Spatial Distribution Of Seeds Research Articles

Horizontal seed dispersal by dung beetles reduced seed and seedling clumping, but did not increase short-term seedling establishment.

Dung beetles are secondary seed dispersers, incidentally moving many of the seeds defecated by mammals vertically (seed burial) and/or horizontally as they process and relocate dung. Although several studies have quantified this ecological function of dung beetles, very few have followed seed fate until seedling establishment, and most of these have focused on the effects of seed burial. We know very little about the effects of horizontal seed movement by dung beetles, though it is generally assumed that it will affect plant recruitment positively through diminishing seed clumping. The objective of our study was to assess the effects of dung beetle activity on the spatial distribution of seeds and seedlings, and on the probability of seedling establishment. In a tropical rainforest in Mexico we carried out two complementary field experiments for each of two tree species (Bursera simaruba and Poulsenia armata), using seeds experimentally imbedded in pig dung and recording their fate and spatial location over time. For both species, dung beetle activity reduced the spatial clumping of seeds and seedlings; however, it did not increase the probability of seedling establishment. We discuss the context- and species-specificity of the combined effects of horizontal and vertical dispersal of seeds by dung beetles, and the need to quantify long-term seedling fates to more accurately determine the effects of seed movement by dung beetles on plant recruitment.

The effects of two abundant ant species on soil nutrients and seedling recruitment in Brazilian Atlantic Forest

Ants can influence soil fertility and the spatial distribution of seeds, with possible effects on seedling recruitment. The ant species Pachycondyla striata Fr. Smith, 1858 and Odontomachus chelifer (Latreille, 1802) co-occur in many forest areas in the Neotropics. We assessed soil fertility and seed bank structure in soil samples close and distant (control) from ant nests in forest fragments. We also assessed the richness and abundance of seedlings on nests and control sites. In soil samples from ant nests, the concentration of phosphorus and potassium were respectively 55.6% and 36% higher than in control sites. Aluminium was 11–15% lower in soil samples from ant nests. In the greenhouse, soils from ant nests had higher plant abundance and species richness, but the same species composition in comparison with control sites. Although more plants emerged from soil samples of O. chelifer nests, in the field, the density and richness of seedlings were similar for the two ant species studied. Seedlings in the nest sites were, on average, 1.8 times more abundant and 1.6 times richer in species than in control sites. Our results showed that ant species can play a key role in seedling recruitment in forest fragments, where other animals with equivalent and positive effects, such as mammals, are missing.

Physical Properties of Maize Seed and its Effect on Sowing Quality and Variable Distance of Individual Plants

Precision sowing has been a major thrust of agricultural engineering research for many years; however, most of the research and development work has dealt with seeders for agronomic crops. Sowing quality means required vertical (depth) and horizontal (spatial) spacing of seeds in the soil with minimal damage to the seeds. The spatial distribution of seeds (horizontal spacing) is given by distance between the rows and spacing of the seeds in a row. The objective of the study was the evaluation and comparison of the sowing quality of sowing machine JD MaxEmerge XP with mechanical filling of ladle holes with holding fingers within usage of seeds with different dimensional and shape attributes in agricultural conditions. As it was observed that shape attributes have a significant effect on seeding quality and at the end it affects emergency and whole further growth of maize seeds. Sowing machine JD MaxEmerge XP reached good results for the quality especially when using the angular seeds. It reached the sowing quality expressed by the standard deviation of 46.95–50.76 % when seeds PR 37 N01 and for seeds PR 37 N01 it is 47.43–48.73 %. Another evaluative criterion is dual sowing of seeds or plants (DRR), it was reached the average value 3.65–2.45 % for angular seeds and the average of 1.54–3.71 % for approximately spherical seeds.

