Empty seeds are not always bad: simultaneous effect of seed emptiness and masting on animal seed predation

Temperature and soil nutrients drive seed traits variation in Pterocarpus erinaceus (African rosewood) in Ghana

Among plant populations, variation in seed traits has important consequences on species recruitment and performance under different environmental conditions. Knowing such variations and understanding its environmental drivers could help with conservation efforts that protect against the loss of diversity. This information is however lacking in the extinction-threatened Pterocarpus erinaceus Poir (African rosewood) in Ghana. Here, we assessed variation in seed set, seed morphological and chemical traits, germinability, and seedling growth of African rosewood from four distant provenances (Tumu, Wa, Carpenter, and Ejura) in Ghana. We sought to answer how local environmental conditions influence the expression of seed traits by examining the relationship between seed traits and maternal environmental factors (temperature, rainfall, soil nutrient, and vegetation index) using regression models and correlation analysis. Seed size, mass, and nutrient composition differed considerably among provenances. Seed size and mass increased as the seed source moved further away from the forest-savanna transition toward the Guineo-Sudanian savanna regions. Temperature mainly accounted for the variability observed in seed traits. Phenology curves of the seed source vegetation expressed a strong correlation with monthly rainfall. Overall, the occurrence of samara containing whole seeds was low (<50%) except for Tumu provenance. Seeds were rich in carbohydrate and crude protein content consistent with most leguminous plants while mean percentage germination ranged between 30 and 62% among provenances. Our results highlight the adaptive strategies of African rosewood to different environments through the expression of their seed traits and suggest the need for priority action to maintain its conservation.

The morphological and ecological variation of Arctostaphylos (Ericaceae) fruit: A link between plant ecology and animal foraging behavior

Persistent soil seed banks are characteristic of Arctostaphylos (Ericaceae) species in the Mediterranean-climate California Floristic Province. While most species are obligate seeders, regeneration of stands of all Arctostaphylos species ultimately depends on post-fire seedling recruitment. Arctostaphylos seed banks are created, in large part, by scatter-hoarding rodents. Variation in fruit morphology, therefore, is expected to impact the Arctostaphylos-rodent interaction. Seeds produce sufficient rewards (nutritious mature embryo) to entice rodents to disperse and ultimately bury seeds in the soil. Hard seed coats increase the time required to extract the embryo, encouraging rodents to choose storage over immediate predation, and nutlets are frequently empty. We assessed the variation of fruit nutlet fusion and seed viability among 38 Arctostaphylos taxa. Factors such as latitude, elevation, life history, ploidy, and phylogenetic position were also analyzed. Generalized mixed-effects models were used to determine the factors contributing to variation in fruit nutlet fusion and seed viability. Our results indicate that fruit volume and shape are the most important variables affecting nutlet fusion and seed viability. Additionally, other potential influences only show a weak correlation and are not predicted to significantly impact nutlet fusion or seed viability. These findings provide insights into evolved strategies used by plants to increase reproductive success via scatter-hoarding rodents. Our study benefits the conservation and restoration of Arctostaphylos stands by emphasizing the importance of animal-mediated dispersal and providing estimates of seed viability for different species. With the anticipated effects of climate change, such as departures from historic fire regimes, the preservation of the relationship between plants and animal foragers is crucial for the continued survival of Arctostaphylos and California's evergreen chaparral.

Jack pine of all trades: Deciphering intraspecific variability of a key adaptive trait at the rear edge of a widespread fire-embracing North American conifer

PREMISE

Understanding mechanisms fostering long-term persistence of marginal populations should provide key insights about species resilience facing climate change. Cone serotiny is a key adaptive trait in Pinus banksiana (jack pine), which shows phenotypic variation according to the fire regime. Compared to range-core populations within the fire-prone boreal forest, low and variable serotiny in rear-edge populations suggest local adaptation to uncommon and unpredictable wildfire regime. We assessed environmental/physiological factors that might modulate intraspecific variation in cone serotiny.

