Soil seed bank composition and diversity in a managed temperate deciduous forest

Little is known about the influence of forest management on the interaction between seed bank and aboveground vegetation. We surveyed seed banks and vegetation in 10 forest stands under similar abiotic conditions but submitted either to a coppice-with-standards treatment (n=5) or to a selective-cutting system (n=5). We analyzed species composition and diversity, community ecological profile, and distribution of taxa among different life forms, strategy, morphology and functional type categories. A total of 2085 seedlings (8296 seedsm^–2) germinated-corresponding to 28 species, among which Juncus effusus was the most abundant. Fifty-seven percent of the species were also recorded in the aboveground vegetation, the dominant species being Rubus fruticosus agg., but only 28% of the aboveground species were present in the seed bank. Our results suggest that (1) vernal geophytes and shade-tolerant perennials, which group most true forest species, are not incorporated in the seed bank, (2) parent plants of most seeds were present either in the stand in an earlier dynamic stage or apart from the stand and long-distance dispersed, (3) as expected, early-successional species are well represented in the seed bank, (4) forestry vehicles seem to be a major means of dispersion for stress-tolerant species normally found in forest lanes and wheel tracks. We conclude that seed banks contain species that have a potentially negative impact on the true forest flora and, thus, forest management should minimize soil disturbance and retain remnants of old-coppice woods to conserve disturbance-sensitive true forest species.     

Seed reserve composition in 19 tree species of a tropical deciduous forest in Mexico and its relationship to seed germination and seedling growth

Background and Aims

The size and composition of seed reserves may reflect the ecological strategy and evolutionary history of a species and also temporal variation in resource availability. The seed mass and composition of seed reserves of 19 co-existing tree species were studied, and we examined how they varied among species in relation to germination and seedling growth rates, as well as between two years with contrasting precipitation (652 and 384 mm).

Methods

Seeds were collected from a tropical deciduous forest in the northwest of Mexico (Chamela Biological Station). The seed dry mass, with and without the seed coat, and the concentrations of lipids, nitrogen and non-structural carbohydrates for the seed minus seed coat were determined. The anatomical localization of these reserves was examined using histochemical analysis. The germination capacity, rate and lag time were determined. The correlations among these variables, and their relationship to previously reported seedling relative growth rates, were evaluated with and without phylogenetic consideration.

Key Results

There were interannual differences in seed mass and reserve composition. Seed was significantly heavier after the drier year in five species. Nitrogen concentration was positively correlated with seed coat fraction, and was significantly higher after the drier year in 12 species. The rate and lag time of germination were negatively correlated with each other. These trait correlations were also supported for phylogenetic independent contrasts. Principal component analysis supported these correlations, and indicated a negative association of seedling relative growth rate with seed size, and a positive association of germination rate with nitrogen and lipid concentrations.

Conclusions

Nitrogen concentration tended to be higher after the drier year and, while interannual variations in seed size and reserve composition were not sufficient to affect interspecific correlations among seed and seedling traits, some of the reserves were related to germination variables and seedling relative growth rate.     

Consequences of frugivore-mediated seed dispersal for the spatial and genetic structures of a neotropical palm

The idiosyncratic behaviours of seed dispersers are important contributors to plant spatial associations and genetic structures. In this study, we used a combination of field, molecular and spatial studies to examine the connections between seed dispersal and the spatial and genetic structures of a dominant neotropical palm Attalea phalerata. Field observation and genetic parentage analysis both indicated that the majority of A. phalerata seeds were dispersed locally over short distances (<30 m from the maternal tree). Spatial and genetic structures between adults and seedlings were consistent with localized and short-distance seed dispersal. Dispersal contributed to spatial associations among maternal sibling seedlings and strong spatial and genetic structures in both seedlings dispersed near (<10 m) and away (>10 m) from maternal palms. Seedlings were also spatially aggregated with juveniles. These patterns are probably associated with the dispersal of seeds by rodents and the survival of recruits at specific microsites or neighbourhoods over successive fruiting periods. Our cross-cohort analyses found palms in older cohorts and cohort pairs were associated with a lower proportion of offspring and sibling neighbours and exhibited weaker spatial and genetic structures. Such patterns are consistent with increased distance- and density-dependent mortality over time among palms dispersed near maternal palms or siblings. The integrative approaches used for this study allowed us to infer the importance of seed dispersal activities in maintaining the aggregated distribution and significant genetic structures among A. phalerata palms. We further conclude that distance- and density-dependent mortality is a key postdispersal process regulating this palm population.     