<b>Ferns influence on the woody species seedling bank in semi-deciduous forest, Southern Brazil

Different species of plants can act as environmental filters affecting the composition, structure and spatial distribution of seeds and seedlings of forest species. This study aimed to examine whether the presence of the fern stratum acts as a biotic environmental filter on the woody species seedling bank in a semi-deciduous forest remnant (Atlantic biome), Southern Brazil. Two experiments were developed in areas with ferns (F) and ferns-free (NF): a) richness and abundance of woody seedling bank were determined in 10 plots (25 m 2 ) in each area; b) seedlings abundance of Actinostemon concolor (Spreng.) Mull. Arg., Aspidosperma polyneuron Mull. Arg., Euterpe edulis Mart., Guarea kunthiana A. Juss. and Inga marginata Kunth were monitored for a year in three areas F, NF and an area with ferns whose fronds were cut (CF). In this experiment, it was used five plots (4 m 2 ) in each area. The abundance of seedlings (considering all species) was higher in F areas and ferns positively and negatively affected the abundance of A. polyneuron and G. kunthiana , respectively. It is possible to conclude that ferns worked as a selective environmental filter for some woody species.

Production components of corn as function of seed distribution along the planting row

ABSTRACT High corn yields in high-tech systems are related to proper crop implementation. This study aimed to evaluate the effects of variability in the distribution of seeds along the planting row on corn production components. The study was conducted under Cerrado conditions in the municipality of Urutaí, GO, Brazil. The experimental design was randomized blocks in a 5 x 2 factorial scheme, with four replicates. The effects of five coefficients of variation (0, 20, 40, 60 and 80%) of non-uniformity in the spatial distribution of seeds along the planting row of two corn hybrids (P30F53HX and P3646HX) were evaluated. No interactions were observed for the analysed corn variables. However, as the non-uniformity in seed distribution along the planting row increased, stalk diameter, hundred-grain weight, number of rows per ear, number of kernels per row and ear length decreased. Additionally, linear reductions were observed in corn grain yield with the increase in the coefficient of variation of the spatial distribution of seeds along the planting row. Between the hybrids, the 30F53HX showed higher hundred-grain weight.

Could recently locally extinct population patches of Astragalus nitidiflorus regenerate from the soil seed bank?

Persistence, distribution, and dynamics of Astragalus nitidiflorus soil seed bank, a critically endangered species of southern Spain, were studied during four consecutive years to determine their importance to regenerate locally extinct patches of the only known A. nitidiflorus population worldwide. The spatial distribution of seeds in the soil was highly influenced by the presence/absence of adult plants and by the indehiscent trait of the fruit. Results showed that most seeds were present on the soil surface, inside the fruit and close to the mother plant. Seed longevity was low, but higher for seeds protected by fruit than for single ones after two years buried in the soil. This species is able to form a short-term persistent soil seed bank that is strongly influenced by environmental factors and population fluctuations. Based on these results, natural regeneration of local patches where plants of A. nitidiflorus are no longer present is unlikely from the soil seed bank and population recovery should be attempted by sowing seeds or planting new specimens.

Effect of Resting Patterns of Tamarins (Saguinus fuscicollis and Saguinus mystax) on the Spatial Distribution of Seeds and Seedling Recruitment

The spatial distributions of dispersed seeds have important evolutionary consequences for plants. Repeated defecations in sites frequently used by seed dispersers can result in high seed concentrations. We observed the resting behavior of a mixed-species group of tamarins in Peru and recorded the occurrence of seed dispersal (over 8 mo) and seed fate (over 11–22 mo) to determine whether the location and use of resting sites influenced the spatial distribution of dispersed seeds and seedlings. The tamarins rested mostly on trees (Saguinus fuscicollis: 60.6%, S. mystax: 89.2%) and dead trunks (S. fuscicollis: 24.4%) and used 61% of their resting sites repeatedly. During both the dry and wet seasons, tamarins dispersed significantly more seeds within resting areas (0.00662 and 0.00424 seeds/m2, respectively) than outside them (0.00141 and 0.00181 seeds/m2). Seed survival and seedling recruitment did not differ significantly between resting and other areas, resulting in a higher seedling concentration around the resting sites. Seed density did not increase with the duration or the frequency of use of the resting sites but did increase when we pooled the seasonal resting sites together in 50 m × 50 m quadrats, ultimately causing a clumped distribution of dispersed seeds. The use of resting sites in secondary forest, particularly during the dry season, allows the creation of seedling recruitment centers for species coming from the primary forest. Our findings show that tamarin resting behavior affects the spatial distribution of dispersed seeds and seedlings, and their resting sites play an important role in plant diversity maintenance and facilitate forest regeneration in degraded areas.

