For information: belelli@unitus.it

1. Climatic and management influence on the carbon sequestration capacity of a deciduous oak coppice forest in Italy Luca Belelli Marchesini (1),L. Ana Rey Simó (2), Riccardo Valentini (1) and Dario Papale (1) (1) Department of Forest Environment and Resources, University of Tuscia, Viterbo (Italy ); (2) Arid Zones Research Station, Spanish National Scientific Council, Almeria (Spain) B41A-0286 1.Background and objectives Recent updated estimates of the carbon balance of European forests based on a suite of ecological inventories and models confirmed their active role as sink (Ciais at al. 2008, Luyssaert et al. 2010), determined primarily by the management applied in the last decades with wood removals being lower than net primary productivity (NPP). Eddy covariance continuous measurements of CO2 fluxes can detect responses of the carbon dynamics to environmental or management factors in the short term, overcoming the limitation of inventories representing a snapshot of the carbon pools typically at temporal resolution of several years or decades. However the majority of eddy covariance studies, so far performed mostly on middle-aged or mature stands, still have poorly investigated the role of actively managed forest types such as coppices, the changes in the net ecosystem productivity (NEP) over long chronosequence data and ultimately their capacity to store the uptaken atmospheric carbon in the long term. Here we present an analysis of net ecosystem exchange (NEE) data from a deciduous oak (Quercus cerris L.) coppice forest in central Italy (Roccarespampani site) monitored during the years 2000-2008 over two differently aged forest stands and covering almost all the stages of the 20 years rotation period. Main objectives were: i) the interpretation of seasonal and inter-annual variability of CO2 fluxes under the combined effect of climate and forest management; ii) the evaluation of the carbon sequestration capacity along the forest rotation cycle. 3.Temporal variability of annual NEEm Reco , GPP After coppicing the forest ecosystem turned into a net CO2 source for 1 year only, then it intensified its sink strength along with stand age (R2=0.66; P<0.001) up to a maximum observed annual NEE of -1077.9 gC g C m-2 yr-1 (fig.1c). This trend was explained by a decreasing Reco/GPP ratio (R2=0.70; P<0.001), with Reco also showing a negative trend over time (R2=0.41; P<0.01) which underlines the noticeable effect of coppicing on the enhancement of soil CO2 effluxes (fig.1b). Forest microclimate undergoes indeed a drastic change after the forest is cut, consisting mainly in warmer soil temperature getting lower as long as the canopy closure is complete. However, the temporal pattern of Reco was also the result of changes in the availability of decomposable substrate and nutrients, as witnessed by the negative correlation of temperature independent Rref with age (fig.2b) (FW: R2=0.61, P<0.001) (WS: R2=0.48, P<0.01). GPP was most weakly correlated to time since coppicing (fig.1a) and the values of Amax show actually the prompt recover of the almost full photosynthetic capacity already at the second year of growth (fig.2a). 4. Drivers of CO2 fluxes The multiple regression analysis (tab.1) confirmed that time since coppicing, or forest age, was a significant factor influencing the annual NEE, GPP and Reco. Moreover it highlighted the relevant role of temperature, particularly of annual mean air temperature in modulating GPP and that of mean annual soil temperature and air temperature anomalies in the June-July-August period in affecting NEE. During dry periods, typical of sub-Mediterranean climate, warm conditions associated to high VPD and low soil water content limited the processes of photosynthesis and respiration (fig.3). As a consequence, we generally observed a marked drop in both GPP and Reco levels, which translated into a reduction of CO2 uptake or even a release to the atmosphere for the younger forest stands (fig.3). The dry periods, which can last up to 90-120 days with variable severity, resulted the second most important “functional season” of the year (fig.4) to influence the annual carbon balance (ρ=0.91, P<0.001) after the whole growing season (tab.2). dry period (a) Fig.3 (left): response of daily Reco (a), NEE (b) and GPP (c) to daily mean air temperature (Ta) and corresponding variation of vapour pressure deficit (VPD), global radiation (Rg) and soil water content (SWC) (d). A dry period associated to depression of both Reco and GPP is observed when mean daily Ta exceeds 20.3±1.1 °C. Values represent mean± standard error of 1°C bins. (b) R²= .91 Adjusted R²= .86 F(4,8)=20.421 p<.00029 Std.Error: 115.96 NEE (left): chorography of the Roccarespampani site, location of EC towers and details of coppice forest structure. (below): view of a forest stand 2 years after coppicing R²= .66 Adjusted R²= .53 F(4,10)=5.0438 p<.01736 Std.Error : 197.72 (c) 1 SWC 3 R²= .82 Adjusted R²= .76 F(3,9)=13.993 p<.00098 Std.Error: 89.93 Rg VPD (d) “Rocca-1” site: coppiced in 1999 Eddy covariance records: (2000-2008) 2 (a) Fig.1 (left): temporal variability of annual sums of GPP (a), Reco (b) and NEE (c) with observations starting just after coppicing. The time trends are evidenced by linear regressions (solid lines) with 95% confidence intervals (dashed lines). Fig.2 (above): temporal variation following coppicing of (a) maximum photosynthetical capacity (Amax) estimated under non limiting conditions and (b) base respiration at 10°C for spring-winter and fall-winter periods. Tab.1 (above): results of multiple feed forward regressive analysis highlighting the significant drivers acting on annual NEE, Reco and GPP among climatic factors and forest age. GPP • 2.Methods • Processing of eddy covariance data according to standardized CarboEurope procedure (Papale et al., 2006; Reichstein et al., 2005), including gap-filling and partitioning of NEE into ecosystem respiration (Reco) and gross photosynthetic (GPP) fluxes. • Individuation of the main drivers of the NEE, GPP and Reco seasonal variability by analysis of functional relations and identification of different “functional periods”, importantly when heat and water stress limit CO2 exchanges. • Evaluation of the weight of each “functional season” on the annual carbon balance by non parametric correlation analysis (Spearman). • Multiple regression (forward stepwise) between annual NEE, Reco, GPP and several factors accounting for climate and management effect: Ta -air temperature (annual mean, JJA anomaly), Ts- soil temperature, PPT-precipitation (annual sum, JJA and Jan-May anomaly), Rg-sum of global radiation (annual, growing season), time since coppicing. • Analysis of the temporal variation of: a) the temperature independent basal ecosystem respiration (Rref), derived from the parameterization of the Reco-Ts relation by means of a Q10 function. b) the maximum photosynthetic capacity (Amax) under “non limited conditions”(10 days time window before maximum observed daily GPP) derived from half-hourly GPP-PPFD light curves (Ruimy et al., 1995). • Assessment of the net biome productivity (NBP) considering the potential export of carbon in form of harvestable biomass derived from a combination of yield tables and allometric relations developed specifically for the site (Rizzo, 2002). “Rocca-2” site: coppiced in 1991 Reco Eddy covariance records: (2002-2008) (b) [%] 5.Fate of sequestered carbon The cumulated NEP (NEP=-NEE) over the analyzed time period of 18 years, based on linear regression in fig. 1c, yields 82.9 tC ha-1. The net biome productivity (NBP), obtained after subtraction of the potential carbon export by wood removal (34.9), still denotes an average carbon sequestration capacity of 190 g C m-2 yr-1 along the rotation cycle (fig.5). Forest coppicing cannot take place at time intervals shorter than 16 years by law. Rg source 4 sink Tab.2 (above) Spearman correlation analysis (Tab. 1, right) determining the weight of mean Reco, NEE and GPP of each “functional period” on the annual balance. Fig.4 (left): scheme of the subdivision of the annual biological cycle into main “functional seasons” derived from functional relations in fig. 2. Dormancy is defined when daily GPP<2 gCm-2. Fig.5: cumulated NEP along 18 years of the forest rotation period and NBP after forest coppicing. Pattern of standing biomass carbon stock changes (including shoots and reserve trees) applying a forest rotation period of 20 years, as prescribed by the management plan of Roccarespampani forest. 5 harvest CONCLUSIONS: (1) Coppice management of Roccarespampani forest associated to high carbon sequestration rates and limited duration of net CO2 release following clear cuts (annual NEE <0 already after 2 years). (2) Enhanced ecosystem respiration after coppicing, independently of the altered microclimate (input of C,N through biomass residuals/root mortality). (3) Sink strength increases primarily with stand age, but negatively impacted by warmer temperatures. For information: belelli@unitus.it DepartmentofForestEnvironment and Resources Dipartimento di Scienze dell’Ambiente Forestale e delle sue Risorse