Because productivity in most arctic ecosystems is constrained by low nutrient availability, the most common responses to nutrient addition are increases in nutrient uptake and plant nutrient mass followed by increased plant production and biomass. Similar effects on growth can be expected in response to warming through direct responses of increased productivity in a warmer environment and through enhanced nutrient uptake as a result of increased nutrient supply rate, as mineralization is likely to increase in the warmed soils (Nadelhoffer et al., 1992).
As expected, addition of fertilizer has almost always led to increase in nutrient uptake, tissue nutrient mass, and net primary production. Tissue turnover rates generally have increased because of community changes toward increased proportion of species with short leaf-longevity. In several cases, fertilizer addition has also led to transient responses in biomass. For instance, while the biomass increased during the first two years of fertilizer addition to subarctic, northern Swedish forest-floor vegetation (Parsons et al, 1994), the response did not continue after five years (Press et al, 1998a) because the grass Calamagrostis lapponica expanded strongly and affected growth of dwarf shrubs and mosses negatively (Potter et al, 1995; Press et al, 1998a). Similarly, the canopy density and mass of the deciduous Betula nana increased in Alaskan tussock tundra over nine years of fertilizer application and reduced the biomass of most other species (Chapin et al, 1995). As a result of the increase of B. nana and decline of other species, the mass of vegetation C underwent no, or only small changes. In contrast, NPP increased because species with long leaf-longevity were replaced by the deciduous B. nana, implying that the tissue turnover rate also increased. However, aboveground biomass increased strongly after another six years, i.e., after 15 years of treatment, because B. nana continued to accumulate biomass (Shaver et al, unpublished data). Much of this increase was due to wood formation resulting in a decline of tissue turnover rate in comparison to the response after nine years. Hence, it appears that the transient responses of the vegetation are coupled to changes in the dominance of single species and particularly of those that form a dense or elevated canopy. Indeed, in vegetation types without any pronounced change in relative proportions of dominant species or life forms following fertilizer addition such as in Swedish treeline and high-altitude heaths and in Alaskan wet sedge tundra, most dominant life forms increased. This resulted in up to a doubling of biomass after 5-9 years of treatment (Jonas-son etal, 1999b; Shaver ct al, 1998).
The nutrient content in the fertilized vegetation increased strongly in all vegetation types where nutrient analyses were done (no analyses have been done in the forest floor vegetation). In tire strongly responding Swedish treeline and high-altitude heaths, the increase in N and P was only slightly higher than the proportional increase in biomass due to relatively small effects on vegetation nutrient concentration, except for a strong increase in nutrient concentration of mosses (Jonasson ct al, 1999b). In contrast, the nutrient incorporation in the tussock tundra doubled (N) or tripled (P) after nine years of fertilizer addition, in spite of unchanged biomass, due to an increase of tissue nutrient concentration in the vegetation. The increase of nutrient concentration was particularly high in the mosses, as at the Swedish sites (Chapin et al, 1995). Also the nutrient requirement, i.e., the uptake into the new growth, increased strongly and tripled (N) or increased seven- to eight fold (P) in graminoids and deciduous shrubs, indicating that at least the transient changes in turnover were much more pronounced than llie changes in standing slocks.
A different response pattern was found at a polar semidesert. Fertilizer addition increased plant coverage strongly during the first years of treatment but was set back after an exceptionally warm and wet autumn and winter with strong winter injury and high mortality of the dominant Dryas octopetala. This effect was probably because winter hardening was delayed in plants that had received extra nutrients (Robinson et al, 1998), and it highlights the importance to the vegetation of "unusual" climatic events. However, the moss cover increased strongly, contrasting with the usually decreased coverage in fertilized Alaskan tundra sites and in the Swedish sites, particularly in those where the canopy density of the vascular plants increased. Indeed, it appears that the cryptogams generally increase in coverage and biomass after fertilizer addition until a point is reached where the negative effects of increasing vascular plant cover and litter override the positive effect of fertilizer addition (Jonasson, 1992; Jonasson et al, 1999b; Molau and Alatalo, 1998; Chapin et al, 1995). This nonlinear response in mosses and lichens has a particular relevance because the cryptogams are important regulators of heat and water exchange between the soil and the atmosphere (Tenhunen et al, 1992; McFadden et al, 1998). At the same time, they affect the N cycle through the N-fixation ability of many dominant lichen species and blue-green algae associated with mosses. They also affect the turnover rates of organic matter because of low decomposability of their tissues (Hobbie, 1996).
