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Question Can the co‐existence of the cactus and the shrub in the community be driven by a cyclic succession mechanism? Location Mapimí Biosphere Reserve (southern Chihuahuan Desert). The vegetation is scrub, dominated by Larrea tridentata. Methods Interspecific spatial association between adult L. tridentata and differently sized Cylindropuntia leptocaulis plants was analysed using Ripley′s K function, exploring the hypothesis that facilitation causes differential recruitment of cactus below the shrub and a subsequent interspecific association, the intensity of which declines with the cactus size due to competitive elimination of the shrub. Competition for water among adult plants was investigated by irrigating solitary and associated plants of both species. The proportion of vigorous (measured by the percentage of dead branches) and dead L. tridentata plants associated with adult cacti was compared to that of solitary L. tridentata plants, testing the hypothesis that competition among associated adults reduces the vigour and increases the proportion of dead shrubs, relative to that of solitary plants. Results In 14 out of 16 spatial analyses (four cactus sizes in four plots), C. leptocaulis was found to clump around the L. tridentata adults. Larrea tridentata suffered the effects of interspecific competition for water, but these diminished with increased water provision. In contrast, C. leptocaulis was unaffected by interspecific competition for water. The proportion of vigorous shrubs was higher in solitary than in associated individuals; however, the proportion of dead shrubs was lower in associated than in solitary individuals. Conclusion The irrigation experiment and the proportion of dead branches suggest interspecific competition. However, spatial association of the shrubs with every cactus size and the proportion of dead shrubs associated with the cacti both suggest that replacement of the shrub by the cactus is an infrequent event. Cyclic succession therefore appears to be the exception rather than the rule, and does not account for the co‐existence of the species in the community. The possible scenarios in which cyclic succession may occur are discussed.

The interaction between the nurse plant Larrea tridentata and the cacti Cylindropuntia leptocaulis is thought to follow cyclical‐replacement dynamics. However, the required changing nature of their interaction, from facilitation to competition, has not been investigated through their full life cycle. In order to test the hypothesised cyclical dynamics, we compared the demography of four subpopulations (LA = Larrea associated to Cylindropuntia; CA = Cylindropuntia associated to Larrea, LS = solitary Larrea; CS = solitary Cylindropuntia) in the Mapimí Biosphere Reserve, Mexico, over the period 2008–2015. This allowed us to compare their demography at varying levels of the generally low and unpredictable limiting resource, water. Given the protection afforded to C.  leptocaulis seedlings by L.  tridentata and their presumed increasingly competitive effect on L.  tridentata, which would justify the expectation of cyclic replacement dynamics, we expected to find a zero or positive per capita rate of population growth (r ≥ 0) in CA and LS, and a negative one in CS and LA. The overall r over the study period was close to equilibrium (r = 0) for both LA and LS, with no significant difference between them. In contrast, and as expected, r of CA was consistently and significantly larger than that for CS. In both species, λ (=er) was positively correlated with annual rainfall with no significant difference between LA and LS and a significant difference between CA and CS in this relationship. L.  tridentata can exist alone and coexist with C.  leptocaulis across the levels of precipitation experienced at the study site, while C.  leptocaulis cannot persist in the absence of L.  tridentata. The cyclical dynamics of replacement does not occur in this system and, given the static nature of the evidence on which the original conclusion was based, it is unlikely to exist as a general rule across the species’ geographic range. Synthesis. Our findings stress the importance of elucidating the demographic mechanisms that allow species competing for a common limiting resource to coexist when the resource varies temporally, and expose the difficulty of identifying a species’ ecological niche in the absence of detailed, long‐term demographic information.

The increase and preservation of the socio-environmental functions of urban green spaces (UGS) through suitable management is part of the actions of the United Nations' Sustainable Development Goals. UGS offer benefits to the population in function of their quantity, availability and accessibility. Therefore, we developed a methodology to measure and classify UGS within the urban center of the Queretaro Metropolitan Area in central Mexico. We established one UGS category: public green space and polygon digitization was conducted at 1:1000 scale through on-screen digitization using visual image interpretation. Spatial analysis was carried out in terms of ( 1) extent (urban green space area); (2) density of UGS (m2 of green area/city block; and (3) accessibility to UGS (access for the population at block level as a unit of analysis). Furthermore, cartographic accuracy assessment was conducted in order to validate the generated data. The results show not only the spatial distribution of UGS in the study area but also their spatial relations with the population, in terms of accessibility and density measured against conventional standards. These results may contribute to urban planning regarding UGS, for the improvement of their functions and contributions to the cities' populations.

Here, we present an assessment of long-term trends in the O3 weekend effect (WE) occurrences and spread within the Mexico City (MCMA), Guadalajara (GMA), and Monterrey (MMA) metropolitan areas, which are the three largest metropolitan areas (MAs) of Mexico and concentrate around 33% of the total population in the country. Daytime averages and peak differences in O3 concentrations from weekdays to weekends were used as a proxy of WE occurrence. All MAs exhibited the occurrence of WE in all years at least in one monitoring site. Substantial differences in O3 daytime averages and peaks from weekdays to weekends have decreased over time in all MAs, and since 1998 and 2013 for the MCMA and GMA, respectively, higher O3 levels during weekends are typical during most of the year. The largest variations in the O3 WE were observed at downwind and urban core sites of the MCMA and GMA. Significant increasing trends (p < 0.05) in the O3 WE magnitude were observed for Sundays at all sites within the MCMA, with trends in annual averages ranging between 0.33 and 1.29 ppb O3 yr−1. Within the GMA, for Sundays, fewer sites exhibited increasing trends in the WE occurrence and at lower growth rates (0.32 and 0.48 ppb yr−1, p < 0.1) than within the MCMA, while within the MMA no apparent trends were observed in marked contrast with the MCMA and GMA. Our findings suggest that policies implemented have been successful in controlling weekday ground-level O3 within the MCMA and GMA, but further actions must be introduced to control the increases in the O3 WE magnitude and spread.