A Tree Ring Proxy Evaluation of Declining Causes in Pinus sylvestris L. and Pinus nigra JF Arnold in Northeastern Romania

Drought-induced dieback has been extensively studied in various forests habitats. We used a retrospective tree ring width (TRW), basal area increment (BAI), oxygen isotope ratios in tree ring cellulose (delta O-18(TR)) and carbon isotope ratios in tree ring cellulose (delta C-13(TR)) to assess causes in declining Pinus sylvestris L. and Pinus nigra J.F. Arnold. The climate data analysis indicates a significant increased trend occurred after 1980 in minimum, mean and maximum temperature and a reduced amount of precipitation compared to the 1920-1980-time scale. According to the Palmer Drought Severity Index, we found two extreme drought years (1946 and 2000) and three years with severe drought (1990, 2003 and 2012). One-way ANOVA indicated no significant difference between P. nigra and P sylvestris tree ring width, basal area increment, but a considerable difference between delta C-13(TR) and delta O-18(TR). Basal area increment evaluated the climate-growth relationship most accurately, comparing to delta O-18(TR) and delta C-13(TR,) which explained the influences of environmental factors in tree rings formation. The delta C-13(TR) was mainly negatively correlated with high temperatures from April-August current growing seasons. The negative correlation between delta C-13(TR) and NDVI indices (June, August) shows a decreased carbon uptake induced by drought from summer to early autumn. The low delta O-18(TR) signal was associated with a complex of factors, including the strong influence of heavy precipitation occurring in the growing season and a weak reaction of declined trees to resources. Species-specific responses to drought in 1990, 2003 and 2012 indicated P. sylvestris as more sensitive to drought whit higher demand for water supply in the optimal compared with P. nigra. Weak and unstable correlations in time with increasing/decreasing values in drought periods were obtained more accurately using delta O-18(TR) compared to delta C-13(TR). The species-specific resilience response to drought years showed a weak resilience and resistance in P. sylvestris occurred more evident after the 2012 event compared to less sensitive P. nigra trees. Decision-makers can use presented results to reinforce specific management plans capable of protecting and changing local compositions where is the case with species more resistant to drouth.

Automated summary

This study assessed the causes of declining Pinus sylvestris and Pinus nigra using various indicators. The results showed that there has been a significant increase in temperature and a decrease in precipitation since 1980. The basal area increment was the most accurate indicator of climate-growth relationship, and the delta C-13(TR) was negatively correlated with high temperatures. The species-specific response to drought indicated that P. sylvestris is more sensitive to drought compared to P. nigra.