Optimal Carbon Storage in Mixed-Species Size-Structured Forests

Aino Assmuth,Olli Tahvonen,Janne Rämö

doi:10.1007/s10640-021-00559-9

Abstract

We extend the study of economically optimal carbon storage to a previously unexplored forest type, mixed-species size-structured stands. The ecological model applied in the study is a transition matrix model with growth functions for boreal Norway spruce (Picea abies (L.) Karst.), birch (Betula pendula Roth and B. pubescens Ehrh.), and other broadleaves. The other broadleaved trees are assumed to have no commercial value. We maximize the sum of timber revenues and the value of carbon storage by optimizing the timing and intensity of thinnings and the potentially infinite rotation age. The optimization problem is solved in its general dynamic form using gradient-based interior point methods and a genetic algorithm. We present results for a mixed stand of Norway spruce and birch, and a mixed stand of Norway spruce, birch, and other broadleaves, and compare these to a pure Norway spruce stand. We show that carbon pricing increases stand volume by postponing harvests and limiting them to larger trees, and changes the optimal species composition by increasing the share of Norway spruce relative to birch. Further, carbon pricing incentivizes maintaining other broadleaves in the stand despite their lack of commercial value, thus increasing tree species diversity. We find that sawlog and total yields increase with carbon price. We show that the higher the number of tree species in a stand, the lower the marginal cost of carbon storage.

Highlights

Forest sinks, currently sequestering approximately 30% of global emissions, are likely to play a crucial role in limiting global warming to 1.5 °C by the end of the century (Pan et al 2011; Walsh et al 2017)
The beginning of thinnings is postponed by carbon pricing and hastened by a larger number of tree species naturally regenerating on the site (Table 1)
The stand-level economics of carbon storage has heavily focused on single-species evenaged forestry

Summary

Introduction

Currently sequestering approximately 30% of global emissions, are likely to play a crucial role in limiting global warming to 1.5 °C by the end of the century (Pan et al 2011; Walsh et al 2017). The value of carbon storage may well exceed that of timber production and other provisioning services (Chiabai et al 2011) Management that supports structurally diverse stands may become necessary even in areas currently dominated by single-species even-aged stands (Dymond et al 2014). Economic research on carbon storage in structurally diverse stands is very scarce. This study presents a dynamic bioeconomic model, where the combined production of timber and carbon storage in boreal mixed-species size-structured stands is optimized using an empirical growth model and a detailed economic setup

Methods

Results

Conclusion