Prior studies by Starr et al (2000) and Arft et al (2001) have confirmed a direct effect of soil temperature on maximum thaw depth, which is clearly occurring in this study (Table 4.1, Fig 4.2). The control plot on sampling day 2 had a greater tussock thaw depth than in the OTC (Table 4.1), which was interesting because control plots generally exhibit colder temperatures, so a shallower thaw depth was expected. Starr and Ahlquist (2008) put such anomalies down to lateral displacement of the heat through the soil from the OTCs into the control plot area. Or more likely in the case of this study, chance microtopographic variations, such as water tracks, which are common in areas surrounding Toolik Lake, causing advanced thaw.
Changes in micro climate are important as soil temperature and thaw depth may have both direct and indirect effects on the photosynthetic capacity of E.vaginatum and B. nana. Direct effects include release from photosynthetic limitations through increased stomatal conductance by improved root and water status. Indirect effects include improved nutrient availability through increased mineralization rates and greater exploitable soil volume due to greater thaw depth (Shaver et al., 1991).
5.2.1 Control plots
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Previous studies (Sullivan et al, 2008 and Starr and Ahlquist, 2008) have shown that photosynthetic capacity is dependent on time of day therefore a time frame of readings between 09:00am - 15:00pm was chosen (Fig 3.6) to ensure less interference by external variables. These studies have also reporting that photosynthesis and related fluxes are highly dependent on date, with seasonal peak being mid-end of July. Therefore this study probably did not measure the highest summer rates as it took place at the beginning of July.
5.2.2 Effects of Chamber Warming
Observations of CO2 flux demonstrate that our site is highly sensitive to changes in temperature (Fig 4.3), supporting the hypothesis that photosynthetic rates of both E. vaginatum and B. nana will be higher in open top chambers than control plots, at each temperature manipulation (10, 15, 20 and 25°C). Photosynthesis was significantly greater in OTC than control plots for both B. nana and E. vaginatum (Fig 4.4), likely driven by changes in soil nitrogen availability as the increase in temperatures occurred primarily in the soil (Fig 4.2), which was also found by Sullivan et al (2008), with air temperature alone insufficient to explain photosynthetic increase.
The mid-day growing season air temperature in control plots in this study was well below the temperature optimum for AMAX in arctic vegetation at 11.6°C (Table 4.1) (Chapin 1983 in Sullivan and Welker 2005). OTCs warmed mid-day air temperatures by an average of 0.6°C and probably reduced wind speeds within the chambers, which promotes a deeper leaf boundary layer, raising leaf temperatures above air temperatures. It is likely that chamber warming increased leaf temperatures from below the temperature optimum, to near optimum levels (Sullivan et al, 2008), contributing to the observed increase in AMAX in OTCs in Fig 4.3a.
5.3 Mechanisms Responsible for Difference in Species
E. vaginatum exhibited a response to chamber warming greater than that of B. nana (Fig 4.4) therefore the pronounced effects of chamber warming (Fig 4.3a) affected E. vaginatum more than B. nana. This can be explained by the fact that E. vaginatum grows on tussocks, whereas B. nana typically grows in inter-tussock regions. Tussocks are composed of low-density organic matter and elevated above the water table, making the soils subject to evaporative water loss (Sullivan and Welker, 2007), providing E. vaginatum with increased ability to acquire nutrients, as root kinetics improves with warmer soil temperatures. B. nana, however, is accustomed to saturated soils meaning water is more limiting to this species. Therefore warmer air temperatures increase E. vaginatum photosynthesis but may actually have restricted photosynthesis of B. nana due to water limitation, as a result of increased evapotransipration with warming.
B. nana does not respond as strongly to warming as E. vaginatum (Fig 4.4). This could be because the measurements were taken 9 years after warming was initiated and as described in Shaver et al (2001), woody shrub species, such as B. nana, are much more responsive to warming during the first few years. This is because the number of leaves during the first year of warming is predetermined by the previous year; therefore B. nana allocates extra nutrients acquired by warming towards enhanced photosynthesis, but in following years, the additional nutrients are allocated toward increased total leaf area without an increase in photosynthetic capacity. As growth is enhanced and soil nutrients begin to constrain growth, leaf nutrient content may be reduced which could explain the lower photosynthetic response of B. nana to warming.
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The non-significant photosynthetic increase in B. nana in response to warming compared to the large increase of E. vaginatum evident in Fig 4.4 may be indicative of resource limitations besides temperature. E. vaginatum and B. nana respond differently over time, which may be explained by the 'Transient maxima hypothesis' described by Seastedt and Knapp (1993). This hypothesis notes that when the availability of limiting resources varies, such as decreased litter quality (increased C:N ratio) in low arctic warming experiments, a transient maxima, or short-term elevated response of key system processes will occur under non-equilibrium conditions. Furthermore, E. vaginatum is a faster growing species than B. nana, and is able to maintain uptake of nutrients even in periods of nutrient limitation (Arft et al, 2001) to increase photosynthesis; therefore E. vaginatum continues to respond significantly to warming during lower nutrient availability 9 years into the warming experiment, whereas B. nana does not (Fig 4.4).
As the temperature treatment did not result in any significant increase in ASAT in B. nana from control to OTC (Fig 4.6b), it appears that any extra C gained via increased nutrient availability during warming, could not be diverted to photosynthesis. Given B. nana's higher nutrient requirements and that nutrients were probably in short supply, it appears B. nana could only afford to invest C in growth and phenolics, rather than photosynthesis. Condensed and hydrolysable tannins, flavonoids, phenolic glucosides and secondary metabolites which make leaves unpalatable for herbivores and influence decomposition are all phenolic responses crucial to herbivore defense and demand high concentrations of C (Graglia et al, 2001). As a consequence, part of the 'extra' C gained through warming seems to have been incorporated into condensed tannins which was also noted by Chapin et al (1983) in Sullivan and Welker (2005). Their study also describes how B. nana is favorable to herbivores therefore it must use more C to protect against herbivory, explaining the lower photosynthetic response of B. nana compared to E. vaginatum (Fig 4.6b and 4.6a respectively).
