Vascular plants kingdom on stumps
Previous studies related to tree regeneration on stumps in temperate forests have been restricted to only a few tree and stump species such as Picea abies (Hörnberg et al. 1997), Abies alba, and Fagus sylvatica (Szewczyk and Szwagrzyk 1996). Thus, we have expanded the current level of knowledge through reporting for the first time on the growth of different herbaceous, tree and shrub species on conifer stumps. We have examined the composition of tree species on stumps in diverse understorey plant communities. Of the total 23 vascular plant species that we identified in the three sites, 19 were found growing on stumps. It is likely that these stumps created a suitable habitat for the establishment of these plant species by providing additional moisture through the water-holding mosses that regularly colonize coarse woody debris (During 1979). As stumps are elevated from the ground, they provide vascular plants with growing conditions that receive more sunlight and have higher temperatures than the forest floor, which may help to promote additional plant growth (Nelson 1951). In addition to this, animals have been shown to select the best seeds of vascular plants to store in stumps (Breen-Needham 1994). These factors may help tree and understorey regeneration on stumps within second-growth temperate rainforests in spite of the thick overstorey canopy and cool temperatures (Wirth et al. 2009). Stumps have been researched for their importance for animal biodiversity conservation and management (Lindelöw et al. 1993; Hörnberg et al. 1997; Waldien et al. 2000; Prescott 2002; Konuk et al. 2007; Laitila et al. 2015), while the focus of our assessment adds new information regarding vascular plants and tree regeneration on stumps.
Vegetation-stump basal area relationship
The relationship between vegetation-stump basal area has not been fully studied and it was among the goals of the present study. Re-sprouting on stumps research has shown that the age class of a stump is one of the major factors affecting tree regeneration. A study on the relationship between vegetation-stump basal area conducted in India has indicated that the median basal diameter may have maximized the regeneration of tree species (Khan and Tripathi 1986). This pattern was detected in one out of the three study sites, PSRP, suggesting that stumps of median basal area had maximized established vascular plant quantities. In other words, for PSRP we did not find an explicit “individual species-area relationship”, i.e., the number of individuals increase with increasing habitat area as reported by “Individual Species Area Relationship (ISAR)” by Tsai et al. (2015). The Gaussian distribution of vascular plant species with basal stump area may be due to the understorey species mortality caused by competition with the canopy of tree species on larger stumps, as the mean basal diameter of stumps in PSRP (42.77 cm) is much smaller than that in MKRF (89.78 cm) and SP (74.90 cm) (Table 2). In contrast, for MKRF and SP, there was a clear ISAR, with increasing numbers of individuals as basal area increases. To our knowledge, this is the first time that a study has examined the ISAR concept in a small area (74.90-89.78 cm in diameter). Stumps are ubiquitous in many forests, but still under researched, especially with respect to vascular plants and tree regeneration; thus, we suggest additional research efforts be dedicated to epixylic communities on stumps. Interestingly, our data did not show a correlation between vegetation diversity and stump height. However, this may be due to our exclusion of stumps greater than 200 cm, which we described as snags. Further research is also needed to illustrate the role of high stumps on the various steps and processes in vascular plant establishment.
Species competition patterns on stumps
Species interactions on stumps can be influenced by factors related to natural enemy, density-dependence, inter and intra-specific competition, and species coexistence (Chesson 2013). Competition is one of the most fundamental interactions of ecological organization (Solé et al. 1992; Chesson 2013). Species competition patterns on stumps may help unveil tree regeneration processes on stumps. In the present study we focused on interspecific competition and species coexistence. As previously mentioned, the overall species associations on stumps were positive in MKRF and SP but negative in PSRP which could be caused by factors related to the smaller average basal diameter in PSRP. Within all the studied sites, tree species seemed to significantly compete with each other, such as ACCI vs. ILAQ (species abbreviations in Table 1), ACCI vs. TSHE, ILAQ vs. TSHE, ILAQ vs. PSME, ACCI vs. PSME, and THPL vs. ACCI (Table 3). Tree species also compete with other highly occurring species such as GASH and VAPA. However, in SP, two positive pairs of tree species are ACMA and TSHE, THPL and TSHE; this may due to the large occurrence of TSHE. It should to be noted that ILAQ, an invasive species, successfully colonized stumps; mitigating invasive species is under management consideration in Vancouver (Mosquin 1997).
