Showing papers on "Shrub published in 1992"

Effect of Shrubs on Recruitment of Quercus Douglasii and Quercus Lobata in California

Abstract: Biologically derived safe sites appear to strongly affect spatial and temporal patterns in terrestrial plant communities; however, few experimental field studies have been conducted to investigate the importance of such interactions or the mechanisms by which they operate. Quercus douglasii and Quercus lobata are winter-deciduous oaks en- demic to California. I examined associations between seedlings of these oaks and several species of shrubs and conducted field experiments that tested for facilitative effects of two shrubs, Salvia leucophylla and Artemisia californica, on seedling survival. Naturally oc- curring Q. douglasii seedlings were relatively common (280 seedlings/ha) and were strongly associated with shrub canopies. In two experimental plantings, 30% and 55% of Q. douglasii seedlings that emerged under shrubs survived for > 1 yr, whereas no seedlings in the open survived in either experiment. Survival of Q. douglasii where shrubs were removed was similar to survival in the open grassland, indicating that association between shrubs and oak seedlings was not due to shared microsite requirements. Q. douglasii seedling survival was not different between the two shrub species. Only three Q. lobata seedlings were found in 26 study plots, although reproductive adults were present. The seedlings were under shrub canopies. However, the survival of Q. lobata was not facilitated by shrub cover or shrub simulation and no other evidence was found for nurse-shrub interactions with this oak species. Artificial shade was critical to the survival of Q. douglasii seedlings in simulated shrub environments. When grown in artificial shade, photosynthetic capacities and root elon- gation rates of Q. douglasii seedlings were significantly higher than those of Q. lobata. These differences may partially explain why nurse-plant interactions exist between shrubs and Q. douglasii seedlings but not between the same shrubs and Q. lobata seedlings. Causes of acorn and seedling mortality differed between the shrub and open grassland habitats. Under shrubs, acorn predation was the primary cause of mortality, whereas mortality due to shoot herbivory was much more frequent in the open grassland. Thus, propagules under shrubs that avoided predation until shoot emergence benefitted from canopy shade and protection from herbivores. The shade tolerance and nurse-plant utilization of Q. douglasii seedlings have parallels with shade tolerance and late successional roles of other oak species in temperate deciduous forests, and, as in these forests, may affect the long-term spatial dynamics in California woodlands.

Effects of Moose Browsing on Vegetation and Litter of the Boreal Forest, Isle Royale, Michigan, USA

Abstract: Large mammalian herbivores can influence the dynamics and structure of ecosystems by selectively removing tissues of specific plant species. The plant community composition can be altered as animals feed on some species but not others, changing the biomass, production, and nutrient cycling of an entire ecosystem. We used four paired moose (Alces alces) exclosures and browsed plots (built between 1948 and 1950) on Isle Royale, Michigan, to examine the influence of moose on aboveground biomass, production, and annual litterfall of boreal vegetation in 1987. Tree biomass was significantly greater (X = 230 vs. 150 Mg/ha, df = 3, P < .05), shrub biomass was significantly less (X = 1.9 vs. 3.1 Mg/ha, P < .05), and herb biomass was significantly less (X = 0.2 vs. 0.8 Mg/ha, P < .05) in exclosures than in browsed plots. Tree production was greater in exclosures than in browsed plots (X = 7.9 vs. 5.0 Mg.ha—1.yr, P = .05), but there was no difference in the production per unit biomass between exclosures and browsed plots. Shrub production in exclosures was similar to that of browsed plots (X = 3.5 vs. 2.3 Mg.ha—1.yr—1, P < .05), despite total vegetation biomass differences between paired plots. There was significantly greater herb litter produced in the browsed plots than in the exclosures (X = 0.7 vs. 0.1 Mg.ha—1.yr—1, P < .05). Moose browsing prevented saplings of preferred species from growing into the tree canopy, resulting in a forest with fewer canopy trees and a well—developed understory of shrubs and herbs. In addition, browsing may have altered the eventual balance of white spruce (Picea glauca) was balsam fir (Abies balsamea), causing an increase in the former and a decrease in the latter. Thus, browsing by moose influences in long—term structure and dynamics of the boreal forest ecosystem, which has important implications for forest ecosystem management, especially where the population dynamics of moose are regulated.

