Spectral reflectance and photosynthetic properties of Sphagnum mosses exposed to progressive drought

ECOHYDROLOGYEcohydrol. 1, 35–42 (2008)Published online 20 February 2008 in Wiley InterScience(www.interscience.wiley.com) DOI: 10.1002/eco.5

Spectral reflectance and photosynthetic properties ofSphagnum mosses exposed to progressive drought

Angela Harris*School of Geography, University of Southampton, Highfield, Southampton, S017 1BJ, UK

ABSTRACT

This article explores the utility of spectral reflectance signals to assess changes in the photosynthetic efficiency (PSII) ofSphagnum mosses exposed to reductions in water availability. Reflectance was measured in parallel to moisture content andchlorophyll fluorescence in five species of Sphagnum exposed to progressive drought. Decreases in moisture availabilitycaused a significant reduction in PSII for all samples tested. An objective was to ascertain whether Sphagnum PSII wasbetter correlated with (i) spectral indices directly related to photosynthetic processes through association with xanthophyllcycle pigmentation (the photochemical reflectance index; PRI), or (ii) indices indirectly correlated with photosynthetic activitybut able to detect changes in canopy morphology (the normalized difference vegetation index; NDVI), chlorophyll stability(the structure insensitive pigment index; SIPI), or canopy moisture content (the floating water band index; fWBI). Strongestcorrelations were found between PSII and indices which were indirectly related to PSII. Both the SIPI and the NDVI exhibitedlinear correlations although these relationships were sometimes sample-specific. The fWBI was the index least affected bysample-specific relationships and showed a strong curvilinear correlation with PSII. Photosynthetic efficiency was correlatedwith the PRI but relationships were much weaker than for the other indices used, and in some cases negative. The NDVI andthe fWBI show the most potential for monitoring Sphagnum photosynthetic activity at the ecosystem scale. The results fromthis study will help to monitor and understand the responses of these key species to hydrological disturbances. Copyright 2008 John Wiley & Sons, Ltd.

KEY WORDS peatlands; remote sensing; photosynthetic efficiency; moisture; chlorophyll fluorescence; Sphagnum

Received 6 September 2007; Accepted 17 December 2007

INTRODUCTION

Sphagnum mosses are a major component of the surfacevegetation of northern peatlands, which cover approxi-mately 2% of the world’s land surface (Gorham, 1991).The importance of peatland environments is multi-faceted, ranging from their role as biological refugesfor species of wetland-dependent fauna and flora, totheir global significance as long-term soil-carbon storesand their continuing sequestration of anthropogenic CO2

emissions (Gorham, 1991; Oechel et al., 1993; Belyeaand Malmer, 2004; Smith et al., 2004). Sphagnummosses are the key species responsible for the seques-tration of large amounts of carbon into peat and thusplay an important role in the global carbon cycle. Therate at which they are able to fix carbon (i.e. photosyn-thesize) is highly dependent on water availability (e.g.Titus et al., 1983; Rydin and McDonald, 1985; Gerdol,1995; Williams and Flanagan, 1996; Schipperges andRydin, 1998) and many Sphagnum species are specifi-cally adapted to living in particular microtopographicallocations within the peatland, often in relation to theirwater transport capabilities (e.g. Clymo, 1973; Clymo and

* Correspondence to: Angela Harris, School of Geography, University ofSouthampton, Highfield, Southampton, S017 1BJ, UK.E-mail: [email protected]

Hayward, 1982; Andrus et al., 1983; Rydin and McDon-ald, 1985). Any changes in the hydrology of a peat-land, either due to (future) climate changes, or due toanthropogenic disturbance, are likely to alter the photo-synthetic efficiency in these key species with possibleconsequences for the carbon sequestration function ofthese environments (Dorrepaal et al., 2003).

The photosynthetic efficiency of photosystem II (PSII)is commonly measured by chlorophyll fluorescence(Maxwell and Johnson, 2000). When photosynthesis islimited, as is common during drought, plants absorbmore light than is necessary for photosynthesis (Demmig-Adams and Adams, 1992) but are often unable to dis-sipate this excess light energy in an orderly fashion,potentially causing harm to their photosynthetic appara-tus. Under environmental stresses, such as progressivedrought, plants must therefore employ specific mecha-nisms to safely dissipate this excess energy. The exactphysiological processes that occur in mosses, whichenable energy dissipation, are poorly understood (Del-toro et al., 1998), although it is widely accepted thatplants dissipate excess energy either as heat or re-emitthe energy as light, a process known as chlorophyll flu-orescence (Demmig-Adams and Adams, 1992, 2006).Heat dissipation (or non-photochemical quenching) iscommonly thought to occur via conversion of xantho-phyll cycle pigments into their photoprotective state(Demmig-Adams and Adams, 1992; Horton et al., 1996).

