Early Successional Vegetation Research Articles

Ecological principles in agricultural policy: but which principles?

There are increasing calls to base farming and agricultural research policy on ecological principles. One such principle, the relation between crop species diversity and stability, is reviewed. Parallels are drawn between species-poor early successional vegetation and the management of annual crops by traditional farmers. Traditional management is based on the control of succession, and the exploitation of the competitive strategies characteristic of early-successional species. The existence of apparently stable pure stands in natural vegetation confirms the sound ecological strategy of simple and productive annual cropping. Farmers' ecological knowledge also encompasses the value of introduced crops and varieties. Changes are recommended in international agricultural research policy, with the need to redefine sustainability and to adopt ecological principles more relevant to annual crop production. Suggestions are made to apply specific traditional ecological knowledge of farming to agricultural research and policy.

Effects of Foliar Insecticides on Survival of Northern Bobwhite Quail Chicks

Reduced survival of chicks may result from exposure to insecticides and may explain declines in northern bobwhite quail (Colinus virginianus) populations on agricultural landscapes. To determine the risk insecticides pose to quail, we quantified exposure rates and hazard, Exposure rate depends on quail habitat use in relation to insecticide applications, whereas hazard depends on susceptibility to a toxin and the dose an individual receives, Because providing early-successional vegetation around row-crop fields is a typical habitat management recommendation, we determined rates of exposure of quail and their broods to insecticides applied to soybean fields with and without vegetated field borders, Radiocollared quail (n = 69) used soybean fields extensively (64% of telemetry locations) at the time of year insecticides were applied. Quail used soybean fields twice as often when vegetated borders surrounded crop fields (P = 0.04). Ten of 18 broods monitored by telemetry were located in soybean fields 88% of the time. In 1993. 4 of 6 broods 0.49). However, ChE activity was depressed (P < 0.001) and body mass was lower (P = 0.02) in chicks exposed to methyl parathion, which historically (pre-1980) received significant use in production of row crops in North Carolina. Our results, along with data on use and toxicity of other insecticides applied to row crops, collectively suggest direct affects to survival of quail chicks from use of foliar-applied insecticides does not explain reduced quail densities on agricultural landscapes.

Effects of above- and below-ground competition of shrubs and grass on Calophyllum brasiliense (Camb.) seedling growth in abandoned tropical pasture

Early-successional vegetation is often a major factor that limits recovery of tropical forest in abandoned pastures by outcompeting seedlings of forest trees. The goal of this study was to compare the effects of above- and below-ground competition of pasture grasses and shrubs on the growth of tree seedlings in abandoned tropical pasture in Costa Rica. Seedlings of Calophyllum brasiliense were planted in areas of dense pasture grass and below shrub patches. In each vegetation type seedlings were subjected to one of four treatments: control, root trenching, above-ground clearing, or trenching and clearing. Seedling height was measured and above- and below-ground biomass was harvested after one year. Seedling height and biomass were higher for seedlings grown under grass than under shrubs. Above-ground clearing had a strong positive effect on seedling height and all biomass measurements (stem, leaves, tap root, and fine roots). Trenching had a significant effect on height and all biomass measurements except tap root mass; however, trenching had a weaker effect on plant growth than above-ground clearing. Root:shoot ratios were significantly affected by trenching. These results suggest that both pasture grasses and early-colonizing shrubs may slow succession of forest in abandoned pasture. Moreover, the results highlight the importance of reducing above-ground competition to improve the success of reforestation efforts.

