Is Empodisma minus the ecosystem engineer of the FBT (fen–bog transition zone) in New Zealand?

Figures & data

Table 1  Parameters which allow differentiation between allogenic and autogenic mechanisms for crossing the fen–bog transition (FBT), including the role of ecosystem engineers (EE), and some examples of papers which apply these parameters to mire studies.

		Allogenic		Autogenic		Example of use in mires
Parameters‘	Tools	Fen → bog		Fen → bog
Climate	Prevailing climate	Not applicable or relevant		Moist		Winkler 1988; Foster & Wright 1990
	Regional climate change	Norm	Cooler and/or wetter	Not applicable		Tallis 1983; Kuhry et al. 1991; Langdon et al. 2003
Catchment history	Mire expansion	Consider		Not applicable		Korhola 1992
	Hydrological changes	Consider		Not applicable		Kulczynski 1949; Heinselman 1970
	Tephra or sediment layers	Absent	Present (buried)	Not applicable		Lowe et al. 1999
Water table indicators	Desiccation surfaces on peat	Present	Absent	Absent		Casparie 1993
	Charcoal fragments	Present	Absent	Absent		Hughes et al. 2000; Barber et al. 2003
	Diatom assemblages (in recent peat)	Differences, especially in Eunotia		Differences		Foster & Fritz 1987; Dell'uomo 1992
	Collembola	Differences		Differences		Slawska 2000
	Testate amoebae assemblage	Low and/or fluctuating water table	Stable, high water table	High water table	Stable, high water table	Charman 1997, 2002; Charman & Warner 1997; Wilmshurst et al. 2003; Lamentowicz & Mitchell 2005; Lamentowicz et al. 2008
	Macrofossil analysis	Low	High	Fluctuating	Stable, high	Johnson et al. 1990; Nicholson & Vitt 1990; Kuhry et al. 1993
Peat characters	Humification	Sapric	Sapric/Hemic	Sapric	Hemic	Shearer 1997; Ellis & Tallis 2001
	Carbon:Nitrogen ratio	Increasing		Stable, high		Kuhry et al. 1991; Turunen & Turunen 2003
Inferred pH		Abrupt transition from high to low		Steady decline		Nicholson & Vitt 1990; Kuhry et al. 1993
Vegetation structure	Presence of EE	EE absent or at low densities	EE absent or present	EE present	EE dominant	Payette 1988; Nicholson & Vitt 1990; Kuhry et al. 1993; Hughes & Dumayne-Peaty 2002; Barber et al. 2003

Table 2  Characters required for a species to be an ecosystem engineer (EE) across the fen–bog transition (FBT). The match to requirements of four potential EE taxa is categorized as good (√), bad (X), mixed (±), and unknown (?).

