Assessment of uncertainty in functional-structural plant models

Background and Aims

Constructing functional–structural plant models (FSPMs) is a valuable method for examining how physiology and morphology interact in determining plant processes. However, such models always have uncertainty concerned with whether model components have been selected and represented effectively, with the number of model outputs simulated and with the quality of data used in assessment. We provide a procedure for defining uncertainty of an FSPM and how this uncertainty can be reduced.

Methods

An important characteristic of FSPMs is that typically they calculate many variables. These can be variables that the model is designed to predict and also variables that give indications of how the model functions. Together these variables are used as criteria in a method of multi-criteria assessment. Expected ranges are defined and an evolutionary computation algorithm searches for model parameters that achieve criteria within these ranges. Typically, different combinations of model parameter values provide solutions achieving different combinations of variables within their specified ranges. We show how these solutions define a Pareto Frontier that can inform about the functioning of the model.

Key Results

The method of multi-criteria assessment is applied to development of BRANCHPRO, an FSPM for foliage reiteration on old-growth branches of Pseudotsuga menziesii. A geometric model utilizing probabilities for bud growth is developed into a causal explanation for the pattern of reiteration found on these branches and how this pattern may contribute to the longevity of this species.

Conclusions

FSPMs should be assessed by their ability to simulate multiple criteria simultaneously. When different combinations of parameter values achieve different groups of assessment criteria effectively a Pareto Frontier can be calculated and used to define the sources of model uncertainty.     

Basal reiteration improves the hydraulic functional status of mature Cinnamomum camphora trees

We compared soil-to-leaf hydraulic conductance (G _T), hydraulic conductivity and water-relations characteristics of leaves between reiterated axes (produced by sprouting of suppressed buds) and sequential axes (produced by elongation of terminal buds) on the same branch to investigate how basal reiteration affected the hydraulic architecture of mature Cinnamomum camphora (L.) Sieb. trees. Given similar light conditions, G _T was higher for leaves on reiterated shoots than for those on sequential shoots. However, where leaves on sequential shoots received more light, G _T was similar to that of leaves on reiterated shoots, suggesting that some compensatory mechanism worked to increase hydraulic conductance to the more distal sequential shoots, which have higher potential for carbon gain. Both xylem- and leaf-specific conductivities were higher for reiterated than sequential shoots. Pressure–volume measurements indicated that leaves on reiterated shoots were more vulnerable to water stress, suggesting that they developed under favorable water status. Because basal reiteration occurs on lower-order branch axes, reiterated shoots have better connectivity to higher conducting xylem and this may contribute to favorable water status. As trees grow larger, hydraulic pathlength and hydraulic resistance both increase as numbers of branch junctions and nodes increase. Our results suggest that basal reiteration improves the hydraulic functional status of mature C. camphora trees by shortening the hydraulic pathway and increasing hydraulic conductance to transpiring leaves.     

Allometric analysis of Sitka spruce branches: mechanical versus hydraulic design principles

The geometry of tree branches can have considerable effect on their efficiency in terms of carbon export per unit carbon investment in structure. The purpose of this study was to evaluate different design criteria using data describing the form of Picea sitchensis branches. Allometric analysis of the data suggests that resources are distributed to favour shoots with the greatest opportunity for extension into new space, with priority to the extension of the leader. The distribution of allometric relations of links (branch elements) was tested against two models: the pipe model, based on hydraulic transport requirements, and a static load model based on the requirement of shoots to provide mechanical resistance to static loads. Static load resistance required the load parameter to be proportional to the link radius raised to the power of 4. This was shown to be true within a 95% statistical confidence limit. The pipe model would require total distal length to be proportional to link radius squared but the measured branches did not conform well to this model. The comparison suggests that the diameters of branch elements were more related to the requirements for mechanical load. The cost of following a hydraulic design principle (the pipe model) in terms of mechanical efficiency was estimated and suggested that the pipe model branch would not be mechanically compromised but would use structural resources inefficiently. Resource allocation among branch elements was found to be consistent with mechanical stability criteria but also indicated the possibility of allocation based on other criteria, such as potential light interception by shoots. The evidence suggests that whilst branch topology increments by reiteration of units of morphogenesis, the geometry follows a functional design pattern.     

