Transpiration and stomatal behaviour of Quercus ilex plants during the summer in a Mediterranean carbon dioxide spring

Abstract

Variations in the water relations and stomatal response of Quercus ilex were analysed under field conditions by comparing trees at two locations in a Mediterranean environment during two consecutive summers (1993 and 1994). We used the heat-pulse velocity technique to estimate transpirational water use of trees during a 5 month period from June to November 1994. At the end of sap flow measurements, the trees were harvested, and the foliage and sapwood area measured. A distinct environmental gradient exists between the two sites with higher atmospheric CO 2 concentrations in the proximity of a natural CO 2 spring. Trees at the spring site have been growing for generations in elevated atmospheric CO 2 concentrations. At both sites, maximum leaf conductance was related to predawn shoot water potential. The effects of water deficits on water relations and whole-plant transpiration during the summer drought were severe. Leaf conductance and water potential recovered after major rainfall in September to predrought values. Sap flow, leaf conductance and predawn water potential decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid-summer. These relationships are in agreement with the hypothesis of the stomatal control of transpiration to prevent desiccation damage but also to avoid 'runaway embolism'. Trees at the CO 2 spring underwent less reduction in hydraulic resistance for a given value of predawn water potential. The decrease in leaf conductance caused by elevated CO 2 was limited and tended to be less at high than at low atmospheric vapour pressure deficit. Mean (and diurnal) sap flux were consistently higher in the control site trees than in the CO 2 spring trees. The degree of reduction in water use between the two sites varied among the summer periods. The control site trees had consistently higher sap flow at corresponding values of either sapwood cross-sectional area or foliage area. Larger trees displayed smaller differences than smaller trees, between the control and the CO 2 spring trees. A strong association between foliage area and sapwood cross-sectional area was found in both the control and the CO 2 spring trees, the latter supporting a smaller foliage area at the corresponding sapwood stem cross-sectional area. The specific leaf area (SLA) of the foliage was not influenced by site. The results are discussed in terms of the effects of elevated CO 2 on plant water use at the organ and whole-tree scale.

Influence of pruning and the climatic conditions on acorn production in holm oak (Quercus ilex L.) dehesas in SW Spain

“…The Calañas plot was subject to additional measurements of this type in July 2002. On each date, five trees per treatment type and plot (including the selected trees chosen for acorn harvesting in both plots) were subjected to two different determinations, namely: xylem water potential (Ψ ), which was measured in twigs of the year by using a Model 1000 pressure chamber from PMS Instruments (Corvallis, OR) immediately after sunrise; and photosynthetic activity, which was measured on adult leaves with an Lci Portable Photosynthesis System from ADC BioScientific (Hoddesdon, UK) between 10 and 12 am (local time) in order to obtain an estimate of the maximum daily rates at the time (especially in mid-summer, when the drought was at its peak) [33].…”

Comparative stem‐growth rates of Mediterranean trees under background and naturally enhanced ambient CO2 concentrations

“…Furthermore, studies of Q. ilex and Q. pubescens at another CO # spring (called ' Bossoleto ') have indicated that the decrease in stomatal conductance caused by elevated CO # may be less evident during severe summer drought stress at high vapour pressure deficits (Tognetti et al, 1996(Tognetti et al, , 1998b(Tognetti et al, , 1999 than during periods of relatively low drought stress (Chaves et al, 1995). Quercus ilex and Q. pubescens also showed a lower foliage area at the corresponding sapwood area under elevated CO # (Tognetti et al, 1998b(Tognetti et al, , 1999. This may counterbalance the increased assimilation rate at the leaf level.…”

Growth responses of Quercus petraea, Fraxinus excelsior and Pinus sylvestris to elevated carbon dioxide, ozone and water supply

“…A small number of whole tree chamber studies (Kellomaki & Wang, 1996 ;Roberntz & Stockfors, 1998) and the free air CO # enrichment facility (FACE) at Duke University in the north-eastern United States address how trees might respond to elevated CO # in the natural environment, although the length of exposure in these studies is still relatively short compared with the life-span of a tree. Only the natural CO # springs in Italy represent a long-term natural elevated CO # experiment (Tognetti et al, 1998), although even here problems associated with large, rapid variations in CO # concentrations are apparent. In many of the traditional growth and open-top chamber experiments, the observed down regulation or acclimation of the photosynthetic apparatus might be attributable to reduced rooting volume (Farrar & Williams, 1991 ;Thomas & Strain, 1991 ;Curtis & Wang, 1998).…”