The impact of Site Changes and Management Methods on Dieback of Common Spruce (Picea abies Karst.) in Croatia

Obična je smreka (Picea abies Karst.) vrsta koja prirodno uspijeva u visokom gorju u dolinama i mrazištima u kojima nema ozbiljnu konkurenciju drugih vrsta drveća, a u Europi se najviše koristila za osnivanje šumskih kultura. Obilježena je svojstvima pionirskih i prijelaznih vrsta drveća, osjetljiva je na ljetnu sušu, a tijekom razvoja na staništima stvara uvjete za razvoj klimatogenih šuma. Razvija se u regularnim i raznodobnim sastojinama, a u prebornim je manje zastupljena u omjeru smjese. Danas je izložena propadanju, tako da joj je postotak osutih krošanja u Hrvatskoj 59 %, i to u sastojinama koje se nalaze u najvišim debljinskim razredima, što upućuje na njihovu visoku dob i fiziološko slabljenje. Današnje sušenje uzrokuju velika suša, nepovoljna »kemijska klima« i kisele kiše te širenje na neodgovarajuća staništa uz izostanak nužne njege i obnove. Izmjerene su temperature u posljednjih 15 godina najviše u razdoblju od 150 godina otkada se mjere. Temeljem prosječnih godišnjih temperaturnih odstupanja za Europu od 1850. do 2009. godine uočavaju se dva hladna i tri topla razdoblja. Sve to smanjuje areal na kojem smreka ima dobre uvjete za razvoj. Obična je smreka značajna i vrlo upotrebljiva vrsta kojoj moramo u gospodarskim zahvatima uvažavati biološka svojstava, ekološke zahtjeve i gospodarske vrijednosti, što će joj osigurati veću stabilnost i bolji razvoj na pogodnim staništima i u sastojinama.Common spruce (Picea abies Karst.) is a tree species that occurs naturally in the valleys and other frost sites in high mountain regions, where it is free of serious competition of other tree species. Being resistant to frost, the winter colds of the continental and mountainous climate suit it. It is highly tolerant of winter chills of the continental and mountainous climate, it likes areas with abundant precipitation and snow in particular and avoids those with little precipitation and low relative air humidity. In Croatia, it naturally occurs in high mountain areas of Gorski Kotar, Velebit and other parts of Lika, and especially in frost spots where it is free of competition with other tree species. In Europe, spruce is a favourite tree species for the establishment of forest cultures. It has the properties of both pioneer and transitional tree species. It is highly sensitive to summer droughts. During its growth, in its habitats it creates the conditions for the development of tree species of a climatogenous forest. It develops in regular and uneven-aged stands, while its percentage in selection stands with stable structural and stand conditions is lower. The total growing stock amounts to 13,200,000 m³, which accounts for 5.57 m³/ha of the forested area in Croatia. Its volume share in the highest age and diameter classes is 80%, which indicates its advanced age and physiological weakening. Regular and uneven-aged spruce forests of high silvicultural form cover an area of 6,300 ha, of which spruce accounts for 52% in the composition mixture, silver fir for 35%, common beech for 11%, and other species for 2%. Spruce is currently significantly affected by dieback; at the level of Croatia, the percentage of defoliated crowns is 59.3%, while in the Gorski Kotar area alone it is 96.2%. Droughts are one of the most important factors contributing to the physiological weakening and dieback of spruce in Croatia, while in Europe they are the major cause of dieback of catastrophic proportions. Temperatures measured in the past 15 years are the highest in the 150-year period, with a linear trend of temperature increase amounting to 0.74 °C. There is controversy as to the causes of droughts: according to many, they are the consequence of global warming, whereas others disagree. Data are given (Fig. 1) on climate change from the past warm period, over the cold one, to the present warm period. Based on average annual temperature deviations for Europe from 1850 to 2009 (Fig. 2), the current warm period has had two cold and three warm sub-periods, which contradicts the frequently emphasised negative impact of man’s activity on climate. Apart from extreme droughts, another cause of spruce dieback is attributed to the impact of the unfavourable »chemical climate« and acid rains arriving most probably from the industrially developed western countries. This is the reason why the lead content in the soils of Gorski Kotar reaches the extreme values of 71 to 128 mg/kg. Accordingly, the unfavourable »chemical climate« and acid rains are the second most important reason for the dieback of not only spruce but other tree species as well (fir, beech). The third cause of dieback is the artificial and natural expansion of spruce into unsuitable sites, coupled with the absence of timely tending and regeneration procedures in natural and artificially established forest cultures. Spruce cannot be managed with the selection method because it is a pioneer species, which does not regenerate in climatogenic and coherently forested sites. It creates the conditions for the return of fir and beech and simultaneously disappears from such stands. Its pioneer properties enable it to expand naturally in all the empty spaces within a broad complex of beech-fir forests in particular, as well as in deforested and degraded forest areas, frost sites, abandoned pastures, meadows, degraded forests such as thickets and scrub and in other areas at higher altitudes. Most of these sites are unsuitable for spruce, causing it to physiologically weaken and decline. Methods of common spruce management used to date have in general not been adjusted to its biological properties and ecological requirements or to the newly created site conditions. In our opinion, in addition to dry periods and unfavourable impact of »chemical climate«, management methods also represent an important cause of dieback. When all the three causes take place simultaneously, the ensuing result is catastrophic dieback. Low intensity dieback occurring in every stand, including a spruce stand, is a normal phenomenon characteristic of management with natural forests and forest cultures. It is usually the consequence of competition between certain species, particularly because tending operations such as cleaning and thinning are not performed, or it is the reaction to minor local site changes. Spruce is the desirable »third species« in selection forests of fir and beech, but with a lower percentage share in the composition mix. It creates the conditions for the return of fir and beech to the sites from which they have disappeared for different reasons. With the return of these species, the spruce gradually undergoes physiological weakening and should generally be removed from the composition mix of selection stands by means of tending and regeneration procedures. Spruce is a tree species whose stands have been subjected either to very little tending or none at all. In regular and uneven-aged stands, regeneration procedures have generally been postponed, while in selection stands spruce has been treated like fir in the selection of trees for cutting. All this has led to the currently unfavourable status of spruce: the spruce is old, over-mature, physiologically weak and prone to dieback in all stand forms. In view of the newly created unfavourable climate conditions, as well as air, water and soil pollution, common spruce should not be expanded into inadequate sites beyond its natural distribution range. If established in southern expositions and on stony and shallow soils, it is subject to premature dieback, so timely measures should be taken to replace it with another species that tolerates such conditions. Common spruce is an important, noble and highly usable species and it should not be neglected in future management. Better knowledge should be acquired of its biological properties, ecological requirements and commercial value. These silvicultural-management properties should be fully observed in the application of management procedures