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Manipulation Experiment (manipulation + experiment)
Selected AbstractsAge-dependent reproductive costs and the role of breeding skills in the Collared flycatcherACTA ZOOLOGICA, Issue 2 2007Joanna Sendecka Abstract This study addressed whether there are any age-related differences in reproductive costs. Of especial interest was whether young individuals increased their reproductive effort, and thereby their reproductive cost, as much as older birds when brood size was enlarged. To address these questions, a brood-size manipulation experiment with reciprocal cross-fostering of nestlings of young and middle-aged female Collared flycatchers, Ficedula albicollis, was performed on the Swedish island of Gotland. Nestlings' body mass, tarsus length and survival were recorded to estimate the parental ability and parental effort of the experimental female birds. Female survival and clutch size were recorded in the following years to estimate reproductive costs. We found that middle-aged female flycatchers coped better with enlarged broods than younger females or invested more in reproduction. In the following year, young female birds that had raised enlarged broods laid smaller clutches than the females from all the other experimental groups. This result shows that the young female birds pay higher reproductive costs than the middle-aged females. Both young and middle-aged female flycatchers seemed to increase their reproductive effort when brood size was increased. However, such an increase resulted in higher reproductive costs for the young females. The difference in reproductive costs between birds of different ages is most likely a result of insufficient breeding skills of the young individuals. [source] Effects of nitrogen deposition on the interaction between an aphid and its host plantECOLOGICAL ENTOMOLOGY, Issue 1 2008CARALYN B. ZEHNDER Abstract 1.,Anthropogenic increases in nitrogen deposition are impacting terrestrial ecosystems worldwide. While some of the direct ecosystem-level effects of nitrogen deposition are understood, the effects of nitrogen deposition on plant,insect interactions and on herbivore population dynamics have received less attention. 2.,Nitrogen deposition will potentially influence both plant resource availability and herbivore population growth. If increases in herbivore population growth outstrip increases in resource availability, then increases in the strength of density dependence expressed within the herbivore population would be predicted. Alternatively, if plant resources respond more vigorously to nitrogen deposition than do herbivore populations, a decline in the strength of density dependence would be expected. No change in the strength of density dependence acting upon the herbivore population would suggest equivalent responses by herbivores and plants. 3.,A density manipulation experiment was performed to examine the effect of nitrogen deposition on the interaction between a host plant, Asclepias tuberosa, and its herbivore, Aphis nerii. Aphid maximum per capita growth rate (Rmax), carrying capacity (K), and the strength of density dependence were measured under three nitrogen deposition treatments. The effect of nitrogen deposition on the relationship among these three measures of insect population dynamics was explored. 4.,Simulated nitrogen deposition increased aphid per capita population growth, plant foliar nitrogen concentrations, and plant biomass. Nitrogen deposition caused Rmax and K to increase proportionally, leading to no overall change in the strength of density dependence. In this system, potential changes in the negative feedback processes operating on herbivore populations following nitrogen deposition appear to be buffered by concomitant changes in resource availability. [source] Summer drought decreases soil fungal diversity and associated phenol oxidase activity in upland Calluna heathland soilFEMS MICROBIOLOGY ECOLOGY, Issue 2 2008Hannah Toberman Abstract Natural moisture limitation during summer drought can constitute a stress for microbial communities in soil. Given globally predicted increases in drought frequency, there is an urgent need for a greater understanding of the effects of drought events on soil microbial processes. Using a long-term field-scale drought manipulation experiment at Clocaenog, Wales, UK, we analysed fungal community dynamics, using internal transcribed spacer-denaturing gradient gel electrophoresis (DGGE), over a 1-year period in the 6th year of drought manipulation. Ambient seasonality was found to be the dominant factor driving variation