Introduction

Of all biome types, forests have the greatest potential for carbon (C) storage (Canadell and Raupach 2008). Consequently, requirements for signatory countries for reporting changes in emissions and C stocks to the Intergovernmental Panel on Climate Change (IPCC) and United Nations Framework Convention on Climate Change (UNFCCC) on greenhouse emissions associated with land use, land‰ÛÒuse change and forestry (LULUCF) and the Kyoto protocol have, over the last decade, placed heavy demands on environmental research in an attempt to mitigate some of the more severe impacts of climate change. This has sparked a wash of policy-driven research into sequestration capabilities of many land-use types, particularly in forestry.

Pan et al. (2011) reported on the increase of the temperate region’s carbon sink in the last decade, underlining the role played by forests in the sequestration of significant quantities of atmospheric CO₂, as well as its storage in ecosystem pools. Conversely, forests are also strongly affected by climate change and extreme climatic events (Ciais et al., 2005; Pautasso et al., 2010; Siedl et al., 2011).

The ability of forests to sequester and store carbon places a burden on anthropogenic management systems. When there is more carbon in forest ecosystems globally than is present in the atmosphere (FAO, 2011), the responsibility of protecting such a vast store, which is anything but immune to the threat imposed by increasingly rapid changes to the climate, is considerable. Boundaries placed on the natural world by an ever-burgeoning requirement for space to accommodate the living, feeding and employing of mankind’s growing population, have substantially reduced the potential for ecosystems to exploit natural and age-old migratory and adaptive strategies against changed environments. Trees, being long-lived organisms, display far lower evolutionary rates than species with shorter life cycles (Smith and Donoghue, 2008), and therefore require informed and sympathetic land management.

There is a danger that strained financial circumstances seen across the globe could cause some to lose sight of a bigger picture. While the mechanics of carbon storage credits, and the minutiae of complex accounting systems attempting to satisfy every possible international circumstance, are important as mitigation measures, the original intent and spirit can be lost. Essentially, adaptation is a process that should run concurrently with mitigation for maximum efficiency.

CARBiFOR and carbon research in Ireland

Forest carbon sequestration was the focus of work funded by The Irish Council for Forest Research and Development, or COFORD, in the CARBiFOR project (2001-2004; Black and Farrell, 2006). Inter-comparisons of methodologies were used to develop and improve inventory-based estimates of forest carbon sequestration, as well as provide a basis for fuller carbon stock change reporting using a chronosequence, or an age sequence, of the most commonly occurring forest type in Ireland. Sitka spruce (Picea sitchensis (Bong.) Carr.) is the most widely planted commercial tree species and accounts for 52.3 percent or 327,000 ha of the total forest estate (NFI, 2007).

A combination of methodologies was used to investigate the sequestration rates of sites afforested with Sitka spruce. An ecosystem approach was used to examine stocks and fluxes between the soil, tree and debris pools. Biometric inventories of chronosequence sites were used in conjunction with biomass models to estimate tree biomass and carbon accumulation (Green et al., 2007; Tobin and Nieuwenhuis, 2007). Soil (Reidy et al., 2006) and deadwood (Tobin et al., 2007) stocks were also examined, as were soil (Saiz et al., 2006a and b; 2007) and tree respiration. Eddy covariance measurements from a tower at a core research site (Dooary, County Laois) were used to estimate net ecosystem exchange, which was compared with inventory-based measurements (Black et al., 2007; 2009). Such inter-comparisons were also used for an investigation into the uncertainties of carbon sequestration calculations. The work has been instrumental in providing estimates of annual sequestration rates over the first Kyoto commitment period from 2008 to 2012, as well as building national research expertise capacity.

In 2007, a second phase, CARBiFOR II, was initiated to extend the range of forest types to include a greater range of species and soil types, but also to include management effects. While adding to what will soon amount to decadal eddy covariance and inventory datasets at the Dooary site (a Sitka spruce stand on mineral soil, Figure 1), further chronosequences encompassing Sitka spruce on a peat soil and ash on mineral soil have been investigated with the same suite of measurements. Augmentations to the data types have included a roving eddy covariance system, as well as measurements of N₂O and CH₄ soil fluxes. Although these aspects have not been fully completed, the aim is to untangle the temporal effects of management from those of climatic variability on ecosystem sequestration. Initial results prove these two factors are difficult to separate.

Saunders et al. (2012) reported a climate dependent impact of thinning on forest carbon-uptake. While an initial drop off was expected following a thinning intervention, no significant effects on the sequestration rate were found. Higher air temperatures and solar radiation in the growing season following the thinning were responsible for an immediate compensatory increase in photosynthesis by the remaining trees. Olajuyigbe et al. investigated the effects of thinning on forest soil respiration (2012b) and climatic factors on CWD respiration from abandoned logs (2012a). These studies demonstrate the value of the combination of long term biometric and flux data. With the availability of flux data, an immediate physiological response to specific climatic circumstances can be measured, which otherwise would have been masked by growth lags and the results of naturally adaptive tree reactions in biometric data. Investigations into the impacts of severe climatic extremes such as flooding, early and late summer droughts, and winter and spring snow and freezing periods are planned in anticipation of projected changes to the Irish climate (McGrath and Lynch, 2008).

