Climate Change and Managed Ecosystems - Chapter 10 pptx

213 10.1 INTRODUCTION Peatland ecosystems are characterized by the accumulation of organic matter in soil and, if following Joosten and Clark’s1 definition of having at least 30 cm of pe

Past and Future

Carbon Legacy

D.H Vitt

CONTENTS

10.1 Introduction 201

10.2 Limitations on Carbon Sequestration in Boreal Peatlands 203

10.3 The Ecology of Boreal Peat Accumulation 204

10.3.1 Bog Accumulation 205

10.3.2 Poor Fen Accumulation 205

10.3.3 Rich Fen Accumulation 205

10.4 Northern Peatlands: Sinks or Sources of Carbon? 206

10.5 Potential Climatic Effects on Peatland Form and Vegetation 207

10.6 Permafrost Melting in the Boreal Forest 209

10.7 Global Climate Change vs Cumulative Disturbance 210

10.8 Mitigation 211

Acknowledgments 213

References 213

10.1 INTRODUCTION

Peatland ecosystems are characterized by the accumulation of organic matter in soil and, if following Joosten and Clark’s1 definition of having at least 30 cm of peat with a minimum organic content of 30%, then peatlands cover over 4 million km2

— about 3% of the Earth’s land surface Nearly 70% of this peatland area lies in the boreal regions of Canada and Russia Canada alone contains just over 1,200,000

km2 of peat Peatlands are significant in that they provide a wide diversity of ecosystem services, not the least of which is the accumulation of large stores of carbon Joosten and Clark1 estimated that since 1800, 10 to 20% of the world’s peatlands have been lost, but it has been the view of many that Canada’s peatlands remain in pristine condition, undisturbed by human activities.2 However, as we will see, this is certainly not the case

Globally, wetlands (especially peatlands) represent a large carbon stock, with estimates varying from 200 to 860 Pg (= Petagrams) of carbon (see for example

Gorham,2 Bohn,3,4 Sjörs,5 Post et al.,6 Houghton et al.,7 Armentano and Menges,8 and Markov et al.9) Generally, carbon-rich peatland soils are thought to represent about one third of the world’s soil carbon, yet cover only about 3% of the land surface Release of this store of carbon into the atmosphere would increase atmo-spheric CO2 concentrations by more than 50% Canada’s peat inventory has been estimated to contain up to 170 Pg of carbon2 and is approximately 38% of the carbon stock in northern peatlands The western boreal forest region of Alberta, Saskatchewan, and Manitoba contain 365,157 km2 of peatlands and along with British Columbia these four provinces have about 40% of Canada’s peatland area, while eastern Canada (Ontario eastward) contains about 37%, and northern Canada (the three territories) contains approximately 23%.10 In terms of carbon, western and northern Canada store at least 83 Pg, whereas eastern Canada has a minimum of 52

Pg In Alberta, peatlands may contain as much as 70% of the province’s soil carbon (13.5 Pg C in peatlands; 2.3 Pg C in lakes, 2.7 Pg C in forests, and 0.8 Pg C in grasslands (data from J Bhatti11) In general, the western Canadian peatlands have sequestered about 48 Pg of carbon during the past 10,000 years, with about half of this accumulated in the last 4000 years.12

Peat accumulates on the landscape when annual net primary production exceeds the sum of annual decomposition and the loss of carbon that is dissolved in the pore water and exported from peatlands The initiation, development, and succession of ecosystems that sequester carbon, as well as the rate of peat accumulation in boreal peatlands, are dependent on regional allogenic factors such as climate, substrate chemistry, and landscape and hydrological position These regional driving factors

in turn determine a suite of local factors that influence the form and function of individual peatland sites (Figure 10.1) These local factors include water chemistry,

FIGURE 10.1 Diagrammatic representation of the interactions between regional, local, and

ecological factors that control function and form of peatlands.

