Springer Old Growth Forests - Chapter 21 doc

Chapter 21 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 increas ed drastically over the last 10 years (Chap. 2 by Wirth et al.), and yet papers on old-growth forests make up only about 1% of all forest- or forestry-related articles listed in the Web of Science. The availability of process information as reviewed in this book decreases exponentially with stand age irrespective of the ecosystem function considered (Fig. 21.1). One likely reason for the scarcity of information on old- growth forests is their seemingly low economic relevance and consequently limited research funding. Another possible reason is the scarcity of old-growth fore sts themselves 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. Like anyone else, ecologists are fascinated by tall, majestic forests. This is clearly reflected 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 make old-growth forests attractive (tall trees, complex structure, organismic diversity, remoteness) pose tremendous challenges to ecosyste m research. Old trees are usually tall, and access to the canopy requires expensive infrastructure such as canopy cranes or towers, not to mention the difficulties involved in studying root systems. Old-growth forests are highly heterogeneous, in both the vertical and horizontal dimensions. Spatial heterogeneity in soil conditions is caused by tree falls (Chap. 10 by Bauhus). Thus, soil sampling aimed at a reliable estimate of element stocks and fluxes requires a large number of spatial replicates (Chap. 11 by Gleixner 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 as snags, dead branches, rotting heartwood in live trees and detritus produced by epiphytes (Zabel and Morrell 1992). Epiphytes are difficult to reach, but may contribute significantly to net primary production (Clark et al. 2001). Micro- meteorological investigations of water, energy and CO 2 exchange using the eddy covariance 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 on complex sloped terrain unsuitable for agriculture or forestry operation (Chap. 7 by Knohl 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 more different from each other than old stands. Under this scenario, the sampling effort should concentrate on young stands and the data scarcity in old stands would be of little concern. However, temporal and between-stand variability may indeed be substantial in old-growth forests. At annual time-scales, old-growth stands may switch 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 0 200 400 600 Stand age (yrs) 0 200 400 600 Stand age (yrs) 0 200 400 600 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 466 C. Wirth the legacy of synchronised mortality influence the net exchange of carbon over many centuries. The model analysis and the chronosequence data presented in Chap. 5 suggest that it may take 400 years or more before ecosystem carbon stocks eventually equilibrate, if they ever do so. In contrast to Clements’ view, the longer the time-scale over which a forest ecosystem develops, the higher the likelihood that it is affected by stochastic perturbations. This is likely to induce a divergence of successional pathways and associated trajectories of matter pools with stand age (Chapin III et al. 2004). In fact, variability may increase with stand age as the biomass chronosequences for Pseutotsuga menziesiiand Tsug a heterophylla illus- trate (see Fig. 5.8 in Chap. 5 by Wirth and Lichstein). Under this scenario, sampling effort should increase with successional age, which is in stark contrast with the actual situation. Finally, the above-mentioned sequences illustrate another severe problem in old-growth forest research. The quantific ation of successional time since stand initiation becomes extremely difficult after the even-aged founder cohort has completely 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 a category (‘old-growth’). Despite data limitations and the many difficulties in carrying out research in old- growth forests, the reviews and novel analyses presented in this book shed new light on how old-growth forests function 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 that existing definitions of ‘old-growth forest’ are based largely on structural criteria (Chap. 2 by Wirth et al.). Although there might be pronounced differences in forest structure between biomes some of which are reviewed in this book old-growth forests across the world share a number of common structural features: Old-growth forest canopies are usually tall. Single tree death and subsequent gap phase dynam- ics create a higher spatial heterogeneity in the horizontal and vertical dimension as compared to managed or younger stands. This aboveground heterogeneity is partly reflected in the forest floor and soil properties. This poses the question of whether, and how, greater stature and spatial heterogeneity translate into differences in functioning. 