First dendrochronological studies of Quercus protoroburoides

Abstract

Quercus protoroburoides Donchev and Bouzov ex Tashev and Tsavkov (Rila oak) is a species with extremely limited and localized distribution. It is found only on several locations on slopes above Rila Monastery in Rila Mountains (Bulgaria) at elevations between 1500 m and 1750 m a.s.l. The trees are in small groups or scattered, usually situated on ridges slightly higher than the local population of sessile oak (Quercus petraea (Matt.) Liebl.) and above mixed fir-beech forests. The species was first discovered and described in 1968 by Boris Buzov, but was officially validated in 2017 by Tashev and Tsavkov. Up to this moment there were no dendrochronological studies of the species and there is general lack of data on how climate affects its growth. Our aim was to contribute to filling this knowledge gap. In our study we analyzed 42 series (22 trees) from Q. protoroburoides from two locations (Drushlyavitsa and Brichebor) and compared them to cores from Q. petraea from the same region. The Rila oak trees varied in age from 30 to 170 years. Nearly half of them were about 50 years old. The composed tree-ring chronology spans from 1856 to 2019 with more than 5 included series after 1870. The sessile oak trees were generally older and the chronology spanned from 1743, having more than 4 series after 1786. The climate-growth analysis with local and gridded data revealed positive effects on growth of previous August and current April-June precipitation, positive correlations with previous May-June temperatures and negative with August temperatures. Most of the pointer years with low growth were characterized by extreme climate conditions, such as summer droughts, unusually cold summers or delayed start of the vegetation period due to late frost events at the end of May or early June. Our data indicate that Q. protoroburoides and Q. petraea trees at this unusually high location for oaks are sensitive to climate conditions including summer droughts.