Character	Northern hemisphere Sphagnum spp.	Eriophorum vaginatum	Empodisma minus	Chionochloa rubra
Ecosystem engineering ability present
EE present both sides of the FBT	√ S. fuscum (Heinselman 1970; Payette 1988; Kuhry et al. 1993) S. fuscum has wide edaphic range (Bragazza 1997) S. imbricatum (Hughes & Dumayne-Peaty 2002)	X Temporary precursor only to Sphagnum-dominated raised bog (Hughes & Dumayne-Peaty 2002)	√ Common in mesotrophic to oligotrophic stages (Cranwell 1953; Shearer 1997; Clarkson 2002; McGlone 2009)	√ Wide edaphic range (Connor 1991)
Persistent macrofossil occurrence of EE throughout peat core	√ Preserved remains in 52% of samples in near continuous bands (Tallis 1994)	± Periodic peaks in leaf sheath bundles and spindles (Tuittila et al. 2007) May be a major peat former (Hughes pers. comm.)	√ EE persistent (Cranwell 1953; Campbell et al. 1973; Walker et al. 2001)	X Not a significant peat former in a mire largely lacking Empodisma (Walker et al. 2001)
Accumulation of peat precursors
Low palatability to herbivores	√ Unpalatable (Clymo & Hayward 1982)	X Palatable; grazed by sheep, cattle, lemmings, ground squirrels, geese and caribou (Chapin & Slack 1979; Grant et al. 1987; Wein & MacLean 1973)	√ Unpalatable (van Rees & Hutson 1983; van Rees & Holmes 1986; McDougall 2007)
	√ Unpalatable; ‘worthless’ character as sheep feed (Cockayne 1967, p. 121) Fertilization increases palatability (Connor et al. 1970) Organic matter digestibility is low (Fenner et al. 1993)
Inherent inertness of litter	√ Low rate of decomposition (Aerts et al. 1999) High phenolic content (Dickinson & Maggs 1974) High refractory nature of peat (Scheffer et al. 2001) Low N content impacts on decay (Aerts et al. 2001) Presence of sphagnan may cause a form of C limitation as consequence of humification (Thomas & Pearce 2004)	√ Low rate of decomposition (Coulson & Butterfield 1978; Cornelissen 1996; Hughes & Barber 2004) Non-significant differences between nutrient content in rain-protected and unprotected leaves (Jonasson & Chapin 1985) Litter of rhizomes, leaf bases and roots accumulates as peat (Hughes pers. comm.)	√ Inherently low tissue nutrients (Agnew et al. 1993; Sharp 1995; Clarkson et al. 2005) Decay resistant outer cortex, central strand and endodermis in root (Campbell 1981) High polyphenol and lignin in stems; anhydro-galactosan in root hairs (Kuder et al. 1998) Heavily lignified mature roots (Sorrell et al. 2000)	√ Lignin 6.8–10.3% (oven dry weight) in shoot material (Connor et al. 1970) High mechanical strength (Connor & Bailey 1972) High mean tiller weight and leaf dieback, and low mineral content in low nutrient soil (Lee & Fenner 1989)
Inhibition of microbial activity	√ Microbial inhibition (Dickinson 1983; Verhoeven & Toth 1995; Borsheim et al. 2001; Bonnett et al. 2006; Opelt et al. 2007; Pankratov & Dedysh 2009) Toxic organochemicals (Verhoeven & Liefveld 1997) Sphagnol and sphagnan inhibit decomposition (Smidsrød & Painter 1984; Rudolph & Samland 1985; Painter 1991; Stalheim et al. 2009)	?	√ Microbial inhibition due to allelopathic properties of tissues (Kuder et al. 1998) Low microbial biomass (Schipper et al. 1998) Litter and moss protected from degradation through incorporation into negatively geotropic root mass (Campbell 1981; Agnew et al. 1993)	?
Maintenance of high and stable water table
Reduced evapotranspiration	√ In hummocks, internal conduction of water re-wets hummocks (Titus & Wagner 1984) In hummocks, inrolled cucullate leaves and compact form reduce surface area for evaporation (Green 1968) In hollows, dehydrated S. cuspidatum hyaline cells have high albedo, reflecting light and reducing temperature (van der Molen & Wijmstra 1994)	√ Increased relative humidity under canopy (Lavoie et al. 2005)	√ Low evapotranspiration rates (<2.5 mm day^-1) due to thick litter and dense canopy (Campbell & Williamson 1997) Stomatal conductance maximized in morning (Sharp 1995)	√ Low transpiration losses c.f. Festuca novae-zelandiae and Chionochloa australis (Espie 1999) Few stomata, located in deep adaxial furrows, preventing desiccation (Espie 1999)
Water-holding capacity	√ High water holding capacity (10–25× own weight) (Andrus 1986) High effective porosity with depth (Holden 2009) Maintenance of macropore structure via litter quality (Turetsky et al. 2008)	√ Peat mass efficient at retaining water during periods of desiccation (Hughes et al. 2000)	√ High water holding capacity (15× own weight) in sponge-like root weft (Campbell 1964) Dense canopy retains moisture at deeper canopy levels (Campbell & Williamson 1997) Moisture retained in peat (Campbell et al. 1995)	?
Decreased vertical drainage	√ Hydraulic conductivity decreases with depth (Adema et al. 2006; Price et al. 2008) Sphagnum structure collapses and bulk density increases with depth (Hayward & Clymo 1982; Johnson et al. 1990) Variable infiltration rates with depth (Holden 2009)	X No change in infiltration rates with depth (Holden 2009)	√ Decreasing hydraulic conductivity with depth; most water exchange in surface 1.5 m of peat layer (Maggs 1997) Acrotelm drainable porosity halved within 200 mm of surface (Miller 1994)	?
Drainage barriers	√ Iron hardpan formation (Andrus 1986) Fe-Mg hardpan formation (Klinger 1996; Kusel et al. 2008)	√ Iron oxide deposits in soil surrounding roots (Koch et al. 1991)	?	?
Adaptations to water table fluctuation	√ Ability to grow under both wet and dry conditions of mire surface (Green 1968; Casparie 1972; Barber 1981; van der Molen & Hoekstra 1988)	√ Wide tolerance to water table depth (Gore & Urquhart 1966; Barber 1981)	√ Structural elements may protect chlorenchyma from collapse in dry spells (Linder 2000) Surficial roots avoid anoxia by negatively geotropic growth habit (Campbell 1981; Sorrell et al. 2000) Surface rhizomes maintain atmospheric O2 concentrations (Sorrell et al. 2000)	√ Wide tolerances (MacIntosh et al. 1983)
Development of mire microforms	√ Hummocks (Green 1968; Titus & Wagner 1984) S. imbricatum has two morphologically distinct ecads, allowing it to grow in both hummocks and semi-aquatic habitats (Green 1968)	?	√ Hummock and hollow topography forms near edges of mire (Rogers 1984)	X No microtopography known (pers. obs.)
Features altering pH
Acidification	√ pH of approximately 4 (Sonesson 1970; Braekke 1981; Glaser et al. 1981) Acidification via polyuronic acids (Clymo 1963; Bellamy & Reiley 1967) Polyuronic acid content 10–30% of dry weight; larger values for hummock species (Clymo 1964, 1967; Vile et al. 1999) Organic acid production acidifies subsurface waters (Hemond 1980)	± Low cation exchange capacity (Hughes & Dumayne-Peaty 2002) Acidification of substrate occurs under continental climate conditions (Hughes pers. comm.)	√ Acidification via polyuronic acids, at higher levels than for Sphagnum (Bannister 2000)	?

