Neighbor Removal Research Articles

Effects of nutrient availability and neighbours on shoot growth, resprouting and flowering of Erica multiflora

Abstract. To test if low soil fertility and competition limit the performance of Mediterranean shrubs, and if the effects of competition on plant performance were modified by soil fertility, we subjected shrubs of Erica multiflora to a factorial field experiment of fertilization and removal of neighbours around target plants. After 18 months of treatment, fertilization had stimulated the growth of pre‐existent sprouts and biomass allocation to stems into new sprouts, but decreased the frequency of sprout flowering. Removal of neighbours increased the number and biomass of new sprouts, the probability of sprout flowering and the biomass of flowers. Fertilization slightly enhanced sprout recruitment and the probability of sprout flowering when neighbours were removed, but did not modify the other parameters of plant performance. According to our results, both low soil fertility and competition limited plant performance. Competition was slightly more intense in fertilized plants, but only in determining sprout and flowering bud stimulation.

Influence of Gap Disturbances and Type of Microsites on Seedling Establishment in Bouteloua Gracilis

1 We examined the effects of patch disturbance leading to removal of conspecific neighbours, and of microsite type on the emergence, survival, and performance of seedlings of Bouteloua gracilis at a shortgrass steppe site. Our hypothesis was that the death and/or removal of a full-size B. gracilis plant, would result in increased availability of below-ground resources and thus promote the establishment of conspecific seedlings. 2 In 1990 and 1991, experimental plots were treated to mimic the effects of some common patch disturbance types. Caryopses were then planted into both plant microsites (i.e. located in the hummock formed by adult plants of B. gracilis) and bare soil. Seedlings were watered until they were past the stage of adventitious root development. Soil water contents were recorded during and after the cessation of watering. 3 In 1991, results clearly showed that the effect of removal of established neighbours was significant in favouring seedling establishment. Differences among disturbance types were of lesser magnitude. Seedling responses were associated with soil water relations. Following cessation of watering, control plots with intact canopies showed more rapid depletion of soil water than disturbed plots. 4 The positive effect of bare soil microsites during emergence and early survival was associated with changes in superficial (0-5 cm) soil-water relations. Type of microsite was important for plant responses only during the early stages of seedling development, although soil water differences remained significant after the cessation of watering. 5 These results support the idea that gap-disturbances must occur to achieve the regeneration of the dominant species. The detrimental effect of microsites associated with the plant hummock may affect the pattern of emergence, resulting in a tendency for seedlings to establish in areas that were not occupied by their predecessors. Interference of established plants via below-ground competition and microsite favourability plays a key role in the regeneration of B. gracilis in this shortgrass community.

Plant Competition in Relation to Neighbor Biomass: An Intercontinental Study with POA Pratensis

A standardized neighbor removal experiment was conducted in 12 plant communities located on three continents to test the null hypothesis that competition intensity (CI) was independent of the amount of plant biomass present. Six plots were chosen in each community to cover the range of local variation in plant biomass. In each plot the relative growth rate (RGR) of transplanted Poa pratensis (Poaceae) seedlings was compared in the presence and absence of neighbors. Neighbors were removed experimentally using herbicide. Removing neighbors increased RGR of transplants significantly in most plots. CI increased with an increase in the amount of neighbor biomass present in one community where the range of neighbors biomass was greater than in any other community. In contrast, CI did not change significantly with an increase in neighbor biomass in other communities where the range of neighbor biomass was smaller. For the communities combined, CI was not related to neighbor biomass in a consistent fashion. These results indicate that competition may reduce growth over a wide range of habitat productivity, but the relationship between CI and neighbor biomass may differ among communities.

Neighbour effects on gender variation in Ambrosia artemisiifolia

In this study we tested the prediction that male gender allocation in natural populations of common ragweed (Ambrosia artemisiifolia L.) depends more on relative plant height within the neighbouring canopy than on absolute plant height. This is consistent with the assumption that success as a pollen donor for an anemophilous plant within a crowded population will be greater when neighbours are shorter than when neighbours are taller. Data on height and proximity of neighbours, irradiance, and target plant height, biomass, and gender allocation were collected from two natural populations. In one population, these data were also recorded for a group of target plants that had local neighbours artificially removed when the target plants were seedlings. Allocation to male flowers was most strongly positively correlated with height relative to that of close neighbours and with percent irradiance in natural populations. Numerous all-female plants were recorded among the smallest individuals that were suppressed by a dense overhead canopy of neighbours. No relationships were found between plant size and gender when plants were taller than their neighbours or when neighbours were artificially removed. Hence, previously reported size-dependent gender variation in this species may depend on the presence of neighbours. It is postulated that ragweed individuals may sense the presence of neighbours through the phytochrome system, and that effects of neighbours on light quantity and quality cues a shift to increased female function. However, plants that grew from the seedling stage without neighbours were heavier and more female but were not taller than plants with neighbours left intact. The interpretation of this effect is unclear but may reflect a change in plant architecture corresponding with the removal of neighbours. Key words: gender, plant height, plasticity, pollen dispersal, phytochrome, shading.

