NSFDEB-NERC: Addressing the plant growth C source-sink debate through observations, experiments, and modelling
Lead Research Organisation:
University of Cambridge
Department Name: Geography
Abstract
Fossil fuel burning is causing atmospheric concentrations of the greenhouse gas CO2 to rise, the main driver of man-made climate change. However, the rate of CO2 rise is much slower than emissions suggest it should be. It appears that the land surface and oceans are together absorbing about 50% of annual CO2 emissions. Some field studies indicate that a large portion of the land surface uptake is due to increasing tree growth. However, the causes, locations, and future behaviour of this CO2 uptake remain highly uncertain.
A plausible hypothesis is that this land uptake is occurring because higher levels of CO2 increase plant photosynthesis, meaning more carbon in plants. However, a number of scientists believe that tree growth is not commonly limited by the rate of photosynthesis itself, but is instead controlled by other factors such as rates of cell division, nutrients, or water supply. If this is true, it implies lower future uptake of CO2 on land than is currently assumed, and so greater rates of climate change. Improving our knowledge of plant responses to CO2 is clearly essential for policy makers to be able to forecast with confidence the impacts of any controls on CO2 emissions on future climate.
When tree growth is limited by photosynthesis, we talk about 'carbon-limited' growth, whereas when it is limited by non-photosynthetic factors, it is 'sink-limited'. So, the extent to which trees are carbon-limited, and under which circumstances, is fundamental to understanding how they will respond to rising levels of CO2. Advocates of the importance of sink-limited growth point to findings of high concentrations of non-structural (storage) carbon observed in wood as evidence that carbon is abundant and not limiting, and thus growth is dominated by sinks, rather than by photosynthesis.
In this project, we propose to significantly improve our understanding of this fundamental issue using a unique combination of observations, experiments, and modelling. We will focus on mature individuals of white pine, red maple, and red oak growing in Harvard Forest, Massachusetts. This is an international collaborative project, with the modelling and detailed wood development work led by the University of Cambridge and the field and laboratory carbon measurement work led by Harvard University. The Harvard team will measure non-structural carbon concentrations and photosynthetic rates, and take microcores from tree trunks for wood development measurements at high temporal and spatial resolutions. These cores will be analysed, under the management of Cambridge, in a Swiss laboratory that is expert in studying cellular development in wood. These observations will enable us to determine the relationships between carbon sources and sinks over time.
In a highly innovative experiment, the Harvard team will also manipulate the supply of carbon to growing wood in our three experimental species at Harvard Forest by cooling the trees at particular points on their trunks. This cooling will be applied using cold collars, in which antifreeze will be circulated around the trunks of the experimental trees. Cooling will reduce the flow of sugars and we will conduct detailed measurements of the effects of changed carbon supply on wood development, and thus the extent to which growth is carbon limited.
At Cambridge, we will use these various measurements to develop a computational model of tree growth, which will be incorporated into a global model of the terrestrial carbon cycle. This model will then be used to assess the consequences of sink-limited growth for historical and future global land carbon uptake. This work has the potential to revolutionise our understanding of the role of vegetation in the global carbon cycle, the impacts of environmental change on plants, our interpretations of past climates as recorded in tree growth rings, and, because of the effect plants have on atmospheric CO2, our predictions of future climate change.
A plausible hypothesis is that this land uptake is occurring because higher levels of CO2 increase plant photosynthesis, meaning more carbon in plants. However, a number of scientists believe that tree growth is not commonly limited by the rate of photosynthesis itself, but is instead controlled by other factors such as rates of cell division, nutrients, or water supply. If this is true, it implies lower future uptake of CO2 on land than is currently assumed, and so greater rates of climate change. Improving our knowledge of plant responses to CO2 is clearly essential for policy makers to be able to forecast with confidence the impacts of any controls on CO2 emissions on future climate.
When tree growth is limited by photosynthesis, we talk about 'carbon-limited' growth, whereas when it is limited by non-photosynthetic factors, it is 'sink-limited'. So, the extent to which trees are carbon-limited, and under which circumstances, is fundamental to understanding how they will respond to rising levels of CO2. Advocates of the importance of sink-limited growth point to findings of high concentrations of non-structural (storage) carbon observed in wood as evidence that carbon is abundant and not limiting, and thus growth is dominated by sinks, rather than by photosynthesis.
In this project, we propose to significantly improve our understanding of this fundamental issue using a unique combination of observations, experiments, and modelling. We will focus on mature individuals of white pine, red maple, and red oak growing in Harvard Forest, Massachusetts. This is an international collaborative project, with the modelling and detailed wood development work led by the University of Cambridge and the field and laboratory carbon measurement work led by Harvard University. The Harvard team will measure non-structural carbon concentrations and photosynthetic rates, and take microcores from tree trunks for wood development measurements at high temporal and spatial resolutions. These cores will be analysed, under the management of Cambridge, in a Swiss laboratory that is expert in studying cellular development in wood. These observations will enable us to determine the relationships between carbon sources and sinks over time.
