Recent evidence has accumulated suggesting an alarming rate of climate warming, with global air temperature predicted to rise by 1Â°C-4.5Â°C (Arft et al, 2001 and Starr and Ahlquist, 2008) and increases in the arctic region (Fig 1.1) ranging from 2.8Â°C-7.8Â°C by the end of the 21st century (Christensen et al, 2007). Globally, tundra ecosystems play a fundamental role in the climate system as reservoirs for carbon (C) as a large amount is stored belowground, with estimates ranging from 250 to 455 Pg of C (Oechel and Billings, 1992) trapped in the frozen soil. Therefore shifts in the balance between respiration and photosynthesis due to climate change, potentially could have a major impact on C fluxes between the ecosystem and the atmosphere.
Figure 1.1. Annual surface temperature change between 1980 to 1999 and 2080 and 2099 in the Arctic. Christensen et al, 2007.
In tundra ecosystems, where mean growing season temperatures are low, an increase of a few degrees can produce a significant rise in warmth available to plants and decomposers. Previous studies indicate long-term warming may turn tundra ecosystems that have long been sinks into net sources of CO2 because rates of decomposition could overtake rates of plant production (Sullivan et al, 2008), creating large stocks of soil C. For instance, the plant response to warming could affect processes such as nitrogen (N) cycling (Quested et al, 2003), litter composition and energy exchange with the atmosphere (Sullivan and Welker, 2005). This may cause increases in C uptake, slowing increase in atmospheric CO2, or could result in more C efflux creating a positive feedback, accelerating increase in atmospheric CO2.
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As well as air and soil warming, Starr and Ahlquist (2008) predict a 40% increase in growing season length which is likely to be important for C assimilation of tundra plant species, as an earlier onset or higher rates of early season growth may enhance nutrient uptake and light interception. Also, delayed senescence may enhance resource capture during the late summer period when soil thaw is greatest (Sullivan and Welker, 2005).
In the arctic, where few species dominate, the potential for individual species to mediate climate change is greater as their functional characteristics appear to be important controls on ecosystem processes (Sullivan and Welker, 2005). The capacity of tundra species to fix CO2 is low as nitrogen mineralisation and photosynthetic rates are strongly limited by temperature. Rising temperature is likely to increase the rate of photosynthesis in fragile arctic environments by increasing the availability of nitrogen and nutrients, which may lead to greater carbon sequestration or increased ecosystem respiration which would differentially alter the current carbon balance.
1.2 Overview of the study
Figure 1.2. Map showing location of Toolik Lake Field Station, Alaska. (Walker et al, 1994). This study was carried out in the low arctic at Toolik Lake Field Station (68Â°38'N, 149Â°36'W, elevation 720 m), Alaska (Fig 1.2). The surface microtopography of the site is a mosaic of tussocks (Fig 1.2), which extend above the water table (Walker et al, 1994), and inter-tussocks, which lie at a lower elevation beneath 5-15cm of water during the summer growing season (Sullivan et al, 2008). Tussock tundra (Fig 1.3) occurs throughout arctic regions and is dominated by graminoids, as well as deciduous and evergreen shrubs. The soils in this community are unevenly covered with an organic mat and underlain by permafrost about 50 cm below the Tussock (Corradi et al, 2005).
Figure 1.3. Photograph illustrating the formation of tussock tundra. 'Tussock' refers to the physiognomy of the tussock-forming sedge Eriophorum vaginatum. (Walker et al, 1994). Photograph by author.
This study focuses on how leaf level photosynthetic exchange responds to long term warming using Open Top Chambers (OTCs) in the field, and how this differs between two species from different functional groups. The two species chosen for sampling were graminoid Eriophorum Vaginatum (E. vaginatum) and woody deciduous shrub Betula Nana (B. nana), as these are two of the most abundant vascular plant species in tussock tundra of the North Slope (Chapin, 1996).
This research is important as relatively little is known about the nature of variation in growth, reproduction and phenological response among species to warming (Arft et al, 2001). Understanding this is crucial to our ability to adequately predict and understand low arctic tundra ecosystem response to a changing climate, at leaf, whole plant, and community and ecosystem levels.