Trema micrantha (L.) Blume (Ulmaceae): A promising neotropical tree for site amelioration of deforested land

The genus Trema includes many species that are fast growing, short living pioneer trees common in early successional vegetation and large recent gaps of many tropical moist forests. They establish themselves and grow in open sites with microclimatic and soil conditions which are contrasting to those prevailing in dense forests. With time they change the environmental conditions by attenuating the wide alternations of soil temperature and lower midday air relative humidity characteristic of open places. At the same time, soil moisture and organic matter content, as well as nutrient levels increase due to the reestablishment of an organic cycle in the soil. Consequently they help generating an environment that permits the establishment of mature forest species that often do not survive under the environmental conditions of open places. The propagules of mature forest trees can be reintroduced from conserved areas nearby. The biological and ecological traits related to the potential use of T. micrantha for site amelioration previous to the reintroduction of mature forest trees are presented in this paper. Furthermore, some preliminary experiences in the field are cited. The same use may apply to other fast growing short living pioneers.

Prolonged leaf senescence in Clusia multiflora H.B.K.

Leaf aging and senescence in Clusia multiflora H.B.K. was investigated by artificial treatments, such as floating leaf discs on water in darkness, or darkening leaves attached to the parent plant in situ in trees living in a tropical cloud forest. In both cases several parameters modified by age were evaluated such as nitrogen levels, chlorophyll content, succulence and carbohydrates levels. A prolonged senescence (nearly 3 months in floating leaf discs) was observed, contrasting with species such as Heliocarpus americanus (5 days) and Cecropia palmatisecta (20 days), characterized by low values of leaf weight per area, but similar to species with high leaf weight per area and with high levels of organic acids such as Clusia minor and Fourcroya humboldtiana, where acids may act as a reserve of C and energy. After 30 days in darkness C. multiflora leaf samples collected in the field did not show differences in comparison to non-darkened opposite leaves with respect to chlorophyll, titratable protons and carbohydrates, and leaves performed photosynthesis after 2 months in darkness. The effect of age in leaves was evaluated in a gradient of leaves, sampled at different positions from the apex and ranging in age from 15 days to 2 years old. The study of senescence in tropical wild plants is uncommon, but it is important knowledge for understanding foliar development, and response to internal rather than environmental regulation in climates where seasons are not strongly marked as is the case in the tropical mountain forest, where C. multiflora constitutes an important component in the early successional vegetation.

Motor–manual, mechanical, and herbicide release affect early successional vegetation in northwestern Ontario

Cover and height of vegetation before and one growing season after: 1) motor-manual cutting, 2) mechanical brush cutting (Silvana Selective/Ford Versatile), 3) aerial application of Release® (a.i. triclopyr) herbicide, 4) aerial application of Vision® (a.i. glyphosate) herbicide, and 5) control (no treatment) were quantified. Multivariate analysis permitted the study of vegetation response as a whole, while accounting for correlations that exist among the individual vegetation groups. Univariate analysis were used to study the responses of individual vegetation groups. Although no pre-treatment differences in percent cover were observed (P = 0.128), deciduous tree, shrub, forb, grass, and sedge groups responded differently to the treatments after one growing season (P &lt; 0.018). Post-treatment cover of deciduous tree and shrub groups was lower in herbicide treated plots than in cut plots. Forb, grass and sedge covers varied greatly among treatments. Brush saw and Silvana Selective treatments decreased cover of deciduous trees. Release® decreased cover of deciduous trees and shrubs. Vision® decreased cover of deciduous trees, shrubs and ferns. Cover of all vegetation groups increased on the untreated control. Among the conifer release treatments examined, Vision® reduced woody and herbaceous vegetation most. Key words: clearing saw, conifer release, Falingsnow Ecosystem Project, glyphosate, herbicides, Release®, Silvana Selective, spruce, triclopyr, vegetation management, Vision®.

