Our results suggest dynamic influences of large, surviving trees on ecological succession in the years immediately following a volcanic tephra disturbance event. While we initially hypothesized strong trends in growth and colonization with distance from the base of trees (hypothesis one and two), differences in responses among growth forms (hypothesis three), an increase in cover through time (hypothesis four), and differences in soil based on distance from tree (hypothesis five), results were more complex. Rather than demonstrating a pattern of uniform positive (facilitative; Bertness and Callaway 1994) or negative (inhibitive; Connell 1983) effects, in general, surviving trees played changing roles depending on the species and growth form in question and time since the disturbance, while effects were also variable among residual and colonizing (seedling) vegetation. While results for some species may also be consistent with tolerance models of succession (Connell and Slatyer 1977), most patterns were variable by species, growth form, and time, rather than simply independent of proximity to surviving trees. Thus, our results were somewhat consistent with our third hypothesis, but more variable than we expected. Initially, large trees may have played an especially strong role where they may have protected moss, epiphytes, and some shrubs and small trees from the mechanical damage associated with scouring tephra deposition disturbance. However, as time and vegetation recovery progressed, colonization of new individuals seemed to occur without regard to proximity to large trees. In fact, in some cases colonization was higher away from the base of trees. This may indicate successful colonization beyond the immediate competitive influence (e.g., space, light) of dominant trees, however the short distance of our transects (0–3 m) should encourage cautious interpretation of these data. For epiphyte and moss species though, the influence of large trees as islands of residual survival and abundance was generally consistent across all years.
Large trees play an important role in ecosystem recovery from disturbance (Franklin et al. 2000; Manning et al. 2006; Schlawin and Zahawi 2008; Miao et al. 2013). Studies in other systems have found these biological legacies may drive recovery and successional processes via seed production and dispersal by the surviving species (Turner et al. 1998; Keeton and Franklin 2005) or by surviving individuals providing habitat and perches for seed dispersing birds and mammals (Wunderle Jr 1997; Albornoz et al. 2013). Legacy trees may also provide refugia, protection, or structural support for other remnant or colonizing taxa, especially bryophytes, lichens, and epiphytic ferns and angiosperms (Miao et al. 2013). Additionally, surviving species may influence the structure and chemistry of soils (Zinke 1962; Franklin et al. 2000) leading to changes in plant community composition following disturbance. As suggested by Elmqvist et al. (2001), the influence of large trees in our study may be transient for non-epiphytic species, and the influence is also variable by plant growth form. However, this compounding temporal influence may also be long-lasting due to radiating effects of influence through the process of nucleation (Schlawin and Zahawi 2008), or ‘zones of influence’ associated with changes in soils (sensu Zinke 1962; see discussion below). While we saw increases in total cover in many of our plots (hypothesis four), these increases generally occurred rapidly, and were most apparent in the first and second years of sampling. As such, our data show a more complex model where close proximity to large trees is influential on short-term succession for some species following disturbance, but such effects are intertwined individualistic responses within plant communities.
In Valdivian rainforests, the role of large trees in ecological succession and community composition has been heavily explored with regard to canopy gap dynamics (Gutiérrez et al. 2008). Characteristics of treefall gaps (gap area, aspect, species, and size of gap-maker, etc.) influence the recruitment of tree species during recovery in Andean montane forests (González et al. 2015). Dominant graminoids represented by Chusquea spp. are likely strong competitors in treefall gaps (Gutiérrez et al. 2008), and inhibit the establishment of seedlings under their canopies (González et al. 2015). Following fire disturbance, Albornoz et al. (2013) found regeneration radiated from surviving individuals that provide seed as well as micro-environments more suitable for propagule deposition and plant survival. In northern hemisphere temperate rainforests with high levels of rainfall and rapidly growing plant species, species composition, density of surviving understory vegetation, as well as regrowth determined by growth form, can set the stage for understory recovery from disturbance (Halpern 1988). This is especially relevant where initial patterns of succession shift as vegetation density increases and light availability declines relatively quickly due to aggressive vegetation growth responses after seemingly catastrophic disturbances. However, such patterns are dependent on the survival of at least some components of the original plant community (including seeds), and in that sense large surviving trees may play a pivotal role in tephra disturbance sites by providing a physical barrier to disturbance and a nucleus of source propagules for understory re-establishment following gap fall succession or canopy re-establishment (Del Moral and Grishin 1999; Schlawin and Zahawi 2008; Cook and Halpern 2018). Nevertheless, some species (such as small shrubs and trees in this study), may disperse rapidly in years following disturbance, and may be adapted to rapid growth in new volcanic substrates independent of large residual trees. For example, Antos and Zobel (1986) found increased density of colonizing conifers (esp. Tsuga spp.) in tephra deposits surrounding Mount St. Helens (Wa, USA) in the years immediately following the eruption, and such initial responses were still reflected in the community over 36 years later (Fischer et al. 2019). Dispersal patterns may also be reflected in our results, and many berry-producing small shrubs and trees may benefit from bird dispersal where dispersal from dead tree branches (Fig. 1) could result in more seedlings away from tree bases.
