Two coastal Pacific evergreens, Arbutus menziesii, Pursh. and Quercus agrifolia, Née show little water stress during California's exceptional drought

Abstract

California's coastal climate is characterized by rainy winters followed by a dry summer season that is supplemented by frequent fog. While rising temperatures and drought caused massive tree mortality in central California during the 2011-2015 extreme drought, dying trees were less common in the central coast region. We hypothesized that cooler, maritimeameliorated temperatures reduced the effects of drought stress on coastal vegetation. To test this, weekly measurements of water potential and stomatal conductance were made on two coast evergreen tree species, Arbutus menziesii and Quercus agrifolia, throughout the summer 2014 dry season. Water potential remained generally constant during this period but stomatal conductance declined in both species as the dry season progressed. Species' resistance to embolism was determined using the centrifuge method, and showed Q. agrifolia to be more vulnerable to embolism than A. menziesii. The stem vulnerability curves were consistent with species' seasonal water relations as well as their anatomy; the ring-porous Q. agrifolia had substantially larger conduits than the diffuse-porous A. menziesii. Leaf turgor loss points differed significantly as did other pressure-volume parameters but these data were consistent with the trees' seasonal water relations. Overall, the two species appear to employ differing water use strategies; A. menziesii is more profligate in its water use, while Q. agrifolia is more conservative, with a narrower safety margin against drought-induced loss of xylem transport capacity. Despite the extended drought, these species exhibited neither branch die-back nor any obvious symptoms of pronounced water-stress during the study period, implying that the maritime climate of California's central coast may buffer the local vegetation against the severe effects of prolonged drought.

Primary tissues may affect estimates of cavitation resistance in ferns

“…For example, resilience and persistence in the face of drought, does not always equal exceptional embolism resistance, as our field data have shown. There are many unexpected ways in which plants can survive or avoid massive droughts (Jacobsen et al ., 2007; Holmlund et al ., 2016, 2019, 2020; Chacon et al ., 2020), and there may be others. Characterising and understanding variation in species vulnerability to cavitation, xylem architecture, xylem anatomy, phenology and other factors will continue to enrich our understanding of how plants have evolved to manage the most acute stress they face on land.…”

“…Some studies have used the centrifuge method to generate vulnerability‐to‐cavitation curves (VCs) on fern petioles (Pittermann et al ., 2011; Burns et al ., 2016; Holmlund et al ., 2016), in which the petioles were spun at progressively higher speeds corresponding with more negative water potentials. With this and other hydraulic methods such as air injection (Sperry & Saliendra, 1994) or bench drying (Jacobsen & Pratt, 2012; Martin‐StPaul et al ., 2014; Chacon et al ., 2020), segments are removed from the instrument after reaching a desired water potential, and the per cent loss of conductivity (PLC) is determined as (1 − K h /K MAX ) × 100, where K h is the segment hydraulic conductivity (K) standardised for length at a given water potential, and K MAX is the maximum hydraulic conductivity at which all xylem conduits are water filled (Sperry et al ., 1988). While air injection pushes air into the xylem rather than pull it in as in the centrifuge method, both approaches should, in theory, yield very similar results.…”

Functional xylem characteristics associated with drought‐induced embolism in angiosperms

“…However, it has also been suggested that the centrifuge method does not overestimate vulnerability to embolism in stem segments with cut‐open vessels (Jacobsen & Pratt, 2012; Sperry et al ., 2012; Tobin et al ., 2012; Hacke et al ., 2014), and that there is no accidental formation of gas‐filled vessels in xylem segments that are cut in water whilst under negative pressure (no cutting artefact observed in Venturas et al ., 2015, 2019; Nardini et al ., 2017). These alternative viewpoints suggest that exponential vulnerability curves may be valid (Sperry et al ., 2012), and consequently support previous reports of high levels of native embolism (up to 60–90%) in functional sapwood, even under mild drought (Pockman & Sperry, 2000; Domec et al ., 2006; Jacobsen et al ., 2007; Chacon et al ., 2020; Percolla et al ., 2021). This discrepancy in xylem vulnerability continues to cause confusion about the frequency of drought‐induced embolism events in planta .…”

Three Censuses of a Mapped Plot in Coastal California Mixed-Evergreen and Redwood Forest

“…UC Santa Cruz data have already been used in a number of comparative analyses (e.g., [27,45,60,[69][70][71][72][73][74][75][76][77]), and the availability of the complete data sets should allow for more widespread use. Equally importantly, we demonstrate that the development of the FERP, as an integral part of experiential learning in forest ecology for hundreds of novice student scientists and as an accessible platform for undergraduate and graduate research (e.g., [78][79][80][81]), is supportive of generating highquality ecological data of value to the broad research community. E120_N240, E160_N040, E160_N120, E160_N240, and E160_N280; values from across the stations were averaged to represent climate patterns within the FERP (Figure A2).…”