Differential use of winter precipitation by upper and lower elevation Douglas fir in the Northern Rockies

Abstract

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.

Global sinusoidal seasonality in precipitation isotopes

“…Furthermore, the variability in tropical precipitation isotope ratios we show here may be the result of different storm sources and cloud types (Bailey et al, 2017;Scholl et al, 2009). Thus, precisely predicting precipitation isotope cycles at low latitudes without calibration data may (especially) require consideration of circulation patterns and their temporal variability (Cai et al, 2018;Martin et al, 2018b); an alternative option would be using regional multiple regression equations, which performed well in those regions (Fig. 6).…”

“…Quantifying seasonal precipitation isotope cycles also facilitates identification of the proportion (and over-or underrepresentation) of precipitation from different seasons in samples such as surface waters (Bowen et al, 2019;DeWalle et al, 1997;Halder et al, 2015), groundwa-ter (Jasechko et al, 2014;Kalin and Long, 1994;Lee and Kim, 2007), or plant and soil water (Allen et al, 2019). Similarly, ecological and physiological inferences can be drawn by observing how seasonal variations in water isotope signals are incorporated into (or propagate through) plant and animal tissues (Csank et al, 2016;Gessler et al, 2014;Martin et al, 2018a;Vander Zanden et al, 2015;Yang et al, 2016). Even where phase values are poorly constrained, amplitude and offset values are still useful identifiers of typical mean values and magnitudes of seasonal variation.…”

Persistence and Plasticity in Conifer Water‐Use Strategies

“…There are limited data to test how these dynamics might evolve over longer timescales in response to stressors such as decreases in spring snowpack or rising evaporative demand (Mote et al, 2018;Restaino et al, 2016). For example, existing data on water use by conifers in the western United States have indicated that snowmelt, the predominant source of regional recharge, is the primary reservoir to support forest water demand (Bowling et al, 2017;Hu et al, 2010;Marshall & Monserud, 2006;Martin et al, 2018;Phillips & Ehleringer, 1995). However, the snapshots provided by these studies do not indicate how reliance on this water source has varied in response to region-wide changes in the snowpack or lengthening of the growing season.…”

17O‐excess as a detector for co‐extracted organics in vapor analyses of plant isotope signatures

“…The use of the stable isotope ratios of hydrogen and oxygen (δ 2 H and δ 18 O values) as a tracer of plant water uptake is increasing rapidly . Investigations of ecohydrological processes using such techniques have improved our understanding of soil water dynamics and patterns of plant water use .…”