A seed predator drives the evolution of a seed dispersal mutualism

Although antagonists are hypothesized to impede the evolution of mutualisms, they may simultaneously exert selection favouring the evolution of alternative mutualistic interactions. We found that increases in limber pine (Pinus flexilis) seed defences arising from selection exerted by a pre-dispersal seed predator (red squirrel Tamiasciurus hudsonicus) reduced the efficacy of limber pine's primary seed disperser (Clark's nutcracker Nucifraga columbiana) while enhancing seed dispersal by ground-foraging scatter-hoarding rodents (Peromyscus). Thus, there is a shift from relying on primary seed dispersal by birds in areas without red squirrels, to an increasing reliance on secondary seed dispersal by scatter-hoarding rodents in areas with red squirrels. Seed predators can therefore drive the evolution of seed defences, which in turn favour alternative seed dispersal mutualisms that lead to major changes in the mode of seed dispersal. Given that adaptive evolution in response to antagonists frequently impedes one kind of mutualistic interaction, the evolution of alternative mutualistic interactions may be a common by-product.     

Effects of seed release timing on plant life-history and seed production in a population of a desert annual,shape Blepharis sindica (Acanthaceae)

Growth, phenology, survivorship, and seed production were observed in a population of a desert annual, Blepharis sindica, with reference to the variation in the timing of seedling emergence. The population consisted of several cohorts induced by rain-cued seed release within a growing season. The fate of 100 individuals of six cohorts was monitored throughout the growing season. Earlier-established cohorts had significantly larger plant sizes and higher reproductive outputs than later cohorts. The time and duration of each phenological stage varied among the cohorts, and they were also influenced by plant size. Mortalities at the seedling stage, vegetative stage, and reproductive stage increased with the delay of seed release. Seed release was concentrated in the early growing season. Fecundity was highest in the earliest cohort and decreased monotonically in later cohorts. The results suggested that even in temporally varying environments, the superiority of early emergent plants was evident. The seed release patterns in temporally fluctuating desert environments are discussed as a compromise between 'diversified bet-hedging' and an optimal timing for maximizing the reproductive success in a growing season.     

Interaction between ants and seeds of a nonmyrmecochorous neotropical tree, Cabralea canjerana (Meliaceae), in the Atlantic forest of southeast Brazil

On the forest floor of two Atlantic forest sites in southeast Brazil, we recorded 26 ant species (12 genera) interacting with the seeds of Cabralea canjerana (Meliaceae), a typical ornithochorous tree whose seeds are covered by a lipid-rich aril. The ants treat the arillate seeds in three different ways: (1) the large ponerine ants Pachycondyla striata and Odontomachus chelifer individually remove the seeds to their nests, (2) many species (Pheidole spp.) recruit workers to remove the aril on the spot, or (3) Solenopsis spp. recruit nestmates and cover the seeds with soil before removing the aril on the spot. The ants remove the aril exceptionally rapidly, and removal greatly facilitates seed germination. Seed predation by insects below fruiting trees is severe, and field experiments using vertebrate exclosures showed that rodents also prey heavily upon seeds found near parent trees. Ponerine ants actively remove seeds from this predation-prone zone. By removing bird-manipulated and naturally fallen seeds, ants can play a key role in the fate of medium-sized seeds like those of C. canjerana.     

Seed Dispersal by Salvin's Curassow,Mitu salvini (Cracidae), in a Tropical Forest of Colombia: Direct Measurements of Dispersal Distance1

Seed dispersal by Salvin's curassows (Mitu salvini) was studied in a lowland tropical forest at La Macarena on the border of the Macarena and Tinigua National Parks, the Department of Meta, Colombia. Continuous observations were made on the feeding and ranging behavior of a well-habituated pair of birds from 0600 to 1800 h for 19 days. Ibtal observation time was 222 h. The daily distance traveled by the birds ranged from 630 to 3750 m, with a mean of 1959 m (± 776 m). The total home range was ca 34.4 ha. Although curassows were observed consuming fruits of 13 species belonging to the families Rubiaceae, Meliaceae, Moraceae, Burseraceae, Leguminosae, and Lecythidaceae, only seeds of Geophila re fens (Rubiaceae) and Picas spbenophylla (Moraceae) were found in their feces. For G. repens, the mean and maximum retention times were 1 h 52 min (± 1 h 20 min) and 6 h 08 min, and the mean and maximum direct dispersal distances were 245 m (± 164 m) and 633 m. More than half the seeds were dispersed in canopy gaps. For F. sphenophylla, the mean and maximum retention times were 3 h 1 5 mm (± .37 min) and 7 h 08 min, and the mean and maximum direct dispersal distances were 329 m (± 46 m) and 451 m. Nearly 60 percent of the estimated seed intake of G. repens and 92-94 percent of F. sphenophylla were digested or damaged. The retention times and the dispersal distances for Streptogyna americana, which has adhesive burrs, were also measured. The mean and maximum retention times were 1 h 55 min (± 1 h 56 min) and 9 h 11 min, and the mean and maximum direct dispersal distances were 128 m (± 68 m) and 280 m. This is the first study in which direct measurements were made for retention times and dispersal distances of the epizoochory and endozoochory for birds in the field.     

