26 Feather , nitrogen fixation and the boreal biome 27 Feather mosses, nitrogen fixation and the boreal biome

Feather mosses, nitrogen fixation and the boreal biome

> T. H. DeLuca School of the Environment and Natural Resources, Bangor University, Bangor, UK [email protected]

Introduction

Nitrogen (N) is the primary limiting nutrient for both productivity and decomposition in boreal forests (Tamm, 1991) yet we lack a clear understanding of the influence of N inputs from nitrogen fixation on forest ecosystem dynamics, productivity and carbon (C) cycling. Although our atmosphere is 78% N2 gas, none of that N is available Figure 1. A late succession Scots pine, Norway spruce forest in directly to land or animals; it must first be converted northern Sweden. Feather mosses make up the light green carpet on the forest floor. to ammoniacal (NH3) N by nitrogen-fixing microbes, often acting in symbiosis with species, and then further transformed to useful amino acids in plants or bacteria. As there is a general lack of woody or herbaceous N-fixing plant-microbe associations in upland boreal forests, the primary source of useful N supply in these ecosystems has until recently remained a mystery.

There is currently a great deal of interest in boreal forest ecosystems and the vital role they play in global N and C balance. The boreal ecosystem is the second largest biome in the world and one of the largest terrestrial C sinks on Earth, storing approximately 30% of total terrestrial carbon stocks. Feather mosses are an ever-present component of the boreal forest floor (see Figs 1, 2), accounting for as Figure 2. A close up view of schreberi, pic: © Uwe much as 95% of ground cover, and a major contributor to the Drehwald annual productivity of the boreal biome. While the ecology formed from both feather mosses and sphagnum mosses, and ecosystems of these bryophytes within the boreal forest this dense insulting layer controls both heat flux from have been widely described by numerous researchers, the incoming radiation and losses from the largely mineral boreal contribution of bryophytes and, in particular, feather mosses, soils (Beringer et al., 2001), thereby directly controlling soil to global ecosystem models often goes unobserved. temperatures and indirectly regulating surface processes such as carbon cycling. The feather ‘bottom layer’ is a vital organ of the boreal forest, functioning to filter water, insulate against Over the past 10 years, we have investigated the notion temperature change, regulate nutrient uptake and minimise that feather mosses also serve as hosts for N-fixing nutrient losses. It provides the breathing, protective skin cyanobacteria, aiming to expand our knowledge of their of the forest floor (Lindo and Gonzeles, 2010). Potentially beneficial properties. Our results show that, for the biota 28 Feather mosses, nitrogen fixation and the boreal biome