The broader range of temperature optimum of B. nana in OTC and control plots (Fig 4.6) might be due to the developmental flexibility regarding the fate of its buds allowing it to radically change its branching pattern when resource limitation, such as temperature and nutrient availability is alleviated, as outlined in Bret-Harte et al (2001). During warming, B. nana induces axillary buds that would normally grow as short shoots, to grow instead as long shoots, which have a greater photosynthetic return and cause a wider temperature window for response. This earlier budding also provides yet another reason for the smaller effect of temperature on B. nana (Fig 4.4). Production of long-shoot branches requires a much greater investment of both N, in leaves, and C, in stems and leaves, than production of short shoots and the increased whole plant C capture due to greater leaf area is insufficient to allow C allocation to both increased branch production and increased photosynthesis per unit leaf area (Bret-Harte et al, 2001) resulting in lower photosynthetic rates.
One observed but not measured response was that leaf expansion of B. nana occurred earlier in the season in OTCs than it did in control plots B. nana but had little effect on the production of the aboveground biomass, Chapin (1996) observed similar trends. By contrast, E. vaginatum began the growing season with more leaves in OTCs than control and temperature had little effect on the timing of leaf expansion of E. vaginatum, but increased its aboveground growth, leading to increased production of aboveground biomass by early July which may explain the increase in LAI in OTC plots (Fig 4.7).
Although the increase in LAI in OTC (Fig 4.7) is consistent with findings of increased photosynthesis in the warming treatment, it did not determine which species contributed most to the change, though it is likely both species contributing to this increase. Studies such as Chapin (1996) found that with continued summer growth warming of over 4 years, a 25% increase in shoot density, larger leaf areas and a greater canopy height occurred in B. nana, with more above ground biomass observed for E. vaginatum. Therefore it stands that B. nana contributed via enhanced growth of leaf area and E. vaginatum contributed by producing more abundant aboveground biomass resulting in both species caused the increased in LAI.
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The results of this study, and many other studies with similar findings, could have implications for predicting the carbon balance of tundra ecosystems as the arctic climate changes. Modeling efforts may be simplified in that they can focus more on leaf area changes of species in response to climate change rather than on acclimations of area-based physiological activity (Starr and Ahlquist, 2008), as this appears to have less of an effect than warming.
The significant effect of warming found in this study could have several implications for the tundra species of the North Slope, Alaska and progressive warming could lead to shifts in species dominance and community structure. This would have implications for the herbivores that feed on them, such as caribou, affecting their feeding, roaming and breeding habits which could have an impact on their numbers (Joly et al, 2009), in turn affecting the wolf population in the region. Composition changes such as these are a major concern for native and rural people of the North Slope who rely on being able to harvest caribou and wolves for subsistence.
If species differences in photosynthetic enhancement caused by warming translate to increased growth there may be shifts in total leaf area, aboveground biomass and community composition. For example, the bud break of B. nana is dependent on North Slope spring temperatures, and in a warmer climate bud break will occur earlier than under current temperature regimes (Arft et al, 2001 and Pop et al, 2000), while E. vaginatum appears to produce more above ground biomass. Both of these processes require more C uptake and result in greater C storage. Therefore, photosynthesis is likely to increase due to increased growth, enhancing the sink capacity of the North Slope vegetation to store carbon.
However, as Starr and Ahlquist (2008) discuss, temperature is not the only variable responsible for change on the North Slope. Nutrients and water loss can also be extremely limiting to photosynthesis. Water stress occurs when extended dry and warm periods are coupled with increased thaw depth, leading a decrease in water within the rhizosphere. The photosynthetic activity of dominant deciduous shrubs such as B. nana diminishes when exposed to prolonged water stress, as seen in this study. If stress such as this occurred throughout the North Slope region, there could be a reduction in photosynthetic rate and therefore a reduction in the carbon sink capacity at ecosystem level. Moreover, C stored in permafrost may escape as warmers temperatures cause permafrost to thaw to greater depths each year, increasing global atmospheric CO2 concentrations and which may then feedback into the system to exacerbate warming further.
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 25°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.
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.
Greenland, and repeating it there cos the betula pops are diff there and they might react diff to this kind of warming plus paddy and jeff did that in 2005 but just otc no manips and in eri only.
collect leaf samples to run n analysis, greater insight into n and C use
maybe soil samples, to see how temps increase soil n
A, versus calculated substomatal CO2 concentration, Ci). a significant relationship between A/Ci and chlorophyll fluorescence estimates of carboxylation is achieved. The use of the Vcmax parameter to describe accurately the Rubisco activity from the A/Ci curve analysis is also dependent upon the assumption that Ci is approximately equal to chloroplast CO2 concentrations (Cc). In
many species, the optimal temperature that maximizes
the photosynthetic rate increases with increasing
growth temperature. In this minireview, mechanisms involved
in changes in the photosynthesis-temperature
curve are discussed. Based on the biochemical model
of photosynthesis, change in the photosynthesis-
temperature curve is attributable to four factors: intercellular
CO2 concentration, activation energy of the
maximum rate of RuBP (ribulose-1,5-bisphosphate)
carboxylation (Vc max), activation energy of the rate of
RuBP regeneration (Jmax), and the ratio of Jmax to
Vc max. In the survey, every species increased the activation
energy of Vc max with increasing growth temperature.
Other factors changed with growth temperature,
but their responses were different among species.