Significant increase in natural disturbance impacts on European forests since 1950

Over the last decades, the natural disturbance is increasingly putting pressure on European forests. Shifts in disturbance regimes may compromise forest functioning and the continuous provisioning of ecosystem services to society, including their climate change mitigation potential. Although forests are central to many European policies, we lack the long-term empirical data needed for thoroughly understanding disturbance dynamics, modeling them, and developing adaptive management strategies. Here, we present a unique database of >170,000 records of ground-based natural disturbance observations in European forests from 1950 to 2019. Reported data confirm a significant increase in forest disturbance in 34 European countries, causing on an average of 43.8 million m3 of disturbed timber volume per year over the 70-year study period. This value is likely a conservative estimate due to under-reporting, especially of small-scale disturbances. We used machine learning techniques for assessing the magnitude of unreported disturbances, which are estimated to be between 8.6 and 18.3 million m3/year. In the last 20 years, disturbances on average accounted for 16% of the mean annual harvest in Europe. Wind was the most important disturbance agent over the study period (46% of total damage), followed by fire (24%) and bark beetles (17%). Bark beetle disturbance doubled its share of the total damage in the last 20 years. Forest disturbances can profoundly impact ecosystem services (e.g., climate change mitigation), affect regional forest resource provisioning and consequently disrupt long-term management planning objectives and timber markets. We conclude that adaptation to changing disturbance regimes must be placed at the core of the European forest management and policy debate. Furthermore, a coherent and homogeneous monitoring system of natural disturbances is urgently needed in Europe, to better observe and respond to the ongoing changes in forest disturbance regimes

Presoja varovalnega učinka gozda pred drobirskimi tokovi ob Savi Bohinjki v Soteski

Varovalni gozdovi imajo pomembno vlogo pri zmanjševanju učinka različnih naravnih nevarnosti. V Sloveniji imamo 10 % varovalnih gozdov, 30 % teh gozdovima zaščitno funkcijo. Preučili smo učinkovitost varovalnih gozdov na severozahodu Slovenije, v Soteski med Bledom in Bohinjem, kjer sta ogroženi državna cesta in železnica. Na podlagi geološke karte in karte podvrženosti drobirskim tokovom smo ugotovili vplivno območje drobirskih tokov. Pri modeliranju njihovega širjenja smo uporabili program TopRunDF. Podatke o gozdu smo zbrali na 47 vzorčnih ploskvah, kjer smo izmerili vsa živa drevesa s prsnim premerom ? 10 cm. Podrobno smo opisali sestoje in jih ovrednotili po metodi NaiS. Ugotovili smo, da ima gozd ključno vlogo pri zaščiti infrastrukturnih objektov. Za trajno zaščitno vlogo v enomernih sestojih je potrebno gojenje mozaično raznomernega gozda. V sestojih, kjer gozdnogojitveni ukrepi ne zadostujejo, je potrebno uporabiti tehnične ukrepe. Ker s sestoji niso gospodarili več desetletij so motnje (najpogosteje v obliki vetrolomov) pogoste. Izsledki kažejo, da je potrebno ovrednotenje varovalne in zaščitne funkcije teh gozdov. Priporočamo tudi aktivno nego, kar je v nasprotju z dosedanjo prakso negospodarjenja teh gozdov.Protection forests have an important role of mitigating the influence of various natural hazards. Only 10% of all forests in Slovenia have an indirect protection function, while 30% of them perform a direct protection role. Studyof protection efficiency of beech dominated forests in the Soteska gorge in NW Slovenia, where a main state road and railway are endangered was done. We assessed the starting impacts of the debris-flow natural hazard based on a small-scale geological survey of the terrain characteristics and local debris flow susceptibillity map. For determination of the run-out zones we used the TopRunDF model. Forest structure data was obtained from 47 sample plots where all trees with DBH ? 10 cm were measured. A detailed description and delineation of forest stands was performed. Results showed that the forests stands play a crucial role in protection of infrastructural objects. For long-term protection efficiency, spatially-explicit regeneration patches are needed in uniform forest stands. In areas where silvicultural measures could not provide sufficient protection, technical measures are needed. Since these forests have not been managed for several decades, natural disturbances are frequent. Research findings suggest that assessment and management of these beech dominated protection forests is necessary, contrary to the current practice of non-management in protection forests in Slovenia

