lntroduction 
ACTA BIOLOGICA SLOVENICA 
LJUBLJANA 2004 Vol. 47, Št. 1: 27-30 
Sprejeto (accepted): 2003-11-21 
Harmful impact of exceptional cold air outbreak in april 1997 
on silver fir in Croatia 
Danko DIMINIC1, Boris HRAŠOVECI , Milan GLAVAŠI , Nenad POTOČIC2 
1 University of Zagreb, Faculty of Forestry, P.O. Box 422, 10002 Zagreb, 
Croatia; e-mail: danko.diminic@zg.htnet.hr 
2 Forest Research Institute, Cvjetno naselje 41, 10450 Jastrebarsko, Croatia 
Abstract. After a severe outbreak of cold air in mid and late April in 1997, 
locally occurring damages on twigs and needles of silver fir were recorded in some 
forests in Croatia. Field examinations in the first half of July 1997 revealed the 
damages on one-year and older needles while fully developed twigs of current year 
growth were left intact. Southem and western slopes with fir growing forests rep-
resented typically affected sites. Further laboratory analyses confirmed the field 
observation that no harmful insect or pathogens were the cause of these damages. 
Spread of symptoms, expressed more heavily at lower elevations, indicated a prob-
able connection with recent exceptionally cold air outbreak in mid-April. 
Physiological disturbances caused by potassium deficiency are discussed in the 
light of specific occurrences of damage symptoms on silver fir trees. 
Key words: Abies alba, cold air damages, potassium. 
Abiotic damages on forest trees are well documented and known in forest science and practice 
for a long tirne (EDLIN & NIMM0 1956). Among the commonest are injuries caused by cold air and 
late or early frost (REIF & PAPP 1995). Very often, such damages occur on tree species not fully adapt-
ed to their artificial habitat (such as plantations) or plants growing in the border zones of their natu-
ra! area of spread. In the case of silver fir (Abies alba Mill.) cold air damages are not common but 
are documented and known in the literature (HARTMANN & al. 1988). This paper deals with the rare 
event of atypical damages on sil ver fir in natural forest stands of central Croatia in the spring of 1997. 
Change of needle colour and dieback of twigs and branches was recorded in a relatively isolated area 
of roughly 40 ha with no visible cues to known biotic agents. At the same tirne, it was the year of 
extraordinary atmospheric events and cold air outbreak which occurred during mid and late April and 
hit the whole country, especially the maritime and mountainous region (BRzovrc 1999). As with the 
other tree species, the nature of cold hardiness of silver fir is strongly correlated with potassium 
physiology as potassium is known to increase frost resistance. Potassium deficient plants are more 
prone to winter damage (BAULE & FRICKER 1970, MENGEL & KrRKBY 1987, BERGMANN 1992). 
28 Acta Biologica Slovenica, 47 (1 ), 2004 
Based on the <lata of Diagnostic and Prognostic Service of Forest Research Institute J astrebarsko, 
on reported atypical symptoms on silver fir trees in May 1997, in the north Vinodol area (NW 
Croatia), this research was initiated to find out the possible causes. Impacts of phenological, climat-
ic and physiological factors were analysed in relation with spatially and temporally occurring dam-
ages in the research area. 
Materials and methods 
Samples were collected and trees were analysed on July 10, 1997, at two locations Treskavac and 
Kalic in the north Vinodol area. Both sites are located at altitudes between 900 and 960 m, with incli-
nation from O and 10 °. The site Treskavac covers an area of 17 .11 ha, with various expositions, and 
the site Kalic covers an area of 19.92 ha, with SW exposition. Sampled fir trees were randomly 
selected, felled and debarked to find out the possible presence of defoliators, suckers or xylophages 
(expectedly, bark beetles) and pathogenic fungi (wood decay and root rot fungi). 
Samples of needles, twigs and branches, with observed visible symptoms, were collected from 
sampled trees and randomly selected young firs. Standard laboratory analyses were carried out to 
examine the presence of possible harmful insect fauna or pathogenic fungi. 
In the year 2000, the second study was conducted to determine the possible influence of nutri-
ent dynamics on cold hardiness of silver fir, 5 plots (each 100 x 100 m) were chosen in the region of 
Gorski kotar: Belevine - at altitude of 800 m, SW exposition; Kupjački vrh - at altitude of 1000 m, 
SW exposition; Leska S - at altitude of 697 m, NW exposition; Leska D - at altitude of 708 m, NW 
exposition. Aerial distances between these plots and sites Treskavac and Kalic are approximately 20 
km. The 5th plot Sljeme was chosen on Medvednica (near Zagreb), at altitude of 954 m, S exposi-
tion. Current and 1-year old needles from a total of 141 trees were sampled monthly from February 
to November. Samples (24-36 trees per location, representing a 30 % size sample of all dominant and 
co-dominant trees) were pooled by location into a composite sample. Samples were dried at 105 °C 
and ground. After wet digestion (H2SO4 / HC1O4) potassium was deterrnined through flame pho-
tometer Eppendorf (AOAC 1996). 