Regional

Local

Disturbance Function Form

Autogenic Processes

Vegetation and Flora

Water Flow

Nutrients

Water Chemistry Water Level Fluctuation

Production

Decomposition

Carbon Sequestration

Succession Bog Development Pattern Development Landform Development Substrate

water level fluctuation, water flow rates, and nutrient inputs Peatland form is deter-mined by this interacting suite of local and regional factors through the development

of ombrotrophy wherein the peatland receives all water and nutrients from the atmosphere, evolution of internal landforms and landscape pattern, and the direction

of succession Additionally, once established, peatlands have strong autogenic con-trols (acidification, eutrophication; Vitt13) that also help regulate form and function.14 The functioning of peatland ecosystems centers on the process of peat accumu-lation Yu et al.15 provided conceptual diagrams of carbon cycling in peatlands Peat accumulation is dependent on the rate of input of organic matter into the anaerobic peat column (the catotelm) and on the rate of the slow decomposition of this material over time.16 Climate is the most important regional factor, mainly through its regu-lation of local water regimes Among climatic variables, Halsey et al.17 demonstrated for wetlands of Manitoba that temperature and aridity are the most critical limiting factors at the landscape scale for peatland ecosystems

Climate affects carbon sequestration by limiting photosynthesis and aerobic (acrotelm) decomposition rates, thus influencing the amount and quality of the organic material reaching the catotelm Climate also affects carbon stocks within the catotelm by limiting anaerobic processes (methenogenesis, sulfate reduction, and

N2O production), as well as controlling the position of the acrotelm–catotelm bound-ary Thus, climatic change can affect current peat accumulation as well as persistence

of the peat column itself

10.2 LIMITATIONS ON CARBON SEQUESTRATION

IN BOREAL PEATLANDS

Four factors contribute to limiting carbon sequestration in pristine boreal peatlands: (1) The formation of permafrost in boreal peatlands reduces the input of carbon to the peatland (2) Ground layer biomass contributes high-quality organic matter that

is resistant to decay to the peat column and, along with vascular plant roots and litter from aboveground vascular plant biomass, compose the carbon inputs to peat-form-ing ecosystems These inputs are limited by net annual primary production (NPP) (3) Rates of aerobic respiration (occurring in the acrotelm) limit peat accumulation Rates of aerobic decomposition may be determined by substrate quality and by temperature (4) The amount of time the decomposing plant material spends in aerobic conditions In summary, cold, dry climatic conditions favor permafrost aggra-dation; warm, dry (arid) conditions limit ground-layer NPP and increase acrotelm depth, while warm, wet conditions increase microbial respiration Peat accumulation decreases with aridity and increases under cool, moist conditions (Figure 10.2) Corollaries to these relationships provide us with four mechanistic statements:

1 Increases in precipitation increase the ground layer production, and these, coupled with a rise in water table, decrease residence time in the acrotelm lowering initial catotelmic bulk densities: Carbon sequestration increases

2 Decreases in precipitation decrease ground layer production and are cou-pled to a lowering of the water table These factors increase the residence

time in the acrotelm, thus increasing the initial catotelmic bulk densities: Carbon sequestration decreases

3 Increases in temperatures increase acrotelmic respiration, hence increas-ing initial catotelmic bulk densities: Carbon sequestration decreases

4 Decreases in temperatures decrease acrotelmic respiration, hence decreas-ing initial catotelmic bulk densities: Carbon sequestration increases

10.3 THE ECOLOGY OF BOREAL PEAT ACCUMULATION

Accumulation of peat in the boreal region appears to occur under three somewhat different ecological regimes

FIGURE 10.2 Factors limiting carbon sequestration in boreal peatlands plotted over climatic

space represented by temperature (inverse on y axis) and precipitation (x axis) Shading of

central ellipse indicates increased rates of carbon accumulation A = Direction of increase in ground-layer NPP B = Direction of bulk density decrease White circle = Estimated peat accumulation during Holocene wet period 43 Black circle = Estimated peat accumulation during Holocene dry period 43 Details of long-term peatland dynamics are available in Yu et

al 15,50 Dotted line is degrading permafrost.

A

B

Cold

Cool

Warm

Precipitation

Acrotelm Depth Increase

Decreased

Ground layer

NPP

Permafrost

Aggradation

10.3.1 B OG A CCUMULATION

Bogs are ombrogenous, receiving their nutrients and water supply solely from the atmosphere as precipitation Bogs that occur in the boreal region are generally

treed and possess a continuous ground layer of Sphagnum (peat mosses) These

peat mosses develop an extensive, undulating microrelief of hummocks and hol-lows Hummocks attain heights of nearly 1 m above the water surface Thus the aerobic zone of decomposing peat (the acrotelm) is well developed and organic matter spends a relatively large amount of time in this zone; however, rates of

decomposition are reduced here largely due to factors inherent in the Sphagnum

species themselves.18–20 Furthermore, catotelmic bulk densities are relatively low

due to the fibrous nature of the hummock-occurring Sphagnum species and low

number of graminoid roots

10.3.2 P OOR F EN A CCUMULATION

Fens are geogenous, receiving waters and nutrients that have been in contact with the surrounding uplands as well as from precipitation Poor fens have ground layers

