Persistence and Plasticity in Conifer Water‐Use Strategies

Abstract

The selective use of seasonal precipitation by vegetation is critical to understanding the residence time and flow path of water in watersheds, yet there are limited datasets to test how climate alters these dynamics. Here, we use measurements of the seasonal cycle of tree ring δ18O for two widespread conifer species in the Rocky Mountains of North America to provide a multi‐decadal depiction of the seasonal origins of forest water use. The results show that while the conifer tree stands had a dominant preference for use of snowmelt, there were multi‐annual periods over the last four decades when use of summer precipitation was preferential. Utilization of summer rain emerged during years with increased snowfall and tree growth, suggesting that summer rain enhanced the transpiration stream only during the periods of highest water use. We hypothesize this could be explained through shallowing of the root profile during wetter periods and/or through the influence of changing water table depths on the residence time of summer precipitation in the root zone. We suggest the tree ring proxy approach used here could be applied in other watersheds to provide critical insight into the temporal dynamics of plant water use that could not be inferred from short measurement campaigns. These data on the seasonal origins of forest water are critical for understanding forest vulnerability to drought, the processes that affect precipitation pathways and residence time in watersheds and the interpretation of tree ring proxy data.

Soil Respiration Response to Rainfall Modulated by Plant Phenology in a Montane Meadow, East River, Colorado, USA

“…Additionally, the EVI trends during the 2012 drought indicate earlier senescence and a lower peak EVI despite significant monsoon precipitation. These observations support emerging work on the importance of snowpack amount and timing over summer precipitation in sustaining plants through times of increased water demand (Berkelhammer et al, 2020;Knowles et al, 2018;Sloat et al, 2015;Wieder et al, 2017).…”

Montane forest productivity across a semiarid climatic gradient

“…Whereas abundant snow melt moisture stimulated the highest overall rates of warm season vegetation productivity (Figures 2f and 3), monsoon rains were critical to mid‐summer vegetation recovery at the bimodal GPP sites characterized by foresummer drought (Figure 6b; Kolb, Dore, & Montes‐Helu, 2013; Peltier & Ogle, 2019). These competing results reflect a variety of biophysical processes including deeper infiltration and storage of snow melt water in the soil profile where it can be accessed for a longer period of time (Fan et al., 2017; Hu et al., 2010), a “priming” effect of snow melt moisture on the persistence of fine root activity (Berkelhammer et al., 2020; Martin et al., 2018), and/or seasonal changes in atmospheric moisture demand (e.g., Novick et al., 2016). Given forecasted changes to both precipitation timing and type, the degree to which vegetation can continue to exploit seasonal water resources represents a key control on the persistence of the regional carbon sink (e.g., Grossiord et al., 2017).…”

“…Summer precipitation can compensate for snow water deficits, especially in areas affected by monsoon precipitation, but climate models have predicted future delays in the onset of the North American Monsoon (Cook & Seager, 2013; Grantz, Rajagopalan, Clark, & Zagona, 2007) and the potential for large reductions in monsoon precipitation (Pascale et al., 2017). Moreover, the timescales over which evergreen conifers may be capable of modifying their use of winter versus summer precipitation remain highly uncertain (Allen et al., 2019; Berkelhammer et al., 2020). To project the impact of forecasted climate change on montane forest GPP, the current study takes a synthetic approach that considers the potential for current cross‐site biophysical relationships to parallel regional trends.…”

“…At higher latitude and/or elevation locations that are characterized by consistent seasonal snow cover, the spring snow melt moisture pulse is often sufficient to satisfy the bulk of vegetation water demand throughout the growing season (Hu, Moore, Burns, & Monson, 2010; Knowles, Lestak, & Molotch, 2017; Trujillo, Molotch, Goulden, Kelly, & Bales, 2012). However, recent work has also highlighted the sensitivity of montane forest gross primary productivity (GPP) to summer rain, especially at high elevations or following high snowpack years (Berkelhammer, Stefanescu, Joiner, & Anderson, 2017; Berkelhammer et al., 2020). Under these circumstances, greater sensitivity to summer rain may reflect a longer residence time of precipitation in the root zone when soils are wet or shallower rooting depths when and where precipitation is greater (Fan, Miguez‐Macho, Jobbágy, Jackson, & Otero‐Casal, 2017; Martin, Looker, Hoylman, Jencso, & Hu, 2018; Szejner et al., 2016).…”

Increasing plant water stress and decreasing summer streamflow in response to a warmer and wetter climate in seasonally snow‐covered forests

“…Together, these findings demonstrate the plasticity in tree water use in response to climate variability. Although snowmelt water has been shown to be used by trees throughout the year (Hu et al, 2010) highlighting the importance of deep water storage (Goulden & Bales, 2019; Rempe & Dietrich, 2018; Schwinning, 2010), many species are able to shift to shallow water sources in response to summer rain (e.g., Berkelhammer et al, 2020). These temporal and spatial patterns in plant water use are reflected in forest productivity and carbon balance across the landscape and across years (e.g., Swetnam et al, 2017; Anderson‐Teixeira, DeLong, Fox, Brese, & Litvak, 2011; Knowles, Blanken, Lawrence, & Williams, 2019; Perdrial et al, 2018: Knowles et al, 2019).…”