A cactus in a mosaic of canopies
The low stem of Lophophora diffusa may fit beneath Larrea tridentata, among shale fragments under a shrub or in an open patch of scrub. Each location has its own pattern of incoming light and heating of the plant surface. A sheltering shrub, also called a nurse plant, changes conditions in the immediate surroundings. The benefit to the cactus depends on the magnitude of that change, season and developmental stage. Two types of study connect the appearance of the site with the functioning of the photosynthetic apparatus. A survey shows where plants persist, while chlorophyll fluorescence describes their physiological response. [Arias and Aquino 2019] [Jiménez-Sierra et al. 2026] [González-Salvatierra et al. 2024]
A Querétaro study published in 2026 found 380 cacti in ten plots of 49 m² each. Sheltered areas occupied 57,5% of the total but contained about 78% of individuals. Comparing abundance with the proportion of area distinguishes simple availability of a microhabitat from a stronger association with it. If cacti were distributed in proportion to area, far fewer specimens would be expected beneath vegetation. The population studied was selected for its high abundance, so these proportions describe a particular patch of scrub. [Jiménez-Sierra et al. 2026, pp. 3–5, Table 2]
Different shrubs, different conditions
| Microhabitat | Area | Proportion of area | Individuals |
|---|---|---|---|
| Beneath vegetation | 281,57 m² | 57,5% | 296 |
| Open space | 208,43 m² | 42,5% | 84 |
| Total | 490 m² | 100% | 380 |
Of the 296 sheltered cacti, 153 grew beneath Larrea tridentata. Under a distribution proportional to this shrub's canopy area, about 109 would be expected. This excess indicates a positive spatial association. A plant census itself records the outcome of many earlier processes, including seed dispersal, germination, growth and survival. Their contributions to cluster formation need to be established through separate observations. In the 2026 study, the physiological explanation for the pattern was based on earlier papers. [Jiménez-Sierra et al. 2026, pp. 4–8, Table 4]
The association with a shrub also depends on canopy properties. In the abstract of a 2005 study, the authors described less frequent occurrence of L. diffusa beneath Celtis pallida. This shrub provided stronger shading and temperature reduction than the other plants compared. The authors identified light limitation as a possible cause of poorer seedling development there. This hypothesis arises from the spatial pattern and microclimate. It concerns the cost of deep shade alongside the benefit of reduced heating. [Zúñiga et al. 2005, abstract]
Reading light through chlorophyll
Chlorophyll absorbs light, and some of the transferred energy is used in photochemical reactions. Some is dissipated as heat, while a small amount leaves the photosynthetic apparatus as fluorescence. Measuring this last signal allows inferences about the state of photosystem II, a complex of proteins and pigments involved in transferring light energy to photosynthetic reactions. Changes in fluorescence reveal regulation of energy use. The resulting index describes a particular aspect of the process and the measurement conditions. [González-Salvatierra et al. 2024, pp. 1–3]
Fv/Fm describes the maximum photochemical efficiency of photosystem II after dark adaptation. Effective yield, designated ΦPSII, concerns its operation under current illumination. Non-photochemical quenching, NPQ, is associated with dissipation of excess energy. It allows assessment of one mechanism limiting overload of the photosynthetic apparatus. Electron transport, ETR, is estimated in such an experiment from fluorescence and incoming photons, using assumed absorption coefficients. The rate of carbon dioxide uptake requires a separate gas-exchange measurement. [González-Salvatierra et al. 2024, pp. 1–3]
Four seasons in the same population
González-Salvatierra and colleagues studied adult L. diffusa about 4,5 cm in diameter. The methods describe six plants beneath Larrea tridentata, one cactus per shrub, and six in open sites. Sheltered specimens occupied the dense part of the canopy close to the shrub's stem. Exposed plants grew without protection from shrubs or rocks. Measurements were taken in autumn 2011 and then in winter, spring and summer 2012. Each season was represented by one date. Results concern adult plants and four observed moments in their annual cycle. [González-Salvatierra et al. 2024, pp. 2–3]