Copyright 2008 John Wiley & Sons, Ltd.

36 A. HARRIS

Photosynthesis, heat dissipation and chlorophyll fluores-cence are competing pathways; therefore, an increase inthe efficiency of any one of these processes will cause adecrease in the yield of the remaining two (Demmig-Adams and Adams, 1992). Consequently, chlorophyllfluorescence can provide information on both photosyn-thetic efficiency and the level of heat dissipation. Thefluorescence parameter PSII is often used to estimatephotosynthetic efficiency (Genty et al., 1989). PSII is anindicator of the proportion of light absorbed by chloro-phyll associated with PSII that is subsequently used inphotochemistry (Maxwell and Johnson, 2000) and can bestrongly correlated to CO2 fixation (Genty et al., 1989;Edwards and Baker, 1993; Laing et al., 1995; Gamonet al., 2001).

Although highly detailed, measurements of photosyn-thetic efficiency via chlorophyll fluorescence often pro-vide data over relatively small spatial scales and aretime-consuming and expensive to collect. In order tomonitor photosynthetic efficiency of Sphagnum acrossentire peatland complexes there is a need to explore alter-native high-resolution synoptic, economical approaches.Optical remote sensing, by way of spectral reflectanceindices, is one approach which may enable assessmentof photosynthetic function over larger spatial scales. Oneof the most commonly used spectral indices for the deter-mination of photosynthetic efficiency via remote sensingis the photochemical reflectance index (PRI) (Gamonet al., 1992; Penuelas et al., 1995a). The PRI incorporatesreflectance changes at 531 nm, which are directly relatedto the amount of xanthophyll cycle pigments in theirphotoprotective state (Gamon et al., 1992). The PRI hasbeen correlated with photosynthetic efficiency at the leafscale (e.g. Penuelas et al., 1995a; Gamon et al., 1997;Richardson and Berlyn, 2002; Richardson et al., 2004;Guo and Trotter, 2006; Nakaji et al., 2006); the canopyscale (e.g. Gamon et al., 2001; Nichol et al., 2006); andthe ecosystem scale (e.g. Nichol et al., 2002; Drolet et al.,2005; Fuentes et al., 2006), and is able to track both diur-nal (Gamon et al., 1992) and seasonal variation (Stylin-ski et al., 2002) in photosynthetic activity, although thestrength of these correlations often vary with species, siteand time of year.

Whilst not a direct measure of plant photosyntheticfunction, a number of researchers have found strongcorrelations between photosynthetic status and spec-tral indices related to resource availability (e.g. waterand nutrients). These relationships are largely based onthe assumption that vegetation in stressed environmentshas lower photosynthetic efficiencies than that living instress-free environments (Sims et al., 2006). Such spec-tral indices may indicate changes in biochemical con-tent, such as the structure insensitive pigment index(SIPI; Penuelas et al., 1995b), which track changes inthe carotenoid/chlorophyll a ratio, or changes in green-ness and structure, for example the normalized differ-ence vegetation index (NDVI) or the enhanced vege-tation index (EVI; Huete et al., 2002; Rahman et al.,2005; Sims et al., 2006), which make use of changes

in reflectance in the red portion of the electromagneticspectrum (¾600–700 nm) due to chlorophyll absorption,and in the near infrared (NIR; ¾750–1400 nm) due tochanges in the internal structure of the leaves. Spec-tral indices which are able to track changes in canopymoisture content, such as the water band index (WBI),formulated from water absorption features located in theNIR, may also prove useful for monitoring photosyn-thetic efficiency where water availability is directly linkedto photosynthetic activity (Claudio et al., 2006; Simset al., 2006). Such a link is likely to occur in Sphagnummosses due to the absence of stomata in their photo-synthetic tissue, which means that conductance to CO2

