Vulnerability of xylem to embolism in relation to plant hydraulic resistance in Quercus pubescens and Quercus ilex co‐occurring in a Mediterranean coppice stand in central Italy

Abstract

The seasonal patterns of xylem embolism and xylem transport properties in Quercus pubescens Willd. and Quercus ilex L. trees growing in a natural mixed coppice stand in conditions of severe water stress were investigated. Xylem embolism was evaluated in both dehydrating branches and in apical twigs during a whole year. Measurements of xylem water potential were conducted from predawn to sunset on selected sunny days. On the same days, diurnal courses of leaf conductance were monitored. Measurements of half‐hourly sap flow were made by the heat‐pulse technique throughout the summer. At the onset of summer, a sharp decrease in water potential was observed in both species. Full recovery of water potentials was observed for both species after the first major rainfall event in September. Both experienced serious embolism throughout the year, ranging between minima of c. 60% (expressed as percentage loss of hydraulic conductivity) after the rains in autumn and after bud burst in spring, and maxima of c. 80% during summer and after freezing‐thawing events during the winter season. A significant negative linear relationship was found between water potential and xylem embolism in branches dehydrating in air for Q. pubescens and Q. ilex. Q. pubescens had greater efficiency in hydraulic transport (higher specific conductivity and leaf specific conductivity) by the xylem than Q. ilex. In June, leaf conductance was high early in the morning and decreased gradually during the day. Midday depression of leaf conductance, as a result of high evaporative demand combined with water deficit, was observed in both species. In August, leaf conductance of both species was greatly reduced, as water potential dropped to extremely low values, and the stomata were almost completely closed during the afternoon. No hysteresis resulting from plant capacitance was observed in the relationship between shoot water potential and sap flow. Q. pubescens exhibited very high values of whole‐tree hydraulic resistance between July and September, whereas Q. ilex generally showed lower values. The effect of soil moisture depletion on the relationship between sap flow and shoot water potential appears as a lowering of water potential at zero flow. A significant decrease of whole‐tree hydraulic resistance in both species was observed with the onset of the autumn, preceding the partial recovery of twig hydraulic conductivity. The results demonstrate that both Q. pubescens and Q. ilex, although highly tolerant of severe water stress and tissue dehydration, operate at the limits of safety which are surpassed under severe droughts, and prolonged climatic stress might predispose these Quercus species to decline.

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

“…I), with heavy frosts in the first quarter that were more influential on the trees in the Calañas plot as revealed by the physiological measurements. These frosts could have led to bud freezing [10] and to a considerably decreased xylem hydraulic conductivity [34,36]. Notwithstanding the unfavourable climatic conditions of that year, the water potential of the oaks never fell below -3.70 MPa in San Bartolomé, but reached -4.10 MPa in Calañas.…”

Response of Scots pine (Pinus sylvestris L.) and pubescent oak (Quercus pubescens Willd.) to soil and atmospheric water deficits under Mediterranean mountain climate

“…While much lower leaf water potentials (ca. −4.5 MPa) have been recorded for pubescent oak throughout the Mediterranean region [14,43], the values found for Scots pine are among the lowest ever measured, with the exception of even drier populations in NE Spain [28].…”

“…Nevertheless, maximum conductivity losses of ca. 70% were observed under severe water stress (minimum ψ l, pd of −3.6 MPa) [43].…”

The importance of xylem constraints in the distribution of conifer species

“…As the xylem water potential (ψ x ) decreases, air is believed to enter xylem elements by breaking the surface tension of water at the inter-conduit pits (a process known as ' airseeding '), rendering them non-functional (Sperry & Tyree, 1988). Xylem vulnerability to cavitation is highly variable among taxa, with significant differences recorded even in closely related species growing under identical conditions Jarbeau et al, 1995 ;Tognetti et al, 1998). However, intraspecific variation in vulnerability of stem xylem is quite small, even amongst individuals from contrasting environments (Franks et al, 1995 ;Alder et al, 1996).…”