Recruitment near conspecific adults and the maintenance of tree and shrub diversity in a neotropical forest.

Abstract: According to the Janzen-Connell hypothesis for the maintenance of species diversity, recruitment is inhibited in the immediate vicinity of adults by herbivores and pathogens. This reduces the per capita ability of abundant species to reproduce, relative to less common species, and gives rare or competitively inferior species a greater chance to persist. We tested this hypothesis in a 50-ha mapped plot of tropical moist forest on Barro Colorado Island, Panama, by investigating the spatial patterns of sapling recruitment in 80 species of trees and shrubs. Two censuses of adults and saplings were carried out, in 1982 and in 1985. Recruits were defined as saplings of 1-8 cm dbh (diameter breast height) appearing in the 1985 census that were not present in 1982. The distance from each recruit to its nearest conspecific adult neighbor was measured. At various distances from adults, the number of conspecific recruits and the number of recruits of all species were tallied. The ratio of recruits of species i to al...

The invasive potential of Australian banksias in South African fynbos: A comparison of the reproductive potential of Banksia ericifolia and Leucadendron laureolum

Abstract: Several taxa of the Australian Proteaceae have invaded South African fynbos and require costly management programmes to bring under control. Banksia spp. have been introduced only recently to fynbos regions. The invasive potential of Banksia ericifolia (Proteaceae) was investigated by comparing its recruitment potential with that of an indigenous proteaceous shrub, Leucadendron laureolum. Both species are overstorey shrubs that are killed by fire and rely on canopy-stored seeds (serotiny) for recruitment. Eight year old B. ericifolia shrubs produced an average of 16 500 seeds per plant, which is thirty times more than the average of 570 seeds produced by 10 year old L. laureolum shrubs. The seed bank of B. ericifolia was not only larger than that of L. laureolum (1098 vs 525 viable seeds m−2 projected canopy cover), but also considerably larger than that described for the species in its native environment (200–330 seeds m−2 in a 9 year old stand north of Sydney). Leucadendron laureoleum released most of its seed a few days after the cones were burnt, whereas seed release in B. ericifolia was spread over 12 weeks. The seeds of B. ericifolia had lower wingloading and fall rates than L. laureolum and were dispersed over greater distances. The relative seedling growth rates of the two species were very similar (0.03 g per day), but below-ground biomass was greater and proteoid roots were more developed in B. ericifolia seedlings than in L. laureolum after 100 days. Four year old B. ericifolia plants growing in the field had attained over twice the height of indigenous pro-teoids and accumulated up to 10 times the fresh biomass of L. xanthoconus, a species which is ecologically similar to L. laureolum. The Bioclimatic Prediction System (BIOCLIM) was used to create a bioclimatic profile of B. ericifolia and identify climatically suitable areas in the Cape Province. Results show that its potential distribution covers most fynbos areas in the southwestern Cape. It is concluded that B. ericifolia has the potential to be highly invasive in fynbos.

Species richness and seed dispersal to secondary woods in southern Poland.

Abstract: Twenty-seven secondary woods, 32-51 years old, occurring on a limestone hill near Krakow, Poland, were investigated. Twenty-five woods were Scot's pine woods; twelve of them were planted. They differ in area, shape and isolation from the primary woods. 195 species of vascular plants were found in the secondary woods, including fifty-nine small shrub and herb woodland species (out of the seventy woodland species growing in the nearby primary woodlands). Only thirteen woodland species occuffed in more than a half of the secondary woods. Statistical analysis included ten independent variables (area, age, shape, distance to primary woodlands, cover of tree and shrub layer, and others) and two dummy variables. The results of a stepwise multiple regression analysis show that the number of all species depends only on area. The number of woodland species is significantly dependent on distance from the source of diaspores, area, shape, age and type of soil. The number of conifer woodland species is independent of wood age, which affects the number of broadleaf woodland species. A close relationship was found between the type of dispersal of woodland species and their distribution in secondary woods. Hovering and flying anemochores and endozoochores are the best colonizers, the worst are heavy anemochores, myrmecochores and barochores. The slow rate of the secondary succession in the study woods is due to poor colonizing ability of many woodland species as well as to the effect of the xeric conditions on the establishment and growth of seedlings.