How best to quantify soil seed banks in arid rangelands of the Nama Karoo?

Sampling design and three sample treatments prior the application of the seedling emergence method were tested in order to find the best method for seed bank quantification in arid Nama Karoo rangelands. I analyzed species composition and seed densities by contrasting undercanopy and open-matrix samples from two soil depths and by comparing the effects of cold-, heat-, and no stratification on germination rates of species in a greenhouse setting. The soil seed bank showed minimal similarity to the standing vegetation, with only 20 plant species germinated. Spatial distribution of seeds was highly heterogeneous. Nearly 90% of germinated seeds were located in 0- to 4-cm compared to >4- to 8-cm soil depth. Undercanopy seed banks contained significantly more species and seeds than open-matrix seed banks. Neither the number nor the diversity of seeds germinated differed significantly among the three treatments. Cold stratification tended to detect more species and seeds only at >4- to 8-cm soil depth. The results highlight the importance of spatial heterogeneity in the accurate evaluation of soil seed banks in the arid Nama Karoo and the importance of considering seasonal variability in the availability of readily germinable seeds. Data also suggest that sample pretreatment in germination trials may give little return for cost and effort, which emphasizes that it is more important to choose the sampling design most likely to give a representative number of seed bank species. Further studies are needed to analyze seed bank dynamics and species-specific germination requirements to promote recruitment of plant taxa underrepresented in the seed bank.

Plant fecundity and seed dispersal in spatially heterogeneous environments: models, mechanisms and estimation

Summary Plant fecundity and seed dispersal often depend on environmental variables that vary in space. Hence, plant ecologists need to quantify spatial environmental effects on fecundity and dispersal. We present an approach to estimate and model two types of spatial environmental effects: source effects cause fecundity and dispersal to vary as a function of a source's local environment, whereas path effects depend on all environments a seed encounters during dispersal. Path effects are described by first transforming physical space so that areas of low seed permeability are enlarged relative to others, and then evaluating dispersal kernels in this transformed ‘movement space’. Models for source and path effects are embedded into the established inverse modelling (IM) framework. This enables the statistical estimation of environmental effects from easily available data on the spatial distribution of seeds, seed sources and environmental covariates. The presented method is applied to data from a well‐studied population of the wind‐dispersed Aleppo pine (Pinus halepensis). We use local tree density as an environmental covariate, model fecundity as a function of a tree's basal area, and consider four dispersal kernels: WALD (a closed‐form mechanistic model for seed dispersal by wind), log‐normal, exponential power and 2Dt. The inclusion of source and path effects of tree density markedly improves IM performance. IM analyses and independent data agree in the parameter range of the mechanistic WALD kernel and in suggesting weak negative density‐dependence of fecundity. Of 64 IMs considered, the best four involve the WALD kernel and negative source effects on its shape parameter. The best IM predicts that increasing tree density at the source shortens median dispersal distance while enhancing long‐distance dispersal (LDD). Additionally, path effects lead to lower seed permeability of high density areas. These results shed light on the mechanisms by which environmental variation affects fecundity and dispersal of P. halepensis. Moreover, the predicted density‐dependent dispersal causes a pronounced lag‐phase in simulations of population spread. Synthesis. The presented method can quantify environmental effects on fecundity and dispersal in a wide range of study systems. The movement space concept may furthermore promote a unified understanding of how various organisms move through spatially heterogeneous environments.