of the relatively pristine (low N deposition) environments in creating bioavailable N in the boreal forest. As stated of northern Scandinavia, feather mosses are a dominant previously, the conspicuous lack of any significant quantity source of N (DeLuca et al., 2002b; Zackrisson et al., 2004; of herbaceous or woody N-fixing plant-microbe associations DeLuca et al., 2007; Zackrisson et al., 2009), thus providing a across much of the boreal forest had long puzzled forest first step towards a holistic understanding of the role which ecologists, who therefore questioned the source of available natural N fixation plays in these high latitude boreal forests. N supply in fire-maintained or harvested boreal forest This research has also provided an insight into the means by ecosystems (Tamm, 1991). which N subsequently accumulates in such pristine forest ecosystems and the following paragraphs describe some of Repeated efforts to measure N fixation associated with the history of our studies and raise some future questions feather mosses have failed because most researchers would that have yet to be addressed. take their samples from high latitude ecosystems in late June to mid-July, a time of uniquely low N fixation activity carpets in forest feather moss ecosystems (DeLuca et al., 2002b; Zackrisson et al., 2004; Zackrisson et al., 2009). Also, the Upland moss communities of the boreal forest contain spatial variation of N fixation in feather mosses obligates hundreds of species of bryophytes but are dominated an intensive sampling strategy, which was not the norm in by two pleurocarpous species (in which the female surveys of bryophyte N fixation. Our work has therefore archegonium reproductive structures are on a short side opened up the possibility that feather mosses are actually branch rather than on the main stalk): Pleurozium schreberi responsible for a great deal of the N accumulation resident and , and one acrocarpous species in boreal forests and has underscored the importance of (in which the archegonia are at the top of the stem), feather mosses in the total N economy of the boreal biome Polytrichum commune. The photosynthetic capacity of these (DeLuca et al., 2002b; Zackrisson et al., 2004). feather mosses is reflected in an ecosystem productivity that rivals, and at times (during early to mid-succession) We have found cyanobacteria (see Fig. 3) residing in the can surpass, that of the overstorey (Bond-Lamberty et al., leaf incurve of some of the pleurocarpous moss species, 2007). Hylocomium and Pleurozium often account for 30- leaving them both difficult to observe and difficult to 95% of the average cover of the boreal forest floor and yield culture. Nevertheless, we have commonly retrieved three a net primary productivity (NPP) of 200 to >400 kg ha-1 yr-1, genera of cyanobacteria (Nostoc, Stigonema and Calothryx) comprising a total biomass of between 0.1–2.0 Mg C ha-1 associated with Pleurozium schreberi alone (Gentili et al., as live moss and 2.0–4.0 Mg C ha-1 as dead moss (Vogel 2005). These cyanobacteria demonstrate several unique and Gower, 1998; Turetsky, 2003). Their photosynthetic properties, including having biphasic temperature optima contribution to C fixation capacity has been found to be of for productivity at 13oC and 22oC, which makes them ideally particular importance when the overstorey is under stress, suited to N fixation in boreal ecosystems. The feather moss as, for example, in the large-scale bark beetle outbreak that carpets are thus a uniquely convenient system for studying recently raged across the Canadian boreal forest, where N fixation, as either individual shoots or whole moss carpets bryophyte productivity remained relatively consistent with can be placed in a vessel for manipulation or measurement the opening of the forest canopy in the absence of fire of fixation activity and this has allowed us to conduct (Olsson and Staff, 1995). With the dip in photosynthetic C intensive monitoring. fixation by trees and the net C emissions associated with dead tree decay, the carbon assimilation associated with Distribution of N fixation inPleurozium the moss bottom layer becomes much more important for schreberi in Fennoscandia ecosystem maintenance. The carbon fixed by the mosses is slowly incorporated into the fibric materials that comprise Over the past 10 years, we have established sampling plots the dense humus and debris mat often associated with for Pleurozium schreberi and Hylocomium splendans at boreal Spodosol soils (Beringer et al., 2001). about 50 sites across northern Sweden, Finland and Norway (Fig.4). The majority of samples have been collected from Although feather mosses have long been known to intercept natural forest preserves to minimise site disturbance and and retain nutrients from forest throughfall, there has all sites occur within the mid- to northern boreal zone of previously been little understanding of the role they play Fennoscandia (59o–69oN; 17o–19oE). When samples from 500 ) -1 Pleurozium

d 400

-2 Hylocomium

m 29 Feather mosses, nitrogen fixation mol and the boreal biome μ 300 (

!

200

these plots were assessed for N fixation activity using the acetylene reduction technique, there was a clear relationship 100 between the latitude and longitude of the sample collected Acetylene reduction and N fixation rate, wherein fixation rates were greater in the north, away from the Botinian Bay section of the Baltic Sea, likely as a result of the high rates of N deposition along 0 this more populated coast. In comparison, the boreal forests 59 59 59 60 61 64 j 65 Ty Br 59Bj So 59Fi Va 60In 60Gr Ab60 Ed60 60Lo o Ta 62 Ku Vi 64 Ki 64Vi 65 Ja 65 T Ku 65u Su65 66 2 Ad67 67 a An67 69 of northern Sweden exhibit exceptionally low rates of N H SonVat 61 61Lon Ro63 64 Mo 64 De 66 Ru Gu65 65 N Ai1Ai2 66Ai3 66Ai 66v3Aiv1 66Ai 67v Ru N67 Site and latitude (name and degrees north) deposition, both compared to southern Sweden and to other ! more industrial regions of the world (Gundale et al., 2011). Indeed, nitrogen deposition in Sweden ranges from about 20 kg N ha-1 yr-1 in southern Sweden to less than 1.0 kg ha-1 yr-1 in Northern Sweden where most of our studies have been performed. Nitrogen deposition can interfere greatly with N fixation in cyanobacteria and this may partially account for the geographic variation in our results.