A Bite is a Bite is a Bite? Constraints on Response to Folivory in Piper Arieianum (Piperaceae)

Abstract: Because resource movement is restricted within and among branches of woody plants, different patterns of folivory are likely to differentially affect plant fitness. I examined experimentally the effects of folivory pattern on growth and seed output in the understory tropical wet forest shrub Piper arieianum (Piperaceae) in Costa Rica. I removed 10% of a plant's leaf area in two constrating manners, either from a single reproductive branch or throughout the canopy. I also conducted the experiment at two times of year, 3 mo before flowering and during flowering, to determine the effects of damage relative to the phenology of reproduction. Leaf area was removed to simulate damage by natural folivores. I followed the effects of the removal for 1 yr, both for experimental plants and for control plants from which no leaf area was removed. Response to experimental folivory varied by treatment both at the branch and whole plant level. When 10% of a plant's leaf area was removed from single reproductive branches 3 mo before the main flowering season, those damaged branches grew less and produced 80% fewer viable seeds than did similar branches on control plants and plants that lost 10% of their leaf area, but with the losses scattered throughout the plant. These differences among treatments for individual branches translated into differences over the entire plant: plants with damage concentrated on one reproductive branch grew less overall and produced fewer seeds than plants of the other two treatments. Thus, restricted movement of resources in P. arieianum following leaf damage to single reproductive branches resulted in decreased whole plant fitness. When this same experiment was conducted at the time of flowering, there were no differences among treatments in seed production or growth, either at the branch or plant level. Results of these two experiments demonstrate that the effects of folivory are dependent on the timing of the loss relative to the reproductive phenology of the plant; resources necessary for fruit production are accumulated before flowering and not subsequent to it in P. arieianum. In 32% of plants in which damage was concentrated, seed production was reduced due to either abortion of the damaged branch (17%) or aborption of its bud inflorescences (15%). In the remaining 68% of branch—defoliated plants, neither level nor timing of flowering was affected by the experimental folivory, suggesting that reduction in seed production was due to a decrease in resource availability and not to a lack of pollinator visitation. Together these results add to the evidence that resource movement is restricted in woody plants, even in the face of a stress such as herbivory. The results further suggest that relatively small amounts of folivory can have major detrimental effects in woody plants and that damage pattern as well as damage level must be measured.

Contrasting water-use patterns among size and life-history classes of a semi-arid shrub

Abstract: Ecophysiological characteristics of plants from different size and life-history classes were investigated in a field population of Crysothamnus nauseosus. Juvenile plants (>1-year-old but pre-reproductive) had higher rates of photosynthesis, stomatal conductance and transpiration than reproductive adults, even though pre-dawn xylem pressure potentials of juveniles were slightly lower. Juveniles were also less water-use efficient than adults based on instantaneous gas exchange (photosynthesis/transpiration) and carbon isotope discrimination (...)

Competition and facilitation in the recruitment of seedlings in Patagonian steppe

Abstract: The Patagonian steppe consists of shrubs encircled by relatively dense stands of grasses; the areas between shrubs have scattered tussocks, interspersed with bare soil. This study investigates the seed bank and the establishment of a perennial grass, Bromus pictus, around shrubs and in the scattered-tussock patches, with special attention to root competition. About 20 times more seeds of B. pictus were found in the soil of the dense grass zone around shrubs than in the scattered-tussock patches. B. pictus seeds were placed in both types of patches, either with the natural level of below-ground competition present, or experimentally reduced by a fine mesh. Seedling survival and growth were reduced by root competition. Where root competition was experimentally reduced, the aerial protection afforded by the shrub increased the growth of seedlings. We conclude that root competition between adult plants and seedlings is mainly for soil water, and is greater near shrubs than in the scattered-tussock patches. Root competition from established plants is greater than the aerial protection afforded by shrubs, in the area close to shrubs.