Post-disturbance treatments and stand restoration success in beech forest sites

Doktorska disertacija je sestavljena iz treh sklopov. V prvem sklopu smo proučili vplive tehnične sanacije (posek in spravilo) na obnovo vegetacije po ujmah manjših do srednjih jakosti v mešanih, pretežno bukovih sestojih na osmih raziskovalnih objektih po Sloveniji, kjer je bila na delu površine opravljena tehnična sanacija, medtem ko preostali del ni bil saniran. Raziskava je pokazala, da med načinoma dela ni bilo bistvenih razlik v gostotah, višinski strukturi in hitrosti rasti mladja. Majhne razlike nakazujejo, da je na predelih brez tehnične sanacije nekoliko več bolj razvitega oz. višjega mladja, ki se je verjetno ohranilo iz prejšnjega sestoja, medtem ko je na saniranih predelih nekoliko več manj razvitega oz. nižjega mladja svetloljubnih drevesnih vrst, ki je lahko vraščalo zaradi odstranjenih drevesnih ostankov in majhnih poškodb tal pri spravilu. Tehnična sanacija po ujmah majhnih do srednjih jakosti v mešanih, pretežno bukovih sestojih ni imela negativnih vplivov na obnovo vegetacije. V drugem sklopu smo proučili razlike med naravno in umetno obnovo na treh raziskovalnih območjih, ki jih je leta 2008 prizadel vetrolom. Osnovali smo ploskve na mestih naravne obnove, kjer smo popisovali dominantne osebke naravnega mladja, na mestih umetne obnove pa sadike. Znotraj obeh tipov ploskev smo postavili ploskvice za analizo vegetacije, kjer smo popisali pokrovnost zeliščne vegetacije in gostote naravnega mladja. Rezultati kažejo, da je bila naravna obnova različno uspešna. Na območju Trnovski gozd je bila pomanjkljiva zaradi zaostrenih ekoloških razmer, na zgornjem delu Črnivca zaradi goste travne ruše vijugave masnice, na Bohorju pa je bila uspešna. Gostote naravnega pomladka so bile na ploskvah z umetno obnovo od tri- do štirikrat nižje kot na ploskvah naravne obnove, kar je verjetno posledica obžetev sadik, s čimer se verjetno odstrani samoniklo mladje. V tretjem sklopu smo proučili dolgotrajno drugotno sukcesijo po požaru na Mozirski Požganiji. Leta 2013 smo ponovili meritve stalnih vzorčnih ploskev, ki so bile prvič premerjene leta 1981. Izsledki kažejo, da so na preživetje dreves ključno vplivale drevesna vrsta, višina, drevesna plast in globina krošnje. Smreka je imela najvišjo stopnjo preživetja, sledijo breza, trepetlika in iva. Visok začetni delež smreke in povečevanje njenega deleža kažeta na pionirske lastnosti smreke, ki v začetnem obdobju brez težav tekmuje z drugimi pionirji, kasneje pa se uveljavi kot poznosukcesijska vrsta, ko se pionirji že umikajo iz sestojev.This dissertation comprises three studies. In the first study we examined the influence of salvage logging on forest regeneration following small to medium-scale moderate severity disturbances in eight mixed beech (Fagus sylvatica L.) dominated forest stands in Slovenia where part of the disturbed area was salvaged and the adjacent part was not salvaged. Results show that there were no significant differences in terms of densities, height structure and regeneration growth between treatments. Minor differences indicate that the non-salvaged sites tended to have a slightly more well-developed (taller) regeneration layer that remained from the previous stand and was not damaged during harvesting, while salvaged sites had a larger proportion of shade intolerant tree species due to the removal of damaged timber, which created minor soil disturbances and space for easier recruitment of regeneration. The results suggest that salvage logging following small-scale intermediate severity disturbances may not hinder forest recovery in mixed beech dominated forests. In the second study we analyzed the differences between planted and natural regeneration on three research areas disturbed by storms in 2008. We established sampling plots of natural regeneration where the dominant trees of the natural regeneration were measured and sampling plots with artificial regeneration where all planted seedlings were measured. Within both plot types, smaller vegetation subplots were established where the cover of each plant species was estimated and natural regeneration was counted. Results show that the success of the natural regeneration varied between research areas and study sites. Natural regeneration densities in Trnovski gozd were not sufficient due to the more extreme ecological conditions, while at the Bohor area the densities were high. On the upper slope of Črnivec, the natural regeneration was inhibited by thick grass of Deschampsia flexuosa (L.) Trin. Average densities of natural regeneration were 3-4 times higher on the plots with natural regeneration, which may be due to collateral damage to natural regeneration during ground vegetation removal on planted sites. In the third part of the dissertation, we studied secondary succession after forest fire at Mozirska Požganija. In 2013 we remeasured the plots established and inventoried in 1981. Results indicate that tree species, height, layer and crown depth positively influenced tree survival. Spruce showed the highest survival probability, followed by Betula pendula (Roth.), Populus tremula (L.) and Salix caprea (L.). The high initial proportion of spruce and its increasing share indicates the pioneer characteristics of a species that can easily compete with pioneers and later establish as a late successional species after pioneers retreat from the stands

