Study site

The study was carried out in commercial stands of two of the most planted clones in the Mediterranean area of Chile during one season (2016-2017). The clones belong to Populus ×canadensis Moench. (P. deltoides × P. nigra) (‘I-214’ and ‘I-488’), established by the Company name “Agrícola y Forestal El Álamo”, near the town “Retiro” in the Maule Region, Chile (36º 05 'LS; 72º 47' LW; 470 m.a.s.l). Both clones were established in 2009 and 2010, respectively. The soil was prepared with a disc plough, and the poplars were planted at a density of 6 m x 6 m. Fertilizers were not applied due to the high fertility of the soil (over 10% organic matter). Homogeneous trees in excellent sanitary conditions were selected for the study.

This area has a Mediterranean climate with a prolonged dry season between November and March (Table S1 [suppl.]). The scarce rain in spring and summer months made it necessary to irrigate the poplar stands, which is carried out by a furrow method. This an efficiency between 50 and 70% depending on the water transport system, which for this test is high, due to the use of Californian pipes to transport the water to each groove. According to the amount of water used by the company and the results of Cañete-Salinas et al., (2019), both clones for this trial were well irrigated and did not present stress conditions.

ETr was estimated using climatic information from an automatic weather station (AWS) (Adcon Telemetría, model A730, Klosterneuburg, Austria) located in reference conditions 2.5 km away from the study site. The AWS measured radiation (W m-2), air temperature (°C), relative humidity (h), precipitation (mm), wind speed (km hr-1) and direction in 15-minute intervals. ETr was calculated from this information by using the Penman-Monteith equation (Allen et al., 1998). T was obtained using the sap flow methodology (Tranzflo New Zeland ltda). Sap f low sensors were installed in 4 trees per clone representative of the stand. For this, homogeneous trees (height and average foliar mass) and in excellent sanitary conditions were selected, in which two temperature sensors were installed asymmetrically on and under a heater (Green et al., 2003). Each of these sensors has 3 transmitters located at 5, 10 and 15 mm. Each of the sensors was coated with aluminium foil to avoid the effect of radiation. The sensors delivered information every 15 minutes. Clone ‘I214’ was measured for 25 days between November and December. Then the same sensors were put in ‘I-488’ for 41 days between January and February. The same sensors were used since the research was a preliminary test, so resources were limited.

Ev was calculated daily using micro-lysimeters located on the irrigation furrows near the selected trees, using a total of 6 devices. The micro lysimeters were constructed with two PVC pipes, the first worked as a soil container, and the second worked as a container to facilitate the extraction of the first one. The container with soil had a mesh in its lower part that allowed the infiltration of water naturally, which was retained in the second recipient, to obtain the real losses by evaporation. The measurement was carried out every day to obtain the daily weight of the wet soil at each point. Then, the differences in weight from one day to the next were transformed to mm of water. It should be noted that soil conditions are homogeneous throughout the stand, having an almost zero slope, with a large percentage of organic matter and soil depths above 80 cm.

ETa was estimated by the methodology proposed by Allen et al., (1998) by adding T and Ev. Finally, for the calculation of the crop coefficient (kc), we used the equation proposed by Allen et al., (1998), where kc is the ratio between ETa and ETr.

Characterization of water status

Furrow irrigation was applied in both stands from November (2016) to March (2017), once or twice per month. The amount of water applied was measured in each of the irrigations carried out during the season using a Parshall-type flume and multiplying the height of the water by a correction factor of 0.2982 (manufacturer's guidelines) to calculate the flow in megaliters per hectare (WFA; ML ha-1). The plant water status was monitored by the xylem water potential (Ψx) using a pressure chamber (PMS Instrument Co., model 1000, Corvallis, Oregon, USA) with the same methodology used by Cañete Salinas et al. (2019). Leaf area index (LAI) was measured using a Nikon Coolpix 4300 digital camera with a Nikon FC-E8 hemispherical converter lens (Macfarlane et al., 2007). A tree calliper was used to determine the diameter of the breast-high increments (DBH-I; the difference of DBH between two consecutive measurements). DBH measurements were performed 3 times during the season, that is, in the beginning, at the middle and at the end of season. Finally, a Water Use efficient Index (WUE-I) was calculated as the ratio between the stem diameter increment (measured by a calliper) and the water applied by irrigation.

Data Analysis

For the statistical analysis, the t-Student test was used, for independent samples, assuming that both transpiration were measured at different times within the same sensors. This test allowed to determine differences between T and kc between two clones ("I-214" and "I-488"). For the normality of the independent samples, the Shapiro-Wilk test was used. On the other hand, to make a graphical comparison, the ETr and vapour pressure deficit (VPD) was used as variables that depend on the climatic conditions and cause modifications on the behaviour of T and kc.

Results and DiscussionTop

As expected, reference ETr showed the highest values during December and January and lower demand at the beginning of the season (November) and at the end of the season (March) (Fig. 1). This is normal behaviour because this period has the greatest radiative demand in the southern hemisphere. No differences were observed in Ev between both clones (Fig. 1). This may be because both experimental devices were close in the same stand with a similar soil texture and both were irrigated with the same amount of water. Daily tree T of clone ‘I-214’ remained relatively constant, between 1- and 3-mm d-1, even on the days of greatest water demand (Fig. 1). On the other hand, T of ‘I-488’ showed values higher than 4.5 mm d-1 during the first days of January to then reach values like those registered in ‘I-214’ (Fig. 1). This behaviour is corroborated by t values (Table S2 [suppl.]), showing that T for I-488 is statistically higher than I-214. Finally, ETa, like the previous variables, remained stable for ‘I-214’ evenduring the end of December. While for ‘I-488’, it showed a higher value for the first days of January, suggesting the effect of climate demand (Fig. 1).

