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Facilitation by tussocks is common in high-altitude tropical environments. It is thought that facilitation results from stress amelioration, but it is unclear which of the many stressors acting in these environments is ameliorated. We aimed at determining the relative importance of different stressors as drivers of facilitation by the tussock Festuca tolucensis in Mexico. We manipulated five stressors in the field: minimum temperatures by using electric radiators that kept plants warm; maximum temperatures by means of reflective sand that precluded temperature build-up during the day; UV radiation by using screens opaque to UV; poor soil properties by comparing soils from beneath tussocks and from bare ground; and low water availability by adding vermiculite to the soil. The performance (survival and growth) of Mexerion sarmentosum (a plant usually associated with Festuca) in these treatments was compared to that recorded under tussocks and in bare ground. Amelioration of extreme temperatures had the largest positive effects on Mexerion survival. UV radiation and increased soil humidity did not affect survival, although humidity increased growth rates. Nevertheless, tussocks reduced the growth of Mexerion, which is consistent with observations of competition between plants and soil microorganisms favoured by tussocks. Our results highlight the importance of the extreme daily fluctuations in temperature that characterise tropical mountains as fundamental drivers of their dynamics.

espanolEl conejo de los volcanes (Romerolagus diazi),tambien conocido como teporingo o zacatuche, es un pequeno conejo endemico de Mexico. En este estudio se caracterizo su habitat potencial en el Area de Proteccion de Flora y Fauna Nevado de Toluca, en Mexico.Entre abril de 2016 y noviembre de 2017, se analizaron 1.807 unidades de muestreo para determinar la presencia de esta especie de forma indirecta. Encontramos excrementos atribuibles a R. diazi en 41 unidades de muestreo (el 2,27%). En el 10% de estas unidades se colocaron camaras de trampeo para confirmar lapresencia de la especie. Los sitios con presencia de excrementos supuestamente pertenecientes a R. diazi estan en terrenos rocosos, donde la especie dominante arborea es Pinus hartgewii y la herbacea, Festuca tolucensis. El porcentaje de cobertura herbacea fue superior al 70%. Se observo que el pastoreo y los incendios inducidos tienen un efecto negativo en la presencia de los excrementos. Los resultados del muestreo con camara no revelaron la presencia de R. diazi en el Nevado de Toluca. EnglishThe volcano rabbit (Romerolagus diazi), also known as teporingo or zacatuche, is a small rabbit that is endemic to Mexico. In this study we characterized its potential habitat in the Area of Protection of Flora, and Fauna Nevado de Toluca, Mexico. Between April 2016 and November 2017, we sampled 1,807 units to determine the presence of this species using indirect evidence. We found dung pellets that could be attributed to R. diazi in 41 (2.27 %) of the sampled units. In 10 % of these units, we set up camera traps to confirm the presence of the species. Sites with presumed R. diazi pellets were characterised by rocky terrain, with Pinus hartwegii as the dominant tree species, and Festuca tolucensis as the dominant grass. Overall herbaceous cover was over 70 %. Sites observed to have a negative effect on the presence of the pellets were areas with livestock grazing and induced burning. The results of camera trapping did not reveal the presence of R. diazi in Nevado de Toluca.

Facilitation by tussocks is common in high-altitude tropical environments. It is thought that facilitation results from stress amelioration, but it is unclear which of the many stressors acting in these environments is ameliorated. We aimed at determining the relative importance of different stressors as drivers of facilitation by the tussock Festuca tolucensis in Mexico. We employed eight experimental treatments to manipulate five stressors in the field: minimum temperatures by using electric radiators that kept plants warm; maximum temperatures by means of reflective sand that precluded temperature build-up during the day; UV radiation by using screens opaque to UV; poor soil properties by comparing soils from beneath tussocks and from bare ground; and low water availability by adding vermiculite to the soil. The performance (survival and growth) of Mexerion sarmentosum (a plant usually associated with Festuca) in these treatments was compared to that recorded under tussocks and in bare ground. Amelioration of extreme temperatures had the largest positive effects on Mexerion survival. UV radiation and increased soil humidity did not affect survival, although humidity increased growth rates. Nevertheless, tussocks reduced the growth of Mexerion, which is consistent with observations of competition between plants and soil microorganisms favoured by tussocks. Our results highlight the importance of the extreme daily fluctuations in temperature that characterise tropical mountains as fundamental drivers of their dynamics.