Tree regeneration process
Tree regeneration and vascular plant biodiversity on stumps have intricate processes and patterns. Our literature searching uncovered that little is known about stump-vegetation relationships other than non-vascular plants, and linking processes to patterns is relatively rare. Our research suggests that stumps can play an important role in tree regeneration in Pacific Northwest temperate rainforests, and a conceptual model for future stump-vegetation research would be useful. To fill this gap, we have included a schematic depicting the regeneration process (Fig. 3), which can be described as:
Disturbance: Stumps are initially created by abiotic disturbances (e.g., wind, lightning, harvesting, or fire) synergistically with biotic disturbance (e.g., a weakened tree by disease could be more likely to be broken by wind). Canopy gaps resulted from the disturbances start allowing for shifts in succession on stumps.
Stump decay: Stumps begin the decay process, which is enhanced by the activity of agents such as insects, fungi, bacteria, etc. (Palviainen et al. 2010). At this stage, the chemistry of the stump and nutrient cycling and accumulation matter especially. After consistent rainfall, bryophytes and lichens appear on stumps, and then, the habitat on the stump becomes suitable for the establishment of tree species and vascular plant biodiversity.
Seed or propagule dispersal: Tree seeds are dispersed on to stumps passively (e.g., falling from nearby trees, wind), or actively by animals (e.g., squirrels, birds, including hiding by animals). Seed survival can be affected by multiple processes, e.g., pathogens, seed predation, facilitation, mutualism, etc. The seeds that move into germination and subsequently other stages will be impacted by stochastic and deterministic processes.
Germination: Tree seeds and other propagules germinate. As water, light, nutrition and other factors support favorable growth, seedlings and other vascular plants can grow. Seedlings compete with bryophytes and lichens, as well as other vascular plants for growing space and vying for light, so they can move into emergent seedling stage.
Seedling stage: Tree seedlings continue in growth (seedlings greater than one year in age but less than 1.3 meter in height) as well as vascular plants. Bryophytes and lichens at this stage likely have reduced effects on tree seedlings but if present, other vascular plants continue to compete and have other species interactions in relation to resources and space. The previous steps may still be relevant at this stage, as the plant starts influencing the stump as well as other organisms that break down the stump (potentially releasing nutrients, depleting nutrients, etc.). As the seedlings grow bigger, they move into the establishment stage.
Tree establishment stage: At this stage, trees are bigger than 1.3 m. Inter and/or intra-specific competition is likely to occur among seedlings growing on a single stump. Years after establishment, one or more trees may eventually grow tall enough to be a canopy tree.
Within our study, we observed a higher density of tree seedling individuals growing on stumps than on the nearby ground, which suggests the stumps in stages one and two underwent processes that provided suitable habitat stages three to six (Fig. 3). The presence of more regeneration on stumps is supported by a study conducted in old-growth boreal swamp forests in Sweden where Picea abies seedlings were more concentrated in elevated micro-relief features (logs, roots, or stumps) than the forest floor (Hörnberg et al. 1995, 1997). For a re-sprouting study, stages four to six are more relevant for the seedling stages as they arise directly from stumps. We can only infer what happened in stages one to four for our study sites because we did not do a long-term study or manipulative experiment. We focused mainly on plant species patterns relevant to stages five and six; whereby we found ISAR with respect to individual plants and stump basal area for two study sites (MKRF and SP) and a Gaussian distribution instead ISAR for PSRP. To our knowledge, our study is the first to evaluate the ISAR with stumps with to plants and any organism on stumps. Another pattern across these stages was species associations on stumps, which suggested species interactions such as competition.
Stump history from stage one to stage three may play a crucial role in setting up the habitat conditions needed to create the association patterns that we found in this study. Further experiments could do manipulative transplant and growth experiments to study this further and look at other patterns such as plant fitness (or survival ratio) and species cover (how much of stump surface area covered by vegetation). These appear to not yet be measured by any researchers to our knowledge. Furthermore, studies could compare how different stump species at different sizes impact regeneration processes. The patterns and processes mentioned with our schematic are not simple and could potentially have downstream effects, with not all stages being mutually exclusive and may happen together. For example, some seeds may land when other seedlings are already established. Very early processes such as insects could affect the entire trajectory; for example, mountain pine beetle attack could modify the nutrition of tree which affect it as a stump relative to non-beetle kill stumps, which potentially affect all the other processes. The nutrition processes of stumps could be further studied with isotope methods that tease apart the different substrate (e.g., redcedar stump vs. other species, etc.) and chemistries. The habitat matrix around stumps potentially has a significant impact on seedling regeneration and vascular plant biodiversity on stumps. Many factors could impact the start of a stump’s trajectory for tree seedling regeneration and vascular plant biodiversity.