In a highly innovative experiment, the Harvard team will also manipulate the supply of carbon to growing wood in our three experimental species at Harvard Forest by cooling the trees at particular points on their trunks. This cooling will be applied using cold collars, in which antifreeze will be circulated around the trunks of the experimental trees. Cooling will reduce the flow of sugars and we will conduct detailed measurements of the effects of changed carbon supply on wood development, and thus the extent to which growth is carbon limited.
At Cambridge, we will use these various measurements to develop a computational model of tree growth, which will be incorporated into a global model of the terrestrial carbon cycle. This model will then be used to assess the consequences of sink-limited growth for historical and future global land carbon uptake. This work has the potential to revolutionise our understanding of the role of vegetation in the global carbon cycle, the impacts of environmental change on plants, our interpretations of past climates as recorded in tree growth rings, and, because of the effect plants have on atmospheric CO2, our predictions of future climate change.
Planned Impact
Who might benefit from this research?
Potential beneficiaries of this research include:
1) Policy-makers and politicians
2) Members of the general public
3) Members of groups traditionally under-represented in science
How might they benefit from this research?
1) Policy-makers will benefit from our project because the results will likely affect the outcome of climate change projections, providing greater scientific certainty for political negotiations and policies to mitigate and adapt to climatic changes. Our work will engage with policy makers through our links to the IPCC process via the ISI-MIP ("Inter-Sectoral Impact Model Intercomparison Project") and TRENDY ("Trends in net land atmosphere carbon exchanges") international collaborations. Our modelling within ISI-MIP has already been used in IPCC reports, and we intend for this to be reinforced in the coming years. Our HYBRID simulations have been used to examine the consequences of different levels of future climate change. Building on this work we are now, for example, using HYBRID to contribute to the COP-21 Agreement request for the IPCC to investigate the impacts of warming of 1.5 degC above pre-industrial levels through collaboration with colleagues at PIK. Our continued strong involvement with ISI-MIP and PIK will ensure that the outcomes of this project will feed through to policy makers.
2) Members of the general public will benefit from our project through our efforts to communicate our science and we will engage in a two-way dialogue with them. Through Science Pub Nights, print and video media, and a project web page, we will have the opportunity to share our stories, explain our fascination and passion for research, and transfer knowledge and understanding about fundamental topics such as "how do trees grow?" and "what does climate change mean for the forests of the future?"
3) Groups traditionally under-represented in science will benefit from our participation in the Harvard Forest Summer Research Program in Ecology, and our mentoring a total of six interns (two per year) in this program. The Summer Research Program has a phenomenal track record of attracting students from geographically, ethnically, and culturally diverse institutions, and cultivating the next generation of ecological scientists and educators. These interns will learn valuable career skills, will engage in hands-on participation in cutting edge research, and will be given responsibility for overseeing specific project tasks and objectives. The Harvard PDRA who mentors the interns will develop important skills related to supervision and personnel management.
Potential beneficiaries of this research include:
1) Policy-makers and politicians
2) Members of the general public
3) Members of groups traditionally under-represented in science
How might they benefit from this research?
1) Policy-makers will benefit from our project because the results will likely affect the outcome of climate change projections, providing greater scientific certainty for political negotiations and policies to mitigate and adapt to climatic changes. Our work will engage with policy makers through our links to the IPCC process via the ISI-MIP ("Inter-Sectoral Impact Model Intercomparison Project") and TRENDY ("Trends in net land atmosphere carbon exchanges") international collaborations. Our modelling within ISI-MIP has already been used in IPCC reports, and we intend for this to be reinforced in the coming years. Our HYBRID simulations have been used to examine the consequences of different levels of future climate change. Building on this work we are now, for example, using HYBRID to contribute to the COP-21 Agreement request for the IPCC to investigate the impacts of warming of 1.5 degC above pre-industrial levels through collaboration with colleagues at PIK. Our continued strong involvement with ISI-MIP and PIK will ensure that the outcomes of this project will feed through to policy makers.
2) Members of the general public will benefit from our project through our efforts to communicate our science and we will engage in a two-way dialogue with them. Through Science Pub Nights, print and video media, and a project web page, we will have the opportunity to share our stories, explain our fascination and passion for research, and transfer knowledge and understanding about fundamental topics such as "how do trees grow?" and "what does climate change mean for the forests of the future?"
3) Groups traditionally under-represented in science will benefit from our participation in the Harvard Forest Summer Research Program in Ecology, and our mentoring a total of six interns (two per year) in this program. The Summer Research Program has a phenomenal track record of attracting students from geographically, ethnically, and culturally diverse institutions, and cultivating the next generation of ecological scientists and educators. These interns will learn valuable career skills, will engage in hands-on participation in cutting edge research, and will be given responsibility for overseeing specific project tasks and objectives. The Harvard PDRA who mentors the interns will develop important skills related to supervision and personnel management.
