ENVIRONMENTAL-REGULATION OF XYLEM SAP FLOW AND TOTAL CONDUCTANCE OF LARIX GMELINII TREES IN EASTERN SIBERIA

Xylem sap flow and environmental variables were measured on seven cons ecutive midsummer days in a 130-year-old Larix gmelinii (Rupr) Rupr. f orest located 160 km south of Yakutsk in eastern Siberia, Russia (61 d egrees N, 128 degrees E, 300 m asl). The site received 20 mm of rainfa ll during the 4 days before measurements, and soil samples indicated t hat the trees were well watered. The tree canopy was sparse with a one -sided leaf area index of 1.5 and a tree density of 1760 ha(-1). On a clear day when air temperature ranged from 9 to 29 degrees C, and maxi mum air saturation deficit was 3.4 kPa, daily xylem sap flux (F) among 13 trees varied by an order of magnitude from 7 l day(-1) for subcano py trees (representing 55% of trees in the forest) to 67 l day(-1) for emergent trees (representing 18% of trees in the forest). However, wh en based on xylem sap flux density (F'), calculated by dividing F by p rojected tree crown area (a surrogate for the occupied ground area), t here was only a fourfold range in variability among the 13 trees, from 1.0 to 4.4 mm day(-1). The calculation of F' also eliminated systemat ic and large differences in F among emergent, canopy and subcanopy tre es. Stand-level F', estimated by combining half-hourly linear relation ships between F and stem cross-sectional area with tree size distribut ion data, ranged between 1.8 +/- 0.4 (standard deviation) and 2.3 +/- 0.6 mm day(-1). These stand-level F' values are about 0.6-0.7 mm day(- 1) (30%) larger than daily tree canopy transpiration rates calculated from forest energy balance and understory evaporation measurements. Ma ximum total tree conductance for water vapor transfer (G(max), includi ng canopy and aerodynamic conductances), calculated from the ratio of F' and the above-canopy air saturation deficit (D) for the eight trees with continuous data sets, was 9.9 +/- 2.8 mm s(-1). This is equivale nt to a leaf-scale maximum stomatal conductance (g(max)) of 6.1 mm s(- 1), when expressed on a one-sided leaf area basis, which is comparable to the published porometer data for Larix. Diurnal variation in total tree conductance (G(t)) was related to changes in the above-canopy vi sible irradiance (Q) and D. A saturating upper-boundary function for t he relationship between G(t) and Q was defined as G(t) = G(max)(Q/[Q Q(50)]), where Q(50) = 164 +/- 85 mu mol m(-2) s(-1) when G(t) = G(tm ax)/2. Accounting for Q by excluding data for Q < Q(85) when G(t) was at least 85% of G(tmax), the upper limit for the relationship between G(t) and D was determined based on the function G(t) = (a + blnD)(2), where a and b are regression coefficients. The relationship between G( t) and D was curvilinear, indicating that there was a proportional dec rease in G(t) with increasing D such that F was relatively constant th roughout much of the day, even when D ranged between about 2 and 4 kPa , which may be interpreted as an adaption of the species to its contin ental climate. However, at given values of Q and D, G(t) was generally higher in the morning than in the afternoon. The additional environme ntal constraints on G(t) imposed by leaf nitrogen nutrition and aftern oon water stress are discussed.