Boreal peatlands harbour large stores of carbon as peat below their surfaces. Climate change is expected to cause drying in northern peatlands, which will in turn impact the carbon balance of these ecosystems that is maintained by high water tables and the hydrologically sensitive plants growing there. This study aims to quantify how vegetation will be structured (I) and photosynthesize (II, III) in a future climate as emulated by long-term water level drawdown (WLD). To do this, changes in the vegetation and its photosynthesis after WLD are linked, and the response of Sphagnum mosses to periodic drought is investigated.
Field measurements were done at a long-term WLD field experiment that contained a rich (mesotrophic) fen, a poor (oligotrophic) fen and a bog (ombrotrophic) site. Measurements included vegetation surveys from existing permanent sample plots and leaf-level carbon dioxide exchange measurements. For an experiment in controlled conditions peatland surface cores from this field experiment were transported to a greenhouse where the photosynthesis of lawn Sphagna during and after an experimental periodic drought was measured.
The field study revealed that the response of peatland vegetation to WLD depend on peatland type. The species composition in the rich fen was the most impacted by WLD, while the bog vegetation demonstrated stability. Similarly, large increases in photosynthesis occurred following WLD on the vascular plant-covered rich fen, while changes were negligible on the Sphagnum-carpeted bog. The vegetation on the two fens shifted from an open sedge-, or sedge and Sphagnum-dominated ecosystem, to a tree-dominated ecosystem. Canopy development following WLD further accelerated vegetation changes by shading and sheltering the understorey vegetation. Vascular plants were the most likely to increase productivity from WLD as they are best suited to utilize the nutrients made available by peat mineralization, while Sphagnum moss photosynthesis was impacted little. The greenhouse study revealed that lawn Sphagnum mosses exposed to long-term WLD were more vulnerable to drought compared to those from wet sites. Large capitula typical to fen Sphagnum species appeared to be beneficial for surviving periodic drought.
This work demonstrated that climate change as emulated by long-term WLD will have a large impact on the vegetation composition of northern peatlands and increase photosynthetic function of these ecosystems, fens in particular. To better predict climate feedbacks from these changes, carbon dynamics including peatland vegetation dynamics should be updated in global process models. Future research to better understand the tipping point of different peatland types after WLD in different climatic regions will help us to predict changes in these diverse and globally important systems.
This dissertation studies the stand-level economics of continuous cover and rotation forestry. The main method of this dissertation is economic-ecological optimization, where statistical-empirical size-structured ecological models are coupled with economic optimization models including fully flexible optimization between continuous cover and rotation forestry. The dissertation consists of a summary section and three original research articles. The first article compares the favourability of continuous cover forestry between pure Norway spruce (Picea abies (L.) Karst.) and Scots pine (Pinus sylvestris L.) stands and studies the effects of ecological models on economically optimal solutions. The second article studies the economics of mixed-species stands with up to four tree species. The third article studies the economics of arctic forestry in the homeland region of the Sámi people using a model that simultaneously includes timber production, carbon storage, and negative externalities of forestry on reindeer husbandry.
According to the results, the differences in continuous cover forestry favourability between tree species are largely dependent on both species-specific differences in natural regeneration and natural regeneration differences between ecological models. Under realistic economic parameters, our model specification yields the result that continuous cover forestry is optimal for Norway spruce in both single- and mixed-species stands. In contrast, Scots pine favours rotation forestry in both single- and mixed-species stands. Physical overyielding of a species mixture does not reveal the economic preferability of that species mixture. In addition, we demonstrate that economically optimal continuous cover forestry avoids “high grading”, i.e. selective harvesting that leads to a completely different and economically inferior outcome. Including the negative externalities of forestry on reindeer husbandry into the economic model favours continuous cover forestry in arctic Scots pine stands. A carbon price between €14–€20 tCO2-1 is enough to imply that saving old-growth forests as carbon storages and reindeer pastures becomes optimal.
This dissertation examines how carbon storage in forests may be increased by changing forest management at the stand level. To extend the economics of forest carbon storage beyond single-species even-aged stands, this dissertation develops a bioeconomic model framework that incorporates the internal structure of the stand, and the optimal choice between continuous cover forestry and forestry based on clearcuts. The studies apply empirically estimated growth models for boreal conifer and broadleaf tree species.
The first article presents an analytically solvable economic model for timber production and carbon storage. Continuous-time optimal control theory is utilized to solve the thinning path and the potentially infinite rotation age: if no optimal finite rotation age exists, thinnings are performed indefinitely while maintaining continuous forest cover. The second article extends this model by applying a size-structured growth model for Norway spruce, a detailed description of revenues and costs, and several carbon pools. The timing and intensity of thinnings, the rotation age, and the management regime are optimized numerically. In the third article, the optimization approach of the second article is extended to mixed-species size-structured stands. Species mixtures include the commercially valuable Norway spruce and birch, and other broadleaves (e.g. Eurasian aspen and maple) that have no market value.
Optimal rotation age is shown to either increase or decrease with carbon price depending on interest rate and the speed of carbon release from harvested wood products. Given empirically realistic assumptions, carbon pricing increases the rotation period and eventually causes a regime shift from rotation management to continuous cover management. Hence, carbon pricing heightens the importance of determining the management regime through optimization.
Optimal thinnings are targeted to the largest size classes of each tree species. Carbon pricing postpones thinnings and increases the average size of harvested and standing trees. Without carbon pricing, commercially non-valuable other broadleaves are felled during each harvesting operation. When carbon storage is valued, some of the other broadleaves are retained standing until they are large, thus increasing tree species diversity and deadwood quantity.
The results suggest that moderate carbon price levels increase timber yields, especially of sawlog. Increasing carbon storage through changes in forest management is shown to be relatively inexpensive, and the marginal abatement cost is the lower, the higher the number of tree species in the stand.
This dissertation aims to develop the economics of even-aged Scots pine (Pinus sylvestris L.) management. In our economic-ecological model, a detailed process-based forest growth model is connected to an economic description of stand management. The process-based growth model is able to describe forest growth in management regimes and climate conditions previously not experienced, because it applies causal relationships and feedbacks instead of statistical correlations. Optimization is carried out with an effective general pattern search algorithm. The optimized variables include rotation length, initial stand density, and the timing, type, intensity, and number of thinnings. Essential model details include the quality pricing of timber and detailed harvesting cost functions. Integration of carbon subsidy systems into the model enables the determination of the economically optimal carbon storage with various carbon price levels. Finally, the growth model is extended to include a direct link between climate change and tree growth, to optimize stand management in a changing climate.
The dissertation thesis is composed of a summary section and three articles, which produce a coherent and comprehensive picture on the optimal stand management of Scots pine in the relevant growth conditions of Fennoscandia. The results demonstrate the necessity to simultaneously optimize all stand management variables, and the advantages of having a detailed model. Optimal stand management is shown to be sensitive to growth conditions, interest rate, and management objective, along with the design of the carbon subsidy system and the subsidy level. The stand-level analysis is additionally extended to the national level, and adapting forest management was found to potentially be a cost-efficient method for carbon abatement in Finland. Furthermore, the optimal adaptation of stand management in a changing climate remarkably improves the economic surplus from forestry.