Multi-scale environmental filters and niche partitioning govern the distributions of riparian vegetation guilds

Abstract

Across landscapes, riparian plant communities assemble under varying levels of disturbance, environmental stress, and resource availability, leading to the development of distinct riparian life-history guilds. Identifying the environmental filters that exert selective pressures and favor specific vegetation guilds within riverscapes is a critical step in setting baseline expectations for how riparia may respond to the environmental conditions anticipated under future global change scenarios. In this study, we ask (1) what functional riparian plant guilds exist across two major North American river basins? (2) What environmental filters shape riparian guild distributions? (3) Does resource partitioning between guilds influence guild distributions and co-occurrence? We identified riparian plant guilds, examining relationships between regional climate and watershed hydrogeomorphic characteristics, stream channel form, and co-occurring riparian guilds. Woody species composition was measured at 703 streams and each species’ traits were extracted from a database in five functional areas: life form, persistence and growth, reproduction, and resource use. We clustered species into guilds by morphological characteristics and attributes related to environmental tolerances, modeling guild distributions as a product of environmental filters (stressors and resources) and guild co-existence. We identified five guilds, i) a tall, deeply rooted, long-lived, evergreen tree guild, ii) a xeric disturbance tolerant shrub guild, iii) a hydrophytic, thicket-forming shrub guild, iv) a low-statured, shade-tolerant, understory shrub guild and v) a flood tolerant, mesoriparian shrub guild. Guilds were most strongly discriminated by one another species’ rooting depth, canopy height and potential to resprout and grow following biomass-removing disturbance. Hydro-climatic variables including precipitation, watershed area, water table depth, and channel form attributes reflective of hydrologic regime were predictors of guilds whose life history strategies had affinity or aversion to flooding, drought, and fluvial disturbance. Biotic interactions excluded guilds with divergent life history strategies and/or allowed for the co-occurrence of guilds that partition resources differently in the same environment. We conclude that riparian guilding provides a useful framework for assessing how disturbance and bioclimatic gradients shape riparian functional plant diversity. Multiple processes should be considered when the riparian response guilds framework is to be used as a land-use decision-support tool framework

Exploring the key drivers of riparian woodland successional pathways across three European river reaches

“…Geographic extrapolation of correlative studies is not straightforward because species responses to stressors such as stem flexibility, root length, or mechanical resistance to flooding (Glenz et al, ; González, González‐Sanchis, Comín, & Muller, ) may be filtered by environmental features at different spatial and temporal scales (e.g., climate; Hough‐Snee et al, ). Nevertheless, different flow regimes have theoretically exerted similar selective pressures on riparian vegetation, resulting in common phenological, morphological, and physiological traits, controlling the spatial and temporal locations and appearance of the associations of riparian species (Bejarano, Marchamalo, González del Tánago, García de Jalón, & Sordo‐Ward, ; Merritt, Scott, LeRoy, Auble, & Lytle, ; Stromberg & Merritt, ).…”

An approach for measuring confinement and assessing the influence of valley setting on river forms and processes

“…Within these imposed conditions, hydrology, sediment regime and vegetation conditions (flux boundary conditions) act on the valley bottom to determine the range of process behaviour that is possible, and the range of types of rivers that can form in that setting (Brierley and Fryirs, ; Hough‐Snee et al ., ). The extent to which various forms of confinement occur determines the degrees of freedom within which the river has the capacity to adjust (Phillips, , ).…”

Functional Traits and Ecological Affinities of Riparian Plants along the Colorado River in Grand Canyon

“…Trait information has been used for a variety of riparian vegetation analyses, including defining functionally relevant groups of plants (guilds or functional groups) that are expected to respond similarly to environmental resource gradients and/or stressors (Merritt et al 2010, Hough-Snee et al 2015, Stromberg and Merritt 2016, evaluating changes in vegetation attributes due to environmental change (Kyle andLeishman 2009, Bejarano et al 2012), and predicting changes to ecosystem services and ecological functions resulting from vegetation change (Merritt and Bateman 2012). In dryland riparian areas, trait-based analyses are beginning to be used to better understand the impacts of flow regulation and groundwater pumping on riparian vegetation (Bejarano et al 2012, Merritt and Bateman 2012, Strom berg and Merritt 2016, the results of which could potentially be applied to future scenarios and restoration efforts associated with climate change or changing human water use demands (Harrison et al 2010).…”

“…Tremendous effort and resources have been focused on gathering data on the biological traits and ecological affinities of thousands of common species throughout the world as part of a number of trait database projects (e.g., PLANTS, LEDA, TRY, MARIWENN, BROT, FLOWBASE, SID, eHALOPH, and GLOPNET) (Kew Royal Botanic Gardens 2008, Kleyer et al 2008, Reich et al 2009, USDA 2010, Baralotto et al 2011, Aguiar et al 2013, Paula and Pausas 2013, Boenisch and Kattge 2014, Flowers et al 2015. Unfortunately, trait data for U.S. dryland riparian areas are not well represented in these databases (Stromberg 2013, Hough-Snee et al 2015. The USDA PLANTS database (USDA 2010) contains information on many desert riparian and upland species, but the data are mostly ecological affinities with categorical or ordinal values that are most applicable in agricultural or cultivation settings.…”

“…Environmental factors interact with plant traits (e.g., leaf area, rooting depth, seed mass) to influence where, when, and to what extent species may occur within a habitat and across landscapes (Tilman et al 1997, Lehman and Tilman 2000, Hough-Snee et al 2015. Analysis of plant traits and environment interactions has provided a way to explore aspects of functional ecology across geographic, environmental, and evolutionary scales.…”