Importance of stemflow to soil water replenishment in a montane forest of Popa Mountain Park, Myanmar

Stemflow (SF), rainwater that reach the ground by flowing down along the branches and trunk, is important in maintaining soil water content (SWC) and recharging groundwater in a forest. This study aims to investigate the importance of SF to soil water replenishment in a montane forest. Measurements of gross rainfall, SF at 9 trees, SWC at two different points for two soil depths and infiltration rates at the two points were carried out in Popa Mountain Park (PMP) in 2019. SF rates in PMP were high ranging from 4.0% to 18.8% of total gross rainfall. Mean SWC near the tree were 18.5% at 5 cm depth and 21.7% at 15 cm depth, respectively, while those outside the canopy area were 11.4% and 9.0%, respectively. SWC near the tree were significantly higher for both soil depths. Similarly, significant higher infiltration rate was found near the tree. Near the tree, infiltration rate exceeded the amounts of individual rain events helped to store more rainwater as SWC in deeper soil layers. In PMP, thus, vegetative cover particularly forested areas are expected to have hydrological advantages in restoring rainwater through a large amount of infiltrated SF into the soil.


Introduction
Stemflow (SF) is rainwater that reaches the ground by flowing down along tree branches and trunks [1].It plays an important role as a point source in recharging the ground water and maintaining soil water content (SWC).Due to a small partition amount from rainfall, however, most of the studies neglected SF [2].Several studies have discussed that even small partition amount of stemflow could restore SWC, especially in arid and semi-arid regions with limited rainfall [3].Price [4] stated that montane forest could provide 60-80% of freshwater despite its coverage areas was only 28% of the world's total forest areas.These water provisioning services by the montane forest could possibly be due to the efficient water replenishment by the trees through SF.Furthermore, some studies proved that SWC and infiltration rates decreased with increasing distance from tree trunks [5].Consequently, this condition generated a large amount of surface runoff with less restored water to the soil [6] and soil erosion.Nowadays, this becomes a common ecological and hydrological issue in most of the tropical countries due to forest degradation and deforestation.
Popa Mountain Park (PMP) is enclosed in central part of Myanmar, which is an arid region with limited rainfall and limited surface resources.Soil erosion is not the main issue in PMP but restoring rainwater is a crucial hydrological service because natural water springs and artificial wells are the main water sources for domestic and other uses [7].Therefore, the study related to the soil water replenishment by the trees is crucial and fundamental research subject for PMP region to understand how the trees restore the soil water content through SF by enhancing the infiltration rate and reducing the surface runoff.However, there has been no scientific studies related to soil water replenishment via SF.In this study, SWC was measured near the tree and at an open space between the trees (the term "outside the canopy area" will be used for this point) to examine how the trees restore the rainwater to the soil through SF.

Study area
Popa Mountain Park (PMP) is located between latitude 20° 47ʹ to 25° 56ʹ N and longitude 95° 12ʹ to 95° 20ʹ E in Kyaukpadaung Township, Nyaung-U District and Mandalay region (Figure 1).PMP was established as a protective area in 1989 for the objectives of protecting Popa catchment area to achieve sustainability for water sources, e.g., natural water springs.Average annual precipitation ranges from 630 mm to 1500 mm but most areas get no more than 750 mm [7].

Field measurements
Diameter at breast height (DBH) was measured using diameter tape (Forestry Suppliers, USA), tree height (Ht.) and crown thickness using a Vertex IV hypsometer (Haglof, Sweden), and crown projection area (CPA) using spacing tape (Forestry Suppliers, USA).Bark texture was visually observed in the study site, and was defined as smooth, semi-rough and rough bark textures.In the same way, crown shape was visually observed in the site, and was defined as erect, semihemispherical and hemispherical crown shapes.Figure 2 illustrates the classification of bark textures and crown shape in this study.
Measurements of gross rainfall (Pg) and SF were conducted during the rainy season in 2019.Pg was measured using a tipping bucket rain-gauge (TBRG) (RainWise, USA) in an open area, 40 m away from the 0.16 ha experimental plot as shown in Figure 1.SF was measured at 9 trees, which were randomly selected from widely distributed deciduous tree species in PMP to be representative ones.Among 9 trees, 3 trees were measured by TBRGs (RainWise, USA) and the other 6 trees were manually measured using hand-made gauges.Size of storage tank used for hand-made gauges was 30 Liters.Hose pipe was attached around the tree trunk using silicone glue, then it was connected to TBRG and hand-made gauge.Measurement interval for Pg and SF by TBRGs was 10 min and the data were stored using a RainLog TM Data Loggers.Collecting time for the SF manual gauges was within one day after the rain had stopped.Collected SF were measured using cylinder cube.Accuracies of manual gauges were tested using linear relationships with TBRGs (R 2 = 0.76 to 0.89).Installation of TBRG and manual gauges for SF measurements and SWC sensors are shown in Figure 3. Detail layout of SWC measurement points is described in Figure 4.In this study, SWC measurement points were decided based on the followings: (1) the importance value index (IVI) of species observed in the experimental plot, and ( 2) an open space between the trees.Two different points, i.e., near the tree (1.6 m distance from the tree trunk) and outside the canopy area (4.0 m distance from the tree trunk) were selected within the same experimental plot.Reason of selecting outside the canopy area was to avoid the direct effect of throughfall to SWC.The 5TE sensors (Decagon) were installed at 5 cm and 15 cm soil depths to measure SWC under the same Pg.Measurement interval was 1 min and the data were stored using Em50 data loggers (Decagon).SWC data from August 15 to August 21 was lacking due to battery error.
As a reference, infiltration rate was measured near the tree and outside the canopy area within the same experimental plot during Feb, 2019 (before the rainy season) using the double-rings infiltrometer (30 cm diameter for inner ring, 60 cm diameter for outer ring and 50 cm height for both rings).About 20 cm height of both rings was put into the soil to measure infiltration rates as shown in Figure 5.

