Ninth International Conference on Permafrost ... - IARC Research

The Effect of Climate and Permafrost on Tree Line Dynamics in Northwest Russia:A Preliminary AnalysisMartin WilmkingSaskia KenterJens IbendorfUniversity Greifswald, 17487 Greifswald, GermanyIntroductionTree line dynamics provide important feedbackmechanisms to the global climate system by altering albedoand carbon storage at high latitudes. A northward movementof the northern tree line is generally assumed to decreasealbedo (Chapin et al. 2005), but increase above-groundcarbon storage. However, ecosystem carbon storage mightbe reduced during tree line advance due to the respirationof (old) soil carbon and thus result in a positive feedback towarming (Wilmking et al. 2006).Establishment of trees north of tree line requires on theone hand favorable climatic conditions (climate driver) andon the other hand suitable microsite conditions (micrositedriver) (Lloyd & Fastie 2002). Suitable microsites are oftenhampered by the existence of permafrost, and thus ultimatelymay depend on permafrost degradation. Thus, one mainfactor controlling carbon storage and tree line advance inmany regions is permafrost.The EU Project “CarboNorth” aims at quantifying thecarbon budget in northwest Russia, and as part of this project,the consortium is studying tree line dynamics. This paperreports preliminary results from the dendroecological partof the investigation. Particularly, we were interested in theclimate sensitivity of established trees at tree line (climatedriver) and in the age structure of seedlings extending beyondtree line from mostly permafrost-free soils to permafrostinfluencedsoils (microsite driver).MethodsField sites were located at northern tree line in northwestRussia at 67.4°N, 62.3°E (R1), 67.2°N, 62.1°E (R2),67.1°N, 59.5°E (Kho). Each site consisted of a gradient fromtree islands on well-drained soil with permafrost pockets,through woodland to treeless tundra underlain by permafrost(conceptional permafrost degradation). Each transectconsisted of 3–4 plots (15 x 15 m): (1) forest, (2) densewoodland, (3) open woodland, and (4) tundra. Permafrostdepth was measured with a 120 cm long probe (July/August2007). We collected penetrating tree cores or disks (at rootcollar level) from every Picea obovata tree or seedlingwithin each plot in 7/2007. Samples were prepared accordingto dendrochronological standards. Age of samples wasdetermined by ring counts and adjusted for sample height.We measured ring-width (LINTAB 5, 1/1000 mm) from allR2 samples including other well-established trees close byand crossdated visually and with COFECHA. We used theprogram ARSTAN (negative exponential, straight line fits,Huggershoff) to standardize the tree ring series, to removethe biological age trend, and to build site chronologies. Wecalculated climate-growth relationships of chronologiesonly, if EPS (expressed population signal) exceeded 0.85.We used mean monthly temperature and precipitation data(1901–2002) from the closest grid point of the griddedCRU-dataset, because the temperature record correlatedvery well (r > 0.98) with the closest station (Khoseda Hard)and records of Pecora (r > 0.90) and Nar Jan Mar (r > 0.87).Correlation and response functions over time were analyzedwith the program DENDROCLIM2002 (Biondi & Waikul2004); moving window length was 50 years.ResultsMicrosite conditionsThe active layer depth was generally significantlyshallower in the tundra plots compared to woodland andforest plots, which showed similar thaw depths (Fig. 1),often exceeding the depth of the probe. While forest plotswere generally well drained, woodland and especially tundraplots were often waterlogged, and trees and seedlings hadoften established on slightly higher terrain (hummocks).Age structureThe age structure revealed a striking similarity at all plotsat all three study sites, where most individuals had establishedbetween 1950 and 1960, and none had after 1982.Climate-growth relationships at R2Only one chronology (Forest) exceeded an EPS value of0.85 during the 20 th century and, subsequently, was usedfor the calculation of climate-growth relationships. Thestandard chronology showed some significant correlationand responses to temperature, but none was very strongor stable over time. No significant correlation existed withprecipitation. The residual chronology showed significant0255075100125R1 R2 Kho TundraopenWoodlanddenseWoodlandForestFigure 1. Minimum thaw depth (in cm) across the forest–tundratransect; error bars are standard deviation.345