High Mountain Plants And Climate Change

Concern is often expressed for the future fate of mountain vegetation should there be sustained and marked climatic warming. The specific sources of danger fall into two categories:

(1) an upward migration of sedge heaths and forest that will eliminate the subnival and high-alpine communities from mountain summits (Grabherr et al., 1994)

(2) the disappearance of snow patches and their associated species in the subnival zone (Guisan et al, 1995).

In the Central European Alps the vertical extent of the nival zone may be sufficient to accommodate much of the expected upward migration of the alpine flora and thus reduce any imminent danger of species extinctions in these regions. Also, within the alpine zone in higher mountains, there is probably sufficient variation in microclimate to accommodate the changes that may arise from global warming. It has therefore been argued that in these areas there are at present few examples of species that are likely to become extinct in the higher mountain ranges within the next century (Korner, 1995).

A situation similar to that observed in the Alps has also been observed in the Jotunheimen range of central Norway. A resurvey in 1998 of sites which had detailed site descriptions and species lists from 1930-31 showed that there had been an increase in species richness on 19 ofthe mountains, a tendency that was most pronounced at lower altitudes and in eastern areas (Klanderud & Birks, 2003). The greatest increases in abundance and altitudinal advances since 1930-31 were found in lowland species, and particularly in dwarf shrubs and species with wide altitudinal and ecological ranges. However, species with more restricted habitat demands, such as some hygrophilous snow-bed species, were found to have declined. High-altitude species have also disappeared from their lower-elevation sites and have increased their abundance at the highest altitudes.

The investigators suggested that recent climatic changes are likely to be the major driving factor for the changes observed (Klanderud & Birks, 2003). Nevertheless, in any assessment of change in mountain floras it is important to take into account that there have also been significant alterations in land use over the past century which are not without their effects on mountain vegetation. In particular, the reduction in grazing by goats in Norway has probably benefited biodiversity in many upland marginal habitats, especially around former summer grazing stations (saeter). In the Alps, this century has witnessed an increase in species richness on mountain summits. On Piz Linard (3411 m), a pyramid-shaped mountain in the Swiss Alps, there was only one summit species of flowering plant in 1835, four in 1895 and 10 in 1992 (Pauli et al, 2003). A similar study on the summit flora of Mount Glungezer (2600 m) showed that in just 13 years, from 1986 to 2000, which included the 10 warmest years on record in central Europe, the total of 55 species recorded in 1986 had increased to 88 without any losses (Bahn & Körner, 2003).

Despite these observations of increasing biodiversity, which relate principally to the European Alps and Scandinavia, there are other regions of the world where climatic warming over the past 100 years has facilitated the invasion of mountain sites of modest elevation and lacking extensive nival zones by certain aggressive species, to the detriment of the diversity of the alpine flora. In particular the range extension uphill of grasses has resulted in increased competition at such sites, leading to a decreased abundance of the less competitive alpine species. Increased deposition of nitrogen during recent years and changes in grazing and tourism might also have influenced some of the species turnover. It is in these lower and less-studied mountains in particular that examples can be found of species that are on the verge of extinction. In the French Massif Central, the second highest mountain is the Mezenc (1754 m). Just under the summit of the mountain on its north-facing slope (Fig. 10.28, upper panel) grows the last French colony of the rare mountain Senecio leucophyllus. The few remaining plants do not exhibit any loss of vigour or reproductive capacity as a result of climatic warming, but nevertheless appear unlikely to survive due to the invasion of their preferred habitat by grasses (Fig. 10.28, lower panel). Fortunately, the species still exists in the eastern Pyrenees and its likely disappearance from its last French locality will not lead to its extinction.

Although the heterogeneity of the larger and higher mountain systems will provide adequate space for many mountain species even in a warmer alpine world there are nevertheless certain high-altitude areas where deleterious changes to high-altitude vegetation are already evident as a result of recent climatic alterations. Where the climate change is towards drier conditions, adverse effects are common irrespective of temperature change (see also Chapter 5). In the high alpine pastures of southern Tibet, between c. 5000 to 5300 m, up to 30 cm thick Kobresia pygmaea mats

Fig. 10.28 Species extinction on the isolated summit of the Mezenc (1754 m) in the French Massif Central. (Above) Just under the north side of the summit the arrow marks the last location in France of the rare Senecio leucophyllus. Increasing grass cover and advancing trees threaten to cover the open scree habitat on which this species grows. (Below) Senecio leucophyllus on the verge of extinction due to environmental change favouring the expansion of grasses onto montane scree.

