Friday, April 17, 2015

The man who planted trees -part I

It was a few years ago that I came across the beautiful and inspiring movie  “The man who planted trees”  (in italian here)[1] . 
Read it or watch to it, it is a pleasure of a eco-novel which I find particularly adapt to Spring time. 
The novel raises several eco-hydrological issues, and in particular it poses a question to me: does planting trees change so greatly the hydrological cycle (and the ecosystem) ?  

Ecosystem Services (from the hydrological point of view)

From another point of view, a modern way to ask the same  question would be: which kind of ecosystem services can be obtained with a careful management of the environment, and  when the hydrological cycle is positively managed ? And, what kind of ecosystems services are activated or dumped as a consequence of soil cover change ?

Ecosystem services cover a broad range of agents, but in this case, I would restrict the focus to  understand the interactions between waters and vegetation (and, possibly opened to consider the carbon fluxes). 

It is believed that vegetation can serve for natural hazard and water cycle regulation. There is a generic consensus that forest ecosystems play a significant role in the prevention of soil erosion.  Specifically by cutting surface run-off and storing water they decrease the effects of extreme weather events and natural hazards like floods, storms, avalanches and landslides. Also they are believed to have an action of filtering waters producing cleaner waters and providing  groundwater recharge.  But how much of these beliefs can be quantitatively assessed ? 

The questions can be moved from forests to agricultural landscapes, a provisioning service themselves, without changing much of the hydrological aspects. The spatial unit, in this case is the cadastral unit, or something similar to it. As well as forests, agricultural fields can contribute to the carbon budget, to water quality, especially when they are riparian. 

Again the question is: how can we quantify it, in order to guide landscape management, precision agricolture, and doing forecasts on the effects of changes of soil use (and BTW the impacts of climate change) ?

So, why do not ask to  hydrologists [2] ;-) what they can say about the effects of vegetation on the hydrological cycle ? 

To any hydrologist it is clear that the key hydrological effects are related to evapotranspiration, of which I discussed in several posts. Dealing with it  there are several aspects to account for, among which I name three:

- Canopy transpiration (which can be differentiated in several layers: for instance, grass and plants are different for the way they uptake water). Usually it depends on the height of canopy, leaf area index (LAI), root depth, phenology  and plant's functional  specific parameters. Now there is quite an amount of literature on those parameters, but its real robustness and applicability is unknown.
Plants are different, but  plant types are really so different or it does exists an underlying optimization principle which ever optimize the use of water resources and therefore evapotranspiration for an ecosystem ? Some experimental evidence is going in this direction: but it is clearly a matter of equilibrium and time scales. Among plants there are differences, but it is not certainly possible to say that, for example, pine woods transpire more than larches: too many factors are playing a role. Dimension, characteristic and locations of the trees play possibly a major role than tree species and, this case may be it is the tree specific phenology the major source of difference among trees that share the same landscape.

- Soil evaporation (from bare soil and from below the canopy). It could appear pretty simple with respect to the thermodynamics of plants. Somehow pretty simple. However, whilst it affects only the surface layer, it is controlled by the water potential gradient of the soil column if atmospheric demand is sufficient. As recent papers by Dani Or and coworkers showed.   So very it is very coupled with bottom conditions. Evaporation from soils,  with their own biology,  is not always less than transpiration. In many conditions could be more, and separating it from canopy behaviour is not as simple. 
However, also the interactions,  with the overlaying atmosphere cannot be given for easily estimated. Either for soils and plants.

- Therefore, it is necessary to considering  a better description of the Boundary layer turbulence (which can be treated at different levels of approximations, also depending on the considered time and spatial scales).
 Per se, ET is a flux, a molecular diffusion driven flux, that we have at the surface of soil, of leaves, of water, or when it comes from sublimation, of snow or ice. The flux obeys to the laws of irreversibile thermodynamics processes, and is commanded by gradients of chemical potential. However, ET, as treated in hydrology, is  lumped together with transport, and theoretically derived from conceptualisations of the fluxes conceived a few decades ago. So, even if not simple, better quantitative estimates could derive by addressing fluxes and transport separately and numerically, which could actually benefit both the description of  stomatal and soil resistances, and of aerodynamics.

When me move out from science to application of science, perception of science products is important, as remarked by the post here

[1] - Giono, J. - The man who planted trees - L’uomo che piantava gli alberi, 1953
[2] - And I asked in particular to Giacomo Bertoldi


  1. I used to use the English translation to teach English, beautifully simple. Maybe I was teaching, and learning, much more...