Variation in foliage morphology and physiology along vertical and geographic gradients in coast redwood and giant sequoia trees

Background/Question/Methods

Forest canopies are characterized by significant vertical gradients in key variables affecting tree structure and function such as light, vapor pressure deficit, and xylem pressure potential (XPP). Understanding how foliage morphology and physiology change in response to these gradients is important for quantifying whole-tree carbon and water fluxes and for predicting tree and forest ecosystem responses to climate change. To improve this understanding, we examined patterns of variation in leaf structural and functional traits in relation to both site (climate) and height in Sequoia sempervirens (coast redwood) and Sequoiadendron giganteum (giant sequoia) trees. Vertical gradients in shoot and leaf morphology (shoot mass per unit area [SMA] and leaf mass per unit area [LMA]), bulk leaf stable carbon and oxygen isotope ratios (δ¹³C and δ¹⁸O), and leaf gas exchange (photosynthesis, stomatal conductance, and transpiration) responses to experimental drought were characterized in trees of different size and age growing at multiple sites throughout the geographic ranges of both species. A total of 67 trees were sampled from 13 sites, with sample heights ranging from 2.0 - 115.0 m.

Results/Conclusions

We observed small but significant differences in vertical patterns of SMA and LMA across sites within each species but large differences between the two species. LMA did not vary with height in giant sequoia but increased with height in coast redwood, whereas SMA changed with height in both species. Leaf δ¹³C increased with height in both species. At a given height, there was a trend towards more positive δ¹³C values for trees inhabiting drier and warmer sites for both species. Giant sequoia generally had more positive δ¹³C values at a given height and site compared to coast redwood. At most sites the bulk leaf δ¹⁸O for both species did not change with height. However, we found a clear influence of site on the average δ¹⁸O for both species, with drier and warmer sites generally showing the highest δ¹⁸O compared with cooler and wetter sites. Photosynthesis, stomatal conductance, and transpiration showed a linear decline in response to decreasing XPP’s at all sites. Gas exchange rates in trees from warmer and drier sites became zero at lower XPP’s compared to cooler and wetter sites of both species. The implications of these patterns of variation on tree performance will be discussed.