Fig. 10.28 Species extinction on the isolated summit of the Mezenc (1754 m) in the French Massif Central. (Above) Just under the north side of the summit the arrow marks the last location in France of the rare Senecio leucophyllus. Increasing grass cover and advancing trees threaten to cover the open scree habitat on which this species grows. (Below) Senecio leucophyllus on the verge of extinction due to environmental change favouring the expansion of grasses onto montane scree.

dominate the south-eastern humid quarter of the Tibetan Highlands where there is a water surplus (Miehe, 1989). This formation developed during younger more humid Holocene phases (the 'Kobresia pygmaea age') but has now been widely destroyed by the Himalayan fohn (warm, dry catabatic wind on the lee side of a mountain range) which creates conditions only suitable for semi-arid alpine steppe on stone pavements. There is a recently observed decreased vigour and lack of regeneration of this alpine turf which is a relict of a past climatic optimum. In the Karakorum Mountains the decrease is particularly evident in plant vigour at climatically sensitive vegetation borders. It is at the upper treeline that drought limits forest survival, as well as in the transition zone between humid alpine mats and alpine steppe. Diminishing winter precipitation during the last decades of the twentieth century has adversely affected these high-altitude plant communities (Miehe, 1996).

10.7.1 Indirect effects of increased temperature on alpine vegetation - reduction in winter snow cover

The second major danger to alpine plants from climatic warming is indirect, and is related to the reduction in snow cover that might take place should global temperatures rise. Plants that are adapted to survive under snow are generally considered to be less frost resistant than those that live in exposed habitats.

There are few detailed studies to corroborate this general assumption. However, a New Zealand study of the annual course of frost resistance of species of native alpine plants from southern New Zealand that are normally buried in snow banks over winter (e.g. Celmisia haastii, C. prorepens, Hebe mora) with other species, typical of more exposed areas, that are relatively snow-free (e.g. Celmisia viscosa, Poa colensoi, Dracophyllum muscoides) has confirmed the assumption that species from snow banks or sheltered areas have the least frost resistance (Bannister et al., 2005).

Away from the snow banks it is the loss of resistance in late winter to early summer (August-December in New Zealand) that is most likely to expose the plants to injury (e.g. Poa colensoi and Dracophyllum muscoides). However, in the principal species examined, seasonal frost resistance was more strongly related to day length than to temperature. This phenological control appears to be sufficient to ensure that frost resistance is likely to be unaltered by climatic warming as the relationship of frost resistance to day length prevents frost damage at any time of year (Bannister et al., 2005).

Notwithstanding the above conclusion, absence or early loss of winter snow is likely to affect water availability to mountain plants and increase the risks of desiccation injury as the season advances and especially if summer temperatures also rise (see below). In Sweden, where the snow-bank-dependent Vaccinium myrtillus suffered lethal injuries during a 5 °C warmer-than-average winter, the damage was associated with a reduction over winter in shoot solute content brought about by the progressive respiratory loss of cryo-protective sugars (Ogren, 1996). There would appear therefore to be two classes of mountain plants in relation to susceptibility to warmer winters at high altitudes: one from exposed ridges where frost tolerance is photoperiodically induced and will therefore persist even in a warmer climate, and a second where cover under snow is essential and where exposure as a result of warmer winters will lead to a loss of frost tolerance due to desiccation.

10.7.2 Effects of increased atmospheric CO2 on high mountain vegetation

The current change in atmospheric greenhouse gases inevitably raises the question as to whether increasing global levels of carbon dioxide will be beneficial to high-altitude vegetation. The low growth rates and small stature of plants from high altitudes do not suggest that growth is limited by carbon dioxide availability. In addition, comparisons of related species from low and high altitude sites in the European Alps have shown that the possession by high-altitude species of thicker leaves, with well-developed palisade layers more than one cell deep (see Section 3.2.1), together with a higher protein content (largely ribu-lose carboxylase, RuBisCO) suggests that the alpine species are well adapted to utilize current levels of atmospheric carbon dioxide even when growing at high altitudes where the partial pressure of carbon dioxide is reduced.

Morphological adaptations are noticeable in relation to gas exchange in high-altitude plants. Alpine species show higher levels of both ab- and ad-axial sto-matal density. These morphological and physiological adaptations enable plants to compensate metabolically for low temperatures, and also the brevity of the growing season. A similar phenomenon with increased enzymatic capacity is seen in the low temperature muscular physiology of fish from cold waters and is described as capacity enhancement irrespective of whether it is due to morphological or metabolic adaptations or both (Hochachka & Somero, 1973). Due to capacity enhancement, many species from high altitudes have higher rates of CO2 uptake as determined on a leaf area basis than their related lowland species (Körner et al., 1989). The greater efficiency of thicker leaves in alpine plants in taking up carbon dioxide at high altitudes can be likened to the greater vital capacity of the lungs of Quechua Indians who have been born and raised at high altitudes, in that they have a greater lung vital capacity (the volume change of the lung between a full inspiration and a maximal expiration) which aids their uptake of oxygen (Heath & Williams, 1977).

