The influence of precipitation and vapor pressure deficit on xylem phenology: a case study in a semi-arid conifer forest

Since the early 2000s, the western U.S has been entrenched in a historic 20 year "megadrought," ranked one of the worst in 1200 years. In addition to decreases in precipitation, atmospheric aridity, or vapor pressure deficit (VPD) has been also increasing. The important role that VPD plays in regulating plant growth has recently been implicated in a handful of studies, but primarily from the ecosystem perspective. The possibility of direct effects of VPD on cellular-to-organismic growth processes has not been adequately explored, beyond the obvious influences on stomatal conductance and photosynthesis. We hypothesize that from a biophysical perspective, persistent seasonal trends of high VPD may also play an important role at the whole tree scale. In a Pinus ponderosa forest in southern Arizona, USA, we collected bi-weekly micro-core samples and found that although earlywood cambial cell expansion occurs during spring (under high soil moisture and low VPD), the carbon and oxygen isotope ratios (¹³C/¹²C_, ¹⁸O/¹⁶O ) of early-wood cellulose reflect the mid-summer climate of low soil moisture and high VPD. Using cellular modeling of xylogenesis, we have shown that the reason for these contrasting observations is a multi-week offset between cell expansion and cell-wall thickening. Thus, the deposition of isotopically-enriched sugars (revealed by ¹⁸O/¹⁶O and ¹³C/¹²C) occurs as a "backfilling" process during the weeks following cell expansion. The cellular modeling of xylogenesis also revealed that between our two sampling years, 2018 (drier winter, wetter summer) and 2019 (wetter winter, drier summer), there were differences in the timing and duration of cell enlargement and cell wall thickening, which correlated with VPD. Our next step is to conduct quantitative wood anatomy measurements to characterize the seasonal rates of carbon accumulation and to assess how this highly variable seasonal climate fluctuation influences seasonal rates of carbon accumulation.