Seed damage tolerance and seedling resprouting ability of Prioria copaifera in Panamá

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.

Endangered plant-parrot mutualisms: seed tolerance to predation makes parrots pervasive dispersers of the Parana pine

Parrots are largely considered plant antagonists as they usually destroy the seeds they feed on. However, there is evidence that parrots may also act as seed dispersers. We evaluated the dual role of parrots as predators and dispersers of the Critically Endangered Parana pine (Araucaria angustifolia). Eight of nine parrot species predated seeds from 48% of 526 Parana pines surveyed. Observations of the commonest parrot indicated that 22.5% of the picked seeds were dispersed by carrying them in their beaks. Another five parrot species dispersed seeds, at an estimated average distance of c. 250 m. Dispersal distances did not differ from those observed in jays, considered the main avian dispersers. Contrary to jays, parrots often dropped partially eaten seeds. Most of these seeds were handled by parrots, and the proportion of partially eaten seeds that germinated was higher than that of undamaged seeds. This may be explained by a predator satiation effect, suggesting that the large seeds of the Parana pine evolved to attract consumers for dispersal. This represents a thus far overlooked key plant-parrot mutualism, in which both components are threatened with extinction. The interaction is becoming locally extinct long before the global extinction of the species involved.

Frugivory in Canopy Plants in a Western Amazonian Forest: Dispersal Systems, Phylogenetic Ensembles and Keystone Plants

Frugivory is a widespread mutualistic interaction in which frugivores obtain nutritional resources while favoring plant recruitment through their seed dispersal services. Nonetheless, how these complex interactions are organized in diverse communities, such as tropical forests, is not fully understood. In this study we evaluated the existence of plant-frugivore sub-assemblages and their phylogenetic organization in an undisturbed western Amazonian forest in Colombia. We also explored for potential keystone plants, based on network analyses and an estimate of the amount of fruit going from plants to frugivores. We carried out diurnal observations on 73 canopy plant species during a period of two years. During focal tree sampling, we recorded frugivore identity, the duration of each individual visit, and feeding rates. We did not find support for the existence of sub assemblages, such as specialized vs. generalized dispersal systems. Visitation rates on the vast majority of canopy species were associated with the relative abundance of frugivores, in which ateline monkeys (i.e. Lagothrix and Ateles) played the most important roles. All fruiting plants were visited by a variety of frugivores and the phylogenetic assemblage was random in more than 67% of the cases. In cases of aggregation, the plant species were consumed by only primates or only birds, and filters were associated with fruit protection and likely chemical content. Plants suggested as keystone species based on the amount of pulp going from plants to frugivores differ from those suggested based on network approaches. Our results suggest that in tropical forests most tree-frugivore interactions are generalized, and abundance should be taken into account when assessing the most important plants for frugivores.

Low redundancy in seed dispersal within an island frugivore community

The low species diversity that often characterizes island ecosystems could result in low functional redundancy within communities. Flying foxes (large fruit bats) are important seed dispersers of large-seeded species, but their redundancy within island communities has never been explicitly tested. In a Pacific archipelago, we found that flying foxes were the sole effective disperser of 57 % of the plant species whose fruits they consume. They were essential for the dispersal of these species either because they handled >90 % of consumed fruit, or were the only animal depositing seeds away from the parent canopy, or both. Flying foxes were especially important for larger-seeded fruit (>13 mm wide), with 76 % of consumed species dependent on them for dispersal, compared with 31 % of small-seeded species. As flying foxes decrease in abundance, they cease to function as dispersers long before they become rare. We compared the seed dispersal effectiveness (measured as the proportion of diaspores dispersed beyond parent crowns) of all frugivores for four plant species in sites where flying foxes were, and were not, functionally extinct. At both low and high abundance, flying foxes consumed most available fruit of these species, but the proportion of handled diaspores dispersed away from parent crowns (quality) was significantly reduced at low abundance. Since alternative consumers (birds, rodents and land crabs) were unable to compensate as dispersers when flying foxes were functionally extinct, we conclude that there is almost no redundancy in the seed dispersal function of flying foxes in this island system, and potentially on other islands where they occur. Given that oceanic island communities are often simpler than continental communities, evaluating the extent of redundancy across different ecological functions on islands is extremely important.

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.

The effect of light, seed size and biomass removal on cotyledon reserve use and root mass allocation in Gustavia superba seedlings

Some large-seeded tree species have cotyledonary reserves that persist for months after seedling establishment. We carried out two screened growing-house experiments with seedlings of Gustavia superba (Lecythidaceae) to test hypotheses proposed to explain why cotyledons are retained. We grew seedlings from large and small seeds in sun and shade to determine if cotyledon reserves supplement photosynthetic carbon gain, and in a second experiment applied defoliation and shoot removal treatments to determine if reserves are allocated to resprout tissue. In each experiment we tracked cotyledonary resource use over time and measured the fraction of seedling biomass allocated to roots and shoots. We found no evidence that light environment, seed size or damage treatment affected the rate of cotyledon resource usage; 20% of the cotyledonary mass remained 9 wk after leaves were fully developed in both sun and shade and 25–30% of the cotyledonary mass remained 6 wk after leaf or shoot removal. Instead, cotyledon reserves appear to be slowly translocated to roots regardless of light environment or seedling damage. Once seedlings are established, lost tissue is replaced using reserves stored in roots; in high light, damaged seedlings had a lower root mass fraction (0.42) than undamaged ones (0.56) when considering the mass of tissue removed and resprout tissue combined. We conclude that cotyledon reserves are important for resprouting during early seedling emergence and establishment, but do not directly contribute to seedling growth or biomass recovery from herbivores at the post-establishment stage. Persistence of cotyledons may ultimately depend on the development of sufficient root mass for reserve reallocation.

