Phenology and competitiveness of three temperate tree species in the juvenile stage under warmer springs and drier summers

Vegetation period of temperate tree species was projected to lengthen with climate warming by both advancing leaf-out during spring and delaying leaf senescence during autumn. However, this longer vegetation period does not necessarily translate into higher growth and carbon sequestration due to the increase in adverse weather conditions (e.g., severe drought) that occur during the growing season. Further, it remains unclear how species mixing might help benefit from a longer season and cope with extreme droughts compared to monocultures, i.e., whether diverse ecosystems are more resilient than monocultures. To tackle these questions, we set up experimental mesocosms using saplings from three species (i.e., Fagus sylvatica, Quercus petraea and Tilia cordata) grown either as monocultures or mixtures of two species. Each experimental unit was exposed to either (i) increased spring temperatures using a passive warming method, (ii) reduced precipitation (~ 50%) using rain shelters all year along, (iii) a combination of the two first treatments, or (iv) ambient conditions.

In spring 2022 we observed significantly earlier leaf-out of Quercus compared to Fagus and Tilia at ambient conditions and advanced leaf-out by about 4 days for all species when exposed to the passive warming treatment. In autumn 2022 we observed 50% senescence of Tilia several weeks before Fagus and Quercus at ambient conditions. Advanced leaf-out due to increased spring temperature and drought exposure did not affect senescence of any species growing in monocultures. However, the presence of Tilia in the same experimental unit delayed senescence of Fagus by 4 days and senescence of Quercus by 5 days, whereas neither species changed senescence when growing in Fagus-Quercus mixtures compared to growing in monocultures. A potential explanation could be the competition release for water and nutrients due to Tilia’s much earlier senescence. Although we expected saplings with earlier leaf-out to grow stronger and saplings exposed to drought to grow less, height increment of neither species was affected by any treatment.