Table 3  Characters required for a species to persist on the bog side of the fen–bog transition (FBT). The match of four potential EE taxa is categorized as good (√), bad (X), mixed (±), and unknown (?).

Character	Northern hemisphere Sphagnum spp.	Eriophorum vaginatum	Empodisma minus	Chionochloa rubra
EE dominance
Competitive?	√ Dominant (Moore & Bellamy 1974)	X Increased relative humidity under canopy facilitates establishment of moss species (Lavoie et al. 2005) Only temporary on raised bogs (Hughes & Dumayne-Peaty 2002)	√ Shrubs stunted by low P concentrations on restiad mires (Clarkson et al. 2005) Clonal resprouter (Meney & Pate 1999)	± Tussocks appear stunted in restiad mires (pers. obs.) and string fens (Rapson et al. 2006)
Nutrient access
Water pre-emption	√ Efficient capture of atmospheric water (Lee & Woodin 1988; Aldous 2002) External capture via capillary water flow (Clymo & Hayward 1982) Buoyancy-driven water flow replenishes nutrients (Rappoldt et al. 2003; Adema et al. 2006)	√ Exploitation of subsurface waters via deep roots overcoming nutrient limitation (Chapin et al. 1988)	√ Pre-emption by high stem flow (Agnew et al. 1993) Accesses atmospheric nutrient sources (Clarkson et al. 2009)	± By fog-water interception (Ingraham & Mark 2000)
Deep root systems	X No roots present	?	X Cluster roots retrieve N from only the top 5 cm of substrate (Clarkson et al. 2009)	√ Deep roots (Craine & Lee 2002)
Nutrient uptake
Cation exchange	√ High cation exchange capacity (Clymo 1963; Daniels & Eddy 1985) S. rubellum takes up amino acids as N source (Kielland 1997) Symbiotic methanotrophs oxidize methane for Sphagnum assimilation (Raghoebarsing et al. 2005) Incorporation of nitrate and phosphate rapidly from solutions of unusually low concentration (Clymo & Hayward 1982; Bragazza et al. 2003)	√ Absorbs organic N and has preferential use of amino acids as N source (Schimel & Chapin 1996) Methanotrophs in Eriophorum/Sphagnum dominant community similar to those of ombrotrophic peatlands (Chen et al. 2008) Very efficient in taking up nutrients in luxury nutrient conditions (Shaver et al. 1986) Root surface phosphatases contribute up to 69% of annual P demand (Kroehler & Linkins 1991; Moorhead et al. 1993)	√ Presence of uronic acids suggests uptake capability (Bannister 2000) High efficiency of nutrient uptake across a range of substrate values (Clarkson et al. 2005)	?
Root adaptations	X None	?	√ Scavenges nutrients with negatively geotropic roots and 1 mm root hairs (Campbell 1975, 1981; Agnew et al. 1993)	?
Leaf life span	√ Long leaf longevity (Chapin et al. 1995)	X Short leaf longevity (Jonasson & Chapin 1985)	√ Stems long-lived, dead shoots retained in canopy layer (pers. obs.)	√ Long-lived (Greer 1979)
Nutrient storage organs	X None	√ Seasonal variation in nutrients of tundra species' organs (Chapin 1980) Storage of limiting nutrients (especially P and K) in perennating organs (Goodman & Perkins 1959; Defoliart et al. 1988)	√ Rhizomes to 1 cm thick (pers. obs.)	√ High nutrient concentrations in leaf sheaths (Williams et al. 1978) High carbohydrate storage in stems and leaf bases of congeners (Payton & Brasch 1978)
Nutrient recycling
	√ Nutrient translocation (Rydin & Clymo 1989; Svensson 1995; Aldous 2002) Internal nutrient transport through perforations in stem parenchyma cells (Rydin & Clymo 1989)	√ 90% leaf P and 80% leaf N resorbed (Jonasson & Chapin 1985) High nutrient use efficiency under low nutrient conditions (Thormann and Bayley 1997) Competitive on cold, infertile sites with other sedges (Shaver & Laundre 2003) Vascular sclerenchyma in corm aids in nutrient recovery and redistribution (Cholewa & Griffith 2004).	√ Nutrient uptake occurs in upper 5 cm of root weft (Clarkson et al. 2005), so leachates can be scavenged	?

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