21.2.1 Tall Stature Tall forests occupy a greater ecosystem volume in which to accumulate carbon. This is why canopy height is a good predictor of biomass carbon stocks (Chap. 5 by Wirth and Lichstein). On the other hand, tree growth usually follows a sigmoidal function, 21 Old Growth Forests: Function, Fate and Value a Synthesis 467 implying reduced growth rates in tall trees and, by the same token, lower biomass increment in tall forests. The most prominent (but still controversial) explanation for 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 water potential 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 productivi ty decline in Sect. 21.3. At this point, it is important to note that tall stature as a structural feature may induce reduced growth rates. Another aspect of forest stature is that tall forests enclose a large volume of air between the soil surface and the canopy. This favours the development of internal convection cells that transport ground-level air, which is enriched in CO 2 from soil respiration, into the canopy where it can be re-fixed and increase growth rates if trees are carbon-limited (see Chap. 17 by Grace and Meir). Without this convection, respiratory CO 2 might be lost with lateral air flow. The belowground analogue of tall stature is a deep rooting depth. In addition to their anchoring function, deep roots provide access to ground water. During summer, when the top soil has dried out, old-growth forests may therefore maintain a higher stomatal conductance and photosynthesis than shallow-rooted young forests (Chap. 7, Knohl et al.). Deep roots thus help overcome the hydraulic limitation of photosynthesis in tall trees by increasing the water supply. In addition, deep roots act as channels for hydraulic lift of ground water (Caldwell et al. 1998). This passive redistribution provides surplus water 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 the vegetation, thereby lowering gross primary productivity (GPP). However, the aver- age light availability at the forest floor does not seem to differ between old-growth and 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 are filled 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-growt h multi-layered beech stand with an othe rwise 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 tropical forests (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 trees with a high degree of shade-tolerance (Table 6.2 in Chap. 6 by Messier et al.; 468 C. Wirth Chap. 17 by Grace and Meir, see also Sect. 21.3.4). The resilience of GPP thus also hinges on diversity of plant function. Another line of argument suggests that GPP should be even higher in old-growth forests (Chap. 7by Knohl et al.): old-growth canopies possess a ‘rough’ topography and exhibit a high contact surface with the atmosphere (up to 12 times higher than the ground surface). Radiation therefore penetrates deeper into the canopy and is trapped more efficiently because radiation back-scattered from lower layers is less likely to escape the canopy (Weiss et al. 2000). A lower surface reflectance (which implies higher radiation absorption) over old-growth forests has inde ed been verified by remote-sensing (Ogunjemiyo et al. 2005). To maintai n non-lethal leaf temperatures in the face of higher net radiation, energy is dissipated by elevating the leaf transpiration rate. This requires a higher stomatal conductance, and thus indirectly induces an increase in GPP. This link between canopy surface roughness and higher transpiration rates represents yet another process that partly offsets a size-related hydraulic limitation of photosynthesis in tall trees (Chap. 4 by Kutsch 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, this could lead to a leakier system with less efficient uptake of water and nutrients in old-growth forests with frequent gaps. Unfortunately, literature on this topic is scarce and results are far from conclusive (Chap. 10 by Bauhus). On the one hand, there is some evidence that belowgr ound gaps are less abrupt and close faster than canopy gaps, mostly because the root systems of adjacent trees overlap more than their crowns. In fact, tree mortality does not seem to punch a hole in the root layer, but merely reduc es fine-root biomass by about 20 40%. On the other hand, high nutrient losses via leaching have been reported even under small gaps (Chap. 10 by Bauhus), and a reduction in stand basal area of only about 10% can increase the water yield of a forest catchment (Chap. 7 by Knohl et al.). This paradox (little structural change , but large increases in ‘leakiness’) may be explained by the fact that, although the uptake capacity is barely affected, the supply of both leachates and 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 tree regeneration, 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 density are idiosyncratic. The mere existence of large trees induces a patchiness in the forest floor structure. Trees grow on top of their own woody litter (i.e. the heartwood) in order to lift their leaves above those of their competitors. This growth strategy concentrates organic matter into a comparatively small volume (i.e. the stem), in which it may be locked up for many centuries. After tree death and subsequent tree 21 Old Growth Forests: Function, Fate and Value a Synthesis 469 fall, the carbon and nutrients contained in the stem are deposited in an area that is significantly 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 m 2 , while the projection area of its downed stem is only about 17 m 2 , i.e. 7.5 times smaller. This ‘concen- tration effect’ increases linearly with tree diameter and is thus most pronounced in old-growth forests and in young forests that are rich