Effect of local competition on resprouting ofArbutus unedoafter clipping

Abstract. To evaluate the effects of local competition on the growth and size variability of sprouts following disturbance in a natural population of Arbutus unedo in Catalonia, plants were cut at the base and a neighbor removal experiment was performed. Removal of neighbors resulted in an increase in the number and biomass of sprouts at 2 and 7.5 months after clipping. Number and biomass of sprouts was also correlated with initial plant size (stump area). Inter-genet competition appeared to be symmetric, and acted to delay the onset of interference among sprouts within a genet. Size variability of sprouts on an individual was positively correlated with their density on the stump, supporting the hypothesis that competition among ramets (sprouts) within a genet is asymmetric. Reduced inter-genet competition from neighbor removal resulted in an increase in the number and biomass of sprouts growing from a stump. This resulted in an increase in the asymmetry of competition among sprouts, and therefore an increase in the size variability of these sprouts.

Behavioural plasticity mediates aggression in choruses of the Pacific treefrog

Abstract. Male Pacific treefrogs, Hyla regilla, produce advertisement and encounter calls in breeding choruses. Both types of call contribute to inter-male spacing. The aggressive threshold of males to the advertisement call, defined as the lowest amplitude of playback that elicits encounter calls, correlates positively with the maximum amplitude of the advertisement calls of neighbours, as measured at the focal male's position. Results of two studies support the hypothesis that this correlation derives from plasticity of a frog's aggressive threshold to the advertisement call, as opposed to the hypothesis that a frog has a fixed threshold and adjusts his spacing from his neighbours so that their call amplitudes fall below his threshold. First, aggressive thresholds decreased following removal of a male's closest calling neighbours. Second, playback of the advertisement call at superthreshold amplitudes elevated a male's aggressive threshold for this call. A third study tested the hypothesis that the mechanism underlying this threshold plasticity was sensory adaptation in the peripheral auditory system. The aggressive thresholds of frogs to both the encounter and advertisement calls, which have similar frequencies but different temporal structures, were measured before and after playback of the advertisement call at a superthreshold amplitude. Contrary to the prediction of the sensory adaptation hypothesis, the playback elevated the aggressive threshold to the advertisement call without significantly increasing the threshold to the encounter call. This result suggests that the plasticity of the aggressive threshold to the advertisement call occurs at higher, more central levels of the auditory system.

Pine vole (Microtus pinetorum) movement toward areas in apple orchards with reduced populations

Using radiotelemetry, the movements and activity of 24 adult pine voles (Microtus pine- toruni) were examined before and during removal of neighbouring voles in three North Carolina orchards. Each site had control and treatment plots that were monitored concurrently for 28 d, the first 7 d defining normal behaviour patterns. For the following 21 d, animals were removed from half of each treatment plot while baited, inoperable traps were tended in half of each control plot. Relative to controls, treatment animals moved significantly toward removal areas and significantly increased activity. Although both sexes responded to removal of neighbours, males tended to move further than females. Pine voles actively invade areas in which populations have been reduced, therefore control measures in orchards cannot be limited to one-time removal of problem animals.

Effects of Supplementary Soil Nutrients on Hummock Grasses

Hummocks of Triodia basedowii E. Pritz, T. pungens R.Br. and Plectrachne schinzii Henr. were treated with ash and with standard and high applications of slow-release fertiliser. Following rain and the ensuing growth and flowering, the soils and the hummocks were sampled for biomass and for nutrient content, and compared to controls. In a similar but separate experiment, seed set in inflorescences was assessed for hummocks treated with standard fertiliser application, for hummocks treated by removal of neighbouring hummocks, and for controls. In the absence of nutrient addition, total topsoil nitrogen concentration was higher under hummocks than outside, whereas total topsoil phosphorus concentration was lower under hummocks than outside. The absolute amount of phosphorus in the hummock itself was insufficient to account for the depletion of phosphorus in topsoil under the hummock. Available soil phosphorus was about 1-4% of total phosphorus, and was generally highest towards the edge of hummocks. Additions of ash did not significantly affect soil nutrients, while addition of fertiliser produced substantial increases in total nitrogen and in available and total phosphorus. The increases did not extend to the area surrounding the hummock. The increases in soil nutrients did not result in significant increases in new green growth, or in inflorescence biomass or number. However, nutrients appear to have been taken up into the plants, including into old tissue, because there were increases in nutrient concentration in plant tissues, which sometimes were statistically significant and sometimes not. In the second experiment, neither increased soil nutrients nor removal of neighbours increased the number of inflorescences produced, or the proportion of florets that gave rise to filled grain.