dominated by species of Sphagnum and are acidic ecosystems The Sphagnum

species of poor fens occur in carpets and lawns forming extensive flat areas relatively close to the water’s surface Thus, the acrotelm is poorly developed and the residence time of organic material in the aerobic zone is low, with the organic matter reaching the catotelm rather quickly Catotelmic bulk densities are higher than in bogs, but

due to the fibrous nature of Sphagnum and the low root biomass, are less than those

of the rich fens

10.3.3 R ICH F EN A CCUMULATION

Geogenous rich fens have ground layers dominated by true mosses (generally referred to as “brown mosses”) These plants, like the sphagna of poor fens, form lawns and carpets, water tables are high, and acrotelms are poorly developed Ground-layer canopies of rich fens differ from those of poor fens and bogs by the difference between true moss and peat moss plant architectures The rich fen acrotelm has denser canopies because it is dominated by true mosses As a result

of the high water table in rich fens, this relatively dense (carbon-rich) ground layer spends little time in the acrotelm, and reaches the catotelm as high-quality peat with high bulk densities Additionally, rich fens have higher abundances of grami-noids, and sedge roots also contribute to the high bulk densities

All three peatland types effectively sequester carbon, each in a somewhat dif-ferent manner Fundamental differences in vegetation, hydrology, and chemistry between these three peatland types13,21 lead to three generalizations about how climate change can affect the ecology of these peatland systems

1 Bogs require a local positive climatic water balance The large Sphagnum

hummocks and well-developed acrotelms must be maintained through precipitation input Nutrient supplies for these ombrogenous peatlands are dependent on atmospheric influxes

2 Fens require a constant groundwater source, and the acrotelm–catotelm boundary (so critical for peat accumulation in fens) must be maintained

at a relatively stable elevation Lowering of water tables or changes in annual water table fluctuations alter the boundary conditions

3 Changes in upland and surrounding nutrient fluxes strongly affect fens, whereas changes in atmospheric nutrient inputs strongly affect bogs

In conclusion, peat accumulation and the sequestration of carbon from the atmosphere (as CO2) to solid organic matter (as CHO) is determined and controlled

by a series of interacting processes I argue that four of these processes are of most importance (Figure 10.2) and that all of these are climatically controlled Two are more affected by temperature, while the other two are more affected by precipitation How these four factors interrelate and how they are individually affected by climate change is still poorly understood and needs to be a priority research goal

10.4 NORTHERN PEATLANDS: SINKS OR SOURCES

OF CARBON?

Although I believe that it is generally acknowledged that northern peatlands are a present-day sink for atmospheric CO2,2 several complicating factors exist that may severely limit their role in maintaining this large carbon sink

Local temporal and spatial variation in carbon sequestration is high Annual changes from net carbon sinks to net carbon sources have been demonstrated for an oligotrophic pine fen in Finland by Alm et al.,22 a Minnesota peatland by Shurpali

et al.,23 a Manitoba rich fen by Suyker et al.24 and Lafleur et al.,25 and a temperate poor fen by Carroll and Crill.26 Likewise, spatially local microhabitats may be either net sinks or sources.27

The concept of the Canadian peatlands and the boreal forest being of a pristine nature is doubtful Long-term carbon accumulation rates have been estimated at between 19.4 g m2 yr–1 for western Canada12 and 28.1 g m2 yr–1 for northern peatlands in general.2 These rates, however, are based on apparent long-term accumulation in pristine peatlands (and as well may not be representative of current net rates), and do not take into consideration peat lost from the direct and indirect effects of fire and other natural disturbances If peat losses due to the effects of the historical fire regime are added back into the 19.4 g m2 yr–1 estimates of peat accumulation the actual rate of peat accumulation is 24.5.28 In the only cumulative effects study of which I am aware, Turetsky et al.28 estimated that 13% of western Canada’s peatlands are disturbed She estimated that of the 8940 Gg C yr–1 of carbon that should be sequestered annually under a no-disturbance regime, 48 Gg