Successional influence on N-fixation rates Figure 3. The cyanobacteria, Stigonema sp., taken from the leaf incurve of Pleurozium schreberi. Fire is the primary form of chronic disturbance in the boreal forest ecosystems of northern Europe and is a potent driver of shifts in nutrient cycling and plant community composition. The recovery of organic N resources lost

Pleurozium during severe wildfire is dependent upon biological N fixation 400 (Smithwick et al., 2005) and, in the boreal forest, this is primarily accomplished by the feather moss-cyanobacterial 300 associations (Fig. 5). We have found that feather mosses recover to pre-fire densities ca 30- 60 years after fire and 200

Boreal Forest Secondary Succession and N Availablity 100 Early Mid Late

Rain / throughfall Nitrogen Shrubs + - xation N 0 0 NH4 , NO3 2 ? NO N 2 CH 4 140 Hylocomium N 0 2 R-NH2

) ? + + - -1 120 Fire - NH , R-NH d NH , NO Moss 4 2 4 3 -2 ?

m 100 mol

µ 80 R-NH NO 2 3 NH tion ( 60 4 Relative soil Relative concentration

educ 40

20

ylene r Throughfall N Moss N xation Acet 0 Relative rate rate Relative Time since re Site and latitude (name and degrees north) Figure 5. Nitrogenase activity in Pleurozium schreberi and Figure 4. A stylized view of forest succession and N cycling in Scots Hylocomium splendans along a latitudinal gradient (59o N to 69o pine forests of northern Sweden. N) in Sweden (from Zackrisson et al., 2009). 30 Feather mosses, nitrogen fixation and the boreal biome

that N fixation rates increase linearly with time during the 400 recovery period (DeLuca et al., 2002a; Zackrisson et al.

2004). The rates of N fixation measured in fire-damaged ) -1 300 locations were scaled up to ecosystem level by factoring yr -2

in the percentage ground cover of P. schreberi (Zackrisson m mol

et al., 2004) and using an experimental ratio of 3 moles of µ

( 200

acetylene reduced to correspond to 1 mole of N2 fixed in the field (15N calibrated value, DeLuca et al., 2002b). Applying these assumptions, we estimated N-fixation rates to be less 100 than 0.5 kg ha-1 yr-1 in early succession sites (aged 25–80 years since fire), between 0.4 and 1.6 kg ha-1 yr-1 on mid-

Acetylene reduction 0 succession sites (100 – 200 years since fire) and between Heath EricoidHerb -1 -1 1.0 and 3.0 kg ha yr in late-succession ecosystems (>200 10 years since fire). ) 1 - 8 yr 1 Early succession forests exhibit relatively high levels of N - availability as a product of recent fire; this N being ultimately 6 recycled through the canopy and resulting in higher rates of N deposition via throughfall (DeLucaet al., 2008). In this 4 throughfall, water percolates through the canopy, picks up nutrients and then delivers them to the moss bottom layer. If the N content of the throughfall is high (as in early 2 succession), N fixation in feather mosses is reduced (DeLuca

Inorganic N deposition (kg ha 0 et al., 2008). Heath Ericoid Herb

Nitrogen deposition Figure 6. Nitrogenase activity in feather mosses and throughfall N deposition (as observed in resin lysimeters) averaged across three natural fertility gradients in Northern Sweden (DeLuca et The influence of N deposition on N fixation rates has been al., unpublished data). studied at several locales in northern Sweden. A fertilisation study at a late successional forest site (Ruttjeheden, 355 years since fire) where N fixation rates had been observed to be relatively high (Zackrissonet al., 2004) demonstrated Numerous natural fertility gradients exist in northern Sweden a very clear elimination of N fixation with N applications where groundwater recharge zones deliver nutrients and as low as 4.25 kg N ha-1 yr-1. In a long-term N enrichment moisture to the soil surface. This results in the creation of a study near Vindlen, we observed the influence of 5 years completely unique forest composition where groundwater application of 0, 3, 6, 12 or 50 kg N ha-1 yr-1 as ammonium discharge zones are dominated by extremely large spruce and birch trees, with an understory dominated by tall nitrate, NH4NO3 (Gundale et al. 2011). Isotopic analyses associated with this study suggested that feather mosses herbaceous species, in contrast to the modest sized pines absorbed much of the N supplied at low applications rates, and dwarf shrubs of normal upland forests. Nitrogen fixation with accumulation of fertiliser N in the dwarf shrub layer rates in the tall herbaceous ecosystems within discharge occurring only at the highest levels of application. These zones are greatly reduced in line with the far greater levels studies support the notion that N deposition directly down- of N in the throughfall (Fig. 6). In contrast, N fixation rates regulates the N fixation associated with feather mosses were maximised in lower fertility dwarf shrub stands and at (Huttunen et al., 1981; Zackrisson et al., 2004; Gundale et intermediate levels in the low fertility lichen heath stands. al., 2011). 31 Feather mosses, nitrogen fixation and the boreal biome

