Seasonal and diurnal water relations during summer drought
Seasonal and diurnal patterns of leaf conductance and xylem water potential at the two sites were similar to those observed previously in the same and other western Mediterranean sclerophyllous species (Tenhunen et al. 1987; Rhizopoulou & Mitrakos 1990; Romane & Terradas 1992; Sala & Tenhunen 1994; Castell, Terradas & Tenhunen 1994; Damesin & Rambal 1995).
As in other Mediterranean species, stomatal closure in Q. ilex constitutes a mechanism to cope with diurnal and seasonal water deficits (Aussenac & Valette 1982; Lange, Tenhunen & Braun 1982; Archerar & Rambal 1992; Castell et al. 1994; Sala & Tenhunen 1994; Damesin & Rambal 1995).
The progressive stomatal closure with decreasing predawn water potential indicates conservative water use and also explains the decrease in ΔΨ during drought (Damesin & Rambal 1995). Yet, the minimum predawn water potential on trees at Rapolano (particularly in 1993) was between – 4 and – 6 MPa, which is similar to that measured on Mediterranean Quercus species in very dry years or on sites close to desert areas (Rambal & Debussche 1995). Extreme stress conditions were, in fact, evidenced by yellowing leaves during August (1993), probably indicating irreversible cell damage (Kyriakopoulos & Larcher 1976). Drought-induced senescence and shedding of leaves is a known means to avoid marked drought stress (Kramer 1980). Overall, the plants recovered to high water potentials and leaf conductances after the return of the September rains, allowing continuation of annual net production (Damesin & Rambal 1995).
At the same sites in late June 1993, Tognetti et al. (1996a) found an osmotic potential of ≈– 3·35 MPa at turgor loss point in Q. ilex trees growing at the CO2 spring and ≈ 3·05 MPa in the control trees. In this study, predawn water potentials were near or below this critical value when the drought was at its peak (July and August). Although water loss was still measurable, stomata opened only for a short period in the early morning. However, strong reductions in stomatal conductance occurred when water potential approached the turgor loss point (Hinckley et al. 1980; Rhizopoulou & Mitrakos 1990; Sala & Tenhunen 1994), coinciding with large decreases in predawn water potential. Osmotic adjustment can be an important complementary mechanism of drought resistance that operates to enable the stomata to open in conditions of severe soil water stress, lowering the water potential at which closure occurs (Rhizopoulou & Mitrakos 1990; Terradas & Savé 1992).
The heat-pulse velocity technique has already been applied successfully on these and other Quercus species (Miller, Vavrina & Christensen 1980; Borghetti et al. 1993; Raschi et al. 1995; Tognetti et al. 1996a,b). Sap velocity and sap flow decreased in parallel with increases in hydraulic resistance, reaching a minimum in mid-summer. Diffuse xylem embolism (up to 80% in branches) occurred in trees at Rapolano at this time (Tognetti et al. 1996a; Tognetti & Raschi unpublished results). The relationships between decreases in stomatal conductance, predawn water potential and sap flow, and increases in hydraulic resistance are consistent with the suggestion by Tyree (1989) that an important role of stomatal control of transpiration is not only to prevent desiccation damage but also to avoid ‘runaway embolism’, despite a lot of xylem redundancy (Jones & Sutherland 1991; Borghetti et al. 1993). The CO2 spring trees had less reduction in hydraulic resistance for a given value of predawn water potential than the control trees. This might confer the capacity to prolong photosynthesis during dry periods.
Mean peak sap flows declined progressively in concert with increasing drought conditions and reached a minimum when water stress was at its maximum (mid-summer). With the onset of late summer rainfall, mean peak sap flows recovered to prestress values, regardless of site. This seasonal pattern, of progressive decline and recovery, may be attributed to increasing and decreasing soil water deficit. The presence of shallow bedrock at both sites may also have prevented trees abstracting water from deeper soil water reserves, so that the trees responded rapidly to soil water deficit and recharge of the upper horizons, which have a low water-holding capacity. On the other hand, the distribution of roots in this kind of substrate is difficult to assess and, although, unlikely, differential access to soil water cannot be excluded.
Effects of high CO2 concentration
It is a general, but not a universal, observation that leaf transpiration is reduced in elevated CO2. However, the majority of measurements published so far are from plants grown in controlled-environment chambers, often in pots, and were made at low or moderate leaf-to-air water vapour pressure deficit. Because of the apparent species specificity of response, predictions of the effects of elevated atmospheric CO2 on leaf conductance, particularly of trees, remain uncertain.