Bird dispersal of the larval stage of a seed predator

Multiflora rose seeds containing larvae of the phytophagous wasp Megastigmus aculeatus nigroflavus were fed to a caged mockingbird (Mimus polyglottos). Ninety percent of the seeds were recovered from the bird's feces, and there was no significant difference in subsequent emergence of the wasps from these seeds when compared to controls. This study indicates the dispersal of seed-feeding hymenoptera by avian frugivores may be a widespread and significant phenomenon.     

Fruit eating and seed dispersal by howling monkeys (Alouatta palliata) in the tropical rain forest of Los Tuxtlas,Mexico

The frugivory and ranging habits of howling monkeys living in the rain forest of Los Tuxtlas, Veracruz, Mexico, were studied for 5 consecutive years with the aim of investigating the role of seed dispersal agents played by the howling monkeys. The howling monkeys' consumption of fruit occupied half of their feeding time, and 80% of this time was spent feeding on mature fruit. Observations confirmed use of 19 tree species and a preference for a few species of Moraceae and Lauraceae. Fruit exploitation was very seasonal; only two species provided fruit year-round. Home range size was about 60 ha, and animals in the troop (N = 16) showed a day range of 10–893 m. Monthly collection of fecal samples during 2 years indicated that 90% of the seeds (N = 7,928) in the samples belonged to eight tree species and to one liana; the rest belonged to 15 unidentified species of vines. Germination success for the seeds in the feces was about 60% and for control seeds was 35%. Howling monkeys created diverse seed shadows in the vicinity of their leaf and fruit sources, and while they dispersed the seeds of some plant species, they also produced a great deal of fruit and seed waste for others.     

Carnivorous mammals as seed dispersers of Myrtus communis (Myrtaceae) in the Mediterranean shrublands

ABSTRACT

The role of carnivorous mammals as seed dispersers of Myrtus communis L. in the Mediterranean environment was studied. Seeds of M. communis were found in faeces of red fox (Vulpes vulpes L.) and carnivores from the genus Martes, at four sites on the Tyrrhenic coast of Southern Italy in the period autumn-winter 1993–1995. All the study sites are covered by Mediterranean shrublands with M. communis as one of the dominant species. Content analysis of fox scats showed that fruits of M. communis constitute a relevant component of the diet of these carnivores, although other food categories were also well represented. Further investigations on the effect of mammal digestion on seed germinability showed that carnivores not only moved the seeds away from the mother plant but also enhanced their germinability. This species was known to be bird- and ant-dispersed and the adaptive implications of a multi-vectorial seed dispersal process is discussed.     

Post-dispersal embryo development, germination phenology, and seed dormancy in Cardiocrinum cordatum var. glehnii (Liliaceae s. str.), a perennial herb of the broadleaved deciduous forest in Japan

In an investigation of seed germination in Cardiocrinum cordatum var. glehnii, embryos in fresh seeds in October were underdeveloped and did not grow until September of the following year. Then, they grew rapidly and had fully elongated by early November. In the second spring after dispersal, radicles emerged under snow in late March and after snowmelt in April. Cotyledons emerged soon after radicles. In several laboratory experiments, embryos grew at 15°/5°C (light 12 h/ dark 12 h) following 25°/15°C. Radicles emerged from seeds with fully elongated embryos at 5°-15°C after cold stratification at 0°-5°C. Cotyledons emerged in 2 wk from seeds with a radicle at 15°/5°C to 30°/20°C. Although seeds require c. 18-19 mo after dispersal to germinate in nature, under controlled conditions, they required only 9 mo with a sequence of 25°/15°C → 15°/5°C → 0°-5°C → 15°/5°C. This is practical knowledge for propagation of plants from seeds. GA(3) treatment partially substituted for the high temperature requirement. Based on dormancy-breaking requirements, the seeds have deep simple morphophysiological dormancy (MPD). A literature review of seed dormancy in taxa of Liliaceae s. str. showed that phylogenetic position in this case is not a good predictor of level of MPD.     

