Silva Fennica 42(2) research articles -6Ê www.metla.fi/silvafennica · ISSN 0037-5330 The Finnish Society of Forest Science · The Finnish Forest Research Institute Tree Mortality Agents in Pristine Norway Spruce Forests in Northern Fennoscandia Antti Lännenpää, Tuomas Aakala, Heikki Kauhanen and Timo Kuuluvainen Lännenpää, A., Aakala, T., Kauhanen, H. & Kuuluvainen, T. 2008. Tree mortality agents in pristine Norway spruce forests in northern Fennoscandia. Silva Fennica 42(2): 151–163. We examined tree mortality agents in pristine old Norway spruce (Picea abies (L.) Karst.) forests in northern Finland and northwestern Russia. The data was collected on nine 40 m × 400 m transects. The primary mortality agents of recently dead trees were recorded and their frequencies were calculated. The pattern of tree growth prior to death was studied based on increment core samples and compared with the growth of healthy dominant trees. Of all recently dead trees, 72% could be associated with a primary mortality agent. In both study areas the most common primary mortality agent was a Coniophora (Mérat) DC. -genus fungi, which was found on average in 33% of trees sampled. The fungi Phellinus chrysoloma (Fr.) Don and Onnia leporina (Fr.) H. Jahn as mortality agents were more common in the Finnish area compared to the Russian area. Analysis on the growth patterns indicated weak differences between different pathogens’ influence on prior-to-death growth of trees, so that fungi rotting the whole tree decreased tree growth more rapidly than fungi rotting only the heart wood. The results demonstrated that in old Norway spruce forests of northern Fennoscandia the most common primary tree mortality agents were wood rotting fungi, which weaken the mechanical stability of tree stems until they fall due to snow or wind, which should be considered only as secondary mortality agents. It is evident that tree death in pristine forest typically results from a long-lasting process involving both biotic and abiotic factors. Keywords Coniophora, mortality agent, northern Fennoscandia, Picea abies, pristine forest Addresses Lännenpää, Aakala & Kuuluvainen: Dept. of Forest Ecology, University of Helsinki, Finland; Kauhanen: Kolari Research Unit, Finnish Forest Research Institute, Finland E-mail [email protected] Received 6 July 2007 Revised 5 December 2007 Accepted 3 January 2008 Available at http://www.metla.fi/silvafennica/full/sf42/sf422151.pdf 151 Silva Fennica 42(2), 2008 research articles 1 Introduction exposed mineral soil. Similarly, tree mortality is important for biodiversity, as it creates the dead wood many organisms depend on (Siitonen 2001). Forest dynamics are driven by three main proc- Therefore, understanding both regeneration and esses: regeneration, growth and death of trees. dead wood dynamics requires understanding the Understanding all these processes is needed for mortality process. a comprehensive picture of forest dynamics. The purpose of this study was to examine the From these three processes, tree death is prob- causes and process of tree mortality in old pris- ably the most difficult one to predict (Dobbertin tine Norway spruce forests in northern boreal and Biging 1998). Many external factors may Fennoscandia. We asked the following specific affect the weakening and eventually the dying of questions: 1) what are the most significant tree a tree. These factors can be divided into primary mortality agents? and 2) how do different mortal- mortality agents that are capable of killing healthy ity agents affect tree growth prior to death? trees, and secondary mortality agents that kill only weakened trees. However, this division is often problematic from an ecological point of view, 2 Material and Methods because in many cases both are needed to finally kill a tree. When taking into account all the agents involved in the death of a tree, it is reasoned to 2.1 Study Areas say that tree mortality often is a complex chain of cause and effect (Franklin et al. 1987, Manion The research was carried out in two locations in 1991, Kuuluvainen 1994). northern Fennoscandia, in northwestern Finland Northern boreal Norway spruce (Picea abies L. in the Pallas-Yllästunturi National Park, and in Karst.) forests are growing in harsh environmental northwestern Russia in the Murmansk Oblast. conditions, where the growing season is short, the The study area in the Pallas-Yllästunturi National angle of solar radiation is low and temperatures Park was chosen because it harbors some pris- remain low throughout the year. In these condi- tine Norway spruce forests. The area belongs to tions all ecological processes are slow and tree the northern boreal vegetation zone (Ahti et al. growth is retarded. Particularly in old Norway 1968). The yearly mean temperature is –1°C, the spruce forests the thick and poorly decompos- effective temperature sum is 650–700 dd. and the ing moss layer decreases soil temperatures and annual precipitation is about 550 mm (Vadja and slows down the nutrient cycle (Sirén 1955). In Venäläinen 2003). such stressful