phases in polygon hydrology: geomorphological and climatic controls
The long-term development of the three cores examined here is characterized by recurrent, contrasting phases in soil moisture and vegetation composition (Figs 2, 3 & 5), supporting a growing body of evidence for the dynamic nature of High Arctic wetland development (LaFarge-England et al. 1991; Ellis & Rochefort 2004) rather than the previously generally perceived view of Arctic vegetation as resistant. The development of three low-centre wetland polygons (cf. Tarnocai & Zoltai 1988; French 1996) was reconstructed up to the present-day (BY-LowC) or until the cessation of sediment growth with the shift to high-centre polygon conditions at c. 305 cal. years bp in BY-HighC and c. 140 cal. years bp in BY-a. Individual low-centre polygons are hydrologically discrete, comprising water-shedding polygonal ridges, raised by the growth of ice-wedges, around water-accumulating, lower and wetter polygon centres. Reconstructed changes in past soil moisture can be considered specific to individual polygons. The water balance (ΔS) of a plot within a given polygon can be approximated as:
- ΔS = P + Qg + Qs − ET( eqn 1 )
influenced by climate, i.e. precipitation (P) and evapotranspiration (ET), and/or geomorphology, i.e. topography, which controls groundwater and surface-water input and output (Qg and Qs, respectively) and modifies inputs from precipitation (P) (Rovansek et al. 1996; Young et al. 1997). Climatic control may also be indirect, through the effect on topography of ice-wedge growth (Kasper & Allard 2001), which will in turn affect soil moisture (P, Qg and Qs) (Rovansek et al. 1996; Young et al. 1997). Garneau (1992) and Vardy et al. (1997) suggested that the long-term development of Arctic wetlands had been sensitive to past climatic variation, and Ellis & Rochefort (2004) observed a possible effect of the Little Ice Age (c. 300–465 cal. years bp) in the development of a High Arctic wetland polygon. However, they reported a generally low correspondence between proxy climate records and reconstructed soil moisture, and LaFarge-England et al. (1991) also suggested that local geomorphology was the principal factor controlling soil moisture during the development of High Arctic peat deposits. These palaeoecological studies (LaFarge-England et al. 1991; Ellis & Rochefort 2004) raise uncertainty about the extent to which a direct climatic effect on tundra vegetation described by short-term studies (Chapin et al. 1995; Arft et al. 1999) might also contribute to inherent, long-term vegetation change, including the direct effects of future climate warming. It is possible that soil moisture conditions and vegetation during development of tundra polygons are controlled principally by autogenic processes, i.e. the balance between sediment accumulation and upward development of syngenetic ice-wedges, which will control the topographic amplitude between polygon ridges and centres, and therefore values of Qg, Qs and P (Ellis & Rochefort 2004).
evidence for climatic control during polygon development
Evidence for the climatic control of soil moisture and vegetation during long-term development is circumscribed by a lack of replication and possible errors in both the radiocarbon dating scheme (Table 1, Fig. 4) and the dating of ice cores used as a paleoclimatic proxy (Fujii 1995). However, dating of ice cores over the past c. 5000 years is expected to be relatively precise (Paterson et al. 1977), and comparisons between ice cores suggest that a general interpretation of palaeotemperature is reasonable for the Holocene (Bradley 1985, 1990). Rates of sedimentation for the three cores appear consistent (Fig. 4), and the 95% confidence intervals for individual radiocarbon dates (75–100 years) are, on average, c. 45 years less than the mean duration of wet and dry phases (130 ± 15 years), suggesting that a circumspect comparison of phases may be justified. Based on the comparison between palaeoecological records and palaeoclimatic values of both percentage melt and δ18O, our evidence suggests that soil moisture of the wetland polygons investigated may have become drier during periods of climatic warming (dry phases associated with higher values of percentage melt and δ18O) and wetter during periods of climatic cooling (wet phases associated with lower values of percentage melt and δ18O). However, the relative importance of climate in controlling soil moisture is unlikely to have been consistent during polygon development and a climatic effect is therefore more strongly supported when similar phases are registered contemporaneously in the three cores (Fig. 7).
Given a combined palaeoecological record of c. 3885 years (i.e. the combined time span of BY-LowC, BY-HighC and BY-a over the period for which a comparison with palaeoclimatic records is possible), 50% of the period shows no evidence for a correspondence between wet and dry phases and palaeoclimatic proxy values in any core. There is evidence for a correspondence in only one core, or else contrasting evidence for a correspondence between phases and palaeoclimatic proxy values in several cores (i.e. significantly higher or lower values of percentage melt or δ18O related to contrasting coeval wet and dry phases) over c. 44% of the palaeoecological record (i.e. c. 1710 years). Over only c. 6% of the palaeoecological record (i.e. c. 225 years) is there evidence for a correspondence between wet and dry phases and consistent change in the records of percentage melt (two to three cores) and δ18O (two cores or one core).
