Igor Drobyshev

Influence of annual weather on growth of pedunculate oak in southern Sweden

Abstract• A network of oak (Quercus robur L.) chronologies containing 49 sites and 635 single trees was analysed to identify weather variables affecting annual tree-ring increment dynamics in southern Sweden during 1860–2000.• We analysed (1) the growth response of oak to non-extreme weather, and (2) the temporal and spatial patterns of regional growth anomalies (pointer years) and associated climatic extremes resolved on a monthly scale.• Growth was controlled by precipitation in the current (June–July) and the previous growing season (August) in 48% and 22% of all sites, respectively. Temperature during July of the current year and August of the previous year was negatively correlated with growth in 29% and 43% of the sites, respectively. Growth was positively correlated with temperature in October of the previous season in 72% of the sites. The most extensive growth anomaly occurred in 1965 and was probably caused by intrusion of cold Arctic air masses into the region at the end of March that year.• During climatically non-extreme years, oak growth is driven mostly by the dynamics of summer precipitation. Many of the negative growth anomalies, however, were associated with temperature extremes. Southern Swedish oak pointer years tend not to coincide with the pan-European oak pointer years.Résumé• Une série de chronologies de chênes (Quercus robur L.) comprenant 49 stations et 635 arbres individuels a été analysée pour identifier les variables climatiques affectant la dynamique de croissance annuelle des cernes dans le Sud de la Suède, pendant la période 1860–2000.• Nous avons analysé (1) la réponse de croissance des chênes aux événements météorologiques non extrêmes et (2) les patrons temporels et spatiaux des anomalies régionales de croissance (années indices) et les extrêmes climatiques associés au pas de temps mensuel.• La croissance était contrôlée par les précipitations de la saison de végétation (juin–juillet) et les précipitations de la saison de végétation précédente (août), respectivement dans 48 % et 22 % de toutes les stations. Les températures du mois de juillet de la saison de végétation et du mois d’août de l’année précédente étaient corrélées négativement avec la croissance, respectivement dans 29 % et 43 % des stations. La croissance était corrélée positivement avec la température du mois d’octobre de l’année précédente dans 72 % des stations. L’anomalie de croissance la plus considérable est arrivée en 1965 et a été causée probablement par l’intrusion de masses d’air froid arctique dans la région, à la fin du mois de mars.• Pendant les années climatiquement non-extrêmes, la croissance du chêne est principalement commandée par la dynamique des précipitations estivales. Cependant, beaucoup d’anomalies négatives de croissance ont été associées avec des extrêmes de température. Les années indices du Sud de la Suède ont tendance a ne pas coïncider avec les années indices pan-européennes.

Increased openness around retained oaks increases species richness of saproxylic beetles

The decrease of old deciduous trees in northern Europe is a threat to the saproxylic fauna. In northern Europe, oak sustains the highest richness of saproxylic invertebrates, among which beetles is a large group. In order to preserve species associated with old trees, it has become common practice in commercial forestry to retain such trees at final felling. However, to create beneficial conditions for species associated with retained trees, the surrounding plantation has to be managed with regard to their specific demands. In the case of oak-associated species, including many red-listed species, several studies have shown that light is an important factor. The aim of this study was to analyze the effects of increased openness around oaks (Quercus robur) in spruce plantations (Picea abies) on species richness and abundance of oak-associated saproxylic beetles. The study was performed in nine spruce plantations located in southern Sweden, with mature oaks standing in a gradient of canopy openness. Beetles were collected from 54 oaks from May to September during two seasons, using window traps. The analyses revealed that increased openness around oaks increases species richness and abundance of oak-associated beetles. By including insolation angle in the analysis, we found that it is mainly the degree of openness directed south from the oak that has positive effects on beetle richness. These findings imply that it is desirable to maintain an open area around retained oaks, and that this area should be placed on the southern side of the oak to maximize the biodiversity benefit in relation to costs for the forest owner.

