Estacion Experimental Forestal Horizontes – a paradise for ecohydrologists (but not only those)

The ‘Estacion Experimental Forestal Horizontes’ (EEFH, https://www.acguanacaste.ac.cr/biodesarrollo/programa-de-restauracion-y-silvicultura/estacion-experimiental-forestal-horizontes), located in Guanacaste, the driest region of Costa Rica (~ 1700 mm/y mean precipitation and about six months without any drop of rain). The station was established in order to investigate and manage the valuable dry forests, that are often recognized less valuable compared to rainforests – which is complete nonsense. In fact, many of the most valuable trees grow in dry forests. The people of Horizontes have dedicated themselves to protect and recover those valuable forests; and eventually cultivate some of the really rare tree species that are highly endangered (such as Guaiacum sanctum L. or Cordia gerascanthum L.). Milena Gutierrez, the director of the EEFH, is a lovely, dedicated person who has a vision: Establishing EEFH as a permanent research venue and create a long-term sustainable management plan for the dry forest.

I would have never imagined that in Costa Rica, the country that is known for its large, partially pristine rainforests. Even more crazy is that many of the trees dropping their leaves start to leave-out in the driest month of the dry season – April – where not a drop of rain was falling. Even the species we are studying were replacing their leaves in a very short period. The question is how, and why?

I start may day with a meditation, check my mails (the early morning is pretty much the only time with good internet) and the head to the ‘Caseta’ – the little wooden equipment hut we have built in the middle of our research plots. I like the silence, the early mornings – this is the time where my mind works best. I am surprisingly happy and calm while walking over – proud on what we have achieved during the last few months as a team.

The goal of this campaign was to monitor precisely – on the ground with highly resolute measurements of water stable isotopes and ecohydrological methods and from above with drones – what happens in those trees that are able to keep their leaves and transpire during the dry season. Therefore, we equipped lateral and deep roots as well as the trunk with ‘stem boreholes’ for measuring water stable isotopes in situ ([see former blog from Kathi]({% post_url 2019-02-05-testing-stem-borehole-method %})). We further installed sap probes to measure sap flow velocity in those tree compartments. We also established an extensive soil monitoring network (TDR probes, matric potential sensors and soil gas probes for water isotopes at two plots at 7 depths each) and installed a weather station (including isotope sampler).

As for the ‘above’ part, a Matrice 210 drone equipped with a thermal and multispectral camera were used to obtain highly resolute images three times a week. Leave temperature sensors were installed in the canopy, pre-dawn and midday leaf water potential was measured and LAI determined frequently (for validation of the drone-derived information and correlation analysis).

It is not easy to investigate deep roots and deep water uptake – and I never said that. But what it really means, we experienced during those months. Constructing, climbing, digging, excavating, soldering things together, building sensors – there are so many things determining our everyday life here that have nothing to do with what somebody would think of when they hear ‘researcher’. It feels like coming back to the ‘real’ world from the sometimes abstract and weird reality of science. Such experience – in my opinion – belong to the education of any PhD student and is essential for keeping your feet to the ground. For the Isodroners’ (Kathi, Malkin and me) and even more for our hardcore field crew, Adrian and Alberto, this is just great! Even John (Marshall) has his big smile back on his face after three days with us in the field (even though part of that can be attributed to John’s first surf session ever we had together). We decided to listen to those locals at Horizontes when selecting our target tree species. Caoba (a type of Mahogany) and Tempisque (locally called ‘The guard of the river’) are two evergreens that somehow seem to manage to stand the six months of zero rain; and we have no idea how they do that. Will they share their secrets with us? We chose two plots of 250 m2 and 150 m2, each one with abundant individuals of each of the two target species. In the middle, we dig ‘The agujero’ (‘The hole’), a four meter long, two meter deep and one meter wide pit at the separation of the two plots (Note of the author: I spare hardcore scientific details here, those can be found in the upcoming publications ;)).

Consequently following Murphy’s law, we encounter volcanic bedrock starting at about 1,5m soil depth, making it impossible to dig deeper than two meters (even with a gigantic excavator – check the pics for a proof). Not really the depth we anticipated, given the target of measuring ‘deep roots’! But wait, what do I see there in the middle of (non-fractured) bedrock – little roots coming out into the pit, even at two meters where it is hard as concrete!!! Relief – deep roots do exists. But how about the mother of all deep roots, that tap root? – Gently excavating around each three of the selected species, we unveil the root system. Ultimately, we have to dig under the tree to find what we were looking for. In terms of tap roots, I can say: Yes, we find something that tries to tap deep. But rather than going straight down, ‘our’ tap roots wind through the compacted and rocky soil, definitely penetrating downwards. And something we can equip with our sap sensors, tree boreholes and other sensors. The complete setup looks like a spaceship, and not only once I am asking myself what the hell are we doing here. It took about one and a half month to install everything, and more than once we were desperate, but: Finally, everything is running! We now are able to measure 16 soil gas probes and 18 stem boreholes on one day, giving us the chance to obtain a dataset that has never (to our knowledge) been recorded previously. Together with all the additional plant and soil related information, this is just great. Will the drone data be able to forecast or identify deep water uptake? How much deep water are our measured individuals using during the dry season? Too be continued…