Treesearch

Interest in investigating the influence of extreme ecological factors on the radial growth of oak (Quercus robur L.) is bound up with oak dieback in the South Urals during the last decade. Factors contributing to this problem in the study area are hard winter frosts, late spring frosts, and Lymantria dispar L. outbreaks. To distinguish the influence of these factors on radial growth, I used a methodological approach involving the analysis of radial increment, anatomical structure of xylem, histograms of ring indexes, and comparative analysis of radial growth in different plots (Kucherov...

We have investigated the population dynamics of some common phyllophagous insects in oak stands of the forest-steppe zone and their impact on common oak (Quercus robur L). Considerable attention has also been paid to mathematical modeling of the studied processes. All field data represent samples taken from the Tellerman oak grove in the Voronezh region, where repeated observations have been made from 1969 up to the present. Our particular concern is the green oak leaf roller (Tortrix viridana L), which propagates in 70 to 100-year-old oak stands in the floodplain of the region.

The most important defoliator of the silver fir, Abies alba, is Choristoneura murinana (Lepidoptera: Tortricidae) (Bogenschütz 1978). The fact that Zeiraphera rufimitrana H.-S. (Lepidoptera: Olethreutidae) (Bovey 1978) often occurs simultaneously with C. murinana has, in practice, frequently led to misidentification. The larvae of both species feed exclusively on juvenile needles in the upper, sun-exposed tree crowns and cause an identical type of defoliation. Foraged needles discolor reddish-brown in the early summer and are later shed. Once the brown needles have been shed, one can...

Explanation of population dynamics is one of the main problems in population ecology. There are two main approaches to the explanation: the factor approach and the dynamic approach. According to the first, an explanation is obtained when the effect of various environmental factors on population density is revealed. Such analysis is performed using well developed regression methods (Poole 1978). The resulting regression equations can be used for prediction of population density and their coefficients indicate the role of each factor in population change. This method of explanation considers...

During the past two decades, the decline of red spruce, Picea rubens Sargent, and balsam fir, Abies balsamea (L), at high elevations (900-1200 m) in eastern North America has evoked concern about the effects of anthropogenic deposition upon terrestrial ecosystems. In many high-elevation forests across New England, as many as 50 percent of the standing red spruce are dead (Hertel et al. 1987). Wood cores indicate that growth has been severely curtailed since the 1960s (Hornbeck and Smith 1985). Although acid rain is most commonly invoked as the principal causal agent of this decline, there...

A key problem in addressing patterns of interaction between forest insects and their host trees is determining the level at which important ecological and evolutionary interactions occur. We commonly view plant-herbivore relations as herbivore species interacting with plant species, tacitly assuming that variation among members of either species is small and one can represent their interaction by the average characteristics of the insect and the tree species (Ehrlich and Raven 1964, Feeny 1976, Rhoades and Cates 1976, Scriber and Feeny 1979). This simplistic view is violated if insects...

Damage inflicted by insects may trigger responses in their host plants resulting either in immediate effects on herbivores either rapidly or in effects upon subsequent herbivore generations. Differentiation between rapid and delayed inducible resistance is essential since the two responses affect the population dynamics of herbivores in fundamentally different ways (Haukioja 1982). Rapid inducible resistance (RIR) tends to stabilize herbivore population dynamics. On the other hand, delayed inducible resistance (DIR) introduces a time-lag into the negative feedbacks regulating the population...

It was discovered more than 10 years ago that toxic and repelling substances may increase in plant tissue in response to damage by phyllophages. Such rapidly inducible chemical changes may appear a few hours after leaf injury and then disappear in a few weeks or months (Walker-Simmons and Ryan 1977, Baldwin and Schultz 1983, Van Hoven 1974, Wratten et al. 1984). This phenomenon has been investigated in detail on mountain birch, Betula pubescens tortuosa, in Finland (Haukioja and Niemelä 1977, 1979, Niemelä et al. 1979, Haukioja 1982, Neuvonen and Haukioja 1991). Research there has demonstra...

It is well known that large differences in performance parameters such as growth rate, survival rate, or fecundity rate are found between various insect guilds, e.g. root feeders and sapsuckers (Slansky and Rodriguez 1987, Slansky and Scriber 1985). Within guilds and even within a given host plant, the variability of the plant material may also result in considerable differences in the performance parameters of insect larvae feeding on the plant (Whitham 1983, Jensen 1988).

Variation in levels of herbivory within and among plants can be attributed to many mechanisms, such as differences in a) host nutritional quality, b) suitability of the physical environment, and c) abundance of competitor consumers or natural enemies (Mattson et al. 1982, Denno and McClure 1983, Mattson and Scriber 1987, Clancy et al. 1988a, 1988b, Mattson et al. 1988). To test the hypothesis that variation in host plant nutritional quality is a significant mechanism in plant resistance against herbivores, I have selected western spruce budworm, Choristoneura occidentalis (Lepidoptera:...