorns Show Higher Seed Dispersal Effectiveness Than Dormant Ones

Seed dormancy has been thought to be an important survival strategy to tune the seed dispersal timing. Although a theoretical trade-off between seed dormancy and dispersal is often proposed, empirical field evidence of the trade-off between seed dormancy, spatial dispersal, and seedling recruitment is still lacking. Here, we tracked seed dispersal of several Fagaceae species exhibiting different levels of dormancy both in artificial enclosures and in the field. We presented evidence that oak species with nondormant acorns rather than those bearing dormant species exhibit reduced spatial dispersal. Despite the empirical evidence that seed germination is negatively correlated with spatial dispersal, nondormancy rather than dormancy showed higher seed dispersal effectiveness, demonstrating a negative correlation between seed dormancy and dispersal fitness. Our study, using the oak-rodent system, may provide solid evidence of a dispersal–germination trade-off between spatial and temporal correlation, highlighting the ecological role of seed germination schedule in seed dispersal and life-history evolution.

Mutual cheating strengthens a tropical seed dispersal mutualism

While cheating can cause the degradation or collapse of mutualisms, mutualisms may theoretically stabilize or strengthen if the cheating is mutual. Here, we present an asymmetric two-player game model to explore the evolutionary dynamics of mutual cheating in a mutualistic interaction. We found that the interaction evolved towards mutual cheating if cheating can help both partners obtain higher benefits or if counter-cheating yields more benefits to victims than simply tolerating exploitation by partners. Then, we present empirical evidence for such mutual cheating strengthening a seed dispersal mutualism in which rodents disperse seeds by scatter hoarding, rodents sabotage seed germination by pruning radicles, and seeds escape rodents by resprouting. By tracking >8000 Pittosporopsis kerrii seeds throughout the dispersal process in a tropical forest in southwest China, we found that rodents provided better dispersal to seeds that they pruned, i.e., pruned seeds were dispersed farther and were more likely to establish seedlings than unpruned seeds. Compared to unpruned seeds, pruned seeds retained more of their nutrients, i.e., dry mass of pruned seeds was greater than that of unpruned seeds, and were stored for longer by rodents. These findings indicate that mutual cheating benefited both partners. Payoffs estimated from the field experiments indicated that mutual cheating was indeed favored in rodents and plants P. kerrii, and that neither partner was enslaved by the other under mutual cheating. Rather, the mutualism remained stable because the partners were able to exploit each other, and each partner attempted to gain the maximum benefits from the interaction. Our findings indicate that mutual cheating between two mutualists can enhance and stabilize mutualisms.

Radicle pruning by seed-eating animals helps oak seedlings absorb more soil nutrient

Although radicle pruning has well been observed in plant–animal interactions, research has not been conducted to determine how radicle pruning by seed‐eating animals regulates nutrition mobilization of cotyledonary reserves and absorption of soil nutrients. We used stable nitrogen isotopes to test how acorns of early‐germinating oak species (Quercus variabilis, Q. aliena, and Q. mogolica) trade off nutrients in the cotyledons and those in the soil in response to radicle pruning by seed‐eating rodents. Radicle pruning by rodents resulted in root branching in the 3 early‐germinating oak species. Moreover, radicle pruning increased shoot dry weight and substantially reduced the root‐to‐shoot ratio of oak species. Corresponding to the decreased dry weight of roots and root‐to‐shoot ratio, the dry weight of the remnant cotyledons was higher after radicle pruning in the 3 oak species. We provided first evidence that radicle pruning by seed‐eating animals improved seedling performance of early‐germinating oaks by increasing absorption of nutrients from soil. The results indicate that early‐germinating oak seedlings trade off nutrition budget by altering nutrient absorption from soil and reserve mobilization from cotyledons in response to radicle pruning by seed‐eating animals. Our study provided new insight into the nutrition allocation mechanism of young seedlings in response to radicle pruning by seed‐eating animals, reflecting a mutualistic interaction between early‐germinating oak and food‐hoarding animals.

One acorn produces two seedlings in Chinese cork oak Quercus variabilis

Although the early germination of white oaks has long been considered an adaptation to counter animal predation, the role of the robust radicles of white oak acorns has been largely neglected in the interaction between acorns and seed-eating animals. Here, the regeneration capabilities of the pruned radicles of intermediate oak Quercus variabilis left by the acorn-eating animals were compared to test the 'one acorn produce two seedlings' hypothesis stating that the remnant radicles exhibit the capability to produce normal seedlings owning to their atypical nutritional and anatomical characteristics. Our results first showed that both pruned radicle and cotyledons from single acorn successfully established seedlings, providing strong evidence on an old puzzle that the robust radicles of early-germinating oak acorns act as reserved propagules, allowing them to escape animals' predation. Our study also suggests that much of the investment in the food reserves of an acorn may be important as a food reward for the animals that disperse the acorns, rather than simply serving as energy for the young seedling.