in fungal community dynamics. The summer drought manipulation resulted in a significant decline in the abundance of dominant fungal species, both independently of, and in interaction with, this seasonal variation. Furthermore, soil moisture was significantly correlated with the changes in fungal diversity over the drought manipulation period. While the relationship between species diversity and functional diversity remains equivocal, phenol oxidase activity was decreased by the summer drought conditions and there was a significant correlation with the decline of DGGE band richness among the most dominant fungal species during the drought season. Climatically driven events such as droughts may have significant implications for fungal community diversity and therefore, have the potential to interfere with crucial ecosystem processes, such as organic matter decomposition. [source] Drought changes phosphorus and potassium accumulation patterns in an evergreen Mediterranean forestFUNCTIONAL ECOLOGY, Issue 2 2007J. SARDANS Summary 1Climate models predict more extreme weather in Mediterranean ecosystems, with more frequent drought periods and torrential rainfall. These expected changes may affect major process in ecosystems such as mineral cycling. However, there is a lack of experimental data regarding the effects of prolonged drought on nutrient cycling and content in Mediterranean ecosystems. 2A 6-year drought manipulation experiment was conducted in a Quercus ilex Mediterranean forest. The aim was to investigate the effects of drought conditions expected to occur over the coming decades, on the contents and concentrations of phosphorus (P) and potassium (K) in stand biomass, and P and K content and availability in soils. 3Drought (an average reduction of 15% in soil moisture) increased P leaf concentration by 18·2% and reduced P wood and root concentrations (30·9% and 39·8%, respectively) in the dominant tree species Quercus ilex, suggesting a process of mobilization of P from wood towards leaves. The decrease in P wood concentrations in Quercus ilex, together with a decrease in forest biomass growth, led to an overall decrease (by approximately one-third) of the total P content in above-ground biomass. In control plots, the total P content in the above-ground biomass increased 54 kg ha,1 from 1999 to 2005, whereas in drought plots there was no increase in P levels in above-ground biomass. Drought had no effects on either K above-ground contents or concentrations. 4Drought increased total soil soluble P by increasing soil soluble organic P, which is the soil soluble P not directly available to plant capture. Drought reduced the ratio of soil soluble inorganic P : soil soluble organic P by 50% showing a decrease of inorganic P release from P bound to organic matter. Drought increased by 10% the total K content in the soil, but reduced the soil soluble K by 20·4%. 5Drought led to diminished plant uptake of mineral nutrients and to greater recalcitrance of minerals in soil. This will lead to a reduction in P and K in the ecosystem, due to losses in P and K through leaching and erosion, if the heavy rainfalls predicted by IPCC (Intergovernmental Panel on Climate Change) models occur. As P is currently a limiting factor in many Mediterranean terrestrial ecosystems, and given that P and K are necessary for high water-use efficiency and stomata control, the negative effects of drought on P and K content in the ecosystem may well have additional indirect negative effects on plant fitness. [source] Vegetation responses in Alaskan arctic tundra after 8 years of a summer warming and winter snow manipulation experimentGLOBAL CHANGE BIOLOGY, Issue 4 2005C.-H. A. Wahren Abstract We used snow fences and small (1 m2) open-topped fiberglass chambers (OTCs) to study the effects of changes in winter snow cover and summer air temperatures on arctic tundra. In 1994, two 60 m long, 2.8 m high snow fences, one in moist and the other in dry tundra, were erected at Toolik Lake, Alaska. OTCs paired with unwarmed plots, were placed along each experimental snow gradient and in control areas adjacent to the snowdrifts. After 8 years, the vegetation of the two sites, including that in control plots, had changed significantly. At both sites, the cover of shrubs, live vegetation, and litter, together with canopy height, had all increased, while lichen cover and diversity had decreased. At the moist site, bryophytes decreased in cover, while an increase in graminoids was almost entirely because of the response of the sedge Eriophorum vaginatum. These community changes were consistent with results found in