In a paper published in a recent issue of the Journal of Vegetation Science, we demonstrated that in maritime Antarctica the macrolichen Usnea antarctica (Usnea hereafter) modifies the microclimate, acting as a ‘nurse species’ for co-existing lichens and mosses, and might facilitate the vascular plant Deschampsia antarctica (Deschampsia hereafter) (Molina-Montenegro et al. 2013). Subsequently, Casanova-Katny et al. (2013) (C-K hereafter), apart from providing useful corrections concerning species identification (which we appreciate), pointed out several issues that in their view weakened our study. The main criticisms were focused on three aspects. First, Usnea could not ameliorate the microclimate as vascular cushion plants do, because it is a poikilothermic and poikilohydric organism and therefore is unable to form a discrete microhabitat with constant ‘inside’ conditions different from ‘outside’ conditions. Second, the facilitative effect of Usnea on Deschampsia is only theoretical, because the two species have not been found growing together. Moreover, Usnea is not a pioneer species in succession in Antarctic communities and therefore can hardly facilitate several of the species included in our study. Third, the authors question the comparison between substrates below Usnea and the bare soil surrounding the lichens because – they claim – these substrates are intrinsically different rather than modified by the nurse species. C-K conclude that the role of Usnea as a facilitative species in the maritime Antarctica is merely philosophical and has no ecological relevance. We explain below why these critiques are unsubstantial.
These authors argue that macrolichens cannot be equated to vascular cushion plants concerning facilitative effects because lichens are poikilohydric and poikilothermic, and consequently cannot modify the microenvironment. First, any physical structure (presence of canopy, branches, leaves or asexual lichen structures) may produce some microclimatic amelioration – such as that produced by Usnea thalli – and thus may have facilitative effects on other species. It has been shown that even artificial nurses, which at best are poikilohydric and poikilothermic, can influence the microclimatic conditions within their canopies and thereby provide increased survival for the protégées (Badano et al. 2011). Second, the authors should be aware that the compact cushion plants from high-elevation environments (Körner 2003) are not the only kind of cushion plant (e.g. Hager & Faggi 1990; Molenda et al. 2012) or nurse plant (e.g. McAuliffe 1986; González-Teuber & Gianoli 2008).
Deschampsia was not found associated with the macrolichen Usnea nor was it recorded in the samples outside the macrolichen cushions. However, in the context of climate change, the finding of higher survival of Deschampsia when associated with Usnea may be ecologically relevant. In a future, warmer environmental scenario (Robinson et al. 2003), both species may co-occur, and then Usnea may facilitate Deschampsia. C-K state that Deschampsia is not easily found in the central highlands of the Fildes Peninsula, thus suggesting that such an association is unlikely. Nevertheless, the references supporting this assertion are rather old. In view of the current climate changes and the spread of both species in Antarctica, it would not be surprising for their distributions to overlap in the near future. In Maritime Antarctica both species may be found in (or around) active and abandoned penguin rookeries (Tatur et al. 1997). Enhanced ecophysiological performance and consequent range expansion southwards are expected for Deschampsia in a climate change scenario (Torres-Mellado et al. 2011). Concerning C-K's claim that Usnea is not a true pioneer species in the succession but a multi-stage species, which questions the inclusion of several study species in the facilitation experiment, it is puzzling that they cite Favero-Longo et al. (2012), who specifically indicated that ‘Some species appear very early in the succession (X1) and remain throughout, such as Sanionia georgicouncinata, Usnea antarctica…’ (p. 331). Therefore, published recent evidence supports the potential role of Usnea as nurse of most (if not all) of the species included in our study.
It is unlikely that soil characteristics within Usnea and outside of it are intrinsically different. The majority of species were recorded both within and outside nurses, and many microsite measurements were made just 50-cm apart between a given nurse and its bare soil counterpart. Moreover, the methodology used to assess the nurse effects by Usnea is the most common procedure used to assess positive interactions by nurse species with compact or lax canopies (Badano et al. 2002; Cavieres et al. 2002, 2005, 2006; Molina-Montenegro et al. 2005). The environment beneath Usnea thalli showed higher moisture, milder temperature, less photosynthetically active radiation and lower evaporative water loss than surrounding open areas. Nevertheless, C-K deem these modifications as irrelevant in a biological context. We disagree. Those differences are not only statistically significant but also biologically relevant for other lichens, mosses and a vascular species. We concur with C-K in that soil characteristics at 10-cm depth may be less important for lichens or mosses compared to vascular Deschampsia (the main goal of our study), whose individuals may be 25-cm tall and show a shoot:root ratio of 1:3 (see Lewis-Smith 2003).
Our study provides evidence that a common Antarctic macrolichen can modify the microclimate, ameliorating the harsh conditions prevailing in Antarctica, establishing positive interactions with other lichens and mosses, and eventually facilitating vascular species. It might be suggested that the macrolichen Usnea would act as a ‘nurse species’, playing a key role in structuring the Maritime Antarctic plant community. In this regard, a recent comment to our study stated that in one of the most extreme environments in Antarctica, ‘facilitation remains an important organizing principle of life, even to the point where vascular plants begin to drop out of communities altogether’ (Callaway 2013). However, further research is needed because the fact that a given species establishes positive interactions with the co-existing flora in stressful, cold environments should not be immediately equated to playing a key role in community structure (see Mitchell et al. 2009).