Dry or hot air? Unraveling the growth stressors of European beech during drought periods

Increasing atmospheric evaporative demand is a key driver of drought stress in European forests. Yet, it remains unclear whether high vapor pressure deficit (VPD) arising from elevated temperature or, in contrast, from reduced atmospheric humidity exerts a stronger constraint on tree growth. European beech (Fagus sylvatica L.), a dominant species in Central Europe, is particularly sensitive to drought-induced growth reductions, making it an ideal model to disentangle these mechanisms.

We conducted a unique controlled phytochamber experiment at the TUMmesa facility to isolate the effects of contrasting VPD drivers on intra-annual growth dynamics of beech trees. Six climate chambers simulated (i) control conditions with low VPD (max. ~1.3 kPa), (ii) high-VPD conditions induced by elevated temperature under control relative humidity (“hot air”), and (iii) high-VPD conditions induced by low relative humidity under control temperature (“dry air”). Both atmospheric drought treatments reached the same maximum VPD levels (~2.3 kPa), allowing direct comparison of temperature- versus humidity-driven VPD effects.

Tree growth was continuously monitored using high-resolution dendrometers, providing sub-hourly insights into stem growth. Atmospheric treatments were combined with contrasting soil textures and progressive soil drying to assess whether growth responses to VPD depend on soil hydraulic context.

By disentangling the growth effects of hot versus dry air under equivalent VPD, this study advances mechanistic understanding of how atmospheric drought shapes tree growth under climate change and improves predictions of forest productivity responses to increasing evaporative demand. Moreover, this experiment provides the basis for us developing advanced mechanistic growth models which can incorporate the impact of atmospheric and soil droughts.