Carbon Dioxide and Water Vapour Exchange within a Norway Spruce Canopy

Abstract

Terrestrial ecosystems can act both as sinks and sources in the global carbon cycle. Forests are an important part of this system and a good understanding of their carbon balance is essential for assessments of the future climate and for evaluating mitigation strategies. Much progress has been made in understanding the main processes controlling plant-atmosphere gas exchange and their responses to environmental factors. However, most previous studies that describe the gas exchange have either been based on laboratory experiments or on short field campaigns with measurements representing a limited range of environmental conditions or positions within the canopy. The results may therefore be ecologically unrepresentative. This limits our ability to accurately represent the true interactions between the gas fluxes and their biological and meteorological regulators on a long-term basis. This thesis addresses the questions regarding spatial and temporal variety of gas exchange in a forest canopy. It is based on carbon dioxide (CO2) and water vapour (H2O) exchange measurements carried out between 2007 and 2010 at Skogaryd research site, a 60 year old Norway spruce (Picea abies (L.) Karst.) dominated mixed stand growing on drained peat soil in south-western Sweden. The measurements were conducted once every half-hour at several heights in the canopy on three adjacent trees using continuously measuring automated chambers under naturally occurring meteorological conditions. In addition, the concept of optimality of resource allocation within the canopy, with respect to maximising the canopy productivity, was investigated in two modelling based studies. Strong seasonality and vertical gradients were observed in the shoot-scale gas exchange rates. However, the relative strengths of the vertical gradients were nearly constant over the year. Therefore, no strong seasonal patterns were observed in the vertical variation of resource use efficiencies. This finding supports the use of simple resource use efficiency based models in ecological modelling. Nitrogen is often found to be a key constraint with respect to canopy assimilation in the northern forests. However, it was found that at this nitrogen rich former fen, neither the within-canopy nitrogen allocation pattern, nor the total availability of nitrogen had large effects on carbon assimilation on shoot-scale. Using optimality modelling on the stand-scale it was shown that by constraining the minimum structural allocation to the lower canopy shoots it was possible to accurately predict the observed properties of the studied stand from easily obtainable meteorological and stand properties. Stem gas exchange also exhibited strong spatial variation, and was observed to be considerably higher in the upper part of the stem during the main growing period. Ignoring this vertical variation was shown to result in considerable underestimations of the annual stem-scale gas exchange. Despite the fact that the trees at the site were accumulating large amounts of carbon, the studied stand was not a strong carbon sink on the ecosystem scale, owing to the high carbon emission from the soil. It can be concluded from the findings of this study that variation in resource use efficiency and resource availability within a forest stand is of great importance for the CO2 and H2O exchanges with the atmosphere.