studies of responses to warming and increased nutrient availability in the Arctic. However, during the time period of the experiment, summer temperature did not increase, but summer precipitation increased by 28%. The snow addition treatment affected species abundance, canopy height, and diversity, whereas the summer warming treatment had few measurable effects on vegetation. The interannual temperature fluctuation was considerably larger than the temperature increases within OTCs (<2°C), however. Snow addition also had a greater effect on microclimate by insulating vegetation from winter wind and temperature extremes, modifying winter soil temperatures, and increasing spring run-off. Most increases in shrub cover and canopy height occurred in the medium snow-depth zone (0.5,2 m) of the moist site, and the medium to deep snow-depth zone (2,3 m) of the dry site. At the moist tundra site, deciduous shrubs, particularly Betula nana, increased in cover, while evergreen shrubs decreased. These differential responses were likely because of the larger production to biomass ratio in deciduous shrubs, combined with their more flexible growth response under changing environmental conditions. At the dry site, where deciduous shrubs were a minor part of the vegetation, evergreen shrubs increased in both cover and canopy height. These changes in abundance of functional groups are expected to affect most ecological processes, particularly the rate of litter decomposition, nutrient cycling, and both soil carbon and nitrogen pools. Also, changes in canopy structure, associated with increases in shrub abundance, are expected to alter the summer energy balance by increasing net radiation and evapotranspiration, thus altering soil moisture regimes. [source] Response of Nutrients, Plankton Communities and Macrophytes to Fish Manipulation in a Small Eutrophic Wetland LakeINTERNATIONAL REVIEW OF HYDROBIOLOGY, Issue 5-6 2005István Tátrai Abstract A food web manipulation experiment was started in 1999 in a small shallow eutrophic (Major Lake as a part of the wetland area, the Kis-Balaton Water Protection System, KBWPS). The development of submerged macrophytes, the structure and biomass of phytoplankton and crustacean plankton responded rapidly to the removal of 60% of omnivorous cyprinid fish. An increase in transparency and a decrease in the concentrations of chlorophyll- a, phytoplankton and phosphorus occurred simultaneously with the increased presence of submerged macrophytes, which covered 45% of the lake area compared with <10% during the premanipulation period. The success of fish manipulation demonstrated the potential of this measure as a short-term management strategy. Our data also indicated that the clear water state was not stable in the long term. As long as phosphorus level is high, there is a risk that macrophytes will not successfully establish for longer period. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Gonadotropin-Inhibitory Peptide in Song Sparrows (Melospiza melodia) in Different Reproductive Conditions, and in House Sparrows (Passer domesticus) Relative to Chicken-Gonadotropin-Releasing HormoneJOURNAL OF NEUROENDOCRINOLOGY, Issue 8 2003G. E. Bentley Abstract Gonadotropin-releasing hormone (GnRH) regulates reproduction in all vertebrates. Until recently, an antagonistic neuropeptide for gonadotropin was unknown. The discovery of an RFamide peptide in quail that inhibits gonadotropin release in vitro raised the possibility of direct hypothalamic inhibition of gonadotropin release. This peptide has now been named gonadotropin-inhibitory hormone (GnIH). We investigated GnIH presence in the hypothalamus of two seasonally breeding songbird species, house sparrows (Passer domesticus) and song sparrows (Melospiza melodia). Using immunocytochemistry (ICC), GnIH-containing neurones were localized in both species in the paraventricular nucleus, with GnIH-containing fibres visible in multiple brain locations, including the median eminence and brainstem. Double-label ICC with light microscopy and fluorescent ICC with confocal microscopy indicate a high probability of colocalization of GnIH with GnRH neurones and fibres within the avian brain. It is plausible that GnIH could be acting at the level of the hypothalamus to regulate gonadotropin release as well as at the pituitary gland. In a photoperiod manipulation experiment, GnIH-containing neurones were larger in birds at the termination of the breeding season than at other times, consistent with a role for this neuropeptide in the regulation of seasonal breeding. We have yet to elucidate the dynamics of GnIH synthesis and release