Springer Old Growth Forests - Chapter 21 doc

15 presented an example of an unmanagedold-growth forest where almost all individual large trees grew at high rates, andthe stand accumulated carbon in the above-ground biomass at the ex

Old-Growth Forests: Function, Fate

and Value – a Synthesis

Christian Wirth

21.1 Challenges in Functional Old-Growth Forest Research

The total number of scientific articles on old-growth forests has increased drasticallyover the last 10 years (Chap 2 by Wirth et al.), and yet papers on old-growth forestsmake up only about 1% of all forest- or forestry-related articles listed in the Web ofScience The availability of process information as reviewed in this bookdecreases exponentially with stand age irrespective of the ecosystem functionconsidered (Fig 21.1) One likely reason for the scarcity of information on old-growth forests is their seemingly low economic relevance and consequently limitedresearch funding Another possible reason is the scarcity of old-growth foreststhemselves in the countries where most scientific research is carried out Surely,this may be compensated for by the fact that rarity tends to spark interest Likeanyone else, ecologists are fascinated by tall, majestic forests This is clearlyreflected by the dominance of old-growth studies carried out in the famous temper-ate rainforests of the western United States However, the same features that makeold-growth forests attractive (tall trees, complex structure, organismic diversity,remoteness) pose tremendous challenges to ecosystem research Old trees areusually tall, and access to the canopy requires expensive infrastructure such ascanopy cranes or towers, not to mention the difficulties involved in studying rootsystems Old-growth forests are highly heterogeneous, in both the vertical andhorizontal dimensions Spatial heterogeneity in soil conditions is caused by treefalls (Chap 10 by Bauhus) Thus, soil sampling aimed at a reliable estimate ofelement stocks and fluxes requires a large number of spatial replicates (Chap 11 byGleixner et al.) To complicate matters further, mere soil sampling is not sufficient

to quantify ecosystem processes such as mineralisation or heterotrophic respiration,because decomposition also takes place in aboveground compartments such assnags, dead branches, rotting heartwood in live trees and detritus produced byepiphytes (Zabel and Morrell 1992) Epiphytes are difficult to reach, but maycontribute significantly to net primary production (Clark et al 2001) Micro-meteorological investigations of water, energy and CO2exchange using the eddycovariance method require homogenous vegetation surfaces on level terrain, but

C Wirth et al (eds.), Old ‐Growth Forests, Ecological Studies 207, 465 DOI: 10.1007/978 ‐3‐540‐92706‐8 21, # Springer‐Verlag Berlin Heidelberg 2009

old-growth forests often exhibit structurally irregular canopies and occur often oncomplex sloped terrain unsuitable for agriculture or forestry operation (Chap 7 byKnohl et al.).

According to the classical view, tree species composition and process rates tend

to stabilise with age as forest stands approach the climax state (Clements 1936;Odum 1969), i.e younger stands are expected to change faster and are moredifferent from each other than old stands Under this scenario, the sampling effortshould concentrate on young stands and the data scarcity in old stands would be oflittle concern However, temporal and between-stand variability may indeed besubstantial in old-growth forests At annual time-scales, old-growth stands mayswitch from carbon sinks to sources in response to inter-annual climate variability(Chap 7 by Knohl et al.) At longer time scales, successional species turnover and

Stand age (yrs)

Fig 21.1 Frequency of information on net primary productivity (NPP; top left panel) and net ecosystem exchange (NEE; top right panel) based on the datasets used in Kutsch et al (Chap 4) and Knohl et al (Chap 7), respectively, according to stand age The left lower panel shows the age distribution of inventory plots in the United States forest inventory assessment, and the right lower panel the number of publications indexed in Web of Science referring to either ‘forest’ or

