Amount Of Foliage Research Articles

LIGNUM: a model combining the structure and the functioning of trees

The model LIGNUM treats a tree as a collection of a large number of simple units that correspond to the organs of a tree. The model describes the three-dimensional structure of the tree crown and derives growth in terms of the metabolism taking place in these units. The time step is one year. The structural units are: tree segments, branching points and buds. Tree segments are separated by branching points. The buds produce new tree segments, branching points and buds. The tree segments contain wood, bark and foliage. A model tree consisting of simple elements translates conveniently to a list structure: the computer program implementing LIGNUM treats trees as a collection of lists. The annual growth of the tree is driven by the available photosynthetic products after accounting for respiration losses. The photosynthetic rate of foliage depends on the amount of intercepted light. The amount of photosynthates allocated to the growth of new tree segments is controlled by the light conditions and the amount of foliage of the mother tree segment. The biomass relationships of the tree parts follow, e.g. from a pipe model hypothesis. The orientation of new tree segments results from application of simple branching rules. LIGNUM has been parametrized for young Scots pines (Pinus sylvestris L.).

Growth of Young Trees of Acer platanoides and Quercus robur Along a Gap- Understory Continuum: Interrelationships between Allometry, Biomass Partitioning, Nitrogen, and Shade Tolerance

Growth and biomass partitioning across a natural light gradient were examined in saplings (0.3-2.6 m in height) of relatively shade-tolerant Acer platanoides L. and relatively intolerant Quercus robur L. for dependence on relative irradiance, calculated from hemispherical photographs (RI), and on total tree height (H). H and RI were independent. All allometric relationships between H and the parameters of plant biomass and dimensions were positively affected by RI, signifying that at a common H, saplings increased horizontal growth relative to vertical growth in environments with greater irradiance. Root mass ratio (coarse plus fine roots per total plant biomass) increased with increasing irradiance in both species and was not different between the species. However, Q. robur had a larger stem mass ratio, while A. platanoides had a greater leaf mass ratio (LMR). At a common irradiance, leaf mass per area (LMA) was lower in A. platanoides. As a result of these interspecific differences in fractional allocation of sapling biomass in foliage and biomass requirement for construction of foliar surface area, leaf area ratio (LAR = LMR/LMA) was larger in A. platanoides. Relative growth rate of standing biomass (RGR) was significantly correlated with RI in both species and was not different between the species. In contrast, net assimilation rate per unit leaf area (NAR) was larger in Q. robur. Thus, in these species a similar RGR (NAR x LAR) was achieved in a completely different way: A platanoides increased the intercepting foliar area, while Q. robur increased the productivity per unit area. Growth analysis repeated on a nitrogen basis demonstrated that nitrogen use efficiency (biomass increment per unit nitrogen) was similar between the species, and that the species differences in NAR resulted from different foliar N concentrations; fractional nitrogen investment in foliage was independent of species, but lower nitrogen cost of leaves allowed A. platanoides to construct a more extensive foliar display with the same nitrogen investment in foliage. Biomass allocation between different compartments was also dependent on sapling size. With increasing H, A. platanoides maintained a constant proportion of biomass in foliage, but the relative amount of foliage decreased in Q. robur. Therefore, due to a decreasing investment of resources in foliage construction with advancing sapling ontogeny, saplings of similar height require more light to survive in Q. robur than in A. platanoides. Greater investment of biomass in leaves vs. standing biomass may result in lower volume gain and reduced competitive ability in more open habitats in A. platanoides. These results indicate that interspecific differences in allometry and nitrogen allocation patterns significantly alter species competitive relations during sapling development across the gap-understory continuum.

Crown transparency of beech ( Fagus sylvatica L.) in the Northern Appennines (Italy) — Status, changes and relationships with site characteristics and other indices of tree condition

Explained and unexplained crown transparency (ECT and UCT, respectively) of 2090 beech trees were examined in 1994 and compared with site condition and other indices of tree status. No relationship was found between UCT, social class, age and diameter. Some relationships were found between UCT, canopy closure, the amount of discolored foliage, the growth pattern (a modified Roloff score), fruiting and regeneration. UCT increases with altitude and decrease with site quality. Short-term changes in UCT do not exceed 5 % in approximately 70 % of the observed beeches, although changes of 20 % or more in UCT scores has been recorded on 6 % of the beeches.

