Signals of summer drought in crown condition data from the German Level I network

Abstract

Crown transparency estimates of Scots pine, Norway spruce, common beech, pedunculate and sessile oak, annually surveyed between 1990 and 2004 within a grid over Germany, provide a suitable response variable to study drought effects on forest trees. Major climatic factors, available on a monthly basis as plot-specifically interpolated values and parameters of site and stand conditions, biotic and other relevant factors were used as predictors in different cross-and length-sectional, and longitudinal models. Stand age is a considerable and most constant driver of crown transparency in all species. Pine, spruce and beech responded-mainly with a delay of 1 year-with some foliar loss in areas where there was a surplus of temperature after the generally hot and dry summer of 2003. Parallel time-series analyses delivered species-specific geographic large-scale patterns with delayed or recent precipitation deficits or temperature surpluses. Even if beech is partly responding in current years with leaf loss towards precipitation surpluses, defoliation is especially high 1 year after hot summers, partly a result of high seed sets after such summers. Crown condition of oak responds in dry and warm areas according to the drought stress hypothesis, however, in cool and wet mountainous ranges oak responds after wet summers with higher defoliation.Longitudinal approaches revealed for all 4-tree species significant relationships between crown condition and deviations from the long-term means of temperature, precipitation but also global radiation and wind speed. Results do not always match the drought stress hypothesis, however, this is not to expect considering the heterogeneous site, stand and climatic conditions across Germany. Complex interactions of climatic and biotic factors also impede simple relationships. Soil-related clusters reveal higher sensitivity of spruce and beech towards climatic drought factors on more acid soils with thin humus layers. Also clusters constructed from plotspecific courses of defoliation reveal groups with rather closer relationships like a group of pine plots in the Oberpfalz, which seems to be especially sensitive to summer drought.

Tree diversity mitigates defoliation after a drought‐induced tipping point

“…The fact that, for both beech and sessile oak, the mean defoliation level was highest after dry years than wet years confirms the results of earlier studies (e.g., Carnicer et al., ; Sánchez‐Salguero et al., ; Seidling, ), as well as our hypothesis, that drought can trigger and expedite defoliation in deciduous forests. But perhaps more importantly, our results reveal that the negative impact of drought on the crown defoliation of the three studied species in deciduous temperate forests was mitigated by species mixing through time (i.e., interaction between species richness and year of observation; Figure b).…”

“…These results are consistent with those of previous studies, which have observed that tree vitality in Europe's forests is substantially driven by climate variability in both the current and preceding years (Neumann et al., ). Defoliation of beech in a given year, for instance, has been consistently associated with low precipitation and high evapotranspiration rates in the year before (e.g., de la Cruz et al., ; Ferretti et al., ; Seidling, ). The previous year's defoliation has also been suggested to enhance the process of defoliation in the current year (de la Cruz et al., ; Ferretti, Calderisi, & Bussotti, ).…”

The seasonal variability of air pollution effects on pine conditions under changing climates

“…Climate change has additional effect on this interaction through changes in air temperature and precipitation amounts (Mayerhofer et al 2001, Wellbrock et al 2005Seidling 2007;Mellert et al 2008). This process is insufficiently understood, because meteorological conditions change over the course of the year.…”

Plant health and global change – some implications for landscape management

“…In regions where temperature, but not precipitation, is predicted to increase, these and other water‐limited forests are likely to experience considerable abiotic stress (e.g. Seidling, 2007; McDowell et al , 2008). Such disturbance is often followed by an increased activity of secondary pests and pathogens (Schwartz, 1992; Brasier & Scott, 1994; Desprez‐Loustau et al , 2006 b ), with subsequent potentially extensive dieback at the landscape scale (Holdenrieder et al , 2004).…”