‘forestry’ (left bar) or one of the terms defined by Wirth et al (Chap 2) related to old growth forests (right bar) The different shadings of the bars (except in the lower right panel) indicate, from left to right, the developmental stages: pioneer, transition, early old growth and late old growth; *stand age not known

the legacy of synchronised mortality influence the net exchange of carbon overmany centuries The model analysis and the chronosequence data presented inChap 5 suggest that it may take 400 years or more before ecosystem carbon stockseventually equilibrate, if they ever do so In contrast to Clements’ view, the longerthe time-scale over which a forest ecosystem develops, the higher the likelihoodthat it is affected by stochastic perturbations This is likely to induce a divergence ofsuccessional pathways and associated trajectories of matter pools with stand age(Chapin III et al 2004) In fact, variability may increase with stand age as thebiomass chronosequences forPseutotsuga menziesiiand Tsuga heterophylla illus-trate (see Fig 5.8 in Chap 5 by Wirth and Lichstein) Under this scenario, samplingeffort should increase with successional age, which is in stark contrast with theactual situation Finally, the above-mentioned sequences illustrate another severeproblem in old-growth forest research The quantification of successional time sincestand initiation becomes extremely difficult after the even-aged founder cohort hascompletely turned over This is why, in many studies, e.g Janisch and Harmon(2002), old-growth stands are assigned an arbitrary high age Alternatively, the age

is given as a range (e.g 200 500 years), a minimum value (>200 years) or acategory (‘old-growth’)

Despite data limitations and the many difficulties in carrying out research in growth forests, the reviews and novel analyses presented in this book shed new light

old-on how old-growth forests functiold-on differently from younger and managed forests

21.2 Functional Consequences of Old-Growth Forest Structure: the Spatial View

The structure of old-growth forest is special This is reflected by the fact thatexisting definitions of ‘old-growth forest’ are based largely on structural criteria(Chap 2 by Wirth et al.) Although there might be pronounced differences in foreststructure between biomes some of which are reviewed in this book old-growthforests across the world share a number of common structural features: Old-growthforest canopies are usually tall Single tree death and subsequent gap phase dynam-ics create a higher spatial heterogeneity in the horizontal and vertical dimension ascompared to managed or younger stands This aboveground heterogeneity is partlyreflected in the forest floor and soil properties This poses the question of whether,and how, greater stature and spatial heterogeneity translate into differences infunctioning

21.2.1 Tall Stature

Tall forests occupy a greater ecosystem volume in which to accumulate carbon This iswhy canopy height is a good predictor of biomass carbon stocks (Chap 5 by Wirth andLichstein) On the other hand, tree growth usually follows a sigmoidal function,

implying reduced growth rates in tall trees and, by the same token, lower biomassincrement in tall forests The most prominent (but still controversial) explanationfor size-related growth reduction is provided by the hydraulic limitation hypothesis:

as trees grow taller, increasing gravitational potential and path length lead

to decreased leaf water potential (Chap 4 by Kutsch et al.) To prevent leaf waterpotential from dropping below the wilting point, stomatal conductance is reduced,and thereby also photosynthesis and growth I will summarise the extensive debate

on age- or size-related productivity decline in Sect 21.3 At this point, it isimportant to note that tall stature as a structural feature may induce reducedgrowth rates

Another aspect of forest stature is that tall forests enclose a large volume of airbetween the soil surface and the canopy This favours the development of internalconvection cells that transport ground-level air, which is enriched in CO2from soilrespiration, into the canopy where it can be re-fixed and increase growth rates iftrees are carbon-limited (see Chap 17 by Grace and Meir) Without this convection,respiratory CO2might be lost with lateral air flow The belowground analogue oftall stature is a deep rooting depth In addition to their anchoring function, deeproots provide access to ground water During summer, when the top soil has driedout, old-growth forests may therefore maintain a higher stomatal conductance andphotosynthesis than shallow-rooted young forests (Chap 7, Knohl et al.) Deeproots thus help overcome the hydraulic limitation of photosynthesis in tall trees byincreasing the water supply In addition, deep roots act as channels for hydraulic lift

of ground water (Caldwell et al 1998) This passive redistribution provides surpluswater to the ground vegetation and thus contributes to the maintenance of under-storey productivity under dry conditions (Dawson 1993)