Notwithstanding the efficiency of high-altitude plants in taking up carbon dioxide there still remains the question of whether or not increased levels of carbon dioxide will have any effect on carbohydrate supply and growth on high-altitude vegetation. Comparisons between related high and low altitude species, e.g. Ranunculus glacialis and R. acris and the similar altitudinally distinct Geum reptans and G. rivale grown in CO2-enriched atmospheres (Fig. 10.29), have demonstrated that alpine plants may be able to obtain, at least initially, greater carbon gains from increased carbon dioxide availability than comparable lowland plants (Körner & Diemer, 1994). Whether or not increases in RuBisCO activity will lead automatically to increased productivity is open to question. Decreased rates of starch accumulation in leaves may actually allow a more efficient use of fixed carbon as lower amounts of RuBisCO can make available nitrogen that would otherwise be limiting growth due to sequestration in Rubisco protein. In perennial plants from arctic and montane habitats, growth is not directly related to photosynthesis and many plants retain a small stature despite their photosynthetic activity. Futile cycles that oxidize sugars without generating ATP can operate and reduce metabolic efficiency, a process that has been studied in the boreal cold desert shrub Erotia lanata (Thygerson et al., 2002). Thus, even though alpine plants may increase photosynthetic activity with elevated CO2 levels, this carbon gain may be dissipated by futile cycles and not necessarily translated into higher biomass production. It follows also that if lower levels of RuBisCO can increase growth then higher levels of RuBisCO may cause a reduction. In many habitats a reduction in growth and an increase in stored carbohydrate from increased photosynthetic activity could have a significant effect on survival.

In uncertain environments, such as the neighbourhood of snow banks that do not always melt promptly in spring, starch accumulation without growth is important in aiding plants to overcome long non-productive periods. High levels of soluble carbohydrates are also necessary to provide cryo-protectants for plants in cold regions. Some plants that live in snow patches can survive 2-3 years without emerging from the snow bank (Pielou, 1994) and such a feat is dependent on adequate carbohydrate reserves. If, in addition to being covered in snow the plants have to endure the hazard of also being encased in ice, this can induce a prolonged period of anoxia which places additional demands on carbohydrate reserves (see Section 3.6.3).

It would appear that the enhanced capacity adaptation (whether of enzymatic or morphological origin) that ensures that mountain species have adequate supplies of carbon dioxide even at high altitudes would make it unlikely that any increased in levels of atmospheric carbon dioxide will have any great effect either on growth or survival given their intrinsic low growth rates. In a number of ways the plants that live at high altitudes present an example of the long known phenomenon of the Montgomery effect, which asserts that 'ecological advantage is conferred by low growth rates in areas of low environmental potential' (Montgomery, 1912)'.

In this respect it is relevant to examine the results that have been obtained experimentally by providing increased levels of atmospheric carbon dioxide to trees growing at the treeline. Historically, it has been argued that carbon, through a shortage of photo-assimilates, limits the growth of trees at the upper altitudinal treeline. Re-examination of this possibility in a wide range of alpine habitats has failed to reveal any clear metabolic evidence of carbohydrate limitation to plant growth either in alpine vegetation or in trees at the upper limits of distribution at the treeline (Körner, 2003).

Fig. 10.29 Effects of increased atmospheric CO2 on pairs of comparable high and low altitude species. (a) Ranunculus glacialis and (b) Geum reptans from high altitudes; (c) R. acris and (d) G. rivale from low altitudes. (Reproduced with permission from Körner & Diemer, 1994.)

Despite this apparent satiation of the current needs for photo-assimilates in high-altitude plants there nevertheless remains the possibility that future increases in atmospheric concentrations of carbon dioxide may stimulate plant growth at high altitudes where the reduction in partial pressure of all constituents of the atmosphere might make additional CO2 beneficial. In a three-year free-air CO2 enrichment (FACE) experiment, two species of 30-year-old alpine conifers (Larix decidua and Pinus uncinata) were studied in situ in the Swiss Central Alps (2180 m above sea level). Elevated CO2 enhanced photosynthesis and increased non-structural carbohydrate (NSC) concentrations in the needles of both species (Fig. 10.30). While the deciduous larch trees showed longer needles and a stimulation of shoot growth over all three seasons when grown in situ under elevated CO2, pine trees showed no such responses. The study also involved the removal of needles to determine if defoliation stimulated photosynthesis either in the

Timberline Vegetation Mountains
Fig. 10.30 Effect of increased levels of atmospheric carbon dioxide on lateral shoot extension (n — 5) over 2 years in undefoliated and defoliated trees of Larix decidua and Pinus uncinata growing at the timberline in the Swiss Alps. (Reproduced with permission from Handa et al, 2005.)

current-year needles or those produced in the following year. Defoliated larch trees had fewer and shorter needles with reduced NSC concentrations in the year following defoliation and showed no stimulation in shoot elongation when exposed to elevated CO2. By contrast, defoliation of the evergreen pine trees had no effect on needle NSC concentrations, but stimulated shoot elongation when defoliated trees were exposed to elevated CO2.

The conclusion after three years of this study suggests that deciduous larch is carbon limited at treeline, while evergreen pine is not (Handa et al., 2005). Whether this extends to other treelines where both deciduous and evergreen trees can be found is an intriguing question. If the response to additional CO2 depends on whether the trees are evergreen or deciduous it is probable that other factors such as growing season length, temperature, moisture and nutrients will also be involved.

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