Fractal analysis of foliage distribution in loblolly pine crowns

A new method for estimating fractal characteristics (fractal dimension and foliage density) of a single crown or its portions is developed. The proposed method operates with volume and mass of natural units of the crown, such as shoots and branches, rather than with numbers of regular cubes. Fractal dimension alone is not sufficient to describe foliage distribution in the crown because it says nothing about the density of foliage at a given point. The density is defined as the ratio of foliage mass to fractal volume it occupies. Fortunately, the intercept of the regression, which contains fractal dimension as the slope, provides a measure of foliage density. Thus the method makes it possible to separate purely spatial factors represented by fractal dimension from ecophysiological effects characterized by foliage density. Application of the method showed that neither fractal dimension nor foliage density of the studied loblolly pines (Pinus taeda L.) correlates with current diameter increment. At the same time, there is a pronounced negative correlation between fractal dimension and crown size. These results suggest that as crowns become larger, the amount of foliage located at the crown periphery increases in proportion to the foliage amount inside the crown. As a spin-off of this analysis, a method for estimating relative foliage density (defined as the ratio of actual to maximal foliage mass for a given branch) is developed.

Stem xylem features in three Quercus (Fagaceae) species along a climatic gradient in NE Spain

Stem xylem features in two evergreen Quercus species (Q. coccifera and Q. ilex) and a deciduous one (Q. faginea) were analysed along an Atlantic-Mediterranean climatic gradient in which rainfall and winter cold experience strong variation. Mean maximum vessel diameter, vessel density, vessel element length, xylem transverse sectional area, Huber value (xylem transverse sectional area per leaf area unit), theoretical leaf specific conductivity (estimated hydraulic conductance per leaf area unit) and total leaf area were determined in 3-year-old branches. Q. faginea presented the widest vessels and the highest theoretical leaf specific conductivity while Q. coccifera showed the lowest total leaf area and the highest Huber value. Studied features did not exhibit significant correlations with mean minimum January temperature in any species but did show significant relationships with rainfall. In Q. coccifera, mean maximum vessel diameter, vessel element length and theoretical leaf specific conductivity increased with higher rainfall while vessel density decreased. Mean maximum vessel diameter and total leaf area in Q. ilex increased with precipitation whereas variables of Q. faginea did not show any significant trend. Results suggest that aridity, rather than minimum winter temperature, controls stem xylem responses in the studied evergreen species. Q. faginea traits did not show any response to precipitation, probably because this species develops deep roots, which in turn makes edaphic and topographic factors more important in the control of soil water availability. In response to aridity Q. coccifera only exhibits adjustment at a xylem level by reducing its water transport capacity through a reduction of vessel diameter without changing the amount of xylem tissue or foliage, whereas Q. ilex adjusts its water transport capacity in parallel to the foliage area.

Predicted leaf area growth and foliage efficiency of loblolly pine plantations

Foliage dynamics research is helpful for better understanding the process of forest production and improving silvicultural practice. However, the difficulty of measuring foliage amount has slowed down the research progress. Since leaf area of an individual tree can be reliably predicted from its diameter, growth and yield models that provide detailed information for each diameter class can be used to benefit foliage dynamics research. Simulation results from a growth and yield system for unthinned loblolly pine plantations indicated that foliage area increased with stand age, peaked between ages 36 and 51, and decreased after that. Volume growth increased with leaf area for young stands and decreased for older stands, whereas foliage efficiency consistently decreased with age. Better sites supported higher levels of leaf area index, volume growth, and foliage efficiency. Higher planting densities led to higher maximum leaf area indices and shorter time to reach that level. Initial density had no effect on foliage efficiency through time.