1.3 Aims of the research
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The aims of this research were fourfold. First, was to further the understanding of ways in which low arctic moist tussock tundra ecosystem responds to OTC warming by recording environmental data for the season, such as air and soil temperatures, and soil moisture and thaw depth, from control and OTC plots. Second, was to test the sensitivity of B. nana and E. vaginatum after 9 years of warming during the growing season, by taking leaf level measurements of photosynthetic exchange in both control and OTC plots. Third, was to gain insight into the temperature optimum of E. vaginatum and B. nana to 9 years of OTC warming and to specific temperature manipulations of 10, 15 20 and 25Â°C. Fourth, to analyse data to discover the extent of plant responses after 9 years of experimental warming and isolate the role of species in controlling ecosystem functioning.
Given the results of previous studies (Walker et al 1994, Chapin 1983, Arft et al, 2001 and Sullivan et al, 2008) there were several key trends we expected to see in our study:
1 - Photosynthetic rate will be greatest (for both species) at 20Â°C as this most closely resembles mean peak summer temperature.
2 - Photosynthetic rates of both E. vaginatum and B. nana will be higher in OTCs than control plots, at each temperature manipulation (10, 15, 20 and 25Â°C).
3 - Responsiveness of photosynthesis will differ between plant functional groups.
The main objective of this research is to pinpoint and explain any interesting results and apply them on the broader scale, considering how these trends may impact whole plant and community level interactions with the surrounding ecosystem, and whether this is strong enough to influence the C cycle.
Review of Current Knowledge
2.1 Arctic Climate Change
Air temperatures have been predicted to increase during this century, with global air temperature predicted to rise by 1Â°C-4.5Â°C (Arft et al, 2001 and Starr and Ahlquist, 2008) and increases in the arctic region ranging from 2.8Â°C-7.8Â°C (Fig 2.1) by the end of the 21st century (Christensen et al, 2007), indicating temperature will play an important role in arctic region.
Figure 2.1. Observed and predicted temperature increase in the arctic region. Christensen et al, 2007.
Starr and Ahlquist (2008) hypothesise that recorded and predicted temperature increases are likely due to anthropogenic practices that continue to enhance concentrations of carbon dioxide in the atmosphere. This warming is having a considerable impact on the physical environment that drives weather patterns in northern latitudes, including thinning of the pack ice in the Arctic Ocean (Maxwell et al, 1992). Reduction in the sea ice has increased surface water movement therefore increased ocean mixing, which directly affects precipitation events and other weather patterns within the arctic region. The combination of increasing winter temperatures and changing weather patterns may lengthen the growing season and cause warmer peak summer temperatures, which, in turn, will likely influence the depth of permafrost thaw, ecosystem productivity and community composition. It is still uncertain though, as to how this will impact the arctic C balance, which is an important issue that needs to be addressed, as tundra ecosystems play a fundamental role in the global climate system, as reservoirs for C.
2.2 Climate Change and the carbon balance in the arctic
Arctic climate warming is expected to strongly affect the regional C balance; of particular concern are the very large stores of carbon present as peat in the frozen soil of arctic ecosystems, estimated at being between 250 to 455 Pg (Oechel and Billings, 1992). In most terrestrial ecosystems, warming would result in a net sink of CO2 as more C is sequestered for growth and production creating a large C stock (Fig. 2.2). However, in the arctic tundra region, 90% of carbon is stored below the soil surface (Jenkinson et al, 1991), as aboveground biomass is less, and efflux of carbon through soil respiration is known to depend strongly on temperature. Therefore, the balance between C sequestration gains, microbial respiration losses and potential release of stored soil C, as the arctic warms and dries, will determine the extent of C balance change in the arctic, and if these changes will affect the global C balance. However, the arctic encompasses a wide range of tundra ecosystems with differing productivity (Oberbauer et al, 2007); therefore the C balance could be affected in different ways, depending on site specific characteristics.
Figure 2.2. Idealised response functions of (A) plant photosynthesis and ecosystem respiration and (b) net ecosystem exchange to temperature (Luo, 2007).