Effect of natural disturbances on the abundance of Norway spruce (Picea abies (L.) Karst.) regeneration in nemoral forests of the southern boreal zone

The impact of natural disturbances on the canopy (trees 14 m high) and sapling stratum (>0.3 and 14 m high) composition was studied in nemoral old-growth forests located within the southern boreal zone in Central Russia (Central Forest Reserve, 32829 0 ‐33801 0 E, 56826 0 ‐56831 0 N). I hypothesized that the current disturbance regime does not allow the maintenance of current spruce abundance in the canopy, and, as a result, there is a continuous shift in the canopy composition towards a greater abundance of deciduous species. Three 30020 m 2 transects were established to estimate the proportions of stand under non-closed unexpanded canopy gaps. Data on sapling composition of 49 canopy gaps were used to analyze pattern of gap refuting in these forests. Additionally, data from three forest inventories showed changes in canopy composition over a period from 1972 to 1990. The current status of nemoral forests is characterized by the high proportion of stand area under treefall gaps (71%). The loss of spruce from the canopy caused by treefalls (53% of the total basal area of gap-makers) was slightly greater than its canopy abundance (45%). Canopy gaps of all sizes encouraged spruce regeneration which might be due to a decrease in sapling mortality and/or more active recruitment of spruce seedlings. After a gap was formed, the presence of spruce in sapling strata increased. However, within both small ( 200 m 2 ) gaps, tall (>6 m) spruce saplings did not reach the level of its abundance in the tree canopy. In gaps, tall (>6 m) saplings of lime (Tilia cordata) and elm (Ulmus glabra) grew more quickly than those of spruce and maple. These data suggested a decrease in canopy spruce and an increase in deciduous species in the near future which supported the original hypothesis. Analysis of forest inventory records revealed similar changes in the canopy structure over the past two decades. However, the observed high proportion of stand area under gaps implies that for the next few decades large areas of nemoral communities will be occupied by relatively young stands. This may, in turn, decrease the frequency of large-scale treefalls revegetated mainly by deciduous saplings. # 2001 Elsevier Science B.V. All rights reserved.

Lifespan and mortality of old oaks – combining empirical and modelling approaches to support their management in Southern Sweden

Abstract• Old oaks (Quercus robur L.) play an important role in the southern Scandinavian landscape by providing habitat for a wide range of species, a large proportion of them being currently on the National Redlists.• To provide support for the management of these trees, we review data on oak mortality and formulate a mortality-driven stochastic model analysing interactions between mortality rate, oak recruitment rate into 100–150 age class, and amount of oaks older than 200 years.• Empirical annual mortality rates varied between 0 and 13% with average 1.68%. Trees older 200 years had an average mortality rate of 1.1%. Oaks in the high density forests showed higher mortality (3.2%) as compared to the trees growing in the low density forests (1.2%). A 400-year long modelling exercises indicated that under current mortality rates (regular mortality being centred around 1% annually; and irregular mortality 7% with average return time of 13 years) the long-term maintenance of 20 trees older than 200 years per ha would require an input rate of 1 to 5 trees × year−1 × ha−1 into the 100–150 years old class.• The modelling highlighted the importance of initial oak abundance affecting amount of old trees at the end of shorter (100 years) simulation period.Résumé• Les vieux chênes (Quercus robur L.) jouent un rôle important dans le paysage du sud de la Suède en procurant un habitat pour un large éventail d’espèces, une grande proportion d’entre elles étant actuellement dans les Listes rouges nationales.• Pour appuyer la gestion de ces arbres, nous avons examiné les données relatives à la mortalité des chênes et formulons un modèle stochastique de mortalité analysant les interactions entre taux de mortalité, taux de recrutement des chênes dans les classes 100–150 ans et total des chênes plus vieux que 200 ans.• Les taux empiriques de mortalité ont varié entre 0 et 13 % avec une moyenne de 1,68 %. Les arbres de plus de 200 ans présentaient un taux de mortalité de 1,1 %. Dans les plus fortes densités forestières les chênes montraient une mortalité plus élevée (3,2 %) comparativement aux arbres poussant dans des forêts de densité plus faible (1,2 %). Des exercices de modélisation sur 400 ans ont indiqué qu’en dessous d’un taux courant de mortalité (mortalité courante annelle centrée autour de 1 % et mortalité irrégulière de 7 % avec un temps de retour de 13 ans) la maintenance à long terme de 20 vieux arbres par hectare demanderait un taux d’apport de 1 à 5 arbres par an et par hectare dans les classes d’âge de 100 à 150 ans.• Cette modélisation met l’accent sur l’importance de l’abondance initiale des chênes qui affecte la totalité des vieux arbres à la fin d’une période courte de simulation (100 ans).