Seasonal and spatial variance of seed bank species composition in an oligotrophic wet meadow

Seasonal and small-scale spatial variation of the seed bank in an oligotrophic wet meadow ( Molinion) was assessed by two methods: (1) direct determination of seeds extracted from soil samples (6 cm in diameter, 10 cm depth) and (2) germination of seeds according to Ter Heerdt et al. [1996. An improved method for seed bank analysis: seedling emergence after removing the soil by sieving. Funct. Ecol. 10, 144–151]. Samples were taken three times during the year (July and November 2001, March 2002) from three distinct microhabitats (moss layer, gaps, Molinia caerulea tussocks). Using the extraction method, more seeds were found in the upper 0–3 cm soil layer than in the deeper 3–7 cm layer, significant differences in the number of seeds were found among microhabitats (with the lowest seed bank densities in Molinia tussocks). The highest number of seeds was found in the spring census, but the test was not significant ( P<0.10). However, seasonal variation was found when damaged seeds were included in the analysis. Changes in damaged/undamaged and light/heavy seed ratios, and in numbers of grass/herbs/sedges seeds are also discussed. The spatial distribution of seeds is probably influenced to a great degree by abiotic factors, such as microtopography (round and heavy seeds of sedges prevail in gaps and in deeper soil layers). The species composition of the seed bank varied among microhabitats, depth, and throughout the year. No differences were found by the seed germination method.

The Role of a Seed Bank in Establishment and Persistence of Aeschynomene virginica, a Rare Wetland Annual

Abstract Seed banks of Aeschynomene virginica may have significant implications for population dynamics and conservation of this rare, freshwater wetland, annual plant. Little work, however, has been done on the size and extent of the seed bank of this threatened plant. Experimental seed banks were placed at three sites to estimate overwinter seed survivorship and the potential for long-term seed dormancy. The soil beneath 5 extant populations was sampled after current-year seed germination and before seed fall to determine the presence and spatial distribution of seeds surviving more than one season. An average of 47.4 ± 2.8 (± 1 SE) of the 125 seeds placed in the field survived over winter. Of those seeds that were recovered from experimental seed banks, an average of 37.3 ± 1.7 germinated under greenhouse conditions. When the seed coats of ungerminated seeds recovered from experimental seed banks (10.3 ± 0.3) were scored, 4.9 ± 0.8 seeds germinated. Analysis of soil cores from the field showed high var...

Spatial distribution of seeds and juveniles of Enterolobium glaziovii Bentham (Leguminosae, Mimosoideae) in the Atlantic forest, Brazil

The spatial distribution of fruits and juveniles of Enterolobium glaziovii Bentham were evaluated in four circular plots of 30 m radius, centered around reproductive individuals to determine whether the size of the parental crowns, topography, and canopy coverage influence these patterns. Each crown was subdivided into eight irregular triangles and the area of each was calculated. Each plot was subdivided into four sub-plots, two of them in an up-slope direction, and the other two in a down-slope direction. The canopy coverage for each plot was estimated at breast height (1.30 m) and at ground level, every three months during 1998. The numbers of fruits and juveniles of E. glaziovii were recorded. Although there were more fruit under the parental crown (F1,12 = 0.01; p = 0.92), there was no difference in the number of juveniles under and beyond the reproductive trees (F1,12 = 0.88; p = 0.36). There was a positive correlation between the areas of the crowns and the numbers of fruits and juveniles under them (r = 0.62; df = 14; p &lt; 0.05). Most of the fruits and juveniles were found in the down-slope directions. The canopy coverage was lower in spring, but higher under parental crowns (only at breast height). These results indicate that the parental crown, topography and canopy coverage influenced the pattern of spatial distribution and probably also the dynamics of E. glaziovii seeds and juveniles.

