Seasonal growth dynamics in pure and mixed oak–pine stands under drought: insights from hierarchical generalized additive models

Global warming is expanding the length of the growing season. However, within this longer window, the number of effective growing days is decreasing due to drought, so that warming often leads to growth losses on dry sites. Tree species mixtures can enhance productivity and biodiversity and may mitigate climate-change impacts. To better understand the effects of climate and species mixing, high-resolution seasonal growth observations are valuable. Nevertheless, mixing effects have rarely been examined at the seasonal scale.

In pure and mixed oak–pine stands at Maissau (475 m a.s.l.; mean annual temperature 10.1 °C; precipitation 475 mm year⁻¹), hourly radial growth of 48 trees has been monitored since 2017 using band dendrometers installed on dominant, intermediate, and suppressed trees. A hierarchical generalized additive model (GAM) with components for seasonal growth, shrinkage and swelling following rainfall, and diurnal water uptake was fitted to the data.

The model explained 95% of cumulative diameter increment. Increment differed significantly among years, species, dominance classes, and mixtures. There was pronounced interannual variability, with lower growth rates and a shorter growing season in drier years. Growth of Quercus robur and Q. petraea started earlier and lasted longer than that of Pinus sylvestris. Dominant trees grew for approximately one month longer than suppressed trees. Mixture effects were small. Both species exhibited pronounced diurnal cycles of water uptake, which were stronger around the summer solstice than in spring and more pronounced in P. sylvestris than in Quercus spp.; differences due to mixture were again minor.

Pinus sylvestris was more affected by drought than Quercus spp., which can be traced to their differing physiology. As a conifer, P. sylvestris pursues a conservative water-use strategy, closing stomata earlier than Quercus spp.; under drought this leads to strongly reduced photosynthesis and lower growth. Mixture had only a small beneficial effect for the two species studied, likely because niche complementarity is limited.