CZECH POLAR REPORTS 4 (1): 73-79, 2014 Constraints on dendrochronological dating of Salix polaris from central Spitsbergen Short communication Agata Buchwal1, 2* 1Institute of Geoecology and Geoinformation, Adam Mickiewicz University, Dzięgielowa 27, 61-680 Poznań, Poland 2Dendroecology Group, Swiss Federal Research Institute WSL, Zürcherstrasse 111, 8903 Birmensdorf, Switzerland Abstract Application of tundra shrubs in dendrochronological studies is recently increasing. However their growth rings are characterized by high irregularity which cause a com- mon tree-ring dating problems. The goal of the study is to demonstrate a dendro- chronological potential of common tundra species Salix polaris from central Spitsbergen (Ebbadalen, Petuniabukta) while (i) presenting its radial growth pattern and irregularities and (ii) discussing on its example a common problems of Arctic shrubs radial growth cross-dating. The results revealed that in average 25% of growth ring measured per single shrub was irregular and therefore might be problematic to detect and cross-date. It was found that even on a short distance (i.e., along a main root axis) cambial activity is not equal and missing and partially missing rings occurred alternately in a longitudinal profile. Common growth irregularities, such as missing and wedging rings, must be taken into account while measuring and cross-dating such a difficult dendrochrono- logical woody material as shown on the example of a Salix polaris dwarf shrub. The study revealed a high importance of serial sectioning and intra-plant cross-dating in dendrochronological studies of tundra shrubs. Key words: tundra shrubs, radial growth, cross-dating, serial sectioning, Salix polaris DOI: 10.5817/CPR2014-1-8 ——— Received May 31, 2014, accepted August 22, 2014. *Corresponding author: Agata Buchwał <[email protected]> Acknowledgements: This work was supported by the Scientific Exchange Program Sciex through the project ArcDendro – Arctic Shrub Dendroecological Potential (grant number 09.045). The polar expeditions in Svalbard were funded by the Polish Ministry of Science and Higher Education (grant number N N306 009139). 73 A. BUCHWAL Introduction Tundra shrubs become more and more 2013). Since shrubs can become old (more important for dendrochronological studies than 100 years old) and its annual ring in the last two decades. Annual radial growth register valuable environmental in- growth of shrubs, analogically like in the formation, dendrochronological studies on trees, provide a great opportunity for a tundra shrubs are expected to be of great tree-ring based studies and high-resolution importance in the future polar studies dating. The use of so called “shrub-rings” (Myers-Smith et al. 2011). (alternatively named as “tree-rings”) has Although a great application of shrubs been widely demonstrated in both dwarf in dendrochronological studies has been and erect shrubs species across a vast indicated so far all studies have deeply Arctic area, i.e., from Canadian Arctic stressed a common problems in ring-width (Woodcock et Bradley 1994, Rayback et measurements of tundra shrubs. This limi- Henry 2005, Au et Tardiff 2007); Alaska tation is mainly linked with a radial (Tape et al. 2012) or Greenland (Schmidt growth irregularities, such as partially and et al. 2006, Schweingruber et al. 2013). completely missing rings, which make an Recently we observed a deep interest in accurate ring-width dating of shrubs very dendrochronological studies in Spitsber- problematic. Thus a high dendrochrono- gen, were dominant tundra species like logical potential of tundra woody plants is Cassiope tetragona (Weijers et al. 2010) contrasting with its limitations and re- or Salix polaris (Buchwal et al. 2013, quires a closer look. Owczarek 2009, Owczarek et al. 2014) The goal of the study is to demonstrate and Salix reticulata (Owczarek 2009, a dendrochronological potential of com- Owczarek et al. 2014) are used for tree- mon tundra species Salix polaris from cen- ring dating with an annual resolution. This tral Spitsbergen while (i) presenting its is happening because the highest interest radial growth pattern and irregularities and of climate change studies is located in the (ii) discussing on its example a common polar regions where, as predicted, the problems of Arctic shrubs radial growth warmer conditions will influence a terres- cross-dating. trial environment significantly (i.e., IPCC Material and Methods The study material was collected in rather stable growth conditions for Salix Ebbadalen located at the Petuniabukta in polaris in the post-glacial valley. Such central Spitsbergen. Sampling plot was High Arctic location was recently found to situated ca. 1,5 km apart eastwards from represent rather opportunistic vegetation the Adam Mickiewicz University Polar change response to a warming climate Station. The study area represents Dryas (Jónsdóttir 2005), associated with limited octopetala community (Elvebakk 1999, succession of new species (Prach et al. Prach et al. 2012) with Salix polaris as a 2010, Prach et Rachlewicz 2012). How- co-dominant shrub species (Fig.1A). Ten ever the study area is characterized by entire below-ground and above-ground high geomorphic activity (Rachlewicz et parts of Salix polaris shrubs were sampled. al. 2013) lately characterized by almost The material was collected from a valley triple higher mechanical over a chemical bottom and were assign to represent a denudation rates (Szpikowski et al. 2014). 