Persistence of<Emphasis Type="Italic">Pseudotsuga menziesii</Emphasis> (Douglas-fir) in temperate coniferous forests of the Pacific Northwest Coast,USA

Old-growthPseudotsuga-Tsuga forests of the Pacific Northwest Coast of North America are characterized by the presence of large, old trees ofPseudotsuga menziesii var.menziesii (Douglas-fir). Colonizing soon after a stand-replacing disturbance,P. menziesii persists in these forests, coexisting for centuries with the late-successional species.P. menziesii survives by maintaining emergent status in the uppermost part of the forest canopy, above the crowns of competing late-successional species. After reaching maximum tree height and crown size,P. menziesii maintains shoots and foliage of the established crown by epicormic shoot production. In this review, we propose that attaining emergent status in the upper canopy combined with the process of crown maintenance contributes to the persistence ofP. menziesii into later stages of succession, making this species a long-lived pioneer that between infrequent disturbances can coexist with late-successional species for centuries.     

Competition-dependent modelling of foliage biomass in forest stands

Currently, foliage biomass is estimated based on stem diameter or basal area. However, it is questionable whether the relations between foliage and stem observed from plantations of a single tree species can be applied to stands of different structure or species composition. In this paper, a procedure is presented to simulate foliage and branch biomass of tree crowns relative to crown size and light competition. Crowns are divided into layers and segments and each segment is divided into a foliated and an unfoliated fraction. Depending on the competitive status of the segment, leaf area density, specific leaf area and foliated branch fraction are determined. Based on this information, foliage biomass is calculated. The procedure requires a crown shape function and a measure to characterise competition for light and space of each individual segment within the canopy. Simple solutions are suggested for both requirements to enable an application with data that can be measured non-destructively in the field; these were stem position, tree height, crown base height, crown radii and some general crown shape information. The model was parameterised from single trees of Norway spruce and European beech and partly evaluated with independent data close to the investigation plot. Evaluations showed that the model can attribute the ecology of the different crown forms. Modelled foliage distribution for beech and spruce as well as total needle biomass of spruce agreed well with measurements but foliage biomass of beech was underestimated. The results are discussed in the context of a general model application in structured forests.     

Functional–structural models optimize the placement of foliage units for multiple whole-canopy functions

I present examples of plant functional–structural models (FSMs) that are used to evaluate how foliage units affect whole-canopy functions, and I show that multi-criteria optimization is an effective tool for these models. FSMs produce plant structures through the repeated application of a set of rules for the placement of foliage units. The models are blind (rules are the same regardless of dynamic simulation conditions), sighted (rules change with interference from other foliage units) or self-regulatory (rules change depending on the conditions of the simulation, i.e., internal conditions). In the examples presented, the models are used to optimize plant morphology for one or more measures of plant performance; these measures include movement of materials and associated hydraulic functions, foliage display, light interception and net carbon, mechanical support and stability, and reproductive success. It is consistently found that no morphology is optimal for any single measure of plant performance, and the rules for plant development are not stationary in space and time. In multi-criteria optimization, alternative morphologies are compared against multiple measures of plant performance; these are optimized simultaneously using Pareto optimality, which yields the set of mutually co-dominant solutions not dominated by any other solution. Two solutions are considered to be mutually co-dominant if improvement with respect to one criterion is at the expense of another criterion. I conclude that multi-criteria optimization is an essential tool for the use of FSMs to relate processes at the foliage level to whole-canopy function and to explain the structural diversity of old-growth forests.     