diffusion is largely controlled by a passively variablewater layer (Silvola, 1990; Williams and Flanagan, 1996).The WBI and variants of the WBI [e.g. floating waterband index (fWBI); Penuelas et al., 1997; Strachan et al.,2002] have been shown to be strongly related to bothSphagnum moss water content (Van Gaalen et al., 2007)and measures of near-surface moisture [i.e. volumetricmoisture content (VMC); Bryant and Baird, 2003; Har-ris et al., 2005, 2006]. With the exception of Van Gaalenet al. (2007), previous studies have not correlated spectralreflectance with actual measures of Sphagnum photosyn-thetic efficiency. The work of Van Gaalen et al. (2007)suggests that the PRI may be a useful proxy for moni-toring photosynthetic activity in Sphagnum, although thestudy was constrained to a single species where spectralreflectance was measured from single Sphagnum plants,as opposed to natural Sphagnum canopies.

This article seeks to address aspects of this gap inthe knowledge by examining correlations between PSIIphotosynthetic efficiency (PSII) and spectral propertiesof natural Sphagnum moss canopies sampled from fivedifferent species, as near-surface moisture content is sig-nificantly reduced. Spectral indices directly (PRI) andindirectly (SIPI, NDVI and fWBI) related to photosyn-thetic processes are compared in terms of their abilityto monitor changes in the photosynthetic efficiency ofSphagnum mosses. The findings of this study provide abasis for evaluating the potential of remote sensing forassessing the impact of hydrological disturbances on thephotosynthetic function of peatland environments.

MATERIALS AND METHODS

Species and sample collection

Five species of Sphagnum were chosen for this studybased on their spanning different peatland microtopo-graphic locations ranging from dry hummocks and rela-tively dry (but often inundated) lawns to edges of wet-boghollows. A total of six samples (five different speciesand one control sample) were collected from peatlandsin west Wales and the South of England between lateMarch and early May 2006 (Table I). The samples ofSphagnum moss, 8 ð 10 ð 15 cm3 deep, were carefullycut from the peatland surface and placed in plastic trays.Each of the samples was homogeneous with respect to

Copyright 2008 John Wiley & Sons, Ltd. Ecohydrol. 1, 35–42 (2008)DOI: 10.1002/eco

SPECTRAL REFLECTANCE AND PHOTOSYNTHETIC PROPERTIES OF SPHAGNUM 37

Table I. Collection locations of Sphagnum samples used in the experiment, the microtopographic location at which they were foundin a peatland and the date on which they were collected.

Species Collecting location Microtopographiclocation

Collectiondate

Sphagnum pulchrum West Wales; Cors Fochno (52°320N 04°000W). Lawn/hollow March 2006Sphagnum tenellum West Wales; Cors Fochno (52°320N 04°000W). Lawn March 2006Sphagnum capillifolium West Wales; Cors Fochno (52°320N 04°000W). Hummock March 2006Sphagnum subnitens Southern England; New Forest (50°490N 01°430W) Lawn/low hummock May 2006Sphagnum papillosum Southern England; New Forest (50°490N 01°430W) Lawn/low hummock May 2006

both moss species and physical appearance. The sam-ples were kept moist and transferred to the University ofSouthampton within 24 h where they were kept outsideunder natural daylight and well watered until commence-ment of the experiment in July 2006.

Experimental design

All samples were maintained in a controlled environ-ment, under a photoperiod of 16 h, with a photosyntheticphoton flux density (PPFD) of 350 µmol m�2 s�1 andtemperatures maintained at ca 24–18 °C day/night. Theplants were placed in the controlled environment 7 daysprior to the experiment in order to acclimatize to theconditions and were sprayed daily with distilled waterand appeared healthy during this time. After the drought-treatment samples were watered for the last time, theywere left to dry naturally for 23 days. The control samplewas sprayed with distilled water each day. All sampleswere rearranged daily to account for possible edge effectsand to equalize the amount of light for each sample. Atthe same time every day each sample was weighed, andchlorophyll fluorescence and spectral measurements weretaken.

Canopy moisture

Canopy moisture was not measured directly in orderto prevent disturbance and destruction of the Sphagnumcanopies. Instead, the spectral index fWBI was used as anindirect indicator of moisture in the top few centimetresof the canopies to reveal sample-specific patterns inthe relationship between progressive drought and PSII.Research has shown the WBI to be a good indicatorof canopy moisture content in Sphagnum mosses (VanGaalen et al., 2007). Although a direct comparison ofwater contents between samples is not possible with thisindex, due to species-specific relationships, the indexdoes provide a measure of the relative canopy watercontent of a given sample.