Regulation of clonal growth and dynamics of Panicum virgatum (Poaceae) in tallgrass prairie: effects of neighbor removal and nutrient addition

Clones of the perennial grass Panicum virgatum were studied on the Konza Prairie in northeast Kansas to determine the effects of neighbors, nutrient availability, and physiological integration on ramet population dynamics and clonal growth and architecture. Opposite halves of established clones in the field were subjected differentially to treatments including neighbor removal and nitrogen addition, with intact or severed rhizome connections between halves. Neighborhood competition strongly influenced clone architecture and expansion rates. Removal of neighbors resulted in a >;95% increase in radial clone expansion, intraclonal ramet densities, ramet population growth rates, ramet biomass, and percent of stems flowering, averaged over a 4‐year period relative to halves or clones with intact competitors. Plant responses suggest that effects of interclonal neighbors are mediated through alteration of the light environment in the clone canopy and water availability. Addition of nitrogen did not affect lateral spread or clone structure, but resulted in significant increases in ramet size, flowering, and seed production. ANOVA revealed no significant effect of rhizome severing or treatment × severing interactions, suggesting that the size of the integrated physiological unit is much smaller than clone size and/or that physiological integration had no effect on clone responses to environmental heterogeneity at the scale of the diameter of established clones.

Regulation of Clonal Growth and Dynamics of Panicum virgatum (Poaceae) in Tallgrass Prairie: Effects of Neighbor Removal and Nutrient Addition

Regulation of Clonal Growth and Dynamics of Panicum virgatum (Poaceae) in Tallgrass Prairie: Effects of Neighbor Removal and Nutrient Addition

Plasticity of reproductive components at different stages of development in the annual plant Thlaspi arvense L.

This study examines the effect of different densities and the removal of all neighbours at different stages of development on all components of reproduction in the inbreeding annual Thlaspi arvense L. A 64-fold increase in density significantly reduced all repooductive components. The number of flower buds per plant was decreased most strongly; the order of decreasing plasticity among the other components was number of capsules per flower, individual seed weight, ovule number per capsule, flowers per flower bud and seeds per ovule. Removing neighbours at all stages of development increased seed yield of plants in comparison to the control without density reduction, but patterns of plasticity depended strongly on time of treatment. The main effect of the removal of neighbours at the vegetative stage was to increase the number of flowers per plant, but the number of ovules per capsule and seed weight increased also, and abortion of capsules decreased. Removing neighbours at the onset of flowering initially failed to affect flower number per plant, instead it resulted in a strong reduction of capsule abortion and an increase in seed weight. However, several weeks after flowering had initially ceased, fresh lateral inflorescences were produced, resulting in a second flush of reproduction. Removing neighbours at the stage of fruit ripening resulted at first only in an increase in seed witht, but later a second reproductive phase occurred. Fresh lateral branches were produced, but the apical meristem was also reactivated. The overall pattern of plasticity among all reproductive components in response to a removal of neighbours was the same as in response to density. The position of a capsule along the inflorescence influenced its number of ovules, the rate of seed abortion and the mean weight of seeds, with the type of effect depending on the developmental stage at which neighbours were removed. Significant negative correlations were found between the mean weight of seeds and the number of seeds in a capsule under all treatments.