METHODS

We experimentally subjected closed cones to incrementing temperatures, then tested seed germination to determine whether and how various ecological factors (cone age, branch height, tree size, tree age) are related to cone dehiscence and seed viability in jack pines from rear-edge and range-core populations in eastern Canada.

RESULTS

Cones from rear-edge populations dehisce at a lower opening temperature, which increases with cone age. Cones from range-core stands open at a more constant, yet higher temperature. Cones from rear-edge stands take between 13 and 27 years to reach the level of serotiny achieved at the range core. At the rear edge, seed viability is steady (51%), whereas it decreases from 70% to 30% in 20 years at the range core.

CONCLUSIONS

We inferred the mechanisms of a bet-hedging strategy in rear-edge populations, which ensures steady recruitment during fire-free intervals and successful postfire regeneration. This capacity to cope with infrequent and unpredictable fire regime should increase the resilience of jack pine populations as global changes alter fire dynamics of the boreal forest.

Endozoochory of frugivorous birds as potential dispersal of seed parasites from Pistacia chinesis: experimental evidence

The plant-disperser-fruit pest triads involve 3 interacting animals or groups (plants, vertebrates and seed parasites), and the dispersal of both seeds and seed parasites, which can both benefit from endozoochory via defecation or regurgitation by frugivorous vertebrates. However, we have very limited knowledge about the ecological and evolutionary consequences of these plant-disperser-fruit pest triads. Across central Northern China, several seed wasps (mainly Eurytoma plotnikov attack Pistacia chinensis fruits, and seed wasp larvae can develop, diapause and finally emerge as adults inside a seed during the following 1-3 years. In this study, we experimentally investigated whether frugivorous birds discriminated P. chinesis fruits with or without seed parasites, and whether bird endozoochory (by defecation or regurgitation) affected larval survival of seed parasites. The infestation rate by seed parasites was 37% of the P. chinesis fruit crop but with up to 48% of aborted fruits. We found that all 5 bird species can discriminate and then reject all unhealthy fruits (including aborted and insect-infested). However, 4 of the 5 bird species, in particular bulbul species, consumed 15-41% of aborted and insect-infested fruits as complementary food only when these unhealthy fruits were provided. Moreover, all larva of seed parasites remained alive after bird defecation or regurgitation. In conclusion, our study demonstrates that endozoochory by frugivorous birds could lead to potential dispersal of seed parasites of P. chinensis but with a very low probability.

Fruit development of Lithocarpus (Fagaceae) and the role of heterochrony in their evolution

Stone oaks, or Lithocarpus species of Fagaceae are ecologically important canopy trees in the tropical and subtropical forests over East Asia, and the fruits of which are important food sources for insects and vertebrates there. The great fruit morphological variation of this genus represents two fruit types, acorn and enclosed receptacle fruit types. However, the evolutionary mechanisms of differentiation into these two fruit types with contrasting morphology remain a puzzle. To reveal the morphogenetic properties of two fruit types, we observed tissue differentiation and development among 20 Lithocarpus species from fruit set to maturity. Unlike in fruits of Quercus, the endocarp differentiation in Lithocarpus fruits occurred later than exocarp and mesocarp. Cupules provided further protection of developing seeds, particularly of acorn-type fruits. Fruits of Lithocarpus and Quercus acorns share similar insect predators. At fruit set, both acorn and enclosed receptacle types were largely identical, with similar tissue morphology and the sequence of differentiation. The distinct difference between two fruit types at maturity came from varied rates and degrees of development between the pericarp and receptacle tissues. We found that heterochrony between two tissues could create substantially divergent ecological strategies for protection and dispersal of their seeds, which is essential for the evolution of two fruit types.