21.2.2 The Imprint of Aboveground Structural Complexity

It may appear that canopy gaps in old-growth forests reduce the overall light use of thevegetation, thereby lowering gross primary productivity (GPP) However, the aver-age light availability at the forest floor does not seem to differ between old-growthand secondary growth forests (Chap 6 by Messier et al.), suggesting that light is not

‘wasted’ in old-growth forests, but simply harvested across a wider height gradient:gaps quickly fill from below with understorey herbs and tree regeneration, or arefilled laterally by the expanding crowns of surrounding trees This represents a form

of resilience of GPP against canopy mortality and gap formation This is supported

by Knohl et al (Chap 7), who compared an old-growth multi-layered beech standwith an otherwise similar mono-layered managed stand and detected no differences

in GPP Given the above evidence, it is not surprising that the mean light experienced

by the understorey vegetation in old-growth forests is low in temperate and tropicalforests (Table 6.1 in Chap 6 by Messier et al.) Under these conditions, the above-mentioned ‘gap filling’ is enhanced by the presence of sapling banks formed by treeswith a high degree of shade-tolerance (Table 6.2 in Chap 6 by Messier et al.;

Chap 17 by Grace and Meir, see also Sect 21.3.4) The resilience of GPP thus alsohinges on diversity of plant function.

Another line of argument suggests that GPP should be even higher in old-growthforests (Chap 7 by Knohl et al.): old-growth canopies possess a ‘rough’ topography andexhibit a high contact surface with the atmosphere (up to 12 times higher than theground surface) Radiation therefore penetrates deeper into the canopy and istrapped more efficiently because radiation back-scattered from lower layers isless likely to escape the canopy (Weiss et al 2000) A lower surface reflectance(which implies higher radiation absorption) over old-growth forests has indeedbeen verified by remote-sensing (Ogunjemiyo et al 2005) To maintain non-lethalleaf temperatures in the face of higher net radiation, energy is dissipated byelevating the leaf transpiration rate This requires a higher stomatal conductance,and thus indirectly induces an increase in GPP This link between canopy surfaceroughness and higher transpiration rates represents yet another process that partlyoffsets a size-related hydraulic limitation of photosynthesis in tall trees (Chap 4 byKutsch et al.)

21.2.3 The Imprint of Belowground Structural Complexity

Single tree mortality may create not only canopy gaps but also root gaps If so, thiscould lead to a leakier system with less efficient uptake of water and nutrients inold-growth forests with frequent gaps Unfortunately, literature on this topic isscarce and results are far from conclusive (Chap 10 by Bauhus) On the one hand,there is some evidence that belowground gaps are less abrupt and close faster thancanopy gaps, mostly because the root systems of adjacent trees overlap more thantheir crowns In fact, tree mortality does not seem to punch a hole in the root layer,but merely reduces fine-root biomass by about 20 40% On the other hand, highnutrient losses via leaching have been reported even under small gaps (Chap 10 byBauhus), and a reduction in stand basal area of only about 10% can increase thewater yield of a forest catchment (Chap 7 by Knohl et al.) This paradox (littlestructural change, but large increases in ‘leakiness’) may be explained by the factthat, although the uptake capacity is barely affected, the supply of both leachatesand water is strongly increased as a consequence of higher mineralisation rates(warmer, wetter soil) and reduced interception losses and transpiration In any case,this leakiness calls into question the uptake efficiency of mycorrhizal networks.Such leakiness is likely reversed as the gap is re-colonised by herbs, shrubs and treeregeneration, but whether fine-root density is higher in such vegetated patches than

in the surrounding matrix of old trees is unclear, as age-trends of fine-root densityare idiosyncratic

The mere existence of large trees induces a patchiness in the forest floorstructure Trees grow on top of their own woody litter (i.e the heartwood) inorder to lift their leaves above those of their competitors This growth strategyconcentrates organic matter into a comparatively small volume (i.e the stem), inwhich it may be locked up for many centuries After tree death and subsequent tree

fall, the carbon and nutrients contained in the stem are deposited in an area that issignificantly smaller than the area from which these elements were initially gath-ered For example, a deciduous broad-leaved tree with a breast-height diameter of

