Evaluating climate variability and management impacts on carbon dynamics of a temperate forest using a variety of techniques

Abstract:

Climate variability, extreme weather events, forest age and management history impacts carbon sequestration in forest ecosystems. A variety of measurement techniques such as eddy covariance, dendrochronology, automatic soil CO₂ chambers and remote sensing are employed fully understand forest carbon dynamics. Here, we present carbon flux measurements from 2003-2014 in a 76-year old managed temperate pine (({\it -Pinus strobus L.}) forest, near Lake Erie in southern Ontario, Canada. Forest was partially thinned (30% tree harvested) in 1983 and 2012. The thinning in 2012 did not significantly impact carbon fluxes as post-thinning fluxes were within the range of inter-annual variability. Mean annual post-thinning (2012-2104) gross ecosystem productivity (GEP) measure by the eddy covariance technique was 1518 ± 78 g C m⁻² year⁻¹ as compared to pre-thinning (2003-2011) GEP of 1384 ± 121 g C m⁻²·year⁻¹. Over the same period, mean post-thinning net ecosystem productivity (NEP) was 185 ± 75 g C m⁻² year⁻¹ as compared to post-thinning NEP of 180 ± 70 g C m⁻² year⁻¹, indicating that pre-thinning NEP was not significantly different than post-thinning NEP. Only post-thinning mean annual ecosystem respiration (Re; 1322 ± 54 g C m⁻² year⁻¹) was higher than pre-thinning Re (1195 ± 101 g C m⁻² year⁻¹). Soil CO₂ efflux measurements showed similar trends. We also evaluated the impacts of climate variability and management regime on the full life cycle of the forest using annual radial tree-ring growths from 15 trees and compared them with historical climate (temperature and precipitation) data. While the annual growth rates displayed weak correlation with long-term climatic records, the growth was generally reduced during years with extreme drought (-36% of mean annual precipitation) and extreme temperature variability (±0.6 – 1.0°C). Overall, forest was more sensitive to management regime than climate variability. It showed higher growth stress during low light condition after crown closure. When partial thinning was introduced in 1983, it responded slowly and took about 5 to 7 years to show measureable increase in its growth, despite favorable climatic conditions. This study will help to advance our understanding of carbon dynamic of forest ecosystems.