Differential effects of Soil Drought Alleviation on mitigating the Effects of Atmospheric Drought in mature Pine Forest Trees

Forests play a vital role in the earth’s ecosystems by regulating global water and carbon cycles and carbon assimilation. While significant advancements have been made in understanding the impacts of drought on tree physiology and gas exchange, the extent to which alleviation of soil drought mitigates the impact of high vapor pressure deficit (VPD) on leaf net photosynthesis (Anet) remains unclear. In a six-year study of soil drought alleviation (SDA; using dry-season supplement irrigation), we investigated its mitigating effects on the response of branch-scale net photosynthesis (Anet) to high atmospheric drought in a mature pine forest (Pinus halepensis). We demonstrate that while SDA improves Anet response to VPD, VPD remains a major factor in shaping the daily Anet cycle and overall tree productivity. In fact, in summer, SDA trees show an early Anet peak, followed by a sharp decrease reflecting a relative sensitivity to VPD greater than that of the soil-droughted (SD; control) trees. Specifically, we show peak Anet of around 7 AM and 9 AM in the SD and SDA trees, compared to ~noon in winter. The data also indicate that increasing VPD above a threshold of ~4 kPa, the SDA trees show enhanced sensitivity to VPD, shaping the above-noted daily peak and limiting productivity. The analysis of the partial dependence of Anet on key microclimatic variables and soil moisture content, using generalized additive models (GAMs), confirmed that the branch-scale Anet in the SDA trees improved under VPD only up to 4 kPa compared to SD trees. Above this apparent threshold, Anet in SDA trees declined sharply, associated with reduced stomatal conductance, and with increased respiration due to the elevated temperatures at these times. Further analysis, across the entire observed VPD range, showed that SDA trees do, in fact, have a greater sensitivity of Anet to VPD (and in particular to extreme atmospheric drought). The results indicated that while SDA effectively buffers trees from moderate atmospheric drought, it does not provide efficient mitigation towards extreme conditions. Our findings underscore the complex interplay between soil and atmospheric drought impacts in shaping tree physiological responses in pine forests, offering a important basis for predicting their response to different climate change scenarios.