A junction that can break
The stem of Tephrocactus aoracanthus consists of distinct segments. In the mechanical study, 30 branches were collected, each with three successive segments. The junction at the youngest segment was compared with the one lower down. Lower junctions had a larger cross-section and carried a greater load before breaking. [Almirón et al. 2016]
Force was applied transversely using a weighted setup, recording deflection and the point of junction failure. Mean loading mass at breakage was 1.16 kg for the apical junction and 2.19 kg for the lower one. These are masses in the described measurement setup. The result also depends on the length of the force's lever arm. [Almirón et al. 2016]
Sixty labelled stem segments
In a subsequent experiment on an alluvial fan in San Juan, 30 labelled segments were placed in the main runoff channel and 30 outside its reach. Fragments were located after rainfall, and their GPS positions recorded, along with whether they remained in the channel, lay between channels or became caught in vegetation. [Almirón et al. 2019]
Of five rainfall episodes, three generated runoff that moved the material. No control segment changed position. Segments in the channel moved unevenly, and some remained unlocated after successive rains. The reported means describe the retained record of movements. [Almirón et al. 2019]
| Episode | Rainfall and duration | Mean displacement |
|---|---|---|
| First | 35 mm in 2 days | 480.4 ± 210.0 m |
| Second | 100 mm in 4 days | 103.0 ± 138.9 m |
| Third | 20 mm in several hours | 33.2 ± 52.3 m |
Initially, 30 segments were placed in the channel. After the first event, 27 were found, and at the end of the experiment 21, or 70% of the initial number. Some previously lost fragments were found again after the next rainfall. Simply adding the three means therefore does not reproduce the mean journey of the same thirty plants. [Almirón et al. 2019]
Landforms slow transport
In the upper part of the fan, differences between channels and ridges were greater. Lower down, the runoff network divided and the ground became gentler. Vegetation or surfaces between channels stopped increasing numbers of segments. The greatest later rainfall did not produce the greatest displacement, because the material was already in different terrain conditions. [Almirón et al. 2019]
No significant, simple relationship between displacement distance and segment mass or diameter was demonstrated in the comparisons studied. Some combinations of conditions had few observations. Position before rainfall and the shape of the route were essential to interpreting the result. [Almirón et al. 2019]
From transport to a new plant
Here, water transports a fragment capable of vegetative reproduction. Becoming an independent plant still requires living tissues to survive, the fragment to stop on suitable substrate and roots to develop. The field experiment primarily documented transport. The authors related it to earlier knowledge of rooting in this species, and a segment may move again before rooting. [Almirón et al. 2019]
Prepared from the publications read. Metrics, experimental conditions and tabular data were checked against the source material.
Morphology and mechanical properties of junctions: Implications on the success of clonal spread of cacti
Almirón M. G., Martínez Carretero E. E., Navas M. D., Pantano M. V., Morphology and mechanical properties of junctions: Implications on the success of clonal spread of cacti, Journal of Arid Environments, 2016, 128:8-11, doi: 10.1016/j.jaridenv.2016.01.001
A biomechanical experiment on 30 three-segment branches of Tephrocactus aoracanthus. The authors measured segment geometry, joint area, deflection and the load causing separation. The results link the greater brittleness of young joints to clonal reproduction.
Linking hydrological, geomorphological and biological properties in hyper-arid conditions: Distribution processes of a dominant clonal cactus by run-off
Almirón M. G., Martínez Carretero E. E., Navas Kaluza M. D., Rothis L. M., Linking hydrological, geomorphological and biological properties in hyper-arid conditions: Distribution processes of a dominant clonal cactus by run-off, Austral Ecology, 2019, 44(2):209-215, doi: 10.1111/aec.12665
A field experiment on the transport of Tephrocactus aoracanthus segments by water runoff after downpours. It tracked 30 marked segments in a channel and 30 beyond its reach, taking microrelief and interception by shrubs into account. The paper describes transport over hundreds of metres and changes in position after successive rains.