80 cm occupies a horizontal growing space of about 130 m2, while the projectionarea of its downed stem is only about 17 m2, i.e 7.5 times smaller This ‘concen-tration effect’ increases linearly with tree diameter and is thus most pronounced inold-growth forests and in young forests that are rich in legacy deadwood after stand-replacing disturbances A special situation arises when trees are uprooted following

a windstorm (Chap 10 by Bauhus) Root plates are tipped up and, with progressivedecay, the elevated stem bases and any attached roots and soil sink down to formmounds, whereas the exposed mineral soil remains as pits This process disruptsany continuous layering of the soil and accumulates carbon and nutrients inmounds Up to 33% of the forest floor might by covered by pits and mounds Thequestion arises whether and how this heterogeneity affects the net ecosystembalances of carbon and nutrients Given the same total amount of organic material,does a forest floor with a patchy distribution lose more or less carbon and nutrientsper unit area than one with homogeneous layering? The chapters in this volume

do not directly answer this question, but they do allow us to formulate hypotheses:(1) the high concentration of easily degradable carbohydrates in and under woodydetritus may help to overcome an energy-limitation of decomposition, therebyinducing a ‘priming’ effect (Chap 12 by Reichstein et al.) This effect will bemost pronounced around coarse roots in deeper soil layers where energy-limitation

is most severe As a result, the heterotrophic loss per unit organic matter would behigher in patches with high loads of carbon, which would translate into higherlosses in ecosystems with a clumped distribution of fresh organic matter (mounds,logs) (2) We further hypothesise important interactions with site conditions Snags,logs and mounds represent elevated structures that tend to be drier than thesurrounding forest floor (Chap 10 by Bauhus) For a patchy distribution in a dryclimate, this could mean that a large amount of organic matter is locked up in placestoo dry for microbial activity In a wet climate the opposite is true: elevatedmicrosites might be the only places providing the oxic conditions required fordecomposition (Chaps 8 by Harmon, and 11 by Gleixner et al.)

21.2.4 Habitat Structure

Of all functions, the provision of habitat for plants and animals is the most obviousand by far the best studied in old-growth forests There is a massive literature on thissubject: out of 1,347 original papers in the Web of Science referring to ‘old-growth’1,125 (or 83.5%) were published in the fields of either conservation biology orgeneral ecology (Chap 2 by Wirth et al.) The chapters by Frank et al (Chap 19)and Armesto et al (Chap 16) suggest that the complex horizontal and verticalstructure created by gap phase dynamics provides a diverse array of habitat structures,and thus probably allows more and different species to dwell in old-growth forests

More specifically, because of the high spatial variability of light and temperature,the fine scale of these patterns ensures that moist microhabitats with a low temper-ature amplitude are never far apart from each other This allows typical old-growthspecies with low desiccation tolerance and limited dispersal distances, such aslichens, mosses, snails or newts, to form viable populations Large old trees createstructures that cannot be provided by smaller trees, such as a fissured bark, cavities,small canopy ponds, and branches strong enough to carry high loads of epiphytes.The development of epiphyte communities further diversifies the habitat, as doesthe activity of woodpeckers and other habitat-structuring organisms As reviewed

by Bauhus (Chap 10), the process of tree fall itself forms special microsites forplant growth and tree regeneration Uprooting exposes mineral soil and creates aseedbed for those species that cannot germinate in organic substrates Microsites onelevated root plates have higher light availability and allow shade-intolerant plantspecies to establish The impenetrable tangle of branches where the crown hits theforest floor is usually avoided by ungulate herbivores and thus provides safe sitesfor tree regeneration The dead trees themselves add significantly to the mosaic ofhabitat Snags and logs support a great variety of specialised organisms that depend

on decaying wood as food sources, hideouts and hunting territories, and nesting orrooting substrates (Chap 8 by Harmon)