Publications
Zhang X
(2021)
Dryness decreases average growth rate and increases drought sensitivity of Mongolia oak trees in North China
in Agricultural and Forest Meteorology
Friend A
(2019)
On the need to consider wood formation processes in global vegetation models and a suggested approach
in Annals of Forest Science
Zhang X
(2021)
High risk of growth cessation of planted larch under extreme drought
in Environmental Research Letters
Rademacher T
(2019)
Using Direct Phloem Transport Manipulation to Advance Understanding of Carbon Dynamics in Forest Trees
in Frontiers in Forests and Global Change
Eckes-Shephard AH
(2021)
Direct response of tree growth to soil water and its implications for terrestrial carbon cycle modelling.
in Global change biology
Rademacher T
(2021)
The Wood Image Analysis and Dataset (WIAD): Open-access visual analysis tools to advance the ecological data revolution
in Methods in Ecology and Evolution
Rademacher T
(2021)
Manipulating phloem transport affects wood formation but not local nonstructural carbon reserves in an evergreen conifer.
in Plant, cell & environment
Babst F
(2018)
When tree rings go global: Challenges and opportunities for retro- and prospective insight
in Quaternary Science Reviews
Rademacher T
(2022)
Insights into source/sink controls on wood formation and photosynthesis from a stem chilling experiment in mature red maple.
in The New phytologist
Chen Y
(2022)
Inter-annual and inter-species tree growth explained by phenology of xylogenesis.
in The New phytologist
Description | Identification of controls on wood growth, showing large impact of carbon supply. Results can be used to simulate tree growth in global vegetation models and so contribute to our understanding of the global carbon cycle and hence climate change. |
Exploitation Route | Many. We are continuing to use the results in our modelling work and publications will be forthcoming. |
Sectors | Environment |
Description | Postdoc Tim Rademacher coauthored a publication in the online outlet The Conversation, which describes the Witness Tree project and other related efforts to bring ecological research to the general public through "talking trees": https://theconversation.com/to-save-forests-researchers-are-hooking-trees-up-to-twitter-167649 Rademacher was on the Internet of Nature podcast "on creating the tree that is live tweeting climate change" (https://www.nadinagalle.com/podcasts/s2e1-tim-rademacher-on-creating-the-tree-that-is-live-tweeting-climate-change). Replenish Earth also interviewed Tim Rademacher about the Witness Tree as part of the London Climate Action Week in November 2020 (https://www.youtube.com/watch?v=l1dfdnGSUR0). |
First Year Of Impact | 2018 |
Sector | Environment |
Impact Types | Societal |
Title | Datasets and scripts used for the publication "Direct response of tree growth to soil water and its implications for terrestrial carbon cycle modelling" |
Description | |
Type Of Material | Database/Collection of data |
Year Produced | 2020 |
Provided To Others? | Yes |
URL | https://www.repository.cam.ac.uk/handle/1810/310720 |
Title | Density Anomalies in White Pine at Harvard Forest 2019 |
Description | The density of wood is a primary determinant of the amount of carbon sequestered in forests. For boreal and Mediterranean ecosystems, anomalies from the typical intra-annual density increase in radial growth of conifers are predominantly related to drought. We took 41 wood samples at breast height, ten additional samples near branches, and seven samples from the top of 41 white pines to determine the spatial distribution of density anomalies throughout the stem. We measured the ring width, density anomaly presence, position within a ring, and arc of density anomalies at multiple heights. Even in a mesic forest density anomalies in white pine are predominantly occurring during drier growing seasons. Moreover, we examined the spatial extent of density anomalies within the stems of white pines and discovered density anomalies are more likely to occur near branches, at the top of the tree, and in wider rings. Furthermore, the position of the density anomalies within the rings was remarkably consistent with the anomalies occurring roughly 80% into the fully formed ring at all heights for high-frequency years only as well as all years Density anomalies seem to be triggered by exogenous factors but their distribution within the stem varies along endogenous gradients, thus better understanding these systematic anomalies can help us to understand how wood formation has reacted and will respond to the environment and changes therein. |
Type Of Material | Database/Collection of data |
Year Produced | 2021 |
Provided To Others? | Yes |
Impact | N/A |
URL | https://portal.edirepository.org/nis/mapbrowse?packageid=knb-lter-hfr.347.3 |
Title | Impacts of Phloem Chilling and Compression on Mature Red Maple at Harvard Forest 2019 |
Description | Impacts of Phloem Chilling and Compression on Mature Red Maple at Harvard Forest 2019 |
Type Of Material | Database/Collection of data |
Year Produced | 2021 |
Provided To Others? | Yes |
Impact | Impacts of Phloem Chilling and Compression on Mature Red Maple at Harvard Forest 2019 |
