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ŠUMARSKI LIST 7-8/2020 str. 30     <-- 30 -->        PDF

(Polák et al., 2006). This can be seen clearly from our results.
The negative influence of precipitation in June of the previous year, coupled with the aforementioned influence of summer temperatures in the previous year, may point to the lasting effects of drought on defoliation through divergent mechanisms. The example for this is the uninterrupted rise in defoliation from the dry 2000 until 2004 - the years 2001 and 2002 were warm, but with plenty of precipitation; still, the defoliation continued to rise through the dry 2003, reaching peak values in the cool, moist 2004. Similarly, (Králíček et al., 2017) documented significant effect of temperature in July and August of previous year on beech defoliation. Severe drought limits leaf area production by reducing the number and viability of leaf buds and thus the tree’s ability to recover an efficient crown development after resuming normal water availability (Bréda et al., 2006). Drought during the year of bud formation decreases the number of new leaves formed in the bud and the new stem segments present. Drought then influences the number of leaves, leaf surface area, and twig extension the following year when those buds expand (Coder and Daniel, 1999). In beech trees, all leaves are completely preformed in winter buds, so the number of leaves is predetermined in the preceding year (Uemura et al., 2000). On the other hand, defoliation can be related to drought events of the current year through a change in intensity of physiological processes such as (slower) leaf expansion or (enhanced) senescence of older leaves (Jackson, 1997), but our results do not show this effect: neither SPI nor scPDSI correlated with defoliation in the current year. This may be because the assessments of defoliation were performed during summer, when the effects of drought may have not occurred yet.
We found that drought affects mostly the foliar concentrations of Ca and Mg, and the ratio of K to Ca. According to Bergmann (1992), the uptake of Ca is negatively affected by irregular water supply and, in particular, by prolonged dry periods. Acting as a counterpart to K, Ca plays a key role in the stomatal movement and regulation of water balance of trees (Raghavendra et al., 2010). Ca and K are also competing for uptake and the lack of Ca in dry years can often be associated with enhanced K uptake (Wallace and Mueller, 1980).
While foliar composition is directly related to weather conditions in the current year through the functioning of uptake mechanisms, the links of current mineral element foliar concentrations and last year’s weather conditions have to be considered through storage and remobilization mechanisms which are increasingly recognized as one of the key processes in nutrient conservation in plants and in nutrient cycling in ecosystems (Achat et al., 2018). To maintain growth under a permanently fluctuating availability of soil nutrients, plants use various strategies to optimize nutrient acquisition - nutrient transporters, soil exploration by roots, root exudation, and remobilization of nutrients from storage (Maillard et al., 2015). For mobile nutrients, especially N, remobilization from reserves is very important, as shown in the multiple dependencies of N foliar concentrations on the climate variables of the previous year late summer and autumn months, reflecting the storing of N after the period of intensive vegetative growth is finished (Table 5). In case of elements that are generally not remobilized, such as Ca, we should not consider the effects of storage and remobilization processes: rather, as the growth of roots is dependent partly on the Ca availability (Emanuelsson, 1984), climate conditions in the previous year may modulate the uptake capacity of trees in the current year. However, this effect was not recorded in our study. Also we found no correlations of previous year climate variables and current year P leaf concentrations, although P can be used from reserves stored in the root (Marschner, 2002). Perhaps this is due to the generally very good P nutrition of beech on this site.
Jonard et al. (2010) found that beech defoliation levels were associated with lower foliar Ca and Mg concentrations in a study in Belgian Ardennes, but this relation could not be confirmed in our study: no significant relationship was observed between defoliation and foliar nutrient concentrations. According to Simon and Wild (1998), if the concentration of a certain element remains in the normal range, the decrease in mineral nutrition should be regarded more as a consequence than as the cause of damage. If, on the other hand, the concentrations are inadequate, we can suspect nutrition to be the cause of tree decline. Therefore, the lack of P, Ca and Mg caused by drought in year 2003, may have resulted in enhanced defoliation in the following year (i.e. the lag effect on defoliation).
The growth of trees is a key ecological parameter of forests and thus of high importance as an indicator of forest condition (Dobbertin et al., 2013). Trees generally respond to environmental stresses by increment decrease as a consequence of a reduced photosynthetic activity and altered carbon allocation. Most tree ring studies have observed that trees predisposed to die have lower mean growth rates or greater growth sensitivity to climate in the years proceeding mortality (McDowell et al., 2008). It is clear from our analysis that May average, minimum or maximum monthly temperature plays an important role in the tree-ring formation of the beech. May is critical month for the growth of beech on Medvednica massif, in particular for beech that grows above 800 m a.s.l. Above average temperature in May has a positive influence on tree growth as well as on the tree phenology (Tikvić et al., 2006), this was observed not only at Medvednica massif, but also in Slovenia (Prislan et al., 2019, Čufar et al., 2008) Bosnia and Herzegovina