Neighbour manipulations in natural vegetation a review

Abstract. This paper reviews the literature on neighbour manipulation experiments on species intemctions in natural of semi‐natural vegetation. Three major approaches have been used: 1) introductions into vegetation, 2) trenching, and 3) vegetation removals. Some studies have used a combination of approaches, especially 1) and 3). The removal approach has been used most commonly and is given the greatest emphasis in this paper accordingly.Details of over 50 such studies are summarized here, 48 of which have appeared within only the last two decades. We compare and contrast the experimental designs of these studies and examine their contributions to the understanding of species interactions in vegetation. Evidence for competition is virtually universal among these studies. Several studies have also detected evidence of beneficence between plant species. Numerous factors may confound interpretations in removal experiments including: life stage dependent species responses, species‐dependent timing and speed of response, inadequacy of controls due to temporal and/or spatial variability in site quality, indirect effects of treatment on soil moisture content or nutrient levels, or on the activities of predators or decomposers, and several constraints inherent in particular experimental designs. Another level of complexity arises as a consequence of several variables related to plant attributes that directly determine the nature of interactions between neighbours (e.g. relative competitive abilities, the magnitude of beneficial interactions, the extent to which neighbours make demands on the same resource units). These may interact in a complex manner to affect the response of a ‘target’ plant to the removal of neighbours. Recommendations for future studies are considered and a neighbourhood experimental design is proposed which enables analysis of the extent to which the fates of naturally established individuals following vegetation removal can be accounted for (in multiple regressions) by several variables that reflect different properties and circumstances of interaction with immediate Thiessen neighbours in the field.

Crested Wheatgrass Growth and Replacement following Fertilization, Thinning, and Neighbor Plant Removal

The growth and annual replacement of crested wheatgrass (Agropyron desertorum (Fisch. ex Link) Schult.) tillers are affected by resource availability. Fertilization and grazing affect the resources available to crested wheatgrass plants directly, by increasing nutrient supply or by reducing photosynthetic area and root elongation, and indirectly, by changing the competitive status of neighboring plants. To determine the time and manner of crested wheatgrass response to alterations in resource availability, we assessed the growth, flowering, and replacement of tillers on plants treated as follows: tiller thinning, neighbor plant removal, combined thinning and neighbor plant removal, and nitrogen fertilization. These treatments were repeated on different sets of plants in early spring 1984 and 1985. Plant response was inferred from tiller heights, number of flowering culms, and new spring tiller production within the season of manipulation, and the number and heights of replacement tillers the following spring. Neighbor removal, and in 1 year fertilization, increased the size of tillers and stimulated the emergence of new spring tillers. Fertilization effects did not persist into the following year whereas neighbor removal increased annual tiller replacement at least two-fold on target plants. Tiller-tiller competition was not important in plants of crested wheatgrass because tiller growth and replacement on thinned plants did not differ from that of intact plants. This result occurred when thinned and intact plants were compared when both were growing with or without neighbors. Thinned plants replaced only the remaining tillers by the following spring. They did not regain their pretreatment status, presumably because of the encroachment of neighbors. These results indicate that thinned plants probably would lose their position in a plant community unless their neighbors are affected similarly.

Field Experiments on Desert Ants: Testing for Competition Between Colonies

Our previous observations and short—term experiments on two granivorous ant species in the California desert indicated that established colonies may compete intraspecifically. Here we report the results of 3—yr neighbor—removal and food—addition experiments that tested for the presence of intraspecific and interspecific competition between colonies. We followed the responses of territory use, diet breadth, foraging activity, and reproductive output (alate production) to the treatments. Both territory use and alate production patterns were consistent with the predictions of intraspecific resource competition. We found to evidence for interspecific effects between colonies. Specifically, Veromessor pergandi "neighbor—removed" colonies foraged randomly throughout the treatments, while control colonies tended to forage away from their nearest conspecific neighbor when resource levels were low. Both food—supplemented and neighbor—removed V. pergandei colonies produced proportionately more alates than control colonies. There were no treatment effects on diet breadth or foraging activity at V. pergandei or Pogonomyrmex californicus colonies. Since other experiments on desert granivores have shown significant competitive effects on colony density over the same time scale, the lack of consistent effects of competition was most likely due to: (1) differences between the communities in relation to seed storage and productivity and (2) the fact that our experiments only tested for competition between established colonies and did not consider potential competitive effects of established colonies on alate survival (colony regeneration).

Rapid shifts in phosphate acquisition show direct competition between neighbouring plants

Direct demonstrations of competition between higher plants for specific soil resources are seldom possible1–3. Previous neighbour plant removal experiments in desert environments have shown increased growth and improved water status of the remaining plants4–6. However, these responses were only measured several months after the removal of neighbours, after which period there would have been ample time for major adjustments in the root distribution of surviving plants. We report here field experiments using dual-isotope labelling to measure opportunistic phosphate acquisition by shrubs within 2 weeks of partial defoliation of neighbouring grass plants. Phosphate isotope uptake from interspaces shared with defoliated grasses increased to as much as six times that of uptake from interspaces shared with unperturbed neighbours. The experiments indicate immediate competition for phosphate and the influence of root physiological activity on this competition.

Feeding selectivity in relation to territory size in a herbivorous reef fish.