21.3 Old-Growth Forests in the Context of Succession:

the Temporal View

Old-growth forests are the result but not the end result of primary or secondarysuccession Succession is a process that unfolds over time, and the underlyingtemporal view recognises that old-growth forests have a history Their structureand function is a transient manifestation of various processes that operate ondifferent time-scales but are nevertheless interdependent For example, the decay

of legacy woody detritus after disturbance is completed within several decades(Chap 8 by Harmon), successional tree species replacement may take centuries(Chap 5 by Wirth et al.), and the development of phosphorous limitation mayrequire millennia (Chap 9 by Wardle et al.) The rate of change is highest initially,with later transformations being more subtle Nevertheless as will be arguedbelow the time since stand initiation matters at any successional stage, includingold-growth This dynamic view of old-growth contradicts the equilibrium view,according to which forests reach a self-perpetuating condition without long-termmemory While the equilibrium view is most likely incorrect for any old-growthforest, this view is certainly incorrect for most forests labelled ‘old-growth’ inexisting studies These have a mean age of only 300 years (Chap 2 by Wirth et al.)and are thus strongly influenced by the legacy of earlier developmental stages(Chaps 5 by Wirth and Lichstein, and 6 by Harmon) The notion that old-growthforest functioning can be understood only in the context of successional history iscommon to the sections that follow

21.3.1 Long-Term Trends in Tree and Stand Productivity

The debate about the so-called ‘age-related decline’ in forest net primary tivity (NPP) had its peak in the 1990s and was spearheaded by ecophysiologists.Discussions about age-trends of stand biomass (B) were lead by forest ecologistsand started much earlier Although these discussion have been largely separate inthe literature, simple differential equation models from classical ecosystem theoryshow that productivity and biomass dynamics are tightly linked (Olson 1963; Odum1969; Shugart 1984) For example:

produc-dB

dt ¼ NPP  mB ! BðtÞ ¼NPPm ð1 exp mtÞ 21:1wheret denotes time, and NPP and m (the loss rate per unit biomass) are assumedconstant The equation on the right illustrates that, under the assumption of constantproductivity and loss rate, biomass equilibrates atNPP/m Should either of the twoterms change over time after equilibrium has been reached, as would be the case in

‘age-related decline’ for NPP, this would cause biomass to change over time aswell In short, given a constant loss rate, an ‘age-related decline’ in productivitywould induce a biomass decline

According to Binkley et al (2002), age-related NPP declines are one of the mostuniversal patterns in the growth of forests Do such declines actually exist outsideplantations, and, if so, do they have anything at all to do with age? Growth rates ofindividual trees usually decline after some peak, but it may take a long time beforethis peak is reached There are numerous examples of tree-ring sequences that showconstant or even increasing ring widths over many centuries, indicating increasingvolume growth rates with age (Chaps 3 by Schweingruber and Wirth, and 15 bySchulze et al.) Moreover, old trees remain responsive to sudden improvements ingrowing conditions (e.g Fig 7.1 in Chap 7 by Knohl et al.; Wirth et al 2002; Mund

et al 2002) Schulze et al (Chap 15) presented an example of an unmanagedold-growth forest where almost all individual large trees grew at high rates, andthe stand accumulated carbon in the above-ground biomass at the exceptional rate

of 232 g C m–2year–1 However, we also know that trees do not grow forever.Hypotheses on the age and size constraints on tree productivity are discussed byKutsch et al (Chap 4) The original hypothesis, which stated that an increasingrespiratory burden suppresses the growth rate of large trees, was not supported byexperimental data From the early 1990s on, the hydraulic limitation hypothesisbecame popular According to this hypothesis, stomata close because hydraulicconductivity decreases with tree height (not age!) Since then, two lines of argumenthave challenged the hydraulic limitation hypothesis (Chap 4 by Kutsch et al.),namely: (1) that trees can adjust their hydraulic architecture and fine-root biomass

to compensate for size-related reductions in hydraulic conductivity, and (2) thatreduction in growth in old trees might not be driven by supply (i.e by changes in

carbon assimilation rates) but by demand (i.e the ability to create carbon sinksthrough growth).

