<p>In the boreal forests of the Northern Hemisphere, time series of tree-ring width (TRW) and maximum density in the latewood (MXD) are highly correlated to local instrumental summer-temperature data and are thus widely used as proxies in high-resolution climate reconstructions. Hence, much of our present knowledge about climatic variability in the last millennium is based on tree-rings. However, many tree-ring records have a lack of data in the most recent decades, which severely hampers our ability to place the recent temperature increase in a longer-timescale perspective of natural variability.</p><p>The main objective of this thesis is to update and extend the Torneträsk TRW and MXD records in northern Sweden. Local instrumental climate-data is used to calibrate the new tree-ring records. The results show that TRW is mainly forced by temperature in the early growing season (June/July) while MXD has a wider response window (June – August) and has a higher correlation to temperature. Two reconstructions of summer temperature are made for (i) the last 7,400 years based on TRW, and (ii) the last 1,500 years based on a combination of TRW and MXD. The reconstructions show natural variability on timescales from years to several centuries. The 20th century does not stand out as a notably warm period in the long timescale perspective. A medieval period from AD 900 – 1100 is markedly warmer than the 20th century.</p><p>The environmental impact from a large explosive volcanic eruption in 1628/1627 BC is analysed in the tree rings of 14C-dated bog pines in south-central Sweden and in absolutely-dated subfossil pines from Torneträsk. The results show evidence of an impact in the southern site at approximately this time but no detectable impact in the North. </p><p>Subfossil trees of Fitzroya cupressoides in southern Chile were 14C-dated to approx. 50,000 years BP and amalgamated into a 1,229-year TRW chronology. This tree-ring record is the oldest in the world. The variability in this Last-glacial chronology is similar to the variability in present-day living trees of the same species. These results suggest that the growth–forcing mechanisms 50,000 years ago were similar to those at present.</p>

Among the most compelling and least well-understood tree species growing in the temperate forests of southern South America is Fitzroya cupressoides, a high biomass species and the second longest-lived tree species in the world. This thesis quantified the main components of the carbon cycle in Fitzroya forests (i.e. net primary productivity (NPP) and soil respiration) and evaluated the environmental variables that are most related to them. The study was focused on medium-age and old-growth forests growing in the Coastal Range (Alerce Costero National Park, AC) and the Andean Cordillera (Alerce Andino National Park, AA) of southern Chile, respectively. The specific objectives of this thesis were to: 1) assess the forest structure, species composition and characterise the environmental conditions of these forests; 2) assess biomass, aboveground NPP, carbon allocation and mean wood residence time in these forests; 3) assess soil respiration and relate it to soil environmental conditions. Additionally, to use a mass balance approach to estimate fine root productivity; 4) estimate total NPP using biometric and indirect estimates of productivity; 5) evaluate the climatic factors mainly related to Fitzroya stem radial change on an intra-annual basis; and 6) evaluate changes in Fitzroya's tree growth and carbon isotopes during recent decades, and determine which environmental factors are more related to them. The last two objectives focus on Fitzroya as the dominant species and the subject of this study. Two 0.6 ha plots were installed within each national park; NPP was estimated for a year and soil respiration and high resolution stem growth measurements were monitored over almost two years. Aboveground biomass estimates for the Andean site are among the most massive reported in the world and carbon fluxes in Fitzroya forests are among the lowest reported for temperate wet forests worldwide. The longevity as well as the particularly rainy and nutrient poor soil conditions where these ecosystems grow may influence their exceptionally slow carbon dynamics. Differences in carbon fluxes between sites seem most probably driven by different environmental conditions rather than by developmental stage. Moreover, carbon fluxes were more sensitive to interannual climate variability in AC than AA. Warmer and drier summer conditions, likely to become more common under future climate change, more significantly affected stem growth and soil respiration in the Coastal Range than in the Andes. Regarding long-term changes, tree growth has been decreasing in the coastal site in the last 40 years and increasing in the Andes since the 1900s. These trends have been accompanied by an increase in intrinsic water use efficiency which is likely caused by rises in CO₂ and changes in climate conditions in both sites. Although Fitzroya grows in particularly wet and cool areas, projected drier and warmer conditions may have a negative effect on Fitzroya stem growth and carbon sequestration in both study sites. This effect would be more critical in the Coastal Range though, because of its more Mediterranean climate influence and more restrictive soil conditions in this area. Adequate resources are needed for the monitoring and conservation of these slow growth and massive forests especially in the Coastal Range, in order to avoid ongoing illegal cuttings and threatening forest fires.