Near-surface moisture

After completion of the experiment, all samples wereoven dried at 70 °C until a constant weight was achievedfor determination of dry mass. Sample weights recordedthroughout the experiment were used to determine near-surface VMC, i.e. the VMC over the upper ¾15 cm of thepeatland soil, including plant canopies. VMC ranges from

0–1 where a value of 1 indicates saturated conditions.VMC was determined using Equation (1).

VMC D [�wet weight � dry weight�/

weight density of water]/total volume of sample �1�

Canopy reflectance measurements

All spectral measurements were collected in the con-trolled environment (at PPFD 350 µmol m�2 s�1).Canopy reflectance measurements were collected usingan ASD FieldSpec3 Handheld VNIR spectroradiometer(Analytical Spectral Devices, Boulder, CO, USA) whichcollects data between 325 and 1075 nm at approximately1 nm intervals. The 10° field of view attachment of theinstrument was used to view an area in the centre of eachsample covering approximately 20 cm2 of the canopy.Multiple spectra were collected and averaged for eachcanopy to acquire a representative value. Reflectance wasdetermined from canopy radiance divided by radiance ofa calibrated and perfectly lambertian spectralon panel.A reference measurement from the spectralon panel wascollected before scanning each sample. Table II lists thespectral reflectance vegetation indices calculated fromeach reflectance spectrum and the property that each aredesigned to detect.

Chlorophyll fluorescence measurements

Chlorophyll fluorescence parameters were recorded at thesame time each day using a Hansatech FMS1 modulatedfluorometer. Each day fluorescence measurements weremade on four randomly selected capitula within each ofthe six samples using the Hansatech leaf clip holder,which exposes 1 cm diameter of the sample to thedetector array. The actual efficiency of PSII (PSII) wasdetermined as:

PSII D �Fm0 � Fs�/Fm0, �2�

Where Fm0 is the maximum fluorescence of a pre-illuminated sample and Fs is the steady state fluorescenceyield of a light-adapted sample (Genty et al., 1989).PSII measurements were made prior to the spectralmeasurements under the ambient light conditions of thecontrolled environment (at PPFD 350 µmol m�2 s�1).

Data analysis

The relationships between reflectance and plant phys-iological attributes were examined with the Pearson


38 A. HARRIS

Table II. Calculation of reflectance indices used in this study and the property that each is designed to detect.

Parameter Index Formula Property detected

Photochemical reflectance indexa PRI �R531 � R570�/�R531 C R570� Xanthophyll cycle pigment activityStructure insensitive pigment indexb SIPI �R800 � R445�/�R800 � R680� Changes in the chlorophyll/carotenoid ratioFloating water band indexc fWBI R920/ min�R960–1000� WaterNormalized difference vegetation indexd NDVI �R850 � R680�/�R850 C R680� Greenness and/or structure

a Gamon et al., 1992.b Penuelas et al., 1995b.c Penuelas et al., 1997.d Rouse et al., 1974. Where Rx represents the reflectance at x nm.

product-moment correlation function (r) (Systat Sigma-Plot 10Ð0). Where possible, average values are presentedas the mean š one standard error of the mean (SE).

RESULTS AND DISCUSSION

Effects of desiccation on the photosynthetic efficiency(PSII ) of Sphagnum mosses

There was a significant (p < 0Ð05) gradual decline inVMC for all water-stressed samples over the course ofthe drought treatment (Figure 1(a)). Reductions in VMCwere accompanied with decreases in PSII and the fWBI(an indicator of canopy moisture content); (Figure 1(b)and (c); respectively). The VMC of the control sampleremained similar throughout the experiment and therewas no apparent time trend in the fWBI or PSII (resultsnot shown). Photosynthetic efficiency (PSII) was signif-icantly (p < 0Ð05) decreased in all water-stressed plantsby the end of the experiment, although sample-specificdifferences in the pattern of PSII reduction were clearlyevident (Figure 1(b)). Over the duration of the exper-iment, the relative change in PSII followed the pat-tern S. pulchrum > S. capillifolium > S. tenellum >S. subnitens > S. papillosum (Figure 1(b)). These pat-terns may, in part, be explained by species differencesand the microtopographic position where each is natu-rally found (Rydin, 1985). Species from low microformsand wet environments often have poorer water transportcapabilities than those occurring in higher, drier loca-tions (e.g. hummocks and firm lawns). Hummock specieshave superior morphological adaptations that enable themto transport water further, retain water in the capitulalonger and maintain the supply of water to the plant headduring drier conditions (Li et al., 1992). As a result, rela-tionships between photosynthetic efficiency and moistureare likely to differ between species occurring in diversehabitats. The relationships observed in the present studygenerally concur with the physiological explanations inthat photosynthetic efficiency was most susceptible todrought in species commonly found in wetter environ-ments, i.e. S. pulchrum, than for those adapted to liv-ing in drier habitats, i.e. S. subnitens and S. papillosum.However, the small number of samples used in the cur-rent study advocate caution in reporting species-specificdifferences. Additional research regarding both intra-and inter-species variations in photosynthetic capacity

