Seminar: Simple model approaches

Patterns in survival-enhancing mechanisms derived from plant water storage

Yair Mau - Senior Lecturer at the Department of Soil and Water Sciences, The Hebrew University of Jerusalem

Internal water storage is of crucial importance for plants under water stress, allowing them to temporarily maintain transpiration higher than root-uptake rate.

With the help of a minimalistic model, Yair Mau and his colleagues investigate the most basic effects that result from the water storage buffering, among them: the time lag between peak daily transpiration and sap flow; the typical reaction time of fluxes to sudden changes in environmental conditions; increased hydraulic safety margin from xylem embolism; and the frequency filtering in the sap flow and water storage recharge.

The analytical results derived for each of these phenomena provide insight into the pivotal role of the capacitance in plant survival under water stress. Yair Mau showcase the predictions and their use by evaluating a model against transpiration and sap flow measurements of a semi-arid pine forest, determining whole-plant and ecosystem-wide characteristics, such as the hydraulic capacitance and hydraulic conductivity.

The potential and limitations of SOC decomposition theories

Speaker: Lorenzo Menichetti - Researcher at the Department of Ecology, Swedish University of Agricultural Sciences

Soil organic carbon (SOC) represents nearly 80% of the total C stored in terrestrial systems. Therefore, understanding its dynamics and response to human actions is crucial for developing effective climate change mitigation strategies. Such dynamics depend on the C coming into the soil through photosynthesis and the C leaving the soil through decomposition.

At least in aerobic conditions, which we find in most soils globally, organic matter decomposition follows relatively simple kinetics over time, influenced mainly by temperature and soil water content which drive microbial activity. Although decomposition is a complex process with many biochemical steps, ecosystems tend naturally to maximize resource utilization efficiency, and the process usually follows its upper limits (the fundamental constraints of microbial growth).

We can already describe most of the variation over time of soil C with a combination of relationships discovered long ago. The fundamental equation used by almost any SOC model was developed in the 1960's and is still the theoretical foundation upon which most SOC decomposition models are built and still does a decent job.

This fundamental "law" of decomposition assumes that microbial biomass will always grow to saturate the available resources, limited by the speed at which organic matter can decompose over time. This defines a linear relationship: the decomposition flux is proportional to the mass of organic matter times its decomposition rate. This relationship emerges from a highly complex ecosystem with thousands of microbial species cooperating and competing.

Reality is more complex, and this relationship cannot describe all the variance of the process over time (although it comes pretty close). In the last two decades, the scientific community working on the problem started to focus on the limits of such a theory. Questioning the linear decomposition assumption has led to experimenting with nonlinear decomposition models, where the decomposition rate or kinetic k is no longer constant but varies over time as a function of different variables.

These models are far more complex objects to deal with than linear ones and have different characteristics. The scientific debate in the field of SOC decomposition models is nowadays exciting, continuously oscillating between these two paradigms: linear or nonlinear decomposition. Both paradigms have pros and cons and are more suited to different purposes. When considering broad temporal and spatial scales (years and whole countries), the linear paradigm is still rather effective.

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