Table of contents
- Plant hydraulics
- Photosynthetic response to droughts
- Mono-specific tree competition leads to power laws between density and biomass
- Peak grain forecasts for the U.S. high plains amid withering waters
- Resolving differences in canopy radiation interception between four species of pine
Plant xylem hydraulics and loss of function by embolism
Plants rely on conduits, miniature pipes made of dead cells, to transport water from the roots to the leaves (shown in the figure to the left). Plant water transport is impressively robust as it has stood the test of time for millennia but is also deceptively prone to failure. Even without a pump like our human hearts, plants lift water tens of meters against gravity . How they do it was a matter of heated debate for centuries until the Cohesion-Tension theory came along in the late 19th century. In a nutshell, it says that when some water molecules evaporate from leaves, others replace them to keep leaves hydrated. The new molecules pull others up using strong cohesive forces between the molecules made possible by their peculiar geometry. This pulling puts water under tension, the same way you can say a rope is under tension when you pull it tight.
This notion is not intuitive because rarely has any of us seen water under tension, when we try to pull on water ourselves, it falls back down and certainly doesn’t form a ‘rope’-like structure. This is because water hates (chemically) tension and, if given the opportunity, prefers letting air break the ‘rope’-like tension. But in a tree, air is hard to come by except when the tree is sick, is under attack, is freezing, or can’t find water in the soil. In this case, some of the plant conduits might attract air to break the water column and to form what are called embolisms.
My research focuses on plants when they struggle to find water in the soil, like after months of drought. In my first published paper on this topic (see CV & publications page), collaborators and I discerned what anatomical features, on the micro- and nano-meter scales, affect how plants lose their efficient water use abilities. In other words, what aspects of plant anatomy is most descriptive of how the whole organism, or one of its organs, responds to this lack of water. By respond, I mean how far can they go without water before giving in and letting air in, potentially jeopardizing the function of that plant or its organs. Doing so helps:
- bridging a gap between the endeavors of anatomists and those of ecologists,
- identifying advantageous anatomical features for different climate conditions, and
- understanding how microscopic phenomena, like water movement in vessels and air bubble formation, upscale to affect whole-plant health and vice-versa.
Currently, I’m working with several collaborators on developing a porous-media type theory and studying conduit grouping using ‘network’ or ‘graph’ theory.
Photosynthetic response to drought
One of the implications of drought is that plants cannot perform photosynthesis quite as well. But predicting how plant photosynthesis reacts to lack of water in the soil is under debate because, as of yet, we lack a complete physical or chemical description of how plants control their stomata. Stomata are pores on the leaf through which carbon dioxide enters and water leaves and plants can actively and/or passively control how open they are. From the trade-off between gaining CO2 and losing H2O stemmed the idea that there is an optimal stomatal opening allowing just enough CO2 in and just enough H2O out. This idea was first proposed in the 1970s by I. Cowan and G. Farquhar and is now referred to as the stomatal optimality principle. Such ideas are of great importance if we are to predict how our environment will respond to climatic change.
Cowan and Farquhar showed that this idea gave accurate predictions for plant photosynthesis under well-watered conditions. That, in itself, was unexpected because there is no physical or biological reason for the plant to follow any ‘optimal’ criterion, but it worked! Since then, various scientists all over the world have been building on this idea and applying it to various other conditions, like arid climates.
This is where my work comes in. Back in 2013, S. Manzoni and collaborators relaxed many of the assumptions on which the Cowan and Farquhar study was based. Collaborators and I built on the 2013 study to attempt to explain the range of different photosynthetic behaviors observed in water-stressed plant (the iso-, aniso-hydricity concept). Our work also elucidates how plants are expected to act under a competitive environment during a long-term drought. The article adds the following to the idea of photosynthetic optimality:
- It resolves the photosynthetic activity at a half-hour timescale
- It includes the effects of plant competition for water
- It provides the necessary means for the addition of extra constraints in new plant ecosystems
- It shows how the additions agree with data from a controlled experiment published elsewhere
Mono-specific tree competition leads to power laws between density and biomass
A curious observation of newly-seeded high seed density tree plots is that, as the trees get older and some of them die, the average tree biomass and the density of trees follow a power-law relation. A power law relation between two variables y and x is characterized by the following form: y = x^a, where a is known as the power law exponent. Identifying such a relation between tree biomass and density in a mono-specific stand could provide an important tool to better manage forests.
In the review that is now published (see CV & Publications), we synthesized the diversity of mechanisms that have historically been offered as explanations of the power-law form observed. That such a simple relation emerges from complex processes defining intense plant competition is a matter of wide interest in the fields of forestry and complexity alike. In fact, a ton of theoretical work is being undertaken to make sense of how the seemingly complex interaction between individuals (i.e., humans, trees, ants, even non-living things like roads) lead to the patterns we see in everyday like, and the problem studied here is no exception.
Our synthesis brings together eight previously proposed mechanisms:
- dimensional analysis and allometric (i.e., related to form) constraints,
- structural constraints,
- metabolic constraints,
- hydraulics constraints on growth,
- spatial constraints,
- a dynamical systems theory,
- size distribution arguments,
- and a neighborhood interaction argument (novel).
The last of those is a novel simulation that ties together much of the previous explanations, constraints, and arguments offered. Many of the constraints offered in the past have to do with the individual (2, 3, 4) and are then scaled up to the whole plot level. The dimensional analysis constraint (1) is a purely mathematical, and deceptively powerful, argument. The remaining arguments are rooted in the interaction among the trees with arguments 5, 6, and 7 more rooted in theory compared to 8 which is a brute force modelling effort that makes relatively more explicit and realistic assumptions.
Our hope was that this study will instigate more research of the same kind in the environment. It shows that it is possible to look for simple emergent dynamics even when the underlying explanations themselves are complex and tough to understand in their entirety.
Peak water as a harbinger for peak grain in the U.S. High Plains
In this article, we advance a dynamical systems approach that models the socio-hydrological system of groundwater withdrawal for crop production. To understand this system means to better be able to make informed decision about the future of food production in much of the world. More than half the human population today lives in countries reliant on heavily stressed aquifers. Groundwater is used worldwide to irrigate crops. In the United States, water from the High Plains aquifer (or the Ogallala) irrigates immense amounts of corn, wheat, and other grains. It forms the subject of our work.
Mining of non-renewable resources grows fast initially, peaks, then collapses. This happened in the case of crude oil production, analyzed in the 1960s in a seminal report by M. King Hubbert. The peak in oil production that occurred is now called ‘peak oil’. This peak occurs in the 1970s followed by a collapse. But technological innovations like horizontal drilling and hydraulic fracturing allowed the crude oil production to boom again in 2008.
Groundwater can be approximated as non-renewable but only in cases where the rate of groundwater recharge is low. Groundwater is recharged when it rains and the water percolates down to the water table, or when nearby streams or lakes lose some water to replenish the aquifer through the soil. Groundwater recharge is low in Texas and Kansas, but high in Nebraska, which is why we chose the High Plains aquifer as our region of interest. This region also enjoys extensive data reporting. Comparing these three states using the new approach revealed that crop production, like groundwater withdrawal, suffers from a peak way after peak groundwater withdrawal which we call ‘peak grain’. This lag in trends is a characteristic of systems sensitive to population dynamics (see Turchin’s work on historic trends of population vs warfare). This lag is larger when the aquifer has a longer lifespan.
This work is currently submitted; more details will appear here once it is published.
Resolving differences in canopy radiation interception between four species of pine