Increased needlefall and defoliation in Norway spruce induced by warm and dry weather

Analyses of a time-series of needlefall data showed enhanced needlefall due to unusually warm and dry weather in southeastern Norway during 1986–2000. Needlefall was sampled routinely in ten stands of older Picea abies as part of long-term forest monitoring. Mixed linear models were developed for brown and green needlefall separately. Both the brown and the green needlefall had clear seasonal variations, peaking in October and May, respectively. In addition, the needlefall was correlated with weather conditions. Unusually dry summers were followed by increased brown needlefall in the autumns and winters, and unusually high temperatures were accompanied by increased amounts of green needlefall, in particular in the winter. Using the models, we found that unusually warm and dry weather during these 15 years likely caused an overall surplus of needlefall. Even though the brown needlefall was the dominant fraction of the needlefall, the surplus of green needlefall was of larger magnitude. The results suggest that unusually warm winters and dry summers were the main cause of increased crown defoliation during these years.