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An extremely important piece of research conducted by Starr and Ahlquist (2008), has led to predictions that the arctic growing season may increase by as much as 40% by the middle of this century, with the increase attributable to an earlier snowmelt during spring, and a later accumulation of snow during the autumn. This large increase in growing season length would affect ecosystem functioning, community structure and physiological activity significantly. One of the most important factors affected would be permafrost, which has controlled the structure and function of arctic plant communities over tens of thousands of years (Maxwell et al, 1992). Permafrost limits plant development, as it keeps soil temperatures in the active layer just above 0Â°C, which restricts root growth and kinetics, restricting the ability of roots to move water and nutrients needed for physiological activity (Starr and Ahlquist, 2008). Changes in soil conditions caused by warming North Slope permafrost would affect nutrient cycling, soil moisture and aeration (Maxwell et al, 1992), affecting photosynthetic rates and eventually could lead to changes in plant communities if warming persists.
Another important conclusion made by Starr and Ahlquist (2008) is the importance of water stress and a decrease in transpiration, which occurs when increases in depth of thaw are combined with longer periods of warm dry weather (Starr et al, 2000), as is the case with a longer growing season. Prolonged water stress could result in either; 1) A decrease in water within the rhizosphere leading to a reduction in plant growth and diminished photosynthetic activity, with a reduction in the C sink capacity at ecosystem level, or, 2) If water is not limiting, physiological activity may increase in response to warming temperatures, creating a C stock and increasing the C sink capacity of the tundra.
2.3 Plant response to climate warming
2.3.1 Whole plant response
Rising air temperatures may differentially alter photosynthesis and respiration, especially in the arctic region where plants and soil composition have greater temperature constraints. The tundra is a fragile ecosystem with low nutrient mineralisation as a consequence of low temperatures; therefore it is more sensitive to rises in temperature than lower latitude environments. As temperatures warm, chemical reactions occurring during photosynthesis speed up (Lloyd & Taylor 1994), resulting in an increase in photosynthesis and plant growth therefore increased C sequestration by plants.
Photosynthetic response to rising air temperatures may be an important response to warming, as climate controls the natural flora and fauna present in the tussock tundra environment. Warming may induce direct short-term changes in the form of phenotypic responses of current vegetation. Such shifts in total leaf area can increase ecosystem C fixation even if the rate of C fixation per unit leaf area remains unchanged (Chapin et al, 2002). Long-term responses may involve changes in species composition such as slow growing evergreen species with low nutrient loss rates being replaced by deciduous and/or graminoid species with high nutrient loss rates (Aerts, 2009), thus altering biodiversity. Since the OTCs have been in operation for 9 years at Toolik field station it would be expected that deciduous and gramininod species will supercede the evergreen species.
The work of Aerts (2006) is particularly relevant, as this study confirms warming leads to higher N availability in the soil, recording an increase of approximately 70% meaning a higher leaf N concentration and greater photosynthesis (Fig 2.3). Aerts's more recent work (2009) detailing the short-term effects of N supply on leaf dynamics in tundra environments, has indicated there is considerable variation between species, and so functional group, in the response of leaf production to an increased N supply. In terms of leaf dynamics and plant response to increasing N supply, it has been suggested that graminoids, such as E. vaginatum, are more responsive than deciduous shrubs, such as B. nana, which in turn are more responsive than evergreen shrubs (Questead et al, 2003).
Figure 2.3. Relationship between leaf N concentration and photosynthetic rate. Chapin et al, 2002.
Studies such as Starr et al (2000) and Arft et al (2001) have confirmed that soil temperature has a direct impact on the depth of permafrost thaw, which as noted by Starr and Ahlquist (2008), enhances the ability of plants to acquire nutrients via improved root kinetics. As arctic plants compensate for lower temperatures by producing leaves with high quantities of leaf nitrogen (N) and photosynthetic enzymes, this research question was concerned with gaining insight into whether leaf area or photosynthesis would increase given the fact that N availability is enhanced by greater temperature and the constraints and tradeoffs associated with this. It may be however that after 9 years other factors such as light and water availability become dominant controls meaning arctic plants can no longer take advantage of higher N availability, producing little or no change in photosynthesis.