Figure 1. Time trend for (a) volumetric moisture content (VMC; downto 15 cm); (b) PSII photosynthetic efficiency (PSII); and (c) fWBI(canopy moisture) over the course of the experiment for each species ofSphagnum. Values of PSII and fWBI are daily means š1 SE �n D 4�.

in relation to drought is required to fully explain theobserved sample variations.

Spectral reflectance and Sphagnum PSII

Table III summarizes the correlation coefficients betweenphotosynthetic efficiency and spectral reflectance indices



Table III. Correlations of remotely sensed vegetation indices and PSII photosynthetic efficiency (PSII) for different species ofSphagnum moss.

S. tenellum S. capillifolium S. pulchrum S. subnitens S. papillosum

SIPI �0Ð92ŁŁŁ �0Ð92ŁŁŁ �0Ð92ŁŁŁ �0Ð85ŁŁŁ �0Ð63Ł

NDVI 0Ð93ŁŁŁ 0Ð90ŁŁŁ 0Ð89ŁŁŁ 0Ð84ŁŁŁ 0Ð56Ł

fWBI 0Ð81ŁŁŁ 0Ð78ŁŁŁ 0Ð87ŁŁŁ 0Ð9ŁŁŁ 0Ð58Ł

PRI 0Ð70ŁŁŁ 0Ð48Ł �0Ð62ŁŁ 0Ð83ŁŁŁ 0Ð66ŁŁŁ

In all cases correlations were significant (Ł p < 0Ð05, ŁŁ p < 0Ð01, ŁŁŁ p < 0Ð001, Pearson’s correlation test).

Figure 2. PSII photosynthetic efficiency (PSII) as a function of the sample means of (a) structure insensitive pigment index (SIPI), (b) normalizeddifference vegetation index (NDVI), (c) floating water band index (fWBI) and (d) photochemical reflectance index (PRI). All PSII values are daily

means (n D 4, p < 0Ð0001).

for the five species of Sphagnum moss observed in thisstudy. Significant relationships were observed between allspectral indices and PSII of the drought-stressed Sphag-num samples. On average, PSII was best correlated withreflectance indices related to resource availability (e.g.SIPI, NDVI and fWBI), most likely driven by the paralleleffects that water limitation has on Sphagnum photosyn-thetic function, structure and pigment content (Bewleyand Krochko, 1982; Rydin, 1985; Gerdol, 1995). Nocorrelations were observed between PSII photosyntheticefficiency and reflectance properties of the control sampleduring the experiment (results not shown).

The SIPI and the NDVI both demonstrated strong lin-ear relationships with PSII, for individual Sphagnumsamples and when all samples were pooled (Table III;Figure 2(a) and (b)). The SIPI was negatively corre-lated with PSII suggesting an increase in the carotenoid/

chlorophyll a ratio as photosynthetic efficiency decreased,whereas NDVI and PSII exhibited a positive correlation.Strong relationships between Sphagnum PSII and NDVIcould be due to synchronous changes in greenness and/orstructure (Sims and Gamon, 2002) since the NDVI uti-lizes vegetation’s typical low reflection in the red (dueto absorption by chlorophyll) and strong reflection in theNIR (due to their cellular structure). As the SIPI is anindirect indicator of chlorophyll stability, and is nearlyindependent of structural changes, the greater decline inNDVI than SIPI suggests that structural changes werethe primary influence on NDVI values in the presentstudy. The weakest relationships between PSII and boththe SIPI and the NDVI were observed in the S. papillo-sum sample (r D �0Ð63 and r D 0Ð56, p < 0Ð05; respec-tively). Significant changes in both indices were observed