The significance of life history strategies in the developmental history of mixed beech (Nothofagus) forests,New Zealand

Size and age structure analysis, dated past disturbances, treefall replacement patterns, and spatial pattern analysis were used to reconstruct the developmental history of two old-growth Nothofagus fusca/N. menziesii stands, South Island, New Zealand. Diameter and height class distributions suggested that N. menziesii was replacing N. fusca, however, stand history reconstruction analysis showed that both species had regenerated intermittently after small-scale disturbances. Although large-scale disturbances such as blowdowns may occasionally generate even-aged stands, gap-phase regeneration maintains the forests in compositional equilibrium. In the absence of other competing tree species and understorey plants the two species appear to coexist by way of different life history strategies, where one species (N. menziesii) has low juvenile mortality and the other (N. fusca) has faster height growth rates and greater longevity and adult survivorship.     

Canopy Carbon Gain and Water Use: Analysis of Old-growth Conifers in the Pacific Northwest

This report summarizes our current knowledge of leaf-level physiological processes that regulate carbon gain and water loss of the dominant tree species in an old-growth forest at the Wind River Canopy Crane Research Facility. Analysis includes measurements of photosynthesis, respiration, stomatal conductance, water potential, stable carbon isotope values, and biogenic hydrocarbon emissions from Douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western red cedar (Thuja plicata). Leaf-level information is used to scale fluxes up to the canopy to estimate gross primary production using a physiology-based process model. Both light-saturated and in situ photosynthesis exhibit pronounced vertical gradients through the canopy, but are consistently highest in Douglas-fir, intermediate in western hemlock, and lowest in western red cedar. Net photosynthesis and stomatal conductance are strongly dependent on vapor-pressure deficit in Douglas-fir, and decline through the course of a seasonal drought. Foliar respiration is similar for Douglas-fir and western hemlock, and lowest for western red cedar. Water-use efficiency varied with species and tree height, as indexed using stable carbon isotopes values for foliage. Leaf water potential is most negative for Douglas-fir and similar for western hemlock and western red cedar. Terpene fluxes from foliage equal approximately 1% of the net carbon loss from the forest. Modeled estimates based on physiological measurements show gross primary productivity (GPP) to be about 22 Mg C m^–2 y^–1. Physiological studies will be necessary to further refine estimates of stand-level carbon balance and to make long-term predictions of changes in carbon balance due to changes in forest structure, species composition, and climate.     

The canopy water relations of old-growth Douglas-fir trees

 We investigated whole tree water relations in 56–65 m tall, old-growth Pseudotsuga menziesii trees within the Wind River Canopy Crane site, Carson, Washington, USA. We measured at predawn and solar noon the vertical gradients in xylem pressure potential using a pressure chamber. On an Abies amabilis sapling located in the understory at the base of one of the study trees, predawn and solar noon xylem pressure potentials were also measured. Xylem pressure potential data were measured from late June through early September 1996 on foliage sampled from 1 to 64.5 m. Over this height gradient, predawn water potentials ranged from –0.23 to –1.10 MPa. Solar noon values showed an even greater range (from –0.44 to –2.51 MPa). At predawn, the water potential gradient approached the theoretical hydrostatic gradient (–0.0105 vs –0.010 MPa m^–1). The gradient at solar noon was steeper (–0.0331 MPa m^–1). Instantaneous stomatal conductances were not greatly different between young, sapling-sized and old-growth trees [0.094±0.033 (SD) vs 0.086±0.045 cm s^–1, respectively]. Stomata of both size classes of trees appeared very sensitive to increasing vapor pressure deficits. A comparison of stable carbon isotope values from the old-growth and sapling-sized trees indicated lower stomatal conductances in the old-growth. This study provides sound documentation regarding the utility of the cohesion theory in the interpretation of water potential gradients. This study also emphasizes inherent differences between sapling-sized and tall, old-growth trees. Received: 10 January 1998 / Accepted: 12 October 1998     

