Effects of height on treetop transpiration and stomatal conductance in coast redwood (Sequoia sempervirens)

Treetops become increasingly constrained by gravity-induced water stress as they approach maximum height. Here we examine the effects of height on seasonal and diurnal sap flow dynamics at the tops of 12 unsuppressed Sequoia sempervirens (D. Don) Endl. (coast redwood) trees 68-113 m tall during one growing season. Average treetop sap velocity (VS), transpiration per unit leaf area (EL) and stomatal conductance per unit leaf area (GS) significantly decreased with increasing height. These differences in sap flow were associated with an unexpected decrease in treetop sapwood area-to-leaf area ratios (AS:AL) in the tallest trees. Both EL and GS declined as soil moisture decreased and vapor pressure deficit (D) increased throughout the growing season with a greater decline in shorter trees. Under high soil moisture and light conditions, reference GS (GSref; GS at D = 1 kPa) and sensitivity of GS to D (-δ; dGS/dlnD) significantly decreased with increasing height. The close relationship we observed between GSref and -δ is consistent with the role of stomata in regulating EL and leaf water potential (ΨL). Our results confirm that increasing tree height reduces gas exchange of treetop foliage and thereby contributes to lower carbon assimilation and height growth rates as S. sempervirens approaches maximum height.