URL | https://harvardforest1.fas.harvard.edu/exist/apps/datasets/showData.html?id=HF421 |
Title | Impacts of Phloem Chilling and Compression on Mature White Pine at Harvard Forest 2018 |
Description | Wood formation is a crucial process for carbon sequestration on land, yet how variations in phloem-transported carbon and temperature affect wood formation, respiration and nonstructural carbon pools remains poorly understood. To better understand the role of carbon supply on allocation to wood formation, we constrained phloem transport using compression and chilling around the stem of 15 mature white pines to monitor the effects of contrasting carbon supply (enriched above and reduced below the manipulations) on local wood formation and respiration, as well as on nonstructural carbon pools in stems and roots. This data set contains all data measured during the experiment. This includes wood anatomical, xylogenetic, dendrochronological, stem CO2 efflux, and nonstructural carbon measurements in coarse roots and stems, as well as pre-dawn water potential measurements of needles and branches. Furthermore, we provide basic allometric measurements for all trees. The code to process these data and reproduce our results is available at https://github.com/TTRademacher/Exp2018Analysis. For more details pertaining to the methods see Rademacher et al. (in review) and contact the investigator. |
Type Of Material | Database/Collection of data |
Year Produced | 2022 |
Provided To Others? | Yes |
Impact | N/A |
URL | https://portal.edirepository.org/nis/mapbrowse?packageid=knb-lter-hfr.422.1 |
Title | Nonstructural Carbon, Phenology and Wood Formation in Three Tree Species at Harvard Forest 2017-2019 |
Description | This data set comprises various observations and measurements across the 2017 to 2019 growing season for seven red maple (Acer rubrum), eight red oak (Quercus rubra), and six white pine (Pinus strobus) in the Prospect Hill Tract of Harvard Forest. The observations include spring and fall leaf phenology and basic allometry, such as diameter at breast height and height. For the leaf phenology, we followed the protocol from John O'Keefe (HF003). Measurements include wood growth data from weekly microcores and a three time characterisation of growing season nonstructural carbon concentrations (soluble sugars and starch) for stems and leaves. Additionally, stem CO2 efflux was measured once a month for the 2018 growing season and weekly for the 2019 growing season. |
Type Of Material | Database/Collection of data |
Year Produced | 2021 |
Provided To Others? | Yes |
Impact | N/A |
URL | https://portal.edirepository.org/nis/mapbrowse?packageid=knb-lter-hfr.361.2 |
Title | White Pine Girdling and Compression Experiment at Harvard Forest 2017-2019 |
Description | Wood formation is a crucial process for carbon sequestration on land, yet how variations in phloem-transported carbon affect wood formation, respiration and nonstructural carbon pools remains poorly understood. To better understand the role of carbon supply on allocation to wood formation, we constrained phloem transport using girdling and compression around the stem of 40 mature white pines to monitor the effects of contrasting carbon supply (enriched above and reduced below the manipulations) on local wood formation and respiration, as well as on nonstructural carbon pools in needles, stems, and roots. This data set contains all data measured during the experiment. This includes wood anatomy, xylogenesis, stem respiration, nonstructural carbon measurements in coarse roots, stems and needles, as well as pre-dawn water potential measurements and pressure measurements from underneath the phloem compression collars. Furthermore, we provide allometric measurements for all trees. The code to process these data and reproduce our results is available in hf348-14-density-anomaly-analysis-code.zip. For more details pertaining to the methods see Rademacher et al. (in review). |
Type Of Material | Database/Collection of data |
Year Produced | 2021 |
Provided To Others? | Yes |
Impact | N/A |
URL | https://portal.edirepository.org/nis/mapbrowse?packageid=knb-lter-hfr.348.3 |
Description | Expert panel following film screening (Cambridge Festival of Ideas). |
Form Of Engagement Activity | A formal working group, expert panel or dialogue |
Part Of Official Scheme? | No |
Geographic Reach | Regional |
Primary Audience | Public/other audiences |
Results and Impact | Film screening of "Before the Flood" on Friday 20th October as part of the Cambridge "Festival of Ideas". I was a member of the expert discussion panel, answering questions from the audience following the screening. The discussion was excellent and good feedback has been received about this event. |
Year(s) Of Engagement Activity | 2017 |
URL | https://www.festivalofideas.cam.ac.uk/events/sold-out-flood-pg |
Description | Witness Tree |
Form Of Engagement Activity | Engagement focused website, blog or social media channel |
Part Of Official Scheme? | No |
Geographic Reach | International |
Primary Audience | Public/other audiences |
Results and Impact | https://twitter.com/awitnesstree |
Year(s) Of Engagement Activity | 2018 |
URL | https://twitter.com/awitnesstree |