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Losses of carbon (C) and nitrogen (N) from the decomposition of woody debris in harvested forest ecosystems is of interest in national C accounting, regional water quality assessment as well as for managing long-term localised forest fertility. The main commercial forest type in Ireland is Sitka spruce dominated plantations and the woody debris from harvesting is routinely gathered into windrows in reforested stands. This study examined woody decomposition losses via two pathways namely respiration to the atmosphere and rainwater leaching to soil water. A chronosequence of forest stands was sampled, ranging in age (of the harvest debris) from three to fifteen years. Respiratory C loss from windrows was found to be 11.5 t C ha−1 year−1 three years after reforestation. This rate had dropped by 29% to 8.1 t C ha−1 yr−1 twelve years later. There was a strong seasonal trend in this efflux and the main driver of this variation was soil temperature, however moisture content and drying/rewetting cycles also played a role. Leached losses of C followed a similar trend to respired losses, but their rate was consistently less than 1% of respiration. Total dissolved N fluxes in woody debris leachate followed a different pattern, where more was retained by the system than lost. Thus, windrowed woody debris acted as a nutrient reservoir for N but as a steady source of C.

a b s t r a c t Commercial forest plantations need to be actively managed, through tree removal, in order to improve wood quality, maintain productivity and provide an economic return, although this could compromise an important role for forests in carbon sequestration and greenhouse gas mitigation. The impact of for- est thinning on net primary productivity (NPP) and net ecosystem exchange (NEE) was assessed using a combination of biometric and eddy covariance (EC) techniques. Two thinning operations were per- formed in close succession, which reduced the basal area of the stand by 17% and 11% and removed a timber volume of 48 m3 ha−1 and 50 m3 ha−1, respectively. Annual rates of NPP ranged from 13.24 (±3.96) to 18.94 (±4.88) t C ha −1 and 13.22 (±3.72) to 17.77 (±5.30) t C ha −1 for the pre- and post-thinning peri- ods, respectively. Estimates of NEE varied between 8.44 (±1.34) to 8.87 (±1.48) t C ha −1 and 6.75 (±1.19) to 10.33 (±1.41) t C ha−1 in the pre- and post-thinning periods. Forest thinning did not have a significant impact on carbon stocks or fluxes when pre-thinning (2002-2006) and post-thinning (2007-2009) esti- mates of NPP and NEE were compared, however the range of inter-annual variability in NEE increased after thinning. The partitioning of annual NEE carbon budgets into gross primary productivity (GPP) and ecosystem respiration (Reco) together with an analysis of key physiological parameters suggested that the impacts of forest thinning are largely dependent on temperature. An expected decrease in GPP after the initial thinning in 2007 was not observed due, in part, to the higher mean annual air temperatures and incident photosynthetic active radiation (PAR) and a compensatory increase in photosynthesis by the remaining trees. A continual decline in Reco, was observed in the years subsequent to the first thinning and was attributed to both biomass removal and climatic factors. Inter-annual variations in climate had a significant impact on NEE, GPP and Reco. Annual mean air temperature, total precipitation and total incident PAR were all shown to influence the processes driving CO2 exchange. Overall, these results suggest that the impacts of the thinning practices, as implemented in this study, are dependent on climate and under similar conditions are unlikely, in the short-term, to compromise a role for forest ecosystems in carbon sequestration and greenhouse gas mitigation.

Forest thinning influences soil processes by altering key microclimatic conditions, root density, microbial communities, organic matter turnover and nutrient budgets. It introduces a large pulse of harvest residues (brash) to the soil surface and can alter the balance between autotrophic and heterotrophic respiration. This study determined the influence of thinning, microclimatic factors and plant productivity on carbon (C) losses through the emission of carbon dioxide (CO 2 ) respired from thinning lines (brash lanes or BL) and the forest floor (FF: without brash) in a first rotation Sitka spruce ( Picea sitchensis (Bong.) Carr.) forest in Ireland. Weekly measurements of CO 2 efflux were carried out using an Infra-Red Gas Analyser connected to static chambers; while soil moisture content and soil surface temperature were measured, using theta probes and data loggers, respectively. The soil respiration measurements were also correlated with the gross primary productivity (GPP) determined by eddy covariance techniques.The highest CO 2 efflux were observed at the peak of summer in July/2010 (FF = 699.20 mg CO 2 m −2 h −1 and BL = 374.22 mg CO 2 m −2 h −1 ) and were associated with maximum soil surface temperatures and higher rates of GPP. Soil temperature had a strong positive influence on the variation of CO 2 from the forest (FF = 75% and BL = 59%), and the temperature sensitivity ( Q 10 ) of soil respiration from the FF (5.47) was higher than from the BL (2.72). Soil moisture was inversely correlated with soil respiration from both FF ( R = −0.73, p R = −0.53, p = 0.003). The combined effect of temperature and moisture gave a better description of the variability in CO 2 respired from both the FF ( R 2 = 0.85, p R 2 = 0.67, p R 2 = 0.73, p R 2 = 0.45, p −2 year −1 ) was significantly higher than BL (351.77 g C m −2 year −1 ). The annual soil respiratory C loss was 435.32 g C m −2 year −1 (calculated based on the contribution of the BL (14%) and FF (86%) to the total forest area).

