Abstract:Evapotranspiration (ET) was measured via the eddy covariance technique at a shrub bog peatland in southeastern Ontario for 5 years. For most of the study period the temperature was above normal. Precipitation was variable, but, in 2 years, late summer dry periods resulted in an extended period of deep drawdown of the water table (WT). Growing-season (May-September) daily ET varied considerably; maximum ET rates were 4 to 5 mm day 1 . Winter ET rates were an order of magnitude smaller than in summer, yet the total winter ET loss was important, accounting for 23 to 30% of the annual ET water loss. Annual precipitation exceeded annual ET by 1Ð55 to 1Ð94 times.During the growing season, daily ET was closely related to daily potential evaporation (PET); however, the slope of this relationship was statistically different in some years. In contrast, ET and WT were only weakly related in most years. When ET was sorted into 5 cm WT classes there was no difference in mean ET across most WT classes; only the two deepest WT classes had significantly smaller mean ET. The ratio ET/PET followed the same pattern. We present a conceptual model of ET that relates WT, soil hydraulic properties and moss and vascular plant processes.
Accounting for water stress-induced tree mortality in forest productivity models remains a challenge due to uncertainty in stress tolerance of tree populations. In this study, logistic regression models were developed to assess species-specific relationships between probability of mortality (P ) and drought, drawing on 8.1 million observations of change in vital status (m) of individual trees across North America. Drought was defined by standardized (relative) values of soil water content (W ) and reference evapotranspiration (ET ) at each field plot. The models additionally tested for interactions between the water-balance variables, aridity class of the site (AC), and estimated tree height (h). Considering drought improved model performance in 95 (80) per cent of the 64 tested species during calibration (cross-validation). On average, sensitivity to relative drought increased with site AC (i.e. aridity). Interaction between water-balance variables and estimated tree height indicated that drought sensitivity commonly decreased during early height development and increased during late height development, which may reflect expansion of the root system and decreasing whole-plant, leaf-specific hydraulic conductance, respectively. Across North America, predictions suggested that changes in the water balance caused mortality to increase from 1.1% yr in 1951 to 2.0% yr in 2014 (a net change of 0.9 ± 0.3% yr ). Interannual variation in mortality also increased, driven by increasingly severe droughts in 1988, 1998, 2006, 2007 and 2012. With strong confidence, this study indicates that water stress is a common cause of tree mortality. With weak-to-moderate confidence, this study strengthens previous claims attributing positive trends in mortality to increasing levels of water stress. This 'learn-as-we-go' approach - defined by sampling rare drought events as they continue to intensify - will help to constrain the hydraulic limits of dominant tree species and the viability of boreal and temperate forest biomes under continued climate change.
To understand how environmental changes have influenced forest productivity, stemwood biomass (B) dynamics were analyzed at 1267 permanent inventory plots, covering a combined 209 ha area of unmanaged temperate-maritime forest in southwest British Columbia, Canada. Net stemwood production (DB) was derived from periodic remeasurements of B collected over a 40-year measurement period in stands ranging from 20 to 150 years old. Comparison between the integrated age response of net stemwood production, DB(A), and the age response of stemwood biomass, B(A), suggested a 58 ± 11% increase in DB between the first 40 years of the chronosequence period (1859-1898) and the measurement period. To estimate extrinsic forcing on DB, several different candidate models were developed to remove variation explained by intrinsic factors. All models exhibited temporal bias, with positive trends in (observed minus predicted) residual DB ranging between of 0.40 and 0.64% yr À1 . Applying the same methods to stemwood growth (G) indicated residual increases ranging from 0.43 and 0.67% yr À1 . Higher trend estimates corresponded with models that included site index (SI) as a predictor, which may reflect exaggeration of the agedecline in SI tables. Choosing a model that excluded SI, suggested that DB increased by 0.40 ± 0.18% yr À1 , while G increased by 0.43 ± 0.12% yr À1 over the measurement period. Residual G was significantly correlated with atmospheric carbon dioxide (CO 2 ), temperature (T), and climate moisture index (CMI). However, models driven with climate and CO 2 , alone, could not simultaneously explain long-term and measurement-period trends without additional representation of indirect effects, perhaps reflecting compound interest on direct physiological responses to environmental change. Evidence of accelerating forest regrowth highlights the value of permanent inventories to detect and understand systematic changes in forest productivity caused by environmental change.
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