Competition for Resources Between Understory Vegetation and Overstory Pinus Ponderosa in Northeastern Oregon.

Abstract: The objective of this research was to determine which environmental re- sources, light, water, and/or nutrients, control understory plant production in a Pinus ponderosa forest in northeastern Oregon. A split-plot experimental design, with three 5.0- ha blocks, four treatments, and 44 plots, was established in the summer of 1985. Twenty plots (4 x 4 m) were trenched (root-reduction treatment) ;1 m in depth, and 24 non- trenched plots (root-control treatment) were used to assess the effects of root competition of overstory trees on understory plants. Trees were commercially thinned (canopy-reduction treatment) in half of each block (2.5 ha) during the winter and early spring of 1986, from a density of 345 to 148 trees/ha to increase light levels to the understory. Thinning sig- nificantly increased photosynthetically active radiation, decreased midday relative humid- ity, and increased midday air temperatures. Xylem potential of the dominant graminoid (Carex geyeri), soil water potential, mineralizable nitrogen, and pH were increased within the root-reduction vs. the root-control treatments. Micro- and macronutrients in C. geyeri and Symphoricarpos albus, the dominant shrub, were influenced in both treatments. In- creasing light did not increase understory biomass production. Reducing root competition for soil water and nutrients increased understory aboveground biomass by 53 and 94% in 1986 and 1987, respectively. This research demonstrated that belowground resources were the primary controlling factors of understory production in P. ponderosa forests in north- eastern Oregon.

Recruitment of seedlings and vegetative sprouts in unburned chaparral

Abstract: Age structure of 12 stands of chaparral, unburned for 56-120 yr, was in- vestigated. All shrubs produced discernible growth rings, and ring counts on stems from stands of known age, plus synchrony in annual growth ring width among species in the same stand, were taken as evidence that growth rings represented annual rings. Species that survive fire by vegetative regeneration from the root crown were capable, in the absence of fire, of continuously regenerating their canopy with basal sprouts. These multistemmed resprouting species had an uneven age structure, indicating continuous recruitment of new stems into the population. The predicted age distribution for resprouting shrub stem pop- ulations illustrated very different patterns for species within the same stand and for the same species in different stands. In most resprouting shrubs, < 15% of the stems dated back to the first decade after fire, and in many species the majority of stems were < 10 yr of age. This contrasts with obligate seeding shrubs that do not initiate new stems from the root crown, and for which, in undisturbed chaparral, 100% of the stems date back to the years immediately following the last fire. Seedling recruitment was abundant in seven of these old chaparral communities. Species with abundant seedling populations, ranging from 1000 to 37 500 individuals/ha, were Quercus dumosa, Q. wislizenii, Rhamnus crocea, Prunus ilicifolia, Heteromeles arbutifolia, and Cercocarpus betuloides. These species share the characteristic of regenerating after fire solely by resprouting, and seldom recruit seedlings during the first few decades after fire. Thus, it is concluded that these postfire obligate resprouters require long fire-free periods for successful reproduction and potential population expansion. For all but the last species, seedlings were most abundant in stands with a closed canopy and deep litter layer. Safe sites for establishment were not in gaps, although gaps may be important for successful recruitment of saplings into the adult population. For all sprouting species the actual age distribution of seedlings and saplings indicated that recruitment did not occur every year; mortality was high but most populations had some saplings that survived beyond the first decade. For all postfire seeding taxa, viz., Adenostoma, Arctostaphylos, and Ceanothus, no significant seeding recruitment was observed in these ancient communities. Seedlings of woodland tree species had colonized several of these old chaparral communities; however, even in stands a century old, successional replacement of chaparral was not imminent.

Correlating genetic variation in carbon isotopic composition with complex climatic gradients.

