Abstract
Water-use efficiency (WUE), weighing the balance between plant transpiration and growth, is a key characteristic of ecosystem functioning and a component of tree drought resistance. Seasonal dynamics of tree-level WUE and its connections with drought variability have not been previously explored in sky-island montane forests. We investigated whole-tree transpiration and stem growth of bristlecone (Pinus longaeva) and limber pine (Pinus flexilis) within a high-elevation stand in central-eastern Nevada, United States, using sub-hourly measurements over 5 years (2013–2017). A moderate drought was generally observed early in the growing season, whereas interannual variability of summer rains determined drought levels between years, i.e., reducing drought stress in 2013–2014 while enhancing it in 2015–2017. Transpiration and basal area increment (BAI) of both pines were coupled throughout June–July, resulting in a high but relatively constant early season WUE. In contrast, both pines showed high interannual plasticity in late-season WUE, with a predominant role of stem growth in driving WUE. Overall, bristlecone pine was characterized by a lower WUE compared to limber pine. Dry or wet episodes in the late growing season overrode species differences. Our results suggested thresholds of vapor pressure deficit and soil moisture that would lead to opposite responses of WUE to late-season dry or wet conditions. These findings provide novel insights and clarify potential mechanisms modulating tree-level WUE in sky-island ecosystems of semi-arid regions, thereby helping land managers to design appropriate science-based strategies and reduce uncertainties associated with the impact of future climatic changes.
Highlights
Climatic variability can profoundly impact carbon and water exchanges between forests and the atmosphere (Frank et al, 2015)
Complex Water-use efficiency (WUE)–drought relationships across broad ranges of biomes and environments have been linked to drought intensity, which alters the degree of coupling between gross primary productivity and evapotranspiration (Xu et al, 2019)
Such discrepancies in WUE–drought relationships can be the result of species-specific evolutionary histories, morphological traits, and/or physiological strategies (Yi et al, 2018), making studies on whole-tree WUE necessary to gain a comprehensive picture of tree physiology under a changing climate (Monson et al, 2010)
Summary
Climatic variability can profoundly impact carbon and water exchanges between forests and the atmosphere (Frank et al, 2015). In arid and semi-arid ecosystems, both productivity and water cycling are co-limited by drought (Knowles et al, 2020) Tree species in such water-limited environments can adapt to climatic changes through a variety of physiological mechanisms, Water-Use Efficiency of Great Basin Conifers spanning from stomatal regulation to whole-tree remobilization of non-structural carbohydrates (Hartmann and Trumbore, 2016). A reduction in WUE has been documented in drought-stressed trees (Linares and Camarero, 2012; López et al, 2021) Such discrepancies in WUE–drought relationships can be the result of species-specific evolutionary histories, morphological traits, and/or physiological strategies (Yi et al, 2018), making studies on whole-tree WUE necessary to gain a comprehensive picture of tree physiology under a changing climate (Monson et al, 2010)
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