Local Dispersion of the Eucalyptus Leaf-Beetle Chrysophtharta bimaculata (Coleoptera: Chrysomelidae), and Implications for Forest Protection

1. Chrysophtharta bimaculata is a native chrysomelid species that can cause chronic defoliation of plantation and regrowth Eucalyptus forests in Tasmania, Australia. Knowledge of the dispersion pattern of C. bimaculata was needed in order to assess the efficiency of an integrated pest management (IPM) programme currently used for its control. 2. Using data from yellow flight traps, local populations of C. bimaculata adults were monitored over a season at spatial scales relevant to commercial forestry: within a 50-ha operational management unit (a forestry 'coupe') and between coupes. In addition, oviposition was monitored over a season at a subset of the between-coupe sites. 3. Dispersion indices (Taylor's Power Law and Iwao's Mean Crowding regression method) demonstrated that C. bimaculata adults were spatially aggregated within and between coupes, although the number of egg-batches laid at the between-coupe scale was uniform. Spatial autocorrelation analysis showed that trap-catches at the within-coupe level were similar (positively autocorrelated) to a radius distance of approximately 110 m, and then dissimilar (negatively autocorrelated) at approximately 250 m. At the between-coupe scale, no repeatable spatial autocorrelation patterns were observed. 4. For any individual site, rapid changes in beetle density were observed to the associated with loosely aggregated flights of beetles into and out of that site. Peak adult catches (>the weekly mean plus standard deviation trap-catch) for a site occurred for a period of 2.0 ± 0.22 weeks at a time (n = 37), with normally only one or two peaks per site per season. Peak oviposition events for a site occurred on average 1.4 ± 0.11 times per season and lasted 1.5 ± 0.12 weeks. 5. Analysis of an extensive data set (n = 417) demonstrated that adult abundance at a site was positively correlated with egg density, but negatively correlated with tree damage (caused by conspecifics) and the presence of conspecific larvae. There was no relationship between adult abundance and a visual estimate of the amount of young foliage on trees. 6. Adults of C. bimaculata are shown to occur in relatively small, mobile aggregations. This means that pest surveys must be both regular (less than 2 weeks apart) and intensive (with sampling points no more than 150 m apart) if beetle populations are to be monitored with confidence. Further refinement of the current IPM strategy must recognize the problems posed by this temporal and spatial patchiness, particularly with regard to the use of biological insecticides, such as Bacillus thuringiensis, for which only a very short operational window exists.

Seasonal variation in activity and diet in a small-bodied folivorous primate, Hapalemur griseus, in southeastern Madagascar.

How small-bodied (500-1,200 g) folivorous prosimian primates cope with large amounts of foliage in their diet seasonally has yet to be determined for many species such as Hapalemur griseus, which specializes on bamboo. To address this issue, we present results on seasonal variation in activity and diet from a wild group of H. griseus in southeastern Madagascar. Throughout the study (which was conducted from July-November 1994 and July 1995-February 1996), H. griseus primarily fed on new growth from three species of bamboo: two species of liana bamboo and Cephalostachyum perrieri. Bamboo species were used in different ways seasonally; liana bamboo was consumed more during the dry, cool season, and C. perrieri was eaten more often during the wet, warm season. H. griseus also spent more of their time feeding and traveling than nocturnal folivores of similar body size during the dry season. During the warm wet season, H. griseus decreased the amount of time spent feeding and traveling and rested more often. We hypothesize that seasonal changes in activity may be primarily related to the distribution and availability of food sources and/or reproductive cycles.

