Ulrich Kohnle

Comparison between the productivity of pure and mixed stands of Norway spruce and European beech along an ecological gradient

Abstract• Existing growth and yield plots of pure and mixed stands of Norway spruce (Picea abies (L.) H. Karst.) and European beech (Fagus sylvatica L.) were aggregated in order to unify the somewhat scattered sources of information currently available, as well as to develop a sound working hypothesis about mixing effects. The database contains information from 23 long-term plots, covering an ecological gradient from nutrient poor and dry to nutrient rich and moist sites throughout Central Europe.• An empirically formed interaction model showed, that depending on the site conditions, dry mass growth in mixed stands can range from −46% to +138 % of the growth yielded by a scaled combination of pure stands at equal mixing proportions.• Drawing from the interaction model, overyielding of the mixed stands appears to be triggered by two separate mechanisms. On poor sites, where significant overyielding is commonly found, facilitation by beech offsets nutrient-related growth limitations in spruce. In contrast, overyielding of mixed stands occurs less frequently on rich sites, and appears to be based on an admixture effect, with spruce reducing the severe intra-specific competition common in pure beech stands.• It was concluded that silviculture can accelerate growth of spruce by beech admixtures on poor sites, while growth of beech can be promoted by admixture of spruce, particularly on excellent sites.

How does silviculture affect storm damage in forests of south-western Germany? Results from empirical modeling based on long-term observations

Storms represent the most important disturbance factor in forests of Central Europe. Using data from long-term growth and yield experiments in Baden-Wuerttemberg (south-western Germany), which permit separation of storm damage from other causes of mortality for individual trees, we investigated the influence of soil, site, forest stand, and tree parameters on storm damage, especially focusing on the influence of silvicultural interventions. For this purpose, a four-step modeling approach was applied in order to extract the main risk factors for (1) the general stand-level occurrence of storm damage, (2) the occurrence of total stand damage, and (3) partial storm damage within stands. The estimated stand-level probability of storm damage obtained in step 3 was then offset in order to describe the damage potential for the individual trees within each partially damaged stand (4). Generalized linear mixed models were applied. Our results indicate that tree species and stand height are the most important storm risk factors, also for characterizing the long-term storm risk. Additionally, data on past timber removals and selective thinnings appear more important for explaining storm damage predisposition than for example stand density, soil and site conditions or topographic variables. When quantified with a weighting method (summarizing the relative weight of single predictors or groups of predictors), removals could explain up to 20% of storm risk. The stepwise modeling approach proved an important methodological feature of the analysis, since it enabled consideration of the large number of observations without damage (“zero inflation”) in a statistically correct way. These results form a reliable basis for quantifying forest management’s direct impact on the risk of storm damage.

Semiochemicals from bark beetles: New results, remarks, and reflections

A brief survey is given about recent results in the identification of semiochemicals in bark beetles: Males ofIps sexdentatus (Boern.), stressed by the attack on resinous trees produce large amounts of 3(S)-1-methyl-5-(1-hydroxyl-1-methylethyl)-cyclohexa-1,3-diene. The compound appears to be derived from Δ3-carene and acts as a repellent. Males ofIps typographus (L.), stressed through the attack on unsuitable host material release 3-methyl-7-methylene-1,3(E), 8-nonatriene, which seems to act as a repellent. The odor bouquet of three species ofPityogenes is described. The occurrence of (+)-grandisol and other compounds related to weevil pheromones points to a close relation between Scolytidae and Curculionidae. Females ofDendroctonus simplex (Le Conte) use (−)-frontalin as the main pheromone. 6-Methyl-6-hepten-2-one, a minor component among the volatile compounds released by the females, is regarded as a possible precursor of frontalin. Similarly, (2R,5S)-2(1-hydroxyl-1-methylethyl)-5-methyltetrahydrofuran, pityol, a pheromone ofPityophthorus spp., is regarded to at least share a common biogenetic precursor with 6-methyl-5-hepten-2-ol, sulcatol. A new bicylic acetal, 2-ethyl-1,5-dimethyl-6,8-dioxabicyclo[3.2.1]octane, is described as an aggregation pheromone of the beech bark beetle,Taphrorychus bicolor (Herbst). Structural relationships between bark beetle pheromones and plant volatiles are discussed.

E‐7‐methyl‐1,6‐dioxaspiro[4.5]decane in the chemical communication of European Scolytidae and Nitidulidae (Coleoptera)

E‐7‐methyl‐1,6‐dioxaspiro[4.5]decane (mdos), a spiroacetal known from male ash bark beetles, Leperisinus varius (Col., Scolytidae), was found in the frass of the fir bark beetle, Cryphalus piceae. In the field, mdos reduces L. varius response to baited traps regardless of chirality but only the naturally produced (5S, 7S)‐(‐)‐enantiomer effects response reduction in C. piceae while racemic mdos attracts an associate, Epuraea thoracica (Col., Nitidulidae) in significant numbers.

The odour bouquet of three pine engraver beetles (Ips spp.)