Sullivan and Welker (2005) performed a detailed analysis of graminoid species E. vaginatum, and its response to warming. They found that E. Vaginatum produced leaves sequentially in ambient tundra conditions, such that late-season cohorts overwintered and resumed photosynthesis the following spring. In the warming treatments however, maximum leaf growth rates occurred early with peak biomass occurring 20 days earlier. Consequently, the plants growing under higher temperatures maintained more live leaf biomass during the period of highest photosynthetically active radiation (PAR). Sullivan and Welker (2005) also identified a period of relatively high nutrient availability in soils, which showed a positive correlation with greater root biomass. This study indicates that both above and belowground biomass of E. Vaginatum responds favourably to warming, taking advantage of higher nutrient levels for increasing photosynthesis and growth, therefore it was expected that more E. Vaginatum would be more abundant in OTCs and that its photosynthesis would be greater.
B. nana, a deciduous shrub species, also produces leaves sequentially, however as wood has a greater C:N ratio than leaves, B. nana stores a greater amount of C per unit of N, than graminoid species (Shaver et al, 2001). This means that the photosynthetic response of deciduous shrubs to warming is great during the first year of study, as the number of leaves is predetermined in the previous year, so they allocate 'extra' nutrients to photosynthesis rather than growth. However in following years, the additional nutrients are allocated toward increased total leaf area and an early seasonal bud break (Pop et al, 2000). This suggests after 9 years of OTC warming at Toolik Lake, B. nana will not use increased N availability for photosynthesis therefore photosynthetic rates in OTC plots were not expected to be significantly altered.
2.3.2 Leaf level interactions
The key axis of this study was centered on the sensitive and complex leaf level interactions described by Chapin et al, (2002). Their research investigated the limiting interactions of temperature on photosynthetic rates and how arctic vegetation compensates for this. Photosynthesis operates most efficiently when the rate of CO2 diffusing into the leaf matches the biochemical capacity of the leaf to fix CO2. When temperature rises or falls, plants adjust the components of photosynthesis so CO2 diffusion and biochemistry are about equally limiting to photosynthesis (Farquar and Sharkey, 1982). They do this by the process of stomatal conductance (Fig 2.4). As photosynthetic rates of tundra species are limited by temperature and the capacity of the leaf to fix CO2 is low, this study was interested in understanding how photosynthetic rates change as a result of temperature, what the photosynthetic temperature optimum might be and how this varies among plant functional type.
Figure 2.4 Schematic cross-section of a stoma of a leaf showing the pathway of CO2 and H2O in daylight. Ci, Cs, Ca: internal, surface and ambient CO2 concentration; ei, es, ea: internal, surface and ambient air humidity. (Van de Geijin for FAO (2008).
Stomatal conductance (gs) regulates the diffusion of CO2 between atmospheric CO2 and chloroplasts, and for efficient functioning of a plant; gs must be tuned to the photosynthetic metabolism of the leaf (Chapin, 2002). Therefore gs plays a key role in controlling ecosystem functional response to rising temperatures. gs responds to external variables within the ecosystem, such as temperature, and it varies over time both throughout the day, and over the growing season (Farquar and Sharkey, 1982), therefore a firm understanding of stomatal conductance was necessary for this study.
Lastly, the current understanding of phenolics of tundra species is comprehensive, with many studies (Pop et al 2000, Graglia et al 2001, Sullivan and Welker 2005 and Aerts 2009) describing the characteristics of individual plant species. Their phenolic response to warming however, is uncertain, as this can change over time as warming continues. As tussock tundra is strongly nutrient limited, the costs to increase leaf area or photosynthetic apparatus are great and thus, plants may limit their growth or phenological capacity. Therefore the way in which a plant responds to warming, strongly depends on the functional group.
The work by Graglia et al (2001), has highlighted the importance of C based secondary compounds, including condensed and hydrolysable tannins, flavonoids, phenolic glucosides and chlorogenic acids, acting as grazer deterrents in heavily herbivorised tundra shrub species. Secondary metabolites make leaves unpalatable for herbivores and their effects can be toxic as the proteins can prevent the herbivore from assimilating N from the forage. Graglia et al (2001) compared phenolic concentrations in two populations of B. nana, one at Toolik Lake, Alaska, one at Abisko, Greenland. Their study found no effect at Abisko but that responses of bulk phenolics to warming were pronounced at Toolik, as condensed tannins increased. These processes are very C demanding and it would be likely that B. nana would allocate extra C made available by warming to produce tannins and other secondary compounds rather than to increase photosynthesis which might be the response of a graminoid species like E. vaginatum, less prone to herbivory. However, species phenological response to warming is complex and depends on many variables making this hard to predict.