C yr–1 are lost to oil sands mining, 80 Gg C yr–1 to flooding from hydro-electric projects, 135 Gg C yr–1 to peat extraction activities, 4704 Gg C yr–1 from the direct effects of fire (carbon released from the fire itself), and 1578 Gg C yr–1 are lost

to the indirect effects of fire (due to decreased sequestration of carbon from vegetation recovery, plus decomposition during recovery) On the positive side,

melting of boreal permafrost yields a return of +100 Gg C yr–1 (see Turetsky et

al.29 for explanation) and undisturbed peatlands sequester 7781 Gg yr–1 of carbon Overall, disturbance and development across western Canada has reduced the annual carbon sequestration to +1319 Gg C yr–1 — only 14% of the long-term carbon sink rate Increases in any of the anthropogenic disturbances or in the future fire regime, or a decrease of only 17% in the carbon sequestered in undis-turbed peatlands because of drought and or temperature increases, will move western Canadian peatlands from a sink to a source of CO2

Additionally, peatland types differ in the forms of gaseous carbon release and have different global warming potentials Anaerobic respiration releases include methane (among other gases) Methane is a greenhouse gas with different absorptive properties and different atmospheric lifetimes from those of CO2 Boreal wetlands release an estimated 34 Tg of CH4 annually.30,31 Joosten and Clark1 provided global warming potentials (GWP) for northern pristine bogs and fens that were calculated for different time horizons into the future Their data indicate that currently pristine fens remove 250 kg C ha–1 yr–1 (as CO2) and release 297 kg C ha–1 yr–1 (as CH4), while bogs currently remove 310 kg C ha–1 yr–1 and release 53 kg C ha–1 yr–1 So, even though a carbon sink is indicated by 560 kg C ha–1 yr–1 being sequestered and only 350 kg C ha–1 yr–1 released, differences in atmospheric properties of CO2 and

CH4 produce a positive GWP when calculated per hectare for bogs and fens over the next 20- and 100-year intervals, but a negative GWP at 500 years due to different atmospheric residence times of the gases involved

10.5 POTENTIAL CLIMATIC EFFECTS ON PEATLAND

FORM AND VEGETATION

Gignac et al.32 established response surfaces for a number of the indicator and keystone species of western Canadian peatlands for climate (using an aridity index),

pH, and height above the water surface table Of the 31 indicator species that were examined, all but 8 are climatically limited in western Canada Using these response surfaces, Gignac and Vitt33 developed peatland indicator bryophyte communities and constructed two peatland communities for contemporary climate; one at Athabasca, Alberta and one at Wandering River, Alberta These communities encompassed a range of peatland types from bogs to rich fens Then, by using the Canadian CCC General Circulation model 2 × CO2 scenario that predicted an increase of 4°C for these southern boreal sites, an increase in the growing season of 19 days, and no increase in precipitation, two future climate peatland bryophyte communities were constructed The resulting indicator communities for these two locations show the complete absence of all peatland species at Athabasca and a reduction of cover at Wandering River from 14 species with 77% cover to 5 species with less than 1% cover Essentially, peatland communities would cease to exist at both of these southern boreal locations

Nicholson and Gignac34 and Nicholson et al.35,36 examined the current and future occurrences of fens and bogs in the Mackenzie River Basin They constructed three-dimensional response surfaces for 21 indicator species spanning the rich fen, poor

fen, boreal bog, and peat plateau (ombrotrophic sites with extensive permafrost) gradient Under 2 × CO2 climatic scenarios (using both the GFDL and CCC General Circulation Models), peatland ecosystems of all types were displaced northward approximately 780 km (Figure 10.3) The southern limit of peat-forming ecosystems was predicted to be at about 60° N latitude Bryophyte species are especially sensitive indicators of water level changes, and Nicholson et al.36 utilized these sensitivities

to predict projected changes in depth to the water table relative to the peat surface Predictions of changes ranged from –7 dm in northeastern Alberta, to –5 dm in north central Alberta, decreasing to a –3 to –1 dm change north of 60˚ N latitude (Figure 10.4) The use of plant indicators to predict water table position is a site-specific modeled response that has much more ecosystem relevance than predictions made from landscape-scale hydrology Present-day vegetation response for drawdown is clearly evident in fens of the Athabasca area, which is situated north of central Alberta Furthermore, the latitudinal position of the parkland–boreal forest boundary may react to increasing temperature through a parallel northward migration

FIGURE 10.3 Geographical locations of extant peatlands (left) and projected distributions

(right) by peatland type of sites in the Mackenzie River Basin as a result of global warming Climatic data that were used by the model to generate the projected distribution of peatlands were obtained from the Geophysics Fluid Dynamics Laboratory (GFDL) Model for 2 × present

CO2 concentrations (From Nicholson, B.J et al., in Mackenzie Basin Impact Study (MBIS), Final Report, Cohen, S.J., Ed., Environment Canada, Downsview, Ontario, 1997, 295 With permission.)