Competitive strategies in adult beech and spruce: space-related foliar carbon investment versus carbon gain

In Central Europe, Fagus sylvatica and Picea abies represent contrasting extremes in foliage type, crown structure and length of growing season. In order to examine the competitive strategies of these two co-occurring species, we tested the following hypotheses: (1) the space occupied by the foliage of sun branches is characterized by greater foliar mass investment compared to shade branches, (2) the carbon (C) gain per unit of occupied space is greater in sun than in shade branches, and (3) annual C and water costs of the foliage for sustaining the occupied space are low, wherever C gain per unit of occupied space is low. These were investigated in a mature forest in Southern Germany. The examination was based on the annual assessment of space-related resource investments and gains of the foliage. The foliated space around branches was regarded as the relevant volume with respect to aboveground resource availability. Occupied crown space per standing foliage mass was higher in shade compared to sun branches of beech, whereas no difference existed in crown volume per foliage mass between sun and shade branches of spruce (hypothesis 1 accepted for beech but rejected for spruce). However, beech occupied more space per foliage mass than spruce. The C gain per occupied crown volume was greater in sun than in shade branches (hypothesis 2 accepted) but did not differ between species. The amount of occupied space per respiratory and transpiratory costs did not differ between species or between sun and shade branches. In beech and spruce, the proportion of foliage investment in the annual C balance of sun and shade branches remained rather stable, whereas respiratory costs distinctly increased in shade foliage. Hence, shade branches were costly structures to occupy space, achieving only low and even negative C balances (rejection of hypothesis 3), which conflicts with the claimed C autonomy of branches. Our findings suggest that competitiveness is determined by the standing foliage mass and the annual branch volume increment rather than annual investments in foliage. Expressing competitiveness in terms of space-related resource investments versus returns, as demonstrated here, has the potential of promoting mechanistic understanding of plant–plant interactions.     

The architecture of Zeylanidium olivaceum (Podostemaceae) inferred from the structure of the primary shoots

Zeylanidium olivaceum (Podostemaceae-Podostemoideae) is the only crustose-rooted species of the genus that still develops prominent primary shoots from the seedling in addition to the secondary (root-borne) shoots forming the clonal plant body. The primary shoots are articulated into an up to 8.5 cm long and 4 mm thick stalk (hypocotyl) and a copiously foliated paint-brush-like shoot which is sympodially branched in the form of a helicoid cyme. The helicoid branching pattern indicates a transversal prophyll position, typical of the dicotyledons, but replaced in most other Podostemoideae by a median prophyll position. The short stems within the leafy head do not separate, but are fused to a dense aggregate (coenosome). Branches are mainly vegetative with a rosette of about 20 elongate subulate leaves. The primary shoots branch in the vegetative stage and thus differ from other Podostemoideae where ramification is confined to the floriferous shoots. The leaves adhere together at the base, forming an apical furrow-like hollow surrounding the shoot tip. The tiny shoot apex is one-layered, radially symmetrical, and develops leaf primordia in a decussate pattern. The erect primary shoots thus differ from the distichously foliated plagiotropic secondary shoots by the decussate phyllotaxis, and by the presence of more than 20 leaves on a shoot as compared to the about six leaves on the vegetative and floriferous secondary shoots. The features observed in the primary shoots are interpreted as primitive as compared to those of the secondary shoots. Z. olivaceum is thus characterised by heterobathmy, i.e., the occurrence of plesiomorphic (primary shoots) and apomorphic features (secondary shoots). The primary shoots exhibit primitive features that apparently have been lost in secondary and primary shoots of most other members of subfamily Podostemoideae.     