Seasonal variation in activity and diet in a small‐bodied folivorous primate, Hapalemur griseus, in southeastern Madagascar

How small-bodied (500–1,200 g) folivorous prosimian primates cope with large amounts of foliage in their diet seasonally has yet to be determined for many species such as Hapalemur griseus, which specializes on bamboo. To address this issue, we present results on seasonal variation in activity and diet from a wild group of H. griseus in southeastern Madagascar. Throughout the study (which was conducted from July–November 1994 and July 1995–February 1996), H. griseus primarily fed on new growth from three species of bamboo: two species of liana bamboo and Cephalostachyum perrieri. Bamboo species were used in different ways seasonally; liana bamboo was consumed more during the dry, cool season, and C. perrieri was eaten more often during the wet, warm season. H. griseus also spent more of their time feeding and traveling than nocturnal folivores of similar body size during the dry season. During the warm wet season, H. griseus decreased the amount of time spent feeding and traveling and rested more often. We hypothesize that seasonal changes in activity may be primarily related to the distribution and availability of food sources and/or reproductive cycles. Am. J. Primatol. 43:211–223, 1997. © 1997 Wiley-Liss, Inc.

Patterns in vertical distribution of foliage in young coastal Douglas-fir

Total amount and vertical distribution of foliage represent important aspects of forest stand structure and its influence on dry matter productivity, forest microclimate, watershed properties, and habitat structure. Variation in foliage distribution was analyzed on trees and plots in a series of even-aged Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) stands scheduled for management under a wide range of silvicultural regimes. Branch-level foliage mass and foliage area equations were developed from a sample of 138 branches. These equations were applied to 27 trees on which the diameter and height of all live primary branches were measured, allowing estimation of both the total amount of foliage and its vertical distribution. A β-distribution was fitted to data describing the vertical distribution of foliage on each tree, and the resulting parameter estimates were modelled as functions of tree height, diameter at breast height, crown length, and relative height in the stand. Foliage area distribution tended to be shifted downward relative to foliage mass because of the expected increase in specific leaf area with depth into the crown. Similarly, the relative foliage distribution in terms of both mass and area was shifted downward as the tree became more dominant, or as relative height in the stand increased. In contrast, foliage on trees of similar relative height was shifted upward in response to the lower stand densities imposed by precommercial thinning. On the stand level, relative vertical distribution of foliage in the canopy was more peaked than would be implied by assuming a constant leaf area/sapwood area ratio throughout the composite tree crowns. Between-stand variation in vertical foliage distribution was dictated by differences in stand top height, height to crown base, and number of trees per hectare.

The ecophysiological and growth responses of Aleppo pine (Pinushalepensis) to controlled heating applied to the base of the trunk

Controlled, localized heating was applied to the trunks of young Aleppo pines (Pinushalepensis Mill.) during the winter. Three levels of burning were applied to the circumference of the trunks (zero, partial, or total), and the effects on tree physiology and growth were monitored during the 7 months following heating. Effects of the heat treatment on tree physiology were assessed by measuring predawn water potential, sap flux density, microvariations in trunk diameter, electrical resistance of the cambium, and gas exchanges from the needles (photosynthesis and stomatal conductance). Morphological response measurements included height and radial growth of the trees and morphological characteristics of the needles. The amount of chlorotic foliage was also measured. Aleppo pine was highly resistant to trunk injury, surviving when 80% of the trunk circumference was destroyed by fire. The behaviour of partially burnt pines was not significantly different from that of controls, for all variables studied. Trees whose trunks were totally burnt died from 1 to 5 months after treatment. Sap flux density of totally burnt trees was already significantly lower than that of other trees in the first week following the treatment. A decrease in net photosynthesis and stomatal conductance in the still surviving totally burnt trees was also recorded during the first measurements made in July. Treatment differences in predawn water potential, microvariations in trunk diameter, and electrical resistance of the cambium only appeared a few days before death of the totally burnt trees.

One and two years impact of commercial thinning on spruce budworm feeding ecology and host tree foliage production and chemistry

