CARBON CYCLE IN MANAGED FORESTS – APPLYING A PROCESS MODEL TO STANDS OF MAJOR TREE SPECIES IN CENTRAL-EUROPEAN FORESTRY EMIL CIENCIALA, FYODOR TATARINOV, MARTIN ČERNÝ Institute of Forest Ecosystem Research, 1544, CZ–254 01 Jílové u Prahy, Czech Republic, [email protected] CIENCIALA, E., TATARINOV, F., ČERNÝ, M.: Carbon cycle in managed forests – applying a process model to stands of major tree species in central-european forestry. Lesn. Čas. – Forestry Journal, 52(1–2): 29–39, 2006, 4 fig., tab. 2, ref. 23. Original paper. ISSN 0323–1046 This study describes the adaptation and application of a process model BIOME-BGC to simulate carbon cycle of forest stands under typical conditions of Central-European forestry. BIOME-BGC is a point model developed to study energy, water, carbon and nitrogen cycling in major natural biomes. BIOMEBGC attracts researchers by its reasonable model simplification of ecosystems, while retaining the spectrum of key physiological processes and feedbacks important for advanced and effective analysis of ecosystem behaviour and responses to imposed factors. To be applicable also for managed forest ecosystems, several new routines were added to the model code. They include thinning and felling routine that can be specific as for time and intensity, and species change after clear-cut. This contribution describes 1) sensitivity analysis of the key model parameters and 2) model application to typical managed stands of spruce, pine, beech and oak, utilizing the long-term measurements of aboveground biomass on permanent research plots. These steps are vital to address the next/final goal of the model adaptation, namely large-scale grid application permitting regional analysis of managed forest ecosystems under defined scenarios of management and changing environmental conditions. Keywords: carbon budget, prediction, forest ecosystem, biomass, forest management, BIOME-BGC 1. Introduction Carbon budget and sink capacity of forest ecosystems are important for mitigating environmental change. Policies on sustainable forest management and climate change stimulate research agendas on forest carbon budget and its interpretation. The recent adoption of Kyoto Protocol further spurred the need of a sound understanding carbon-related processes. For example, countries may voluntarily use forest management Kyoto Protocol Art. 3.4 (more here) to offset part of its emission reduction target. Such a decision requires thorough analysis of the likely development of forest carbon stock. This in turn requires application of several types of prediction tools, which aid analysis and interpretation needed for making the optimal choice. Evidently, this is very challenging, as the analysis should include management scenarios, species-specific differences and interactions of biomass and soil compartments. Several types of models are available to address the issues of carbon budget. A traditional type of models used in forestry are regression models, which are tree and/or stand level growth models based on empirically derived statistical relationships between biometric parameters of trees or stands, and production, which is most often expressed as height and volume growth. Such models are simple and hence easy to apply. However, they include no causality and hence do not provide much explanatory power for ecosystem analysis under changing growth conditions. In contract to regression models, the process models include ecophysiological processes describing ecosystem functioning in terms of key processes and their interactions. They simulate ecosystem development as a result of eco-physiological processes described mechanistically. Such models are able to quantify effects of e.g., change in climate, elevated CO2, nitrogen deposition and land use scenarios. Moreover, ecosystem process models include both soil and biomass components and their interactions. A major drawback of process-based models is their complexity. They usually require a considerable set of ecophysiological and site parameters. Therefore, a critical task for the application of a process- based model is its parameterisation, including sensitivity analysis of model output to the input data and parameters. Secondly, for both types of models, verification of model simulations on real