| dc.description.abstracteng | Bamboos are grasses (Poaceae) that are widespread in tropical and subtropical regions. As monocots, woody bamboos have thin-wall hollow culms
with abundant parenchyma and the culms are connected by rhizomes. These
properties may imply that bamboos have specifc water use characteristics.
For example, the abundant parenchyma in culms provides a water reservoir
for internal water storage, which may influence the response of water use to
environmental factors; the connected rhizome provides passages for water
transfer between bamboo culms. However, only a few water-use related studies
have been conducted with thermal dissipation probe (TDP) and deuterium
(D2O) tracing method until now. Therefore, in the present study, we aimed at
exploring water use characteristics of four tropical bamboo species (Bambusa
vulgaris, Dendrocalamus asper, Gigantochloa atroviolacea and Gigantochloa
apus) with TDP and deuterium tracing method. At the meantime, to accurately
measure bamboo water use, we also tested and calibrated TDP on the studied bamboo species, and assessed the influence of bamboo water content on
TDP. Additionally, as a less used water use measurement method, potential
error sources of deuterium tracing method were explored when applying it on
bamboos to explore water use, storage and transfer.
As the frst step, the TDP method was calibrated with the stem heat balance method (SHB) and gravimetric readings in B. vulgaris culms in a pot
experiment. In this experiment, it was confrmed that the stem heat balance
method is well suited for bamboos but that thermal dissipation probes need to
be calibrated. In situ, TDP was further calibrated with SHB for the four bamboo species and species-specifc parameters for such calibration formulas were
derived. With the calibrated TDP, we monitored sap flux density (Js) of the
four bamboo species along with three tropical tree species (Gmelina arborea,
Shorea leprosula and Hevea brasiliensis) during a dry and a wet period. During
feld monitoring, we found that some bamboo species reached high maximal Js.
Across bamboo species, maximal Js increased with decreasing culm diameter.
In the diurnal course, Js in bamboos peaked much earlier than radiation andiv
vapor pressure defcit (VPD), and also much earlier than Js in trees. There
was a pronounced hysteresis between Js and VPD in bamboos, which was less
pronounced in trees. Three of the four bamboo species showed reduced Js at
high VPD values during the dry period, which was associated with a decrease
in soil moisture content. Possible roles of internal water storage, root pressure
and stomatal sensitivity are discussed.
Bamboos and other plants may substantially rely on stem water storage
for transpiration. Fluctuations in wood water content (θwood) may lead to errors when estimating transpiration based on Js measurements with the widely
used TDP method. To test the effects of θwood on Js, we conducted a culm
dehydration experiment, monitored bamboos with TDP, and implemented a
steady-state thermal model. Central to the calculation of Js, and thus a major
potential source of error, is the maximal temperature difference between probes
(∆Tmax) which are usually assumed to happen under "zero sap flow" condition.
In the culm dehydration experiment, θwood was found to be highly negatively
correlated with ∆Tmax. In the long-term feld monitoring, soil moisture content was also negatively correlated with daily ∆Tmax, indicating changes in
θwood and a seasonal decrease in stem water storage. The steady-state model
reproduced the θwood to ∆Tmax relationship of the dehydration experiment and
underlined a considerable sensitivity of Js estimates to θwood. Fluctuations
in θwood may lead to substantial underestimation of Js, and subsequently of
transpiration, in commonly applied estimation schemes. However, our model
results suggest that such underestimation can be quantifed and subsequently
corrected when key wood properties are known. Our study gives insights into
the relationship between θwood and TDP-derived Js and examines potential
estimation biases.
Deuterium tracing method was also applied on bamboos to explore water
use, storage and transfer, and its potential error sources were explored. The
deuterium-derived sap velocities correlated with the TDP-derived velocities in
two of the three species. In all species, the deuterium residence time in bamboo
culms was little influenced by the contribution of the culm water storage
to transpiration as estimated by TDPs at different culm heights. Potential
reasons include a small water storage volume in the culms, a low estimated
contribution of the storage to transpiration and high sap flux densities. Daily
culm water use rates estimated by the deuterium and the TDP approaches
correlated linearly (R2 = 0.9) but were by 70% in the deuterium estimates. After
the experiment, culms were cut down and analyzed for residual deuterium,
but concentrations were low which indicates that retention did not play av
major role in causing errors of the deuterium tracing approach. In culms
neighboring the deuterium labeled culms of B. vulgaris and G. apus, elevated
deuterium concentrations were detected indicating water transfer between
culms. Based on the differences in daily water use on labeled culms and the
enhanced deuterium concentrations in neighboring culms of these two species,
we inferred that fve neighboring culms might receive water from the labeled
culms. On contrast, in culms neighboring labeled D. asper culms, only slightly
elevated deuterium concentrations were observed which implies a limited role
by water transfer. However, incomplete mixing as indicated by high variation
among three TDP sensors at a given height may be of particular importance
for deuterium tracing in D. asper. In conclusion, species-specifc differences
among big clumpy bamboos are indicated and the deuterium tracing points to
water transfer among culms. | de |