Phenological Responses To Increased Temperatures

This account of arctic vegetation so far has been orientated towards a discussion of the likely long-term effects of climate change in the belief that for an ancient and heterogeneous arctic flora the main impacts of climatic change will be found at a population or species level. There is no doubt that phenotypic responses to increasing temperatures are already taking place. In the Arctic these have already been mentioned in relation to the profuse flowering by the purple saxifrage that can now be found on cold shores (Fig. 6.6). Whether or not these phenological responses will have a long-term effect on the future status of high-latitude vegetation is still a difficult question. In the case of Saxifraga oppositifolia, which everywhere in the Arctic can be found as a highly successful pioneer species, increased flowering, if it leads to increased seed production, should facilitate its spread to newly deglaciated terrain. This species has already demonstrated its ability to persist once glacial conditions have retreated by its well-documented occurrence in areas from which the ice has long vanished. One particularly striking location is the lowland Scottish site at the Falls of Clyde. For other species the question still remains: will unaccustomed heat cause extensive destruction and loss of biodiversity in an ancient cold-adapted flora?

Many experimental studies have been carried out on the short-term phenological responses of vegetation to climatic change by placing plastic shelters in various habitats around the Arctic and making detailed observations of the effects of the artificially induced higher temperatures. In some cases this has also included the addition of nutrients. Most notable among these studies has been the International Tundra Experiment (ITEX), a collaborative experiment using a common temperature manipulation to examine variability in species reactions to increased temperatures across a wide variety of tundra sites. The data recorded the vegetation responses in terms of plant phenology, growth, and reproduction. Details of the results as reported after the first four years of these observations revealed many phenological differences between sites and species. A general conclusion in the report (Arft et al., 1999) indicated that key phenological events such as leaf bud burst and flowering occurred earlier in warmed plots throughout the study period, but that there was little impact on growth cessation at the end of the season. A shift away from vegetative growth and towards reproductive effort and success in the fourth treatment year was taken to suggest a shift from the initial response to a secondary response. The change in vegetative response may be due to depletion of stored plant reserves, whereas the lag in reproductive response may be due to the formation of flower buds one to several seasons prior to flowering.

Warmer, low arctic sites produced the strongest growth responses, but colder sites produced a greater reproductive response. It was suggested that greater resource investment in vegetative growth might be a conservative strategy in the Low Arctic, where there is more competition for light, nutrients, or water, and less opportunity for successful germination or seedling development. By contrast, in the High Arctic, it was speculated that heavy investment in producing seed under a higher temperature scenario may provide an opportunity for species to colonize patches of unvege-tated ground, as appears to be illustrated by the flowering success already mentioned for the purple saxifrage.

The main interest of this research lies in the diversity of responses that can be found between different life-forms and different species, depending on

Fig. 6.22 Aerial view looking to Lake Hazen from above the thermal oasis. Although further north than Alexandra Fiord this oasis supports a flowering plant flora of 117 species. (Photo Professor J. Svoboda.)

whether they are in the Low Arctic or the High Arctic. However, in terms of predicting the long-term consequences of climatic change on arctic vegetation these experiments still leave considerable uncertainty, and consequently reliable predictions are difficult to make.

When experiments are pursued over longer periods of time some of these initial phenological responses can disappear altogether.

In a series of studies carried out manipulating light, temperature, and nutrients in moist tussock tundra

Fig. 6.23 Oblique aerial view showing the eastern portion of Sverdrup Pass. This deglaciated 80-km-long pass studied by Professor Svoboda and colleagues between 1986 and 1994 separates two of Ellesmere Island's major ice fields. The vegetation in the pass is reported as richer at the east end and sustained a resident population of 45-60 musk oxen. (Photo Professor J. Svoboda.)

near Toolik Lake, Alaska, it was found that short-term (3-year) responses were poor predictors of longer term (9-year) changes in community composition (Chapin et al., 1995). Instead the longer-term responses showed closer correspondence to patterns of vegetation distribution along environmental gradients that were evident in changes in the availability of soil nutrients. In particular, nitrogen and phosphorus availability tended to increase in response to elevated temperature, reflecting increased mineralization. The major effect of elevated temperature was to accelerate plant responses to changes in soil resources and, in the long term (9 years), to increase nutrient availability through changes in nitrogen mineralization. It has been suggested that the lag in response is due to the time needed for litter fall to alter the nutrient status of the underlying soil.

In Alaska there is increasing evidence of climatic warming on woody plants. Shrubs trigger several feedback loops that influence their expansion rate (Chapin et al., 2005). The responses of vegetation to increased temperature, as seen in Alaska with its extensive moist tussock-tundra, cannot be taken as indicative of what will happen elsewhere in the Arctic. Detailed discussion on the whole question of the availability of resources in marginal areas including the Arctic is given in Chapter 3 and the diverse effects of

Fig. 6.24 Location ofan arctic biodiversity hotspot along a river course. (Left) Location ofBathurst Inlet and the Hood River. (Right) Satellite NDVI image of Bathurst Inlet and the surrounding area in July. The NDVI image provides a relative comparison of photosynthetic activity between the surrounding barrens (blue) and the river course (green). The more continental location of this relatively southern polar hotspot (66° 49' N — just north of the Arctic Circle) results in a later start to the growing season than either of the two northern sites described above at Lake Hazen and Alexandra Fiord. (Satellite image by courtesy of Dr W. Gould.)

Fig. 6.24 Location ofan arctic biodiversity hotspot along a river course. (Left) Location ofBathurst Inlet and the Hood River. (Right) Satellite NDVI image of Bathurst Inlet and the surrounding area in July. The NDVI image provides a relative comparison of photosynthetic activity between the surrounding barrens (blue) and the river course (green). The more continental location of this relatively southern polar hotspot (66° 49' N — just north of the Arctic Circle) results in a later start to the growing season than either of the two northern sites described above at Lake Hazen and Alexandra Fiord. (Satellite image by courtesy of Dr W. Gould.)

climatic warming on the tundra-taiga interface have already been reviewed in Chapter 5.

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