Surface properties and physiology of Ulmus laevis and U. minor samaras: implications for seed development and dispersal

Plant surface properties influence solid-liquid interactions and matter exchange between the organs and their surrounding environment. In the case of fruits, surface processes may be of relevance for seed production and dispersal. To gain insight into the relationship between surface structure, chemical composition and function of aerial reproductive organs, we performed diverse experiments with the dry, winged fruits, or samaras, of Ulmus laevis Pall. and Ulmus minor Mill. both at the time of full maturity (green samaras) and of samara dispersal (dry samaras). Samaras of both elm species showed positive photosynthetic rates and absorbed water through their epidermal surfaces. The surface wettability, free energy, polarity and solubility parameter were lower in U. laevis than in U. minor and decreased for dry samaras in both species. Ulmus laevis samaras had a high degree of surface nano-roughness mainly conferred by cell wall folds containing pectins that substantially increased after hydration. The samaras in this species also had a thicker cuticle that could be isolated by enzymatic digestion, whereas that of U. minor samaras had higher amounts of soluble lipids. Dry samaras of U. laevis had higher floatability and lower air sustentation than those of U. minor. We concluded that samaras contribute to seed development by participating in carbon and water exchange. This may be especially important for U. minor, whose samaras develop before leaf emergence. The trichomes present along U. laevis samara margin may enhance water absorption and samara floatability even in turbulent waters. In general, U. minor samaras show traits that are consistent with a more drought tolerant character than U. laevis samaras, in line with the resources available both at the tree and ecosystem level for these species. Samara features may additionally reflect different adaptive strategies for seed dispersal and niche differentiation between species, by favoring hydrochory for U. laevis and anemochory for U. minor.

Comparing the Sexual Reproductive Success of Two Exotic Trees Invading Spanish Riparian Forests vs. a Native Reference

A widely accepted hypothesis in invasion ecology is that invasive species have higher survival through the early stages of establishment than do non-invasive species. In this study we explore the hypothesis that the sexual reproductive success of the invasive trees Ailanthus altissima (Mill.) Swingle and Robinia pseudoacacia L. is higher than that of the native Fraxinus angustifolia Vahl., all three species coexisting within the riparian forests of Central Spain. We compared different stages of the early life cycle, namely seed rain, seed infestation by insects, seed removal by local fauna, seed germination under optimal conditions and seedling abundance between the two invasive trees and the native, in order to assess their sexual reproductive success. The exotic species did not differ from the native reference (all three species displaying high seed rain and undergoing seed losses up to 50% due to seed removal by the local fauna). Even if the exotic R. pseudoacacia showed a high percentage of empty and insect-parasited seeds along with a low seedling emergence and the exotic A. altissima was the species with more viable seeds and of higher germinability, no differences were found regarding these variables when comparing them with the native F. angustifolia. Unsuitable conditions might have hampered either seedling emergence and survival, as seedling abundance in the field was lower than expected in all species -especially in R. pseudoacacia-. Our results rather suggest that the sexual reproductive success was not higher in the exotic trees than in the native reference, but studies focusing on long-term recruitment would help to shed light on this issue.

Potential Germination Success of Exotic and Native Trees Coexisting in Central Spain Riparian Forests

We compared potential germination success (i.e., percentage of produced seeds that germinate under optimal conditions), the percentage of empty and insect-damaged seeds, germinability (Gmax), and time to germination (Tgerm) between the exoticsAilanthus altissima,Robinia pseudoacacia,andUlmus pumilaand two coexisting native trees (Fraxinus angustifoliaandUlmus minor) in the riparian forests of Central Spain. Additionally, we tested the effect of seed age, seed bank type (canopy or soil) and population onGmaxandTgermofA. altissimaandR. pseudoacacia, which are seed-banking species. Species ranked by their potential germination success wereA. altissima>U. pumila>R. pseudoacacia>U. minor>F. angustifolia. The combination of a highGmaxand negligible seed insect-damage providedA. altissimawith a potential germination advantage over the natives, which were the least successful due to an extremely high percentage of empty seeds or a very lowGmax.R. pseudoacaciashowed high vulnerability to insect seed predation which might be compensated with the maintenance of persistent seed banks with highGmax.GmaxandTgermwere strongly affected by seed age in the seed-banking invaders, but between-seed bank variation ofGmaxandTgermdid not show a consistent pattern across species and populations.