Beneath the canopy: Linking drought-induced forest die off and changes in soil properties

Climate warming and the occurrence of more severe dry spells are causing widespread drought-induced forest die-off events. Despite research on drought-triggered die-off processes is rapidly increasing, little is known on how soil conditions and rhizosphere features are affected by canopy dieback and tree death. We studied the soils in the rhizosphere of three coniferous forests where die-off was induced by a severe drought in 2012. We selected three forest types subjected to contrasting climatic and edaphic conditions dominated by three different tree species: silver fir (Abies alba; temperate conditions), Scots pine (Pinus sylvestris; continental and Mediterranean conditions) and Aleppo pine (Pinus halepensis; semi-arid and Mediterranean conditions). In each forest, we analyzed soil physical characteristics such as water retention capacity and soil texture, nutrient availability and microbial community structure (Phospholipid fatty acids, PLFA) below non-declining and declining or dying trees. We did not observe differences in nutrient availability between the two vigor classes. Conversely, we found strong differences in soil microbial community structure below non-declining and declining trees in the Silver fir and Aleppo pine stands. Soils in the Scots pine stand presented extremely low values of soil saturated sorptivity indicating a reduction of soil water infiltration after prolonged dry periods which could exacerbate drought stress. We conclude that forest dieback impacts the soil microbial community structure in the short term. Further research is required to understand the linkages between a reduced capacity of soil water infiltration after prolonged droughts, short-term changes in the soil microbiota, long-term nutrient imbalances and tree death. Soil conditions shall be considered as an important part of forest management strategies after drought-induced die off.