‘‘Recalcitrant understory layers’’ revisited: arrested succession and the long life-spans of clonal mid-successional species

In their recent review of arrested succession, Royo and Carson (A.A. Royo and W.P. Carson. 2006. Can. J. For. Res. 36: 1345–1362) demonstrate that ‘‘recalcitrant understory layers’’ are widespread and pervasive modifiers of ecosystems and disruptors of forest regeneration. They rightly point out that many plant species associated with arrested succession are characterized by rapid vegetative spread. Extending their review, we point out that most of such species are clonal or thicket-forming and suggest that an additional reason why these plants so effectively suppress succession for extended periods is their long life-spans. Résumé : Dans leur revue de littérature récente au sujet de l’interruption du processus de succession, Royo et Carson (A.A. Royo et W.P. Carson. 2006. Rev. can. rech. for. 36 : 1345–1362) démontrent que les strates récalcitrantes du sousbois sont des agents envahissants et largement répandus de modification des écosystèmes et de perturbation de la régénération forestière. Ils font remarquer avec raison que plusieurs espèces végétales associées à l’interruption du processus de succession sont caractérisées par une propagation végétative. Poussant plus loin leur revue du sujet, nous soulignons le fait que la plupart des espèces sont clonales ou qu’elles forment des fourrés et nous soumettons l’idée que leur longévité est une raison additionnelle qui explique pourquoi ces plantes interrompent si efficacement le processus de succession pendant de longues périodes. [Traduit par la Rédaction] Succession is one of the core concepts in ecology and one of the oldest and most widespread ecological observations. The idea that communities can often recover from disturbance is also a useful starting point in ecological restoration, which seeks to repair damaged ecosystems. One major exception to the typical successional trend is the occurrence of numerous cases of arrested succession (also called truncated succession), in which earlyand mid-successional species dominate the community so fully that later successional species are suppressed, and succession apparently ceases or is delayed far beyond what is considered typical or desirable. These species are considered major impediments to natural regeneration and ecological restoration (Young et al. 2001), particularly in forest ecosystems (but see Brantley and Young 2007). This form of arrested succession is the subject of a recent synthetic review (Royo and Carson 2006). The authors demonstrate that ‘‘recalcitrant understory layers’’ are widespread and pervasive modifiers of ecosystems and disruptors of forest regeneration. They rightly point out that many species associated with arrested succession are characterized by rapid vegetative spread and suggest two reasons why such species would create recalcitrant understories. First, rapid vegetative spread may help these species to quickly dominate the understory vegetation. Second, their vegetative capabilities may make them less susceptible to elimination by the herbivores (or fire) that co-contribute to the suppression of late-successional species. We would like to suggest that an additional reason why these clonal and thicket-forming species so effectively suppress succession for extended periods is the long life-spans of their genets. There appears to be a rough match between the persistence of species during succession and the characteristic life-spans of their individuals (or genets), perhaps because individuals of many species can establish only during relatively small successional windows and can persist as adults only for their own life-spans. This suggests that many successional species last for only a single generation (see the inhibition model of Connell and Slatyer 1977). Indeed, a common definition of the result of (forest) succession is that the ‘‘climax consists of those plants that can reproduce successfully beneath their own shade and therefore maintain the community indefinitely under prevailing climatic conditions’’ (Kimmins 1997, pp. 403–404). We propose here that many cases of arrested succession and alternative stable states may be anomalous cases of Received 23 March 2009. Accepted 15 March 2010. Published on the NRC Research Press Web site at cjfr.nrc.ca on 14 May 2010. T.P. Young1 and E. Peffer. Department of Plant Sciences and Ecology Graduate Group, University of California, Davis, CA 95616 USA. 1Corresponding author (e-mail: [email protected]). 1184 Can. J. For. Res. 40: 1184–1188 (2010) doi:10.1139/X10-066 Published by NRC Research Press Table 1. Species reported to suppress forest regeneration, and their growth forms (adapted from Royo and Carson (2006), with a few typographic corrections and additional species and references). Species Family Clonal or thicketforming? Reference(s) Ferns Dennstaedtia punctilobula Dennstaedtiaceae Yes See Royo and Carson 2006; Hill and Silander 2001 Pteridium aquilinum Yes See Royo and Carson 2006 Thelypteris noveboracensis Thelypteridaceae Yes See Royo and Carson 2006 Blechnum spp. Blechnaceae Yes See Royo and Carson 2006 Cyathea spp. Cyatheaceae ?? See Royo and Carson 2006 Dicranopteris linearis Gleicheniaceae Yes See Royo and Carson 2006; Ashton et al. 2001 Dicranopteris pectinata Yes See Royo and Carson 2006; Slocum et al. 2004, 2006 Gleichenia bifida Yes See Royo and Carson 2006 Gleichenia linearis Yes See Royo and Carson 2006 Bamboos Tabanez and Viana 2000 Sinarundinaria gangiana Poaceae Yes See Royo and Carson 2006 Chusquea spp. Yes See Royo and Carson 2006 Guadua sarcocarpa Yes See Royo and Carson 2006 Fargesia denudata Yes See Royo and Carson 2006 Sasa spp. Yes See Royo and Carson 2006 Yushania microphylla Yes See Royo and Carson 2006 Other grasses Andropogon scoparius Poaceae Yes Niering and Goodwin 1974 Calamagrostis canadensis Yes See Royo and Carson 2006 Cortaderia spp. Yes See Royo and Carson 2006 Deschampsia flexuosa Long-lived tussock See Royo and Carson 2006 Pennisetum purpureum Yes Zanne and Chapman 2001 Setaria sphacelata Long-lived tussock Sarmiento 1997 Cymbopogon nardus Yes Zanne and Chapman 2001 Panicum maximum Yes Kent et al. 2000