ORIGINAL ARTICLEChanging climate sensitivity of black spruce (Picea mariana Mill.) in a peatland–forest landscape in Interior Alaska
Introduction
Interior Alaska's landscape consists mainly of lowland forests underlain by permafrost, which is sensitive to warming (Swanson, 1996) and thus includes many thermokarst-induced depressions (Jorgenson et al., 2001). These depressions often developed into peatlands with scattered and stunted black spruce (Picea mariana Mill.) trees growing on their surface. Subfossil remains of trees in peatlands have been used quite extensively to develop long tree-ring chronologies used for climate reconstruction (Pilcher et al., 1995; Gunnarson, 1999; Wang et al., 2000; Leuschner et al., 2002), but the current climatic sensitivity of peatland trees has only been studied to some extent (Linderholm et al., 2002) or, in Interior Alaska, not at all. Juday et al. report in the Arctic Climate Impact Assessment (ACIA, 2004) results of climate–growth relationships for P. mariana (Mill.) on four permafrost dominated sites in Interior Alaska, but these sites are not low-lying peaty soils.
In northern Europe, warmer growing season temperatures during the 20th century seemed to increase growth in peatland scots pine (Pinus sylvestris) (Stravinskiene and Juknys, 1998; Linderholm, 2001; Linderholm et al., 2002), while precipitation influences were more regional. In northern Sweden (Linderholm et al., 2002) and Lithuania (Stravinskiene and Juknys, 1998) precipitation influenced growth negatively. In western central Sweden, precipitation did show no significant influence on growth (Linderholm, 2001), while in southern Sweden positive correlations between annual growth and early summer rain exist, a feature attributed to increased evaporative demands there (Linderholm et al., 2002). In New Jersey, USA, red spruce trees on bogs at their lower elevational range limit were insensitive to climate (1951–1989) characterized by monthly means of temperature and precipitation (Webb et al., 1993). Even though the red spruce growing on bogs exhibited slower growth than counterparts on mineral soil, they reached higher ages and seem to utilize bogs as refugia at this lower, more drought prone limit of their range (Webb et al., 1993).
But, how stable is the sensitivity of trees to climate variables over time? Changing sensitivity to climate has been documented for alpine treeline in the European Alps (Carrer and Urbinati, 2006), northern and alpine treeline areas in Alaska (Wilmking et al., 2004; Driscoll et al., 2005) and within the circumpolar boreal forest (Wilmking et al., 2005). Here we explore the stability of the climate–growth relationship of black spruce growing in an open stand and on a peatland in Interior Alaska.
Section snippets
Site description
Isla peatland is situated in the Tanana flats, south of the Tanana river at 64°38′N and 148°20′W, about 30 km SW of Fairbanks (Fig. 1). Continental climate with hot summers and very cold winters prevails, but a pronounced warming trend has been observed since the mid 1970s (Barber et al., 2004).
Scattered individuals of P. mariana (Mill.) occupy the peatland surface, a thermokarst-induced depression, which developed over 100 years ago. Lateral expansion has increased since the last fire and peat
Tree samples
Disks were collected with a handsaw from 21 fire-scarred trees (11 on slightly higher ground and 10 on the peatland). Trees had been killed but not burned severely during the fire so that the outer bark was still visible. We tried to cut the trees as low as possible, as in the lower areas peat accumulation over time causes the root collar of the trees to generally be quite deep below the surface. Disks were sanded as fine as necessary (up to 800 grid) and then ring width was measured using a
Growth trends
Trees growing on slightly higher ground in the surrounding forests (HG) were generally older and larger (average age 120 years, dbh: 13 cm) than trees growing on lower ground (LG) within the depressional peatland (average age 78 years, dbh: 8 cm). The differences in site conditions of the two chronologies also resulted in differing growth trends (Fig. 2). While HG had a period of high growth in the first half of the 20th century and then declined in growth starting around the 1950s, LG does not
Discussion
In the last decade, several studies have reported a changing climate–growth relationship over time in tree ring series of the temperate and boreal zone, mostly at treeline locations (Carrer and Urbinati, 2006; Driscoll et al., 2005, Wilmking et al., 2004, Wilmking et al., 2005; Lloyd and Fastie, 2002, Briffa et al., 1998). Change in sensitivity was mostly attributed to climate change, but Briffa et al. (2004) suggested falling ozone levels as a cause for reduced climate sensitivity of
Conclusions
In this study, black spruce growing in the Tanana flats in Interior Alaska showed distinct growth trends according to the microtopography of their site and thus the distance to the local water table. While trees growing in a low lying depressional peatland were generally not very climate sensitive, trees on slightly higher ground in surrounding open forest displayed strong significant correlations with summer temperature indices consistent with a drought stress signal. However, all
Acknowledgments
We thank two anonymous reviewers for their input, which definitely sharpened the focus of the paper. We thank Glenn Juday for climate data from Fairbanks, Jens Ibendorf for field assistance, and the Bonanza Creek LTER for transportation to the Tanana flats. Funding was provided by a NOAA/UCAR Postdoctoral Fellowship and Emmy Noether Postdoctoral Fellowship of the German Science Foundation (DFG) to M. Wilmking (WI 2680/1-1).
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