We examined the relationships between Sitka spruce ( Picea sitchensis (Bong.) Carr.) site index, windspeed, former land use, soil moisture and soil nutrients with a view to identifying factors limiting the growth potential of the species in three climate regions in Ireland. We selected plantations covering three climate regions (delineated on the basis of ‘growing season’ balance of precipitation and potential evapotranspiration; representing dry, moist and wet climate regions) located on sites representing the range of soil moisture regimes (SMR), soil nutrient regimes (SNR), and land use types found throughout Ireland. Site index of Sitka spruce varied among climate region, with significantly lower site index associated with the wet climate region mainly due to the deterioration in edaphic conditions and adverse climatic conditions. The effect of edaphic variables (SMR, SNR) on site index was consistent across climate regions, site index increasing with increasing SNR, and decreasing with excess moisture or moisture deficit. Site index reached a maximum on fresh/very rich sites in the dry and moist climate regions and on moist/rich sites in the wet climate region. In the dry climate region, water supply (SMR) was the most important variable regulating growth, in wetter windier climates nutrient supply (SNR) was the most important factor, accounting for 69% of the variation in site index. The study has allowed region-specific recommendations to be made for successful plantation establishment in Ireland and for countries with similar climatic regions.

Coarse woody debris (CWD) has become recognised as an important component of the carbon (C) pool in forest ecosystems. In Ireland, managed Sitka spruce (Picea sitchensis (Bong) Carr.) forests account for 52.3% of the total forest estate. To determine the stock and decay dynamics of above and belowground CWD, field surveys using fixed area sample plots, were conducted in six even-aged Sitka spruce stands, representing the young, intermediate and mature stages of a typical commercial rotation. The volume, mass, density loss and C:N ratio of all CWD types (logs, stumps, and coarse roots) were determined using a five-decay class (DC) system. The decay rates and half life of CWD was also determined. To estimate CWD coarse root mass; roots associated with stumps classified in different decay classes were excavated. The coarse roots were categorised into small (2–10 mm), medium (10–50 mm) and large (>50 mm) diameter classes. CWD C-mass ranged from 6.98 to 18.62 Mg ha−1 and was highest in an intermediate forest (D35), while the aboveground volume varied from 6.31 to 42.27 m3 ha−1. Coarse roots accounted for 21% to 85% of the total CWD C-pool in the surveyed stands. The total CWD C-mass was poorly correlated with the number of thinning events (R2 = 0.29), when data from D35 was excluded. The density loss was significant in logs (45%), stumps (58%), and small- (38%), medium- (50%) and large roots (38%) as decay progress from DC 0 to 4. There was a 46%, 41%, 51%, 72% and 57% decline in C:N ratio of logs, stumps, small-, medium- and large roots, respectively, as decay progressed from DC 0 to 4. The density decay rates were 0.059, 0.048 and 0.036 kg m−3 year−1 for logs, stumps and coarse roots, respectively. The size classification of roots did not significantly affect their decay rate. The half life (50% decomposition) of CWD was estimated has 12-, 14- and 19 years for logs, stumps and roots of Sitka spruce. Regression curves showed a strong correlation between the density and C:N ratio (R2 = 0.69, 0.74 and 0.93 for logs, stumps and coarse roots, respectively). The long term storage of C and its slow rate of decomposition make CWD a vital structural and functional component of the CWD C-pool and a major controller of forest ecosystem C-retention.

Significant expansion has occurred in Ireland's forest estate since the 1950s. However, the design of the monocultural plantations established in the 1950s and 60s is now considered insensitive to local landscapes and re-design intervention and transformation is needed to improve integration into the environment. This case study was carried out in Laracus forest, Co. Donegal. The rotation of all stands in this 581-ha property has reached the final production phase, with coupes scheduled for clearfelling from 2003 to 2015. The implications of forest re-design for both the volume production in the current rotation and the financial return of the current and subsequent rotations were examined. Results indicated a volume loss due to design planning for the current rotation of 5.6% compared to the volume produced under the standard regime. In financial terms, this represents a loss of 4.6%. For the subsequent rotation, a significant financial gain of 22% in net present value was achieved as a result of redesigning the plantation. The overall financial out-turn for the property, when both the current and subsequent rotations were considered, was a 3.4% lower net present value for the design plan than for the standard regime. This result represents a lower impact of design planning in Laracus than most other forest restructuring studies have reported.