Seppo Neuvonen

Induced Long‐Term Resistance of Birch Foliage against Defoliators: Defensive or Incidental?

The long—term increase in foliage resistance of white birch subjected to artificial defoliation, which has been previously documented, may be a defensive response against leaf predators, or it may be passive deterioration in foliage quality due to lost nutrients. We tested these hypotheses by two experiments in which foliage quality was assayed by the growth response of a geometrid caterpillar that is the trees major herbivore in our area. Fertilization of the soil around defoliated trees did not eliminate the change in foliage quality caused by mechanical damage, contrary to the prediction of the nutrient—stress hypothesis. Another result consistent with the defensive hypothesis was that insect damage was a more effective inducer of changes in birch foliage than mechanical damages was. Artificial defoliation was an effective inducer in a nutrient—poor but not in a nutrient—rich site; this result can be explained by either of the two hypotheses.

Nutrient stress: an explanation for plant anti-herbivore responses to defoliation

SummaryA hypothesis is put forward that the long-lasting inducible responses of trees to herbivores, particularly lepidopteran defoliators, may not be active defensive responses, but a by-product of mechanisms which rearrange the plant carbon/nutrient balance in response to nutrient stress caused by defoliation. When defoliation removes the foliage nutrients of trees growing in nutrient-poor soils, it increases nutrient stress wich in turn results in a high production of carbon-based allelochemicals. The excess of carbon that cannot be diverted to growth due to nutrient stress is diverted to the production of plant secondary metabolites. The level of carbon-based secondary substances decays gradually depending on the rate at which nutrient stress is relaxed after defoliation. In nutrient-poor soils and in plant species with slow compensatory nutrient uptake rates the responses induced by defoliation can have relaxation times of several years. The changes in leaf nitrogen and phenolic content of mountain birch support this nutrient stress hypothesis. Defoliation reduces leaf nitrogen content while phenolic content increases. These responses of mountain birch to defoliation are relaxed within 3–4 years.

The Autumnal Moth in Fennoscandia

Mountain birch, Betula pubescens Ehrhart ssp. tortuosa (Ledebour) Nyman, forests form the tree line in northwestern Europe and are periodically defoliated by the autumnal moth, Epirrita = Oporinia autumnata (Borkhausen). This chapter analyzes the population dynamics of Epirrita, but many of our conclusions may apply to other defoliators, such as Operophtera brumata (Linnaeus), O. fagata (Scharfenberg), O. bruceata (Hu1st), and Erannis defoliaria (Clerck), which often exhibit synchronous fluctuations in the same or adjacent areas.38,62

The relationship between size and reproductive potential in male and female Epirrita autumnata (Lep., Geometridae)

Abstract. 1. We studied the fecundity of Epirrita autumnata Bkh. (Lepidoptera) in relation to pupal weight of females and males.

Delayed Inducible Resistance in Mountain Birch in Response to Fertilization and Shade

Delayed inducible resistance (DIR) is triggered by artificial or herbivore-caused foliar damage and is manifested as decreased performance of herbivore generation(s) feeding on the trees subsequent to the generation during which the damage took place. DIR is associated with increase in concentrations of foliage phenolics and decrease in nitrogen. The growth-differentiation balance hypothesis, and the carbon-nutrient balance (CNB) hypothesis contained in it, claim that DIR is caused by nutritional stress after defoliation of trees growing on nutrient-poor soils. In these environments, nutrient uptake limits plant growth more strongly than does photosynthesis; that is, carbon-based secondary metabolites (e.g., phenolics) are prone to accumulate. According to the CNB hypothesis, an excess of limiting nutrient(s) or reduced photosynthetic rate should lead to elimination of DIR. We tested this using same-aged Betula pubescens ssp. tortuosa trees of five open-pollinated families growing in a common arboretum in northernmost Finland. In addition to unmanipulated control trees, we had three nutritional treatments during three successive growth seasons: N-fertilization, PK-fertilization adding all nutrients except N, and shading to decrease carbon assimilation. Half of the trees in each treatment were artificially defoliated (50% leaf area) in the second study year, one year before the growth trial with geometrid (Epirrita autumnata) larvae. Tree growth measurements showed that N is the growth-limiting nutrient in our study area. N-fertilization and shading of the trees affected foliage chemistry generally as proposed by the CNB hypothesis. For example, they reduced foliar concentrations of total phenolics and condensed tannins. The birch families differed significantly in foliage chemistry, suggesting genetic differences, but the differences were not associated with fertilizations, shading, or defoliation of the trees. Contrary to fertilizing-shade treatments, changes in leaf chemistry and E. autumnata performance caused by defoliation were not consistent with the CNB hypothesis. For example, defoliation caused significant DIR irrespective of N-fertilization or shading. There were no significant differences among the birch families in performance of the moth larvae or in effects of fertilization, shade, or defoliation on larval performance. These responses to defoliation contrast with those of some other studies, especially those on the Betula resinifera-Rheumaptera hastata system in Alaska, which provide clear support for the CNB as an explanation of DIR. We find methodological differences to be an unlikely explanation for the different results but are unable to propose any single mechanism that will explain the diverse plant responses.