Volcanic ejecta is typically devoid of important ecosystem soil building blocks, specifically carbon and nitrogen. The chemical and physical characteristics of volcanic materials could pose challenges to plant community development in post-eruption systems. In the case of the 2015 Calbuco eruption, the specific features of the tephra itself (coarse texture) may have ameliorated this obstacle to plant growth to some degree by abrading existing vegetation, resulting in significant additions of C and N during the eruption. In our data, C, N, and OM adjacent to tree bases were significantly greater than at 3 m from the tree base, consistent with hypothesis five, while pH was significantly lower. Scoria cover was generally reduced away from canopies, while litter was higher. It was unclear if the higher C, N, OM, and lower scoria were due to 1) less scoria initially deposited near the base of trees, 2) more mixing of scoria with canopy soil deposits, wood, detritus, and epiphyte mats near the base of trees that were deposited during (or shortly after) the eruption, 3) occasional epiphyte slumping (secondary disturbance) during the three years of measurements, or 4) a combination of these factors. These explanations are not mutually exclusive, and in combination they represent unique mechanisms through which trees may produce ‘zones of influence’ (sensu Zinke 1962) at the soil surface.
Such edaphic differences in microsite are consistent with initial responses of the vegetation community. Additionally, as organic surface material increased through time away from tree bases, we also saw colonizers (e.g., M. planipes and A. luma) increase with distance from tree bases. The role of large trees in delivering organic material from the canopy concomitant with tephra deposition is an important but understudied factor that contributes to microsite conditions, which may be important to plant growth and survival on newly deposited volcanic substrate. However, more controlled substrate response studies will be needed to better understand specific species responses. At our study sites, long-term successional impacts on the forest understory may depend upon the continued survival of the dominant, emergent trees through mechanisms that are not limited to immediate effects on edaphic conditions. If these trees continue to persist and regenerate extensive canopies, light availability will decrease and litter will increase to the forest floor, while substrate for epiphytes will also increase. If they eventually succumb due to damages incurred during the eruption, fallen boles will contribute to gap dynamic succession. Additionally, long-term effects of trees on edaphic conditions could include effects on soil microbial communities, soil moisture, and essential micro and macro nutrients.
Regardless, our data suggest that the patterns of succession surrounding large trees can change rapidly in Valdivian rainforests following disturbance by tephra. Initial protection from scouring volcanic deposits provided by surviving trees may be followed by preferential establishment of woody vegetation independent from the base of the same trees. For some species though, surviving trees may serve as consistent nuclei for re-establishment and vegetation recovery. Over-all, our data contribute to the understanding of these forests in that they suggest roles of large trees in vegetation recovery following disturbance by scouring tephra deposits, a disturbance that may be especially common in montane forests of central and southern Chile (Ayris and Delmelle 2012; Swanson et al. 2013; Romero et al. 2016). We show that cryptic moss and epiphyte recovery may be predictably associated with survival of large trees, and survival of residual shrubs in some species (esp. R. magellanicum and F. magellanica) can be equally dependent on large trees. Interestingly, these patterns occur even though expanding cover of several small tree seedings and saplings (colonizers and residuals) occurred either independent of distance from large trees, or increasing with distance from large trees. These patterns through the first three years of measurements (less than four years after the eruption), may be harbingers of longer-term succession trends as have been described in other longitudinal studies (e.g., Antos and Zobel 1986, Fischer et al. 2019). In forest ecosystems that face frequent and intense disturbances, understanding the complex roles of legacy large trees may help land managers further understand ecological resilience patterns through time.