The underground stem and the cost of Lophophora regeneration

Two organs in one swollen structure

In Lophophora williamsii (formerly Anhalonium williamsii), the soft, bluish-green crown projects slightly above ground. Beneath it lies a stem portion covered with brown cork tissue, gradually transitioning into the root. In 2006, Terry and Mauseth studied five plants, from young specimens to a large adult. Sections confirmed that the external continuity of the swollen structure conceals a boundary between two organs. Underground position alone does not determine tissue identity. [Terry and Mauseth 2006, pp. 571–585]

In the stem, the extensive cortex contains its own xylem and phloem bundles, and the centre is filled with parenchymatous pith. In the older root, the cortex-like zone arises from secondary phloem. Central parenchyma expands through divisions of xylem-associated cells and contains scattered xylem conducting elements. This organisation explains why similar-looking sections have different developmental origins. [Terry and Mauseth 2006, pp. 582–589]

Buds remaining underground

An areole is a shortened lateral cactus shoot. Its meristem can produce a new branch. In the plants studied, remnants of areoles and living tubercles also persisted on the underground portion. New crowns grew from these sites after loss of the apex. Some replacement shoots formed their own adventitious roots and gradually became independent of the dying parent portion. Formation of several crowns therefore describes an architectural change and possible clonal development. [Terry and Mauseth 2006, pp. 567–581, 588–591]

One hundred plants observed over years

In March 2008, Terry and colleagues marked 100 plants in one population in the scrub of southern Texas. Fifty were left as controls, while the above-ground crown was removed from the others, retaining the underground part. The study measured the response to crown loss in the natural habitat. After two years, some surviving plants were left to continue regenerating, while others underwent the same damage every two years. Three publications present successive observations from the same experiment. [Terry et al. 2011, 2011, pp. 664–666. 2012, p. 568] [Terry et al. 2012, 2011, pp. 664–666. 2012, p. 568]

The size of the above-ground part was estimated from crown diameters, treating each as a hemisphere and summing their volumes. This allowed plants to be compared without damaging controls. The authors checked that the estimate was strongly related to tissue fresh mass. Volume, crown number and survival measure different consequences of regeneration, so they were assessed separately. [Terry et al. 2011, pp. 665–667]

The first two years of regrowth

After two years, survival was 89,6% in the crown-removal group and 97,9% in the control. The difference was not statistically significant. Two plants uprooted by feral pigs were excluded from each group, leaving 48 per group for survival assessment. Meanwhile, the change in stem size was clear. Mean above-ground volume after back-transformation from logarithmic values was 4,01 cm³ in regrowing plants and 22,75 cm³ in the control, with 90 living specimens analysed. [Terry et al. 2011, pp. 666–670]

More crowns formed after damage, but each was smaller. Branching increased the number of visible units, while total green-tissue volume remained greatly reduced. Recovery is described by the size of the entire above-ground part. Its further growth also depends on the time during which regenerated stems can assimilate carbon. [Terry et al. 2011, pp. 666–670]

Repeated damage changes the balance

The next publication showed that in 2010–2012, plants after a single crown loss had a growth rate similar to the control but began this period with a much smaller above-ground part. Plants damaged again regenerated less effectively. Fresh mass of tissues obtained at successive crown removals fell by an average of 44% and then by a further 32%. These were comparisons of the same plants between dates, covering 20 and 16 specimens, respectively. [Terry et al. 2012, pp. 569–575]

The authors linked this course to expenditure of underground reserves. After loss of the green crown, new tissues form at the expense of resources already accumulated. Their subsequent replenishment requires photosynthesis in the replacement shoots. This explanation matches the observed shrinking of plants, but the experiment did not measure quantities of stored sugars or their flow. Wounds and possible infections could also influence survival. [Terry et al. 2012, pp. 571–574]

Six years of regeneration

In the final interval assessed, 2012–2014, survival was 76,5% in plants damaged every two years, 100% after a single injury and 97,7% in the control. The total sample for calculating survival during this interval comprised 80 plants. These values concern the final two years and refer to specimens alive at the start of the period, after excluding losses attributed to animals. They are not percentages surviving from the beginning of the six-year study. [Terry et al. 2014, pp. 542–545, Table 1]

After six years, mean estimated volume was about 35 cm³ after a single injury and 48 cm³ in the control. The 27% difference was not statistically significant. However, power analysis showed that with this sample and plant variability, only a loss of about 42% or more was detectable. The study indicates continued regeneration with limited ability to resolve its final extent. Repeatedly damaged plants remained clearly smaller. [Terry et al. 2014, pp. 545, 548–549]

Regeneration after natural freezing damage

A similar activation of lateral buds was observed after natural destruction of the apex. After the February 2011 frost, which occurred during drought in western Texas, Terry moved one specimen with a dried crown and living underground part into a greenhouse. Three replacement shoots appeared after eight weeks and a fourth after eleven. The observation documents a particular case of regeneration. Its course already depended on greenhouse conditions, while the temperature beside the plant in the field remained an estimate. [Terry 2011, pp. 330–334]

The subsequent fate of the study site

In 2019, the Cactus Conservation Institute published an account of further visits. In 2018–2019, unplanned collection of some marked plants and disturbance of scrub around the site were recorded. This stage provides observations of a population under additional pressure, with disruption of the controlled experiment's history. [25-WWW]