Bird Community Dynamics along Primary Successional Gradients of an Amazonian Whitewater River

We used spot-map methods to census birds in two plots (40 ha and 80 ha) representing the early and middle stages of primary succession generated by the meandering of the Manu River in southeastern Peru in 1983 and 1985. We distinguished seven distinct successional stages beginning with the first plant communities growing on open beaches (Tessaria: stage 1) and proceeding through mature floodplain forest (stage 7). Each successional stage was more structurally and floristically complex. One plot was dominated by early successional vegetation (stages 1-5) and the other was domi- nated by middle-to-late stages (5-6). We supplemented spot-map data with mist netting to census nonterritorial species, principally nectarivores and frugivores. Because the ear- liest successional stage (Tessaria) covered only small (<3 ha) areas on any given me- ander tongue, we censused six additional stands to characterize their bird communities. Species richness increased along the successional gradient, but not uniformly. Struc- turally simple Tessaria stands (stage 1) contained more diverse communities of breeding birds than the next two successional stages, which appeared to have few resources avail- able to birds. After stage 3, breeding species richness increased by 31-71 species per successional stage as a distinct canopy layer formed (stage 4), fruiting trees became more available (stages 5-6) and the vertical structure of the forest became fully developed (stage 7: 71 more species than stage 6). Early successional bird communities (stages 1- 3) were dominated by a few very abundant insectivores and omnivores, but wanderers from the adjacent forest plus Nearctic and austral migrants seasonally outnumbered breeding residents. Many species that breed in Tessaria stands were restricted to just one or two of the seven stands censused. The middle stages of succession had the highest estimated richness, abundance, and biomasses of nectarivores and frugivores. Nectarivore abundance reflected the huge stands of Heliconia that dominate the understory of mid- successional stages. Bark foragers reached their highest abundance in late successional stages as the dominant canopy tree species of middle successional stages (e.g., Cedrela odorata) began to die off, but remained standing. Middle successional stages lacked many of the insectivores characteristic of more structurally complex mature forest. Among the missing species were understory Mockers, ant-followers, ground foragers, and many arboreal insectivores that participate in canopy flocks. Species of early and middle stages tended to be more abundant on average than those of mature forest. Early-/and late-successional bird communities had few species in common, and many congeners segregated along this successional gradient. Most species characteristic of early successional vegetation, however, were also found in many other kinds of natural and anthropogenic disturbances. The Tessaria beaches of the Manu lacked most species endemic to the same successional stage on the larger rivers in the Amazon system. Mid-successional stages had relatively fewer species restricted to them, but some of these species are rare and endemic to southwestern Amazonia. Many char- acteristic species of floodplains reached their peak abundances in stages 4-6. Mid-suc- cessional stages, therefore, may play a greater role in contributing to the unusually high species richness of western Amazonia than had previously been recognized. These results point to the importance of preserving natural, undammed meandering rivers in conserving the avian diversity of Amazonia. Logging and agricultural devel- opment of mid-successional floodplain forests probably has a negative impact on regional bird communities. Not all species of the early and middle stages of succession have adapted to human-generated secondary succession. Recovery of the full diversity and richness of tropical bird communities following disturbance may take several hundred

Miocene paleosols and habitats of Proconsul on Rusinga Island, Kenya

Abstract Early Miocene (18 Ma) volcaniclastic deposits of the Hiwegi Formation on Rusinga Island include at least six distinct kinds of paleosols, representing different parts of former ecosystems. Four localities (R1 site of L. S. B. Leakey, nearby site of Pickford, R106 of M. Leakey, and Kaswanga site of Walker) for articulated or cranial material of two species of Proconsul are all in the same kind of paleosol. These are here called Chuodho paleosols, and represent woodlands early in ecological succession of streamsides. Proconsul heseloni is best known from the lower Hiwegi and Kiahera Formations, and was replaced in the same habitat by P. nyanzae in the geologically younger middle Hiwegi Formation. In contrast, articulated skeletons of Dendropithecus macinnesi were found in another kind of paleosol (here called Tek), representing dry upland forest including hackberries ( Celtis rusingensis ). Other paleosols represent other kinds of early successional vegetation, woodland and dambo grassy woodland. Paleoclimate was dry (300-650 mm mean annual precipitation) and mildly seasonal. The paleosols formed along braided streams on outwash and airfall ash of a carbonatite-nephelinite volcano. The Hiwegi Formation accumulated during the building of this volcano about 18 million years ago.