10.6 PERMAFROST MELTING IN THE BOREAL FOREST

In 1994, Vitt et al.37 described a series of landforms associated with permafrost features (frost mounds) found in boreal peatlands When these frost mounds melt,

they collapse and form melt features termed internal lawns Boreal permafrost melt

is in disequilibrium with present-day climate,38 owing to the insulative features of

peat and of living Sphagnum, as well as the local microclimatic variation due to tree

and shrub cover Over the last millennium, permafrost distribution in the boreal forest has fluctuated in a sensitive zone that is 672,000 km2 in extent across western continental Canada During the Little Ice Age, about 28,800 km2 of permafrost were present.39 With the climate warming over the past 100 to 150 years, 9% (or 2627

km2) of this permafrost has degraded Additionally, 22% (5813 km2) is currently in disequilibrium and actively degrading Only 69% of boreal permafrost exists today

in an equilibrium undegraded state

Collapse of a frost mound is followed by extremely rapid recolonization of the

resulting internal lawn by sedges and species of Sphagnum that form wet carpets

and lawns.37,40,41 Over the subsequent 100 to 200 years, vegetation of the internal

FIGURE 10.4 Projected minimum mean changes in depth of water table relative to peat

surface (dm) for peatlands in the Mackenzie Basin based on climatic data obtained from the Canadian Climate Centre (CCC) Model for 1 × and 2 × present CO 2 concentrations (From Nicholson, B.J et al., in Mackenzie Basin Impact Study (MBIS), Final Report, Cohen, S.J., Ed., Environment Canada, Downsview, Ontario, 1997, 295 With permission.)

lawn gradually increases in height and the system regenerates to the hummocky microrelief of a continental bog For at least the first 100 years, carbon sequestration

in internal lawns is greater than that of both nonpermafrost boreal bogs and perma-frost mounds,29 and the melting of permafrost results in an increase in the storage

of organic matter Turetsky et al.29 reported organic matter accumulation in internal lawns (formed when permafrost melts) are 1.6 times higher than the close-by frost mounds and boreal bogs Organic matter accumulation in boreal western Canada (where at least 90% of the permafrost has melted) has increased by 5% (or 2 × 10–11

g yr–1) when compared to Little Ice Age amounts.12

10.7 GLOBAL CLIMATE CHANGE VS CUMULATIVE

DISTURBANCE

Across the boreal and subarctic regions of the world large amounts of carbon are sequestered in different places in our natural ecosystems Carbon can be sequestered

in lakes and buried in lake sediments where it is effectively permanently removed from the global carbon cycle In Alberta, Campbell et al.42 estimated that about 2.3

Pg of carbon are stored in this long-term sink

Forests and croplands, on the other hand, sequester new carbon that is released and recirculated to the atmosphere within a short-term time range of decades to a few hundred years and have current soil carbon stocks estimated at 3.5 Pg C.11 These two ecosystem types have carbon stored largely in living biomass and in the upper-most relatively shallow soil profile These systems are generally intensively managed with harvest and planting cycles planned and implemented following tight manage-ment schedules

Northern peatlands contain one third of the world’s soil carbon In Alberta, they contain about 13.5 Pg of carbon, while in continental western Canada they store 48

Pg of carbon of which only 0.1 Pg is found in living vascular plant aboveground biomass.12 Of this large carbon stock, 50% was developed in the last 4000 years Vitt et al.12 estimated that in the last 1000 years, the western Canadian carbon store increased by 7.1 Pg or 14.8% Both rates of peat accumulation and peatland initiation apparently are highly sensitive to natural Holocene climatic changes In western Canada, carbon sequestration has been highly sensitive to millennial wet climate cycles.43,44 These periods of increased moisture, rapid organic matter accumulation, and increased peatland initiation in western Canada appear to be related to warm periods in the North Atlantic,45 as well as to global atmospheric CO2 concentrations

in the past.43 Peat accumulation rates at one rich fen in western Alberta varied from means of about 183 g m2 yr–1 during wet periods to a low of 7 g m2 yr–1 during dry periods (with the long-term time-weighted mean of 31.3).43 These data suggest that even minor climatic fluctuations in the past have had strong affects on peatland function and they may alter the rates of peat accumulation considerably Furthermore, northern peatlands appear to be strongly coupled to natural global climatic changes From these data it is apparent that boreal peatlands have been strongly affected

by climate change in the past; however, it is also important to realize that the cumulative effects of disturbance may actually have more of an impact on these