Territory size in the herbivorous reef fish, Parma victoriae, (Pisces: Pomacentridae) is primarily determined by local density patterns and is independent of changes in food levels (Norman and Jones 1984). However, while individual readily expand territories in response to having their neighbours removed, they appear not to increase total food consumption. Individuals with large territories may benefit from improved food quality. Here we test the hypothesis that expanded territories increase access to preferred food types.A preliminary comparison of the relative proportions of algal species consumed and those present in the field indicated that feeding was highly selective. Strong feeding preferences were indicated by a field 'cafeteria' experiment, in which algal species were presented to individuals in equal amounts. Selectivity indices calculated from diet and availability measures were positively correlated with the feeding preference rankings.Individuals examined from two territory size categories (<10 m2 and > 10 m2) did not exhibit significant differences in feeding prefernces. However, they differed greatly in terms of the makeup of their diets. Higher ranking algae such as Champia zostricola and Rhodoglossum sp. were consumed in greater proportions on large territories. Algal abundances within territories also differed for the two groups. Some of the less abundant, high ranking food species were found in greater quantities per unit area on large territories. The differences in diet were not solely explained by changes in availability. The appropriate comparison showed that whereas individuals on large territories fed in a highly selective manner, those on small territories consumed algae in proportion to their availability. A neighbour removal experiment supported the hypothesis that territory expansion results in an increase in the consumption of preferred algal species.

Electronic structure of idealTiO2(110),TiO2(001), andTiO2(100) surfaces

We report theoretical investigations on the electronic structure of ideal, defect-free surfaces of rutile (${\mathrm{TiO}}_{2}$). The (110), (001), and (100) faces are studied, and for each face, several models are examined, which correspond to different surface atomic compositions. The bulk electronic structure is described by a tight-binding Hamiltonian including interactions up to second-nearest neighbors. From this, the surface electronic structure is determined by making use of the scattering-theoretic method. The results are obtained in terms of surface bound states, surface resonances, and surface densities of states, which are wave-vector, layer, atom, and orbital resolved. This allows a detailed discussion of the origin, the nature, and the localization of the various surface features obtained. For all surface models examined, no occupied states are found in the gap, in accordance with experimental results. The main effect of the surface is to induce O $p$-derived resonances in the valenceband region and Ti $d$-derived resonances in the conduction-band region. These trends are correlated with the relatively strong ionicity of ${\mathrm{TiO}}_{2}$. The nature and the strength of the surface states and resonances, as well as their variation from one face to the other are interpreted in terms of the coordination of the surface cations, the removal of anion second-nearest neighbors, and the reduced screening of cation-cation interactions due to the removal of surface anions.

Competition controls the growth of an identified axonal arborization.

The shape of the axonal arborization was studied in an identified insect sensory neuron. The distribution of presynaptic varicosities within an axonal arbor was shown to be modulated by the density of neighboring terminals. Removal of neighbors near one portion of the axon terminal increased the growth rate in the denervated region and caused a compensatory retraction in other regions. The results support the hypothesis that the size of an axonal arbor is determined intrinsically, whereas the distribution of varicosities within the terminal is determined extrinsically by neighboring terminals. These findings provide a direct demonstration of the effects of competition on an identified nerve cell, as well as one of the first examples of competitive interactions in an invertebrate central nervous system.

Determinants of territory size in the pomacentrid reef fish, Parma victoriae.

Factors governing the size of territories defended by the pomacentrid reef fish, Parma victoriae, were investigated, prompted by contradictory predictions in the literature concerning the effects of food supply and competitors. Observations were carried out over the non-breeding period (March-October) on a medium density population in which territories were partially contiguous. The territory size of adult fish varied between 3 and 26 m2, and was inversely correlated with local densities of conspecifics. The same range in territory size was found for both males and females, which did not differ in the time they spent on territory defence and foraging activities. No correlation existed between territory size and the abundance of algal food, body size, age or time spent on territory defence. Also, there was little variation in territory size over time, despite seasonal changes in the abundance of food algae.Experimental reduction of food supplies on isolated territories of males and females had no effect on territory size. In a higher density habitat an experiment was carried out in which population density and food abundance were simultaneously manipulated. This showed that territory size was primarily determined by intraspecific interactions, as territories exhibited considerable increases in size upon removal of neighbours. No changes in the size of defended areas resulted from either artificial increases or decreases of food levels. There were also no changes in the time spent on defence of territories, foraging time or feeding rates associated with food manipulations or territory expansion, which suggested that food was not a limited resource. This conflicted with current theories proposed to explain territory defence and expansion. It is hypothesized that intraspecific interactions constrain territory size well below the optimum in terms of the abundance of preferred food algal species.