Reinhardt, K, Germino, MJ, Kueppers, LM
NWT Accession Number: NWT1877
Survival of tree seedlings at high elevations has been shown to be limited by thermal constraints on carbon balance, but it is
unknown if carbon relations also limit seedling survival at lower elevations, where water relations may be more important. We
measured and modeled carbon fluxes and water relations in first-year Pinus flexilis seedlings in garden plots just beyond the
warm edge of their natural range, and compared these with dry-mass gain and survival across two summers. We hypothesized
that mortality in these seedlings would be associated with declines in water relations, more so than with carbon-balance limitations.
Rather than gradual declines in survivorship across growing seasons, we observed sharp, large-scale mortality episodes
that occurred once volumetric soil-moisture content dropped below 10%. By this point, seedling water potentials had decreased
below −5 MPa, seedling hydraulic conductivity had decreased by 90% and seedling hydraulic resistance had increased by
>900%. Additionally, non-structural carbohydrates accumulated in aboveground tissues at the end of both summers, suggesting
impairments in phloem-transport from needles to roots. This resulted in low carbohydrate concentrations in roots, which likely
impaired root growth and water uptake at the time of critically low soil moisture. While photosynthesis and respiration on a leaf
area basis remained high until critical hydraulic thresholds were exceeded, modeled seedling gross primary productivity declined
steadily throughout the summers. At the time of mortality, modeled productivity was insufficient to support seedling biomassgain
rates, metabolism and secondary costs. Thus the large-scale mortality events that we observed near the end of each summer
were most directly linked with acute, episodic declines in plant hydraulic function that were linked with important changes in
whole-seedling carbon relations.
carbon balance, hydraulic resistance, non-structural carbohydrates, productivity, respiration, Pinus flexilis
This material is based upon work supported by the National Science Foundation under Cooperative Agreement #DEB-1637686. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necesarily reflect the views of the National Science Foundation.
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