Light and temperature were recorded beside the photosynthetic stem surface, and midday measurements were taken between 12 and 14 hours. Incoming photons beneath shrubs were lower in spring, summer and autumn. The strongest difference occurred in spring. Winter illumination was similar in the two microhabitats. A temperature reduction under shelter was found in spring and autumn. In summer and winter, the comparison showed no significant difference. Shelter properties therefore changed seasonally along with conditions across the site. [González-Salvatierra et al. 2024, pp. 2–4, Fig. 2]
Spring stress and winter slowing
In spring, Fv/Fm in unsheltered plants fell clearly relative to cacti beneath shrubs. In the other seasons, microhabitat differences were smaller and not significant in seasonal comparisons. The authors related the summer and autumn increase in the index to dynamic photoinhibition, a reversible reduction in the activity of the photosynthetic apparatus under stress. The annual pattern is consistent with seasonal regulation. Distinguishing rapid protection against excess light from persistent damage would also require measurements of recovery over a shorter timescale. [González-Salvatierra et al. 2024, pp. 4–8, Fig. 3]
In winter, effective photosystem II yield and estimated electron transport declined in both microhabitats. The physiological background of the light response then changed. Lower temperature can limit the rate of metabolic processes and energy use by reactions following absorption. Midday NPQ measurements in the study did not differ significantly between sheltered and exposed cacti. Individual indices therefore responded to conditions in different ways. [González-Salvatierra et al. 2024, pp. 4–8, Fig. 3–4]
A brief pulse and prolonged exposure
The second part of the experiment comprised rapid light-response curves. A fluorometer illuminated the stem with successive pulses from darkness to 2431 µmol photons m⁻² s⁻¹. Each level lasted 10 seconds. This procedure examines the short response of the photosynthetic apparatus to a change in incoming energy. The authors defined the electron transport saturation point as the level corresponding to 90% of the maximum estimated from the fitted curve. The threshold obtained from this model is useful for comparing seasons and microhabitats. [González-Salvatierra et al. 2024, pp. 3, 5–8]
In winter, maximum estimated electron transport was lower, and saturation occurred at a lower photon input. No significant difference in maximum ETR was found between sheltered and exposed plants across the full set of seasons. In the authors' interpretation, higher NPQ after light curves indicated a response revealed in the short test, while midday measurements showed no significant difference between microhabitats. Tolerance of brief exposure and the conditions of whole-day plant functioning describe different response scales. [González-Salvatierra et al. 2024, pp. 4–8]
Shelter in the life cycle
Adult L. diffusa present in open sites retained the capacity for seasonal photosystem functioning. In spring, however, shelter clearly improved Fv/Fm. The spatial pattern, with most cacti beneath shrubs, and the responses of adult specimens can be connected by considering different life stages. Cluster formation requires successful germination and persistence of young plants, while later photosystem efficiency describes individuals that have already survived those earlier stages. [Jiménez-Sierra et al. 2026] [González-Salvatierra et al. 2024]
A similarly strong spatial association with plant canopies was described for L. williamsii in Mexican rosette scrub. This is a separate species and a different plant community. The comparison indicates that the association with shelter recurs while retaining local differences. In a collection, data on L. diffusa help relate light selection to season and stem temperature. They do not establish a single light dose for seedlings and adults, or a single shading percentage applicable throughout the year. [Schwertner-Charão et al. 2022] [González-Salvatierra et al. 2024]
Based on full texts of the 2024 and 2026 papers and the description of Lophophora diffusa in a 2019 regional flora. The 2005 and 2022 papers are discussed within the scope of their abstracts. Table captions for orientation beneath canopies in the 2026 paper are inconsistent with the counts, so compass-direction data are omitted. In the 2024 paper, the methods describe 6+6 plants, while figure captions give n=3. The supplement was not obtained.
