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Citation for the original published paper (version of record):
Jägerbrand, A. (2007)
Effects of an in situ temperature increase on the short-term growth of arctic-alpine bryophytes. Lindbergia, 32(3): 82-87
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Material and methods The study area
Fieldwork was conducted in northern Swedish Lap-land during the 1995 growing season at the Latnjajaure FieldStation (Fig. 1) at 980 m a.s.l. (68°21′N, 18°30′E). The area is geographically situated in the subarctic part of northern Sweden, but due to the high altitude it is also considered to be alpine. Although the climate at the site is typically low arctic, the area is phytogeo-graphically classified as subarctic-alpine tundra. Dur-ing the summer of 1995, the mean temperature was 4.4, 5.9 and 5.3°C for June, July and August, respec-tively, and the mean precipitation for the same months was 44.7, 81.4 and 142.0 mm, respectively. The study was performed at two different sites without perma-frost on the southwest facing slope of Mt. Latnjatjårro. The bedrock at site 1 was a base-rich mica garnet schist, and the vegetation was a species-rich sedge meadow dominated by Carex bigelowii, Carex vaginata and
Poly-gonum viviparum. Site 2 was near a meltwater flow,
and the bedrock was a transition between base-rich mica garnet schist and acidic glacial deposits. The vegeta-tion was a moist-wet sedge meadow dominated by
Ra-nunculus nivalis, Carex lachenalii, Carex bigelowii, Poly-gonum viviparum, Sphagnum teres and Sphagnum warn-storfii.Therewaslateralsoilwatermovementat both sites. Nomenclature follows Nilsson (1991) for vascular plants, Söderström and Hedenäs (1998) for bryophytes. The species
The mosses selected for the growth analysis were
Aula-comniumturgidum,Tomentypnumnitens (both at site 1),
and Sphagnum teres (site 2). These mosses were chosen mainly because of their high abundance. Aulacomnium
turgidum is a typical arctic and alpine moss, and can be
found in mountains around the world (Nyholm 1981). In alpine areas it occurs in rather moist and
nutrient-rich heaths or meadows. Sphagnum teres is common in northern Scandinavia, and has circumboreal distribu-tion, occurring in minerotrophic peatlands. In the mountains, S. teres occurs in the wetter but still nutri-ent-rich patches found between meadows and small meltwater streams. Tomentypnum nitens occurs in Fen-noscandia from the lowlands to the high alpine zone (Arnell and Nyholm 1954), with a circumboreal distri-bution in the northern hemisphere (Schofield 1980), and can be found in various habitats with intermedi-ate levels of moisture in the mountain regions.
Aula-comniumturgidum and S. teres have orthotrophic growth
forms, whereas T. nitens has a plagiotrophic growth form (Mägdefrau1982).Plagiotrophic(pleurocarpic)mosses can form mats or wefts. Orthotrophic (acrocarpic) mosses can form different kinds of turfs, i.e. short turfs, tall turfs, and dense cushions (Mägdefrau 1982). At Latnjajaure, only S. teres forms well-developed turfs.
Aulacomnium turgidum shoots are found isolated or
mixed with other moss and liverwort species, and T.
nitens grows in rather loose mats.
Simulated climate change experiment
Moss growth was measured in different ongoing ITEX-experiments (International Tundra Experiment) at Lat-njajaure. In these experiments, the temperature was enhanced by using hexagon open-top chambers (OTC; Marion et al. 1997) of ITEX design. The OTC, a poly-carbonate hexagon (ground surface area 1 m2), increased the air temperature by 2–3°C. At site 1, experimental manipulation to study the temperature responses of
Carex bigelowii (Stenström and Jónsdóttir 1997)
start-ed 30 June 1994. This study usstart-ed four and five OTCs and control plots at site 1 to analyse the growth of
Aula-comnium turgidum and Tomentypnum nitens,
respec-tively. At site 2, the experiment to study the responses of Ranunculus nivalis (Molau 2001) was started during the summer of 1993. Only three OTCs and three con-trol plots with high abundance of Sphagnum teres could be used in this study.
Growth measurements
The moss growth was measured in mid-June and to late August 1995, close to the start and end of the grow-ing season, respectively. It was assumed that this peri-od covered the annual growth, because under similar harsh growth conditions in the Antarctic, the growth of Polytrichum alpestre [= P. strictum] occurs mainly during the warmest months (Longton 1970).
Three different methods were used to measure growth of the three moss species.