in legacy deadwood after stand- replacing disturbances. A special situation arises whe n trees are uprooted following a windstorm (Chap. 10 by Bauhus). Root plates are tipped up and, with progressive decay, the elevated stem bases and any attached roots and soil sink down to form mounds, whereas the exposed mineral soil remains as pits. This process disrupts any continuous layering of the soil and accumulates carbon and nutrients in mounds. Up to 33% of the forest floor might by covered by pits and mounds. The question arises whether and how this heterogeneity affects the net ecosystem balances 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 nutrients per 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 woody detritus may help to overcome an energy-limitation of decomposition, thereby inducing a ‘priming’ effect (Chap. 12 by Reichstein et al.). This effect will be most 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 be higher in patches with high loads of carbon, which would translate into higher losses in ecosystems with a clumped distribution of fresh organic matter (mounds, logs). (2) We further hypothesise import ant interactions with site conditions. Snags, logs and mounds represent elevated structures that tend to be drier than the surrounding forest floor (Chap. 10 by Bauhus). For a patchy distribution in a dry climate, this could mean that a large amount of organic matter is locked up in places too dry for microbial activity. In a wet climate the opposite is true: elevated microsites might be the only places providing the oxic conditions required for decomposition (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 obvious and by far the best studied in old-growth forests. There is a massive literature on this subject: 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 or general ecology (Chap. 2 by Wirth et al.). The chapters by Frank et al. (Chap. 19) and Armesto et al. (C hap. 16) suggest that the complex horizontal and vertical structure 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. 470 C. Wirth 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-growth species with low desiccation tolerance and limited dispersal distances, such as lichens, mosses, snails or newts, to form viable populations. Large old trees create structures 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 does the activity of woodpeckers and other habitat-structuring organisms. As reviewed by Bauhus (Chap. 10), the process of tree fall itself forms special microsites for plant growth and tree regeneration. Uprooting exposes mineral soil and creates a seedbed for those species that cannot germinate in organic substrates. Microsites on elevated root plates have higher light availability and allow shade-intolerant plant species to establish. The impenetrable tangle of branches where the crown hits the forest floor is usually avoided by ungulate herbivores and thus provides safe sites for tree regeneration. The dead trees themselves add significantly to the mosaic of habitat. Snags and logs support a great variety of specialised organisms that depend on decaying wood as food sources, hideouts and hunting territories, and nesting or rooting 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 secondary succession. Succession is a process that unfolds over time, and the underlying temporal view recognises that old-growth forests have a history. Their structure and function is a transient manifestation of various processes that operate on different 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 may require millennia (Chap. 9 by Wardle et al.). The rate of change is highest initially, with later transformations being more subtle. Nevertheless as will be argued below the time since stand initiation matters at any successional stage, including old-growth. This dynamic view of old-growth contradicts the equilibrium view, according to which forests reach a self-perpetuating condition without long-term memory. While the equilibrium view is most likely incorrect for any old-growth forest, this view is certainly incorrect for most forests labelled ‘old-growth’ in existing 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-growth forest functioning can be understood only in the context of successional history is common to the sections that follow. 21 Old Growth Forests: Function, Fate and Value a Synthesis 471 21.3.1 Long-Term Trends in Tree and Stand Productivity The debate about the so-c alled ‘age-related decline’ in forest net primary produc- 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 ecologists and started much earlier. Although these discussion have been largely separate in the literature, simple differential equation models from classical ecosystem theory show that productivity and biomass dynamics are tightly linked (Olson 1963; Odum 1969; Shugart 1984). For example: dB dt ¼ NPP À mB ! BðtÞ¼ NPP m ð1 À exp mt Þ 21:1 where t denotes time, and NPP and m (the loss rate per unit biomass) are assumed constant. The equation on the right illus trates that, under the assumption of constant productivity and loss rate, biomass equilibrates at NPP/m. Should either of the two terms 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 as well. In short, given a constant loss rate, an ‘age-related decline’ in productivity would induce a biomass decline. According to Binkley et al. (2002), age-related NPP declines are one of the most universal patterns in the growth of forests. Do such declines actually exist outs ide plantations, and, if so, do they have anything at all to do with age? Growth rates of individual trees usually decline after some peak, but it may take a long time before this peak is reached. There are numerous examples of tree-ring sequences that show constant or even increasing ring widths over many centuries, indicating increasing volume growth rates with age (Chaps. 