Abstract: Genetic variation in both carbon isotope discrimination and the proportions of leaf and photosynthetic twig tissues were observed in ecotypes of Hymenoclea salsola T.G., a common shrub in the deserts of the western United States, when grown under common garden conditions. These variations were correlated with climatic conditions in the habitats of origin through a model that described the leaf-to-air water vapor gradients experienced by plants during the growing season. Both carbon isotope discrimination and the proportion of leaves in the canopy were lower in plants derived from habitats with higher leaf-to-air vapor gradients, despite the fact that some of these sites received relatively high amounts of annual precipitation. These patterns were consistent with the notion that plants are able to maintain substantial control of water-use efficiency over large environmental gradients of temperature and moisture availability.

Effects of Elephants and Goats on the Kaffrarian Succulent Thicket of the Eastern Cape, South Africa

Abstract: 1. Kaffrarian succulent thicket is a dense, semi-succulent, thorny vegetation which is rapidly being eliminated under current pastoral systems. To understand why this occurs, the effect of defoliation by wild herbivores (mostly elephant) was compared with that of domestic ungulates (mostly goats) from surveys inside and outside the Addo Elephant National Park. 2. Both elephants and goats reduced canopy cover and increased shrub density in relation to control areas. Goats reduced the number of dominant shrub species per quadrat. 3. Replacing elephants with goats resulted in a fundamental change in the shrub community to one dominated by small, unpalatable shrubs with a few scattered umbrella-shaped trees

Photosynthetic plasticity of two rain forest shrubs across natural gap transects

Abstract: Photosynthetic plasticity of two congeneric shrub species growing under natural field conditions was compared along transects spanning two canopy gaps in a Costa Rican rain forest. Piper arieianum is a shadetolerant species common in successional and mature forests, whereas P. sancti-felicis is a pioneer species abundant in abandoned clearings and large gaps. Twenty potted cuttings of each species were placed at regular intervals along two east-west transects crossing a small branch-fall gap and a large tree-fall gap. Along the transects, the percent of full sun photon flux density varied from less than 2% to 45%. After six months of growth under these conditions, leaves were monitored for incident photon flux density, photographic measures of light availability, photosynthetic capacity (Amax), leaf nitrogen content, leaf chlorophyll content, and specific leaf mass. Although both species demonstrated considerable plasticity in Amax across gap transects, P. sancti-felicis leaves had a superior capacity to track closely variation in light availability, particularly in the larger gap. For regressions of Amax on measures of light availability, P. sancti-felicis consistently showed a 3.5 to 5-fold higher coefficient of determination (R2) and a 3 to 4-fold higher slope than P. arieianum. In both species leaf nitrogen content per leaf area increased significantly with light availability, although P. sancti-felicis, again, showed a much stronger relationship between these variables. Across the transects, mean chlorophyll content per unit leaf area did not differ significantly between the species, whereas mean chlorophyll content per unit leaf dry mass was 3-times greater in leaves of P. sancti-felicis. Piper arieianum exhibited highly significant increases in chlorophyll a:b ratio with increased light availability, whereas P. sancti-felicis lacked significant variation in this trait across a gradient of light availability. Mean specific leaf mass did not vary significantly between species across the gap transects. The nature of the light acclimatory response differs quantitatively and qualitatively between these species. An important constraint on light acclimation of the shade-tolerant P. arieianum is its inability to increase photosynthetic nitrogen-use efficiency under conditions of high light availability. The lack of plasticity in chlorophyll a:b ratios does not restrict light acclimation of Amax in P. sancti-felicis. Leaves of P. arieianum exhibited symptoms of chronic photoinhibition in exposed microsites within the large gap. Species differences in the capacity to finely adjust Amax across a wide range of light conditions may be attributed to their maximum growth potential. Light acclimation in species with low maximum growth potential may be constrained at the cellular level by rates of protein and chlorophyll synthesis and at the whole-plant level by low maximum rates of uptake and supply of nutrients and water. For P. arieianum, restriction of photosynthetic plasticity is likely to limit competitive abilities of plants in high-light conditions of large gaps and clearings, whereas observed habitat restrictions for P. sancti-felicis do not appear to depend upon the highly-developed capacity for adjustment of Amax observed in this species.

