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Precipitation – Oborine
Seasonal changes in precipitation quantities often lead to physiological stress, changes in the phenology, and reduced increment; however, the impact predominantly depends on trends in changed precipitation quantities through the seasons of the year (Zeppel et al. 2014). Unlike temperature sums, in this investigation precipitation proved to be much more important for budburst date modelling. The importance of precipitation was tested in rain forest and mountain forest ecosystems (Huete et al 2006; Suresh and Sukumar 2011; Grogan and Schulze 2012). Using the example of narrow-leaved ash as one of the representatives of floodplain forest sites in Europe, it can be concluded that it is precisely site ecology that partially attributes importance to precipitation. In forest and grassland ecosystems, changed precipitation quantities in summer and spring periods have a much higher impact on vegetation than changes in autumn and winter periods (Zeppel et al. 2014).
Insolation – Insolacija
In some parts of tropical ecosystems, insolation represents an important factor for phenological modelling (Huete et al. 2006; Asner and Alencar 2010). Fu et al. (2015) points out that neither photoperiod nor insolation should be omitted or considered a less important factor in modelling spring phenological changes. In this investigation, in relation to temperature and precipitation, insolation showed considerable deviations in all the performed analyses. In terms of its light requirement during growth and development, narrow-leaved ash is ranked among heliophilic forest tree species, which allows us to draw a parallel between phenological manifestations and insolation.
We can assume that in heliophilic species and species of floodplain forest sites, precipitation and insolation are the principal activating factors of phenological manifestations, while in species of xerothermal sites this role is played mostly by temperature (Pinto et al. 2011).
CONCLUSIONS
ZAKLJUČCI
Between three investigated meteorological factors (temperature, precipitation, insolation), insolation summed from 1st November (INS_NOV) to the onset of budburst (rs=0.712) had the highest influence on budburst in narrow-leaved ash.
According to the results of linear regression analysis, insolation also digressed from temperatures and precipitation. The highest R2 was recorded in insolation sums, also from 1st November (INS_NOV), where R2 amounted to 0.590.
Using multivariate regression analysis ranked by the R2 criterion, the highest value is obtained if all the studied variables are included, except insolation (INS_DEC) summed from 1st December.
Multivariate regression analysis with stepwise method and step function according to the AIC criterion showed that Model 5 was the best (AIC=173.74; BUD_BURST ~ INS_NOV + TEMP_NOV + INS_DEC + PREC_JAN + TEMP_DEC).
ACKNOWLEDGMENTS
ZAHVALE
This work was supported by the Croatian Science Foundation through the research project „The role of biotic agents on the vitality of narrow-leaved ash (Fraxinus angustifolia Vahl) in Croatian floodplain forests” FRAXINPRO [IP-11-2013].The authors gratefully acknowledge the generous assistance of the employees of Nova Gradiška Forest Office in field measurements.
REFERENCES
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