In temperate regions such as the American west, forest trees often exhibit growth sensitivity to climatic conditions of a particular season. For example, annual tree ring growth increments may correlate well with winter precipitation, but not with summer rainfall, suggesting that trees rely more on winter snow than summer rain. Because both the timing and character of seasonal western climate patterns are expected to change considerably over coming decades, variation in the importance of different seasonal moisture sources for trees can be expected to influence how different forest trees respond to climate change as a whole, with shifts in seasonality potentially benefitting some trees while challenging others. In this study, we inferred patterns of tree water use in Douglas fir trees from the Northern Rockies for 2 years using stable water isotopes, while simultaneously quantifying and tracking precipitation inputs to soil moisture across a vertical soil profile. We then coupled water source use with daily measurements of radial growth to demonstrate that soil moisture from winter precipitation accounted for 87.5% and 84% of tree growth at low and high elevations, respectively. We found that prevailing soil moisture conditions drive variation in the depth at which trees access soil water, which in turn determines which seasonal precipitation inputs are available to support tree growth and function. In general, trees at lower elevations relied more on winter precipitation sourced from deep soils while trees at higher elevations made better use of summer rains sourced from near-surface soil layers. As both the timing of seasons and phase of precipitation (rain vs. snow) are likely to change considerably across much of the west, such patterns in tree water use are likely to play a role in determining the evolution of forest composition and structure in a warming climate.

Inferring whole-tree sap flow rates (Q) with thermometric sap flow sensors requires specification of physiological and structural attributes of trees. Using sap temperature measurements to estimate Q with the heat-ratio method (HRM) requires quantification of the water content (m c), basic density (ρ b), and depth (R s) of sapwood. Values of m c and ρ b serve to estimate sapwood thermal diffusivity (k), a necessary variable in the calculation of heat-pulse velocity (V h) that is often set to a nominal value (k_nom); m c and ρ b are also used to convert V h to sap velocity (V s). The sapwood area across which V s is integrated is often estimated on the basis of R s. Because m c and ρ b are correlated and influence Q through estimation of k and the conversion of V h to V s , we sought to quantify the potential error introduced when Q is calculated with k_nom rather than with k as estimated from measurements of m c and ρ b in five coniferous species. We also examined how variation in m c , ρ b , and R s across sampling scales may contribute to uncertainty in estimated Q. Across the observed range of m c and ρ b , the two traits contributed to a net decline in the process of estimating V s from V h (with k_nom). This suggests that the use of k_nom rather than a calculated k may result in overestimation of Q when m c and ρ b co-vary as they did in our study. Variability in m c and ρ b across sampling scales could induce errors greater than 10% in V s (and hence Q), while within-and among-tree variability in R s could impart even greater errors (up to 130%). We propose that this uncertainty be represented in the expression of error in whole-tree sap flow estimates and in statistical analyses involving those estimates.

Tree radial growth is often systematically limited by water availability, as is evident in tree ring records. However, the physiological nature of observed tree growth limitation is often uncertain outside of the laboratory. To further explore the physiology of water limitation, we observed intra-annual growth rates of four conifer species using point dendrometers and microcores, and coupled these data to observations of water potential, soil moisture, and vapor pressure deficit over 2 yr in the Northern Rocky Mountains, USA. The onset of growth limitation in four species was well explained by a critical balance between soil moisture supply and atmospheric demand representing relatively mesic conditions, despite the timing of this threshold response varying by up to 2 months across topographic and elevation gradients, growing locations, and study years. Our findings suggest that critical water deficits impeding tissue growth occurred at relatively high water potential values, often occurring when hydrometeorological conditions were relatively wet during the growing season (e.g. in early spring in some cases). This suggests that species-specific differences in water use strategies may not necessarily affect tree growth, and that tissue growth may be more directly linked to environmental moisture conditions than might otherwise be expected.

The hydroclimatic controls on transpiration often follow hillslope‐ or catchment‐scale topography in water‐limited ecosystems; however, rates of transpiration may deviate from these patterns due to microtopographic variation in environmental conditions or physiology. Here, we assessed the microtopographic effects on water use of five conifer species within four subcatchments in western Montana along a water availability gradient that was driven by aspect and elevation. To infer physiological processes at both diurnal and seasonal time scales, we analysed the relationship between sap velocity, Vs, and vapour pressure deficit, D, using instantaneous (half‐hourly) and aggregated (daily mean) values of Vs and D. Both within and across species, daily mean Vs was more tightly coupled to D at higher elevation sites (1,720 m), whereas trees 350 m lower in elevation became decoupled from D as snowmelt‐derived soil moisture declined. At the diurnal scale, we found that the degree of decoupling of Vs from D during soil moisture deficits decreased when a time lag between Vs and D was considered. Additionally, contrary to the common inference of plant hydraulic capacitance based on Vs lagging behind D at the diurnal scale, half‐hourly Vs tended to lead D in three of the five conifer species we studied. Predawn and midday branch water potential measurements provided additional evidence that topography influenced plant water status. These results suggest that improved understanding of soil–vegetation–atmosphere coupling in mountainous terrain requires further inquiry into the spatial variability of plant hydraulic regulation.

In stands with a broad range of diameters, a small number of very large trees can disproportionately influence stand basal area and transpiration (Et). Sap flow-based Et estimates may be particularly sensitive to large trees due to nonlinear relationships between tree-level water use (Q) and tree diameter at breast height (DBH). Because Q is typically predicted on the basis of DBH and sap flow rates measured in a subset of trees and then summed to obtain Et, we assessed the relative importance of DBH and sap flow variables (sap velocity, Vs, and sapwood depth, Rs) in determining the magnitude of Et and its dependence on large trees in a tropical montane forest ecosystem. Specifically, we developed a data-driven simulation framework to vary the relationship between DBH and Vs and stand DBH distribution and then calculate Q, Et and the proportion of Et contributed by the largest tree in each stand. Our results demonstrate that variation in how Rs is determined in the largest trees can alter estimates up to 26% of Et while variation in how Vs is determined can vary results by up to 132%. Taken together, these results highlight a great need to expand our understanding of water transport in large trees as this hinders our ability to predict water fluxes accurately from stand to catchment scales.