The response to fertilizer addition, indeed, shows a generally strong sensitivity of arctic tundra to any change that leads to in creased availability of production-limiting nutrients, for instance in N deposition. Furthermore, local disruption of the organic horizon has led to strong increase of soil nutrient mineralization, plant nutrient uptake, and in many plant species a doubling of tissue nutrient concentration in heavily exploited tundra (B. Forbes, unpublished data), mirroring the effects of fertilizer addition.
Warming of tundra vegetation within the range of predicted temperature enhancement of 2-4°C for the next century has generally led to smaller changes than those induced by fertilizer addition and always to greater responses than those after water addition (Shaver and Jonasson, 1999). For instance, temperature enhancement in the high-arctic semidesert increased plant cover within the growing seasons but the effect did not persist from year to year (Robinson et al, 1998). The strongest effect was on sexual and asexual reproduction and seed germinability, which increased strongly (Wookey et al, 1993, 1994). The demonstrated enhanced reproductive success is likely to increase colonization of the present large areas of bare soil surfaces and, hence, increase plant coverage and carbon sequestration.
In the low Arctic, community biomass and nutrient mass changed little in response to warming in two Alaskan tussock sites (Chapin et al, 1995; Hobbie and Chapin, 1998) and in two wet sedge tundra sites (Shaver et al, 1998), coincident with relatively low changes in soil nutrient pools and net mineralization. In the tussock tundra the lack of response basically was because some species increased in abundance and others decreased (Chapin and Shaver, 1985; Chapin et al, 1995), similar to a pattern observed in the subarctic Swedish forest floor vegetation (Press et al, 1998a). In the Alaskan tundra, where nutrients were analyzed, this led to redistribution of nutrients within the vegetation, with increased proportions allocated to the vascular plants and decreasing proportions allocated to the cryptogams.
However, the responses to warming were much stronger in the Swedish tree-line heath and in the fellfield (Jonasson et al, 1999b). The biomass in the low-altitude heath increased by about 60% after air warming by about 2.5°C with little additional effect after a further warming by about 2°C. In contrast, the biomass approximately doubled after the low-temperature enhancement and tripled in the higher temperature enhancement treatments at the colder fellfield. Hence, the growth response increased from the climatically relatively mild forest un-derstory through the treeline heath to the cold, high-altitude fellfield where the response to warming was of the same magnitude as the response to fertilizer addition. Along with the increase in tissue nutrient mass, the nutrient concentration in individual species either remained unchanged, increased or decreased. In some species, particularly at the cold fellfield site, the nutrient concentration declined strongly coincident with increased productivity, suggesting that they responded strongly to the direct effect of warming, and possibly that their nutrient stress increased due to the temperature-induced growth (Jonasson et al, 1999b; Graglia et al, 1997).
After combined warming and fertilizer addition the biomass and vegetation N and P mass increased additively or synergistically at the Swedish sites, but there was a negative temperature X fertilizer interaction in both the Alaskan tussock and wet sedge tundra sites. The negative interaction occurred because plant respiration increased in the combined treatment and led to decreased bionrass and nutrient incorporation in the vegetation (Shaver et al., 1998).
In the warming treatments, net nutrient mineralization increased only slightly at the Swedish sites (Schmidt et al., 1999) and cannot explain the increase of tissue N and P mass and illustrate the discrepancy between net nutrient mineralization and nutrient uptake discussed above. Furthermore, the microbial biomass was largely unaffected by both nutrient addition and warming and the microbial nutrient content increased only when there was also a marked increase of soil inorganic nutrients. That is, the microbes absorbed extra nutrients only when the nutrient sink strength declined in plants (Jonasson et al., 1999b). This speaks against strong plant-microbe competition and suggests that the plants, indeed, are able to sequester nutrients even when there is a substantial microbial sink. On the other hand, the nematode density also increased and the proportion of fungal feeders increased with warming (Ruess et al., 1999). Nematodes are the main predators on the soil microflora, so their increased population density and the changes in their trophic structure suggest that the microbial productivity and activity may have increased but that the biomass was kept at a constant level due to predation. This uncertainty highlights the potential biogeochemical regulation of soil processes by the soil fauna, which so far is almost entirely unknown in the Arctic.
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