Comparison of natural and artificial regeneration after windthrows in 2008

Primerjali smo uspeh naravne in umetne obnove po treh vetrolomih v letu 2008. Na vsakem izmed raziskovalnih območij (Črnivec, Trnovski gozd, Bohor) smo leta 2012 zakoličili in analizirali ploskve z naravno in umetno obnovo ter jih ponovno premerili leta 2014. Znotraj ploskev smo zakoličili manjše vegetacijske ploskvice, na katerih smo popisali zeliščno plast in gostote mladja. Povprečna gostota samoniklega naravnega mladja šest let po ujmi je znašala 14.000 osebkov na hektar. Samoniklo mladje je bilo prostorsko neenakomerno porazdeljeno, kar nakazuje razlike med mikrorastišči. Ugotovili smo slabši uspeh obnove na južnih pobočjih, mestih, ki so bolj oddaljena od semenskih dreves in gozdnega roba, mikrorastiščih brez motenj oz. posebnosti in na večjih nadmorskih višinah. Na takih rastiščih je saditev smiselna in upravičena. Gostote naravnega samoniklega mladja so bile v pozitivni povezavi, preživetje naravnega dominantnega mladja in sadik pa v negativni s pokrovnostjo zeliščne plasti, prav tako je na preživetje negativno vplivalo objedanje. Višina osebka je bila najbolj značilen dejavnik za napovedovanje preživetja. Stopnja preživetja sadik in dominantnega mladja smreke je bila visoka, medtem ko sta sencozdržni bukev in jelka imeli višjo stopnjo mortalitete. Raziskava potrjuje velik potencial samoniklega naravnega mladja na območjih saditev, ki ga lahko z gojitvenimi deli zaviramo. Navedene so gozdnogojitvene usmeritve za ravnanje po vetrolomih.We studied regeneration dynamics in forests disturbed by three windthrows in 2008 to assess the success of natural regeneration vs planting. On each of the study sites (Črnivec, Trnovski gozd, Bohor) plots of dominant naturally regenerated saplings and plots with planted saplings were selected and measured in 2012 and 2014. Herbs and seedling establishment were studied in two subplots within each plot. Average seedling density six years after windthrow was 14,000 per ha. Seedlings were irregularly distributed, which suggests micro-site differences in regeneration dynamics. Results indicated lower seedling success on sites that were southerly exposed, farther from the forest edge and seed trees, on undisturbed micro-sites and at higher altitudes. On such sites planting was justified. While seedling density indicated a positive relation to herb coverage, sapling survival was negatively associated with it as well as with browsing. Sapling initial height was the strongest predictor of their survival. Spruce saplings had the lowest mortality, while shade-tolerant silver fir and beech experienced high mortality. The study revealed the high potential of naturally regenerated seedlings within young plantations, which can be reduced by silvicultural measures. Guidance for the improvement of silvicultural operations for post windthrow restoration is given

