Flora - Morphology, Distribution, Functional Ecology of Plants
Below-ground response of Norway spruce to climate conditions at Mt. Brocken (Germany)—A re-assessment of Central Europe's northernmost treeline
Introduction
Alpine treelines are most abundant in the world's large high-elevation mountain ranges as well as at lower elevations in the boreal regions of the southern and northern hemisphere. Accordingly, the vast majority of studies on alpine treelines were conducted in those regions (see reviews by Wieser and Tausz, 2007, Holtmeier, 2009). In contrast, alpine treelines at medium high mountains are less abundant and thus less frequently studied (e.g. the medium high mountains in Northeast America: Cogbill and White, 1991, Richardson et al., 2004). In Central Europe, only few medium high mountains exist apart from the high-elevation Alps and the Carpathian Mountains that show summits currently free of forest vegetation. Many of these treelines are thought to be anthropogenic rather than climate-driven (e.g. Hermes, 1955). Mount Brocken (1142 m a.s.l.) in the Harz Mountains – the northernmost medium high mountain range in Central Europe – shows a treeline built by Norway spruce (Picea abies). Though some authors have stated that this treeline might be natural (climate-driven), detailed analyses of the prevailing climate conditions and comparisons of the forest vegetation to other alpine treeline sites are still missing to confirm this assessment.
The obvious change in tree height and above-ground forest structure with elevation and near the alpine treeline has over decades stimulated a large number of investigations on the treeline formation (cf. reviews by Troll, 1973, Tranquillini, 1979, Körner, 2003, Wieser and Tausz, 2007, Holtmeier, 2009). Nevertheless, the ecological reasons for the formation of alpine treelines are still not fully understood. Traditionally, the marked decrease in tree height and above-ground tree biomass upslope towards the alpine treeline has been attributed to a reduced carbon gain due to a limitation in photosynthetic activity and/or an imbalance of annual C fixation vs. respiratory losses under the prevailing cold conditions. Other authors have emphasized the effects of strong winds, blowing ice, large snow charge, or winter drought for the formation of the alpine treeline (data reviewed by Stevens and Fox, 1991, Körner, 1998, Körner, 2003, Sveinbjörnsson, 2000, Grace et al., 2002, Holtmeier, 2009). More recent studies by Körner and co-workers have suggested that tree growth at (and above) the alpine treeline might not be source (carbon assimilates) but rather sink limited by reduced metabolic activity of the C allocation (Hoch et al., 2002, Hoch and Körner, 2003, Hoch and Körner, 2005, Shi et al., 2006, Patty et al., in press). Studies by these authors moreover revealed that the elevational position of alpine treelines worldwide coincides with a mean growing season root zone (10 cm soil depth) temperature of 6.7 °C (Körner and Paulsen, 2004). They conclude that the cold root zone temperatures bring about impaired root growth conditions that might significantly account for the adverse tree growth conditions at the alpine treeline (Körner, 1998, Körner, 2003).
In contrast to the large number of studies on the response of the above-ground tree organs to environmental conditions near the alpine treeline, effects of those conditions on the tree root system have been only occasionally studied. Elevational transect studies at boreal, temperate and tropical treeline sites have given evidence that high-elevation forest stands might be worldwide characterized by a particularly high fine root biomass (Kitayama and Aiba, 2002, Leuschner et al., 2007, Hertel and Wesche, 2008, Hertel et al., 2008). This phenomenon is in marked contrast to the response in above-ground tree compartments to the prevailing growth conditions at the alpine treeline and do not support the hypothesis of thermal constraints of root growth activity. However, it is not proven so far whether the fine root system of trees at treelines at medium high mountain ranges responds similarly.
We investigated four mature Norway spruce stands along an extended elevation transect at the northern hillside of Mt. Brocken (Harz Mountains, Germany). The stands were located at altitudes ranging from 390 to 1100 m a.s.l. with the highest stand representing the upper treeline at Mt. Brocken. We aim in our study (i) to investigate the elevational changes in various climatic parameters of Norway spruce stands along the transect, (ii) to thereby verifying the climatic status of the current elevational position of the treeline at Mt. Brocken, (iii) to analyse how the changing growth conditions along the transect affect the fine root biomass of the spruce stands, and (iv) to examine if a general dependence of the standing fine root biomass on elevation exists in Picea abies stands in central and northern Europe.
Section snippets
Location of the study sites
The study was conducted at the northern hillside of Mount Brocken (1142 m a.s.l., 51°48′N, 10°37′E), the highest peak of the Harz mountain range in Central Germany. Four mature stands of Norway spruce (Picea abies (L.) Karst.) were selected along an elevational transect spanning from the mountain basis near Ilsenburg town (ca. 300 m a.s.l., 51°52′N, 10°41′E) up to Mt. Brocken. The four stands were located at 390, 790, 990, and 1100 m a.s.l. with the uppermost forest stand (at 1100 m) representing
Microclimate of the spruce stands
Mean annual air temperature inside the forests decreased with elevation from 6.9 °C in the lowermost spruce stand to 2.1 °C in the treeline stand (Table 1). Air temperature minima also decreased along the elevational transect with the treeline stand experiencing a ca. 4 K lower minimum than the lowermost stand. Similarly, mean annual soil temperature at 10 cm soil depth as well as mean soil temperature during the growing season decreased linearly with elevation (Table 1). The altitudinal change in
Climate conditions of the spruce stands at Mt. Brocken: is there evidence for a natural treeline?
While the presence of natural (i.e. climate-driven) treelines at the prominent large high-elevation mountain ranges of the world (such as the Himalayas, Andes, or the Rocky Mountains) as well as at low-elevation sites at high latitudes are widely recognized, the natural status of treelines in medium high mountains has been the issue of a controversial debate from early decades to date. In Central Europe, this is particularly true for several medium high mountains that currently show treeline
Acknowledgements
We want to thank the national park administration for the permission to conduct the study in the Harz National Park. We are very grateful in particular to Gunter Karste for his skillful support of the study site selection. We thank Mechthild Stange and Marina Röderstein for their help of the soil sampling in the field. The contribution of Felix Norman to the analysis of the measured temperature data is also gratefully acknowledged.
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