To what extent these individual-scale responses translate into a stand-leveldecline in NPP is still subject to debate The 13 chronosequences presented in theseminal review by Ryan et al (1997) clearly exhibited an age-related decline at thestand-level, but these even-aged, mostly managed coniferous monocultures are by

no means representative of the world’s forests The reviews and new data presented

in this book indicate that age-related decline in the productivity of natural stands isnot as ‘universal’ as previously thought At the time-scale of years to centuries(much shorter than the time-scale of ecosystem retrogression; see Chap 9 byWardle), we identified several processes that work against an age-related decline

in NPP These include a stand age-related increase in rooting depth exploring newbelowground resources (Chaps 4 by Kutsch et al., and 7 by Knohl et al.); increasedcanopy roughness in old forests, leading to more efficient light use and higher rates

of transpiration and photosynthesis (Chap 7 by Knohl et al.); and succession fromlight-demanding to shade-tolerant species, resulting in increased leaf area index and

a change in leaf traits suggesting high net carbon gain per unit leaf investment(Chap 4 by Kutsch et al.) Finally, if an age-related decline in productivity weresuch a universal feature, then, according to the equation above, biomass declinesshould also be common However, various chapters conclude that late-successionalbiomass declines are the exception rather than the rule [Chaps 5 (Wirth andLichstein), 14 (Lichstein et al.) and 15 (Schulz et al.) see also below]

The data presented in this book also suggest that physiological processes related

to either size or age are probably less important than structural changes Thereanalysis of the Luyssaert dataset (Chap 4 by Kutsch et al.) revealed only a subtlenegative stand age-effect on NPP in coniferous forests and none in deciduousforests Instead, leaf area index was an important predictor of both aboveground-and total-NPP This suggests that structural changes reducing leaf display, such asgap formation, lateral crown abrasion or increased leaf clumping in bigger crowns,are more likely candidates for driving age-related decline in NPP (if it occurs).Schulze et al (Chap 15) apply the self-thinning rule to identify a minimum standdensity below which the productivity cannot be maintained They argue thatproductivity and biomass might decline with stand age only because large treesare more susceptible to disturbances than small ones The above conclusions are inline with more recent assessments by Smith and Long (2001) and Binkley et al.(2002), who interpret a successional decline in productivity as an emergent stand-level property Taken together, the established term ‘age-related decline’ is mis-leading There are changes in productivity with succession (not necessarily withtree or stand age), some of them with a negative sign The possible causes of theseproductivity declines include age- or size-related limitations of tree physiology,changes in canopy structure, trait-shifts due to species turnover, and interactionsbetween succession and site development The relative importance of factors thatincrease or decrease productivity as succession proceeds is likely to vary betweenbiomes and forest types

21.3.2 Are Old-Growth Forests Carbon Neutral?

This question can generally be approached from two directions (Chap 12 byReichstein et al.) One can monitor carbon stocks over time in different ecosystemcompartments and infer the net ecosystem carbon balance (NECB) (Chapin et al.2006); or one can directly measure the net exchange fluxes, the integral of whichshould, in principle, be equal to the net stock changes if temporal and spatial scalesare similar (Baldocchi 2003) The first, ‘bottom-up’, approach is generally based onrepeated inventories or chronosequences of biomass, woody detritus and soilcarbon, while the second, ‘top-down’, approach uses the micro-meteorologicaleddy-covariance technique Aggregated estimates from these two approaches fordifferent developmental stages are presented in Fig 21.2 It should be noted that theanalyses presented in this book differ from an earlier review by Pregitzer andEuskirchen (2004), who considered dynamics only up to a stand age of 200 yearsfor most pools and fluxes Their study thus does not allow inferences on processesduring the old-growth stage

Knohl et al (Chap 7) and Luyssaert et al (2008) reviewed the evidence for borealand temperate forests from ‘top-down’ eddy covariance studies, and concluded thatmost old-growth forests (eight out of nine stands older than 200 years) remain carbonsinks Not only the sign but also the magnitude was surprising (a mean of 130 42and 257  246 g C m–2

year–1 for boreal and temperate forests, respectively),suggesting that these stands were far from carbon equilibrium For mature humidtropical forests, only seven eddy covariance sites (with unknown ages) are available(Luyssaert et al 2007) Considering upland sites only (six of the seven), the meannet C exchange was 231  249 g C m–2 year–1 This suggests that tropical andtemperate old-growth forests function similarly as carbon sinks (see also Chap 17

by Grace and Meir)