Field rearing experiments of spruce budworm, Choristoneura fumiferana (Clem.), were conducted in conjunction with foliar chemical analyses, one and two years after a commercial thinning (removal of 25% stand basal area) in a 50-year-old balsam fir, Abies balsamea (L.) Mill., stand. The first year after thinning, spruce budworm larvae reared on the residual trees developed five days faster and removed 43% more foliage than those reared on control trees, but in the second year they developed two days faster and removed 37% more foliage. The increase in larval development rate was related to an increase in foliar soluble sugars while a reduction in foliar monoterpenes caused by the thinning apparently accounted for the greater amount of foliage ingested by the larvae. The first year after thinning, trees were more vulnerable to spruce budworm because there was no increase in foliage production and the trees were more heavily defoliated. However, in the second year trees were less vulnerable to the insect because there was an increase in foliage production that exceeded the increase in defoliation, hence a net gain in foliage. Results from this study showed that commercial thinning could reduce the vulnerability of balsam fir trees to spruce budworm if thinning is conducted two years prior to budworm outbreak, but the same silvicultural procedure could increase the vulnerability to the insect if it is conducted during an outbreak. Key words: spruce budworm, balsam fir, chemistry, thinning, defoliation

The Correlation between Plant Growth and Intercepted Radiation: An Interpretation in Terms of Optimal Plant Nitrogen Content

Photosynthesis of leaves is commonly observed to have a saturating response to increases in their nitrogen (N) content, while the response of plant maintenance respiration is more nearly linear over the normal range of tissue N contents. Hence, for a given amount of foliage, net primary productivity ( NPP) may have a maximum value with respect to variations in plant N content. Using a simple analytically-solvable model of NPP, this idea is formulated and its broad implications for plant growth are explored at the scale of a closed stand of vegetation. The maximum- NPPhypothesis implies that NPPis proportional to intercepted radiation, as commonly observed. The light utilization coefficient (ϵ), defined as the slope of this relationship, is predicted to be ϵ=α Y g(1−λ) 2, where α is the quantum yield, Y gis the biosynthetic efficiency, and λ is a dimensionless combination of physiological and environmental parameters of the model. The maximum- NPPhypothesis is also consistent with observations that whole-plant respiration ( R) is an approximately constant proportion of gross canopy photosynthesis ( A c), and predicts their ratio to be R: A c=1− Y g(1−λ). Using realistic parameter values, predicted values for ϵ and R: A care typical of C 3plants. ϵ is predicted to be independent of plant N supply, consistent with observations that long-term growth responses to N fertilization are dominated by increased light interception associated with increased growth allocation to leaf area. Observed acclimated responses of plants to atmospheric [CO 2], light and temperature are interpreted in terms of the model.

Foliar Persistence and Residual Activity of Tebufenozide Against Spruce Budworm Larvae

A field study was conducted to investigate the persistence of tebufenozide in white spruce foliage. An aqueous suspension concentrate formulation, RH-5992 2F, was sprayed over single trees at three dosage rates, 35, 70 and 140 g of the active ingredient (AI), in 2·0 litre ha−1, using ground application equipment. Foliage was collected at different intervals of time up to 64 days after treatment and tebufenozide residues were measured by high-performance liquid chromatography. Foliage was also fed to laboratory-reared 4th- and 6th-instar spruce budworm (Choristoneura fumiferana Clemens). The data indicated that tebufenozide residues in foliage declined with time according to first-order kinetics. The average rate-constant and half-life of disappearance (DT50) were 0·0340 and 20·45 days, respectively. Larval mortality declined gradually, corresponding to the residues, but was still appreciable (49 to 70%) when the larvae were fed with foliage collected 64 days after treatment. The amount of foliage consumed by the larvae decreased when foliar residues of tebufenozide increased, thus indicating anti-feedant activity of the chemical. The LD50 values for both instars were similar and averagedc.25 ng per insect, but the LD90 values were significantly lower for 4th-instar than for 6th-instar, at 63·6 and 96·1 ng per insect respectively. This implies that, theoretically, at a foliar concentration of 1·0 μg tebufenozide g−1 foliage (fresh wt), the spruce budworm larva needs to consume 65 to 100 mg of foliage in 10 days to cause mortality in about 90% of a population of the insect.