3 by Schweingruber and Wirth, and 15 by Schulze et al.). Moreover, old trees remain responsive to sudden improvements in growing 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 unmanaged old-growth forest where almost all individual large trees grew at high rates, and the stand accumulated carbon in the above-ground biomass at the exceptional rate of 232 g C m –2 year –1 . However, we also know that trees do not grow forever. Hypotheses on the age and size constraints on tree productivity are discussed by Kutsch et al. (Chap. 4). The original hypothesis, which stated that an increasing respiratory burden suppresses the growth rate of large trees, was not supported by experimental data. From the early 1990s on, the hydraulic limitation hypothesis became popular. According to this hypothesis, stomata close because hydraulic conductivity decreases with tree height (not age!). Since then, two lines of argument have 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) that reduction in growth in old trees might not be driven by supply (i.e. by changes in 472 C. Wirth carbon assimilation rates) but by demand (i.e. the ability to create carbon sinks through growth). To what extent these individual-scale responses translate into a stand-level decline in NPP is still subject to debate. The 13 chronosequences presented in the seminal review by Ryan et al. (1997) clearly exhibited an age-related decline at the stand-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 is not as ‘universal’ as previously thoug ht. At the time-scale of years to centuries (much shorter than the time-scale of ecosystem retrogression; see Chap. 9 by Wardle), we identified several processes that work against an age-related decline in NPP. These include a stand age-related increase in rooting depth exploring new belowground reso urces (Chaps. 4 by Kutsch et al., and 7 by Knohl et al.); increased canopy 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 from light-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 were such a universal feature, then, according to the equation above, biomass declines should also be common. However, various chapters conclude that late-successional biomass declines are the exception rather than the rule [Chaps. 5 (Wirth and Lichstein), 14 (Lichstein et al.) and 15 (Schul z 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. The reanalysis of the Luyssaert dataset (Chap. 4 by Kutsch et al.) revealed only a subtle negative stand age-effect on NPP in coniferous forests and none in deciduous forests. Instead, leaf area index was an important predictor of both aboveground- and total-NPP. This suggests that structural changes reducing leaf display, such as gap 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 stand density below which the productivity cannot be maintained. They argue that productivity and biomass might decline with stand age only because large trees are more susceptible to disturbances than small ones. The above conclusions are in line 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 with tree or stand age), some of them with a negative sign. The possible causes of these productivity declines include age- or size-related limitations of tree physiology, changes in canopy structure, trait-shifts due to species turnover, and interactions between succession and site development. The relative importance of factors that increase or decrease productivity as succession proceeds is likel y to vary between biomes and forest types. 21 Old Growth Forests: Function, Fate and Value a Synthesis 473 21.3.2 Are Old-Growth Forests Carbon Neutral? This question can generally be approached from two directions (Chap. 12 by Reichstein et al.). One can monitor carbon stocks over time in different ecosystem compartments and infer the net ecosystem carbon balance (NECB) (Chapin et al. 2006); or one can directly measure the net exchange fluxes, the integral of which should, in principle, be equal to the net stock changes if temporal and spatial scales are similar (Baldocchi 2003). The first, ‘bottom-up’, approach is generally based on repeated inventories or chronosequences of biomass, woody detritus and soil carbon, while the second, ‘top-down’, approach uses the micro-meteorological eddy-covariance technique. Aggregated estimates from these two approaches for different developmental stages are presented in Fig. 21.2. It should be noted that the analyses presented in this book differ from an earlier review by Pregitzer and Euskirchen (2004), who considered dynamics only up to a stand age of 200 years for most pools and fluxes. Their study thus does not allow inferences on processes during the old-growth stage. Knohl et al. (Chap. 7) and Luyssaert et al. (2008) reviewed the evidence for boreal and temperate forests from ‘top-down’ eddy