Arctic Oases

In the Arctic it is possible to detect sites which in comparison to adjacent areas are relatively rich either in species or genetic diversity or both. Genera that show high species diversity at high latitudes in such areas include Salix, Saxifraga and Draba. Arctic species-rich hotspots, sometimes referred to as polar desert oases, include Peary Land, Devon and Ellesmere Islands as well as Bathurst Inlet (Figs. 6.20-6.24).

The first polar oasis to be studied in detail was Truelove Lowland at 75° N in Devon Island (Bliss, 1977). Here gently sloping coastal lowlands, protected from the harsher upland climate, enjoy a warmer environment with a locally longer growing season together with availability of water throughout the growing season.

Further north two other outstanding polar oases have been extensively studied on Ellesmere Island. The more southerly, at Alexandra Fiord at 78° 53' N, 75° 55' W (Fig. 6.20) has an extremely short growing season yet supports a vascular plant flora comprising over 90 species (Svoboda & Freedman, 1992). The Sverdrup Pass which lies to the north west is an 80-km-long deglaciated valley that separates the two Ellesmere Island ice fields. The vegetation in the pass is reported as richer at the east end and when studied in the period between 1986 and 1994 by Professor Svoboda and colleagues sustained a resident population of 45-60 musk oxen (Fig. 6.23).

The other notable polar oasis lies further north at Lake Hazen (81° 50' N, 70° 00' W) which in spite of being at a higher latitude has greater biodiversity than Alexandra Fiord. Lake Hazen area is a large open basin with a longer growing season than Alexandra Fiord due to a higher number of 24-hour days and earlier snowmelt compared with Alexandra Fiord. About 117 vascular species were found there compared with 92 at Alexandra Fiord. At this latter more southern location, the lowland is north-facing, losing some radiation energy early in the spring which can delay snowmelt. However, in mid-season the noon temperatures during the sunny, calm days are higher than at Lake Hazen, making Alexandra Fiord a true thermal oasis (J. Svoboda, pers. com.).

Examination of the flora of these polar hotspots reveals that the plants are predominantly aggregates of neoendemic or ancient relict species, which suggests that their existence is a consequence of local moderation of environmental extremes persisting through shifting climatic periods, permitting populations of unique species to survive in these places. There are parallels that can be drawn between the biological oases in Devon and Ellesmere Islands, which have probably had a long history of local environmental amelioration due to local topographic effects on climate, with concentrations of endemic species detected elsewhere. In Central Africa it has been shown that species hotspots are characterized by a local reduction in ecoclimatic variability, or by being located on the boundary to a stable region (Fjeldsa et al., 1997). This is the same situation that exists in the South African Cape flora. In all these cases the floristic hotspot possesses a longer history of occupation by the current vegetation type than is found in neighbouring areas with more variable climatic histories.

At high latitudes, climatic oases are exceptional phenomena and throughout the Arctic the more typical situation is for extensive areas which are poor in species (barrens) to be interspersed with sites where vegetation is both more plentiful and varied. This is most strikingly seen along river courses where sometimes trees can survive many tens of kilometres further north of the main boreal forest. Recent studies using remote sensing have investigated the vegetation of the Hood River region in the central Canadian Arctic. Using satellite imagery (Fig. 6.24) and examining the 'normalized difference of vegetation index' (NDVI) it has been possible to detect sites where high NDVI values corresponded with species richness as estimated from ground surveys. Ground-based sampling showed species richness to vary between 69 and 109 vascular plant species per 0.5 km2 of sample area (Gould, 2000). Sites with the lowest species richness were in the upper reaches of the river, with species numbers generally increasing downstream. Variation in richness along the river was correlated with increasing topographical and microclimatic heterogeneity and reflected changes in the range of site-level variation due to alteration in substrate type and texture, topography, moisture, and

Fig. 6.20 Location of regions studied for botanical diversity in the Canadian Arctic. Note in particular the location of the first arctic biodiversity hotspot to be studied in the Canadian Arctic at Truelove Lowland on Devon Island and also the more northerly locations of subsequent studies at Alexandra Fiord and Lake Hazen in Ellesmere Island. (Reproduced with permission from Svoboda & Freedman, 1992.)

Fig. 6.20 Location of regions studied for botanical diversity in the Canadian Arctic. Note in particular the location of the first arctic biodiversity hotspot to be studied in the Canadian Arctic at Truelove Lowland on Devon Island and also the more northerly locations of subsequent studies at Alexandra Fiord and Lake Hazen in Ellesmere Island. (Reproduced with permission from Svoboda & Freedman, 1992.)

soil pH. The most significant component of the index was an increase in the range of soil pH. Soil pH tends to increase downstream, due to the presence of uplifted marine sediments and tills (Gould & Walker, 1997). The structure and diversity of the vegetation along the arctic river showed also that variation in species richness along the corridor is structured in relation to increasing landscape heterogeneity, with increases in the floristic community distinctiveness (beta diversity) together with species richness within communities.

The facility to use remote sensing to detect species-rich pockets even in distant and uninhabited regions of the Earth should prove a useful conservation tool in providing a definitive global database of areas which could have particular ecological value for their biodiversity.

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