Abundance and stratification of foliage arthropods in a lowland rain forest of Cameroon

Abstract: . 1 Arthropod densities and apparent leaf damage were compared within the canopy ecotone and the shrub layer of a lowland rain forest in Cameroon, using a branch clipping method. 2 Most of the individuals collected consisted of ants (average 44%) and various herbivores (31%). Overall arthropod densities amounted to 17 individuals per sample, which, on average, consisted of 0.85 m2 of foliage area. Arthropod densities were lower than on temperate foliage. 3 Arthropod densities were about 3 times higher in the canopy than within the shrub layer. In particular, ants and herbivores were significantly more abundant in the canopy than within the shrub layer. Usually, layer effects rather than site effects appeared to cause greater variance in arthropod abundance. 4 Arthropod species-richness, as estimated by the number of operational taxonomic units sorted, was higher in canopy samples than in samples obtained from the shrub layer. However, apparent leaf damage was higher within the shrub layer (10.9%) than on the canopy (5.2%). 5 Possible factors responsible for the high densities of ants and herbivores on the canopy and for the high leaf damage within the shrub layer are discussed.

Demographic structure of California chaparral in the long-term absence of fire

Abstract: . Demographic structure of 12 chaparral sites unburned for 56 to 120 years was investigated. All sites were dominated by vigorous shrub populations and, although there was colonization by seedlings of woodland tree species in several stands, successional replacement of chaparral was not imminent. Although successional changes in community composition were evident, there was no indication of a decline in species diversity. Non-sprouting species of Ceanothus suffered the greatest mortality at most, but not all, sites. Sprouting shrubs, such as Quercus and Heteromeles had very little mortality, even in stands more than a century old. All postfire resprouting species had multiple stems of different ages indicating these shrubs were capable of continuously regenerating their canopy from basal sprouts. Ceanothus populations were highly clumped and there was a significant correlation across all sites between variance/mean ratio and percentage mortality. As Ceanothus populations thinned, they became less clumped. In mixed chaparral stands, Quercus and Heteromeles were significantly taller than associated Ceanothus shrubs and overtopped the Ceanothus; at two sites, the density of live Quercus per plot was correlated with the density of dead Ceanothus. Thus, mortality of Ceanothus stems is likely related to both intra and interspecific interations. Seedling recruitment was observed for most shrub species that regenerate after fire by resprouting; seedling and sapling densities ranging from 1000–36 500 ha-1 were recorded for Quercus dumosa, Rhamnus crocea, Prunus ilicifolia, Heteromeles arbutifolia and Cercocarpus betuloides. For all but the last species, seedlings and saplings were most abundant beneath the canopy cover and not in gaps. Across all sites, recruitment was significantly correlated with depth and bio-mass of the litter layer. Cercocarpus betuloides was present in several stands, but seedling establishment was found only in one very open, disturbed stand. Regardless of stand age, taxa such as Adenostoma, Arctostaphylos and Ceanothus, which recruit seedlings after fire, had no significant seedling production.

Importance of the soil seed bank of disturbed sites in Chilean matorral in early secondary succession

Abstract: . The dynamics of the seed bank may provide clues to the process of recovery of the vegetation of disturbed sites. The role of the seed bank may be more important in areas with a seasonal climate than in areas where seedling recruitment is not limited to one season. We studied the seed bank and the seed rain in three sites of the Chilean mediterranean-climate region (33° 48'S) which differed in the degree of anthropic disturbance: a closed-canopy, second-growth forest; an open matorral; and an old-field. Additionally, we tested the germination of seeds from the soil and from the current-year seed crop. The seed bank varied considerably between the two years of study, although no change in the vegetation could be observed. Seed density and species richness were lower in 1989 than in 1988. The seed bank of the second-growth forest changed less between years, while the old-field showed the largest change. The highest seed rain occurred under shrub patches in the open matorral, while few seeds fell in the spaces between shrub clumps or in the old-field. In the forest, seed rain was low and correlated with species cover. Germination was low (0 - 15%) in tests using either soil samples or fresh seeds. These results indicate that matorral succession is a very slow process, limited mainly by low germination and low arrival of propagules to open areas. Most woody species have animal-disseminated fleshy propagules. The presence of established shrubs which may serve as perches or refuges for animals increases species richness in the seed rain and the seed bank.