40 A. HARRIS

for S. papillosum despite only modest decreases in sam-ple PSII, suggesting that photosynthetic activity is notclosely linked to structure or chlorophyll stability over themoisture conditions imposed during this study. Furtherwork is required to determine whether this is a species-specific response. Relationships between SIPI, NDVI andPSII were often stronger for individual Sphagnum sam-ples than for all samples pooled together (r D �0Ð76and r D 0Ð68, p < 0Ð0001; for pooled SIPI and NDVIrespectively). It is likely that these differences are aconsequence of sample-specific changes in structure andpigmentation in response to moisture variation. Suchchanges can have dramatic effects on spectral reflectance,particularly in the visible (380–720 nm) range of theelectromagnetic spectrum (Guyot, 1990). However, thepooled relationships were improved when S. papillosumwas omitted from the analysis (r D �0Ð81 for the SIPIand r D 0Ð89 for the NDVI, p < 0Ð0001), indicating ageneral trend in the remaining samples and the potentialof these indices for monitoring photosynthetic efficiencyin Sphagnum mosses exposed to hydrological distur-bances.

The fWBI was least affected by sample-specific dif-ferences and was the index most strongly correlated withPSII when all samples were pooled (Figure 2(c)). Therelationship between the fWBI and PSII was curvilinearbecause small reductions in canopy moisture when mois-ture content is high (e.g. fWBI >1Ð2, VMC ¾0Ð4), arenot accompanied by a decline in PSII (Figure 2(c)). Fur-thermore, because the fWBI is not directly linked to pho-tosynthetic function, but instead is a surrogate variable,it is also unlikely that the fWBI would be able to detectreductions in PSII as result of super-optimal moistureconditions, i.e. where photosynthetic efficiency decreasesdue to increased resistance to CO2 diffusion (Silvola,1985). Despite limitations at high-vegetation water con-tents, important physiological changes (i.e. decreases inPSII below ¾0Ð6) are linearly correlated with reductionsin fWBI. The close links between vegetation water sta-tus, spectral reflectance and Sphagnum PSII suggest thatthe fWBI could also be used to monitor photosyntheticproperties of Sphagnum mosses via remote sensing.

The PRI was the only spectral index fundamentallyrelated to PSII through its relation to the amount ofxanthophyll cycle pigments in their photoprotective (de-epoxidized) state (i.e. antheraxanthin and zeaxanthin).Higher concentrations of antheraxanthin and zeaxanthinare associated with the dissipation of excess light energyand thus with a reduction in photosynthetic efficiency(PSII). A decrease in the PRI is indicative of an increasein the concentration of xanthophyll cycle pigments intheir de-epoxidized state. Despite this apparent close rela-tionship between PSII and the PRI, correlations betweenthese two variables were generally weaker than for theother indices tested in this study (Table III.). The rela-tionship between the PRI and PSII was complex and allsamples did not fit a single regression (Figure 2(d)). ThePRI was positively correlated with PSII for samples of S.

subnitens, S. tenellum and S. papillosum (Table I). Simi-lar results have been reported by Lovelock and Robinson(2002), who observed significant positive correlationsbetween the PRI and chlorophyll fluorescence parame-ters (Fv/Fm) in arctic mosses, and of Van Gaalen et al.(2007) who demonstrated significant positive relation-ships between PRI and photosynthetic activity in drought-stressed capitula of Sphagnum teres (Schimp.) Aongstr.ex C. Hartm. However, in the current study the rela-tionship between the PRI and PSII for the S. pulchrumsample was markedly different, showing a negative cor-relation (r D �0Ð62, p < 0Ð05) where the PRI decreasedas PSII increased. The reason for such a relationship isunclear, although similar negative relationships betweenPRI and photosynthetic efficiency have also been reportedin senescent Japanese larch needles (Nakaji et al., 2006) aChaparral ecosystem experiencing severe drought (Simset al., 2006) and in Antarctic mosses (Houston, 2004).The PRI and PSII were not well correlated in the S.capillifolium sample (r D 0Ð45, p < 0Ð05), where verylittle change in the PRI was observed throughout theentire experiment, despite significant reductions in PSII.Individual sample correlations with PSII were again gen-erally stronger than when all the samples were pooled(r D 0Ð47, p < 0Ð0001), although when the samples ofS. pulchrum and S. capillifolium were removed from thegroup the correlation increased (r D 0Ð77, p < 0Ð0001)indicating a broadly similar trend amongst the remainingsamples (Figure 3). Variable relationships between thePRI and measures of photosynthetic activity are likelyto be driven by species-specific differences in the reg-ulation of photosynthetic and photoprotective processes,in response to water loss (Deltoro et al., 1998). How-ever, further complexity may be added to the correctinterpretation of PRI/PSII relationships if samples expe-rience differing in situ antecedent environmental condi-tions. The tendency for plants to acclimatize and regulatephotosynthetic processes in response to local or globalenvironmental perturbations (e.g. drought or tempera-ture), can cause the concentration of xanthophyll pig-ments to differ within a single species. Moreover, canopystructure, which varies among and between species, andsoil background interference, can also confound the PRIsignal (Barton and North, 2001; Stylinski et al., 2002;Suarez et al., 2007). Further work is therefore neededto fully understand the relationships between the xan-thophyll cycle, the PRI signal, and canopy moisture ofSphagnum mosses.