covariance studies, and concluded that most old-growth forests (eight out of nine stands older than 200 years) remain carbon sinks. Not only the sign but also the magnitude was surprising (a mean of 130 Æ42 and 257 Æ 246 g C m –2 year –1 for boreal and temperate forests, respectively), suggesting that these stands were far from carbon equilibrium. For mature humid tropical forests, o nly seven eddy covariance sites (with unknown ages) are available (Luyssaert et al. 2007). Considering upland sites only (six of the seven), the mean net C exchange was 231 Æ 249 g C m –2 year –1 . This suggests that tropical and temperate 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 carbon stock changes in biomass, woody detritus, and soil. These numbers represent component 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) 474 C. Wirth [...]... such as wind-throw increases too (Chap 15 by Schulze et al.) Indicators related to diversity or spatial complexity generally increased with stand age, mostly because of smallscale disturbances and gap-phase dynamics 21. 4 21. 4.1 The Fate of Old- Growth Forests Worldwide Current Status of Old- Growth Forests Evaluating the current status of old- growth forests is in fact very difficult because old- growth forest... would be affected (Fenner 2000) 21. 4.2 Politics and the Future of Old- Growth Forests Old- growth forests provide important services to human society, many of which are discussed in this book Destroying old- growth may initially serve individual groups or societies, but it will eventually have a negative impact on humanity as a whole The two services provided by old- growth forests that we probably depend... Estimating the age since stand initiation in old- growth forests is a difficult task (Chap 2 by Wirth et al.), and many chronosequence studies assign arbitrary high ages to old- growth forests or simply refer to them as a separate category However, without a proper age determination of old- growth forest, the age-related shape of functional responses during the old- growth stage cannot be characterised Clearly,... was right or wrong (he probably was wrong see Sect 21. 3.2 above), our book has shown that old- growth forests generally lock up more carbon than any other forest stage or alternative ecosystem Thus, converting old- growth forest will inevitably induce emission of greenhouse gases to the atmosphere If old- growth forests were included under ‘managed’ forests, these emissions would need to be reported an... Can we use old- growth forests sustainably? What is the threshold for acceptable disturbance and management impacts below which the essential features of old- growth forest functioning are maintained? Along the same lines, what are the population densities of ungulate herbivores that allow quasi-natural stand dynamics? To what extent can the findings in this book be transferred to old- growth forests in... regime illustrates how massive the destruction of old- growth forest has been in this region In Central Europe, where forests have been cleared for agriculture since the late Neolithic, old- growth forests are almost non-existent outside nature reserves [Chaps 15 (Schulze et al.) and 19 (Frank et al.)] Forest scientists working in Europe know that old- growth research sites need to be handpicked Only in... climate and the peculiarities of species-specific decay rates Along the four stages, the mean rates of woody detritus stock-change increased from 46 Æ 69 (n = 14; pioneer), to 6 Æ 18 (n = 17; transition), to 12 Æ 24 (n = 14; early old- growth) , to 18 Æ 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 accumulation rate of woody... Chap 20 by Freibauer) This focus on managed forests goes back to Odum’s (1969) ecosystem theory, according to which old- growth forests are supposed to be carbon neutral This implies that humans cannot use old- growth forests to sequester carbon, which in turn implies that there is no reason for carbon accounting schemes to consider oldgrowth This logic has far-reaching consequences Irrespective of whether... biomass, woody detritus, and soil for each stage (Fig 21. 2, asterisks), we find that our bottom-up estimates of NECB remain remarkably constant over succession Old- growth forests appear to have the same sink strength as early-successional stands: roughly 40 Æ 58 g C m–2 year–1 This is true regardless of whether we include the high soil 21 Old Growth Forests: Function, Fate and Value a Synthesis 477 carbon... wide buffer zones) 21 Old Growth Forests: Function, Fate and Value a Synthesis 483 forest, about 60% in temperate deciduous forests, and exceeds 90% in tropical forests (Chap 2 by Wirth et al.) Using a similar method, Bergeron and Harper (Chap 13) estimate that 24% of the Canadian boreal forest would be older than 200 years under a historical fire regime We conclude that old- growth forests have been . of small- scale disturbances and gap-phase dynamics. 21. 4 The Fate of Old- Growth Forests Worldwide 21. 4.1 Current Status of Old- Growth Forests Evaluating the current status of old- growth forests. Chapter 21 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. (Fenner 2000). 21. 4.2 Politics and the Future of Old- Growth Forests Old- growth forests provide important services to human society, many of which are discussed in this book. Destroying old- growth may