Estimating water balance components of Pinus brutia and Cupressus sempervirens trees with observational tree sapflow and soil moisture data

Abstract

In the coming decades, the Mediterranean region is expected to be one of the areas most affected by climate change as models project a reduction in precipitation. It remains a question if some of the Mediterranean forests will be able to adapt and survive. Pinus brutia (pine) and Cupressus sempervirens (cypress) are two important forestry species in the Mediterranean region. Although they are both coniferous, they can have different strategies to cope with water stress. The objective of this study is to estimate water balance components of pine and cypress trees with observational tree sapflow and soil moisture data. The study site is located in Athalassa Forest Park, in Cyprus. The site has a surface area of 10 ha with an average slope of 4%. Average annual rainfall is 315 mm with a mean daily minimum temperature of 5° C during January and a mean daily maximum temperature of 37° C during August. The site was converted in 2011 from rainfed agriculture to a mixed forest by planting seedlings of different tree and shrub species. Hourly observations of sapflow (cm3) and volumetric soil moisture (%) from two pine and two cypress trees and surrounding soil were used for this study. Soil moisture sensors were installed under the tree canopy (0.4 to 0.9 m from the tree trunk), at the edge of the canopy (1.3 to 2.2 m from the tree trunk) and in the open area (midpoint between neighboring tree trunks, 2.6 to 3.5 m from the tree trunk). The sensors were installed at two opposite sides of each tree trunk, in the direction of the neighboring trees. Sensor depths were 10 cm, 30 cm and 50 cm, reaching a total of 60 sensors. Daily water balance calculations were made for the period 06/11/2020 to 29/06/2022 (20 months), in which total rainfall was 581 mm. The extent of the tree root zone area was estimated for different sets of assumptions. For a root zone depth of 60 cm and a root zone area radius of 2.2 m, transpiration amounted to 33.2% of the precipitation for one of the two cypress trees and 40.9% for the other tree, with losses (interception, soil evaporation and drainage) of 60.3% and 53.6% and soil moisture changes of 6.5% and 5.5%, respectively. The pine tree observations indicated a smaller root zone area. For a root zone depth of 60 cm and a radius of 1.7 m, the transpiration of the two pine trees amounted to 30.4% and 48.0% of the precipitation, losses were 60.6% and 50.7% and soil moisture changes were 9.0% and 1.3%. The effect of the different assumptions on the water balance components will be presented.This research has received financial support from the PRIMA (2018 Call) SWATCH Project and the Water JPI (Joint Call 2018) FLUXMED Project, both funded by the Republic of Cyprus through the Cyprus Research and Innovation Foundation. The PRIMA programme is supported by Horizon 2020, the European Union's Framework Program for Research and Innovation.