Several chapters in this volume also present bottom-up estimates for carbonstock changes in biomass, woody detritus, and soil These numbers representcomponent fluxes of the net ecosystem carbon balance Several lines of evidence

Fig 21.2a,b Synthesis of carbon flux estimates based on different approaches presented in the book a Inventory and model based estimates: AGB chrono, CWD chrono, and SOC chrono represent changes in carbon stocks in the aboveground biomass, the woody detritus, and soil, respectively, and were calculated from the chronosequence studies presented in Wirth and Lichstein (Chap 5) and Gleixner et al (Chap 11) AGB FIA mean estimates of change in aboveground biomass based on the Forest Inventory Assessment of the United States (Lichstein

et al Chap 14); AGB model and CWD model estimates from the trait based carbon succession model in Wirth and Lichstein (Chap 5); asterisks sum of the stock changes in the biomass (mean

of chronosequence and FIA estimates), woody detritus, and soil Two different sums are shown, one excluding the high repeated sampling estimates (large filled asterisks) and one including them (small open asterisks), in which case the median of the chronosequence and repeated sampling estimates was used No distinction is made between biomes, but there is a clear dominance of data from the temperate and boreal zone b Comparison of inventory based (bottom up) estimates of the net ecosystem carbon exchange (asterisk) and the estimates from eddy covariance studies in different biomes (Knohl et al Chap 7)

Transition (101 − 200 yrs)

Early OG (201 − 400 yrs)

Late OG (> 400 yrs)

Pioneer (0 − 100 yrs)

Transition (101 − 200 yrs)

Early OG (201 − 400 yrs)

Late OG (> 400 yrs)

emanating from an analysis of forest inventories (Chap 14 by Lichstein et al.), aliterature evaluation (Chap 15 by Schulze et al.), a review of long-term forestchronosequences and a model-data integration based on plant traits and successiondescriptions (Chap 5 by Wirth and Lichstein) suggest that late-successional bio-mass declines are the exception rather than the rule The significance of this findingfor the ecological theory of secondary forest succession is discussed in Chaps 5(Wirth and Lichstein) and 14 (Lichstein et al.) Figure 21.2 shows the mean rates ofbiomass carbon change [as estimated in Chaps 5 (Wirth and Lichstein), 14(Lichstein et al.) and 15 (Schulz et al.)] during progressive stages of succession.Forests in the early old-growth stage (201 400 years) accumulate abovegroundbiomass carbon at mean rates of between 10 and 30 g C m–2year–1(Fig 21.2) Thefour chronosequences containing stands older than 400 years suggest a continued(albeit low) accumulation of about 10 g C m–2year–1.

The temporal course of woody detritus (standing and downed) stocks duringsecondary succession typically shows a ‘reverse-J’ or ‘U’-shape resulting from aninitial decay of legacy woody detritus and the build-up ofde-novo woody detritus asthe stand reaches the old-growth stage (Chap 8 by Harmon) The long-term (>200years) woody detritus chronosequences presented in Wirth and Lichstein (Chap 5)reveal a high variability of stock changes ofde-novo woody detritus that can beexplained partly by climate and the peculiarities of species-specific decay rates.Along the four stages, the mean rates of woody detritus stock-change increasedfrom 46 69 (n = 14; pioneer), to 6  18 (n = 17; transition), to 12  24 (n = 14;early old-growth), to 18 29 g C m–2

year–1(n = 4; late old-growth) (Fig 21.2).The chronosequence data show that, during the early old-growth stage, the accu-mulation rate of woody detritus was slightly lower than the biomass accumulationrate (12 vs 18 g C m–2year–1, respectively), while this trend reverses in the late old-growth phase (18 vs 10 g C m–2year–1) These data indicate that rates of woodydetritus and biomass accumulation are of similar magnitude in old-growth forests,and that the relative contribution of woody detritus increases with forest age.Soil pools are less well defined than biomass and woody detritus pools because