CONCLUSIONS

The data suggests that spectral reflectance is potentially auseful indicator of photosynthetic activity in Sphagnum-dominated peatlands. Spectral indices can help to monitorand understand the response of peatland vegetation tohydrological disturbances. However, further work isrequired to ascertain the applicability of this approachat the ecosystem scale and over longer time periods. The



Figure 3. Curvilinear relationship between photochemical reflectanceindex and PSII photosynthetic efficiency (PSII) for S. tenellum, S.subnitens and S. papillosum (n D 4, p < 0Ð0001). Curve is a simpleexponent, three parameter regression model of the form f D y0 C aŁ

�1 � b ^ x�.

apparent species-specific nature of some reflectance/PSII

relationships deserves particular attention, thus whilstSphagnum patches of a sufficient size can be identifiedfrom remotely sensed data with relative ease (Harriset al., 2006; Sonnetag et al., 2007), identification tospecies level may only be possible from high spatialresolution hyperspectral imagery. A certain degree of apriori knowledge regarding the predominant Sphagnumspecies in a given location may be required. However,the structural characteristics and physiological behaviourof species living in similar microtopographical locationswithin a peatland are often comparable, thus knowledgeof reflectance/PSII relationships of just a few speciesmay be sufficient to predict the responses of many.

Of the spectral indices examined in this study, theNDVI and fWBI show the most promise for ecosystem-scale monitoring. The NDVI is a common index, whichcan be routinely estimated from both airborne and satel-lite sensors (e.g. Landsat). Both indices have strongreflectance signals and were significantly correlated withPSII when all samples were pooled, thus there appearsto be the potential to generalize these relationships acrossSphagnum species. However, at the ecosystem scale, thecalculation of NDVI from reflectance data may be com-plicated by the effects of variable sun/sensor geometryand the atmosphere. Consequently the effective retrievalof PSII from remotely sensed data will require an accu-rate atmospheric correction and an appreciation of thetiming of image acquisition. Inconsistencies in the PRIresults suggest that it may be very difficult to find gen-eral relationships between this index and PSII at theecosystem level. It is likely that the PRI/PSII relation-ship is confounded by a combination of biotic and abioticfactors, which may be exacerbated at increasing spatialscales. Furthermore, the timescale over which measure-ments are recorded must also be considered. The PRImay be responsive to both short term (minutes–hours)changes in xanthophyll cycle pigment concentrations,due to changes in the light climate, and to longer-term(days) changes in the xanthophyll pool size as a result

of environmental acclimatization. Such issues need tobe resolved before the index can be used as a reli-able indicator of Sphagnum PSII. Whilst this study hasshown that empirical relationships exist for a given set ofenvironmental conditions, further research is ultimatelywarranted to understand relationships between spectralreflectance and Sphagnum photosynthetic activity for agreater number of species at increasing spatial and tem-poral scales.

ACKNOWLEDGEMENTS

The Department of Geography, University of Sheffieldand the Department of Biological Sciences, Universityof Essex are thanked for loan of technical equipment.Hakan Rydin, Gustaf Granath and Urban Gunnarssonare thanked for comments on an earlier version of thismanuscript. Andy Carr is thanked for field assistance andthe anonymous reviewers are thanked for their helpfulcomments and suggestions.

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Spectral reflectance and photosynthetic properties of Sphagnum mosses exposed to progressive drought