Forest soil carbon stocks have also been investigated by Wellock et al. (2011b), who found that the afforestation resulted in no significant changes in soil carbon, citing the high degree of uncertainty arising from a low sampling number. However, such a result is not unexpected as the sites sampled only covered half the country. For the estimation of peat soil stocks, Wellock et al. (2011a) found peat depth and type to be significant factors. Byrne and Milne (2006) projected, in a modeling exercise using afforestation rates and the dynamic carbon accounting model carbon-Flow, which the afforestation of peat soils resulted in net emissions of carbon, but this finding depended heavily on a range of assumptions regarding the rate of peat carbon loss. Byrne and Farrell (2005) demonstrated that losses due to afforestation of blanket peat soils could be compensated by carbon taken up by tree growth. The net carbon dynamics of forests growing on peat soils will be a challenge for future management in light of expected changes in climate, and appears to be a key area for combined growth and climate monitoring.

Ecological site classification has been used across the U.K. for assessing the species suitability and yield potential of sites, and more recently for changes in suitability and yield due to projected changes in climate. This methodology was adapted for use in Ireland by Ray et al. (2009) and Black et al. (2010), resulting in a decision support system (http://82.165.27.141/climadapt_client/index.jsp). In an effort to investigate responses of forest tree species to climatic stresses, Tene et al. (2011) developed a methodology to investigate historic tree growth responses to specific and localized droughts from tree ring data, to inform future, species- and site-specific, forest management.

Future

The challenge is not only to demonstrate how much atmospheric carbon can be sequestered — to this point the biometrically calculated stocks and stock changes have been essential — but to show how this can be maintained and best protected in anticipation of stark climatic changes. Because of the rapid nature of forest growth in Ireland (often in excess of 8 t C ha^-1 a^-1; Saunders, 2012), the significant afforestation program in Ireland over the last two decades has resulted in the sequestration of a large quantity of atmospheric carbon. However, the management of this sink and forest resource has acquired a consequent responsibility. Increased efficiencies in carbon and other resource use and management will only be possible in the light of knowledge of the changing interactions between forests and climate. The management of such a resource presents an opportunity for the combined use of remote sensing with inventory measurements to track and monitor land-use changes (e.g. Zheng et al., 2011). Inventory approaches are vital for the calibration and validation of remotely sensed data, but LiDAR and satellite data also provide the potential to optimize stand-level measurements, as well as to reduce uncertainty related to spatial variation in up-scaling calculations (Goetz and Dubayah, 2011). McInerney and Nieuwenhuis (2009) estimated stand-level standing volume and basal area from NFI data and SPOT satellite imagery. In the future, this technique will be invaluable for monitoring changes in forest health and aboveground carbon mass (McInerney et al. 2011).

As pointed out by Hendrick at an EFI conference in Dublin (2010), forest research meets society’s needs only when it contributes to policy aims being met. However, research also has a role in ensuring that goals of policy are well founded, so research must inform policy makers, as well as practitioners, through scientific and timely communicated information. Recent proposals for program-based funding for forest research in Ireland are to be welcomed to ensure continued long-term monitoring of changes in climate and forest growth. This will also contribute to the maintenance and protection of the national research sector’s expertise.

What is still required, in addition to an exploration of the impacts of disturbances caused by management interventions such as thinning and clear-felling operations, but also of the use of less common management systems, as well as abiotic factors like fire and wind and biotic factors, on greenhouse gas dynamics in forests, are adaptive management solutions. It will not be enough to explore the impact of severe climatic events and occurrences such as flooding, fire, wind throw, pest spread and infestation. For adaptation to be successful, a two-pronged approach will be required to include mitigatory steps to reduce emissions and severity of impacts, side by side with adaptive strategies to increase sustainability and efficiency. It is essential to maintain long-term forest monitoring datasets, in particular where both biometric and flux measurements can be directly associated with climatic factors. Adaptive management of the forest resource is required to maintain and develop the diversity of ecosystem goods and services relied upon, and increasingly demanded, by society, as well as to maintain the health and stability of forests, which will be forced to change rapidly to adapt to the expected global alterations in climate (e.g. Figure 2).

Maarten Nieuwenhuis is professor of forestry at University College Dublin and his primary research interest is the planning and implementation of sustainable forest management.

Brian Tobin is a research fellow at University College Dublin and coordinates and carries out research for the CARBiFOR research project. His interests include forest ecosystem carbon sequestration and woody decomposition.

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