Litter effects on plant regeneration in arid lands: a complex balance between seed retention, seed longevity and soil–seed contact

Summary 1The recovery of rare, threatened plant populations can sometimes be achieved through modification of the disturbance regime. Accumulation of litter is likely to follow a reduction in grazing pressure and we therefore examine its effects on seedling recruitment of a threatened species in a temperate arid rangeland. 2We studied the effects of litter on seed longevity in the field and performed glasshouse and field experiments with natural and plastic litter to assess their physical and biological/chemical nature. Seeds were sown on the soil surface, buried or within the litter layer. Published data on spatial distribution of seeds were used to calculate the net effect of litter on seedling recruitment. 3Litter increased seed longevity. In the glasshouse, litter increased seedling emergence and growth for surface, but not for buried, seeds. Seeds within the litter layer (no seed-soil contact) showed reduced seedling emergence and growth. In the field, litter did not have a direct effect but emergence was promoted by burial. 4Integrating the effects of microsite quality and seed density showed that litter microsites recruited c. 50% of Bromus pictus seedlings, double that expected from its cover (25%). The positive effect of litter on seed density and on seed longevity outweighed the negative effect of litter acting as a mechanical barrier to burial. 5Inconsistent effects of litter on plant recruitment in arid environments may be due to responses being dependent on seed size and shape, and thus may represent indirect effects acting via the probability of burial. Alternatively, studies may report effects only on seed retention or emergence and growth rather than net effects on establishment.

FACTORS SHAPING THE SEEDFALL PATTERNOF A BIRD-DISPERSED PLANT

The spatial distribution of seeds can influence several parameters of the natural regeneration of plant populations. Factors shaping seedfall patterns have been typically explored from the tree perspective (seed shadow) or from the population perspective (seed rain). However, the seed rain is actually composed of multiple seed shadows. In this paper, we use this framework to explore the variables shaping the seedfall pattern of the wild olive tree (Olea europaea var. sylvestris), a fleshy-fruited tree of the Mediterranean scrublands. We monitored the movement patterns of avian seed dispersers and the seedfall around each fruit-bearing O. europaea tree in two contrasting sites, differing in the degree of human management and abundance of scrub cover. None of the seed dispersers moved between microhabitats (different shrub species and open interspaces) as a function of microhabitat relative abundance. All dispersers foraged preferentially at O. europaea; only the smaller species visited open interspaces, and these only sporadically. Avoidance of open sites by frugivores, especially larger species, and their attraction to source trees were the major determinants of the seedfall pattern. Regarding seed size distribution among microhabitats, we found that only small seeds were significantly overrepresented in open interspaces. From the seed shadow perspective, seed density under trees was similar in the two study sites, but it decreased with distance at a higher rate in the disturbed scrubland. From the seed rain perspective, seed density was significantly different among microhabitats, with open interspaces collecting few, if any, seeds, and places under source trees receiving the highest densities. Results from seed shadow and seed rain analyses were integrated to construct a path model to explore the relative contribution of spatial, microhabitat, and individual tree features to the seedfall pattern. Factors related to microhabitat were identified as having a major role in shaping seedfall pattern. Open interspaces collected few seeds, points under shrub species with sparse foliage collected moderate seed densities, and points under shrubs with dense foliage collected the highest seed densities. The comparison between sites suggests that habitat alteration (a reduction of the scrub layer) can lead to contrasting seedfall patterns. The occurrence of an abundant scrub layer in the well-preserved scrubland allowed a complex pattern of seed rain. In contrast, the sparse scrub cover in the disturbed scrubland yielded a seedfall pattern composed of multiple seed shadows constrained to the vicinity of tree crowns. Thus, our sampling design and analytical approach have proved useful in describing the relative importance of the set of variables that shapes the complex seedfall pattern of a bird-dispersed plant. This seedfall pattern, in turn, is central to the understanding of the spatial patterns of plant recruitment and the efficiency of the dispersal mutualism.

