Making the best of the worst of times: traits underlying combined shade and drought tolerance of Ruscus aculeatus and Ruscus microglossum (Asparagaceae)

Abstract

Abstract. The genus Ruscus (Asparagaceae) consists of evergreen, woody monocot shrubs with modified photosynthetic stems (phylloclades) that occur in dry, shaded woodland areas of the Mediterranean Basin and southern Europe. The combined drought and shade tolerance of Ruscus species challenges the 'trade-off model', which suggests that plants can be either drought or shade adapted, but not both. To clarify the potential mechanisms that enable Ruscus species to survive in shaded environments prone to pronounced soil drought, we studied form-function relations based on a detailed trait survey for Ruscus aculeatus L. and Ruscus microglossum Bertol., focusing on gas exchange, hydraulics, morphology, anatomy, and nutrient and isotope composition. We then compared these trait values with published data for other species. R. aculeatus and R. microglossum exhibited numerous traits conferring drought and shade tolerance via reduced demand for resources in general and an ability to survive on stored water. Specific traits include thick phylloclades with low rates of maximum photosynthetic CO 2 assimilation, low stomatal conductance to water vapour (g s ), low respiration rate, low light compensation point, low shoot hydraulic conductance, low cuticular conductance, and substantial water storage tissue. Ruscus carbon isotope composition values of -33 ‰ were typical of an understory plant, but given the low g s could be associated with internal CO 2 recycling. Ruscus appears to be a model for extreme dual adaptation, both physiologically and morphologically, enabling its occupation of shaded sites within drought prone regions across a wide geographical range, including extremely low resource understory sites.

Shedding light on shade: ecological perspectives of understorey plant life

“…Tradeoffs between responses to various stresses have been suggested, as contrasting structural and physiological traits are required to tolerate different stresses (Pivovaroff et al 2014;Grubb 2015). These trade-offs are generally considered rather universal, and mainly dependent on physicochemical constraints, whereas biological adaptations are regarded as less important (Sack 2004;Laanisto and Niinemets 2015).…”

“…This has also been shown to be the phylogenetic line of division in other stress tolerance studies (Stahl et al 2013;Coyle et al 2014; but see Hallik et al 2009). Dormancy enables angiosperms to respond more successfully to additional stress factors besides shade and drought; however, gymnosperms, which have lower polytolerance, can tolerate shade and drought more easily when other environmental factors are favourable (Pivovaroff et al 2014;Laanisto and Niinemets 2015).…”

Leaf Shrinkage with Dehydration: Coordination with Hydraulic Vulnerability and Drought Tolerance

“…Instead, we found shrinkage to be closely related to leaf properties principally determined by mesophyll cells, p o and «. Notably, in some species, bundle sheath extensions (especially when fibrous) could play an important role in reducing shrinkage (Cutler, 2005;Pivovaroff et al, 2014).…”

Relationships between overstory and understory structure and diversity in semi-natural mixed floodplain forests at Bosco Fontana (Italy)

“…The understory of mesic and xeric plots analyzed in this study was dominated by Butcher's broom (Ruscus aculeatus, average cover 53%), followed by the shade-tolerant species Anemone nemorosa (16%) and the drought-tolerant Vinca minor (12%). Therefore, we attributed the positive correlation between overstory and Chianucci understory LAI observed in these plots to the large abundance of Ruscus aculeatus, which combines drought and shade tolerance in limiting site conditions (Pivovaroff et al 2014). The high functional dispersion observed in the plots characterized by relatively drier conditions suggests that these stands are undergoing an early successional stage, in which ruderal species with wide ecological amplitude are exhibiting large competition for limited resources.…”

“…Shade tolerance and drought tolerance were the functional traits considered in this analysis. Both these traits are relevant in terms of ecosystem dynamics and management (Roberts & Gilliam 1995, Bartha et al 1998 Mondoni et al 2009, Sobolewska et al 2013, Pivovaroff et al 2014. The full list of species' values for shade and drought tolerance is reported in Tab.…”