Microbial populations on Douglas fir needle surfaces

The surface microflora on Douglas fir (Pseudotsuga menziesii (Mirb.) Franco.) foliage from old-growth trees in western Oregon has been examined by epifluorescence and scanning electron microscopy. Colonies of microorganisms on both upper and lower surfaces of 1-, 3-, 5-, and 8-year-old needles from three heights in the canopy of a single tree have been counted in belt transects, and the relative abundance of various categories of microorganisms has been computed. Aggregations of microbial cells are prevalent in the midrib depression along the upper surface and in stomatal cavities and gutters between rows of epidermal cells on the lower surface. Darkly pigmented hyphae and clumps of cells occur, in general, more frequently on the upper needle surface, a habitat more subject to desiccation and UV exposure.Protococcus colonies become abundant on both upper and lower surfaces of older needles. Microbial cover was found to be significantly higher on the bottom of the needle than on the top. The factors involved in this effect are considered and discussed.     

Biology of the bamboo Chusquea culeou (Poaceae: Bambusoideae) in southern Argentina

Over a period of 7 years the biology and phenotypic variability of Chusquea culeou were studied at 5 locations in cool temperate forests of southern Argentina. Excavated rhizomes had an average of 1.1 successful rhizome buds, and an average of 2.1 years elapsed between successive generations of rhizomes. Rhizome buds usually develop within the first four years after a rhizome forms. Height, volume and weight of a culm can be calculated from its diameter 1 m above the ground. Culm size, length of foliage leaf blades, and pattern of secondary branching differed among study sites. Dead culms were numerous and commonly remained erect for more than 7 years after dying. New culm shoots appear in spring and reach full size within a few months. Shoots can grow more than 9 cm/day. Less than half of the shoots survived a year; most were killed by moth larvae. Multiple primary branch buds emerge through the culm leaf sheaths in the second spring. The mean number of branch buds at mid-culm nodes varied between 34.8 and 81.5, and the mean number of primary branches was between 22.8 and 40.8. Number and length of branches, and number and length of foliage leaf blades at each node is related to the position of the node on a culm. Most branches grow about 3 cm and produce 1 to 3 foliage leaves annually. Foliage leaf blades generally live 2 years or more; few survive 6 years. Relative lengths of foliage leaf blades and their spacing along a branch permit recognition of annual cohorts.Both gregarious and sporadic flowering have been reported, and every year a few isolated plants flower and die. Length of the life cycle is unknown. Seedlings require up to 15 years to produce culms of mature size. Foliage branches may live more than 23 years, and culms may survive 33 years. Extensive loss of new shoots to predation suggests that gregarious flowering may be driven by a need to escape parasitism. C. culeou clumps expand slowly. Average annual rate of increase of the number of live culms in a clump was 4.6%. Methods of seed dispersal are undocumented. A dense stand of Chusquea culeou had an estimated phytomass of 179 tons/hectare (dry weight), 28% of which was underground. Net annual production was about 16 t/ha dry weight.     

Foliage Density of Branches and Distribution of Plant‐Dwelling Spiders1

We compared the abundance of foliage‐living spiders among seven widespread plant species comprising a gradient of levels of structural complexity in a tropical savannah‐like region in southeastern Brazil. Spider abundance among plant species was positively related to the foliage density of their branches. A field experiment using artificial branches was carried out to isolate foliage density effects on spider abundance, thus controlling both biomass and texture effects. Artificial branches were attached to branches of three plant species with similar foliage density, Baccharis dracunculifolia, Diplusodon virgatus, and Microlicia fasciculata. Two treatments were set up: artificial branches with higher foliage density attracted more spiders than those with lower foliage density. The guild structure of hunting spiders was compared among vegetative branches of three plant species with different levels of foliage density: B. dracunculifolia, D. virgatus, and Bidens gardneri. Stalker, and ambusher spiders were more abundant on branches of B. dracunculifolia, which had the highest foliage density. Foliage‐runners constituted the dominant guild on D. virgatus and B. gardneri, which have lower foliage density branches. Our results suggest that branch architecture is the most important factor determining the abundance of plant‐dwelling spiders in the study area independently of branch biomass, leaf surface area or texture.     

