Components of ecosystem evaporation in a temperate coniferous rainforest, with canopy transpiration scaled using sapwood density.

Here we develop and test a method to scale sap velocity measurements from individual trees to canopy transpiration (E(c)) in a low-productivity, old-growth rainforest dominated by the conifer Dacrydium cupressinum. Further, E(c) as a component of the ecosystem water balance is quantified in relation to forest floor evaporation rates and measurements of ecosystem evaporation using eddy covariance (E(eco)) in conditions when the canopy was dry and partly wet. Thermal dissipation probes were used to measure sap velocity of individual trees, and scaled to transpiration at the canopy level by dividing trees into classes based on sapwood density and canopy position (sheltered or exposed). When compared with ecosystem eddy covariance measurements, E(c) accounted for 51% of E(eco) on dry days, and 22% of E(eco) on wet days. Low transpiration rates, and significant contributions to E(eco) from wet canopy evaporation and understorey transpiration (35%) and forest floor evaporation (25%), were attributable to the unique characteristics of the forest: in particular, high rainfall, low leaf area index, low stomatal conductance and low productivity associated with severe nutrient limitation.

Net ecosystem productivity, net primary productivity and ecosystem carbon sequestration in a Pinus radiata plantation subject to soil water deficit.

Tree carbon (C) uptake (net primary productivity excluding fine root turnover, NPP') in a New Zealand Pinus radiata D. Don plantation (42 degrees 52' S, 172 degrees 45' E) growing in a region subject to summer soil water deficit was investigated jointly with canopy assimilation (A(c)) and ecosystem-atmosphere C exchange rate (net ecosystem productivity, NEP). Net primary productivity was derived from biweekly stem diameter growth measurements using allometric relations, established after selective tree harvesting, and a litterfall model. Estimates of A(c) and NEP were used to drive a biochemically based and environmentally constrained model validated by seasonal eddy covariance measurements. Over three years with variable rainfall, NPP' varied between 8.8 and 10.6 Mg C ha(-1) year(-1), whereas A(c) and NEP were 16.9 to 18.4 Mg C ha(-1) year(-1) and 5.0-7.2 Mg C ha(-1) year(-1), respectively. At the end of the growing season, C was mostly allocated to wood, with nearly half (47%) to stems and 27% to coarse roots. On an annual basis, the ratio of NEP to stand stem volume growth rate was 0.24 +/- 0.02 Mg C m(-3). The conservative nature of this ratio suggests that annual NEP can be estimated from forest yield tables. On a biweekly basis, NPP' repeatedly lagged A(c), suggesting the occurrence of intermediate C storage. Seasonal NPP'/A(c) thus varied between nearly zero and one. On an annual basis, however, NPP'/A(c) was 0.54 +/- 0.03, indicating a conservative allocation of C to autotrophic respiration. In the water-limited environment, variation in C sequestration rate was largely accounted for by a parameter integrative for changes in soil water content. The combination of mensurational data with canopy and ecosystem C fluxes yielded an estimate of heterotrophic respiration (NPP' - NEP) approximately 30% of NPP' and approximately 50% of NEP. The estimation of fine-root turnover rate is discussed.

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 consecutive midsummer days in a 130-year-old Larix gmelinii (Rupr.) Rupr. forest located 160 km south of Yakutsk in eastern Siberia, Russia (61 degrees N, 128 degrees E, 300 m asl). The site received 20 mm of rainfall during the 4 days before measurements, and soil samples indicated that 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 maximum 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 subcanopy trees (representing 55% of trees in the forest) to 67 l day(-1) for emergent trees (representing 18% of trees in the forest). However, when based on xylem sap flux density (F'), calculated by dividing F by projected tree crown area (a surrogate for the occupied ground area), there 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 systematic and large differences in F among emergent, canopy and subcanopy trees. Stand-level F', estimated by combining half-hourly linear relationships between F and stem cross-sectional area with tree size distribution 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. Maximum total tree conductance for water vapor transfer (G(tmax), including 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 equivalent to a leaf-scale maximum stomatal conductance (g(smax)) 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 visible irradiance (Q) and D. A saturating upper-boundary function for the relationship between G(t) and Q was defined as G(t) = G(tmax)(Q/[Q + Q(50)]), where Q(50) = 164 +/- 85 micro mol m(-2) s(-1) when G(t) = G(tmax)/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 decrease in G(t) with increasing D such that F was relatively constant throughout 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 continental climate. However, at given values of Q and D, G(t) was generally higher in the morning than in the afternoon. The additional environmental constraints on G(t) imposed by leaf nitrogen nutrition and afternoon water stress are discussed.

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements.

Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day-1 tree-1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m2 of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s-1 that is equivalent to 112 mmol m-2 s-1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m-2 s-1 at the top and 150 mmol m-2 s-1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was -27.76±0.27‰ (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.