Journal
TREE PHYSIOLOGY
Volume 30, Issue 10, Pages 1260-1272Publisher
OXFORD UNIV PRESS
DOI: 10.1093/treephys/tpq064
Keywords
climate change; hydraulic limitation; sapflow; tree water use; water potential
Categories
Funding
- Save-the-Redwoods League, Global Forest Science
- National Science Foundation [IOB-0445277]
- Kenneth L. Fisher Chair in Redwood Forest Ecology at Humboldt State University
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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 (V-S), transpiration per unit leaf area (E-L) and stomatal conductance per unit leaf area (G(S)) significantly decreased with increasing height. These differences in sap flow were associated with an unexpected decrease in treetop sapwood area-to-leaf area ratios (A(S):A(L)) in the tallest trees. Both E-L and G(S) 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 G(S) (G(Sref); G(S) at D = 1 kPa) and sensitivity of G(S) to D (-delta; dG(S)/dlnD) significantly decreased with increasing height. The close relationship we observed between G(Sref) and -delta is consistent with the role of stomata in regulating E-L and leaf water potential (Psi(L)). 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.
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