Predicting angiosperm tree mortality based on embolism resistance, cambium vitality and intervessel pit membrane thickness

Abstract

Drought is one of the most challenging environmental factors inducing world-wide forest die-back and tree mortality. In this thesis, xylem embolism resistance (P50, i.e., the xylem water potential at 50% loss of hydraulic conductivity), cambium vitality and intervessel pit membrane thickness (TPM) was investigated to predict drought-induced lethal water potential in angiosperm tree species. A drought-rewatering experiment was conducted on five common European tree species, and lethal water potential was estimated based on the leaf photosynthetic recovery rate upon rewatering. Cambium ultrastructure during drought stress was visualized under light and transmission electron microscopy (TEM) in the root of C. avellana. Data on TPM and P50 was collected from a broad range of woody angiosperm species. Fresh samples directly fixed for TEM was investigated from 71 species while non-fresh samples were from an additional 60 species. The lethal water potential of studied five species correlated with stem P99 (i.e., the xylem water potential at 99% loss of hydraulic conductivity, PLC). However, several plants that were drought-stressed beyond water potential at 100% PLC showed complete recovery, suggesting that the lethal water potential might be underestimated. C. avellana seedlings that were moderately water-stressed with water potential less negative than P50 (-2.0 MPa), showed intact cambial cells. In contrast, in severely drought-stressed seedlings with water potential more negative than P50, the cell membranes of cambial fusiform cells were detached from the cell wall, with shrunken and/or damaged cytoplasm. However, ray cambial cells remained alive for a longer period than fusiform cells. Death of cambial fusiform cells corresponded to more or less the lethal water potential of C. avellana, suggesting that drought-induced mortality of this species is closely associated with cambial death. A strong relationship betweenP50 and TPM was found on fresh material (r = 0.78, P < 0.01, n = 37), demonstrating that TPM measured on fresh samples provides one of the strongest wood anatomical correlates of drought-induced embolism in angiosperm.