Germination after simulated rat damage in seeds of two endemic Hawaiian palm species

Seed predation by native and alien rodents can limit plant recruitment and ultimately affect forest dynamics and composition (Campbell & Atkinson 2002, Côtéet al. 2003, Hulme 1998, Sánchez-Cordero & Martínez-Gallardo 1998). Even partial consumption of seeds by predators may affect plant community structure, though its importance is poorly understood (Steeleet al. 1993, Vallejo-Marínet al. 2006). Despite consumption of relatively large portions of seeds by herbivores, seeds can retain their ability to germinate if the embryo remains intact (Dalling & Harms 1999, Janzen 1972, Mack 1998). Germination of damaged seeds may be accelerated or prolonged (Karban & Lowenberg 1992, Koptur 1998, Vallejo-Marínet al. 2006). Damage by seed pests also facilitates ageing stress; which manifests as decreased seedling vigour, decreased seed viability, lower germination percentages and slower germination rates (Priestley 1986).

The importance of seed reserves for seedling performance: an integrated approach using morphological, physiological, and stable isotope techniques

To investigate how seed reserves affect early seedling performance, we conducted a factorial greenhouse experiment using Lithocarpus densiflora (Tanoak). Seedlings were grown from large (5.8+/-0.7 g) and small (3.2+/-0.4 g) seeds and, following shoot emergence, seeds were either removed or left attached. Seedlings were harvested for quantification of biomass and delta13C at seven time periods following seed removal (2, 4, 8, 16, 32, 64, 128 days) and seedling photosynthesis was measured three separate time periods (2-4, 49-82, 95-128 days after seed removal). Biomass increased for all seedlings, but the increase was significantly larger for seedlings with attached seeds than with removed seeds. Seed removal just after shoot emergence significantly decreased seedling biomass, but seed removal 64 days after shoot emergence had no effect on seedling biomass. Seedling photosynthesis per unit leaf area varied by time and seed presence, but not by seed size. At the first period, seedlings with attached seeds had significantly higher photosynthetic rates than seedlings with removed seeds, at the second period there was no effect of seed removal, and at the third time period seedlings with attached seeds had significantly lower photosynthetic rates than seedlings with removed seeds. Despite temporal variation in photosynthesis per unit leaf area, seedlings with attached seeds always had significantly greater leaf area than seedlings with removed seeds, resulting in significantly higher total plant photosynthesis at all three time periods. The delta13C values of both the leaves and roots were more similar to that of the seed for seedlings with attached seeds than for seedlings with removed seeds, however, seed removal and seed size strongly affected root delta13C. This study demonstrates that seed reserves have important effects on the early growth, physiology, and delta13C of L. densiflora seedlings.

Propagule size and predispersal damage by insects affect establishment and early growth of mangrove seedlings

Variation in rates of seedling recruitment, growth, and survival can strongly influence the rate and course of forest regeneration following disturbance. Using a combination of field sampling and shadehouse experiments, we investigated the influence of propagule size and predispersal insect damage on the establishment and early growth of the three common mangrove species on the Caribbean coast of Panama: Avicennia germinans, Laguncularia racemosa, and Rhizophora mangle. In our field samples, all three species exhibited considerable intraspecific variation in mature propagule size, and suffered moderate to high levels of predispersal attack by larval insects. Rates of insect attack were largely independent of propagule size both within and among trees. Our experimental studies using undamaged mature propagules showed that, for all three species, seedlings established at high rates regardless of propagule size. However, propagule size did have a marked effect on early seedling growth: seedlings that developed from larger propagules grew more rapidly. Predispersal insect infestations that had destroyed or removed a substantial amount of tissue, particularly if that tissue was meristematic or conductive, reduced the establishment of propagules of all three species. The effect of sublethal tissue damage or loss on the subsequent growth of established seedlings varied among the three mangrove species. For Avicennia, the growth response was graded: for a propagule of a given size, the more tissue lost, the slower the growth of the seedling. For Laguncularia, the response to insect attack appeared to be all-or-none. If the boring insect penetrated the outer spongy seed coat and reached the developing embryo, it usually caused sufficient damage to prevent a seedling from developing. On the other hand, if the insect damaged but did not penetrate the seed coat, a completely healthy seedling developed and its growth rate was indistinguishable from a seedling developing from an undamaged propagule of the same size. Similar to Avicennia, if an infestation did not completely girdle a Rhizophora seedling, it survived, but grew at a reduced rate. In summary, our experiments demonstrated that natural levels of variation in propagule size and predispersal damage by insects translate into significant differences in seedling performance in terms of establishment and/or early growth. Such differences are sufficiently large that they could influence the intensity and outcome of competitive interactions during forest regeneration.