What determines root-sprouting ability: Injury or phytohormones?

PREMISE

Root-sprouting (RS) is an evolutionarily independent alternative to axillary stem branching for a plant to attain its architecture. Root-sprouting plants are better adapted to disturbance than non-RS plants, and their vigor is frequently boosted by biomass removal. Nevertheless, RS plants are rarer than plants that are not root-sprouters, possibly because they must overcome developmental barriers such as intrinsic phytohormonal balance or because RS ability is conditioned by injury to the plant body. The objective of this study was to identify whether phytohormones or injury enable RS.

METHODS

In a greenhouse experiment, growth variables, root respiration, and phytohormones were analyzed in two closely related clonal herbs that differ in RS ability (spontaneously RS Inula britannica and rhizomatous non-RS I. salicina) with and without severe biomass removal.

RESULTS

As previously reported, I. britannica is a root-sprouter, but injury did not boost its RS ability. Root respiration did not differ between the two species and decreased continuously with time irrespectively of injury, but their phytohormone profiles differed significantly. In RS species, the auxins-to-cytokinins ratio was low, and injury further decreased it.

CONCLUSIONS

This first attempt to test drivers behind different plant growth forms suggests that intrinsic phytohormone regulation, especially the auxins-to-cytokinins ratio, might be behind RS ability. Injury, causing a phytohormonal imbalance, seems to be less important in spontaneously RS species than expected for RS species in general.

Does large dam removal restore downstream riparian vegetation diversity? Testing predictions on the Elwha River, Washington, USA

Large dams and their removal can profoundly affect riparian ecosystems by altering flow and sediment regimes, hydrochory, and landform dynamics, yet few studies have documented these effects on downstream plant communities. Ecological theory and empirical results suggest that by altering disturbance regimes, reducing hydrochory, and shifting communities to later successional stages, dams reduce downstream plant diversity. Dam removal could reverse these processes, but the release of large volumes of sediment could have unexpected, transient effects. Two large dams were removed on the Elwha River in Washington State, USA, from 2011 to 2014, representing an unprecedented opportunity to study large dam removal effects on riparian plant communities. Our research objectives were to determine: (1) whether the Elwha River dams were associated with lower downstream plant diversity and altered species composition across riparian landforms pre-dam removal, and (2) whether dam removal has begun to restore downstream diversity and composition. To address these objectives, we compared plant species richness and community composition in river segments above, below, and between the two dams. Plant communities were sampled twice before (2005 and 2010) and four times after (2013, 2014, 2016, and 2017) the start of dam removal, with 2013 and 2014 sampled while the upstream dam removal was ongoing. Prior to dam removal, native species richness was 41% lower below dams compared with the upstream segment; 6 years after dam removal began, it increased ~31% between the dams, whereas nonnative species richness and cover were not apparently affected by dams or their removal. Deposition caused by large volumes of released reservoir sediment had mixed effects on native species richness (increased on floodplains, decreased elsewhere) in the lowest river segment. Plant community composition was also different downstream from dams compared with the upstream reference, and has changed in downstream floodplains and bars since dam removal. In the long term, we expect that diversity will continue to increase in downstream river segments. Our results provide evidence that (1) large dams reduce downstream native plant diversity, (2) dam removal may restore it, and (3) given the natural dynamics of riparian vegetation, long-term, multiyear before-and-after monitoring is essential for understanding dam removal effects.

Disturbance size and frequency mediate the coexistence of benthic spatial competitors

Disturbance plays a key role in structuring community dynamics and is central to conservation and natural resource management. However, ecologists continue to debate the importance of disturbance for species coexistence and biodiversity. Such disagreements may arise in part because few studies have examined variation across multiple dimensions of disturbance (e.g., size, frequency) and how the effects of disturbance may depend on species attributes (e.g., competitiveness, dispersal ability). In light of this gap in understanding and accelerating changes to disturbance regimes worldwide, we used spatial population models to explore how disturbance size and frequency interact with species attributes to affect coexistence between seagrass (Zostera marina) and colonial burrowing shrimp (Neotrypaea californiensis) that compete for benthic space in estuaries throughout the west coast of North America. By simulating population dynamics under a range of ecologically relevant disturbance regimes, we discovered that intermediate disturbance (approximately 9-23% of landscape area per year) to short-dispersing, competitively dominant seagrass can foster long-term stable coexistence with broad-dispersing, competitively inferior burrowing shrimp via the spatial storage effect. When holding the total extent of disturbance constant, the individual size and annual frequency of disturbance altered landscape spatial patterns and mediated the dominance and evenness of competitors. Many small disturbances favored short-dispersing seagrass by hastening recolonization, whereas fewer large disturbances benefited rapidly colonizing burrowing shrimp by creating temporary refugia from competition. As a result, large, infrequent disturbances generally improved the strength and stability of coexistence relative to small, frequent disturbances. Regardless of disturbance size or frequency, the dispersal ability of the superior competitor (seagrass), the competitive ability of the inferior competitor (burrowing shrimp), and the reproduction and survival of both species strongly influenced population abundances and coexistence. Our results show that disturbance size and frequency can promote or constrain coexistence by altering the duration of time over which inferior competitors can escape competitive exclusion, particularly when colonization depends on the spatial pattern of disturbance due to dispersal traits. For coastal managers and conservation practitioners, our findings indicate that reducing particularly large disturbances may help conserve globally imperiled seagrass meadows and control burrowing shrimp colonies that can threaten the viability of oyster aquaculture.

Coexistence of species with different dispersal across landscapes: a critical role of spatial correlation in disturbance

Disturbance is key to maintaining species diversity in plant communities. Although the effects of disturbance frequency and extent on species diversity have been studied, we do not yet have a mechanistic understanding of how these aspects of disturbance interact with spatial structure of disturbance to influence species diversity. Here we derive a novel pair approximation model to explore competitive outcomes in a two-species system subject to spatially correlated disturbance. Generally, spatial correlation in disturbance favoured long-range dispersers, while distance-limited dispersers were greatly suppressed. Interestingly, high levels of spatial aggregation of disturbance promoted long-term species coexistence that is not possible in the absence of disturbance, but only when the local disperser was intrinsically competitively superior. However, spatial correlation in disturbance led to different competitive outcomes, depending on the disturbed area. Concerning ecological conservation and management, we theoretically demonstrate that introducing a spatially correlated disturbance to the system or altering an existing disturbance regime can be a useful strategy either to control species invasion or to promote species coexistence. Disturbance pattern analysis may therefore provide new insights into biodiversity conservation.