Pioneer tree ferns influence community assembly in northern New Zealand forests

Abstract

Cyathea medullaris (Cyatheaceae) is a frequent pioneer of disturbed areas (e.g. landslides) or edge environments, sometimes forming near continuous canopies. We test the hypothesis that colonisation by this species as a pioneer alters the seedling assemblage to favour more shade-tolerant broadleaved trees than that beneath another common native pioneer (Kunzea robusta, Myrtaceae) in the same landscapes. We compared vegetation and abiotic characteristics of 166 sites across the Auckland region where C. medullaris or K. robusta were abundant (≥20% basal area) along successional gradients. Using hierarchical classification and ordination, we identified distinct communities associated with the different pioneers. In the forests we consider, C. dealbata is another common understorey tree fern, which sometimes, but not always, co-occurs with C. medullaris. Cyathea medullaris / C. dealbata successions occurred on steep sites with lower annual water deficit whereas K. robusta / C. dealbata successions were located on flatter, drier sites. The prevalence of macro-charcoal in K. robusta / C. dealbata forest suggests the prominence of that community is in part an outcome of the increased importance of fire disturbance in New Zealand. Dominance of C. medullaris, with C. dealbata understoreys, influences community assembly of tree species towards dominance by shade-tolerant species, whereas seedlings of less shade-tolerant small-leaved species are more prevalent in K. robusta / C. dealbata forests. We provide evidence to suggest that, where present in early forest communities, high tree fern abundance influences the assembly of seedling communities, supporting our hypothesis. Contrary to previous suggestions, a high abundance or basal area of tree ferns on sites not historically affected by fire did not limit the establishment and growth of canopy trees including fleshy-fruited broadleaf species.

Forest seedling community response to understorey filtering by tree ferns

“…Tree ferns are a common component of the understorey of temperate and tropical podocarp-hardwood forests in the Southern Hemisphere, establish early in community development and can dominate the understorey (represent up to 50% of stems >2.5cm diameter; Brock, Perry, Lee, & Burns, 2016;Brock, Perry, Lee, Schwendenmann, & Burns, 2018). Their abundance alone suggests that they may influence the availability of seedling establishment sites for some canopy and understorey species (Brock et al, 2016(Brock et al, , 2018Negrão, Sampaio-e-Silva, Kortz, Magurran, & Silva Matos, 2017).…”

“…Tree ferns are a common component of the understorey of temperate and tropical podocarp-hardwood forests in the Southern Hemisphere, establish early in community development and can dominate the understorey (represent up to 50% of stems >2.5cm diameter; Brock, Perry, Lee, & Burns, 2016;Brock, Perry, Lee, Schwendenmann, & Burns, 2018). Their abundance alone suggests that they may influence the availability of seedling establishment sites for some canopy and understorey species (Brock et al, 2016(Brock et al, , 2018Negrão, Sampaio-e-Silva, Kortz, Magurran, & Silva Matos, 2017). The effects of tree fern macro-litterfall have been evaluated as one potential mechanism influencing vascular species establishment (Gillman & Ogden, 2001;Gillman, Wright, & Ogden, 2002;Gillman et al, 2004); these studies suggest that, via macro-litterfall, tree ferns can filter the regeneration niche in forests, independent of canopy composition.…”

New Zealand forest dynamics: a review of past and present vegetation responses to disturbance, and development of conceptual forest models

“…One notable exception is the demonstration by Coomes et al (2005) that dense tree ferns and ground ferns can affect forest composition by influencing the regeneration success of other taxa, resulting in an advantage for angiosperm canopy tree species over their coniferous counterparts. In addition, recent work by Brock et al (2018) demonstrates that, in northern NZ forests, C. medullarisdominated early successions are associated with trajectories towards podocarp-broadleaved forests, while those dominated by Kunzea robusta are associated with trajectories towards Agathis australis forest; although there may also be potential effects of edaphic conditions and initiating disturbance that are difficult to disentangle. The pathway we present here is derived primarily from descriptions by Blaschke (1988) and Blaschke et al (1992), along with anecdotal evidence (Croker 1953;Silvester 1964;Leathwick & Rogers 1996) and our own observations.…”

“…Our hypothesised pathway through tree fern communities should be treated as provisional and understanding succession through these communities is an important avenue for future research. This pathway appears to be most common in gullies and on moist slopes, while Myrtaceae dominated communities tend to occur on flat, dry sites, and particularly those that have experienced fire (Croker 1953;Brock et al 2018). These two pathways are not mutually exclusive, however, and C. medullaris can form a sub-canopy beneath Kunzea, typically in moist areas (e.g.…”

“…The pathway we present is derived primarily from the descriptions of Blaschke (1988) and Blaschke et al (1992), anecdotal evidence from Croker (1953), and our own observations. This pathway is likely to be important in some NZ forests, yet the paucity of formal descriptions means we are somewhat uncertain in how to depict both the pathway itself, and the conditions (edaphic and disturbance) under which it occurs (see Brock et al 2016Brock et al , 2018.…”