Factors Shaping the Seedfall Pattern of a Bird-Dispersed Plant

The spatial distribution of seeds can influence several parameters of the natural regeneration of plant populations. Factors shaping seedfall patterns have been typically explored from the tree perspective (seed shadow) or from the population perspective (seed rain). However, the seed rain is actually composed of multiple seed shadows. In this paper, we use this framework to explore the variables shaping the seedfall pattern of the wild olive tree (Olea europaea var. sylvestris), a fleshy-fruited tree of the Mediterranean scrublands. We monitored the movement patterns of avian seed dispersers and the seedfall around each fruit-bearing O. europaea tree in two contrasting sites, differing in the degree of human management and abundance of scrub cover. None of the seed dispersers moved between microhabitats (different shrub species and open interspaces) as a function of microhabitat relative abundance. All dispersers foraged preferentially at O. europaea; only the smaller species visited open interspaces, and these only sporadically. Avoidance of open sites by frugivores, especially larger species, and their attraction to source trees were the major determinants of the seedfall pattern. Regarding seed size distribution among microhabitats, we found that only small seeds were significantly overrepresented in open interspaces. From the seed shadow perspective, seed density under trees was similar in the two study sites, but it decreased with distance at a higher rate in the disturbed scrubland. From the seed rain perspective, seed density was significantly different among microhabitats, with open interspaces collecting few, if any, seeds, and places under source trees receiving the highest densities. Results from seed shadow and seed rain analyses were integrated to construct a path model to explore the relative contribution of spatial, microhabitat, and individual tree features to the seedfall pattern. Factors related to microhabitat were identified as having a major role in shaping seedfall pattern. Open interspaces collected few seeds, points under shrub species with sparse foliage collected moderate seed densities, and points under shrubs with dense foliage collected the highest seed densities. The comparison between sites suggests that habitat alteration (a reduction of the scrub layer) can lead to contrasting seedfall patterns. The occurrence of an abundant scrub layer in the well-preserved scrubland allowed a complex pattern of seed rain. In contrast, the sparse scrub cover in the disturbed scrubland yielded a seedfall pattern composed of multiple seed shadows constrained to the vicinity of tree crowns. Thus, our sampling design and analytical approach have proved useful in describing the relative importance of the set of variables that shapes the complex seedfall pattern of a bird-dispersed plant. This seedfall pattern, in turn, is central to the understanding of the spatial patterns of plant recruitment and the efficiency of the dispersal mutualism.

Foraging and community consequences of seed size for coexisting Negev Desert granivores

We examined the effects of seed size on patch use and diet selection for three co‐existing Negev Desert granivores: Allenby's gerbil (Gerbillus allenbyi), greater Egyptian sand gerbil (Gerbillus pyramidum), and crested lark (Galerida cristata). We manipulated size and spatial distribution of seeds in experimental food patches and quantified foraging behavior by measuring giving‐up densities (GUDs: the amount of food remaining in a resource patch following exploitation by a forager). In one experiment, we presented small (&lt;1.4 mm in diameter cracked wheat), medium (2.0–3.3 mm), and large (&gt;3.4 mm) seeds in separate trays; in a second, we presented small and medium seeds separately and mixed together. Gerbils had a higher handling time efficiency on smaller seeds, but a much higher encounter probability on larger seeds (20 times higher on large than medium seeds, and 2–5 times higher on medium than small seeds). This led gerbils to have significantly lower GUDs on larger seeds than smaller seeds and to harvest a higher proportion of the larger seeds. When presented with rich and poor patches, G. allenbyi tended to equalize GUDs in both patches, indicating a quitting harvest rate rule for patch exploitation. In contrast, larks appeared to use a fixed time rule for patch exploitation. For larks, seed size did not influence encounter probabilities, and they showed no seed‐size selectivity. Still, larks had higher handling efficiencies on smaller than larger seeds, and consequently had a significantly lower GUD on small than medium seeds. Despite large differences between the gerbils and larks in their foraging, our results do not support species coexistence via seed‐size partitioning: the larks had much higher GUDs than the gerbils on all seed sizes. Nonetheless, seed size, seed abundance, seed distribution and the animal's patch use behavior all played major roles in determining gerbils’ and larks’ diet selectivities and GUDs.