environment trees are also con- The study area in northwestern Russia was stantly susceptible to fungal diseases (Norokorpi chosen with the help of information from Russian 1979). colleagues and satellite images. This remote area It has been a common belief that tree mortal- is located by the Kazkim-river 68°18’N, 30°22’E. ity and regeneration in old Norway spruce for- The area is in the northern boreal vegetation zone, ests naturally occurs through large and forceful and its climatic key figures are: yearly mean disturbances such as high-severity fires (Sirén temperature –2 °C, the effective temperature sum 1955). However, recent studies demonstrate that 400–430 dd. and the annual precipitation about wildfires are rare in these forests (Syrjänen et 600 mm. (Vadja and Venäläinen 2003). The Rus- al. 1994, Hyvärinen and Sepponen 1988, Wal- sian study area is thus influenced by a consider- lenius et al. 2005) and that their dynamics are ably harsher climate compared to the Finnish driven by small-scale mortality of trees (Kuu- study area. luvainen 1994). The establishment of seedlings in the northern boreal Norway spruce forests is generally limited by seedbed availability (Hof- gaard 1993). Thus, tree mortality plays a major role also in facilitating regeneration by creating suitable seedbeds in the form of nurselogs and 152 Lännenpää, Aakala, Kauhanen and Kuuluvainen Tree Mortality Agents in Pristine Norway Spruce Forests in Norhern Fennoscandia 2.2 Site Selection and Stand Characteristics 2.3 Identification of Mortality Agents In the study areas, the stands for sampling were To study the causes of tree mortality in the searched based on information from maps (Fin- transects, we recorded all standing dead and fallen land) and satellite images (Russia), and the final trees over 10 cm DBH (diameter at 1.3m height). selection was done by field surveys. The for- These trees were classified into decay classes ests that were accepted for sampling fulfilled the (Table 2) following a modification of the classifi- following requirements: 1) The dominant tree cation of Imbeau and Desroches (2002). The most species was Norway spruce, 2) the forest had important mortality agent, i.e. the primary mor- a continuum of decayed trees, and old standing tality agent, was determined for each tree (Table trees, indicating long development without major 1). If signs of several different primary mortality external disturbances, and 3) the forest had no agents were visible, or the tree had died without signs of direct anthropogenic influence. external signs, it was considered as having died The sampling unit was a transect of 40 m × 400 with no clear cause. In practice, identifying a m. In the selected stands the starting points and primary mortality agent was possible only for directions of the transects were randomized so trees that had died relatively recently. Therefore, that the whole transect fitted in the same type of the determination of primary mortality agents was forest. All four Finnish transects were established limited to standing dead trees in decay classes 4 in 2005, and the five Russian transects in 2006. and 5, and fallen trees in decay classes 9 and 10 In the Pallas-Yllästunturi National Park the sites (see Table 2). of sampling are located south of Lake Pyhäjärvi, The identification of biotic mortality agents 67°40’N, 24°22’E. The studied stands are sparse, was mainly done using the presence of polypore with mean volume of 99.5 m3ha–1 (range: 88.9– fruiting bodies and insect galleries. When fruiting 108.2 m3ha–1; unpublished data). Stands are bodies were found the fungi species could easily dominated by Norway spruce (on average, 74% be determined. If the primary agent was clearly a of volume; Picea abies L. Karst), and there is an rotting fungi, but no fruiting bodies were found, admixture of birches (20%; Betula pendula Roth, the identification was based on decay characteris- Betula pubescens Ehrh.) and also a small number tics. In these cases, the agent was identified only of pines (7%, Pinus sylvestris L.) especially on to the genus level. In particular, the typical brown drier locations. The forest floor is dominated by rot of the genus Coniophora (Eriksson 1958) was blueberry (Vaccinium myrtillus L.), lingonberry repeatedly found in the recently dead trees. (Vaccinium vitis-idaea L.) crowberry (Empetrum The observed causes of tree death were summed nigrum L.) marsh ledum (Ledum palustris L.) and and the results were converted into proportional bog bilberry (Vaccinum uliginosum L.). values so that the two study areas could be com- In the sites in northwestern Russia the mean pared. When interpreting the results it should be volume is 65.7 m3ha–1 (range: 54.5–71.5 m3ha–1). kept in mind, however, that the identification of Tree species composition is similar to the Finn- the primary mortality agent involved an unavoid- ish sites, on average 77% of volume is Norway able element of uncertainty, because it was done spruce, 19% birch, and 3% pine (unpublished qualitatively based on external indicators.