The balance of evidence therefore supports an important role for autogenic geomorphological–vegetation change during the long-term development of the three wetland polygons examined (LaFarge-England et al. 1991; Ellis & Rochefort 2004). Periodic changes in vegetation and soil moisture during the development of Arctic wetland polygons, where unrelated to palaeoclimatic proxy records, might instead reflect local periglacial processes (Ellis & Rochefort 2004) resulting in amplitude changes between polygon ridges and centres. Deeper polygons will accumulate more snow in winter (resulting in higher values of P, Young et al. 1997) and will have greater rates of surface (Qs) and groundwater (Qg) input (Rovansek et al. 1996; Young et al. 1997), causing them to be wetter than polygons with a shallower topology. Wet and dry phases can then be explained by a feedback between sediment accumulation, controlled by the input of organic and mineral material, and the upward growth of ice-wedges, which is limited by the rate of sediment accumulation (Harry & Gozdzik 1988; Mackay 2000). Wet phases may correspond to periods when the amplitude between a polygon's ridges and centre is relatively large (higher values for Qg, Qs and P) although upward growth of ice-wedges, and the continued development of ridges, will gradually slow or cease, to be resumed only when sufficient sediments have accumulated in the polygon centre. This intervening period will lower the amplitude between polygon ridges and centre, and therefore lower values of Qg, Qs and P, causing the shift to drier soil moisture conditions evident in the palaeoecological record (Ellis & Rochefort 2004). A process of staggered ice-wedge formation resulting in a chevron pattern of growth (Dostovalov & Popov 1966; Mackay 1974; Lewkowicz 1994) might therefore explain recurrent shifts between wet and dry phases during the long-term development of low-centre polygons (Ellis & Rochefort 2004).
Where evidence for a climatic effect is based on a correspondence between palaeoecological and palaeoclimatic records in one core alone, or points to a contrasting response across several cores, it must be considered equivocal. However, a polygon in a dry state (with lower ridges relative to and surrounding the polygon centre and low values of Qg, Qs and P) may register a shift to a wetter and cooler climate, whereas one in a wet state (with higher ridges and values of Qg, Qs and P) may not. Thus, wet and dry phases in the combined record of palaeohydrological change matching significantly higher and lower palaeoclimatic proxy values (Fig. 7) may point to an underlying long-term climatic influence, which is modified by local geomorphological conditions so that it is only registered in individual cores.
There are, however, two distinct periods (Figs 5 & 6) during which a climatic effect on polygon development is well supported (c. 305–420 and 420–530 cal. years bp). Period I, characterized by contemporaneous wet phases in all three cores, and associated with significantly lower values of percentage melt (BY-LowC, BY-HighC and BY-a) and δ18O (BY-a), is broadly coeval with a period of climatic cooling well documented from sites in the North Atlantic region (Lamb 1965; Williams & Wigley 1983; Millar & Woolfenden 1999) including the Arctic (Williams & Wigley 1983; Gajewski & Atkinson 2003): i.e. the Little Ice Age (LIA). The change from inferred drier soil moisture conditions during Period II to wetter conditions during Period I is also supported by evidence for a pronounced climatic effect at other High Arctic sites. Period I is coeval with geomorphological evidence for the effects of the LIA in the Qunguliqtut Valley (Klassen 1993; Allard 1996) and paleolimnological evidence for LIA cooling from Baffin Island (Hughen et al. 2000) and Ellesmere Island (Lamoureux & Bradley 1996). The direction of change is also consistent with evidence for colder though wetter LIA climatic conditions from Cornwall Island in the Canadian Arctic Archipelago (Lamoureux 2000; Lamoureux et al. 2001) and from central west Greenland (Bennike 1992).
the functional significance of palaeoenvironmental variation
The estimated values of past soil moisture, based on macrofossil mosses, suggest significant variation in hydrology during the development of low-centre polygons (Figs 1 & 5). Soil moisture exerts a control on tundra ecophysiology through production, decomposition and nutrient cycling (Miller et al. 1984), and a lowered water-table and increased thaw might be expected to accelerate the rate of soil decomposition (CO2 source) over photosynthesis (CO2 sink), so that the balance in tundra soils shifts from one of C-input, or storage, to C-output (Billings et al. 1982, 1983; Johnson et al. 1996). If the effect of decomposition were to increase nutrient availability, there may be an additional uptake of CO2 owing to higher rates of photosynthesis (Shaver & Chapin 1986; Shaver et al. 1998; Johnson et al. 2000), although as sink strength in vascular plants decreases, productivity may be offset by substrate-controlled or nutrient-limited CO2 loss from soil respiration by micro-organisms (Nadelhoffer et al. 1991; Hobbie 1996; Jonasson et al. 1999). Larger sinks of CO2 are accordingly associated with lower respiration rates in wetter habitats (Vourtilis et al. 2000), while short-term experiments designed to explain the net effect of climate warming on soil moisture and the C-balance of tundra plots (Johnson et al. 1996) support observational data demonstrating that a shift from net C-input to C-output accompanies the recent drying of tundra habitats (Oechel et al. 1993, 1995; Weller et al. 1995).
Inferred values for past changes in soil moisture are of a sufficient magnitude to affect CO2 flux in wetland ecophysiology, varying periodically between values ranging from c. 705 ± 126.5% d. wt. (the average and SE for DCA axis 1 scores in the range 0–3) to c. 1891 ± 175% d. wt. (range 3–4) (Figs 1 & 5). A corollary is that the observed and predicted effects on tundra soil moisture of human-induced climate warming are, in High Arctic polygon-patterned wetlands, recent additions to previous, long-term fluctuations in ecosystem function. If the inferences of this study are confirmed for Arctic wetlands in general, an inherent variation in ecosystem function controlled by climate change will have to be taken into account when interpreting the results of short-term environmental manipulations (e.g. Arft et al. 1999), and of studies attempting to scale-up results from short-term experiments to longer-term processes (McKane et al. 1997; Epstein et al. 2001).