74 DENDROCHRONOLOGY OF SHRUBS Fig. 1. 50-years old Salix polaris Wahlenb. shrub from (Ebbadalen, vicininity of Petuniabukta, central Spitsbergen) with a main root exposed due to an intense wash-out and wind erosion (A); cross-sectional view of three regular annual growth rings of the main root of Salix polaris with well-developed vessels and fibers visible on the thin-section; ring boundaries marked with red line (B). Climate conditions of the study area are rations followed the methods presented by characterized by high continentality (Przy- Schwiengruber et Poschlod (2005). bylak et al. 2014). Recent studies from the In order to increase a confidence in an- study area revealed that snow cover usual- nual date assignment ring-width measure- ly last from October to mid-June (Láska et ments were conducted both within above- al. 2012), whereas a permafrost thaw ex- ground and below-ground shrub parts. ceed 1 m thickness in late July (Rachlewicz Radial growth of Salix polaris was inves- et Szczuciński 2008). tigated in details tracing annual growth To understand radial growth patterns in rings within whole cross-sections. Measure- Arctic tundra shrubs an extended serial ments of ring widths was performed along sectioning method was applied based on four radii using manual path analysis in principals proposed by Kolishchuck (1990). WinCell (Regents Instruments). To pre- Annual growth rings were investigated cisely date the woody material into calen- both within above-ground (i.e., dominant dar years and to ensure a reliable chro- branches) and below-ground plant parts, nology development cross-dating proce- including main root and tap roots if avail- dure is highly required (Fritts 1976, able. From rather straight and not injured Schweingruber 1988, Cook et Kairiukstis plant parts a sequence, i.e. in average more 1990). A cross-dating procedure of Salix than 10 specimens were sectioned. polaris dwarf shrubs was performed in From each specimen a thin-section was three stages: i) cross-dating of four-radii prepared by using a sledge microtome. tree-ring widths measurements within one Each section was 15-30 µm thick and after cross-section and computing a mean cutting was stained with a mixture of Astra growth series; ii) cross-dating between Blue and Safraning to highlight unlignified mean growth-series of all cross-sections and lignified anatomical structures. A de- taken from one individual shrub, including tailed instruction of micro-sections prepa- below-ground and above-ground parts and 75 A. BUCHWAL computing a mean growth-curve of a establishing a site chronology. Final tree- single shrub; iii) cross-dating between the ring chronology was presented previously mean growth-curves of each shrub and in Buchwal et al. (2013). Results and Discussion Annual ring structure of Salix polaris and between the adjacent cross-sections. shrub is very distinguished with a ring This step ensured confidence in complete boundary usually consisting of 1-2 rows of detection of growth rings and shrub dendro- thick-walled fibers (Fig. 1B). However a chronological dating. tree-ring structure vary from ideally regu- A comparison of raw ring-width lar and complete rings (Fig. 1B) to irregu- measurements taken at a different root lar and erratic configuration completed depth and above-ground shoots length re- with wedging rings (Fig. 2). Detailed serial vealed high incompatibility (Fig. 3). This sectioning studies on Salix polaris sam- was true while considering each shrub and pled from the Petuniabukta area indicated its above-ground and below-ground cross- high intra- and inter-plant variability in its sectional measurements of growth rings. radial growth. It was specified that mean Moreover measurements of growth rings missing rings ratio per individual shrub between two consecutive cross-sections was 24.8% (Buchwal et al. 2013). This taken at a short distance (i.e., 3 cm apart) amount included both completely missing along a main root axis revealed high dis- rings, which were not visible in a whole crepancies in annual ring formation along cross-section and not detected either in the main root axis (Fig. 2). Missing rings some part of the plant or in a whole plant, and wedging rings were very common and as well partially missing rings (i.e. wedging occurred alternated, making a successful rings) detected only in comparison be- cross-dating very challenging. Annual ring tween four-radii measurements within a boundaries were often hard to distin- single cross-section. The oldest part of the guished and in some parts were overlap- plant was always found below-ground, i.e. ping with the following year ring bounda- in the main root and up to 10 cm below the ry.