Inter-annual and decadal changes in teleconnections drive continental-scale synchronization of tree reproduction

Climate teleconnections drive highly variable and synchronous seed production (masting) over large scales. Disentangling the effect of high-frequency (inter-annual variation) from low-frequency (decadal trends) components of climate oscillations will improve our understanding of masting as an ecosystem process. Using century-long observations on masting (the MASTREE database) and data on the Northern Atlantic Oscillation (NAO), we show that in the last 60 years both high-frequency summer and spring NAO, and low-frequency winter NAO components are highly correlated to continent-wide masting in European beech and Norway spruce. Relationships are weaker (non-stationary) in the early twentieth century. This finding improves our understanding on how climate variation affects large-scale synchronization of tree masting. Moreover, it supports the connection between proximate and ultimate causes of masting: indeed, large-scale features of atmospheric circulation coherently drive cues and resources for masting, as well as its evolutionary drivers, such as pollination efficiency, abundance of seed dispersers, and natural disturbance regimes.Climate oscillations affect weather on different temporal-spatial scales, which poses difficulty in understanding how they influence tree reproduction. Here Ascoli et al. show relationships between low- and high-frequency components of the NAO and masting in two European tree species across multiple decades.

Reconstruction of a regional drought index in southern Sweden since AD 1750

We used a network of eight pedunculate oak (Quercus robur L.) sites (n trees = 70) and one Scots pine (Pinus sylvestris L.) site (n trees = 53) to develop drought-sensitive master chronologies for the two areas in southern Scandinavia: a SW-area centred on 57°N 12.7°E and a NE-area centred on 58.8°N 18.2°E. The ratio of actual to equilibrium evapotranspiration (AET/EET) was used as a measure of drought during the growing season defined as the period with average daily temperatures above 9°C. Instrumental data were used to parameterize the relationship between tree-ring data and the drought index (DI) over 1922—2000 for the SW area and over 1922—1995 for the NE area. The DI reconstructions explained 29.7% (SW area) and 43.7% (NE area) of the variance in the observed DI index in the calibration period, and were extended back to AD 1770 for the SW area and to AD 1750 for the NE area. Reconstructed drought dynamics suggested strong decadal- and century-scale temporal variability and limited regional synchronicity over 1770—2000. Large variations in DI were observed in both regions in the second half of the 1700s. Dry conditions were synchronously reconstructed in both sub-regions during 1781—1784, 1853—1855, and, to a lesser degree, during 1974—1978. Over the 1945—1975 period the SW area exhibited a trend towards drier growing seasons, whereas no such trend could be identified for the NE area. Analysis of correlation maps indicated that regional DI dynamics reflected two different climate regimes, associated with the Kattegat area (SW reconstruction) and southeastern Swedish coast of the Baltic sea (NE reconstruction).

Testing for anthropogenic influence on fire regime for a 600-year period in the Jaksha area, Komi Republic, East European Russia

In an attempt to quantitatively evaluate the natural versus anthropogenic signal in site fire histories, the statistical relationship between dendrochronologically dated fire events and tree-ring chronologies (deemed to be an independent proxy for climate variation) was analyzed for 14 sites in a 2600-km(2) area of pine-dominated forests in the Komi Republic (East European Russia) over the period from 1424 to 1954. We developed a cumulative measure of statistical fit between two types of fire events (early- and late-season fires) and ring-width chronologies of Scots pine (Pinus sylvestris L.) (total ring- and latewood-width chronologies). For a given site, the statistical fit between fires and tree-ring data tended to decrease with an increasing proportion of unique fire years. Distance from a site to the nearest village (deemed to be a proxy of human impact) explained 50% of the variation in statistical fit between fires and tree-ring data. The fit decreased in the majority of the sites from the earlier (1424-1700) to the later (1700-1960) periods. We interpret this to be a result of increased human impact on the fire regime since 1700 due to intensified colonization of the area.

