BIOMETRIC-BASED CARBON ESTIMATES AND ENVIRONMENTAL CONTROLS WITHIN AN AGE-SEQUENCE OF TEMPERATE FORESTS

Abstract

<p>Understanding the response of forest carbon uptake and growth to interannual climate variability and forest management practices is important, given the large quantity of carbon stored in forests, and their significant role in the global carbon cycle. Since 2004, biometric and micrometeorological measurements were taken in an age-sequence (10-, 38- and 73-years-old as of 2012) of white pine (<em>Pinus strobes</em> L.) plantation forests in southern Ontario, Canada, providing an 8 year record of carbon sequestration, growth and climate. The 73-year old conifer site was thinned in early 2012, where 25% of trees were removed to improve light and water dynamics of this stand, providing an opportunity to study the impacts of thinning on its carbon cycle. Additionally, in 2012, similar biometric and micrometeorological measurements were initiated in a naturally-regenerated, managed 80-year-old deciduous (Carolinian) forest, located in close proximity to the pine stands. Similar to the conifer sites, the deciduous site is also a managed forest. The objectives of this study were to determine differences in carbon pools and carbon sequestration capacity: (a) across an age-sequence of afforested, managed conifer stands; (b) between similarly-aged managed coniferous and deciduous stands; and (c) in a mature conifer plantation before and after a thinning event. Results show that carbon assimilated in the stem of mature white pine trees follows a linear growth trend, while that of young white pines shows an exponential increase in carbon assimilation over the course of this study. Overall, carbon sequestration increased with stand age across the age-sequence, except when disturbed by an event such as thinning. Thinning substantially reduced the live aboveground carbon pool (by 14%), while increasing woody debris (by 122%) due to logging residue left on-site. Comparison between the mature coniferous and deciduous stands, showed that total aboveground carbon storage within the pine stand (144 t C/ha) was generally higher than in the oak-dominated deciduous stand (83 t C/ha), despite both growing in similar soil and climate. While monthly tree growth exhibited a positive correlation with mean monthly temperature across all sites, tree growth negatively correlated with precipitation at the 10-year old white pine and 80-year old deciduous sites and no apparent correlation existed at the 73- and 38-year old sites. At the three coniferous stands, total annual net primary productivity (NPP) exhibited no correlation with mean growing season temperature or precipitation. This suggested that tree growth in young coniferous stands could be as sensitive as that of mature deciduous stands to precipitation. However, overall NPP seemed to be less sensitive to climatic variables across these stands, irrespective of their age and NPP may be driven more by stand physiology. Finally, eddy covariance and biometric estimations of NPP and NEP were compared, and results showed that although some growth trends do compare between the two techniques, magnitude discrepancies do exist and should be studied further. Results from this study will be informative to forest managers, forest conservationists and those interested in forest carbon sequestration.</p>