Trait-Mediated Facilitation and Stress Tolerance in Two Globose Cactus Species from the Mexican Desert
Jiménez-Sierra C. L., Arroyo-Pérez E., Díaz-Segura O., Matías-Palafox M. L., Flores J., González-Adán M. D. L. A., Trait-Mediated Facilitation and Stress Tolerance in Two Globose Cactus Species from the Mexican Desert, Horticulturae, 2026, 12(4):447, doi: 10.3390/horticulturae12040447
Analysis of Ariocarpus kotschoubeyanus and Lophophora diffusa distribution relative to nurse plants in two Querétaro populations. It covers proportions of open and shrub-covered ground, actual and expected cactus numbers near individual plant species, and clustering. Most Ariocarpus kotschoubeyanus individuals were in the open, while distribution under shrubs depended on shrub species. The authors compare spatial data with earlier physiological studies.
Seasonal chlorophyll fluorescence before and after rapid light curves in the endangered species Lophophora diffusa (Cactaceae) across two microenvironments
González-Salvatierra C., Flores J., Díaz-Segura O., Matías-Palafox M. L., Jiménez-Sierra C. L., Seasonal chlorophyll fluorescence before and after rapid light curves in the endangered species Lophophora diffusa (Cactaceae) across two microenvironments, Flora, 2024, 320:152623, doi: 10.1016/j.flora.2024.152623
A seasonal comparison of chlorophyll fluorescence in adult Lophophora diffusa growing beneath Larrea tridentata and in open sites. Measurements from 2011–2012 include light and temperature near the stem, Fv/Fm, effective photosystem II yield, estimated electron transport and non-photochemical quenching. Rapid light response curves show responses to short pulses of increasing intensity. The clearest decline in Fv/Fm in exposed plants occurred in spring. In winter, photochemical parameters declined in both microhabitats.
Interacciones Planta-Nodriza en Lophophora diffusa (Cactaceae) en un Desierto Subtropical de México
Zúñiga B., Malda G., Suzán H., Interacciones Planta-Nodriza en Lophophora diffusa (Cactaceae) en un Desierto Subtropical de México, Biotropica, 2005, 37(3):351-356, doi: 10.1111/j.1744-7429.2005.00046.x
A SADIE analysis of the distribution of Lophophora diffusa and neighbouring shrubs, combined with assessment of light, temperature and humidity beneath their canopies. The abstract indicates a positive association with Larrea tridentata and a negative association with Celtis pallida. The authors consider limited seedling development in the deepest shade. The publisher's abstract was available.
Familia Cactaceae I
[Arias S., Aquino D., Familia Cactaceae I, Flora del Bajío y de Regiones Adyacentes, fascículo 209, Instituto de Ecología, Centro Regional del Bajío, 2019]
A regional cactus flora of Guanajuato, Querétaro and northern Michoacán. It contains keys, morphological descriptions, distribution maps and herbarium material. It includes Glandulicactus mathssonii under the name Ferocactus mathssonii, Turbinicarpus alonsoi and the current Kadenicarpus pseudomacrochele. Descriptions compare stems, spines, flowers, fruits and seeds. Habitat data cover vegetation, elevation and flowering season. Historical subspecies of the pseudomacrochele complex have separate diagnoses. The description of Lophophora diffusa covers stem, flower, fruit and seed structure, habitats in Querétaro and regional phenology. The description of Mammilloydia candida includes regional stem and spine measurements, flower, fruit and seed traits, and localities in Guanajuato.
Interactions between facilitator species and Lophophora williamsii (Lem. ex Salm-Dyck) J.M.Coult. (Cactaceae) in a rosetophyllus desert scrub in México
Schwertner-Charão L., Delgado-Martínez R., Treviño-Carreón J., Jiménez-Sierra C. L., Astudillo-Sánchez C. C., Osorio-Hernández E., Interactions between facilitator species and Lophophora williamsii (Lem. ex Salm-Dyck) J.M.Coult. (Cactaceae) in a rosetophyllus desert scrub in México, Journal of Arid Environments, 2022, 205:104824, doi: 10.1016/j.jaridenv.2022.104824
The study compares the position of Lophophora williamsii beneath the canopies of neighbouring plants and in open spaces, and the associated light and soil temperature conditions. It covers ten plots in Mexican rosette scrub. The abstract was available.