Multi-temporal influence of vegetation on soil respiration in a drought-affected forest

Aboveground plant activity influences fine roots and rhizosphere activity, which is reflected on soil respiration (SR). However, it is still unclear and poorly understood the nature of plant activity control over SR, especially under drought conditions. We studied the plant activity-SR relationship at different timescales in a water-limited mixed Mediterranean forest where Scots pines (Pinus sylvestris L.) are undergoing drought-induced die-off and are being replaced by the more drought-resistant Holm oak (Quercus ilex L.). Half-hourly sap flow (SF), as a proxy of photosynthesis, coupled with measures of SR using solid-state CO2 sensors, were monitored during nine months in four different trees, representative of the diversity and health condition of the forest. SF was strongly associated with SR at both daily and seasonal timescales. At daily timescales, almost no lags were found between SF and SR, indicating a fast control of photosynthesis on SR. However, the association between SF and SR weakened during the summer drought. These temporal patterns were not constant across the trees representing the die-off and replacement processes. SR beneath living pines was highly controlled by SF at daily scale, whereas Holm oak seemed to be more controlled by SF at seasonal scale. The relationship between SF and SR measured beneath dead pine and Holm oak at the daily and seasonal scales was consistent with the colonization of soil gaps by holm oak roots following Scots pine death and suggests that surviving Scots pines are unable to expand their root system in these gaps. Our results collectively show how drought modulates the link between canopy photosynthesis and soil respiration, and increase our understanding on how belowground processes may be affected by the successional dynamics following drought-induced forest mortality.