Transpiration patterns of Pinus halepensis Mill. in response to environmental stresses in a Mediterranean climate

Resum

Increased frequency of severe drought events, coupled with rising air temperatures and vapor pressure deficits (VPD), pose a great threat to Mediterranean forests. Pinus halepensis Mill. is one of the most widespread species in the countries surrounding the Mediterranean basin. Thus, water use of this species plays a critical role in the regions water balance. Studying transpiration patterns and the mechanisms behind stomatal responses to the combined effects of changing VPD and soil moisture can help us improve estimation of forest water use in a changing climate. To improve the estimation of forest evapotranspiration in the Mediterranean basin, the objective of this thesis is to evaluate the transpiration patterns of Pinus halepensis and the role of this species in the soil-water balance under different environmental conditions. Two pine forests in the Turia river basin, eastern Spain, were monitored over a two-year period. The two locations were selected at contrasting altitudes and distances to the sea but within the same hydrological basin, to evaluate if this placement would change the relationship between environmental conditions and the water use of the pines. Sap flow measurements were obtained on a 30-minute interval together with soil moisture measurements and meteorological variables. A soil-water balance was performed on a forest plot-level using an eco-hydrological model in combination with the transpiration data, to assess the contribution of pine transpiration to actual evapotranspiration. Transpiration was dictated by changes in VPD, relative extractable water (REW) and the interaction between these two variables at both sites, indicating that the pines depended on water in the shallow soil layers, and this was restricted during large parts of the year. Except for low winter temperatures having a decreasing effect on transpiration only at the inland site, no significant differences were found in the relationship between environmental drivers and transpiration patterns between the two sites. Using a predictive model, sap flow was shown to be restricted on days of mean VPD values of 2.5 kPa, even when soil moisture levels were relatively sufficient (REW = 0.30), indicating a VPD threshold that decreases pine transpiration. This could potentially affect performance and survival of the species with predicted increases in air VPD. Transpiration levels were highly restricted throughout the first year demonstrating that physiological stresses were not limited to summer months. Using two-year old seedlings in an experiment under controlled conditions confirmed that high levels of VPD can have a decreasing effect on transpiration of P. halepensis (in response to instant changes from 1.5 kPa to 2.7 kPa), while there is an intermediate VPD range that increases transpiration (between 1.0 kPa – 1.5 kPa). This demonstrate how important it is to incorporate VPD changes when predicting forest water use under future climatic changes. Combining transpiration data with eco-hydrological modelling demonstrated that transpiration levels accounted for 62% of total ETa levels during two years of study. Interception levels where 32% of gross precipitation, representing a large water loss to the forest ecosystem. With increased frequency of drought events, soil moisture levels are predicted to become even more limited. Together with a rise in temperatures and consequently VPD, transpiration and growth are likely to be constrained.