Study zone

The avocado producing region in Michoacan concentrates 148 000 ha (72% of the national surface area). It is distributed mainly in four climates and 40 municipalities. Michoacan represents almost 80% of national production (1.5 million tons per year). The region presents contrasting climates and a phenological behavior of cv. Hass clearly differentiated (^{Rocha-Arroyo et al., 2011}) (Figure 1).

Figure 1 Geo location of the study area. Polygons in green correspond to the parcel pattern, circles in yellow to the selected orchards. 

Description of climates

In the producing region of ‘Hass’ in Michoacan at least 8 types of climate have been identified (^{García, 1964}). For this study were selected the most widely distributed according to the percentage of established area of the crop. The most representative are sub-humid and subhumid temperate (89.21% of orchards) (Table 1).

Selection of orchards Table 2

Reference climatology (current)

The period 1961-2010 was used, for which the climate information system interpolated by INIFAP was used at a resolution of 90 m. The climatic variables obtained from the system were: annual mean minimum temperature (Tmin), mean annual maximum temperature (Tmax), mean annual temperature (Tmed) and annual cumulative precipitation (Pcp).

Climate change scenarios (futures)

We used the INIFAP climate change information system derived from the assembly of 11 general circulation models: BCC-CSM1-1, CCSM4, GISS-E2-R, HadGEM2-AO, HadGEM2-ES, IPSL-CM5A-LR, MIROC-ESM-CHEM, MIROC-ESM, MIROC5, MRI-CGCM3, NorESM1-M for RCPs 4.5 and 8.5 in three temporalities or future climatologies: 2030, 2050 and 2070 in order to characterize the short, medium and long term respectively. The climatic variables were the same ones that were obtained for the reference climatology.

Calculation of climatic variables

For each study site, the value of the climatic variables Tmin, Tmax, Tmed and Pcp were collected. With the Excel Version 14 spreadsheet (Microsoft, Redmond, WA, USA) the mean values for all climate variables were generated by climate type (average of 10 observations).

Phenological schemes

From the diagrams published by ^{Rocha-Arroyo et al. (2011)} chronology was obtained by type of flowering (normal, marceña, surpassed and crazy) and the vegetative flow (winter, spring and summer) by type of climate.

Monthly weather

In order to support the interpretation of the phenological schemes, the minimum monthly temperature of the reference climatology and the climatic change scenarios were included. The monthly data were extracted from the same INIFAP climate information system. Using the Access Version 14 database manager (Microsoft, Redmond, WA, USA) the values of the ten sites of each climate were averaged for each month, scenario and climatology.

Current climatology

The warm subhumid climate (Aw₁) presented the highest thermal indicators of the four climates. Temperatures subhumid (Cbw₂ ) and temperate humid (Cbm(w) were the coldest with Tmín 8.9 °C. Rainfall varied due to the orography of the region. The areas with the lowest precipitation include climates Aw₁ and Cm(w) y thehighest Pcp (1 428.6 mm) is the subhumid temperate climate (Table 3). This coincides with that mentioned by ^{García (1964)} for temperate climates (sub-humid and humid). However, for the warm climate the results were lower (0.3 °C), whereas for the semi-warm climate they surpassed in 1.1 °C the typical value of this climate. The differences found may be due to the data source used, as well as the reference climatology. The result of the analysis of the climatological variables of the present work agrees with the agroecological requirements for the production of ‘Hass’ published by ^{Ruiz et al. (2013)}.

Future climate change scenarios

The behavior of climatic variables under an intermediate GEI emissions scenario (RCP 4.5) surpasses 2 °C increase by the end of the century in the four climates of the Hass avocado producing region in Michoacán. The most representative increase occurs in the Tmax and Tmed in the temperate sub-humid climate. In contrast, Pcp shows decrease in all horizons and climates, with mean values of -40 mm. The mid-century horizon (2050) stands out with a drop in precipitation from 3.56% in the subhumid temperate climate to 4.20% in the semi-humid sub-humid climate (Table 4).

Climate change in a scenario of high GEI emissions (RCP 8.5) shows the greatest differences in reference climatology in all climates (1961-2010). Temperature increases of more than 3 °C, which highlights Tmax with 3.74 °C in the horizon 2070. The exchange rate between the middle and end of the century in all thermal variables is greater than 1 °C. As far as Pcp is concerned, all the climates and all horizons of the decrease, being the 2070 horizon where the decrease (less than 120 mm) is accentuated, which represents on average 10% of the Pcp in each of the four climates (Table 5).

The increase in Tmin according to the scenarios RCP 4.5 and 8.5, would be beneficial to reduce the risk of exposure to low temperatures (<10 °C) which could affect the development of the crop, especially in the flowering phase (^{Whiley and Winston, 1987}; ^{Zamet, 1990}). It is possible that such conditions occur in the four climates for the three horizons.