Multi‐century reconstruction of fire activity in Northern European boreal forest suggests differences in regional fire regimes and their sensitivity to climate

Forest fires are one of the main disturbance agents in boreal and temperate ecosystems. To decipher large-scale temporal and spatial patterns of past fire activity in Scandinavia, we analysed the synchronicity of dendrochoronologically reconstructed fire events in a large network of sites (n=62; 3296 samples, 392 individual fire years) covering a wide geographical gradient (56.5-67.0 degrees N and 9.3-20.5 degrees E) over AD 1400-1900. We identified large fire years (LFY) as years with regionally increased forest fire activity and located the geographical centres of climatic anomalies associated with synchronous LFY occurrence across the region, termed LFY centroids. The spatial pattern of LFY centroids indicated the presence of two regions with climatically mediated synchronicity of fire occurrence, located south and north from 60 degrees N. The return intervals of LFYs in Scandinavia followed a Weibull distribution in both regions. Intervals, however, differed: a period of 40years would carry a 0.93 probability of LFY occurrence in the southern region, but only a 0.48 probability of LFY occurrence in the northern region. Over 1420-1759, the northern region was characterized by significantly higher temporal variability in LFY occurrence than the southern region. Temporal correlation of LFYs with reconstructed average summer temperature and total precipitation was evident mainly for the northern region. LFYs in this region were associated with positive temperature and negative precipitation anomalies over Scandinavia and with colder and wetter conditions in more southern parts of the European subcontinent. Synthesis. Historical patterns of the occurrence of large fire years (LFY) in Scandinavia point towards the presence of two well-defined zones with characteristic fire activity, with the geographical division at approximately 60 degrees N. The northern and mid-boreal forests, although exhibiting lower LFY frequencies, appeared to be more sensitive to past summer climate, as compared to the southern boreal forests. This would imply that fire regimes across Scandinavia may show an asynchronous response to future climate changes.

Variation in local weather explains differences in fire regimes within a Québec south-eastern boreal forest landscape

Variation in natural disturbance regime within a landscape is important for species population dynamics, because it controls spatial arrangement of sites providing regeneration and survival opportunities. In this study, we examinethedifferencesinfireregimeandevaluatepossiblesourcesofitsvariationbetweenthesurroundingmainlandand the islands of Lake Duparquet (44.5km 2 ), a typical boreal lake in north-western Quebec, Canada. Dendrochronological reconstructionssuggestthatfireswerefrequentandofvariableintensityontheislands,whereasfireswerelessfrequenton the adjacent mainland, but were usually large and intense. Islands were significantly drier and warmer than the mainland, and maximum values of Fire Weather Index were significantly higher on the islands during both the early part of the fire season (May-June) and the whole fire season (May-September). The lightning density within the lake perimeter was significantly higher than in the surrounding mainland (0.63 v. 0.48year � 1 km � 2 respectively). This pattern was a result of the differences in lightning density during the first half of the lightning season. The study suggests that more fire-prone localweatherandhigherfrequencyoflightningstrikescouldcauseahigherfrequencyoflow-intensityfiresontheislands, compared with the mainland.

A 400-year history of fires on lake islands in south-east Sweden

Island-lake ecosystems are suitable for testing scale dependence in forests disturbance theories thanks to differences in the potential for fire spread on islands and the mainland. We investigated past fire regime on the mainland and on islands in a large lake in south-east Sweden. We used dendrochronological methods to reconstruct fire disturbances on 18 small islands (0.04–24.1 ha) and in 43 sites in the surrounding 75-km2 landscape over the last 400 years. In the past, fires were frequent on both islands and mainland but not synchronised on an annual scale. Significant temporal changes occurred around the middle of the 18th century. Before 1750, fires were less frequent on islands than on the mainland (median fire return interval 58 v. 25 years respectively). However, an inversion of this pattern was observed during 1750–1860: islands showed even shorter fire intervals than mainland locations, suggesting additional and likely human-related source of ignitions (median fire return interval 15 v. 29 years respectively). A substantial decrease in fire activity in both islands and mainland was apparent in 1860–1890. We suggest that the present fire regime (the last 100 years) on the small islands is largely natural as fire suppression is not present there. The dynamic nature of the fire regime on islands still requires further studies: islands may, at times, attract lightning, humans with fire, or both.