The coastal California climate is expected to become warmer and drier, leading to decreased soil and atmospheric moisture, and increased water stress on redwoods. We have poor basis, however, for predicting its influence on branch hydraulic conductivity, Ks, the feature expected to be most affected. This study investigates the question: How does redwood Ks respond to seasonal variation in major climatic factors?
Numerous lab studies indicate that loss of redwood Ks begins when water potentials, Ψ, drop below a threshold value (c. -2 megapascals). Such threshold Ψ are often observed in summer. We hypothesize, therefore, that: Redwoods experience decreasing Ks during the dry season, especially following episodes of low Ψ driven by low soil moisture and/or high vapor pressure deficit. Given that the dry season generally coincides with the redwood growing season, we also hypothesize: Xylem growth compensates for dry season losses of Ks.
To test these hypotheses, we combine time-series measurements of Ψ, Ks, and xylem growth in stand-dominating redwoods at Big Creek Reserve. We also use stain to distinguish functional xylem. This investigation will expand our knowledge of redwood ecophysiology and provide strong basis for predicting redwood response to a warming and drying climate and decreased Ψ.
Results/Conclusions
On numerous occasions between July, 2017 and February, 2018, Ψ surpassing the accepted threshold for decreasing Ks was observed. In February, 2018, during a heat wave, we measured branch Ψ of -3.04 megapascals—the lowest ever recorded in redwood, to our knowledge. Despite this high degree of water stress, no significant change in Ks has occurred during this period. New growth can functionally replace nonconductive xylem, perhaps masking losses in Ks, but branch diameter measurements also show no growth occurring. Average diameter decreased nearly 2%, presumably due to dry conditions/shrinkage.
Staining revealed that between 14% and 85% of cross sectional area of branch samples was dysfunctional, but no seasonal pattern has emerged. Some degree of Ks is lost at some point, despite its apparent resilience to low Ψ occurring July through February. This resilience contradicts the results of previous lab studies of xylem vulnerability. Our preliminary conclusion, therefore, is that more studies like this, measuring the phenology of Ks and xylem vulnerability in the field, will be essential in advancing our understanding of thresholds for hydraulic dysfunction in redwoods—and other species—and to make predictions about plant response to the changing climate.