1. The tied-thread method (Longton and Greene 1967) measured the growth at a fixed point below the apex. Fig. 1. Map showing site locations at Latnjajaure (68°21′N,
Aulacomnium turgidum at Latnjajaure grew mixed with
other bryophytes and had a somewhat creeping life-form (Jägerbrand unpubl.), which makes the tied-thread method more suitable for this species than the other two methods. On 17 June at site 1, thin cotton threads were tied 5 mm from the apex around the stem, in a total of 40 shoots of A. turgidum in five OTC plots and five control plots. However, since the distribution of
A. turgidum shoots within the site and plots were highly
patchy and thus quite variable, the number of growth measurements in each plot varied from one to nine. In August, individual shoots were measured with a ruler. 2. The cranked-wire method (Clymo 1970) was first applied to Sphagnum species and later to other ortho-trophic mosses (Jónsdóttir et al. 1995). A cranked wire was firmly placed in a moss turf, with a known length of wire above the moss surface. Moss growth was meas-ured as the decrease in wire length above the surface. The method is easily applied to compact orthotrophic growth forms and is therefore suitable for Sphagnum
species. On 17 June, 120 cranked wires were placed in
Sphagnum teres turfs at site 2 in three OTC plots and
three control plots, with 20 wires in each installation. On 23 August, the length increase was measured with a ruler.
3. The shoot-transplanting method is suitable for growth analysis of plagiotrophic mosses, which in this study means Tomentypnum nitens. In the beginning of the measuring period, the moss shoots were cut to a constant length, weighed, and transplanted onto ex-perimental plots. Length and weight of the moss shoots were repeatedly measured after the growing season. Growth was measured as the difference in length and biomass between the two occasions. This method has previously been used in field experiments on
Sphag-num species (Rydin and McDonald 1985, Gerdol 1995)
and for other species as well (Kooijman 1993), and was applied to 40 moss species in laboratory experiments (Furness and Grime 1982a, 1982b). On 18 June, 10 samples (ca 5-10 cm in diameter) of T. nitens were col-lected at site 1, at a maximum distance of 3 m from the plots. Five shoots were randomly chosen; the shoots were then cut to a length of 4 cm (from the main apex), and kept dark in plastic bags prior to measurements of fresh weights in the laboratory. To estimate the fresh water content at the time of collection, one shoot from each sample was dried at 80°C to constant weight. The fresh water content before transplantation and the fi-nal dry weight determined at the end of the experi-ment made it possible to calculate biomass increase. The remaining four shoots per sample were tagged with threads to ensure identification at the end of the ex-periment. The 40 shoots were transplanted randomly within 24 h of collection into four open-top chamber plots and four control plots, i.e. five shoots in each plot. When transplanted into the experimental plots, a small space was created by gently pulling away some of the moss layer with forceps. The moss shoots were then transplanted into the layer of the surrounding moss, and the surrounding moss layer was carefully returned to its original condition. In order to minimize micro-climatic effects, the apex of the transplanted shoot was placed at the same vertical position as that of the sur-rounding moss layer. On 28 August, all shoots were harvested and their fresh and dry weight, and lengths were measured. Unfortunately, 10 shoots were destroyed accidentally because some shoots were very fragile due to the dry conditions during collection, resulting in only one or a few measurements for some of the plots. Data analyses
Because there were only one or a few growth values available for each plot for Aulacomnium turgidum and
Tomentypnum nitens, and as the plots were paired in
Fig. 2. Responses after one growing season during 1995 to temperature enhancement treatments (OTC) in three dif-ferent bryophytes at Latnjajaure, northernmost Sweden. (A) Length increase, mm (± 1 SE) in Aulacomnium turgidum. (B) Length increase, mm (± 1 SE) in Sphagnum teres. (C) Length increase, mm (± 1 SE) in Tomentypnum nitens. (D) Biomass (dry weight increase), mg (± 1 SE) in Tomentypnum nitens. Statistical tests were a two-tailed paired t-test for Aula-comnium turgidum and Tomentypnum nitens, and a nested ANOVA for Sphagnum teres. n.s. = non-significant.
the experiment (i.e. each OTC plot with a correspond-ing control plot in close vicinity), a paired t-test was used. The mean values (if there was more than one measurement available) for each plot were used in the statistical analyses. For Sphagnum teres, up to 20 growth measurements were available for each plot, and a nest-ed ANOVA was performnest-ed to test for significant dif-ferences among plots and between treatments. Growth was assigned to be the dependent variable, while the treatments were OTC and the control that was nested within the blocks, i.e. the three pairs of plots. Statisti-cal tests were performed with StatView 4.12, and Su-perANOVA 1.11.
Results
Aulacomnium turgidum showed a non-significant trend
of a greater length increase in the temperature-treated plots (Fig. 2a). Sphagnum teres showed a significant growth increase in the temperature enhancement plots compared to control plots (Fig. 2b; p = 0.0004). In
Tomentypnum nitens, a non-significant trend of
in-creased growth in length and biomass was found in the control plots (Fig. 2c-d).
