Oak decline induced by mistletoe, competition and climate change: a case study from central Europe.

It is predicted that rising temperatures and extreme summer droughts will adversely affect the growth of pedunculate oak (Quercus robur) and contribute to local population decline. Furthermore, such oaks may become prone to infestation with mistletoe (Loranthus europaeus) and competition from neighbouring trees. We tested these predictions in the warm, south-eastern part of the Czech Republic, a drought-prone area where oaks die prematurely. We compared the radial growth patterns of eight categories of oaks differing in age (younger or older than 60 years), presence of neighbouring trees (solitary versus grouped) and infestation with mistletoe (infested versus healthy), and their responses to variation in both annual and intra-annual temperature (T) and precipitation (P). We analysed long-term data from tree rings and detailed dendrometer records of daily increments using moving correlations and regression trees. Oak growth is affected by (i) dry and cold winters, resulting in root damage and water shortages during summer, (ii) a cool and wet March–April, hampering the onset of earlywood growth, and (iii) a hot and dry May–July period, reducing latewood formation. Latewood width increases when February P > 55 mm, March T > 3.3 °C, April P < 60 mm, May P > 50 mm, June T < 19 °C and August P > 40 mm. Latewood width decreases with tree age, mistletoe infestation and competition from neighbouring trees. These factors are responsible for different climate–growth responses. Infested oaks develop less latewood if T in June–July is high and this is associated with drought. In healthy oaks May P determines how quickly earlywood growth is completed and hence when latewood formation starts. Grouped trees that compete for soil water are more prone to June–July droughts but less harmed by cold springs than solitary oaks. Dendrometer records show that the net daily increments (ΔR) recorded for oaks growing in groups are mainly associated with water deficit, whereas those of solitary oaks with high summer T, which fluctuates more in open landscape than in woods, resulting in a reduced number of days with a positive ΔR. However, under optimal conditions (soil moisture > 20%, mean daily T 10–20 °C), the ΔR phase recorded for solitary oaks is longer than for oaks in groups, which results in greater annual increments. Hence, the differences between solitary oaks and those in groups is due to a difference in the period of time they spend growing rather than the speed of growth. Infested oaks have smaller ΔRs and annual increments because they grow for a shorter period rather than differences in metabolic activity. These results provide support for the crucial role of climate change (decline in rainfall and increase in summer temperatures over the last three decades) and biotic interactions (mistletoe hemiparasites, inter-tree competition) in oak growth and population decline.