Xeromorphy increases in shoots of Pseudotsuga menziesii (Mirb.) Franco seedlings with exposure to elevated temperature but not elevated CO2

Seedling structure influences tree structure and function, ultimately determining the potential productivity of trees and their competitiveness for resources. We investigated changes in shoot structure for seedlings of Pseudotsuga menziesii (Douglas-fir) grown under climate change scenarios of ambient or elevated CO₂ (+180 mol mol^–1) plus ambient or elevated temperature (+3.5°C), for 4 years in outdoor, sunlit chambers. Mass allocation and allometry were measured for buds, leaves, branches, and stems, and anatomy was evaluated for leaves and stems. Seedlings became more xeromorphic with elevated temperature: allocation of total mass to branches over stems and leaves increased, sapwood area to height ratio increased, number of growing points relative to seedling size increased, and stem and branch length and mass decreased for sections initiated during the three full CO₂ and temperature seasons. Neither stem nor leaf anatomy was affected by elevated temperature. Elevated CO₂ increased specific mass of leaves, but had few other effects on mass allocation, allometry, or anatomy for any shoot organ. There were no CO₂ × temperature interactions for any important parameter. Thus, under realistic simulated field environmental conditions representative for in at least some P. menziesii forests (i.e., OR, USA, forests with limited soil nitrogen and summer soil moisture), elevated temperature, but not elevated CO₂, may affect seedling shoot structure and, hence, function.     

Persistent Effects of Short-term, High Exposure to Chlorine Gas on Physiology and Growth of Pinus ponderosa andPseudotsuga menziesii

Following a single acute exposure to chlorine gas, persistenteffects on epicuticular waxes, cuticular transpiration, treegrowth and mortality were studied in foliage of Pinus ponderosaand Pseudotsuga menziesii for three growing seasons. Chlorinegas exposure caused foliar injury to both exposed foliage andfoliage that flushed after exposure (P < 0.05). The tendencyto form films of water rather than droplets was greater in directlyexposed foliage (P < 0.001). Rates of cuticular transpirationwere higher for directly and indirectly exposed foliage of Pinusponderosa up to 1 year after exposure and up to 6 months afterexposure for directly exposed Pseudotsuga menziesii(P < 0.001),after which P. menziesii needles defoliated. Total water content(TWC) and relative water content were significantly correlatedwith foliar injury (P < 0.05). TWC was lower for directlyexposed foliage up to 1 year after exposure (P < 0.001).There was no persistent negative effect on F_v/F_m ratios after1 year. Exposure to chlorine gas did not affect needle lengthor annual shoot increment growth, but exposure was correlatedwith increased bud production. Needle longevity of foliage thatflushed 2 months after exposure was reduced significantly (P< 0.001). Annual stem increment growth for both species decreasedover at least three growing seasons following chlorine gas exposure(P < 0.001), and depended on distance from the spill site.Cone production was lower for exposed Pinus ponderosa treescompared to controls (P < 0.05), and tree mortality was higherwithin approx. 50 m of the release site forPseudotsuga menziesii. Growth responses for both conifers agreed well with predictedpatterns of carbon allocation after defoliation caused by chlorinegas exposure. Copyright 2001 Annals of Botany Company Pinus ponderosa, Pseudotsuga menziesii, conifers, chlorine gas, leaf wettability, cuticular transpiration, water relations, growth, mortality     

A comparison of four different fine root production estimates with ecosystem carbon balance data in a Fagus–Quercus mixed forest

