The same growth trend can be seen for Silver fir and Scots pine for the moving correlation with both drought indices. Silver fir appears to have more climatic sensitivity to SPEI12 and PDSI values. On the other hand, Scots pine does not respond to the drought indices in general, suggesting that some other variables may be influencing its growth. According to Martínez-Vilalta et al. (2012), local climate can have a relatively minor effect on the radial growth response of Scots pine. Its growth response can be primarily determined by tree-level characteristics (age and previous growth rate), stand basal area and other tree species competition. The significant positive correlation between Silver fir growth and the drought indices suggests that during the first half of the 20th century this tree species was significantly influenced by the precipitation sum and consequently soil moisture availability. This was because of its increased growth when moisture availability was greater, and growth decline during drier conditions.
In the second half of the 20th century, both drought indices show a breakdown in the correlations, suggesting that something other than moisture conditions is influencing the tree growth response. Even though Silver fir was, in general, more responsive to climate data than Scots pine, in the period around the 1950s there was no significant response of Silver fir to the drought indices. The studies of Miś and Rączka (2002) and Feliksik and Wilczyński (2003) suggest that this growth decline was due to the atmospheric environmental pollution that
affected the Sudetes area in the second half of the 20th century. This may have contributed to the loss of climatic sensitivity in Silver fir.
The observed negative significant values for the moving correlation comparison between tree growth and temperature for Silver fir and Scots pine (in the first half of the 20th century) are suggesting that high temperatures during some months are causing a decrease in the tree growth, in opposite to lower ones (in both tree species). The positive significant values (in the second half of the 20th century) are suggesting that higher temperatures increase tree growth in opposite to lower temperatures in Silver fir (on March, April, June and September) and Scots pine (on March), respectively. The significant positive values for both species for the moving correlation comparison between tree growth and precipitation over the 20th century show that higher precipitation sum increases tree growth in opposite to lower precipitation sum in Silver fir (March, June, July and August) and Scots pine (June and July), respectively.
5. CONCLUSION
Regarding the methodologies used in this study, it is possible to conclude that the Spline detrending method was the best approach to discern disturbances noise and climate signal in studies of climate sensitivity in natural forests such as the one in Suchý vrch Nature Reserve.
However, each detrending method has its own merits and can show us different aspects of the relationship between growth and climate. The Palmer Drought Severity Index and the Standardized Precipitation Evapotranspiration Index showed also a similar trend for both tree species which indicates that both drought indices can be used quite equally for this kind of studies.
Although there are also some clear differences in the growth patterns of different tree species in certain time periods, the main difference between them was their growth response to climate fluctuations. The relationship with climate fluctuations was generally stronger for Silver fir than for Scots pine. Towards the end of the 20th century, the drought (or precipitation sum) response in Scots pine was more disrupted and lasted a longer time than the Silver fir response. This was perhaps also related to some physiological differences between these species. Also, the response to temperature is seemingly becoming more important during this period compared to the drought (or precipitation sum) response.
The growth response of both tree species to climate showed that moisture stress and low temperature do not affect the growth of these tree species alone, but also the atmospheric pollution likely play an important role in their growth responses. The breakdown in the relationship between drought index or precipitation sum with tree growth, later in the century is almost certainly due to the increased degree of air pollution loading at that time. This possibly has also affected at least partially the growth responses of trees to climatic conditions. In order to better understand the interactive effects of different climatic factors and air pollution on tree growth responses, additional research is still needed in the future.
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Appendix 1
Raw series of Abies alba (Silver fir) and Pinus sylvestris (Scots pine).
Appendix 2
Descriptive statistics for Silver fir and for Scots pine.
Silver fir
Scots pine
Appendix 3
Moving correlation between growth and temperature and precipitation sum, respectively, over the period of 1903 to 2010 for all months. The x-axis indicates the starting year of 30-year moving window with a one-year offset, and the y-axis represents Pearson’s correlation coefficient. Windows with significant correlations (p < 0.05) are displayed in large symbols.
An increase in the trend values indicates an increase in growth, a decrease in the trend values indicates a decrease in growth.