Implications of a dynamic vertical root distribution for modelled water and carbon fluxes across selected ecosystems : incorporating a novel rooting scheme into LPJ-GUESS
(2019) In Student thesis series INES NGEM01 20191Dept of Physical Geography and Ecosystem Science
- Abstract
- Roots play a key role in terrestrial carbon and water cycles, and therefore for the global climate system. They mediate plant evapotranspiration, influence photosynthetic processes and are responsible for atmospheric carbon transport into the pedosphere. Fine roots, in particular, are chiefly regulating water and nutrient uptake. Recent research has suggested that the representation of fine roots in ecosystem models may be too simplistic to accurately represent vegetation responses to predicted environmental changes. Hence, this thesis explores the implications of incorporating a dynamic vertical root distribution into a global dynamic vegetation model (LPJ-GUESS) for the modelled water and carbon fluxes. In contrast to the current static... (More)
- Roots play a key role in terrestrial carbon and water cycles, and therefore for the global climate system. They mediate plant evapotranspiration, influence photosynthetic processes and are responsible for atmospheric carbon transport into the pedosphere. Fine roots, in particular, are chiefly regulating water and nutrient uptake. Recent research has suggested that the representation of fine roots in ecosystem models may be too simplistic to accurately represent vegetation responses to predicted environmental changes. Hence, this thesis explores the implications of incorporating a dynamic vertical root distribution into a global dynamic vegetation model (LPJ-GUESS) for the modelled water and carbon fluxes. In contrast to the current static root representation in LPJ-GUESS, root fractions per soil layers dynamically adapt to permafrost, and to soil water conditions if plants are under water stress. The different scheme outputs are contrasted and compared to observational data for gross primary production (GPP) and actual evapotranspiration (AET) from 15 FLUXNET sites representing a selected set of (Sub-)Arctic, water limited, and non-water limited ecosystems. Furthermore, the sensitivity of the new scheme to precipitation input and root reallocation rate is examined. It was found that the new rooting scheme leads to differences in both modelled fluxes and can locally improve model accuracy with regards to the observational data. The total root-mean-square error (RMSE) for mean annual fluxes is reduced using the new scheme (GPP: 0.62 vs. 0.58 kg C m^-2 year^-1 and AET: 144 vs. 138 mm year-1). However, other sites and biomes were better represented by the static scheme. It is, therefore, crucial to analyse local results carefully as many input factors not directly determined by the root representation influence the accuracy of modelled fluxes (e.g. dominating plant functional types). It must also be distinguished between monthly and annual flux model accuracy. In Arctic sites with low plant productivity, the new initial root distribution and dynamic adaptation to permafrost do not considerably change modelled fluxes. Moreover, a dynamic adaptation due to water stress and availability alone may be too simplistic. Further development of the novel rooting scheme is therefore needed which is aggravated by limited data availability. (Less)
- Popular Abstract
- Roots are often overlooked – not only in the obvious sense that they are usually below-ground and therefore hidden from sight, but also in terms of the amount of scientific attention they have received in comparison with aboveground vegetation properties and processes. Yet roots and the domain within soils where they closely interact with their surroundings, the rhizosphere, play key roles in many ecosystem mechanisms. For instance, fine roots - commonly defined as roots with a cross-cut smaller than 2 mm - mediate plant transpiration (that is water which is lost from the plant to the atmosphere through evaporation) by being chiefly responsible for water uptake from the soil, there-by influencing global and regional water and climate... (More)
- Roots are often overlooked – not only in the obvious sense that they are usually below-ground and therefore hidden from sight, but also in terms of the amount of scientific attention they have received in comparison with aboveground vegetation properties and processes. Yet roots and the domain within soils where they closely interact with their surroundings, the rhizosphere, play key roles in many ecosystem mechanisms. For instance, fine roots - commonly defined as roots with a cross-cut smaller than 2 mm - mediate plant transpiration (that is water which is lost from the plant to the atmosphere through evaporation) by being chiefly responsible for water uptake from the soil, there-by influencing global and regional water and climate systems. They are further mainly in charge of nutrient uptake which is an important factor for plant growth. They transport carbon that was previously drawn from the atmosphere in the process of photosynthesis belowground, where under certain circumstances it can be stored for thousands of years. Consequently, they are also directly linked to the global carbon cycle and the vivid debate of how it will interact with the global climate in the future. Understanding and quantifying the role roots play and how they will, in turn, react to changing environmental conditions is therefore crucial.