Climatic <lata for 1996 and 1997 were obtained from the State Hydrometeorological Institute Zagreb. 
Results 
Field examinations of affected silver firs revealed irregularly distributed dead needles and twigs 
resulting with reddish tint of some parts of the crowns or whole trees. In adult trees some branches, 
randomly positioned in the crowns, showed the abovementioned symptoms. A definitive pattem of 
damage symptoms was observed on smaller trees and fir saplings: significantly highest proportion 
of one-year old needles was affected (those developed a year before). In most cases there was also 
an inexistence of current growth on these shoots. All of these symptoms were recorded with greater 
intensity at lower elevations in the research area. 
Examination of sampled trees in the field obtained no visible attack of sap suckers, defoliators, 
secondary xylophages nor wood decay or root rot fungi. Laboratory analyses of samples confirmed 
the field results. There were no pathogenic fungi, neither insect pests as possible causal agents of 
described symptoms in sites Treskavac and Kalic. 
D. Diminic, B. Hrašovec, M. Glavaš, N. Potočic: Harmful impact of exceptional... 
1,50 
1,30 
1,10 
i 
~ 0,90 
"C 
0,70 
0,30 
II III IV v VI VII VIII IX X 
Months 
XI 
--=i--1-yr.old 
needles 
-"l!t.)lll•Current 
needles 
• • Limit 
value 
29 
Figure 1: Seasonal dynamics of potassium coacentrations in silver tir needles in the year 2000 (averaged 
values from five locations). Limit value (0.50 %) after BERGMANN (1992). 
Dynamics of potassium concentrations in needles revealed a drop of K+ concentration in older 
needles due to its redistribution into younger tissues (Figure 1). Translocation of K+ from one-year 
old needles ( developed in 1999) towards buds is apparent just before the budburst in April, resulting 
in strong potassium deficiency and high K+ concentrations in current-year needles. 
Discussion 
The hypothesis on biotic complex as causal agents for the new case of silver fir dieback, based on the 
aetiology of already recorded damages in the area, was rejected. In spite of the large amount of wind-felled 
trees and woody debris, detailed field analysis of newly affected trees and laboratory check-up could not 
confirm the presence of any significant biotic agent. Spatial distribution of damaged trees, as well as their 
age-class structure, suggest that some abiotic factor must have had been involved in this event. 
A period of exceptionally low temperatures in mid and late April 1997 (both distinct minima, 
April 161h and 21 th , were lower than two standard deviations of the 30-year average) served asa basis 
for case study by which BRzovrc ( 1999) explained the mechanisms of local modifications of larger 
scale atmospheric processes in the area of Northern Mediterranean. 
What we consider highly relevant for this study is (I) the fact that the outbreak of cold air in April 
was one of the strongest ever recorded and (II) the affected area is considered a border zone of nat-
urally growing fir because of the milder climate due to the strong maritime influence from the west. 
This conforms to the conclusions by FoRSTER & al. (1988) and FoRDERER & al. (1990) in the case of 
similar large scale damages recorded in Switzerland after the winter 1986/87. 
30 Acta Biologica Slovenica, 47 (!), 2004 
According to BERGMANN (1992) potassium deficient plants are more prone to winter damage. 
Older leaves (needles) first reveal symptoms of potassium deficiency (ANIC 1973). The dynamics of 
potassium in the silver fir needles, as shown in the second study, reveals the translocation of potas-
sium from one-year old needles towards buds that are becoming physiologically active, leading to 
potassium deficiency in older tissues. This drop of K+ in older needles just before budburst in April 
coincided in 1997 with the outbreak of exceptionally cold air in the area. 
Synthesis of ali available <lata, gathered in the area of affected trees and from the separate study 
analyzing the nutrient status and cold hardines of silver tree in the region, lead to a conclusion that 
it must have been a 1997 exceptional cold air outbreak that initiated such damage symptoms. This is 
further backed up by the fact that at higher elevations, hit by the same climatic phenomenon, were 
no damages recorded. It is reasonable to deduct that the phenological differences of fir trees in dif-
ferent altitudes acted as key factors in the distribution of damage symptoms in the wider area. 
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