In particular, there is relatively little information on how the sensitivity of stomatal conductance to water vapour pressure deficit is affected by either short-term or long-term exposure to elevated CO2. Moreover, the mechanism by which stomata sense humidity is still unclear (Meinzer 1993). Recently, evidence for no stomatal responses to elevated CO2 in tall trees has been reported by several authors (Barton, Lee & Jarvis 1993; Dufrene, Pontailler & Saugier 1993; Ellsworth et al. 1995; Teskey 1995; Körner & Würth 1996; Tognetti et al. 1996a), irrespective of the enrichment method.
Hollinger (1987) found a significantly smaller relative decrease in conductance with increasing leaf-to-air vapour pressure deficit in leaves of two tree species grown and measured in elevated CO2. Leaf conductance in Q. ilex was higher in the control trees than in the CO2 spring trees in the early morning at low vapour pressure deficit, but the difference was small, and this observation was less evident at the peak of water stress in mid-summer. Nevertheless, our data indicate that the decrease in leaf conductance caused by elevated CO2 may be less evident at high atmospheric vapour pressure deficit over the long term. Our observations are consistent with those of Bunce (1993) for soybean (grown outdoors) and orchard grass.
Surprisingly, SLA was not influenced by elevated CO2, in contrast to the reports of many authors [see Ceulemans & Mousseau (1994)]. Again, most studies report data from experiments carried out on seedlings and/or in controlled environments. A different behaviour may be expected by trees in the field, exposed to elevated CO2 for generations (Jones et al. 1995).
Overall, the sap flow measurements showed that water flux from the trees at the CO2 spring was usually less than that from the trees at the control site, but the difference was small. This finding is consistent with the established effects of CO2 on plant water use at the organ and whole-plant scale (Rogers & Dahlman 1993; Bremer, Ham & Owensby 1996; Senock et al. 1996; Dugas, Prior & Rogers 1997). The reduction in the mean daily sap flow was more evident in trees with small foliage and sapwood areas. Half-hourly whole-plant transpiration was larger for trees grown at the control site. However, the degree of reduction in water use between sites varied among the summer periods. There was a small CO2 effect during the peak of water stress in mid-summer, and the degree of reduction caused by drought, that occurred for both the control and the CO2 spring trees because of declining soil water content, was much larger than that attributable to CO2.
Control trees had a consistently larger foliage area at the corresponding sapwood area than CO2 spring trees. In many species, elevated CO2 concentrations have been found to stimulate an increase in transpiring surface area usually associated with the increase in tree size (Ceulemans & Mousseau 1994). However, the majority of experiments have been carried out in conditions of unlimited nutrient and water supply. Chaudhuri, Kirkham & Kanemasu (1990) reported an increased transpiring surface area in wheat grown in elevated CO2, but under free-air CO2 enrichment the same species did not show any increase in leaf area per culm (Senock et al. 1996). The effect of a reduced transpiring surface (lower foliage area) in trees (particularly if of small size) at the CO2 spring site might be equally, if not more, effective than stomatal closure in reducing transpiration and plant water use under elevated CO2.
Hättenschwiler et al. (1997a) found that the regeneration phase of Q. ilex can be accelerated in CO2-enriched atmospheres, while stimulation responses are much less evident when trees are mature; the positive effect of elevated CO2 was relatively larger in years with a dry spring. Hättenschwiler et al. (1997b), studying morphological adjustments to elevated CO2 in mature Q. ilex growing around the CO2 spring of Bossoleto, concluded that the observed enhancement of stem biomass production, but with lower leaf area, may represent an effective mechanism for increased water use efficiency of trees growing in a CO2-enriched atmosphere. These measurements were made in an area of the same CO2 spring, characterized by similar CO2 enrichment but with a different microclimate. Chaves et al. (1995) also reported higher water use efficiency of Q. ilex trees exposed to elevated CO2 for life during the warm hours of the day.
In elevated global atmospheric CO2, the regeneration phase of Q. ilex may be better able to withstand periods of drought than in current ambient CO2 concentrations. This may result in changes in community composition because of varying species response to elevated CO2 and, especially in Mediterranean areas, to enhanced temperature and more severe drought conditions, as forecast by General Circulation Models. A reduction in water usage, which will conserve soil water, might prolong physiological activity during periodic drought. On the other hand, if this reduction in water use is restricted to the smaller trees of a particular group of species, the effect on the water balance of a Mediterranean-type forest will be relatively minor. We may conclude that whole-tree measurements can provide new insights into the consequences of elevated CO2 for transpiration, if coupled with leaf- and ecosystem-scale data.