Stemonitis virginiensis Rex.

Stemonitis virginiensis Rex. View in CoL [1, 0]

Loc 21: On Agl leaves, AET11887, 1 November 2007. [1, 0]

pH of substratum

The resulting pH values in moist chamber cultures differed among species and ranged from slightly acid to alkaline, except for Echinocereus sp. and caudex debris of Agave scabra , which ranged from neutral to alkaline ( Fig. 40 View FIGURES 40–41 ). The development pH for all species recorded in Mapimí Reserve was different. Our findings show that the myxomycetes of Mapimí do not grow in the pH extreme values. Based on the data available, myxomycetes species only occurred in an intermediate range of the entire pH range of each substrate ( Fig. 40 View FIGURES 40–41 ). The highest richness of species was localized in a range of 7.5 to 8.5 (87% of taxa) ( Fig. 41 View FIGURES 40–41 ). The highest pH values of myxomycetes development were recorded in A. scabra , Opuntia rufida , and Yucca sp. , with values close to 9.0 ( Fig. 40 View FIGURES 40–41 ), where species such as Didymium wildpretii and Badhamia melanospora were recorded. Contrarily, the lowest pH development values were recorded in Prosopis glandulosa bark with values below 7.0 ( Fig. 40 View FIGURES 40–41 ).

Water holding capacity of the substratum

In this study, the availability of water varied across all substrates ( Fig. 42 View FIGURES 42–43 ). Water holding capacity (WHC) ranged from 74% of retention in Prosopis glandulosa bark up to 282% in some samples of Agave scabra caudex ( Fig. 42 View FIGURES 42–43 ). There was a strong variation between samples of the same substrate. For instance, Euphorbia rossiana samples showed a narrower variability (144% to 184%), whereas Agave scabra caudex displayed a broader range of variation (83% to 282%). This finding may indicate that the size and structure of the substrates are important in water holding capacity. Our results indicate that the substrates used from Mapimí Reserve can absorb up to three times their water weight, mainly in succulent plant remains ( Cactaceae , Agavaceae , and Euphorbiaceae ), except Prosopis glandulosa bark.

Based on the results, the substrates of Cylindropuntia leptocaulis , C. tunicata , Echinocereus sp. , and Opuntia rufida presented WHC values from 94% up to 236% ( Fig. 42 View FIGURES 42–43 ). Nonetheless, C. leptocaulis and C. tunicata presented the lowest WHC values for Cactaceae , 148 ± 27 and 134 ± 27, respectively. These two species of Cylindropuntia have smaller cladodes than Echinocereus stems (166 ± 24) or Opuntia rufida cladodes (185 ± 31). The substrates of rosulifolious plants also are comparable with Cactaceae . Agave lechuguilla had WHC values from 181 ± 26, whereas Agave scabra had values from 183 ± 24, and Yucca 164 ± 32. The lowest WHC values of all substrates were reported in Prosopis glandulosa bark (from 74% up to 122%), with an average value of 109 ± 23.

Sixty-nine percent of the species developed in substrates with WHC ranged from 140% to 175% ( Fig. 43 View FIGURES 42–43 ). Nonetheless, Agave scabra caudex presented fungus contamination, because the mycelium covered almost all the substrate. This may also have contributed to being among the less productive substrate with lower species richness. During the hydration process of the Euphorbia rossiana samples, the release of exudates was observed in several of them, which turned the supernatant liquid into a dark green colour. In several cultures with E. rossiana there was no evidence for growth or development of myxomycetes, fungi, dipteran larvae or any other organism normally observed in moist chambers.

Based on the average water retention values at 6, 18, and 24 hours, a rapid decrease in the substrates water content was observed ( Table 3). For example, at 6 hours, the substrates that lost most of the water were Prosopis glandulosa bark, followed by the two species of Cylindropuntia and the caudex debris of Agave scabra . On the contrary, those that retained the highest moisture were Echinocereus sp. , Yucca sp. , Euphorbia rossiana and the two Agave species. At 18 hours, the substrates with the highest percentage of water retention were Yucca followed by Agave scabra caudex and Prosopis glandulosa bark; the latter two, being the substrate that lost most of the water in the first 6 hours ( Table 3). All substrates, except Agave scabra , retained less than 25% of water. After 24 hours, all substrates had lost nearly all moisture, retaining only between 2% and 3% of the water gained during saturation. Only Euphorbia rossiana and Agave scabra caudex retained 6 to 10% of water. Curiously, these substrates are among the least productive and have the poorest myxobiota species. It should be noted that these data hardly reflect water loss under natural conditions, where substrates are usually piled in large quantities or under the shade of living parts of plants, retaining water for a longer time. However, this parameter serves as a reference point to determine which substrates have a greater capacity to maintain humidity conditions for a longer period of time.

When comparing the percentage values of water retained at 6 hours with those reported in other studies, it can be noticed that substrates from the Mapimí Reserve, such as Cylindropuntia species, have similar values to those of lianas (64 ± 11) or to the Neotropical bark trees (65 ± 11), while Prosopis glandulosa bark has moisture retention similar to those of lianas bark (56 ± 17). Nonetheless, all Mapimí Reserve substrates retain a greater quantity of water than those of some cacti bark (26 ± 8) or European trees (26 ± 6), which surely provides them with conditions to support development of myxomycetes in the Mapimí Reserve environmental arid conditions. However, most substrates have practically lost all water within 24 hours, in contrast to some Neotropical tree bark, which retained around 26 ± 20% moisture.