Herbs are different: clonal and bud bank traits can matter more than leaf–height–seed traits

Current understanding of functional differences among plant species is based on several key axes of specialization in response to environmental gradients, namely of productivity and disturbance. These axes include leaf economy spectrum, plant size and dispersal ability (Westoby, 1998;Westoby et al., 2002;Wright et al., 2004). These axes are typically proxied by easily measurable traits, namely specific leaf area, height at maturity and seed mass (leaf–height– seed (LHS) traits ofWestoby, 1998)which capturewell variation in a number of correlated plant characteristics (Westoby & Wright, 2006; Laughlin et al., 2010). While the first comprehensive trait-based analysis was dealing primarily with herbs (Grime, 1977), LHS-based analyses turned out to be particularly powerful to describe ecological differentiation of woody plants. In spite of the fact that woody species constitute less than ahalf of existing plant species (FitzJohn et al., 2014),much current understanding of plant functional tradeoffs is based on species sets that contain primarily woody species (Verdu, 2002; Kerkhoff et al., 2014; Lamanna et al., 2014). However, differentiation of herbaceous plants is likely to be shaped by factors different fromwoody species. They donot possess permanent aboveground structures which permits entirely different response to disturbance (Aarssen et al., 2006; Aarssen, 2008; Zanne et al., 2014), but also to other factors, such as drought (Bennett et al., 2015). While strong and infrequent disturbance kills whole plants and generates a long gradient of time-sincedisturbance which favours woody species (Meiners et al., 2015), weaker and regular disturbance events leave belowground regenerative organs intact, favouring plants which can resprout from them (Bellingham & Sparrow, 2000; Mackey & Currie, 2001; Buoro & Carlson, 2014). Such disturbances preclude plants from developing long-lived structures aboveground found in trees and favour resprouting from permanent belowground structures (Bellingham & Sparrow, 2000; Vesk et al., 2004; Dietze & Clark, 2008) as found in shrubs (Bond & Midgley, 2001; Vesk et al., 2004; Dietze & Clark, 2008; Clarke et al., 2013) and herbs (Klime sov a & Klime s, 2007; Meiners et al., 2015). Further, short disturbance intervals (such as winter frost in temperate climates, regular drought or grazing) require rapid resprouting and therefore cheap aboveground tissues that are non-woody (Vesk, 2006; Zanne et al., 2014) and thus largely eliminate selective advantage provided by vertical growth and favour expansion in a horizontal dimension by clonal spread. Selective forces operating on resprouting plants are thus fairly different from those operating on trees. As a result, tradeoffs and correlations of their traits should be different from those known from woody species (Meiners et al., 2015). Still we do not know to what extent the key role of the LHS-differentiation applies to herbs as it does to woody species, and trees in particular. We also do not know whether and how traits that determine ability to resprout and spread horizontally in space (namely traits of belowground bud banks and traits of clonal growth) fit into this differentiation or whether they constitute an axis of specialization independent of the LHS traits.