Six co-occurring conifer species in northern Idaho exhibit a continuum of hydraulic strategies during an extreme drought year

As growing seasons in the northwestern USA lengthen, on track with climate predictions, the mixed conifer forests that dominate this region will experience extended seasonal drought conditions. The year of 2015, which had the most extreme drought for the area on record, offered a potential analog of future conditions. During this period, we measured the daily courses of water potential and gas exchange as well as the hydraulic conductivity and vulnerability to embolism of six dominant native conifer species, Abies grandis, Larix occidentalis, Pinus ponderosa, Pinus monticola, Pseudotsuga menziesii, and Thuja occidentalis, to determine their responses to 5 months of record low precipitation. The deep ash-capped soils of the region allowed gas exchange to continue without significant evidence of water stress for almost two months after the last rainfall event. Midday water potentials never fell below -2.2 MPa in the evergreen species and -2.7 MPa in the one deciduous species. Branch xylem...