From facilitative to competitive interaction between Larrea tridentata and Cylindropuntia leptocaulis in the southern Chihuahuan Desert

Abstract

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.

Coexistence and the niche in a nurse–cactus interaction: Is cyclic dynamics justified?

“…Based on Yeaton () and our own observations (Flores‐Torres & Montaña, ), we expected the change of status from associated to solitary to be a relatively rare event. Indeed, over the 7‐year period, only 17 out of the initial 314 LA died resulting in 17 CA becoming CS (5.4% or <0.8% per year).…”

“…In contrast to this hypothesis, Flores‐Torres and Montaña () found that recruitment of C. leptocaulis under L. tridentata shrubs in the Southern Chihuahuan Desert was mostly via vegetative propagation of stem segments rather than from seed. They also found that, although Larrea facilitates the clonal establishment of the cactus, it is rarely replaced by it (Flores‐Torres & Montaña, ). Long‐term coexistence of the individual plants would suggest that the hypothesised cyclical dynamics of replacement might in fact not occur.…”

“…The individuals of each species were categorised into two subpopulations defined by the presence or absence of an interacting individual of the other species. This allowed us to test the hypothesis of replacement and possible cyclical dynamics between the two species, which our recent study suggested is artifactual (Flores‐Torres & Montaña, ). More broadly, it allowed us to identify the components of their population dynamics that allow them to coexist, that is to say, the features of their dynamics that furnish them with a specific niche.…”

Global Environmental Change in Drylands: Ecohydrological Controls on Plant Communities

“…The rate of hydraulic descent increases as the root depth of woody plants increases; in these conditions trees exert a competitive rather than a facilitative effect on CAM plants in access to soil water. In fact, it has been proposed that CAM plants that are facilitated during their establishment stage competes with and even excludes their nurse plants (woody plants) as an adult (e.g., Withgott, 2000) due to ontogenetic changes in the interacting species, but the experimental evidences are scare (e.g., Flores-Martinez et al, 1994;Flores-Torres and Montana, 2015). The role of hydraulic descent in turning facilitation of CAM plants by woody plant to competition as shown in this study may explain why CAM plants (which benefit from the presence of trees during their establishment), once they become adults they are not strong enough to outcompete their nurse plants (i.e., trees), consistent with field observations (i.e., Reyes-Olivas et al, 2002;Flores-Torres and Montana, 2015).…”

“…In fact, it has been proposed that CAM plants that are facilitated during their establishment stage competes with and even excludes their nurse plants (woody plants) as an adult (e.g., Withgott, 2000) due to ontogenetic changes in the interacting species, but the experimental evidences are scare (e.g., Flores-Martinez et al, 1994;Flores-Torres and Montana, 2015). The role of hydraulic descent in turning facilitation of CAM plants by woody plant to competition as shown in this study may explain why CAM plants (which benefit from the presence of trees during their establishment), once they become adults they are not strong enough to outcompete their nurse plants (i.e., trees), consistent with field observations (i.e., Reyes-Olivas et al, 2002;Flores-Torres and Montana, 2015). In view of the universal ability of woody plants to perform hydraulic redistribution (e.g., Neumann and Cardon, 2012;Prieto et al, 2012), the role of hydraulic descent benefiting trees may contribute to my understanding tree-CAM association/coexistence widely documented in different arid and semiarid zones including the Sonoran and Chihuahuan deserts (e.g., Hutto et al, 1986;Drezner, 2007;Flores-Torres and Montana, 2015), the Zapotitlán de las Salinas or Tehuacan valley, central…”