The increase in maximum temperature could represent a limiting factor for cv. Hass in Michoacán. The floral opening periods of ‘Hass’ are usually 40 to 90 days, with the periods being short for high temperatures during flowering. However, high temperatures (33 °C) during flowering not only shorten the period of flower opening, but also reduce the number of flowers that open (^{Sedgley and Annells, 1981}) and decrease the viability of pollen. The short periods of flowering reduce the probability of harvesting of fruit and consecutively the production of fruit.

After the pollination and fertilization of the ovule the formation and development of the embryo begins. In cv. Hass initial embryo growth is also sensitive to high temperatures obtaining maximum initial growth at 25/20 °C (day / night) and death of embryos at temperatures 28 - 33 °C (^{Sedgley and Annells, 1981}). The fruit fall (< 5 mm) in avocado is also caused by the maximum extreme temperatures (R2= 0.72), as well as the low relative humidity (R2= 0.87) associated with high temperatures (^{Lahav and Zamet, 1999}). The conditions mentioned will only be presented in the warm sub-humid climate under the RCP 8.5 scenario in the 2070 horizon. However, increasing temperature in combination with a decrease in precipitation will increase crop stress (increased evapotranspiration), impact of temperature increase (^{Ruiz et al., 2011}).

Thermal threshold for floral development

The climatic change only allows to affect the process of floral development in the climates of the producing region of avocado in Michoacán (warm subhumid and the semicalido subhumid). Temperate climates still remain below the critical threshold (Tmín ≤ 16 °C) (^{Álvarez-Bravo and Salazar-García, 2015}). In the warm sub-humid climate in the three horizons of the intermediate stage in GEI emissions the Tmin exceeds the threshold for seven months (april - october) (Figure 2A). In the sub-humid semi-warm climate, the threshold of 16 °C in june will be exceeded by the end of the century horizon (Figure 2B).

Figure 2 Tmin monthly for RCP 4.5 in three horizons and ‘Hass’ avocado phenology in climates: a) warm subhumid Aw₁; b) sub-humid sub-fluid (A)Ca(w₁)+(A)Ca(w₂); c) subhumid tempering Cb(w₂); and d) subhumid monsoon tempering Cbm(w). “F” refers to flowering and “FV” to vegetative flow. 

However, in the RCP 8.5 scenario not only increase the months of exposure to the critical threshold (april - december for the 2070 horizon) but also a Tmin of 20 °C in summer (Figure 2A). As far as the sub-humid semi-warm climate is concerned, it is only in the 2070 horizon that the line of 16 °C in the period june-october is expected to surpass (Figure 2B). This shows the vulnerability of cv. Hass report on climate change, coinciding with other studies concluding the negative impact of climate change on phenological stages sensitive to high temperatures (^{Howden et al., 2005}; ^{Putland et al., 2011}).

This vulnerability of cv. Hass is recommended to be included in the future near a line of research aimed at the search of germplasm with the best adaptive response to high temperatures and its incorporation into genetic improvement programs, in order to obtain various best points to face the climate change of the present century.

Inflection of the minimum temperature

It is accepted that the transition from a warm period to a fresh one (decrease in minimum temperature) promotes the onset of floral development by inhibiting vegetative growth (^{Salazar-García et al., 2013}). This inflection occurs in the four climates in the month of july in the reference climatology. However, in the average emissions scenario, there are increases in temperature in the three horizons but July prevails as the month of inflection (Figures 2A, 2B, 2C and 2D). While the monthly climatology in the high emissions scenario will have a negative impact on the four climates. In the warm and semi-humid sub-humid the inflection will move one month forward (august) (Figure 3A and 3B).

Figure 3 Tmin monthly for RCP 8.5 in three horizons and ‘Hass’ avocado phenology in climates: A) warm subhumid Aw₁. B) sub-humid sub-fluid (A)Ca(w₁)+(A)Ca(w₂); C) subhumid tempering Cb(w₂); and D) subhumid monsoon tempering Cbm(w). “F” refers to flowering and “FV” to vegetative flow. 

While in the subhumid and subhumid temperate monsoon the inflection continues until two months later (september) (Figures 3C and 3D) with consequences in the possible disappearance of the marceña flowering. ^{Rocha et al. (2010)} indicated that the irreversible determination to flowering occurs earlier in temperate than in warm climates.

Considering the departure of the scenario RCP 8.5 that assertion will change in the horizon of mid and end of century. These changes in the climatic pattern will promote phenological alterations that will surely impact the quantity and quality of the fruit to be harvested, as shown by works carried out in Michoacán (^{Salazar-García et al ., 2011}; ^{Salazar-García et al., 2016a}; ^{Salazar-García et al., 2016b}).