Testing the Efficiency of the Push-and-Pull Strategy during Severe Ips typographus Outbreak and Extreme Drought in Norway Spruce Stands

Protection of Norway spruce stands using anti-attractants was tested during an outbreak of bark beetles (Ips typographus) in their spring flight. The aims of this study were as follows: (1) to test the proposed experimental design for tree protection; (2) to evaluate height-specific alternatives for dispenser installation on trees; and (3) to evaluate the efficiency of tree protection measures using anti-attractants under bark beetle infestation and drought stress. The experiment was conducted at the forest edges adjacent to recent clearcuts on 10 blocks in the eastern Czech Republic. Each block had three adjacent experimental areas, with 20 trees growing in two rows at the recently cut forest edge (10 trees per row). In front of a block in each of the three areas, four pheromone traps were installed. The treatment area was protected by anti-attractants. The second area served as a so-called switch area, where beetles from the treatment area, as the outflux redirected from the anti-attractant, would start new attacks if not caught in nearby pheromone traps. The third area was a control. We attached anti-attractant tube dispensers on each tree trunk of the treated area at two heights. The results suggest a redirecting effect of anti-attractants, pushing beetles into the switch area and causing subsequent attacks, which was greater than in areas containing treated trees. There was no difference between two dispensers placed at 1 and 8 m height and both at 1 m. A switching effect of beetle attacks occurring outside of the treated areas was observed. Mounting anti-attractant dispensers on tree trunks at one low position above the ground can be substantially less labour-intensive and as efficient as positioning them at two different heights. For areas affected by severe drought and extremely dense bark beetle populations, the use of anti-attractants did not prove effective

The Last Trees Standing: Climate modulates tree survival factors during a prolonged bark beetle outbreak in Europe

Plant traits are an expression of strategic tradeoffs in plant performance that determine variation in allocation of finite resources to alternate physiological functions. Climate factors interact with plant traits to mediate tree survival. This study investigated survival dynamics in Norway spruce (Picea abies) in relation to tree-level morphological traits during a prolonged multi-year outbreak of the bark beetle, Ips typographus, in Central Europe. We acquired datasets describing the trait attributes of individual spruce using remote sensing and field surveys. We used nonlinear regression in a hypothesis-driven framework to quantify survival probability as a function of tree size, crown morphology, intraspecific competition and a growing season water balance. Extant spruce trees that persisted through the outbreak were spatially clustered, suggesting that survival was a nonrandom process. Larger diameter trees were more susceptible to bark beetles, reflecting either life history tradeoffs or a dynamic interaction between defense capacity and insect aggregation behavior. Competition had a strong negative effect on survival, presumably through resource limitation. Trees with more extensive crowns were buffered against bark beetles, ostensibly by a more robust photosynthetic capability and greater carbon reserves. The outbreak spanned a warming trend and conditions of anomalous aridity. Sustained water limitation during this period amplified the consequences of other factors, rendering even smaller trees vulnerable to colonization by insects. Our results are in agreement with prior research indicating that climate change has the potential to intensify bark beetle activity. However, forest outcomes will depend on complex cross-scale interactions between global climate trends and tree-level trait factors, as well as feedback effects associated with landscape patterns of stand structural diversity