Summary and Ongoing Studies References Ongoing and proposed studies are designed to test the fate of the N fixed in feather moss carpets and to assess Beringer, J., Lynch, A. H., Chapi, F. S., Mack, M. and Bonan, G. B.. (2001). The representation of arctic soils in the land surface model: the the role of organic N uptake on the overall N balance of importance of mosses. Journal of Climate 14, 3324-3335. feather moss ecosystems. More detailed investigations Bond-Lamberty, B., Peckham, S. D., Ahl, D. E. and Gower, S. T. (2007). are being undertaken on the interaction of a range of Fire as the dominant driver of central Canadian boreal forest carbon external N sources on N fixation rates, including studies balance. Nature 450, 89-92. on the influence of roadside pollution on feather moss DeLuca, T. H., Nilsson, M.-C. and Zackrisson, O. (2002a). Nitrogen productivity and ecology. Mosses near busy highways have mineralization and phenol accumulation along a fire chronosequence in northern Sweden. Oecologia 133, 206-214. been observed to have few cyanobacterial associates and suppressed nitrogenase activity, whereas mosses located DeLuca, T. H., Zackrisson, O., Gentili, F., Sellstedt, A. and Nilsson, M.-C. (2007). Ecosystem controls on nitrogen fixation in boreal feather moss 100 m or more from the highway express normal rates of N communities. Oecologia 152, 121-130. fixation. These studies suggest that feather moss N fixation DeLuca, T. H., Zackrisson, O., Nilsson, M.-C. and Gundale, M. J. (2008). could serve as a sensitive indicator of N pollution as the Ecosystem feedbacks and nitrogen fixation in boreal forests. Science nitrous oxides emitted from vehicles apparently inhibit both 320, 1181. the presence and activity of cyanobacteria. DeLuca, T. H., Zackrisson, O., Nilsson, M.-C. and Sellstedt, A. (2002b). Quantifying nitrogen-fixation in feather moss carpets of boreal forests. Nature 419, 917-920. In summary, boreal feather mosses clearly play an important role in the ecological function of boreal forests. They function Gentili, F., Nilsson, M.-C., Zackrisson, O., DeLuca, T. H. and Sellstedt, A. (2005). Physiological and molecular diversity of feather moss associative to filter forest nutrient throughfall, to insulate the largely N2 fixing cyanobacteria. Journal of Experimental Botany 56, 3121-3127. mineral-based soils, and to perform the historically important Gundale, M. J., DeLuca, T. H. and Nordin, A. (2011). Bryophytes process of adding plant-available N to the ecosystem. The N attenuate anthropogenic nitrogen inputs in boreal forests. Global provided by feather mosses remains crucial to the function Change Biology 17, 2743–2753. of pristine boreal forest ecosystems and, given their broad Huttunen, S., Karhu, M. and Kallio, S. (1981). The effect of air pollution ecological amplitude, Pleurozium schreberi and Hylocomium on transplanted mosses. Silva Fennica 15, 495-504. splendens, along with their cyanobacterial associates, may be Lindo, Z. and Gonzeles, A. (2010). The bryosphere: an integral and Ecosystems the most broadly distributed N-fixing association on Earth. influential component of the Earth’s biosphere. 13, 612-627. Olsson, B. A. and Staff, H. (1995). Influence of harvesting intensity of logging residues on ground vegetation in coniferous forests. Journal of Applied Ecology 32, 640-652. Smithwick, E. A., Turner, H. M., Mack, M. C. and Chapin, F. S. (2005). Post fire soil N cycling in northern conifer forests affected by severe, stand replacing wildfires. Ecosystems 8, 163-181. Tamm, C. O. (1991). Nitrogen in terrestrial ecosystems. Springer, Berlin. Turetsky, M. R. (2003). The role of bryophytes in carbon and nitrogen cycling. The Bryologist 106, 395-409. Vogel, J. C. and Gower, S. T. (1998). Carbon and nitrogen dynamcis of boreal jack pine stands with and without a green alder understory. Ecosystems 1, 386-400. Zackrisson, O., DeLuca, T. H., Gentili, F., Sellstedt, A. and Jäderlund, A. (2009). Nitrogen fixation in mixed Hylocomium splendens moss communities Oecologia 160, 309-319. Zackrisson, O., DeLuca, T. H., Nilsson, M.-C., Sellstedt, A. and Berglund, L. (2004). Nitrogen fixation increases with successional age in boreal forests. Ecology 85, 3327-3334.