of varying depth and definition of horizons Gleixner et al (Chap 11) presenteddifferent estimates of changes in soil organic carbon stocks This included esti-mates from chronosequences containing stands older than 150 years but also fromrepeated inventories and carbon balance approaches Averaging across biomes anddifferent methods, the chronosequence-based accumulation rates during the old-growth phases were in the order of a few grams (1.5 2.7 and 2.8  2.1 g C m– 2

year–1in the early and late old-growth phases, respectively) In stark contrast, thethree old-growth studies using repeated sampling report rates that are higher by afactor of 50, namely, 61, 76, and 165 g C m–2year–1(cf Table 11.4 in Chap 11 byGleixner et al see discussion below)

If we add the component fluxes of carbon accumulation in biomass, woodydetritus, and soil for each stage (Fig 21.2, asterisks), we find that our bottom-upestimates of NECB remain remarkably constant over succession Old-growthforests appear to have the same sink strength as early-successional stands: roughly

40 58 g C m–2year–1 This is true regardless of whether we include the high soil

carbon sequestration rates obtained by repeated sampling (Fig 21.2, small isks) or exclude them (large asterisks) The higher biomass accumulation ratesduring the first 100 years are compensated by the decomposition of legacy dead-wood During the two old-growth stages, a continued accumulation of biomass and

aster-an increasing accumulation rate ofde-novo woody detritus maintain a net carbonsink The top-down eddy covariance estimates show a similar age pattern ofsustained sink activity but at far higher levels (Fig 21.2b)

What is the nature of this discrepancy? The eddy covariance method is known tooverestimate carbon fluxes as it misses lateral advection of carbon dioxide undernon-turbulent conditions (Chap 17 by Grace and Meir) However, the bias intro-duced by this tends to be lower than 30%, and therefore explains only part of thediscrepancy (Chap 7 by Knohl et al.) On the other hand, our bottom-up estimateignores some pools We did not include the accumulation of carbon in coarse roots,and, for the most part, soil carbon estimates did not include the forest floor organiclayer or deep soil horizons (see below) The chronosequence approach is also blind

to the continuous export of dissolved organic carbon in groundwater There is also amore generic difference: the eddy covariance method quantifies instantaneousfluxes and thus unlike chronosequence approach captures high-frequencytemporal variability For example, forest ecosystems that are influenced by recur-ring disturbances (fire, herbivory) are in a permanent state of recovery Suchecosystems follow a steeper carbon trajectory than suggested by chronosequencefits, which cut through the characteristic saw-tooth pattern of carbon stock changescreated by repeated carbon losses and subsequent recovery of pools (Wirth et al.2002) And, finally, most stands might be forced onto a transient steeper trajectorybecause of ubiquitous carbon dioxide and nitrogen fertilisation via the atmosphere(Schimel 1995; Mund et al.2002; Vetter et al 2005) This is difficult to detect usingthe chronosequence approach since carbon stocks that are compared along the timeaxis are the result of century-long ecosystem history, most of which was unaffected

by contemporary atmospheric changes The inability of chronosequences to capturetransient dynamics might also partly explain the discrepancies in soil carbonstorage between the low chronosequence estimates, and the high rates suggested

by repeated soil sampling Irrespective of the nature and extent of the discrepancy,both estimates independently suggest that old-growth forest maintain their capacity

to sequester carbon, i.e they are not carbon neutral The various bottom-up methodssuggest that the continued carbon sink is almost equally distributed between thebiomass and the woody detritus The actual and potential contribution of the soilpool is still unclear (cf second paragraph in Sect 21.5.2)

21.3.3 Nutrient Dynamics

Secondary forest succession starts with an exceptionally high availability of ents (Peet 1992) This is because stand-destroying disturbances result in a largeinput of necromass to the forest floor that is then rapidly mineralised In the case of