Sap flow for beech (Fagus sylvatica L.) in a natural and a managed forest-effect of spatial heterogeneity

Beech (Fagus sylvatica L.) is an important species in natural and managed forests in Europe. This drought-sensitive species dominates even-aged stands as well more natural stands composed of a mixture of tree species, age and size classes. This study evaluates the extent that heterogeneity in spacing and tree diameter affect the seasonal availability and use of water.

Two stands were evaluated: (i) a heterogeneous forest remnant (NAT) with trees up to similar to 300 years old, a mean top height of 28.4 m and a total of 733 stems ha(-1)with stem diameters averaging 18 cm and (ii) an even-aged 80-year old stand (MAN), with a height of 25 m, and a total of 283 stems ha(-1) with diameters averaging 38 cm.

Stem sap flow, J(s) (g m(-2) s(-1)), was continuously measured in 12 (MAN) and 13 (NAT) trees using 20-mm long heat dissipation sensors. Individual tree measures of sap flow were correlated using non-linear statistical methods with air vapour pressure deficit (D, hPa) and global radiation (R-g, J m(-2) day(-1)), along with constraints imposed by reductions in soil water content (SWC).

SWC was measured as volumetric % using time domain reflectometry. The daily integrated J(s) (J(s-sum)) for trees growing in the evenly spaced MAN stand and trees in canopy and closed forest positions in NAT stand decreased as the availability of soil moisture was reduced. In the heterogeneous NAT stand, SWC in a recently formed canopy gap remained high throughout the vegetation period.

Based on regression models, the predicted relative decrease in J(s-sum) for dry relative to moist soil water conditions in the closed forest (at mean daily D = 10 hPa) was 7-11% for trees near the gap and 39-42% for trees in the closed forest. In MAN, the reduction in J(s-sum) was 29% in dry relative to moist conditions.

J(s-sum) in the outer 20 mm of the xylem in NAT was lower than that in MAN and the rate of decline in J(s) with xylem depth was less in NAT than in MAN. In MAN, J(s-sum) in deep and outer xylem was negatively affected at low soil moisture availability; in NAT, this was the case for only the outer xylem indicating that deep roots could be important in supplying water at times of low soil moisture in the upper soil.