at different times of year, but the data imply temporal regulation of this peptide. In summary, GnIH has the potential to regulate gonadotropin release at more than one level, and its distribution is suggestive of multiple regulatory functions in the central nervous system. [source] DOC leaching from a coniferous forest floor: modeling a manipulation experiment,JOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 3 2005Edward Tipping Abstract The DyDOC model simulates the C dynamics of forest soils, including the production and transport of dissolved organic matter (DOM), on the basis of soil hydrology, metabolic processes, and sorption reactions. The model recognizes three main pools of soil C: litter, substrate (an intermediate transformation product), and humic substances. The model was used to simulate the behavior of C in the O horizon of soil under a Norway spruce stand at Asa, Sweden, that had been subjected to experimental manipulations (addition and removal) of above-ground litter inputs and to removal of the Oi and Oe layers. Initially, the model was calibrated using results for the control plots and was able to reproduce the observed total soil C pool and 14C content, DOC flux and DO14C content, and the pool of litter C, together with the assumed content of C in humic substances (20% of the total soil C), and the assumed distribution of DOC between hydrophilic and hydrophobic fractions. The constant describing DOC exchange between micro- and macropores was estimated from short-term variations in DOC concentration. When the calibrated model was used to predict the effects of litter and soil manipulations, it underestimated the additional DOC export (up to 33%) caused by litter addition, and underestimated the 22% reduction in DOC export caused by litter withdrawal. Therefore, an additional metabolic process, the direct conversion of litter to DOC, was added to the model. The addition of this process permitted reasonably accurate simulation of the results of the manipulation experiments, without affecting the goodness-of-fit in the model calibration. The results suggest that, under normal conditions, DOC exported from the Asa forest floor is a mixture of compounds derived from soil C pools with a range of residence times. Approximately equal amounts come from the litter pool (turnover time 4.6 yr), the substrate pool (26 yr), and the humic-substances pool (36 yr). [source] Generalist predators in organically and conventionally managed grass-clover fields: implications for conservation biological controlANNALS OF APPLIED BIOLOGY, Issue 2 2008K Birkhofer Abstract Organically managed agroecosystems rely in part on biological control to prevent pest outbreaks. Generalist predators (Araneae, Carabidae and Staphylinidae) are a major component of the natural enemy community in agroecosystems. We assessed the seasonal dynamics of major generalist predator groups in conventionally and organically managed grass,clover fields that primarily differed by fertilisation strategy. We further established an experiment, manipulating the abundant wolf spider genus Pardosa, to identify the importance of these predators for herbivore suppression in the same system and growth period. Organic management significantly enhanced ground-active spider numbers early and late in the growing season, with potentially positive effects of plant cover and non-pest decomposer prey. However, enhancing spider numbers in the field experiment did not improve biological control in organically managed grass,clover fields. Similar to the survey results, reduced densities of Pardosa had no short-term effect on any prey taxa; however, spider guild structure changed in response to Pardosa manipulation. In the presence of fewer Pardosa, other ground-running spiders were more abundant; therefore, their impact on herbivore numbers may have been elevated, possibly cancelling increases in herbivore numbers because of reduced predation by Pardosa. Our results indicate positive effects of organic farming on spider activity density; however, our survey data and the predator manipulation experiment failed to find evidence that ground-running spiders reduced herbivore numbers. We therefore suggest that a positive impact of organic fertilisers on wolf spiders in grass,clover agroecosystems may not necessarily improve biological control when compared with conventional farming. [source] Responses of two species of heathland rodents to habitat manipulation: Vegetation density thresholds and the habitat accommodation modelAUSTRAL ECOLOGY, Issue 3 2010VAUGHAN MONAMY Abstract The abundance of two native rodent species, Rattus