Relationships between bird assemblages, vegetation structure, and floristic composition of mosaic patches in riparian forests

The vegetation structure and the plant taxonomic composition of riparian forests, together with the birds, were mapped and analysed. The forest plots were 100 m wide and 1 600 to 2 000 m long, and were located near lake Grand Lieu, Brittany, France. The plots were not homogeneous, but a mosaic of different vegetation types. The patches of the same vegetation type were combined. We recognized 21 vegetation types, but we retained only 16 that were of sufficient size to neutralize species-area effects. Variables of vegetation structure as well as variables of plant taxonomic composition associated with plant succession showed the strongest correlations with the variables of bird community. Disturbances caused by forestry is correlated with reduced bird species richness in forest patches of all ages. Canonical correspondence analyses of the bird data constrained by the vegetation data allowed us to identify that variables that explain most of the variance in bird data are those related strongly to succession. Partial CCA revealed the relative influence of plant structure versus floristic variables as well as their combined influence on bird community structure (17, 17, 21 % respectively). When the same analysis is applied after removal of early-successional vegetation types, the importance of floristic variables increased.

North Island serai tussock grasslands: 2. Autogenic succession: Change of tussock grassland to Shrubland

Abstract The physiognomic stability of seral tussock grassland and tussock-shrubland was assessed by relating species cover and shrub density to environmental and temporal factors using ordination and regression analyses. Four small-leaved, sclero-phyllous shrub species—Dracophyllum subulatum, D. longifolium, Leptospermum scoparium, and Kunzea ericoides—are rapidly invading Chiono-chloa rubra ssp. rubra tussock communities, changing tussock grassland to shrubland within 45-55 years. Elapsed time since the last fire disturbance, slope, and topographical position are reliable factors for predicting rates of vegetation change. Rates of shrub invasion are fastest on steep slopes and on topographically exposed sites, and slowest on flat mesic sites dominated by close-canopied Chiono-chloa rubra ssp. rubra tussock grassland. The cover of wind-dispersed shrubs did not decrease with distance from seed source, but the cover of bird-dispersed shrubs did. The study highlighted the transient nature of some North Island tussock grasslands and the implications for management policy in terms of maintaining floristic, community, and landscape diversity, and problems with deliberate burning to maintain early successional vegetation.

Revegetation of harvested peat surfaces in relation to substrate quality

Abstract. The relationship between substrate quality and pattern of revegetation of harvested peat surfaces was studied by means of a survey and a field experiment examining influences of modest NPK‐fertilization on plant colonization of an initially bare peat surface. The harvested peat surfaces varied a great deal in their chemical and physical characteristics and the sites differed in revegetation pattern. Early successional vegetation was dominated by perennial species native to nutrient‐poor habitats on all sites. Soluble phosphorus and ash content, mean particle size of surface peat, and thickness of peat layer had the strongest influence in a CCA‐ordination of species. The species composition depended on the amount and form of soluble nitrogen in the surface peat. Sites with a high content of phosphorus and ammonium nitrogen, and with a thick peat layer were usually densely revegetated by Eriophorum vaginatum alone, while sites characterized by thin peat layers associated with a high ash content, large particle size and a high content of nitrate nitrogen were mainly dominated by different grass and weed species. Deschampsia cespitosa clearly favoured sites with a high potassium content and small particle sizes of the peat.The importance of nutrient availability for the rate and pattern of colonization was also demonstrated by the field experiment. Application of 20 g/m2 of NPK‐fertilizer resulted in a significant increase in the number of established plant individuals and marked differences in species composition compared to unfertilized plots.

