1. Introduction
In the Mediterranean region of limited water resources, reliable climate predictions on seasonal to decadal timescales are essential in developing effective strategies for mitigating the environmental and socioeconomic impacts of climate change. The increase in temperature and decrease in precipitation, as forecast by climate change scenarios for the Eastern Mediterranean region, pose one of the main challenges for the sustainable management of natural resources in countries of the Adriatic and Aegean regions [
1,
2].
The Balkan Peninsula is known for its relict tree species, due to late glacial refugia and some more or less untouched forest sites [
3,
4,
5]. Two Balkan tree species,
Pinus heldreichii H. Christ. (synonym:
Pinus leucodermis Ant.; Bosnian pine) and
Pinus peuce Grisb. (Macedonian pine), are particularly interesting in terms of dendroclimatology because they can get very old and grow on extreme sites.
Pinus heldreichii and
Pinus peuce are long-lived, relict Balkan species growing close to the alpine timberline, where climatic conditions are harsh and limiting for tree growth. Trees growing at a high elevation are valuable for dendroclimatological research and this has been clearly demonstrated in a large number of reconstructions available for the upper timber line. Opala-Owczarek [
6] studied
Juniperus tree rings in Pamir and found May–September minimum temperatures to be highly correlated with juniper growth. Corona et al. [
7] studied the climate–growth relationship in
Larix decidua and
Pinus cembra at the upper timber line in the Alps and were able to reconstruct the climate for the past 1000 years. Cerrato et al. [
8] made a May–September mean temperature reconstruction for the Rhaetian Alps based on the maximum density of
Pinus cembra tree-rings. Maximum latewood densities correlate better with climate than tree-ring widths [
8]. Saulnier et al. [
9] studied the climate–growth response in
Larix decidua in the French Alps and found that, for high-elevation sites, current summer temperatures significantly correlate with tree growth. All of the above, and many other climate reconstructions at the upper timber line, demonstrate climate sensitivity and a close connection between the climate at the upper timber line and growth of trees.
There have been a limited number of studies focused on the tree-ring characteristics of
P. heldreichii and
P. peuce. Most of these studies have focused on the Pirin mountains in southwestern Bulgaria. Panayotov et al. [
10] analyzed the climate signal in tree-ring chronologies of
Pinus peuce and
Pinus heldreichii in the same region and concluded that a climate signal exists but is weak. Trouet et al. [
11] presented a summer temperature reconstruction (1768–2008) based on maximum latewood density measurements of
P. heldreichii from the Pirin mountains. Panayotov et al. [
12] studied the variation of anatomical leaf parameters of
P. heldreichii. A more recent study by Scheithauer et al. on
P. heldreichii from the northern Pirin mountains suggests that width growth is humidity limited and more sensitive to high summer temperatures on the southern flanks [
13,
14]. Other published studies include an analysis of the climate–growth relationship using tree-ring widths of
P. heldreichii in Kosovo [
15,
16] and Albania [
17,
18], in which a relatively weak but stable climate signal was reported. A similarly strong climate signal was found on Mt. Smolikas in Greece [
19] and in the Pindus mountains, Greece, where they also found the oldest living
Pinus heldreichii to date [
20].
Stable carbon isotope (δ
13C) chronologies for endemic
P. heldreichii and
P. peuce, to the best of our knowledge, do not exist at all. However, there are δ
13C chronologies for other tree species in the wider region, although the number of δ
13C chronologies is smaller than in Western Europe, Fennoscandia, or parts of the Mediterranean region. Nagavciuc et al. [
21] developed a stable carbon and oxygen isotope chronology for
Pinus cembra for the eastern Carpathian timberline and found that δ
13C was negatively correlated with summer precipitation and three-month SPEI (Standardized Precipitation-Evapotranspiration Index) index. Szymczak et al. [
22,
23] developed a 560-year summer temperature reconstruction for Corsica (France) based on a stable carbon isotope from
Pinus nigra tree-rings. They found a statistically significant, positive correlation for temperature in August and September. Hafner et al. [
24] developed a 520-year long stable carbon isotope chronology for
Larix decidua for the south-eastern European Alps and found significant positive correlations with July–August temperature. Kern et al. [
25] analyzed δ
13C in tree rings of
Quercus robur in Hungary and found a strong negative correlation between δ
13C in tree-rings and June precipitation.
In our study, we focused on extracting and assessing the climate signal of tree-ring widths (TRW) and stable carbon isotope ratios (δ
13C) in cellulose extracted from
P. peuce and
P. heldreichii tree rings, growing at the upper timber line in Montenegro, at the northern edge of the species distribution range. Previously developed TRW chronologies of
P. peuce and
P. heldreichii typically possess a weak climate signal in TRW [
19]. In our study, in addition to the TRW chronology, we developed a δ
13C chronology to test it as a potential proxy for the reconstruction of climate in the Balkan Peninsula.
5. Conclusions
Two long chronologies for endemic, long-lived Balkan pines, Pinus heldreichii and Pinus peuce from two locations in Montenegro were constructed: a 443-year long P. heldreichii chronology covering the period 1571–2013 and a 493-year long P. peuce chronology covering the period 1521–2013.
The P. heldreichii chronology from the studied site Murina (Montenegro) cross-dates well with P. heldreichii chronologies from Albania, Kosovo, and Bosnia and Herzegovina, which makes it suitable for dating wooden constructions from the wider region of the chronology. The P. peuce chronology, on the other hand, does not match well with chronologies from not too distant sites in Albania and northern Macedonia and did not even match the P. heldreichii chronology from the nearby site Murina. P. peuce therefore has weak potential for dating wooden structures.
The climate signal in tree-ring widths (TRW) of both tree species is relatively weak to non-existent. In P. heldreichii TRW, we found only a weak climate signal in May and June temperature, which, when merged into the seasonal variable May–June, gave a weak but significant correlation of −0.31 (p < 0.05). We could not find any climate signal in P. peuce TRW. The temporal and spatial stability of the climate signal in P. heldreichii and P. peuce TRW chronologies was weak and limited only to the vicinity of the sampling locations.
We developed the first stable carbon isotope ratio chronology for endemic Balkan pines for the region of the Western Balkans and tested its potential for climate reconstruction. The best results were obtained when correlating the δ13C chronology with the July–August and June–August combinations of mean monthly temperature. Both combinations also passed all statistical tests that are commonly applied prior to climate reconstruction, i.e., RE, CE, and MSE.
Analysis of the temporal stability of selected seasonal variables (July–August and June–August mean monthly temperature) gave stable relationships with the δ13C chronology, without drastic climate signal loss. Analysis of the spatial outreach of the δ13C chronology compared with gridded meteorological data (July–August and June–August temperature) showed a very wide spatial outreach of both seasonal variables, with June–August having the broadest.
The δ13C chronology of endemic Balkan pines, P. peuce and P. heldreichii, was shown to be a reliable proxy for climate reconstructions of summer temperatures, while TRW chronologies of P. heldreichii could potentially be used for dating and provenience purposes.