lutreolus and Pseudomys gracilicaudatus, has been shown to correlate with vegetation density in coastal wet heath. Fox's habitat accommodation model relates relative abundances of such small mammal species to heathland vegetation regeneration following disturbance. Implicit in the model is recognition that it is successional changes in vegetation, not time per se, that drives the responses of small mammal species along a regeneration axis. Using a brush-cutter we deliberately removed approximately 85% of vegetation around trapping stations and recorded significant reductions in the abundance of both P. gracilicaudatus (an earlier-stage colonizing species) and R. lutreolus (a late seral-stage species). A significant decrease in the abundance of only the latter had been demonstrated previously when 60,70% of the vegetation had been removed. Following the brush-cutting both species re-entered the mammalian secondary succession at different times, first P. gracilicaudatus followed by R. lutreolus after the vegetation cover thresholds of each species had been reached. The impact of this habitat manipulation experiment was to produce a retrogression of the small mammal succession, experimentally demonstrating causality between changes in vegetation density and subsequent small mammal habitat use. [source] Fitness consequences of floral variation in Senecio jacobaea (Asteraceae): evidence from a segregating hybrid population and a resource manipulation experimentBIOLOGICAL JOURNAL OF THE LINNEAN SOCIETY, Issue 1 2001STEFAN ANDERSSON The present study examines some of the ecological factors that might exert selection on floral morphology in Senecio jacobaea, a self-sterile composite which exhibits geographic variation in the frequency of rayed and discoid individuals. Regression analyses of phenotypic data from a large, segregating hybrid population, established in a semi-natural (garden) environment and studied over a 2-year period, revealed a negative relationship between the size of the rays and the average germination rate of the maternal seed crop, a pattern that can be attributed to the reduced germination speed of achenes from ray florets. There was no effect of ray size on the amount of cross-pollination achieved, the proportion of heads infested by larvae of seed flies (Pegohylemyia) and the amount of resources retained for the next flowering season. The lack of resource costs was also apparent in a manipulation experiment with greenhouse-grown plants of the rayed phenotype: artificial removal of all rays at the early bud or flowering stage had no detectable effect on subsequent flower and fruit development, regardless of whether the plants experienced high or low water stress. Given these and other observations, I hypothesize that plant-animal interactions and resource costs sometimes play a minor role in exerting selection on flower morphology and that spatially varying selection on germination behaviour accounts for some of the morph frequency variation in S. jacobaea. [source] Maximum growth rates and possible life strategies of different bacterioplankton groups in relation to phosphorus availability in a freshwater reservoirENVIRONMENTAL MICROBIOLOGY, Issue 9 2006Karel, imek Summary We investigated net growth rates of distinct bacterioplankton groups and heterotrophic nanoflagellate (HNF) communities in relation to phosphorus availability by analysing eight in situ manipulation experiments, conducted between 1997 and 2003, in the canyon-shaped ,ímov reservoir (Czech Republic). Water samples were size-fractionated and incubated in dialysis bags at the sampling site or transplanted into an area of the reservoir, which differed in phosphorus limitation (range of soluble reactive phosphorus concentrations , SRP, 0.7,96 µg l,1). Using five different rRNA-targeted oligonucleotide probes, net growth rates of the probe-defined bacterial groups and HNF assemblages were estimated and related to SRP using Monod kinetics, yielding growth rate constants specific for each bacterial group. We found highly significant differences among their maximum growth rates while insignificant differences were detected in the saturation constants. However, the latter constants represent only tentative estimates mainly due to insufficient sensitivity of the method used at low in situ SRP concentrations. Interestingly, in these same experiments HNF assemblages grew significantly faster than any bacterial group studied except for a small, but abundant cluster of Betaproteobacteria (targeted by the R-BT065 probe). Potential ecological implications of different growth capabilities for possible