Discussion
This study shows that Sphagnum teres in the wetter habitat had significantly increased growth in response to the experimental warming, while the other two spe-cies in the drier habitat did not show any significant trends. The positive growth increase in the tempera-ture enhancement treatment shown by S. teres was in accordance with previous results of studies on S.
fus-cum in a subarctic region (Sonesson et al. 2002,
Dor-repaal et al. 2003). In a boreal poor fen, Gunnarsson et al. (2004) showed that S. balticum had decreased bio-mass production in temperature enhancement treat-ments, but also that the cover of S. papillosum increased under the same treatment. Hence, the current infor-mation indicates the possibility of species-specific re-sponses, or that Sphagnum species in subarctic areas are currently growing below their optimal temperatures. The results from this study therefore contribute new information on tundra responses to the intensifying global warming for one species belonging to the eco-logically important Sphagnum genus.
Theothertwo species studied here did not show the same positive responses to the temperature treatments. Even though Aulacomnium turgidum did show a non-significant trend of increased growth to increased tem-perature, a non-significant trend of decreased growth was found in Tomentypnum nitens. The lack of data on growth responses to temperature increases in situ on
the species studied prevents direct comparisons, how-ever. Nevertheless, in a previous study, the abundance of A. turgidum did not significantly change even after three years of experimental manipulations of tempera-ture factored by increased nutrient availability (Jäger-brand et al. 2003), and in Alaska, Aulacomnium spp. did show a biomass decrease to temperature increase and fertilization after three years (Chapin et al. 1995). Since bryophytes are poikilohydric and highly de-pendent on water availability for their growth and pho-tosynthesis (Proctor 1982), water conditions are of great importance. It is likely that the different responses in this study are mainly explained by different water avail-ability at the two sites and the indirect effects of an increased temperature on evaporation.
In the moist-wet meadow, soil water was probably more available throughout the growing season than in the sedge meadow. Thus, the higher water availability in combination with sub-optimal temperature condi-tions may explain the increased growth of S. teres. Sim-ilarly, a positive growth response of Pohlia
wahlenber-gii in warming experiments in a wet snowbed was
shown by Sandvik and Heegaard (2003), indicating the great importance of acknowledging the interaction be-tween the reduced water availability caused by the tem-perature manipulations coupled with the water availa-bility at the sites examined (see also discussion in Sand-vik and Heegaard 2003). Furthermore, SandSand-vik and Heegaard (2003) showed that P. wahlenbergii was ca-pable of changing its growth form with increased nu-trients,becomingmore“lax” and desiccation-intolerant. I did not observe any changes in the growth form of S.
teres in this study.
Increased temperatures from manipulation inevita-bly affect moisture conditions, resulting in somewhat lower levels of relative humidity (Hollister and Webber 2000) due to increased evaporation (Sandvik and Heegaard 2003). These effects on relative humidity will also occur when natural warming takes place. The lower levels of relative humidity in the experiments may af-fect the growth and modify the growth forms of bryo-phytes, but the amount, frequency, and duration of precipitation should be less affected in the OTCs since they are open at the top.
In northern Europe, precipitation has already in-creased because of the changing climate and is predict-ed to increase even more during this century (EEA 2004). However, precipitation is mainly increasing in the winter, so the overall responses of bryophytes in the future will depend on their ability to grow in re-sponse to the enhanced growing season and to increased temperatures during the growing period in the sum-mer, as well as to respond to the effects of changed species composition of vascular plants. In experiments that simulate global climate change in subarctic areas (by increasing temperatures and nutrients), bryophytes
as a functional group often decrease. This has been assumed to be an indirect effect of the great increase in vascular plants (Van Wijk et al. 2003). However, the destiny of tundra bryophytes in the future depends on a number of integrated factors, including other aspects of global climate change, such as the increased abun-dance of new species of vascular plants in alpine and tundra areas (see discussion in Jägerbrand et al. 2006), site-specific conditions (Jónsdóttir et al. 2005), higher CO2-concentrations, enhanced UV-B radiation, all of which may possibly be factored by altered humidity and nutrient availability. At present, vegetation shifts were observed as a response to the global warming at northern high latitudes (Myneni et al. 1997), in the arctic (Chapin et al. 1995, Sturm et al. 2001), alpine (Grabherr et al. 1994, Gottfried et al. 1999), and sub-arctic areas (Jägerbrand 2005). Thus, it is likely that studies currently undertaken will reveal the actual re-sponses of bryophytes to natural global warming.
Acknowledgements – I would like to thank Dillon Chrimes for valuable comments. This study was supported by the Abisko Research Station Scholarship Fund.
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