observations is needed to gain further confidence in predictions. This contribution describes application of a process model BIOME-BGC (RUNNING and HUNT, THORNTON 1998) that was specifically adapted for application to managed forest ecosystems. It addresses the topics of sensitivity analysis as performed in TATARINOV and CIENCIALA (2005) and model application to long-term observations of above-ground production from permanent research plots based on the study of CIENCIALA and TATARINOV (2006). A sensitivity analysis of BIOME-BGC was previously conducted by WHITE et al. (2000). These authors were mostly interested in parameters important for daily net primary production (NPP), while this study is focused on individual carbon pools and key parameters that affect changes of these pools. Other studies also exist that previously applied BIOMEBGC to managed forest ecosystems (e.g. PIETSCH et al. 2003, PIETSCH and HASENAUER 2002, CHURKINA et al. 2003, VETTER et al. 2005), but they did not include a wider set of management options apart from final cut. The study presents the predicted and observed aboveground production for individual tree species and discusses the topics important for model application to managed stands. 2. Material and Methods 2.1. Model description, adaptation and application We used the Biome-BGC (RUNNING and HUNT 1993, THORNTON 1998) model version 4.1.1 adapted so as to include key management routines (thinning, felling, tree species selection) with some additional changes to interception, evaporation and throughfall (TATARINOV and CIENCIALA 2006). BIOME-BGC is a process-based model operating with a daily time step. It describes distribution of energy and cycles of water, carbon and nitrogen for a specific type of terrestrial ecosystem. The calculation of gross primary production follows FARQUHAR et al. (1980), distinguishing illuminated and shaded foliage. Autotrophic respiration is separated into maintenance respiration calculated proportionally to nitrogen content of living tissues (RYAN 1991) and growth respiration that is handled as a function of carbon allocated to the different plant compartments. Other details on the applied model can be found in TATARINOV and CIENCIALA (2006). The model requires site parameters, ecophysiological parameters and series of daily meteorological data as input information. The meteorological data series including minimum and maximum daily temperatures and daily precipitation were extrapolated from a closest reference weather station to a given locality via MTClim simulation model (RUNNING et al. 1987, THORNTON and RUNNING 1999). Altogether 40 weather stations situated all over the Czech Republic with data series from 1961 to 2000 were available for this purpose. The mean annual precipitation totals for each plot required by MTClim for the extrapolation of the actual precipitation from the base station to the individual plots were obtained from the spatial data set of the annual mean temperatures and precipitation totals all over Czech Republic with the regular grid of 1 to 1 km (KVĚTOŇ 2001). The ambient CO2 concentration for each simulation year was taken from the Mauna Loa record (since 1959, KEELING and WHORF 2004) and from Law Dome ice cores (before 1959, ETHERIDGE et al. 1998). The industrial nitrogen deposition was set in the range from 6 to 14 g⋅m-2year-1 at reference year 2000 for different plots based on the maps of the Czech Hydrometeorological Institute (2001). BIOME-BGC simulation procedure always included spin-up simulation and pre-defined historical land-use scenario. The simulation of the current stands started with the stand felling and planting that matched the year of stand establishment. The model was run with species-specific parameter sets (CIENCIALA and TATARINOV 2006), while site parameters varied for individual sites. The applied current and historical scenarios are described below. 