Going Back to the Roots: The Need to Link Plant Functional Biology with Vadose Zone Processes

Procedia Environmental Sciences 19 ( 2013 ) 379 – 383

1878-0296 © 2013 The Authors. Published by Elsevier B.VSelection and/or peer-review under responsibility of the Scientific Committee of the Conferencedoi: 10.1016/j.proenv.2013.06.043

Available online at www.sciencedirect.com

Four Decades of Progress in Monitoring and Modeling of Processes in the Soil-Plant-

Atmosphere System: Applications and Challenges

Going back to the roots: The need to link plant functional biology with vadose zone processes

Martine J. van der Ploega*, Adriaan J. Teulingb aSoil Physics and Land Management, Wageningen University, the Netherlands

bHydrology and Quantitative Water Management, Wageningen University, the Netherlands

Abstract

Soil plant atmosphere continuum concepts used in vadose zone hydrology approach plants as physical entities. This approach has proven very valuable in the past decades. The need to upscale such concepts provokes the question if plant functional biology should be considered, as larger scales also imply variation in (micro)climate and soil composition. Habitat manifestation is an expression of its evolutionary history and although the spatial distribution of habitats is largely driven by current climates, a soil’s water holding capacity and hence its formation over time may have played a role. Separate mechanisms involved in the soil plant atmosphere continuum are often understood and incorporated in numerical models for predictive purposes, yet interrelationships pose challenges, especially when such relationships cross traditional scientific disciplinary boundaries. In fact, the exact driver for root water uptake is itself subject of scientific debate, as there is no consensus on whether the driver for root water uptake is soil moisture content (e.g. [1]) or soil water potential (e.g. [2]). To evaluate soil water availability in relation to crop yield prognoses and the stability of natural vegetation, integrated concepts are sorely needed, especially in the perspective of climate change [3] and global water scarcity [4]. We present considerations and possible approaches for linking plant functional biology and vadose zone processes. © 2013 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Scientific Committee of the conference. Keywords: Roots, upscaling, ecosystems

* Corresponding author. Tel.: +31-317-483714 E-mail address: [email protected]

© 2013 The Authors. Published by Elsevier B.VSelection and/or peer-review under responsibility of the Scientific Committee of the conference

380 Martine J. van der Ploeg and Adriaan J. Teuling / Procedia Environmental Sciences 19 ( 2013 ) 379 – 383

1. Introduction

The last four decades have seen the development of a suite of conceptual approaches to model root water uptake in the vadose zone. To describe water flow through the soil-plant-atmosphere system, different conceptual approaches can be distinguished. An electrical analogue has been used [5, 6] where water flows from soil-root interfaces to a plant’s collar. Coupled with root architecture and a three-dimensional water flow model to represent the soil, has resulted in an intricate description of root water uptake based on hydraulic principles [7, 8]. Another approach is to add a transpiration based sink term to Richards’ equation [9, 10]. The sink term is linked to a macroscopic root parameter and a macroscale function based on root water uptake response to water potentials (e.g. [11]). Hybrid approaches include root density, root permeability and empirical root water extraction relationships [10]. More recently, attempts have been made to derive a macroscopic approach from an approximate analytical solution of a microscopic model [12-15]. These attempts have yielded a myriad of insight into root water uptake especially for agricultural crops. At the same time, it is clear that plants do more than act as a sink in vadose zone models. Processes in the soil plant atmosphere continuum are often modeled separately, yet the intricate relations between multiple processes are highly challenging, especially when these processes are studied in separate scientific disciplines. With the increasing awareness that global systems may influence local circumstances, and vice versa (e.g. [16]), the need to understand and upscale root water uptake of complete ecosystems is clear. Currently, models used in simulation of the terrestrial water cycle have rather crude representations of root water uptake [17]. In particular, parameters that control root water uptake and plant transpiration are assumed to be a property of the plant functional type, and are assigned accordingly. This procedure does not allow for local plant adaptation to be reflected in the parameters, nor does it allow for correlations that might exist between root parameters and soil type. Here, we explore the experimental evidence from natural vegetation to formulate possible alternative modeling concepts.

2. Plant’s relation with environment

Larger scales imply variation in (micro)climate and soil composition, and as a result species have adapted to local conditions [18]. Correlations between water availability and species distribution were already recognized by the turn of the last century [19]. Differences in drought sensitivity shape tree and shrub distribution in tropical forests at local and regional scales [20]. Sperry and Hacke [21] showed that desert species in sand employed clear water-use strategies that corresponded with root-depth index, cavitation resistance, vegetative phenology and seasonal regime in water potential, while strategies of desert species in loam were less clear, probably as a result of the more favorable soil water characteristic of loam soils (same water content at lower potentials compared to sand). Plants may exhibit adaptivity as a plastic trait depending on environmental conditions, as shown for African savannah grasses [22] and an alpine perennial herb [23], or even develop habitat specific, symbiotically-conferred stress tolerance [24].

Plants scan their environment by producing small GTPases, second messengers and a thousand protein kinases, resulting in a myriad of internal information that specify its ecological niche [25]. It is becoming increasingly clear that root-sourced signals appear to play a key role in regulating stomatal aperture in response to soil water availability [26]. Constant exposure to environmental stresses, biotic or abiotic, influences plant physiology, gene adaptations, and flexibility in gene adaptation [27-31]. Transgenerational increased genomic dynamics were found in Arabidopsis thaliana for ultraviolet-C and flagellin [32]. Such mechanisms possibly explain drought resistivity between genotypes of a species (Fig. 1). In the light of all the behavioral adaptations of plants described above, describing root water uptake at larger scales warrants the inclusion of adaptive concepts.


Fig. 1. Example of differences in drought response by genotypic variation.

Fig. 2. Conceptual evapotranspiration patterns with and without optimized plant’s root water uptake.


3. Possible approaches

From an evolutionary perspective plants that inhabit their optimal ecological niche will have optimized their adaptation to local conditions. Species that are subjected to the same climate, but grow in different soils may have optimized their root water uptake for that particular soil. Hence, comparing two conceptual approaches (Fig. 2), (evapo)transpiration modeled with uniform root parameters will yield differences for the loam and the sand soil, while the concept accounting for local optimization of root parameters to soil water availability may even yield a uniform evapotranspiration. Other approaches to include parts of plant adaptive behavior in models are found in literature. Incorporation of plant root cooperation in a model showed improvements in drought resistivity [33]. With an adaptive finite element method Wilderotter [34] modeled a root system that elongated in response to soil water pressures. At the same time it is likely that rooting depth will be as shallow as possible for any biome, and root parameters can be modeled rather than assigned from databases [35, 36]. The need to come up with a more holistic approach is of course counterbalanced by the availability of data of the soil-plant-atmosphere system for complete ecosystems. Nevertheless, the knowledge on root water uptake already accumulated within the vadose zone community warrants development of robust concepts in the future, and may provide a solid base for vadose zone approaches in land surface modeling.

Acknowledgements

Part of this research was funded by EU project soilCAM (FP7, Environmental Technologies contract number 212663), and The Netherlands Organisation for Scientific Research through Veni Grant 016.111.002.

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Going Back to the Roots: The Need to Link Plant Functional Biology with Vadose Zone Processes