Challenges for uneven-aged silviculture in restoration of post-disturbance forests in Central Europe

Forest managers are often required to restore forest stands following natural disturbances, a situation that may become more common and more challenging under global change. In parts of Central Europe, particularly in mountain regions dominated by mixed temperate forests, the use of relatively low intensity, uneven-aged silviculture is a common management approach. Because this type of management is based on mimicking less intense disturbances, the restoration of more severe disturbance patches within forested landscapes has received little attention. The goal of this paper is to synthesize research on the restoration of forests damaged by disturbances in temperate forests of Slovenia and neighbouring regions of Central Europe, where uneven-aged silviculture is practiced. Research indicates that active management aimed at favouring mixed uneven-aged forest reduces the risk of disturbance and improves the resilience of stands. Salvage logging may have positive or negative effects on regeneration, much of which is due to the method applied and the quality of work. The most prominent factors that negatively affect restoration are: lack of advanced regeneration and decomposed woody debris, high altitude, steep slopes, dense ground vegetation, and overbrowsing. Planting or sowing should be applied in post-disturbance forests where many negative factors interact and where a high demand for sustainability of forest ecosystem services is present

Drivers of regeneration dynamics following salvage logging and different silvicultural treatments in windthrow areas in Slovenia

We studied regeneration dynamics in forests disturbed by three different windthrow events in 2008 in Slovenia to assess the success of natural regeneration vs planting and subsequent silvicultural treatments. Fifty-three plots with planted saplings of Norway spruce and sycamore maple and 50 plots with dominant naturally regenerated saplings were selected in a randomly stratified manner. Sapling characteristics (height, height increment, root collar diameter, free-to-grow score, browsing and micro-site) were assessed in 2012 and 2014. Herbs and seedling establishment were studied in two subplots within each plot. Average seedling density was 13,074 and 14,674 ha%1 four and six years after the windthrows, respectively. Seedlings were irregularly distributed, which suggests micro-site differences in regeneration dynamics. The results indicated lower seedling success on sites that were southerly exposed, farther from the forest edge and seed trees, on undisturbed micro-sites and at higher altitudes. On such sites planting was justified. While seedling density indicated a positive relation to herb coverage, sapling survival was negatively associated with it as well as with browsing. Initial sapling height was the strongest predictor of survival. Spruce saplings had the lowest mortality, while shade-tolerant silver fir and beech experienced high mortality. The study indicated a high density and species diversity of naturally regenerated seedlings within windthrow areas, which was reduced by planting, mowing of competing vegetation and protection measures favouring planted saplings. Recommendations for the improvement of silvicultural operations for post windthrow restoration are given

Conversion of Pinus nigra plantations with natural regeneration in the Slovenian Karst

Since the mid-19th century, Pinus nigra plantations have played a key role in the restoration of degraded European landscapes. Nowadays, these plantations are aging and prone to natural disturbances, insect infestations, and diseases. For their successful gradual conversion, knowledge of optimal gap spatiotemporal dynamics is crucial. We studied herb and natural regeneration patterns along with site factors on 477 subplots within 44 plots distributed over four stand types: closed stand (14% diffuse light), open stand (21%), gap edge (23%), and gap (57%). Despite the abundant Quercus petraea, Q. cerris, and Q. pubescens mast year, no one-year seedlings were recorded, which is likely due to the summer drought. Short seedlings (h < 20 cm) of Quercus sp., Fraxinus ornus and Ostrya carpinifolia were more successful within closed stands. Short Quercus seedlings were positively associated with soil depth and negatively associated with soil nutrients, distance to seed trees, and Sesleria autumnalis coverage. Taller Quercus seedlings required more light than both of its strongest competitors and were positively related to humid soils and less rocky sites. Ungulate overbrowsing significantly impeded natural regeneration. The results indicate a satisfactory Quercus density for conversion and the importance of advanced regeneration, which should be gradually, but persistently, released by progressively widening gaps