| 其他摘要 | The semiarid region of loess hilly region of China is characterized by its fragile
ecosystems and severe soil erosion. Water resource is limited and precipitation is less
but concentrated. Improving the regional ecological environment is one of the primary
problems. Estimating forest water consumption, especially the plantation transpiration
of exotic fast-growing tree species is important for understanding characteristic of
water use, regional vegetation restoration and the choice of suitable afforestation tree
species in low water use. The Granier-type thermal dissipation probes (TDP) were
applied to measure the tree sap flow dynamics in a natural mixed Quercus
liaotungensis forest and a pure Robinia pseudoacacia plantation at Mount
Gonglushan located in the southern suburb of Yan’an city of Shaanxi Province in the
central part of the Chinese Loess Plateau (36°25.40’N, 109°31.53’E). Air temperature,
relative air humidity, solar radiation, wind speed, and soil water content were
monitored at the same time. In this paper, the characteristics of water use in two
typical forest were investigated, including the diurnal course of sap flow
characteristics and the relationships between sap flux density and environmental
factors, azimuthal and radial variations of sap flux density on xylem trunk and effects
on tree-scale transpiration estimates, estimation stand transpiration of two typical
forest during the growing season of 2009, interannual variations of stand transpiration
and relationships between stand transpiration and biological factors with
environmental factors in 2008-2010. The main conclusions were as follows:
(1) Sap flux densities (F d ) in Q. liaotungensis and R. pseudoacacia reached their
daily peaks earlier than solar radiation and vapor pressure deficit, usually around
10:00 am and 12:00 am, respectively, though the diurnal courses of sap flux density
were generally similar to the daily course of environmental factors. As the season and
leaf phenology progressed, the overall performance of sap flux density was relatively
low at early stage (April-June), high in the mid and late period (July-September), and
rapidly declining in the last stage (October). Exponential saturation function was applied to the data of sap flux density and vapor pressure deficit, and the fitted curves
effectively reflected the sap flow characteristics in different months. The differences
in fitted curves and parameters among months suggested that the transpiration process
in these forest trees was also affected by soil moisture conditions or other
environmental factors.
(2) Sap flux density at four aspects (north, south, east and west) on the trunk
were significantly different, but were highly linearly correlated in two typical species.
Daily whole-tree transpiration throughout the growing season (May to October) could
be well fitted to the corresponding daily total solar radiation and average daily
daytime air vapor pressure deficit using exponential saturation functions. The
differences relative to tree transpiration estimates based on sap flux densities for four
aspects of Q. liaotungensis were typically 30% and 18% in accordance with the sap
flux densities for one and two measurement aspects, respectively. In R. pseudoacacia,
the errors of tree transpiration estimates based on one, two and three measurement
aspects could be 9.00%, 16.21% and 24.50%, respectively, compared with the sap
flux densities for four measurement aspects.
The pattern of radial variations of F d (0-1 m, 1-2 cm and 2-3 cm) of Q.
liaotungensis was falling from a maximum just below cambium to the smaller value
at the inner sapwood. Different types of sap flow radial patterns were observed in
Platycladus orientalis (L.) Franco. F d values at different depths within an individual
were highly correlated with each other both in two species. We evaluated the errors in
estimating daily water use based on single-depth (0-1 cm) and multi-depth (i.e., every
1 cm in the sapwood) measurements. Using a single 1-cm sensor and omitting the
radial variations, errors of daily tree water use may result in overestimation of 27%-90%
of Q. liaotungensis. The outermost sapwood annuli (0-1 cm) contributed water use for
up to 74%±6% in trees with about 2-cm sapwood depth and 65%±7% in trees with
about 3 cm sapwood depth and may be applied to the estimation of tree water use
incorporated with correcting coefficients. On the other hand, omitting the radial
variations of sap flux density, errors of tree water use could be between
underestimation of 42%±3% and overestimation of 21%±3% and application of
relatively long sensors (e.g. 2 cm) may reduce the estimate errors in P. orientalis (L.)
Franco.
(3) In tree-scale water use, DBH could be well fitted with tree water use of Q.
liaotungensis and R. pseudoacacia in different months during the growing season of 2009. Within a monthly timescale, water use by daily stand transpiration was
correlated with daily solar radiation and daily daytime average vapor pressure deficit
in these two forests. Daily average stand transpiration of Q. liaotungensis and R.
pseudoacacia during the growing season of 2009 was 0.53 mm day -1 and 0.33 mm
day -1 , with the maxima of 0.98 mm day -1 and 0.74 mm day -1 , respectively. Total stand
canopy transpiration of two species was 97.38 mm and 71.7 mm, respectively. The
difference of stand transpiration between these two typical species was 35.68 mm.
The relatively low stand transpiration we estimated might be attributed to a relatively
low leaf area index (LAI) and low stand sapwood area.
(4) During the growing seasons of different years between 2008-2010, stand
transpiration of the natural Q. liaotungensis forest were 127.85, 97.37 and 94.86 mm,
respectively. Among the stand transpiration, omitting the radial variations of sap flux
densities the water use of Q. liaotungensis were 85.32, 69.59 and 77.73 mm,
respectively. Considering the radial variations of sap flux densities in Q. liaotungensis,
the water use was 71.10, 53.14 and 46.62 mm, respectively. Ignoring the radial
variations of F d , the water use of Q. liaotungensis would be overestimation of 20.0%,
30.95% and 42.44%, respectively. The proportions of the water use of Q.
liaotungensis and Armeniaca sibirica in the stand water use were basically at a stable
level, indicating that the natural secondary Q. liaotungensis forest was at a stage of
stable growth in these recent years. During the growing seasons of different years
between 2008-2010, stand transpiration of the R. pseudoacacia plantation were 92.3,
61.7 and 66.1 mm. respectively. Daily average stand transpiration of this plantation
during the different years were 0.49, 0.33 and 0.33 mm, with the maximum of daily
stand transpiration of 1.07, 0.74 and 0.90 mm over the 3 years, respectively. Monthly
stand transpiration of two forests showed a significant correlation with LAI,
indicating that LAI could be a good scalar for estimating stand water use in this
region. However, there was no obvious relationship between monthly stand
transpiration and either soil water content or monthly total precipitation even during
the full-leaf periods in these two typical forests. Further to analysis the relationship
between precipitation and stand transpiration of Q. liaotungensis forest, there was a
significant linear correlation between monthly cumulative precipitation and monthly
cumulative stand transpiration (all days before the end day of one month in one year)
in 3 years. However, the slopes of the linear were varied in different years, maybe
attributed to the temporal and spatial distribution or intensity and amounts of precipitation over these 3 years. In an annual timescale, a positive correlation was
detected between yearly total stand transpiration and average soil water content over
each whole growing season, although it was not statistically significant in the R.
pseudoacacia plantation. |
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