life strategies of different bacterial phylogenetic lineages are discussed. [source] Field experiments testing for apparent competition between primary parasitoids mediated by secondary parasitoidsJOURNAL OF ANIMAL ECOLOGY, Issue 2 2001Rebecca J. Morris Summary 1,Two field manipulation experiments were performed to look for apparent competition between primary parasitoids of aphids, mediated by shared secondary parasitoids. In each case the frequency of secondary parasitism on a ,focal' species was compared in the presence or absence of a ,treatment' species and its aphid host. 2,In both experiments the focal species was Aphidius ervi (Haliday) attacking Acyrthosiphon pisum (Harris) (pea aphid). In the first experiment the treatment species was Aphidius rhopalosiphi (De Stefani-Perez) attacking Sitobion avenae (Fabricius) (a grass aphid) and in the second Aphidius microlophii (Pennachio & Tremblay) attacking Microlophium carnosum (Buckton) (nettle aphid). 3,No significant effects of either the presence of A. rhopalosiphi or of its host on secondary parasitism of A. ervi were detected in the first experiment. 4,In the second experiment, rates of secondary parasitism were estimated twice: a significant effect of treatment was found on the first occasion but not the second. In the former, parasitism was highest in the presence of the treatment species and its host, intermediate when just the host was present, and lowest when both were absent. 5,We discuss the evidence for apparent competition in host-parasitoid communities. [source] Shading delays bud break in Brachsyegia spiciformisAFRICAN JOURNAL OF ECOLOGY, Issue 4 2008R. A. Richer Abstract Whole tree manipulation experiments were performed in the common southern African tree species, Brachystegia spiciformis to test a novel hypothesis that decreasing Total nonstructural carbohydrates (TNC) in the stem could cause bud break in Brachystegia spiciformis. The experimental treatments included fertilization, canopy defoliation, shading and stem heating to decrease stem carbohydrates. None of the treatments significantly decreased mean stem TNC. Likewise the heating, fertilization and defoliation treatments did not significantly affect the date of bud break. However, shading significantly delayed bud break. This delay in bud break could not be attributed to changes in leaf level photosynthetic traits, stem water content, leaf predawn water potential or delayed leaf fall. These results question widely accepted hypotheses about the mechanisms controlling bud break and suggest a carbohydrate homeostatic mechanism. Résumé Des expérimentations de manipulations d'arbres très complètes ont été réalisées sur l'espèce très commune en Afrique du Sud Brachystegia spiciformis pour tester une nouvelle hypothèse selon laquelle une diminution des TNC, les hydrates de carbone nonstructuraux, dans le tronc pourrait causer l'éclosion des bourgeons chez cette espèce. Les traitements expérimentaux comprenaient une fertilisation, le défeuillage de la canopée, la mise à l'ombre ou le chauffage des troncs pour diminuer les hydrates de carbone. Aucun de ces traitements n'a diminué significativement les TNC dans les troncs. Le chauffage, la fertilisation ou le défeuillage n'ont pas non plus affecté la date de l'éclatement des bourgeons. Le fait d'être à l'ombre a, par contre, significativement retardé l'éclosion des bourgeons. Ce retard ne pouvait pas être attribué aux changements du taux de photosynthèse des feuilles, à la teneur en eau des troncs, au potentiel aqueux des feuilles avant l'aube, ni au retard de la chute des feuilles. Ces résultats remettent en question les hypothèses largement acceptées au sujet des mécanismes qui contrôlent l'éclosion des bourgeons et suggèrent l'existence d'un mécanisme homéostatique lié aux hydrates de carbone. [source] Analyzing single-molecule manipulation experimentsJOURNAL OF MOLECULAR RECOGNITION, Issue 5 2009Christopher P. Calderon Abstract Single-molecule manipulation studies can provide quantitative information about the physical properties of complex biological molecules without ensemble artifacts obscuring the measurements. We demonstrate computational techniques which aim at more fully utilizing the wealth of information contained in noisy experimental time series. The "noise" comes from multiple sources e.g., inherent thermal motion, instrument measurement error, etc. The primary focus of this paper is a methodology that uses time domain based methods to extract the effective molecular friction from single-molecule pulling data. We studied molecules composed of eight tandem repeat titin I27 domains, but the