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Climate change challenges forest vitality both directly by increasing drought and heat periods and indirectly, e.g., by creating favorable conditions for mass outbreaks of phyllophagous insects. The large forests dominated by Scots pine (Pinus sylvestris L.) that cover the lowland regions in northeast Germany have already been affected regularly by cyclic mass propagations of defoliating insect species in the past with climate projections implying an even more advantageous environment for devastating outbreaks in the future. To improve predictive and responsive capacities we have investigated a wide range of ecological parameters to identify those most strongly related to past outbreak waves of three central species. In total, we analyzed 3,748 variables covering stand and neighborhood properties, site quality, and climatic conditions for an area of roughly 750,000 ha of pine forests in the period 2002–2016. To reflect sensitivity against varying climate, we computed “floating windows” in relation to critical phenological phases of the respective insects. The parameters with the highest explanatory power resulted from the variable importance measures of the Random Forest (RF) methodology and have been evaluated by a 10-fold cross-validation process. Our findings closely reflect the known specific gradation patterns and show that relative variable importance varies with species. While Lymantria monacha L. feeding was mainly dependent on the surroundings of the respective stand, Diprion pini L. proved to be almost exclusively susceptible to climatic effects in its population dynamics. Dendrolimus pini L. exhibited a mixed pattern of variable importance involving both climatic and forest structure parameters. In many cases the obtained statistical results support well-known ecological cause-effect relations and long-term population change dynamics. The RF delivered very high levels of sensitivity and specificity in the developed classifications and proved to be an excellent tool to handle the large amounts of data utilized for this study. While the presented classification approach may already support a better prognosis of the amplitude during the outbreak culmination, the obtained (most important) variables are proposed as preferable covariates for modeling population dynamics of the investigated insect species.
Recent studies have identified strong relationships between delayed recovery of tree growth after drought and tree mortality caused by subsequent droughts. These observations raise concerns about forest ecosystem services and post-drought growth recovery given the projected increase in drought frequency and extremes. For quantifying the impact of extreme droughts on tree radial growth, we used a network of tree-ring width data of 1689 trees from 100 sites representing most of the distribution of two drought tolerant, deciduous oak species (Quercus petraea and Quercus robur). We first examined which climatic factors and seasons control growth of the two species and if there is any latitudinal, longitudinal or elevational trend. We then quantified the relative departure from pre-drought growth during droughts, and how fast trees were able to recover the pre-drought growth level. Our results showed that growth was more related to precipitation and climatic water balance (precipitation minus potential evapotranspiration) than to temperature. However, we did not detect any clear latitudinal, longitudinal or elevational trends except a decreasing influence of summer water balance on growth of Q. petraea with latitude. Neither species was able to maintain the pre-drought growth level during droughts. However, both species showed rapid recovery or even growth compensation after summer droughts but displayed slow recovery in response to spring droughts where none of the two species was able to fully recover the pre-drought growth-level over the three post-drought years. Collectively, our results indicate that oaks which are considered resilient to extreme droughts have also shown vulnerability when droughts occurred in spring especially at sites where long-term growth is not significantly correlated with climatic factors. This improved understanding of the role of drought seasonality and climate sensitivity of sites is key to better predict trajectories of post-drought growth recovery in response to the drier climate projected for Europe.
In der vorliegenden Untersuchung wurden nach fernerkundungsbasierter Vorauswahl Probeflächen in durch Raupenfraß der Nonne (Lymantria monacha L.) geschädigten Kieferreinbeständen mittleren Alters angelegt. Es wurden drei Schädigungsklassen (D40, D60, D85) und eine Referenzfläche (D15) berücksichtigt. Neben klassischen ertragskundlichen Aufnahmen wurde sich hauptsächlich auf die Entnahme von Bohrkernen an 15 Baumindividuen pro Probefläche und deren Auswertung konzentriert.
Durch die Vermessung der Bohrkerne konnten Zeitreihen der Jahrringbreite (JRB) und nachfolgend des Jahrringindex (JRI) berechnet werden. Während für das Hauptfraßjahr 2003 alle vier Probeflächen mit
einer ähnlichen Zuwachsreduktion reagierten, ergaben sich für das Folgejahr deutliche Unterschiede zwischen den geschädigten Flächen und der Referenz. Letztere erholte sich bereits wieder und wies
einen JRI von 1,06 auf. Die geschädigten Flächen waren hingegen in ihrem Zuwachs noch stärker eingebrochen, was in einem JRI zwischen 0,20 (D60) und 0,30 (D40) bzw. 0,33 (D85) resultierte. Die folgende Berechnung von Resilienzkomponenten nach Lloret et al. (2011) verdeutlichte ebenfalls die Abhängigkeit zwischen Zuwachs und Entnadelungsgrad. Insbesondere die Recovery weist in diesem Zu-
sammenhang darauf hin, dass durch die Freistellung der überlebenden Individuen und aufgrund gewisser Düngungseffekte nach dem Fraß ein Zuwachsniveau erreicht wurde, welches über dem der vierjährigen Periode vor dem Schadereignis liegt.