Chemistry Reference
In-Depth Information
tree, water and nutrients must be transported over increasing distances between the root
and the apex [7]. The water supply to the leaves of the apex becomes constrained by gravity
and hydraulic conductance. These restrictions require a greater stomatal closure to maintain
a minimum water potential to prevent xylem cavitation [15], resulting in decreases in gas
exchange to a point where a positive carbon balance can not be achieved [4]. Some authors,
by measuring transpiration by sap flow observed that the fall in productivity is
accompanied by a decline in gas exchange rates [35, 2]. [28] studied the effects of age on the
transpiration of a forest of
Pseudotsuga menziesii
of about 40 to 450 years of age in Oregon,
USA, and by the sap flow methodology, also attributed to the hydraulic limitation
hypothesis lower transpiration in individuals from the older forest, being that the highest
transpiration of the 40 year-old forest provides further evidence of change in the local water
balance because of its higher transpiration. In [2], on their turn, reported that the hydraulic
limitation hypothesis proposes that the increased distance to be traveled by water inside the
plant reduces the hydraulic conductance of the leaf. If the stoma closes to regulate the status
of leaf water potential, taller trees will close their stomata at low vapor pressure deficits
when compared to younger or shorter trees. Again, this report confirms the behavior
observed in the plot and watershed scales to
Gs = f (VPD)
(Fig. 2d, Table 3), however it was
not the behavior observed for transpiration.
It is recognized that low pressure of water vapor between the leaf interior and the outside
air (VPD) is an important environmental factor that affects the functioning of stomata.
However, the causes for this event are still much discussed in the literature. The [44]
examined the stomatal response to VPD in higher plants and the possible mechanisms
proposed to explain this response. According to the author, the results are conflicting. When
there is stomatal response to VPD, the mechanism that causes this response is also not well
understood, being two hypotheses proposed for this mechanism. The hypothesis of
"feedforward," which considers the decrease of Gs directly with increasing VPD, and
abscisic acid (ABA), the signal for the response. In the event of feedback, Gs decreases with
increasing VPD due to the increase in leaf transpiration, which lowers the water potential in
the leaf. That is, the increase in E could be responsible for stomatal closure due to increased
water potential gradient between guard cells and other epidermal cells or simply by
reducing the leaf water potential [11, 27, 43]. These two mechanisms have been the subject of
debate in the scientific community, for there are results published in the literature to
support both hypotheses.
In any case, our results agree with the behavior explained by the hypothesis of feedback,
even because we did not analyze the ABA during the study. [44] concludes his work as an
unresolved issue, justifying the continuation of research in this area.
The hydraulic limitation hypothesis in some other studies failed to explain the reduced
growth [2, 36] and the mechanism responsible for this fact was not identified. The [33]
believes that there is no universal mechanism to explain the decline in productivity with
increasing tree height, but that there are various components involved.
In searching for the characterization of the ecophysiological behavior of eucalyptus at
different ages, [10] related to leaf area and rate of growth of
Eucalyptus globulus
Labill at the