2.2. Management scenarios The current management included intensity and timing of thinning events corresponding to the recommendation of the Czech Forestry Act. The actual timing (stand age) and thinning intensity for the analysed plots was derived from the recorded data from the individual research plots and complemented from the Czech growth and yield tables (ČERNÝ et al. 1996) for the earlier period of stand development that was not covered by the long-term plot inventory. Thinning volumes were converted to corresponding share of biomass removed from the sample plot at given age. Besides thinning regime, the mortality rates were changed for the current stand generation: fire mortality was set to zero and the whole plant mortality was reduced to 0.002 yr-1. This expressed the partial compensation of natural mortality by the imposed thinning regime. To reflect land-use management history of the studied sites, we applied two historical management scenarios based on site elevation. This was based on the available historical land-use records (NOŽIČKA 1957). For elevations below 800 m, we applied clear-cut in the Middle Age (XIV century) with temporal transformation of forest into the grassland, followed by the forest restoration in XVII century with four 100-year long forest rotations. For elevation above 800 m, we applied two 100-year long forest rotations starting in XVIII–XIX century followed by the simulation for the actual stand rotation. 2.3. Sensitivity analysis The sensitivity analysis was focused on the effect of individual site and eco-physiological parameters on the set of the key output variables under steady state. The output variables included carbon content in plants, litter and soil and total carbon content (abbreviated as Cp, Cl, Cs, Ct, respectively) and mean daily net primary production (NPP). The following text provides only basic description of the performed procedures. The sensitivity of output variables (y) to input parameters (x) (or the effect of parameter x on the variable y), ∆y/∆x was calculated as ratio of output variable change to parameter change (both in %). The sensitivity to qualitative variables, as forest type and aspect, was calculated as a ratio of two output variable values corresponding to two parameter values y(x1)/y(x2). The ranking of parameter sensitivity was based on the effect magnitude (|∆y/∆x|, in absolute values), distinguishing a) very sensitive parameters (|∆y/∆x| > 0.2) ii) sensitive parameters (effect between 0.1 < |∆y/∆x| < 0.2) and iii) parameters with low sensitivity (|∆y/∆x| < 0.1). Sensitivity analysis focused on Norway spruce (Picea abies [L.] Karst.) and European beech (Fagus sylvatica L.). Other details and methodological approaches of sensitivity analysis are described in TATARINOV and CIENCIALA (2006). 2.4. Observed stand biomass The observed stand production was obtained from the database of the Permanent Research Plots (PRP). The plots selected for the model analysis represented mono-specific stands of spruce, pine, beech and oak with the share of basal area for the individual species above 90%. Other criterion for plot selection was a long-time series of recurring growing stock inventory. The final set of stands included 22 plots of four major species with different elevation and site classes. Each included information on plot altitude, stocking density, stand volume, age, mean stem diameter and height. The inventory measurements were performed repeatedly four to eight times with intervals of about five years. Available inventory data were presented before and after thinning (being equal if no thinning occurred), which also gave information on the actual thinning volumes. Stand volume was converted to total above-ground biomass using age-dependent species-specific biomass expansion factors, which were derived from a large dataset of tree-level information of the PRP database and species-specific biomass equations of WIRTH et al. (2004), CIENCIALA et al. (2005 and 2006) and PAŘEZ et al. (1990). 3. Results 3.1. Sensitivity analysis – effect of site and ecosystem parameters The assessed effects ∆y/∆x (for continuous x) and y(x1)/y(x2) (for discrete x) from the sensitivity analysis are presented in Table 1 and 2. The negative sign of ∆y/∆x means that parameter increase results in a decrease of the variable. Table 1. Sensitivity (ratio of % of variable change to % of parameter change) of the selected model output variables at steady-state to the most important site parameters, namely elevation (H, m), soil texture – fraction of sand particles (βs, %), effective soil depth (d, m), CO2 concentration and nitrogen input (sum of nitrogen background and