The controversy on how to measure fine root production of forests (P) most accurately continues. We applied four different approaches to determine annual rates of P in an old-growth temperate Fagus sylvatica–Quercus petraea stand: sequential soil coring with minimum–maximum calculation, sequential coring with compartmental flow calculation, the ingrowth core method, and a recently developed root chamber method for measuring the growth of individual fine roots in situ. The results of the four destructive approaches differed by an order of magnitude and, thus, are likely to introduce large errors in estimating P. The highest annual rates of P were obtained from the sequential coring approach with compartmental flow calculation, intermediate rates by sequential coring with minimum–maximum calculation, and low ones by both the root growth chamber and ingrowth core approaches. A carbon budget for the stand was set up based on a model of annual net carbon gain by the canopy and measurements on carbon sink strength (annual leaf, branch and stem growth). The budget implied that a maximum of 27% of the net carbon gain was available for allocation to fine root growth. When compared to the carbon budget data, the sequential coring/compartmental flow approach overestimated annual fine root production substantially; whereas the ingrowth core and root growth chamber approaches grossly underestimated P rates. With an overestimation of about 25% the sequential coring/minimum–maximum approach demonstrated the best agreement with the carbon budget data. It is concluded that the most reliable estimate of P in this temperate forest will be obtained by applying the sequential coring/minimum–maximum approach, conducted with a large number of replicate samples taken on a few dates per season, in conjunction with direct root growth observation by minirhizotrons.     

Recovery of a Betula pubescens forest in northern Sweden after severe defoliation by Epirrita autumnata

Abstract. Mountain birch (Betula pubescens ssp. czerepanovii) forest in the Abisko valley of northern Sweden was completely defoliated by Epirrita autumnata caterpillars during an outbreak in 1954–1955. The defoliation resulted in an 80–90% mortality of the leaf‐carrying shoots of birches in 1956 and triggered a rejuvenation of stands. The subsequent regrowth of foliage was studied in two damaged birch stands and in one unattacked stand. The number of leaves approximately doubled in the damaged stands between 1961 and 1987, while the number on the reference plot fluctuated without significant increase. Regrowth started with increased production of long shoots from surviving shoots and basal sprouts. Basal sprouts were a substantial source of new shoots in the recovery of the foliage, especially on the most damaged plot. Trees of seed origin constituted a minor fraction of the regrowth. Initial rapid growth of foliage reduced gradually and the annual leaf production in 1986/1987 was 75% of that of the reference plot. Comparison between the recovery curve and data from the reference plot indicates that the shoot population of the damaged forest will, after more than 30 years, need many more years to reach the assumed size of a mature forest. The degree of rejuvenation varied between stands, with different consequences for future dynamics of E. autumnata populations.     

The possible roles of nutrient deprivation and auxin repression in apical control

Apical control is the suppression of growth in lower branches by a higher dominant branch or leader shoot. We investigated possible mechanisms involved in this developmental response in three widely diverse species (Japanese morning glory, Ipomoea nil, hybrid poplar, Populus trichocarpa, × P. deltoides, and Douglas-fir, Pseudotsuga menziesii). The following two hypotheses were tested: (1) the mineral nutrient-deprivation hypothesis, which is that the continued growth of the lower branches is repressed by the diversion of nutrients to the upper dominating branch or shoot, and (2) the auxin-repression hypothesis, which is that auxin produced in the upper dominating branch or shoot moves down to the lower branches where continued growth is repressed. The results of experiments involving the manipulation of available nutrients by dominant branch removal and fertilization were consistent with the first hypothesis for morning glory, poplar, and for second- or third flushing of lateral branches in Douglas-fir. The results of the experiments involving auxin (NAA, 1-naphthalene acetic acid) replacement treatments on decapitated shoots bearing growing lateral branches were inconsistent with the second hypothesis in morning glory, poplar and in first-flushing Douglas-fir. However, despite concerns about possible NAA toxic effects, there was evidence of auxin repression of second flushing in Douglas-fir. Overall, the data supported a significant role for nutrient availability but not for auxin repression in apical control of morning glory and poplar. In Douglas-fir, apical control in first-flushing lateral branches from over-wintered buds was largely insensitive to both nutrient availability and auxin repression; however, second flushing was sensitive to both.