Habitat selection by breeding Black-throated Blue Warblers at two spatial scales

Abstract: New Hampshire, U.S.A. I found different responses to shrub density at two different spatial scales (plot and territory). At the scale of 15 ha study plots, the number of breeding birds on seven plots was positively related to shrub density, suggesting that areas having a high density of shrubs were selected by the birds. Selection of shrubs was further supported by an experiment in which the density of Black-throated Blue Warblers declined to zero in the three years after leaves were removed from shrubs on a 14 ha plot. In contrast to the plot scale results, the territory scale analysis demonstrated that shrub density was not greater inside than outside territories in any of 11 analyses (five plots, each studied for two or three years). Thus, within plots, territories were not located in areas where shrub density was high. No other vegetation variable consistently differed between inside and outside of territories within plots. The plot scale analyses provide an example of how the vertical structure of a habitat can affect the distribution of bird species. The conflicting results at the two scales point out the importance of considering several scales in studies of distribution and abundance, but they also raise the question of why a bird would preferentially select plots with dense shrubs and then not locate its territory so that shrub density is maximized. I argue that all territories on plots where average shrub density is above a threshold level will contain enough small dense patches of shrubs for the birds, especially if these patches are important for nesting rather than for foraging. If this argument is valid, then habitat selection at the scale of the territory may not be necessary for Black-throated Blue Warblers.

Root distribution of Acioa barteri, Alchornea cordifolia, Cassia siamea and Gmelina arborea in an acid Ultisol

Abstract: A major constrait to alley cropping is the competition of tree or shrub roots with those of companion food crops for available water and nutrients in the topsoil. Root distribution patterns of Acioa barteri, Alchornea cordifolia, Cassia siamea and Gmelina arborea grown on an acid Ultisol at Onne in the humid forest zone of southeastern Nigeria were examined to a depth of 120 cm and laterally to 200 cm from the tree trunk to study the suitability of the species for alley cropping. The four woody species have roots throughout the soil profile examined but differ in the concentration of roots both laterally and vertically. Alchornea cordifolia, Cassia siamea and Gmelina arborea, in spite of higher underground biomass production, most of their fine roots (<2 mm diameter) were in the top 20 cm of the soil. This soil layer had 73%, 76%, and 74% of the total Alchornea cordifolia, Cassia siamea, and Gmelina arborea fine roots in the profile examined, respectively. Such root systems would compete with food crops for nutrients and moisture in the surface soil. Alchornea cordifolia and Gmelina arborea have many large woody roots in the surface soil which will make any tillage operation or seedbed preparation difficult. Acioa barteri in contrast, has the desirable rooting system with fewer fine roots in the surface soil (49%), and roots that are concentrated close to tree trunk and decrease markedly away from the tree base. In addition, Acioa barteri roots penetrate deeper soil horizons and can result in more efficient nutrient cycling from these layers, and reduced competition with shallow-rooted food crops. The rooting distribution patterns of Acioa barteri indicated that the species is a promising alley shrub in acid soils of the humid forest ecology. Therefore, consideration of rooting characteristics of potential tree/shrub species is recommended for the development of agroforestry systems such as alley cropping.

Among-tree variation in leaf phenology and morphology and its correlation with insect performance in the mountain birch

Abstract: We studied variation in leaf morphology and phenology in a population of the mountain birch (Betula pubescens ssp. tortuosa) in northernmost Finland over two summers. We tested tree quality with bioassay using several leaf-eating insect species to study the impact of plant variation on herbivore performance. Leaf size and phenology varied considerably between years. Differences among trees were significant, but undividual trees retained their leaf-character-ranking within and between seasons. Positive correlations between tree height and leaf size and differences in bud growth patterns indicate that the mountain is genetically related to the dwarf birch, B. nana (...)