industrial deposition and nitrogen fixation, Nd). The following default values of parameters were applied: βs = 30 %, slope = 0, albedo = 0.2, H = 450 m and no slope (ϕ = 0). Bottom index indicates the ranking of sensitivity (0.1 to 0.2 – index 1, medium sensitivity, above 0.2 – index 2, high sensitivity). No index means low sensitivity. Parameter Constant H (450–550 m) H (450–550 m) βs (10–30%) βs (30–50%) βs (50–80%) βs (30–50%) βs (50–80%) d (1.0–1.5 m) d (0.5–1.0 m) d (0.3–0.5 m) d (1.0–1.5 m) d (0.5–1.0 m) d (0.3–0.5 m) CO2 (298–350 ppm) CO2 (298–350 ppm) Nd (0.5–0.7) Nd (0.3–0.5) Nd (0.2–0.3) Nd (0.5–0.7) Nd (0.3–0.5) Nd (0.2–0.3) Spruce Beech Spruce Spruce Spruce Beech Beech Spruce Spruce Spruce Beech Beech Beech Beech, 450 m Spruce, 450 m Beech, 450 m Beech, 450 m Beech, 450 m Spruce, 450 m Spruce, 450 m Spruce, 450 m Sensitivity (% variable change) / (% parameter change) Carbon pool Plant Litter Soil Total 0.352 0.352 0.402 0.442 0.312 0.272 0.262 0.302 -0.03 0.06 0.06 0.01 -0.01 -0.01 -0.09 -0.151 -0.412 -0.402 -0.442 -0.472 -0.02 -0.01 -0.10 -0.141 -0.422 -0.402 -0.562 -0.622 0.482 0.482 0.322 0.181 0.552 0.552 0.412 0.302 0.432 0.592 0.592 0.502 0.352 0.342 0.272 0.242 0.452 0.482 0.492 0.462 0.922 0.892 0.802 0.762 0.562 0.782 0.762 0.622 0.482 0.662 0.642 0.542 0.04 0.05 0.06 0.05 0.262 0.372 0.402 0.292 0.612 0.862 0.892 0.682 0.00 -0.01 0.00 0.00 0.622 0.672 0.782 0.682 0.572 0.702 0.782 0.662 Spruce 1.5 1.5 NPP 0.432 0.302 -0.02 -0.161 -0.482 -0.141 -0.642 0.171 0.282 0.452 0.191 0.452 0.782 0.572 0.462 0.05 0.252 0.632 -0.01 0.602 0.582 Beech Plant Soil Sensitivity (-) Sensitivity (-) Plant Soil 0.5 -0.5 -1.5 0 20 40 60 Sand fraction (%) 80 100 0.5 -0.5 -1.5 25 30 35 40 Specific leaf area (m 2/kg C) 45 Figure 1. Sensitivity of simulated steady-state carbon pools to the model parameters: example of site parameter (sand fraction, Spruce, left) and ecophysiological parameter (specific leaf area, Beech, right). Among the site parameters, the most significant effect on steady-state carbon pools and NPP were site elevation, soil texture (sand fraction), soil depth, ambient CO2 concentration and the total nitrogen deposition (Table 1). The effect of elevation was positive for the tested tree species. Specific dynamic can be observed for the effect of soil texture that increased with increasing share of sand for both species (Figure 1). The effect of soil depth was strong and increasing with more shallow depths (Table 1). The effect of industrial nitrogen deposition was rather strong, but quickly saturating above certain threshold. Table 2 Sensitivity of simulated steady state carbon pools to single eco-physiological parameters. All simulations were performed for elevation 450 m, soil with 30% of sand and ϕ = 0. Bottom index indicates the ranking of sensitivity (0.1 to 0.2 – index 1, medium sensitivity, above 0.2 – index 2, high sensitivity). No index means low sensitivity. Parameter abbreviations: gs, max – maximum stomatal conductance, SLA – specific leaf area, VPD – vapour pressure deficit, WP – leaf water potential. * = Dimensionless. Parameter Tree species Beech Fine roots C: N Leaf C: N gs,max gs,max N in Rubisco New fine rootC: leafC New stemC: leafC SLA VPD full reduction Spruce Fine roots C: N Leaf C: N Total mortality Fire mortality gs, max Leaf turnover N in Rubisco New fine rootC: leafC New stemC: leafC SLA WP full reduction Unit Range Sensitivity (% variable change)/(% parameter change) Carbon pool NPP Plant Litter Soil Total Dim. * Dim. * m⋅s-1 m⋅s-1 Dim. * Dim. * Dim. * M2kgC Pa 50–72 19–25 0.004–0.005 0.005–0.006 0.07–0.088 1.43–2.0 2.0–2.71 35.0–37.9 2500–3000 0.832 -0.522 0.08 -0.201 1.552 -0.942 0.482 -0.09 -0.07 1.202 -0.882 0.652 0.201 2.192 -1.282 0.121 0.412 0.252 1.202 -0.822 0.642 0.212 2.172 -0.772 -0.372 0.442 0.262 0.932 -0.612 0.232 -0.09 1.722 -0.932 0.282 0.06 0.02 0.632 -0.622 -0.252 -0.312 1.512 -0.372 -0.372 -0.141 -0.161 Dim. * Dim. * Year-1 Year-1 m⋅s-1 Year-1 Dim. * Dim. * Dim. * M2kgC MPa 27.6–37.1 27.7–43.0 0.005–0.006 0.002–0.005 0.004–0.006 0.19–0.24 0.04–0.05 0.66–1.4 1.45–2.02 7.8–9.4 -2.5–2.3 0.692 -0.181 -0.832 -0.872 -0.532 0.402 0.682 -1.552 0.822 -0.252 0.06 0.802 -0.262 0.04 -0.252 -0.212 0.322 0.812 -2.062 0.742 -0.121 0.272 0.812 -0.222 0.03 -0.242 -0.181 0.312 0.812 -1.522 0.342 -0.111 0.272 0.742 -0.201 -0.452 -0.572 -0.392 0.362 0.732 -1.582 0.662 -0.191 0.141 0.722 -0.151 -0.08 -0.151 -0.512 0.482 0.742 -1.062 0.452 -0.362 0.232 The most significant ecophysiological parameters are listed in Table 2. From the tested parameters, the nitrogen content in Rubisco and the allocation ratio of new fine roots carbon to new leaves carbon had the highest effect on NPP and output carbon pools, reaching up to 2. C : N ratios of leaves and fine roots, new stem carbon to new leaves carbon allocation ratio, specific leaf area (SLA), leaf turnover and maximum stomatal conductance (gs,max) mostly high effect on the tested carbon pools and NPP. The effect of SLA for plant and soil carbon pools of beech is shown in Figure 1. 3.2. Predicted and observed aboveground biomass The predicted and observed carbon held in aboveground biomass (CAB) for the set of analysed permanent research plots (n = 22) is shown in Figure 2. The model was generally able to match the observed data well, with slightly better results for conifers as compared to broadleaved species. For the stands of broadleaved species (Figure 2 left), the coefficient of determination (r2) and standard error of estimate reached 0.87 and ±1.2 kg C/m2, respectively (n = 49 observations). This analysis excluded oak plot No. 6061602 site-located in a floodplain region. As for coniferous stands (Figure 2 right), the corresponding results were r2 = 0.95 and SE ±0.87 kg C/m2 (n = 68 observations). The example plots of the individual species with the simulated and observed CAB on time axis are shown in Figure 3. 25 Beech Oak -2 20 CAB modelled (kg C m ) -2 CAB modelled (kg C m ) 25 15 10 5 0 0 5 10 15 20 CAB measured (kg C m -2) Pine Spruce 20 15 10 5 0 0 25 5 10 15 20 CAB measured (kg C m -2) 25 Figure 2. The scatter of the measured and simulated carbon pool of aboveground tree biomass (CAB) for the analysed permanent research plots of broadleaved species beech and oak (left) and coniferous species pine and spruce (right) with inserted regression line. The points of the oak plot No. 6061602 (left, open symbols) situated in a floodplain area are excluded from the regression. 25 Pine 500622 Spruce 501625 2 CAB (kg/m ) 20 2 CAB (kg/m ) 20 25 Beech 501118 Oak 501069 15 10 5 0 0 15 10 5 20 40 60 80 Age (years) 100 120 0 0 20 40 60 Age (years) 80 100 Figure 3. Simulated (lines) and observed (symbols) aboveground biomass carbon (CAB) pool during current rotation – the examples of permanent research plots of broadleaved (left) and coniferous (right) tree species. 4. Discussion The sensitivity analysis of the model identified the key site and eco-physiological parameters important for tree species-specific model application to managed forest stands in CentralEuropean conditions. It must be noted that the interpretation of parameter sensitivity analysis is not easy, because of many interactions involved. With several key site parameters and additional 34 eco-physiological parameters, BIOME-BGC is still considered as being still relatively robust process model, hence also suitable for large-scale application. The sensitivity analysis is the required step for efficient model application. The results demonstrated here and those included in TATARINOV and CIENCIALA (2006) basically correspond to the fundamental BIOME-BGC parameter sensitivity analysis performed for NPP by WHITE et al. (2000). The extension of analysis to effect on carbon pools helped to identify some specific differences. For example, the effect of specific leaf area (SLA), which is a critical parameter for LAI- related variables, was decreasing with larger values for beech, being either positive or negative with respect to plant carbon, while it remains positive for soil carbon (Figure 1, right). This stands out against the observation of WHITE et al. (2000), who reported strong negative effect of SLA on NPP for broadleaved and coniferous woody species. With respect to site parameters, one of the key problems represents the effective soil depth, which is seldom available, although it strongly affects both water and carbon budget fluxes. The model has a simplified treatment of soil hydrology and does not consider drainage or ground water supply. The latter was actually found to be the reason for model failure for the oak