Relationships between crown dimensions and stem development in Fraxinusexcelsior

In this study, the relationships between crown development and stem volume increment of ash (Fraxinusexcelsior L.) were considered in the framework of a project where modelling and computer simulation should be used to gain insight into the dynamics of even-aged ash stands. Twenty trees, of various development stages and competition status, were felled and subjected to stem and branch analysis. The relationships established in a previous study between stem elongation, branch growth, and branching angle (based on these measurements), together with the characteristic pattern of variation of stem cross-sectional area increment along the stem (Pressler's Law), allowed the reconstruction of the past development of tree crowns. The quantity of foliage in tree crowns was estimated by a "foliar volume" calculated as the product of crown surface projection area and height growth, for each growing year considered. This foliar volume was used together with a relative foliar volume based on the stem surface area to obtain a prediction equation for annual stem volume increment.

Growth Phenology and Crown Structure of Selected Loblolly Pine Families Planted at Two Spacings

Abstract Growth phenology and crown structure of five open-pollinated loblolly pine (Pinus taeda L.) families, planted at two spacings (0.92 and 1.83 m) and grown in pure plots near Summerville, SC, were investigated using a split-plot (main plots = spacing, subplots = genotype) field design. Shoot, branch, and foliage characteristics were quantified during the fourth growing season and related to standing volume. Genotype variation in shoot and branch characteristics was generally small and nonsignificant except for differences in annual height increment (from 135 to 152 cm) and current-year branch production (from 11.1 to 14.0). Significant genotype variation in foliage amount, distribution, and longevity was associated with contrasting growth performances and stage of stand development. Foliage biomass differed by as much as 60% among genotypes (from 4.8 to 7.7 Mg ha-1), with 38% of the variation related to family differences in total height. The large range in foliage biomass was the major contributor to leaf area index differences (LAI; from 7.9 to 13.6 m² m-2), although significant genotype differences in weighted specific needle area (SNA; from 175 to 199 cm² g-1) were also a factor. Genotype variation in foliage biomass and LAI was related to the vertical distribution of foliage, and both were significantly correlated with standing volume at age 3 yr (r = 0.64 and 0.59). The best performing families (3, 7, and 49 based on progeny testing) held > 60% of their total foliage biomass in the middle crown third (nonuniform distribution), whereas the poorest families (37 and 138), had < 50% in the same region (more uniform distribution). Compared to the other genotypes, family 37 exhibited reduced foliage longevity, which may have contributed to its low LAI and poor growth performance. Results of this study suggest that foliage display, amount, and duration are the crown characteristics important in conferring differential family performance for loblolly pine. For. Sci. 42(1):46-57.

Effects ofPuccinia carduorumon Musk Thistle Herbivores

Puccinia carduorum Jacky, a rust fungus recently introduced into the United States for biological control of musk thistle ( Carduus thoermeri Weinmann), may interact with three established thistle herbivores, Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), Rhinocyllus conicus Froelich (Coleoptera: Curculionidae), and Cassida rubiginosa Müller (Coleoptera: Chrysomelidae). Tests were conducted to determine the effects of P. carduorum on feeding, oviposition, longevity, egg production, egg hatch, and larval development of the three insects. When offered both rust-infected and healthy musk thistle leaves, adults of C. rubiginosa and T. horridus consumed significantly more healthy than infected foliage. On infected leaves, feeding and oviposition were confined largely to pustule-free areas. The amount of rust-infected and healthy foliage consumed by C. rubiginosa larvae was not significantly different, but feeding on the infected leaves was again confined to rust-free leaf areas. Rust infection did not reduce oviposition by the three insects, and R. conicus oviposited only on healthy portions of the bracts. There were no significant differences in longevity, egg production, percentage of egg hatch, and larval development for any of the three insects fed continuously with both healthy and rust-infected leaves. Thus, effects of P. carduorum on the herbivores were slight, and among the interactions that were found, none is expected to be detrimental to biological control.