Postfire Plant Establishment: The Effects of Environment, Prefire Vegetation, Fire Severity, and Landscape Position

The initial species composition and conditions following disturbance are crucial to determining the long-term development of vegetation (Egler 1954, Glenn-Lewin et al. 1992). Thus, an understanding of early succession can provide insights into the mosaic of vegetation types that will emerge on a landscape. Observations in the Rocky Mountains and elsewhere indicate that postfire vegetation patterns vary considerably. It is now known that numerous factors simultaneously contribute to the species composition of pioneer vegetation (Stahelin 1943, Cattelino et al. 1979, Glenn-Lewin 1980, Noble and Slatyer 1980, Keever 1983, Walker and Chapin 1987, Pickett 1989, Westoby et al. 1989, Glenn-Lewin et al. 1992), and often different patterns of early succession are possible even on sites that are similar. While research on the distribution of late successional vegetation along environmental and disturbance gradients has been a dominant theme in plant ecology (Daubenmire 1943, Whittaker 1956, Despain 1973, Loope and Grueii 1973, Whipple and Dix 1979, Peet 1981, Romme and Knight 1981, Wentworth 1981, Veblen 1986, Allen and Peet 1990), few studies have directly investigated the factors that affect the distribution of pioneer plant communities. Moreover, the influence of landscape structure on the development of vegetation mosaics has only recently been considered (Milne and Forman 1986, Turner et al. unpublished manuscript). Studies designed to understand vegetation patterns following disturbance could be improved by considering site history and the character of the surrounding landscape as well as the effects of site features. We have initiated research to consider the effect of various spatial, historical and site factors on early postfire vegetation in Grand Teton National Park (GTNP) and the adjcent Bridger-Teton National Forest. Specifically, we are studying the relationships between characteristics of early succession and fire severity, geologic substrate, soil characteristics, mean burn patch size, prefire canopy composition, cone serotiny, topographic position, distance to unburned vegetation, and the vegetation composition and fire severity of the surrounding landscape. Our research objectives are to: 1) determine the range of variability in the patterns of early postfire succession 2) determine which environmental and historical variables are most important in controlling vegetation development following fire 3) elucidate the importance of landscape position and between-patch interactions during early postfire succession 4) develop a series of multivariate models that will predict the characteristics of early succession in different situations; 5) project, based on existing literature, the most probable trajectories of each pioneer community type. An important component of our research is to examine the influence of landscape context on early successional vegetation. Understanding and predicting the behavior of organisms requires the consideration of a broad environmental matrix, not simply the autecology of the organisms (Rowe 1981). According to Glenn-Lewin and van der Maarel (1992), spatial attributes such as landscape complexity, community isolation, and the spatial scale of disturbance are important factors influencing succession. We will use GRASS-GIS, to better understand the relationship between site characteristics and pioneer vegetation pattern (Davis and Dozier 1990, Davis and Goetz 1990) as well as to elucidate the importance of landscape position in early succession.

Resource availability at three presumed stages of a heathland succession on the Lüneburger Heide, Germany

Abstract. The availability of maj or plant resources was investigated in three vegetation types that were assumed to represent different stages of a secondary succession on heathland on the Lüneburger Heide, northwestern Germany. Canopy transmission and absorption of photosynthetically active radiation (PAR), soil‐water availability, and nutrient (Ca, K, Mg, N, P) availability were monitored with high spatial and temporal resolution in (1) a Calluna vulgaris heathland, (2) a pioneer birch‐pine forest and (3) a late‐successional oak‐beech forest, situated close to each other on comparable geological substrate (diluvial). Mean fractional transmission of PAR during summer decreased from 0.48 in the heathland to 0.04 in the oak‐beech forest while the fractional canopy absorption increased from 0.49 to 0.92.Soil‐water availability as indicated by the soil‐water potential, was significantly influenced by differential canopy interception loss and characteristic rooting patterns in the three vegetation types. Annual mean nutrient concentrations in the equilibrium soil solution were similar or, for some elements, increased from the heathland to the birch‐pine and the oak‐beech forest despite a growing demand. A marked increase was found for the total nutrient pools in the soil‐organic layer between early and late successional vegetation types. On the poor glacial parent material, nutrient pools seem to be strongly dependent on stand productivity and litter production which increased with succession. Thus, for nutrients, facilitation seems to be important in this type of succession.

