From branch to forest to globe. How do tree strategies regarding wood growth and turnover affect forest responses to increased carbon dioxide levels?

van Wijngaarden, Klaske ORCID: 0000-0001-8255-491X (2025). From branch to forest to globe. How do tree strategies regarding wood growth and turnover affect forest responses to increased carbon dioxide levels? University of Birmingham. Ph.D.

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Abstract

Forests make a vital contribution to the terrestrial carbon (C) sink, by assimilating more C from the atmosphere than is respired back, utilising the available C in both short-lived and long-lived biomass. The role of rising CO2 levels in the atmosphere for the Earth’s radiative balance and climate change has intensified research into the long-term stability of the forest C sink. While CO2 fertilisation enhances growth in young trees, it is also hypothesised to accelerate tree mortality, with canopy dieback and subsequent branchfall acting as precursors as stand age advances. To improve understanding of the role of smaller woody material (branches, barks and twigs) in these dynamics, this thesis examines the effects of elevated carbon dioxide (eCO2) levels on wood distribution within the canopy of mature oak-dominated woodland. It compares woody turnover, defined as the faction of total woody biomass in the canopy lost throughbranch and twig litterfall over time, in two mature forests exposed to eCO2. This research was conducted across oak woodland and phosphorus-limited eucalypt forests using terrestrial laser scanning (TLS), allometric scaling, transect intercept sampling, and traps to measure litterfall. The oak woodland and the eucalypt forests were exposed to eCO2 in Free Air Carbon Enrichment (FACE) facilities, running for four and ten years, respectively.

Chapter 2 explores the impact of eCO2 on the distribution of branch biomass within the canopy of oak (Quercus robur) trees of the BIFoR Free-Air Carbon Dioxide Enrichment (FACE) Experiment in the United Kingdom. It was hypothesised that alleviating C limitation on growth would present itself in increased light foraging. The branch biomass in the canopy was quantified by a single time point Terrestrial Laser Scanning campaign at the start of 2022. Results indicate a significant increase in upper canopy volume, branch volume, and fine branch density in the upper canopy in response to four years of eCO2 exposure preceding these measurements. This supports the hypothesis that additional C availability, resulting from recently published photosynthetic enhancement, is partly directed towards further optimising light capture by trees. This chapter underscores the importance of considering small woody material in predicting mature forest responses to changing atmospheric conditions. Chapter 3 delves into branch and twig litterfall dynamics in an oak-dominated forest under eCO2. The expectation was that the branch and twig litterfall would increase with previously published observations in increased stem growth and observed branch density in Chapter 2.

Despite an observed increase in leaf litterfall, there was no increase in twig, small and larger branchfall evident. Using the branchfall observation to calculate the percentage of branch biomass in the canopy lost over time also showed no significant enhancement in woody turnover after four years of treatment. This finding highlights the complex dynamics of forest ecosystems and the need for ongoing research into woody material turnover after longer exposure to eCO2. Chapter 4 examines branchfall and litterfall in two mature forest stands exposed to eCO2, which exhibit diverse responses of stem growth to eCO2 treatment. It was theorised that the mature forests would be in a steady state and branch turnover would follow previously observed or not observed growth response to eCO2. In addition, the eCO2 response of understory and dominant species wood turnover was expected to differ due to varying growth-mortality trade-offs. Moreover, effect size was expected to scale with exposure time, with eCO2 effect size most prominent in small-sized branch turnover. Branchfall at BIFoR was monitored over 12 months between 2022 and 2023 and was compared to observations on branchfall in the eucalyptus FACE (EucFACE) experiment in Australia between 2023 and
2024. Contrary to expectations, increased branch or twig litter fall was not observed in the oak forest despite increased stem growth under eCO2. In contrast, the eucalypt forest exhibited positive effect sizes for branch and litterfall across multiple size classes under eCO2 despite the lack of a treatment effect on stem growth. This did not disprove the hypothesis that the effects of eCO2 on wood turnover may be more pronounced over longer exposure periods.

Overall, this thesis reveals complex responses of mature forests to eCO2, with implications for understanding forest dynamics, future carbon sequestration, and the potential for increased carbon capture through light foraging strategies. The findings suggest for the two experimental forest sites examined that increases in wood biomass due to eCO2 do not directly translate into increased woody turnover or that such effects may be delayed longer than the ~5-10-year duration of these experiments. However, in one of the two studies (EucFACE in Australia), woody turnover increased even though stem biomass increase had not occurred after ~10 years of eCO2 exposure. A notable finding was that branches larger than 3 cm in diameter constituted a sizeable proportion of total turnover production under eCO2 and ambient conditions, particularly in the eucalyptus Forest. Branches this size are rarely quantified and constitute a ‘blind spot’ in most forest ecosystem experiments. Taken together, the results of this thesis point to a significant role of wood growth and woody turnover of the smallest size classes in the responses of mature forest ecosystems to elevated CO2 levels. This emphasises the need to reassess predictions of carbon sequestration benefits of forests under future climate scenarios.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):

Supervisor(s)	Email	ORCID
Smith, Benjamin	UNSPECIFIED	orcid.org/0000-0002-6987-5337
Medlyn, Belinda	UNSPECIFIED	UNSPECIFIED
Larsen, Joshua	UNSPECIFIED	orcid.org/0000-0002-0650-7369
Pugh, Thomas	UNSPECIFIED	orcid.org/0000-0002-6242-7371