LIGNUM: A Tree Model Based on Simple Structural Units

The model LIGNUM treats a tree as a collection of a large number of simple units which correspond to the organs of the tree. The model describes the three dimensional structure of the tree crown and defines the growth in terms of the metabolism taking place in these units. The activities of physiological processes can be explicitly related to the tree structures in which they are taking place. The time step is 1 year. The crown of the model tree consists of tree segments, branching points and buds. Each pair of tree segments is separated by a branching point. The buds produce new tree segments, branching points and buds. The tree segments contain wood, bark and foliage. A model tree consisting of simple elements translates conveniently to a list structure: the computer program implementing LIGNUM treats the tree as a collection of lists. The annual growth of the tree is driven by available photosynthetic products after respiration losses are accounted for. The photosynthetic rate of foliage depends on the amount of light. The amount of photosynthates allocated to the growth of new tree segments is controlled by the light conditions and the amount of foliage on the mother tree segment. In principle, the biomass relationships of the tree parts follow the pipe model hypothesis. The orientation of new tree segments results from the application of constant branching angles. LIGNUM has been parametrized for young Scots pine ( Pinus sylvestrisL.) trees. However, the model is generic; with a change of parameter values and minor modifications it can be applied to other species as well.

Larval feeding behaviour affects the impact of staminate flower production on the suitability of balsam fir trees for spruce budworm.

Laboratory rearing of spruce budworm, Choristoneura fumiferana, in conjunction with field rearing indicated that the feeding behaviour of the larvae, which is affected by the insect population density, significantly influenced the impact of balsam fir, Abies balsamea, staminate flowering on spruce budworm biology. At low budworm density, the production of pollen in the midcrown of host trees reduced the insect development time by 5 days without affecting pupal weight, fecundity and survival. However, at high budworm density, the small amount of current-year foliage produced by flowering branches forced old larvae (sixth instar) either to feed on 1-year-old foliage (backfeeding) or to move from the midcrown to the lower crown section where staminate flowers are absent and more current-year foliage is available. When old larvae fed on old foliage, they exhibited reduced pupal weight and fecundity without losing the advange in development time that they obtained from feeding on pollen during their early stages of development. On the other hand, when old larvae moved to the lower crown section, they avoided the negative effects of backfeeding but lost the advantage in development time that was gained from feeding on pollen. Results from this study indicated that the production of staminate flowers by balsam fir trees could have opposite effects on spruce budworm population dynamics depending upon the insect population density when flowering occurs.

Modelling the epizootiology of gypsy moth nuclear polyhedrosis virus

Abstract Qualitative understanding of the dynamics of epizootics of the nuclear polyhedrosis virus of gypsy moth has become complete enough to justify attempts to quantitatively predict the timing and intensity of epizootics within a season. In earlier work (Dwyer and Elkinton, 1993), we compared the predictions of a simple differential equation model derived from Anderson and May (1981) to time series of virus mortality in each of eight gypsy moth populations (Woods and Elkinton, 1987). The model's predictions were very accurate for high density populations, but seriously under-estimated virus mortality in low density populations. Here we compare the predictions of the simple model to those of the gypsy moth life system model (GMLSM, Sheehan, 1985; Colbert and Racin, 1991), a highly complex computer simulation of gypsy moth population dynamics and forest stand growth that incorporates much of the existing knowledge of the many factors influencing gypsy moth populations. In particular we looked at two different versions of the GMLSM that incorporate two different models of virus transmission. One was identical to that of the simple model (Anderson and May, 1981; Dwyer and Elkinton, 1993), in which the rate of transmission was a constant (the transmission coefficient) times the product of the densities of healthy larvae and the densities infectious virus particles on foliage. The other approach, developed by Valentine and Podgwaite (1982) was a detailed model of production and consumption of infective particles on foliage. The model took account of age-related changes in the amount of foliage consumed and the variation in susceptibility of larvae to virus (LD50). The Anderson and May version of the GMLSM performed about as well as the simple differential equation model, but only for values of the transmission coefficient about 250 times higher that those we had determined experimentally (Dwyer and Elkinton, 1993). The Valentine and Podgwaite (1982) version of the GMLSM gave a much better fit, but only for values of LD50 that were 100 times higher than those determined experimentally. Future research will focus on efforts to refine our understanding of virus transmission in order to explain and reduce the discrepancies between model predictions and observed mortality from virus in fieldpopulations.