Lepidoptera outbreaks in response to successional changes after the passage of Hurricane Hugo in Puerto Rico

ABSTRACTFifteen species of Lepidoptera occurred in large numbers in spring and early summer after the passage of Hurricane Hugo over the north-east of Puerto Rico.Spodoptera eridania(Noctuidae) was the most common of the larvae and fed on 56 plant species belonging to 31 families. All the Lepidoptera fed on early successional vegetation. Some of the plants represent new host plants for these species. The outbreaks appeared to be based on the flush of new foliage that developed in the Luquillo Mountains after the passage of the hurricane. The end of theS. eridaniaoutbreak was concurrent with the consumption of its preferred host plants and to an apparent increment in parasitism by ichneumonids (Hymenoptera). Parasitism by tachinids (Diptera) may have contributed to the reduction in abundance of other Lepidoptera species that were temporarily very abundant. Natural enemies ofS. eridaniawere recorded for the first time in Puerto Rico.

Introduced versus native plants in austral forests

Despite the concern of some nineteenth- and early twentieth-century biologists, relatively few alien species have become established in undisturbed forests of Australia, Madagascar or New Zealand. All three countries have large 'naturalized' floras adapted to frequent disturbance. The short-term advantages of these alien plants - short life cycles, fast growth, abundant seed and plasticity of habit - have allowed them to cover large areas of disturbed ground and to coexist with native plants in early successional vegetation. Very few persist in mature forest. The threat to the austral forests is human disturbance, and there is need for research on the native plants which perform the specialized role of repairing canopy after disturbance.

Relationship Between Volume and Biomass of Early Successional Vegetation and the Prediction of Loblolly Pine Seedling Growth

Abstract A simple and nondestructive method of measuring plant volume was developed to test the following two hypotheses: (1) plant volume is an effective substitute for plant biomass in the prediction of competitive potential; and (2) the plant biomass-volume relationship is affected by plant growth form. In 1983, above-ground volume and biomass were determined for all plants in 40 1-m² plots in an experimental loblolly pine (Pinus taeda L.) plantation having varying degrees of successional regrowth. After natural log transformations of plant biomass and volume, separate regression equations were developed for grass, forb, shrub, and tree growth forms. A test of homogeneity indicated that the slopes and intercepts for the four regression equations were not all identical. Specific contrasts among the regression equations were also tested for common slopes and intercepts to determine the source of overall significance. Results of the contrasts were explained by differences in plant allocation patterns. The variation in plant biomass accounted for by the volume models ranged from 78% for grasses to 94% for trees. As an application of this approach, volume was measured and biomass was estimated from the regression equations for successional plants within a 2m radius (the "neighborhood") of each of 69 loblolly pine seedlings. Total plant volume and total estimated biomass each accounted for approximately 40% of the variation in pine growth. For. Sci. 34(4):939-947.

Forage quality and use of reforested habitats by snowshoe hares

The forage quality of habitats associated with mature conifer plantations and the use of these habitats by resident snowshoe hares (Lepus americanus) were evaluated. The winter forage of hares was composed of deciduous stems that were predominantly low in protein content and not preferred. The crude protein content of the principal browse species, Viburnum dentatum, was only 4.6–6.4% during winter months. However, snowshoe hares feeding on the low-protein browse maintained their body weight. The intake of protein was maximized by the consumption of stem ends, which compensated for the low-protein forage. Use of habitat by snowshoe hares was closely related to the vegetative cover but did not correlate well with forage availability. Sites preferred by hares had a well-developed overstory (mature spruce) but only sparsely available forage during the winter. Thickets of early-successional deciduous vegetation had high use and contained most of the low-protein browse.

