Evaporation partitioning of forest stands: The role of forest structure

Forest evaporation (Ei) is considered the main source of water vapor at a continental scale. Its quantification has been carried out in many ecosystems worldwide, applying the classical partitioning method to differentiate among sources of water vapor. This partitioning differentiates between transpiration (Et), soil evaporation (Es), water intercepted by plant and ground surfaces (Ei), and open water evaporation (Es) in flooded forests and mangroves. The partitioning of evaporation has been carried out by applying different methodologies such as eddy-covariance, conventional micro-meteorological measurements, stable water isotopes, and the combination of some of these methodologies. However, the classical partitioning approach can have large uncertainties in specific forest ecosystems as a consequence of the canopy structure. Instead, including canopy structure into the evaporation partitioning allowed us to better understand this flux. Forest canopy structure is difficult to assess and is determined by latitude, altitude, water availability, and growing stage of the forest. However, using the canopy layering (overstory, understory, ground layer, and forest floor layer) we can assess the contribution from the structural point of view.

Forest succession is one factor affecting the classical partitioning in Tropical Deciduous Broadleaf Forest. Using cumulative daily collectors in three different stages of Tropical Dry Forest in Costa Rica, we were able to depict how the increment in forest complexity affects the interception of precipitation. Also, the Plant Area Index was the only structural parameter significantly correlated with the estimates of both, interception and effective precipitation. The capacity of the other parameters (e.g., tree densities, tree heights, number of species) was not enough to describe the effect of a growing forest on the interception of precipitation.

Tropical forests with less water stress during the dry season allocate more biomass to their canopies. This increases the forest complexity in terms of the number of species, canopy height, and plant types. Tropical Evergreen Broadleaf forests have a more complex canopy structure than the Deciduous ones. The tropical wet forest in Costa Rica has a canopy of 45 m height and a large number of plant species including trees, lianas, palms, and bushes that provide a completely different canopy structure than mono-specific forests. Here, we were able to define three canopy layers according to canopy height (overstory, lower and upper understory) and monitor the evaporation process during one dry season. Applying conventional micro-meteorological measurements we were able to determine that the lower and upper understory layers contributed 9 % and 15 % of the evaporation, respectively. Meanwhile, the use of water stable isotopes did not allow us to determine the contribution of transpiration using the keeling plot method. However, the signatures of the stable water isotopes allowed us to determine that the source of water used by the plants depends on its type (liana, tree, palm, or bush). Also, we quantified the evaporation during precipitation events as one-third of the amount measured during dry sunny days. The proportion did not change during rain events per canopy layer. This water vapor was produced by the "splash droplet evaporation" process, that together with the energy convection and low air temperature produced the visible vapor plumes. We were able to identify the conditions during which the visible vapor plumes can be spotted. These conditions are the presence of precipitation, air convection, and a lifting condensation level at the top of the canopy with values lower than 100 m.

Plants growing in arid environments developed strategies that help them to cope with the scarcity of water. Usually, these plants grow lumped in patches and the introduction of tree species to fight desertification changed the landscape introducing a forest-like land cover. In a Temperate Shurbland in China, we evaluated the effect of Willow trees (Salix matsudana) and Willow bushes (Salix psammophila) on the soil water after summer. Using stable water isotopes we identified the redistribution of groundwater beneath the plants through the hydraulic lift process.

Mono-specific forest ecosystems such as the Temperate Evergreen Needleleaf Forest may modify the micro-meteorological conditions beneath their canopies. In Speulderbos, we monitor the evaporation process through eddy-covariance and stable water isotope techniques in a Douglas-Fir (Pseudotsuga menziesii) stand. Also, the evaporation process in the forest floor layer was analyzed in detail under laboratory conditions. Different forest floor layers evaporates up to 1.5 mm d-1, differing from field conditions, where the evaporation from these layers do not exceed the 0.2 mm d-1. This evaporation, represents only the 5.5 % of the total measured during the monitoring period. However, there is no evidence that the forest floor evaporation move upwards to contribute to the total evaporation measured above the overstory. This was confirmed by the eddy-covariance footprint and stable water isotopes signatures of the air measured continuously on the forest. Finally, the partitioning of evaporation based on canopy structure is suitable for complex ecosystems with a large number of species and a multilayered canopy. This leaves the classical partitioning for more homogeneous ecosystems where it can be carried out with a smaller monitoring investment.