In the boreal forests of the Northern Hemisphere, time series of tree-ring width (TRW) and maximum density in the latewood (MXD) are highly correlated to local instrumental summer-temperature data and are thus widely used as proxies in high-resolution climate reconstructions. Hence, much of our present knowledge about climatic variability in the last millennium is based on tree-rings. However, many tree-ring records have a lack of data in the most recent decades, which severely hampers our ability to place the recent temperature increase in a longer-timescale perspective of natural variability. The main objective of this thesis is to update and extend the Torneträsk TRW and MXD records in northern Sweden. Local instrumental climate-data is used to calibrate the new tree-ring records. The results show that TRW is mainly forced by temperature in the early growing season (June/July) while MXD has a wider response window (June – August) and has a higher correlation to temperature. Two reconstructions of summer temperature are made for (i) the last 7,400 years based on TRW, and (ii) the last 1,500 years based on a combination of TRW and MXD. The reconstructions show natural variability on timescales from years to several centuries. The 20th century does not stand out as a notably warm period in the long timescale perspective. A medieval period from AD 900 – 1100 is markedly warmer than the 20th century. The environmental impact from a large explosive volcanic eruption in 1628/1627 BC is analysed in the tree rings of 14C-dated bog pines in south-central Sweden and in absolutely-dated subfossil pines from Torneträsk. The results show evidence of an impact in the southern site at approximately this time but no detectable impact in the North. Subfossil trees of Fitzroya cupressoides in southern Chile were 14C-dated to approx. 50,000 years BP and amalgamated into a 1,229-year TRW chronology. This tree-ring record is the oldest in the world. The variability in this Last-glacial chronology is similar to the variability in present-day living trees of the same species. These results suggest that the growth–forcing mechanisms 50,000 years ago were similar to those at present.

In the boreal forests of the Northern Hemisphere, time series of tree-ring width (TRW) and maximum density in the latewood (MXD) are highly correlated to local instrumental summer-temperature data and are thus widely used as proxies in high-resolution climate reconstructions. Hence, much of our present knowledge about climatic variability in the last millennium is based on tree-rings. However, many tree-ring records have a lack of data in the most recent decades, which severely hampers our ability to place the recent temperature increase in a longer-timescale perspective of natural variability. The main objective of this thesis is to update and extend the Torneträsk TRW and MXD records in northern Sweden. Local instrumental climate-data is used to calibrate the new tree-ring records. The results show that TRW is mainly forced by temperature in the early growing season (June/July) while MXD has a wider response window (June – August) and has a higher correlation to temperature. Two reconstructions of summer temperature are made for (i) the last 7,400 years based on TRW, and (ii) the last 1,500 years based on a combination of TRW and MXD. The reconstructions show natural variability on timescales from years to several centuries. The 20th century does not stand out as a notably warm period in the long timescale perspective. A medieval period from AD 900 – 1100 is markedly warmer than the 20th century. The environmental impact from a large explosive volcanic eruption in 1628/1627 BC is analysed in the tree rings of 14C-dated bog pines in south-central Sweden and in absolutely-dated subfossil pines from Torneträsk. The results show evidence of an impact in the southern site at approximately this time but no detectable impact in the North. Subfossil trees of Fitzroya cupressoides in southern Chile were 14C-dated to approx. 50,000 years BP and amalgamated into a 1,229-year TRW chronology. This tree-ring record is the oldest in the world. The variability in this Last-glacial chronology is similar to the variability in present-day living trees of the same species. These results suggest that the growth–forcing mechanisms 50,000 years ago were similar to those at present.