Data analysis
Importance value index (IVI) gives the dominant tree species in a forest or a plot.It is determined by the relative dominance (RD) and relative abundance (RA) of each species in a forest or within a plot.In this study, RD, RA and IVI were calculated by the following equations [8]: Basal area (BA) is calculated by the following equation: In the experimental plot, stemflow occurred even without rainfall but under the dense foggy conditions.Therefore, partitioning of total SF into SF by rainfall and SF by fog were carried out in order to emphasize the SF funneling ratio just by rainfall (SFRrain) by excluding the effect of fog to SF. Detail partitioning method and calculations can be referred in the study of Zaw and Oue [9].The  SFRrain for the respective sampling trees was calculated by referring the equation in previous studies [10][11] as follows: , where Pg is rainfall. ( Kostiakov is the most used infiltration equation due to its simplicity and capability of fitting with the field data.Infiltration rate was calculated by the following equation [12]: where, I is the cumulative infiltrated water (mm), t is the infiltration opportunity time (min), and a and b are parameters determined from the plot of cumulative infiltration versus cumulative time in log forms.Basic infiltration rate (IB) is a constant infiltration capacity under the saturated soil condition [13] and determined directly from the measurements on the site.Hydrographs of Pg with SWC for near the tree and outside the canopy area were drawn to investigate SWC decreases with increasing in distances from the tree trunk.Student t-tests were conducted to examine the significant differences not only for SWC but also for infiltration rates.

Characteristics of stemflow sampling trees
SF sampling trees were randomly selected from widely distributed deciduous tree species in the experimental site [14].According to IVI, Terminalia bellerica was the most dominant species in the experimental plot.Detail descriptions for characteristics of the respective stemflow sampling trees are summarized in Table 1.

Stemflow and stemflow funneling ratios of the respective trees
Total Pg was 753.6 mm by 29 events during the entire observation period (30 th Jun to 2 nd Nov, 2019).Individual rain events ranged from 0.8 mm to 67.2 mm with the rainfall intensities ranging from 0.5 mm/h to 16.8 mm/h, while the individual SF events ranged from 0.0 mm to 28.2 mm of the total Pg.Table 2 describes the partitioning of total SF (SFall) into SF from rainfall (SFrain) and SF from fog (SFfog), and SF funneling ratio just by rainfall (SFRrain) for each tree.In the experimental site, SFrain rates ranged from 4.0% to 18.8% of total Pg under large SFRrain which ranged from 3.6 to 22.5.This result showed that the funnel effect of all SF trees was efficient to catch rainfall and deliver SF to the base of tree.Stemflow funneling ratio (SFR > 1) indicates that the volume of stemflow generated by the tree exceeds the rainfall volume which have been captured by a rain-gauge having an equivalent area as the tree basal area [15].

Comparison of infiltration rates near the tree and outside the canopy area
Basic infiltration rate (IB) was determined from the measurements at the experimental site as shown in Figure 6.IB near the tree and outside the canopy area were 138.0 mm/h and 54.0 mm/h, respectively (Table 3).IB near the tree exceeded the amounts of the individual Pg and SF events, which had been described in section 3.2, confirming that the possibility to occur surface runoff was rare.In case of outside the canopy area, however, the possibility to have a large amount of surface runoff would be high.Higher infiltration rate near the tree could be due to high porosity of the soil, especially by root penetration [16][17].In PMP, areas with vegetative cover particularly forests are expected to have hydrological advantages in restoring soil water through stemflow.This result corresponded with the findings of previous studies that vegetation cover, particularly the trees mitigated the surface runoff and consequently prevented the sedimentation in mountain areas due to high infiltration rates [18].Infiltration was measured for 190 min at both points (near the tree and outside the canopy area).Infiltration parameters were determined from the plot of cumulative infiltration versus cumulative time in their log forms as shown in Figure 7. Infiltration rates estimated using Kostikov equation were in good agreement with the measured data.Infiltration capacity near the tree was significantly higher than that of outside the canopy area (p < 0.05) (Figure 8).
Table 3 Basic infiltration rates (IB) for near the tree and outside the canopy area in this study.