Ecological significance of the aerial seed pool of a desert lignified annual,Blepharis sindica (Acanthaceae)

Reproductive traits of a lignified annual plant, Blepharis sindica were studied in relation to the formation of an 'aerial seed pool' on dead plants in an arid grassland in the Thar Desert of northwestern India. The dead plants remained standing on the soil surface and retained fruits for more than one year. Aerial seed pools developed about 6 cm above the ground. There were no seed pools on or below the ground surface. Only 5.7% of seeds died on dead plants because of insect predation or fungi infection during one year. Seed release was cued by rainfall, and a fraction of seeds on the aerial seed pools was released in each rainfall event. After 13 rainfall events during the monsoon season, 25% of seeds was still retained on the plants. Seed predation on the ground surface was intensive; all cones placed on the soil surface were removed within four days, and 97% of fruits were removed within 10 days. Fifty percent of seeds germinated within 3.5 h, and there were no differences in viability and time required for germination between first year seeds and older seeds. The results indicate that the aerial seed-holding on dead plants is an available way to avoid seed predation in harsh desert environments where seed predation is intense and favorable periods for growth are temporally limited and unpredictable.     

Ecology of seed germination of eight non-pioneer tree species from a tropical seasonal rain forest in southwest China

We compared various aspects of the seed biology of eight non-pioneer tree species from a tropical seasonal rain forest in Xishuangbanna, SW China, that differ in time of dispersal, size and fresh seed moisture content (MC). Seeds were tested for germination under laboratory conditions after dehydration to different moisture levels and under 3.5, 10 and 30% solar irradiances in neutral-shade houses. For six species, germination was also compared in forest understory (3.5% light) and center of a forest gap (32.5% light). Under continuous dehydration over activated silica gel, 100% of seeds of four species had lost the ability to germinate after 48 h, and those of all species except Castanopsis hystrix (decreased from >90 to 30% germination) had lost the ability to germinate after 120 h. Four species did not differ in final germination percentages at the three irradiances (i.e. uniform germination). However, final germination percentages of Horsfieldia pandurifolia and Litsea pierrei var. szemaois were significantly lower in 30% than in 10 or 3.5% light, and seeds of Antiaris toxicaria and C. hystrix germinated to higher percentages in 30 and 10% than in 3.5% light. Mean time to germination (MTG) of the eight species (forest and shade house data combined) ranged from 5–5 days for Pometia tomentosa to 72–207days for L. pierrei; MTG for four species was ≤21 days. There was no obvious relationship between relative desiccation resistance and either time of dispersal, MTG or uniformity of germination at the three light levels, or between seed size and MC or MTG. However, the relationship between seed MC at maturity (25–60% fresh mass basis) and MC at 50% loss of seed viability (12.4–42.5%) was significant. Seven of the species fit Garwood’s (Ecol Monogr 53:159–181, 1983) rapid-rainy germination syndrome and one, L. pierrei, either her delayed-rainy or intermediate-dry germination syndrome. However, fresh, non-dehydrated seeds of all eight species germinated in ≤30 days at constant 30°C in light.     

Thermal plasticity of photosynthesis: the role of acclimation in forest responses to a warming climate

The increasing air temperatures central to climate change predictions have the potential to alter forest ecosystem function and structure by exceeding temperatures optimal for carbon gain. Such changes are projected to threaten survival of sensitive species, leading to local extinctions, range migrations, and altered forest composition. This study investigated photosynthetic sensitivity to temperature and the potential for acclimation in relation to the climatic provenance of five species of deciduous trees, Liquidambar styraciflua, Quercus rubra, Quercus falcata, Betula alleghaniensis, and Populus grandidentata. Open‐top chambers supplied three levels of warming (+0, +2, and +4 °C above ambient) over 3 years, tracking natural temperature variability. Optimal temperature for CO₂ assimilation was strongly correlated with daytime temperature in all treatments, but assimilation rates at those optima were comparable. Adjustment of thermal optima was confirmed in all species, whether temperatures varied with season or treatment, and regardless of climate in the species' range or provenance of the plant material. Temperature optima from 17° to 34° were observed. Across species, acclimation potentials varied from 0.55 °C to 1.07 °C per degree change in daytime temperature. Responses to the temperature manipulation were not different from the seasonal acclimation observed in mature indigenous trees, suggesting that photosynthetic responses should not be modeled using static temperature functions, but should incorporate an adjustment to account for acclimation. The high degree of homeostasis observed indicates that direct impacts of climatic warming on forest productivity, species survival, and range limits may be less than predicted by existing models.