DREWRIA POTOMACENSIS GEN. ET SP. NOV., AN EARLY CRETACEOUS MEMBER OF GNETALES FROM THE POTOMAC GROUP OF VIRGINIA

Drewria potomacensis gen. et sp. nov. from the Lower Cretaceous Potomac Group of Virginia (Zone I, probably Aptian) provides the first definite Mesozoic megafossil record of Gnetales. The stems are slender, display no evidence of secondary growth, and show axillary monopodial branching. Attached leaves are opposite and decussate, borne at swollen nodes, and have clasping sheathing bases. Each leaf is oblong, up to 20 mm long, and has a dense network of longitudinally aligned subepidermal fibers. Leaf venation consists of two, or possibly three pairs of longitudinal parallel veins that form a reticulum at the apex and higher‐order crossveins that form apically oriented chevrons. Reproductive structures consist of short, loose spikes that are borne in dichasially arranged groups of three. The dichasia are either terminal or lateral in the axil of a leaf. Reproductive units of the lateral spikes in a dichasium contain seeds, each surrounded by at least one pair of opposite, broadly elliptical or ovate bracts. Characters of D. potomacensis that suggest a gnetalean relationship include the opposite bracts surrounding the seeds, the network of subepidermal foliar fibers, and the distinctive leaf venation, which is very similar to that seen in the cotyledons of extant Welwitschia. Other features consistent with a gnetalean relationship include opposite and decussate leaves, swollen nodes, and the dichasial arrangement of the reproductive spikes. Masses of polyplicate gnetalean pollen comparable to that of extant Welwitschia occur in the same bed as the megafossils. The morphology of D. potomacensis indicates that it was an herb or possibly a shrub. The growth habit of D. potomacensis and associated plants, combined with the sedimentary occurrence of the fossils, indicate that this species and perhaps related taxa were important components of early successional vegetation during the mid‐Cretaceous.

Dispersal of ornithochorous seeds from forest edges in central Florida

Dispersal of ornithochorous seeds from isolated forests in the phosphate mining region of central Florida, U.S.A., were studied for two seasons (July to November) to determine patterns of dispersal. Three separate studies included (1) dispersal directly from forest edges to early successional herbs; (2) standardized perches (2 m high post with a crossbeam); and (3) snags (dead trees, 11.3±2.8 m high, 21.4±23.3 stem branches). Perch availability was more important than distance in predicting seed types and quantities of bird-dispersed seeds. The greatest concentration of bird-dispersed seeds were found under standard perches (255±249 seeds/m2/fall) followed by snags (147±85 seeds/m2/fall) and early successional vegetation (3.0±4.1 seeds/m2/fall). Seed quantities dispersed to snags were positively correlated with the number of stem branches on snags. Seeds beneath standard perches were significantly lighter and less diverse than those found beneath snags. This difference is attributed to the larger percentage of forest or later successional species found beneath snags (73%) as compared to the standard perches (15%). Perches and vegetative structural development should facilitate ecological succession by increasing the quantity, quality and diversity of seed deposition.

Plant Dominance Along an Experimental Nutrient Gradient

Fertilization experiments in an 8—yr—old field demonstrated that N was the major limiting nutrient of N, P, K, Ca, and Mg, and suggested that Mg became limiting when N was added. After the fertilization experiments, this field was disturbed via thorough disking and divided into 36 plots for a Latin square design experiment on the effect of N:Mg fertilization ratios on vegetation patterns. By the second year, the major species had separated along the imposed N:Mg gradient, with Agrostis scabra dominant at the low Mg but high N end, followed by Agropyron repens, Berteroa incana, Oenothera biennis, and Aristida basiramea, which was dominant at the high Mg but low N end of the gradient. An unmanipulated resource, light availability at the soil surface, was significantly affected by the treatments. The results demonstrate that spatial heterogeneity in the relative availability of soil nutrient may be one cause of spatial heterogeneity in early successional vegetation.