Succession on an Alaskan Tundra Disturbance with and without Assisted Revegetation with Grass

Abstract: The long-term effects of planted grass on the rate and pattern of succession were evaluated on a construction-disturbed alluvial fan tundra site in the central Brooks Range of Alaska. Plots were evaluated 4 and 11 yr after the site was scarified and partially fertilized and seeded in strips with a grass mix. Cover of the surviving planted grass, Festuca rubra, was < 5%. In comparison with the no-grass treatment, the grass treatment inhibited or delayed establishment and growth of native plants. Many species, particularly nonlegume forbs such as Epilobium latifolium, failed to establish on the grass treatment; establishment of other nonlegume forbs and some shrubs, including Artemisia tilesii and Salix glauca, was delayed; and growth of the dominant shrub Salix alaxensis was inhibited. In contrast, legumes such as Astragalus alpinus established and grew equally well on both treatments. Grass treatment effects were attributed to the very dense shallow root system of the planted grass and to higher moss cover, which probably reduced soil moisture in the upper portion of the soil profile, decreased the number of "safe sites" for seedling establishment, and competed for limited nutrients.

Resource heterogeneity in oldfields

Abstract: Spatial heterogeneity of photosynthetically active radiation (PAR), soil moisture and soil N mineralization in a range of oldfield communities was analyzed using semi- variograms for transect data. Percent transmittance of PAR was examined in eight communities, and soil moisture and N mineralization were examined in the two most common com- munities. In general, the sites with the highest resource avail- ability had the smallest coefficients of variation. For light, 63 % of the transects showed significant non-zero spatial autocorrelation with sills at 1 - 70 cm distance intervals. Al- though the grass-dominated communities had the highest av- erage PAR levels and the lowest coefficients of variation, they also were among those with the greatest percentage of transects that had non-zero spatial autocorrelation. For light, the average semivariogram range, or distance within which samples were autocorrelated, was 19 cm and different among communities atp = 0.07. For soil moisture, two of three shrub sites and one of three grass sites showed non-zero spatial autocorrelation with sills at 10 - 300 cm distance intervals. Average semivariance range was 249 cm for the shrub com- munity and 170 cm for the grass community. For N-minerali- zation, none of the shrub and one of three grass sites showed non-zero spatial autocorrelation with a sill, with a semivariance range of 60 cm. Soil moisture and N-mineralization were higher in the shrub than in the grass communities. Overall, these results indicate that resource variability occurred within patches of uniform vegetation, and the range of resource spatial dependence was different among resources even within a single community type. Our results suggest that different seedlings invading these communities will experience very different patterns of microsite heterogeneity as a result of their specific resource requirements.

The effects of light environment on herbivory and growth in the dioecious shrub Lindera benzoin (Lauraceae).

Abstract: -Measurements of total leaf area, herbivory, number of herbivores, and growth on male, female and vegetative individuals of the dioecious shrub Lindera benzoin were made in different light environments. Herbivore abundance did not differ across habitat types and sexual morphs. Herbivory was greater in shade habitats than in sun habitats, and vegetative plants suffered greater herbivory than did flowering males or females which did not differ in the amount of leaf material eaten. Branch growth rates were higher in the sun than in the shade, and females grew less than male or vegetative plants. Because allocation to defense, growth and leaf characteristics are likely to be confounded, rates of herbivory may not always be predicted based on growth rate or gender alone.

Effects of rainfall, competition and grazing on flowering of Osteospermum sinuatum (Asteraceae) in arid Karoo rangeland.

Abstract: The shrub Osteospermum sinuatum, an important fodder plant in Karoo rangelands, flowered after heavy rain in autumn, winter and spring. The number of flowers produced per bush was positively conelated with basal stem diameter and rainfall in the 12 weeks before anthesis. Flowering increased when neighbouring plants of all species were removed. Grazing during flowering and seed set, by sheep stocked at recommended densities, reduced potential seed set by as much as 90%. Birds, hares and small antelope consumed 10–50% of the flowers. Size‐class distributions indicated that little recent recruitment had taken place on a ranch where palatable plants were scarce and where O. sinuatum flower production was severely depressed by grazing sheep.