Nearly 50 volatile compounds are identified from hindguts of the pine engraver Ips acuminatus, I. sexdentatus and I. lecontei. Among these, α‐phellandren‐8‐ol, ipsdienon, ipsenon and others are described from bark beetles for the first time. The quantitative composition of the odour bouquet of both sexes largely depends on the phase of colonization; after the initial phase of attack, they are particularly rich in oxidized monoterpenes. I. acuminatus and I. lecontei contain the male specific terpene alcohols (+)‐ipsdienol and (—)‐ipsenol in high optical purity while I. sexdentatus produces racemates. Racemic ipsdienol proves attractive to I. sexdentatus under field conditions, but does not seem to promote the subsequent steps in colonization such as landing or penetrating the bark.

Chemical communication in the European pine engraver, Ips mannsfeldi (Wachtl) (Col., Scolytidae)

Host selecting males of Ips mannsfeldi release (S)‐(+)‐ipsdienol, (S)‐(‐)‐ipsenol, amitinol and 2‐methyl‐6‐methylene‐1,3E,7‐octatriene (“ipstriene”) as major beetle specific volatiles and obviously employ the terpene alcohols in a frass pheromone system. A vertex‐pronotal stridulatory organ is present in the females.

Coupling a tree growth model with storm damage modeling - Conceptual approach and results of scenario simulations

The purpose of this study was to develop, test and evaluate a software prototype capable of modeling forest growth in consideration of winter storm disturbance and to simulate storm damage in forests under different forest management regimes. The results of a test application showed that simulated storm damage was more strongly influenced by the input data (e.g. tree species and tree height) than by the different forest management regimes. However, early, intense thinnings as well as reducing target diameters by 10% led to reduced storm damage, with decreases as large as 50% of the damage in certain forest stands. The coupled modeling framework was able to simulate interactions between forest growth, storm damage and forest management regimes. Further testing of the prototype appears necessary to investigate a wider range of tree species, soil and site conditions. Also, the use of computational system resources needs improvement. We establish and evaluate a coupled forest growth and storm damage modeling system.The impact of forest management on storm damage risk can be simulated.Measures to stabilize forest ecosystems against storm disturbance were identified.Heavy thinnings and a 10% reduction of target diameters proved effective.Storm damage in coniferous forests could be lowered up to 50% by adapted management.

Host and non-host odour signals governing host selection by the pine shoot beetle, Tomicus piniperda and the spruce bark beetle, Hylurgops palliatus (Col., Scolytidae)

Abstract:  Tomicus piniperda and Hylurgops palliatus colonize susceptible host trees by responding to host‐specific odour signals as well as by avoiding volatiles emanating from non‐host conifers. In the field, the pine shoot beetle, T. piniperda, responded in high numbers to natural odour sources provided by their host tree, Pinus sylvestris, while the non‐host conifers Larix decidua, Picea abies, or Pseudotsuga menziesii were significantly less attractive. In contrast, the spruce bark beetle, Hylurgops palliatus, preferentially responded to its main host, P. abies. Furthermore, T. piniperda attacks on P. sylvestris bolts decreased in presence of bark and wood particles from the non‐host P. abies, whereas particles from P. menziesii appeared not to affect T. piniperda attacks. Apparently, tree‐specific volatiles act at close range as specific signals that lead to the successful discrimination and colonization of the respective host tree species.

Is Silver fir (Abies alba) less vulnerable to extraction damage than Norway spruce (Picea abies)

Inspection of Norway spruce and Silver fir on experimental plots in south-western Germany showed that Silver fir had suffered significantly less bark injuries than Norway spruce. Data from both federal forest inventories (1987, 2002) showed a similar species-specific vulnerability. Additional visual inspections of the basal cross-sections of trees removed from the experimental plots showed rather high proportions of butt rot in uninjured Norway spruce (51%). The proportion further increased to 93% in trees, which had sustained bark injuries. In contrast, decay symptoms were almost absent in uninjured Silver fir and less enhanced in trees with bark injuries (27%). Management implications for risk rating of tree species, as well as the necessity of implementing low-damage harvesting regimes, are discussed.

Odour production and pheromone response in the European engraver bark beetles, Ips amitinus (Eichh.) and Ips amitinus var. montana Fuchs (Col., Scolytidae)

Pheromone producing males of the two subspecies, Ips amitinus and Ips amitinus var. montana, respectively, are fully cross‐attractive to field populations of both subspecies and produce identical patterns of male‐specific terpene alcohols upon feeding in the phloem tissue of their respective host trees, Picea abies and Pinus cembra: major amounts of ipsdienol along with lesser amounts of amitinol, cis‐, trans‐verbenol, myrtenol and traces of ipsenol. The absolute configuration of ipsdienol and ipsenol produced by feeding I. a. montana males was determined by gaschromatographic enantiomer separation of the natural products as (S)‐(+)‐ipsdienol and (S)‐(‐)‐ipsenol, respectively. After joining the males, females release 2‐methyl‐3‐butene‐2‐ol which is produced by males only in trace amounts.