Regarding kc, the clones showed a different behaviour, thus in ‘I-214’ we observed more stable kc values than ‘I488’ (Fig. 1). In this sense, kc of ‘I-214’ fluctuated within values of 0.3 and 0.6 (Fig. 1). On the other hand, kc values of ‘I-488’ were higher than 0.7 during the first days of January (high radiation demand). This could be explained by the limited information collected for I-214 (25 days), while for I-488 there was a total of 40 days allowing a better characterization of this variable. This is corroborated by the independent sample test (t-value), which shows no statistical differences between the kc for both variables, although I-488 is slightly higher (Table S2 [suppl.]).

Although the sample test (t-value) shows differences for T and kc, these may be affected by the climatic conditions that occurred on measurement dates, considering that the data recorded for ‘I-214’ and ‘I-488’ were taken at different times. To solve this, it is necessary to know the behaviour of relevant climatic variables to compare, such as VPD and ETr (Fig. 2). When the environmental demand is between 6 to 7.8 mm d-1, the transpiration of ‘I214’ does not exceed 3 mm, while that of ‘I-488’ is considerably higher and variable, reaching values above 5 mm. Something similar happens for the evaporative demand of the medium represented by the VPD. When this is between 1 and 1.3 kPa, the transpiration of I-214 does not exceed 3 mm, while for the same range, the transpiration of I-488 is variable and manages to reach values above 5 mm. On the contrary, when ETr is less than 6 mm d-1 and VPD less than 1.3 kPa, no differences are observed between both clones. Therefore, when the environmental conditions are more demanding, ‘I-488’ increases its transpiration to a greater extent, showing to be more sensitive for these types of variations.

To further highlight the differences between both clones, the Ψx was studied (Fig. 3). The amount of water applied for both clones was the same, considering some small differences due to the inefficiency of the irrigation system used (furrows). Despite this, the values of Ψx indicated that ‘I-488’ always appeared more stressed than ‘I-214’, with differences of 0.2 MPa between both clones (Fig. 3). These differences have already been observed by Cañete Salinas et al., (2019) in experiments with both clones growing under water restriction conditions, showing that ‘I-488’ is more susceptible to lack of water. O’Neill et al., (2010) mentioned that P. nigra would grant its descendants a certain tolerance to drought conditions. However, this condition would have been inherited only to clone ‘I-214’. All results mentioned above show that ‘I-214’ not only does it have a better transpiration rate than I-488, but it also looks less stressed.

When comparing the values of kc obtained in this study with the information collected in the literature, it can be indicated that there are no works carried out in commercial poplar plantations at low densities (6 m x 6 m = 277 trees ha-1). As for the existing information, in studies carried out by Xi et al., (2017) in poplar plantations at higher density (833 trees ha-1) and five years old, kc values were observed that exceed the value of 1.0.

For similar weather conditions (Mediterranean climates), in Piedmont, Italy, Giovanelli et al., (2007) compared a management situation without irrigation (only contribution by precipitation) and another scenario under irrigation, showing that clone I-214 would be sensitive to lack of water. The Ψx values reached were around -0.9 MPa during the period of greatest drought for I-214, which were much higher than those recorded in this trial, where the trees were always kept under irrigation. In turn, the soil moisture, for the management without irrigation, reached 10%, while for our test the lowest was 20% for I-214. Therefore, the conditions of both experiments were different, explaining the dissimilar behaviour for I-214.

To know the water behaviour of different forest species, it is important to know values such as ETr and kc, but these must be complemented with information such as LAI and increments in growth. The average LAI of both clones during the study season (Table 1), shows that I-214 LAI is higher than I-488. In turn, the growth in diameter (DBH-I) during the season, was double for I-214 and I-488. As previously discussed, I-214 would be more tolerant to adverse conditions in the environment, and it would also be much more efficient, since despite having a higher LAI, it is capable of maintaining a stable water demand concerning the environment and at the same has to maintain high growth rates. Additionally, the WUE-I was calculated, using the amount of water applied through irrigation, concerning the growth in diameter. In this, both clones were applied with similar amounts of water during the season, however I-214, with the same amount of water, was able to grow twice as much (Table 1).

Figure 1.  Reference evapotranspiration (ETr), transpiration (T), evaporation (Ev), actual evapotranspiration (ETa), and crop coefficient (kc), for the clone Populus ×canadensis Moench. (P. deltoides × P. nigra), ‘I-214’ (A) and ‘I-488’ (B).

Figure 2. A) Relationship between Transpiration and Reference evapotranspiration (ETr) for the clones Populus ×canadensis Moench. (P.deltoides × P. nigra), ‘I-214’ and ‘I-488’. B) Relationship between Transpiration and Vapour-Pressure Deficit (VPD), for the clones Populus ×canadensis Moench. (P.deltoides × P. nigra), ‘I-214’ and ‘I-488’.

Figure 3. Water flow rate per month (Q; m3 ha-1) and xylematic water potential (Ψx; MPa), for two clones of Populus ×canadensis Moench. (P.deltoides × P. nigra) (‘I-214’ and ‘I-488’).

Table 1. Water Flow Applied (WFA; ML ha-1), Leaf Area Index (LAI), Diameter of the Breast High Increments (DBH-I; cm), Water Use Efficiency Index (WUE-I; cm ML-1), for two clones of Populus ×canadensis Moench. (P.deltoides × P. nigra) (‘I-214’ and ‘I-488’)