Turgor loss in large trees: Foliar water relations in Sequoiadendron giganteum

Background/Question/Methods

The Earth’s largest tree species, giant sequoia (Sequoiadendron giganteum), is restricted to locations in the Sierra Nevada Mountains containing abundant melt water from snowpack, but climate models for California consistently predict a substantial decline in this critical water source in the coming decades.  To help maintain favorable hydration status in a changing environment, trees build leaf tissues to reduce the probability that leaf water potential will fall below the threshold where physiological damage is irreversible.  This threshold, known as the turgor loss point (TLP), is a strong metric of drought tolerance that varies both within and among species.  However, it is virtually unknown how the TLP varies within individuals and especially in very large trees.  Our objectives were to (1) understand how close to the TLP giant sequoia operates, and (2) explore within-crown variation of the TLP.  Using a pressure-volume curve approach, we quantified TLPs along a height gradient from treetop to crown base in 12 giant sequoia trees occupying four different groves from Calaveras (Northern end of the range) to Freeman Creek (Southern end).  Midday water potential measurements at upper and lower crown positions provided comparative references for the TLPs.

Results/Conclusions

Turgor loss points systematically decreased with height in all trees at a rate approximating the gravitational potential gradient of -0.01 MPa/m and remained 0.3 to 0.5 MPa below reference midday water potentials, although some TLPs were above the most extreme water potentials reported for giant sequoia.  Variation in the TLP was controlled more by tissue osmotica than by tissue elasticity, corroborating recent research on the determinants of TLPs.  Substituting the tissue osmotic and elastic properties of lower crown foliage into the upper crown foliage would result in the loss of tissue turgor at midday water potentials, highlighting the importance of within-crown variation in leaf tissue properties.  The ability of leaf tissues to donate stored water with changes in water potential (i.e., hydraulic capacitance) was not correlated with height, but appears to play an important role in buffering transient swings in environmental conditions over the short term.  Compared to recent research on giant sequoia stems, tree foliage appears to be more vulnerable to desiccation leading to irreversible physiological damage.