modeling approaches have applicability to other single-molecule mechanical studies. The merits and challenges associated with applying such a computational approach to existing single-molecule manipulation data are also discussed. Copyright © 2009 John Wiley & Sons, Ltd. [source] DOC leaching from a coniferous forest floor: modeling a manipulation experiment,JOURNAL OF PLANT NUTRITION AND SOIL SCIENCE, Issue 3 2005Edward Tipping Abstract The DyDOC model simulates the C dynamics of forest soils, including the production and transport of dissolved organic matter (DOM), on the basis of soil hydrology, metabolic processes, and sorption reactions. The model recognizes three main pools of soil C: litter, substrate (an intermediate transformation product), and humic substances. The model was used to simulate the behavior of C in the O horizon of soil under a Norway spruce stand at Asa, Sweden, that had been subjected to experimental manipulations (addition and removal) of above-ground litter inputs and to removal of the Oi and Oe layers. Initially, the model was calibrated using results for the control plots and was able to reproduce the observed total soil C pool and 14C content, DOC flux and DO14C content, and the pool of litter C, together with the assumed content of C in humic substances (20% of the total soil C), and the assumed distribution of DOC between hydrophilic and hydrophobic fractions. The constant describing DOC exchange between micro- and macropores was estimated from short-term variations in DOC concentration. When the calibrated model was used to predict the effects of litter and soil manipulations, it underestimated the additional DOC export (up to 33%) caused by litter addition, and underestimated the 22% reduction in DOC export caused by litter withdrawal. Therefore, an additional metabolic process, the direct conversion of litter to DOC, was added to the model. The addition of this process permitted reasonably accurate simulation of the results of the manipulation experiments, without affecting the goodness-of-fit in the model calibration. The results suggest that, under normal conditions, DOC exported from the Asa forest floor is a mixture of compounds derived from soil C pools with a range of residence times. Approximately equal amounts come from the litter pool (turnover time 4.6 yr), the substrate pool (26 yr), and the humic-substances pool (36 yr). [source] Evaluating different soil and plant hydraulic constraints on tree function using a model and sap flow data from ponderosa pinePLANT CELL & ENVIRONMENT, Issue 7 2001M. Williams Relationships between tree size and physiological processes such as transpiration may have important implications for plant and ecosystem function, but as yet are poorly understood. We used a process-based model of the soil,plant,atmosphere continuum to investigate patterns of whole-tree sap flow in ponderosa pine trees of different size and age (36 m and ,250 years versus 13 m and 10,50 years) over a developing summer drought. We examined three different hypothetical controls on hydraulic resistance, and found that size-related differences in sap flow could be best explained by absolute differences in plant resistance related to path length (hypothesis 1) rather than through different dynamic relationships between plant resistance and leaf water potential (hypothesis 2), or alterations in rates of cumulative inducement and repair of cavitation (hypothesis 3). Reductions in sap flow over time could be best explained by rising soil,root resistance (hypothesis 1), rather than by a combination of rising plant and soil,root resistance (hypothesis 2), or by rising plant resistance alone (hypothesis 3). Comparing hourly predictions with observed sap flow, we found that a direct relationship between plant resistance and leaf water potential (hypothesis 2) led to unrealistic bimodal patterns of sap flow within a day. Explaining seasonal reduction in sap flow purely through rising plant resistance (hypothesis 3) was effective but failed to explain the observed decline in pre-dawn leaf water potential for small trees. Thus, hypothesis 1 was best corroborated. A sensitivity analysis revealed a significant difference in the response to drought-relieving rains; precipitation induced a strong recovery in sap flow in the hypothetical case of limiting soil,root resistance (hypothesis 1), and an insignificant response in the case of limiting plant resistance (hypothesis 3). Longer term monitoring and manipulation experiments are thus likely to resolve the uncertainties in hydraulic constraints on plant function. [source] | ||||||||||||||||||||||