Rock as protection from heat. Mammillaria microclimate

A hot region and a cooler place on the rock

Mammillaria crucigera is an accepted species occurring in Puebla and Oaxaca in Mexico. Martorell and Patiño conducted their study on a gypsum cliff in Oaxaca, at an elevation of 1050 m above sea level. The surrounding vegetation was seasonally deciduous tropical forest. Annual precipitation reported for the area was 743,4 mm. The experiment took place in April 2000, at the end of the dry season, when local temperatures are particularly high. [Martorell and Patiño 2006, p. 543]

In the area described, the cactus occupied rock walls and vertical parts of boulders. Stems were approximately 7 cm high and 5 cm in diameter, and many plants were highly branched. A small stem lies close to the substrate and exchanges heat with the layer of air immediately around it. On an exposed surface, the ground may be much warmer than the air measured higher up. A position on a cliff changes this local arrangement. [Martorell and Patiño 2006, pp. 542–544]

Six plants and measurements inside the stem

Six unbranched, vertically growing plants were selected from an area approximately 2,3 m below the upper edge of the cliff. Three were moved to flat ground above, and three were replanted on the rock wall. The cliff studied had a slope of 80° and an aspect with an azimuth of 283°, close to west-facing. Replanting plants on the cliff served as a control for the transfer procedure itself. [Martorell and Patiño 2006, pp. 543–544]

Thermocouples were placed in the outer 3 mm of the stem, near the apex and on the north side of the flower-bud zone. Over two days, temperature was read hourly during the day and until 22.00, with additional nighttime readings at 2.00 and 6.00. The temperatures of the substrate surface and air at distances of 1, 3 and 6 cm from it were also measured. Air sensors were shielded from direct radiation while retaining airflow. [Martorell and Patiño 2006, pp. 543–544]

The result describes a short field experiment with three plants in each position. Temperature changed over the day, so repeated measurements were analysed. This arrangement allowed the effect of location on heating at a given moment to be identified, retaining the same plant and stem part for successive readings. Survival in subsequent seasons and photosynthetic rate were outside the scope of this measurement. [Martorell and Patiño 2006, pp. 543–546]

The largest difference occurred at thirteen hours

At 13.00, plants on flat ground reached a mean of 47,6°C, and the highest recorded measurement was 51,4°C. The mean temperature of plants on the cliff was 9,9°C lower at that time. The substrate surface above the wall averaged 52,2°C and was 14,4°C warmer than the substrate on the cliff. The plant temperature difference and the ground temperature difference are two separate results. [Martorell and Patiño 2006, pp. 546–547, Fig. 2]

On flat ground, air temperature increased closer to the ground surface. Beside the rock wall, layers at distances of 1–6 cm had more similar temperatures, and the plants remained closer to ambient temperature. The strongest separation between the two positions concerned the daytime maximum. At night, the direction of the difference reversed, and plants on the cliff were slightly warmer. [Martorell and Patiño 2006, pp. 546–547, 550, Fig. 2]

The authors link the nighttime difference to the exchange of longwave radiation. An open horizontal surface loses heat towards the sky, while the rock wall also receives thermal radiation from the ground below. They also consider wind-driven air mixing near the cliff edge as an additional component of daytime cooling. The measured temperature pattern supports these explanations, but the experiment does not quantitatively separate the contribution of each process. [Martorell and Patiño 2006, pp. 549–550]

Changes in light timing and the radiation balance

On the measurement days, the western wall received direct light only after 10.52. Near noon, it received less than one third of the radiation calculated for horizontal ground. Sunlight struck the steep surface at a large angle to its perpendicular, reducing energy input per unit area. Rock geometry limited heating when the thermal load on flat ground was greatest. [Martorell and Patiño 2006, pp. 544, 547, Fig. 2]

Annual direct radiation input was estimated in a model for a spherical cactus 5 cm in diameter. Assuming cloudless skies, it was 23,3 MJ per plant on flat ground and 14,4 MJ per plant on the wall studied. The difference corresponds to approximately 38,2% of energy input. These are modelled totals for the whole plant. They are not light-intensity readings or values in MJ per square metre. [Martorell and Patiño 2006, pp. 544, 547–549]

Separate simulations of walls facing exactly east and west gave an annual reduction of 35,8%, north-facing walls 43,4%, and south-facing walls 7,5%. The seasonal distribution of light also changed. At this latitude, northern and southern walls had opposing illumination patterns in summer and winter. The azimuth of the cliff studied and the azimuths of the four modelled aspects are different cases, so their annual values are not identical. [Martorell and Patiño 2006, pp. 544–549, Fig. 3]

Lower radiation input reduces the thermal load and also limits the amount of light available for photosynthesis. In the actual habitat, clouds and periodic shadows from neighbouring plants add further effects. The authors compare the measured temperatures with the physiology of other cacti to discuss possible benefits for assimilation. CO₂ exchange in M. crucigera was not measured in the experiment itself, so improved photosynthesis remains a physiological interpretation of the microclimate change. [Martorell and Patiño 2006, pp. 547–550]

The relationship between habitat elevation and steepness

The second part of the paper compared ten Mammillaria species from the Tehuacán Valley. Ten sites were examined for most species, and nine, five and six for M. crucigera, M. supertexta and M. huitzilopochtli, respectively. Slope was measured with a clinometer, and elevation was determined from maps and GPS. Lower elevation was treated as a proxy for a warmer climate because comparable climate measurements were unavailable for all sites. [Martorell and Patiño 2006, p. 545, Table 1 on p. 548]

Species at lower elevations occupied steeper sites on average. The correlation between mean elevation and mean slope for the ten species was −0,721, with p = 0,009. Separate within-species relationships were not significant after the multiple-test correction used. The comparative conclusion concerns differences between the species included. The same pattern was not demonstrated between sites of each species. [Martorell and Patiño 2006, pp. 547–549, Table 1, Fig. 4]

An independent-contrasts analysis accounting for relationships among species also indicated an association between low elevations and steep habitats. The tree contained six species from an earlier molecular analysis, and the positions of the others were added using taxonomic classifications. The resulting pattern is consistent with the hypothesis of a thermal benefit of living on rocks. Its scope is defined by the species selected, the use of elevation as a temperature proxy and partly reconstructed relationships. [Martorell and Patiño 2006, pp. 545, 547, 550–551]

Mammillaria crucigera and M. huitzilopochtli were the two species in this sample strictly associated with rocks and also occupying the lowest elevations. Steep surfaces create a distinct microclimate for a small cactus. When interpreting a natural site, describing its aspect, slope and timing of direct light therefore helps reconstruct plant conditions better than regional temperature alone. [Martorell and Patiño 2006, pp. 550–551]