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What Does It All Mean?
Our results for coast redwood support elements of the hydraulic limitation hypothesis of Ryan and Yoder (1997). We found that water potential increases with height as expected from the cohesion-tension model of water transport in plants (see Web Essay 4.2). Because the slope of this relationship was generally close to the gravitational potential gradient, it appears that the long transport path in tall redwoods does not impose a large amount of resistance to water flow. We note, however, that our study trees are all located in sites where soils are very moist throughout most years. The intercepts of the predawn water potential curves estimate that soil Ψ remained close to 0 MPa even during the dry season. It is likely that redwood trees experience larger soil water deficits on hill slopes and ridge tops, in very dry years, and toward the southern and inland limits of the species' distribution. In such conditions we expect the water potential gradient to be steeper and tree top values of water potential to be lower (more negative) than the minimum of about –2 MPa we measured.
Values of foliar δ13C were strongly correlated with height in all study trees, indicating that stomatal conductance becomes increasingly limiting to photosynthesis with height. Strong vertical gradients of light are common in dense forests, and this alone would tend to increase photosynthetic capacity in the upper crown and drive Ci lower even if stomatal conductance was invariant with height. While we do not yet know how stomatal conductance and photosynthesis vary with height, we expect that maximum values are higher in the upper crown. Our gas exchange data indicate that conductance is strongly regulated in the treetop, and we believe stomatal regulation is the main reason behind the high δ13C values in the upper crown. The δ13C values at the tree top approached –21‰, remarkably high considering redwoods occur in very moist environments.
A major component of the hydraulic limitation hypothesis is that stomatal conductance declines in response to low water potential. The observation of a decline in conductance at the time of day of minimum water potential supports this hypothesis. Stomatal conductance may decrease to reduce the risk of xylem cavitation. Given this, a surprising aspect of our study is the high resistance to cavitation observed in coast redwood. Essentially no loss of hydraulic conductivity (i.e., cavitation) was observed at water potentials near the minimum experienced during the dry season.
The large apparent safety margin for xylem cavitation in redwood contrasts with a number of other studies (Sperry, 1995) and has several possible explanations. First, xylem cavitation may be irreversible at great heights in tall trees because re-dissolution of gas emboli requires low xylem tensions, within a few tenths of a MPa, a situation that is precluded by the water potential necessary simply to hold water against gravity above a few tens of meters of height. Thus, redwood may be "over engineered" because the consequences of cavitation are particularly severe. Second, the high cavitation resistance of redwood may be advantageous during extreme and long-lasting droughts, which impose water potentials much lower than we have measured in the field. Such dry periods are known to have occurred during the past millennium and within the lifetime of our study trees. Finally, it may be that the hydraulic characteristics of redwood stems are in part byproducts of other features related to extreme height growth, (e.g., wood mechanical properties).
A final interesting note on redwood physiology concerns the role of fog in the water relations of this species. Summer fog characterizes much of redwood's geographic distribution. Dawson (1998) has clearly documented the importance of fog drip for soil moisture and root uptake. Recently, we have measured the capacity for direct foliar uptake of fog in coast redwood. Simulated fog events resulted in a significant increase in water potential compared to foliage covered with aluminum foil to prevent transpiration (Koch et al., unpublished data). Although we do not yet know the quantitative importance of direct absorption of fog, it does represent a means of acquiring water that circumvents the energetic requirement of lifting it from the soil. It is noteworthy that we have observed xylem pressures higher than the hydrostatic gradient following nights of rain or fog. Could it be that foliar absorption provides a mechanism for xylem tension to relax to the point where cavitation reversal is possible? We are confident that further study of coast redwood across a range of environments will answer this question and reveal the critical elements of physiology and functional morphology that affect and are affected by plant size.
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