plot from a flood-plain area (Figure 2). In such cases, model might be enhanced with additional routines, which in the case of groundwater supply was demonstrated by PIETSCH et al. 2003. Other details on sensitivity analysis can be found in TATARINOV and CIENCIALA (2006). The comparison of model prediction with the observed stand biomass indicates the model potential for analysis of carbon cycling in managed forests. The overall match of the modelled and observed aboveground biomass is considered promising. It should be stressed that one identical set of eco-physiological parameters was applied for all stands of given tree species, while adapting site parameters only. Secondly, the observed data usually covered only a fraction of the simulated rotation, while the other vital information for predicting the growing stock, such as thinning volumes, was derived from the growth and yield tables (ČERNÝ et al. 1996), the application of which remained uncertain for the actually analysed plots. Other source of uncertainty represents the applied land-use history for given sites. It can be demonstrated that the residual effect can be long-term, especially on soil carbon pools (Figure 4). 100 80 60 Spruce site 2 80 Carbon stock (kg/m ) 2 Carbon stock (kg/m ) Beech site Plant Litter Soil 40 20 0 1300 1400 1500 1600 1700 1800 1900 2000 Year 60 Plant Litter Soil 40 20 0 1300 1400 1500 1600 1700 1800 1900 2000 Year Figure 4. Effect of land-use management scenario on carbon pools on beech and spruce stands (note: different site class). Virgin forest with carbon pools in equilibrium felled in XIV century replaced by grassland and afforested in XVII century with planted forest of 100 year long rotation circle; thinning applied only during the last rotation. Hence, it is apparent that model predictions must be interpreted cautiously. CIENCIALA and TATARINOV (2006) provided additional check of model performance using an independent set of the observed soil carbon data. These were derived from the permanent research plot database including those plots, where such information was available. The comparison of these independently observed data and model prediction of soil compartment for the selected stands used in analysis of above-ground biomass showed general agreement on the level of individual species. It was concluded, however, that soil compartments represent the most uncertain components of carbon budget. Specifically challenging in this respect is the reporting under Climate Convention (UNFCCC) and its Kyoto Protocol required by individual pools. The methodological guidance of IPCC (2003) for emission inventory from the sector including forestry justifies excluding those pools from reporting that do not loose carbon. However, it is good practice to provide evidence for this, which is problematic considering the uncertainty of evidence on historical land use, uncertainty in actual effect of land-use change on carbon stocks and overall high variability of soil carbon stock and high sampling requirements for any detection of significant carbon stock changes. BIOME-BGC provides only indications of the likely trends in carbon compartments and more analytical effort and/or alternative models are needed to gain confidence in soil carbon stock change detection. It is apparent that the current model can be further improved. At the same time, the model must remain relatively pragmatic in order to be practically applicable for regional analysis, which is our major aim of the effort spent on testing and adaptation of the model. The distinguished feature to be improved is handling of multilayer vegetation, which would be needed for a more realistic modelling of transition between old and new stand. Recently, BOND-LAMBERTY et al. (2005) demonstrated an elegant adaptation of BIOME-BGC for such situations. Other improvement should concern model parameterisation, which should be further simplified for regional application. Specifically important are the site parameters, some of which could be generalized by additional functional relationships. 5. Conclusions This contribution highlighted some items of the sensitivity analysis identifying key ecosystem and site parameters for species-specific model parameterisation. 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