Acorn size and tolerance to seed predators: the multiple roles of acorns as food for seed predators, fruit for dispersal and fuel for growth

Fitness of parents and offspring is affected by offspring size. In oaks (Quercus spp.), acorns vary considerably in size across, and within, species. Seed size influences dispersal and establishment of oaks, but it is not known whether size imparts tolerance to seed predators. Here, we examine the relative extent to which cotyledon size serves as both a means for sustaining partial consumption and energy reserves for developing seedlings during early stages of establishment. Acorns of 6 oak species were damaged to simulate acorn predation by vertebrate and invertebrate seed predators. Seedling germination/emergence and growth rates were used to assess seedling performance. We predicted that if cotyledons are important for dispersal, acorns should show tolerance to partial seed consumption. Alternatively, if the cotyledon functions primarily as an energy reserve, damage should significantly influence seedling performance. Acorns of each species germinated and produced seedlings even after removing >50% of the cotyledon. Seed mass explained only some of the variation in performance. Within species, larger acorns performed better than smaller acorns when damaged. Undamaged acorns performed as well or better than damaged acorns. There was no pattern among individual species with increasing amounts of damage. In some species, simulated invertebrate damage resulted in the poorest performance, suggesting alternative strategies of oaks to sustain damage. Large cotyledons in acorns may be important for attracting seed dispersers and sustaining partial damage, while also providing energy to young seedlings. Success of oak establishment may follow from the resilience of acorns to sustain damage at an early stage.

Seedling tolerance to cotyledon removal varies with seed size: A case of five legume species

It is generally accepted that seedlings from large seeds are more tolerant to defoliation than those from small seeds due to the additional metabolic reserves present in the large seeds. However, information on the effects of amount of seed reserves (cotyledon removal) from seedlings resulting from large vs. small seeds on seedling growth and long-term survival in the field is limited. Five legume species with different sizes of seeds were sown in the field and none, one, or both cotyledons removed 7 days after seedling emergence. Seedling biomass, relative growth rate (RGR) and survival were determined at different time. Cotyledon removal, species, and their interaction had significant effects on seedling growth and survival. During the period between 33 and 70 days, seedlings from large seeds had a significantly lower RGR than those from small seeds. Biomass, RGR, and survival of seedlings from large seeds were significantly reduced by removal one or both cotyledons, whereas those of seedlings from small seeds were not affected. Seed energy reserves are more important for the early growth of seedlings from large seeds than for those from small seeds. The overall effect of cotyledon removal on growth and survival varies with seed size (i.e., energy reserves) with seedlings from small seeds being less sensitive than those from large seeds under field conditions.

Acorn cotyledons are larger than their seedlings' need: evidence from artificial cutting experiments

Although the consequences of cotyledon removal have been widely studied in oaks producing large acorns, we have little knowledge of at what level cotyledons can be removed without affecting acorn survival and seedling development. In this study, we aimed to test the hypothesis that the amount of energy reserves in cotyledons is more than the demands of seedlings and that large acorns can tolerate seed predation and/or attract seed predators for seed dispersal. Acorn germination rates were not affected even when 60% of cotyledons were cut off at the basal end, suggesting that the energy reserves contained in cotyledons are not essential for acorn survival. Post-cut acorn mass, more than initial acorn mass, appear to be a better predictor of seedling performance, indicating that the energy reserves in cotyledons are sufficient for seedlings. Acorns with large masses sustained cotyledon damage better than small ones with respect to seedling performance. Large acorns were more likely to be dispersed and cached by animals, implying that producing large acorns is more important for oaks to manipulate seed predators and dispersers rather than provide a seedling with cotyledonary reserves.

Retention of cotyledons is crucial for resprouting of clipped oak seedlings

Although resprouting plays an important role in facilitating persistence of damaged seedlings, the functional attributes of cotyledons and taproots during resprouting of 1-year oak seedlings are not well explored. In this study, cotyledons were removed from Quercus mongolica seedlings to explore resprouting in response to simulated disturbance as a function of shoot clipping, and to examine the resprouting ability in relation to timing of clipping and cotyledon removal. Isotope labeling experiments were also performed to evaluate contribution of the cotyledons and taproots to resprouting. Regardless of timing of shoot clipping, seedlings successfully resprouted provided their cotyledons were not detached. Clipped seedlings were less likely to resprout when cotyledons were removed. Seedlings clipped at earlier development stage exhibited higher resprouting capacity than those clipped at later stage. Cotyledon removal, more than timing of clipping, decreased the dry masses of newly-resprouted shoots. However, no significant influences of cotyledon removal and timing of clipping were found on the dry masses of roots, suggesting the importance of cotyledons for resprouting. Roots became functional and accumulated more soil nitrogen after shoot clipping and cotyledon removal, representing a double security-based strategy for the clipped seedlings to resprout despite the importance of cotyledons.

Acorns containing deeper plumule survive better: how white oaks counter embryo excision by rodents

Several squirrel species excise the embryo of acorns of most white oak species to arrest germination for long-term storage. However, it is not clear how these acorns counter embryo excision and survive in the arms race of coevolution. In this study, we simulated the embryo excision behavior of squirrels by removing 4 mm of cotyledon from the apical end of white oak acorns differing in embryo depths to investigate the effects of embryo excision on acorn germination and seedling performance of white oak species. The embryo depth in the cotyledons was significantly different among white oak acorns, with Quercus mongolica containing the embryo most deeply in the acorns. We found that artificial embryo excision significantly decreased acorn germination rates of Quercus variabilis, Quercus acutissima, Quercus aliena, Quercus aliena var. acutiserrata, Quercus serrata. var. brevipetiolata but not Q. mongolica. Artificial embryo excision exerted significant negative impacts on seedling performance of all oak species except Quercus aliena. Our study demonstrates the role of embryo depth of acorns in countering embryo excision by squirrels and may explain the fact that squirrels do not perform embryo excision in acorns of Q. mongolica with deeper embryos. This apparent adaptation of acorns sheds light on the coevolutionary dynamics between oaks and their seed predators.