Transpiration rates of pine (Pinus brutia) and cypress (Cupressus sempervirens) trees in a Mediterranean mixed plantation forest

Mixed species forest stands have been advocated over monoculture stands for afforestation around the globe as they can be more resilient to pests and diseases. However, in arid and semi-arid regions, whether such forests are suitable for future climate conditions remains to be addressed. The overall objective of this study is to analyze ecohydrological processes of indigenous, drought-tolerant tree species in a Mediterranean mixed plantation forest. The specific objectives are i) to quantify transpiration of pine (Pinus brutia) and cypress (Cupressus sempervirens) trees and ii) to analyze the effects of environmental variables (meteorology and soil moisture) on transpiration. 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 winter and a mean daily maximum temperature of 37° C during summer. The site was converted in 2011 from rainfed agriculture to a mixed forest by planting seedlings of different tree and shrub species. The study site is located on two sedimentary formations: Athalassa (calcarenites interlayered with sandy marls) and Nicosia (siltstones and layers of calcarenites). Soil depths up to 1 m can be found on top of the impervious and semi-pervious strata. 

The research field was stratified in two spatial geological units (strata). In each stratum, two P. brutia and two C. sempervirens trees were randomly selected (total eight trees) for sap flow monitoring with sensors (heat ratio method) attached to the tree trunks. In addition to the random trees, two representative (one per species) neighboring trees were selected where sap-flow sensors were installed and mid-day leaf water potential (pressure chamber) and stomatal conductance (porometer) were measured. Forty-five soil moisture sensors were installed between the representative trees at depths of 10 cm, 30 cm and 50 cm.

Data from November 2020 to January 2021 indicated that mean sap flow rate per tree (cm³ h^-1) is higher for C. sempervirens (min: 161, max: 503) than P. brutia (min: 68, max: 266). Total rainfall during these months was 88 mm, most of which fell in three main rainfall events (between 20 and 30 mm per event). Mean soil moisture before rain (15-day average) was 5% for all soil depths. After the rain, soil moisture was 12% for 10 and 30 cm depths and 8% for 50 cm. The increase in soil moisture resulted in 1.6 times higher transpiration for C. sempervirens and 1.4 times higher transpiration for P. brutia. The leaf water potential of C. sempervirens increased from -2.6 MPa before the rain to -0.8 MPa after the rain, whereas it remained near -0.5 MPa for P. brutia. This research of the different plant water-use strategies can contribute to an improved selection of species for afforestation in arid and semi-arid regions.

This research has received support from the Water JPI (Joint Call 2018) FLUXMED Project, funded through the Cyprus Research and Innovation Foundation.