Tree and Shrub Seedling Colonization of Old Fields in Central New York

The main objective of this study was to determine the important mechanisms regulating woody plant establishment in recently abandoned agricultural fields in central New York. The study focused on the early life stages, from the dispersal of seeds to the fate of seedlings at the end of the second growing season, of common old—field invaders, primarily Acer rubrum, Pinus strobus, Cornus racemosa, and Rhamnus cathartica. We investigated patterns of survivorship and causes of mortality during each of these life stages in microsites dominated by previously established herbs and in experimental openings in the herb canopy. Establishment of a tree or shrub seedling was much less likely from seeds landing among herbs than in openings. During the fall, seed predation by mice, especially under the cover of herbs (median of averages across all species = 6% in open, 85% under herbs), was an important process regulating the relative abundance and spatial distribution of seeds after dispersal. Seedling emergence rates were generally quite low, but for most species tended to be greater in the presence of perennial herbs (12%) and were lower under annuals and biennials (7%) and in bare soil (6%). For cohorts of seedlings followed for two growing seasons (Experiment 6A), some mortality occurred due to frost heaving (17% in open, 4% under herbs) and apparent heat stress or desiccation during the summer (5% in open, 0% under herbs); however, the largest source of mortality, especially under the herb canopy, was predation by rodents, especially meadow voles (6% in the open, 62% under herbs). Competition with herbs, especially aboveground competition, significantly reduced growth of woody seedlings. In spite of growth reduction in the presence of herbs, there was no significant relationship between seedling size and survivorship within any treatment, indicating that mortality could not be directly attributed to competition. For different species, and at different life stages, support was found for all three of the succession models of Connell and Slatyer (1977). The tolerance model was supported in some cases of seed predation (Experiment 2), seedling emergence (Experiments 4 and 5, in part), and seedling predation (Experiment 7). The facilitation model was supported in some cases by increased seedling emergence (Experiments 4 and 5, in part) and increased seedling survivorship in the presence of herbs during periods of environmental stress (Experiments 6A and 8). The inhibition model was also supported in some cases of decreased growth (Experiment 6B) and decreased survival of seeds and seedlings (Experiments 1, 3, 6A, and 8) in the presence of herbs. The lower survivorship under herbs was largely the result of seed and seedling predation, which were the two most important processes regulating tree and shrub colonization of old fields in this study. Reduced growth of later colonists due to competition with earlier colonists is considered direct inhibition, while increased mortality due to predation on later colonists caused by animals whose activity is necessarily associated with the presence of earlier colonists is considered indirect inhibition. Seed and seedling predation are potentially important mechanisms regulating succession and should be explicitly included in studies of succession.

Explosive discharge of jack pine dwarf mistletoe (Arceuthobiumamericanum) seed in Manitoba

Seeds of the dwarf mistletoe, Arceuthobiumamericanum Nutt. ex Engelm., explosively discharged from plants parasitizing 40- to 60-year-old jack pine trees (Pinusbanksiana Lamb.), were collected on cheesecloth traps at two sites in Grand Beach Provincial Park at 2-h intervals during daylight hours. Microclimatic variables were continuously recorded during the discharge period. Seeds were recovered up to 18 m from the canopy margin. Seasonal fluctuations in numbers of seeds trapped were attributed in part to fungal hyperparasitism of fruits and high spring temperatures. The spatial distribution of seeds was strongly leptokurtic, with most landing within 12 m of the canopy margin. Neither the onset nor the distance of discharge could be ascribed to any single microclimatic parameter. With one apparent exception, no discharge occurred during the hours of darkness. The number of seeds trapped rose rapidly between sunrise and 11:00 central standard time, when temperature was increasing and relative humidity was falling. Stepwise regression analysis confirmed that change in relative humidity was predominantly responsible for the diurnal periodicity of seed discharge. Based on our observations, we recommend a minimum 20-m buffer zone to protect pine regeneration from A. americanum infected residual trees.

The Use of Torpor by Perognathus amplus in Relation to Resource Distribution

Small (11 g) pocket mice ( Perognathus amplus ) were tested in a laboratory study for their use of torpor in relation to the distribution of food resources (seeds). Generally, the rodents spent more time in torpor the deeper the seeds were buried, but there was essentially no relationship between the dispersion of the seeds and the time spent in torpor. Regardless of the depth or spatial distribution of seeds, the pocket mice maintained similar weight loss patterns, indicating that they can adjust the amount of time spent in torpor to the environmental conditions under which they find themselves.