Long-term inducible resistance in birch foliage: triggering cues and efficacy on a defoliator

Heavy damage of the mountain birch foliage, as well as application of small amounts of insect frass to the soil beneath the trees, reduced growth of Epirrita autumnata larvae reared in these trees in the following year. Foliage damage in the previous year decreased larval survival, too. Both foliage damage and insect frass in the soil decreased a fecundity index which combined the effects of size and survival. Because application of small amounts of fertilizers had an effect indistinguishable from that of insect frass, the effect of the frass may base on responses of trees to an increase in soil nutrient concentration in mid-summer. In previously untreated control trees, all performance indices (growth, survival, and egg production) of Epirrita correlated positively with the distance of the birch from the closest birch defoliated in the previous year, indicating “communication” between adjacent trees. Epirrita egg production in trees that had been both defoliated and treated with frass in the previous summer was at least 70% lower than in previously unmanipulated control trees.

Performance of moth larvae on birch in relation to altitude, climate, host quality and parasitoids

Abstract We studied topographical and year-to-year variation in the performance (pupal weights, survival) and larval parasitism of Epirrita autumnata larvae feeding on mountain birch in northernmost Finland in 1993–1996. We found differences in both food plant quality and parasitism between sites ranging from 80 m to 320 m above sea level. Variation in food plant quality had particularly marked effects on larval survival. The advanced phenology of the birches in relation to the start of the larval period reduced pupal weights. Parasitism rates were different between years and between sites. The clearest site differences were in the proportions of different parasitoid species: Eulophus larvarum was most abundant at the lowest-altitude sites, and Cotesia jucunda at the highest. Differences in the performance of E. autumnata were related to temperature conditions: at higher temperatures, survival and the egg production index were lower, and larval parasitism was higher than at lower temperatures. The higher parasitism at higher temperatures was probably due to greater parasitoid activity during warmer days. In the comparison of different sources of spatial and annual variation in the performance of E. autumnata, the most important factor appeared to be egg mortality related to minimum winter temperature, followed by parasitism and, finally, the variation in food plant quality. If, as predicted, the climate gradually warms up, the effects of warmer summers on the outbreaks of E. autumnata suggest a decrease in outbreak intensity.

Growth and Reproduction of Dwarf Shrubs in a Subarctic Plant Community: Annual Variation and Above-Ground Interactions with Neighbours

1 The effects of coexisting shrub species on the growth, branching, survival and berry production of three dominant dwarf shrubs (Empetrum nigrum, Vaccinium vitis-idaea and V. uliginosum) were examined in northern Finland in a six year experiment. The measurements covered both natural, unmanipulated plots as well as plots from which ramets of one of five dwarf shrub species had been removed. 2 Annual patterns of growth and reproduction of ramets of the three species were different, suggesting that the plants were affected by different combinations of environmental factors and internal patterns of ramet growth. Neither cumulative summer temperature nor summer precipitation accounted for the variations in annual growth of any dwarf shrub species; however, ramet age contributed significantly to the observed variation in growth of shoots of V. vitis-idaea and V. uliginosum. 3 The growth of ramets of E. nigrum in unmanipulated plots was positively correlated with the cover of V. myrtillus and total cover of dwarf shrubs; the growth of V. uliginosum was similarly correlated with the cover of V. myrtillus in undisturbed plots. Furthermore, the growth of both E. nigrum and V. uliginosum was reduced by the removal of V. uliginosum and V. myrtillus, respectively. These observations probably indicate some kind of positive relationship between these species. By contrast, the shoot growth of V. vitis-idaea was less and mortality greater in unmanipulated plots where V. uliginosum was abundant, while both branching and reproduction of V. vitis-idaea were increased by the removal of other dwarf shrubs. These results indicate that interspecific competition may limit the growth and reproduction of V. vitis-idaea.

Low nutritive quality as defence against herbivores: induced responses in birch

SummaryThe effects of artificial defoliation of birch trees in the previous year on the consumption and utilization of food by a geometrid larva, Epirrita autumnata, were studied in laboratory. The leaves were collected from two sites on a slope of a fell. Defoliation had a significant retarding effect on approximate digestibility, efficiency of conversion of ingested food, relative consumption rate and relative growth rate but not on efficiency of conversion of digested food. The effects were to the same direction with leaves from the two sites, but the response was stronger with leaves from the zone defoliated by Epirrita during mid-sixties. The defensive nature of the response(s) of birch to defoliation is discussed in the light of these results. It is concluded that contrary to the hypothesis of Moran and Hamilton (1980) no increase in the consumption incurred by individual trees could be shown in this system.

Species Richness of Macrolepidoptera on Finnish Deciduous Trees and Shrubs

SummarySpecies richness of Macrolepidoptera on Finnish trees and shrubs was analysed by means of stepwise regression analysis. The explaining variables were plant frequency, geographical range, plant height, number of relatives and leaf size.Total frequency of the host plant, which correlated strongly with range, explained 57% of the observed variance of lepidopteran species richness on deciduous trees and shrubs. Height of plant and number of relatives explained significantly the residual variation and altogether these three variables explained 71% of the variance of species richness.Analyses at the plant genus level gave similar results and frequency, height and number of relatives explained 78% of the variance of species richness of Macrolepidoptera on deciduous plant genera.When conifers were included in the analysis leaf size also becomes a significant variable. Leaf size can, however, act as a dummy variable which effectively distinguishes conifers from deciduous trees.The validity of different models explaining herbivore species richness on plants is discussed. The results of this study favoured more than earlier studies the importance of relatedness of host plants as a factor which determines the species richness of herbivores.