One common approach to studying the often very complex mechanisms that govern plant growth and distribution are dynamic global vegetation models. The relationships between environmental variables, such as e.g. temperature and precipitation, with plant growth and behaviour are abstracted by mathematical approximations, usually derived from “real life” experiments. The competition for e.g. light and other resources between different plants is thereby simulated over time using computers. In this thesis, the representation of fine roots in one such model, the Lund-Potsdam-Jena General Ecosystem Simulator (LPJ-GUESS), is revamped. In the previous version, plant resource uptake from the soil is determined by the fraction of fine roots that is assigned to one of 15 soil layers. However, these fractions are static and thus stay identical over time. The new approach developed here makes the fraction of roots adaptive to environmental conditions. If a plant is under water stress, root fractions get shifted to soil layers where more soil water is available. Additionally, roots cannot be located in permanently frozen soil layers anymore. The model outputs for gross primary production (GPP, i.e. carbon assimilated through photosynthesis) and actual evapotranspiration (AET, i.e. evaporation plus transpiration) are contrasted with the old representation. Additionally, it is compared to observational data from 15 exemplary FLUXNET sites, chosen to represent a set of Arctic (mainly for the influence of permafrost), water-limited, and non-water-limited ecosystems.
The main finding of the work is that the vertical root distribution has an important influence on both output parameters. The overall error (RMSE, i.e. a measure of mean differences be-tween modelled values and observations) when considering the annual fluxes is reduced with the new root algorithm. However, while the error is reduced in some of the sites, other sites are better represented by the old root implementation. It is therefore very important to analyse individual locations and further to differentiate between monthly and annual model accuracy - an annual error reduction does not necessarily mean an improved overall model agreement. Due to the number of parameters that influence the output variables, it is difficult to draw general conclusions for specific ecosystems. In Arctic sites with low plant productivity, the dynamic adaptation to permafrost does not considerably change the modelled fluxes. Further development of the novel rooting scheme is therefore needed which would ideally be supported by additional observational root data. (Less)
Please use this url to cite or link to this publication:
http://lup.lub.lu.se/student-papers/record/8991174
- author
- Keetz, Lasse LU
- supervisor
-
- Paul Miller LU
- David Wårlind LU
- organization
- course
- NGEM01 20191
- year
- 2019
- type
- H2 - Master's Degree (Two Years)
- subject
- keywords
- Physical Geography and Ecosystem Analysis, Ecosystem Modelling, LPJ-GUESS, Vertical Root Distribution, Dynamic Root Distribution, Carbon and Water Fluxes
- publication/series
- Student thesis series INES
- report number
- 493
- language
- English
- id
- 8991174
- date added to LUP
- 2019-08-01 23:21:29
- date last changed
- 2019-08-01 23:21:29
@misc{8991174,
abstract = {{Roots play a key role in terrestrial carbon and water cycles, and therefore for the global climate system. They mediate plant evapotranspiration, influence photosynthetic processes and are responsible for atmospheric carbon transport into the pedosphere. Fine roots, in particular, are chiefly regulating water and nutrient uptake. Recent research has suggested that the representation of fine roots in ecosystem models may be too simplistic to accurately represent vegetation responses to predicted environmental changes. Hence, this thesis explores the implications of incorporating a dynamic vertical root distribution into a global dynamic vegetation model (LPJ-GUESS) for the modelled water and carbon fluxes. In contrast to the current static root representation in LPJ-GUESS, root fractions per soil layers dynamically adapt to permafrost, and to soil water conditions if plants are under water stress. The different scheme outputs are contrasted and compared to observational data for gross primary production (GPP) and actual evapotranspiration (AET) from 15 FLUXNET sites representing a selected set of (Sub-)Arctic, water limited, and non-water limited ecosystems. Furthermore, the sensitivity of the new scheme to precipitation input and root reallocation rate is examined. It was found that the new rooting scheme leads to differences in both modelled fluxes and can locally improve model accuracy with regards to the observational data. The total root-mean-square error (RMSE) for mean annual fluxes is reduced using the new scheme (GPP: 0.62 vs. 0.58 kg C m^-2 year^-1 and AET: 144 vs. 138 mm year-1). However, other sites and biomes were better represented by the static scheme. It is, therefore, crucial to analyse local results carefully as many input factors not directly determined by the root representation influence the accuracy of modelled fluxes (e.g. dominating plant functional types). It must also be distinguished between monthly and annual flux model accuracy. In Arctic sites with low plant productivity, the new initial root distribution and dynamic adaptation to permafrost do not considerably change modelled fluxes. Moreover, a dynamic adaptation due to water stress and availability alone may be too simplistic. Further development of the novel rooting scheme is therefore needed which is aggravated by limited data availability.}},
author = {{Keetz, Lasse}},
language = {{eng}},
note = {{Student Paper}},
series = {{Student thesis series INES}},
title = {{Implications of a dynamic vertical root distribution for modelled water and carbon fluxes across selected ecosystems : incorporating a novel rooting scheme into LPJ-GUESS}},
year = {{2019}},
}