Point location
IB Near the tree 138.0 mm/h Outside the canopy area 54.0 mm/h

Comparison of soil water content near the tree and outside the canopy area
Hydrographs of Pg with SWC measured near the tree and outside the canopy area during the whole observation period are shown in Figure 9.In the figure, higher SWC was found near the tree.Student t-tests showed that SWC near the tree were significantly higher than those of outside the canopy area for both soil depths (p < 0.05) as described in Table 5.In some periods, SWC near the tree continuously increased for several days even without rainfall (Figure 9a) compared to those of outside the canopy area (Figure 9b).This was due to SF observed during no rainfall but under foggy conditions, confirming that trees had hydrological advantages in holding and restoring soil water in deeper soil layers compared with non-vegetation area.This result corresponded with the finding of Jian [5] that more rainwater could be conserved in deeper soil layers near the tree compared with an open space.According to timely variation of Pg and SWC near the tree (Figure 9a), SWC at 5 cm depth was higher than at 15 cm depth at the beginning of the observation period.This might be due to the unsaturated conditions of the soil.The delivered SF by the trees was absorbed by the uppermost layer (soil at 5 cm depth), rich in organic matters and litters.This condition could hinder the infiltration process if the soil was unsaturated after very long dry period.This study also found that different distribution pattern of SWC in soil profile at two different points.Near the tree, SWC at 5 cm depth was lower than that at 15 cm depth (p < 0.05) as summarized in Table 4.This could be explained by soil properties and types.In the experimental site, the dominant soil types at 0-15 cm and 15-30 cm are sandy loam and clay loam.Sandy loam soil has high infiltration rate than clay loam soil, which can keep soil water for long hours [19].This property of clay loam soil led to have a high SWC at15 cm depth near the tree.In case of outside the canopy area, however, SWC at 15 cm depth was significantly higher than SWC at 5 cm depth (p < 0.05) as described in Table 5.In an open area without vegetation cover, there was no point source to intercept rainwater and deliver them to the ground as SF like near the tree.This condition consequently led to increase the amounts of surface runoff [20][21].

Conclusion
Findings of this study revealed that more soil water content was stored in deeper soil layers near the tree due to its high infiltration rates.Root penetration could be one of the main reasons for high infiltration rate which exceeded the amounts of individual rain events.Meanwhile, a high probability to have a large amount of runoff was observed outside the canopy area (an open space) due to its poor infiltration rate.This study concluded that areas with vegetative cover, in particular, forested areas are expected to have hydrological advantages in restoring water and maintain soil water content through a large amount of infiltrated stemflow into the soil.This study also suggested that conservation of montane forest should be urged to be less disturbances in Popa Mountain Park.

Figure 1
Figure 1 Locations of Kyaukpadaung township in Myanmar (Left) and Popa Mountain Park in Kyaukpadaung township (Right).

Figure 2
Figure 2 Illustrations for classification of bark textures and crown shapes in this study: (a) smooth bark texture, (b) semi-rough bark texture, (c) rough bark texture, (d) erect crown shape, (e) semihemispherical crown shape, and (f) hemispherical crown shape.

Figure 3
Figure 3 Installation of (a) TBRG, (b) hand-made gauges for stemflow measurements, and (c) soil water content sensors within the experimental plot.

Figure 4
Figure 4 Layout of the soil water content measurement points within the experimental plot.

Figure 5
Figure 5 Measurement of infiltration rate using double-rings infiltrometer in the experimental site.

Figure 6
Figure 6 Infiltration capacity for two different points measured at the experimental site.

Figure 7
Figure 7 Cumulative infiltration versus cumulative time for two different points.

Figure 8
Figure 8 Cumulative time versus (a) cumulative measured infiltration and (b) cumulative infiltration near the tree and outside the canopy area calculated by Kostiakov equation.

Figure 9
Figure 9 Timely variations of rainfall and (a) soil water content near the tree, (b) soil water content outside the canopy area at 5 cm and 15 cm depths under the same rainfall.The period with a lack of data was caused by battery error.

Table 1
Characteristics of stemflow sampling trees in the experimental plot of Popa Mountain Park

Table 4
Comparison of soil water at two different points and different soil depths measured in this study.Standard level of significance is 0.05.

Table 5
Comparison of soil water at different soil depts of the same point measured in this study.Standard level of significance is 0.05.