10:45 AM - 12:15 PM
[AHW19-P01] Effect of groundwater depletion caused by extreme drought on transpiration activity in a dry evergreen forest
Keywords:ENSO, transpiration, dry evergreen forest
Climate change can influence the biosphere-atmosphere exchange of forests. Cambodia is located on the Indochina Peninsula and has distinct wet and dry seasons, owing to the effects of the Asian monsoon. It has been suggested that the climate change enhances the difference in moisture conditions between dry and wet seasons. Indeed, a huge flood occurred in 2011 (Komori et al., 2012), but a decline of rainfall was observed in the dry season from 2015 to 2016 when a strong El Nino-Southern Oscillation (ENSO) was detected (Kabeya et al., 2020). Despite the high seasonality of soil moisture conditions, the dry evergreen forests, which maintain green leaves during the dry season, exist in areas with deeper soil (Ohnuki et al., 2008). Here, we investigate the effect of the ENSO-induced droughts on the transpiration activity of trees in a dry evergreen forest in Cambodia.
Our measurements were carried out in a dry evergreen forest located in Kampong Thom province, Cambodia (12o44'N, 105o28'E). On the top of the measurement tower (60 m height), shortwave radiation (S) and rainfall (P) were measured. The vapor pressure deficit (D) was measured by a ventilated psychrometer installed at a height of 34 m. The groundwater table depth (GWD) was measured daily. We measured the sap flux density for a Calophyllum thorelii Pierre tree (diameter at breast height, DBH: 23.9 cm) and three Annonaceae sp. trees (DBH No.1: 25.8, No.2: 17.0, No.3: 10.1 cm) using the thermal dissipation method (Granier, 1985). We obtained the amount of transpiration of a tree (Q) as the product of sapwood area and sap flux density in the shallow sapwood (i.e., 0-2.0 cm depth of sapwood). It should be noted that the contribution of sap flow in the deeper part (2.0-4.0 cm) was taken into account as correction coefficients (Delzon et al., 2004). Then, we calculated the crown-level stomatal conductance (Gs) from Q and D (e.g., Iida et al., 2013). To estimate the effect of GWD and the micrometeorological factors of S and D separately, we used the Jarvis type model to fit the Gs behaviors (e.g., Kumagai et al., 2008): Gs = Gsref . f(D) . f(S) . f(GWD) = Gsref . (1 - a/b . lnD) . (S/(S + c)) . f(GWD), where Gsref is Gs when D = 1.0 kPa, and coefficients of a, b and c are the fitting parameters. The analysis period was from 2011 to 2017.
The long-term variations in micrometeorological factors and GWD were analyzed based on their averages for periods, dividing each month into three terms (1st to 10th, 11th to 20th, 21th to the end of the month). The maximum daytime mean D was generally around 2.0 kPa for periods in the normal dry seasons, but at around the end of 2015-2016 dry season, in which the strong ENSO was detected, it reached 3.0 kPa. Although GWD did not drop deeper than 80 cm depth in normal dry seasons, GWD of deeper than 170 cm was found at the end of 2015-2016 dry season, during which all trees showed decreasing trends in Q. For Calophyllum, the declining variations in Q were well explained by the effect of D, however, for Annonaceae, the Jarvis type model did not calculate Gs suitably based on the micrometeorological factors. Annonaceae showed a linearly decreasing trend in Gs with respect to GWD, especially for GWD was deeper than 100 cm. These results clearly show that ENSO-induced droughts affect the transpiration activities of evergreen trees. It is necessary to obtain long-term measurements of transpiration, micrometeorological factors, and groundwater conditions to better understand the effects of climate change on forest-water interactions when considering options for the conservation of dry evergreen forests in Cambodia.
Cited papers:
Delzon et al. (2004) Tree Physiol., 24, 1285-1293.
Granier (1985) Ann. Sci. For., 42, 193-200.
Iida et al. (2013) JARQ, 47, 319-327.
Kabeya et al. (2020) JARQ, 55, 177-190.
Komori et al. (2012) HRL, 6, 41-46.
Kumagai et al. (2008) Agric. For. Meteorol., 148, 1444-1455.
Ohnuki et al. (2008) Hydrol. Process., 22, 1272-1280.
Our measurements were carried out in a dry evergreen forest located in Kampong Thom province, Cambodia (12o44'N, 105o28'E). On the top of the measurement tower (60 m height), shortwave radiation (S) and rainfall (P) were measured. The vapor pressure deficit (D) was measured by a ventilated psychrometer installed at a height of 34 m. The groundwater table depth (GWD) was measured daily. We measured the sap flux density for a Calophyllum thorelii Pierre tree (diameter at breast height, DBH: 23.9 cm) and three Annonaceae sp. trees (DBH No.1: 25.8, No.2: 17.0, No.3: 10.1 cm) using the thermal dissipation method (Granier, 1985). We obtained the amount of transpiration of a tree (Q) as the product of sapwood area and sap flux density in the shallow sapwood (i.e., 0-2.0 cm depth of sapwood). It should be noted that the contribution of sap flow in the deeper part (2.0-4.0 cm) was taken into account as correction coefficients (Delzon et al., 2004). Then, we calculated the crown-level stomatal conductance (Gs) from Q and D (e.g., Iida et al., 2013). To estimate the effect of GWD and the micrometeorological factors of S and D separately, we used the Jarvis type model to fit the Gs behaviors (e.g., Kumagai et al., 2008): Gs = Gsref . f(D) . f(S) . f(GWD) = Gsref . (1 - a/b . lnD) . (S/(S + c)) . f(GWD), where Gsref is Gs when D = 1.0 kPa, and coefficients of a, b and c are the fitting parameters. The analysis period was from 2011 to 2017.
The long-term variations in micrometeorological factors and GWD were analyzed based on their averages for periods, dividing each month into three terms (1st to 10th, 11th to 20th, 21th to the end of the month). The maximum daytime mean D was generally around 2.0 kPa for periods in the normal dry seasons, but at around the end of 2015-2016 dry season, in which the strong ENSO was detected, it reached 3.0 kPa. Although GWD did not drop deeper than 80 cm depth in normal dry seasons, GWD of deeper than 170 cm was found at the end of 2015-2016 dry season, during which all trees showed decreasing trends in Q. For Calophyllum, the declining variations in Q were well explained by the effect of D, however, for Annonaceae, the Jarvis type model did not calculate Gs suitably based on the micrometeorological factors. Annonaceae showed a linearly decreasing trend in Gs with respect to GWD, especially for GWD was deeper than 100 cm. These results clearly show that ENSO-induced droughts affect the transpiration activities of evergreen trees. It is necessary to obtain long-term measurements of transpiration, micrometeorological factors, and groundwater conditions to better understand the effects of climate change on forest-water interactions when considering options for the conservation of dry evergreen forests in Cambodia.
Cited papers:
Delzon et al. (2004) Tree Physiol., 24, 1285-1293.
Granier (1985) Ann. Sci. For., 42, 193-200.
Iida et al. (2013) JARQ, 47, 319-327.
Kabeya et al. (2020) JARQ, 55, 177-190.
Komori et al. (2012) HRL, 6, 41-46.
Kumagai et al. (2008) Agric. For. Meteorol., 148, 1444-1455.
Ohnuki et al. (2008) Hydrol. Process., 22, 1272-1280.