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Invasive species are a major cause of biodiversity loss worldwide, but their impact on communities and the mechanisms driving those impacts are varied and not well understood. This study employs functional diversity metrics and guilds—suites of species with similar traits—to assess the influence of an invasive tree (Tamarix spp.) on riparian plant communities in the southwestern United States. We asked: (1) What traits define riparian plant guilds in this system? (2) How do the abundances of guilds vary along gradients of Tamarix cover and abiotic conditions? (3) How does the functional diversity of the plant community respond to the gradients of Tamarix cover and abiotic conditions? We found nine distinct guilds primarily defined by reproductive strategy, as well as growth form, height, seed weight, specific leaf area, drought and anaerobic tolerance. Guild abundance varied along a covarying gradient of local and regional environmental factors and Tamarix cover. Guilds relying on sexual reproduction, in particular, those producing many light seeds over a long period of time were more strongly associated with drier sites and higher Tamarix cover. Tamarix itself appeared to facilitate more shade-tolerant species with higher specific leaf areas than would be expected in resource-poor environments. Additionally, we found a high degree of specialization (low functional diversity) in the wettest, most flood-prone, lowest Tamarix cover sites as well as in the driest, most stable, highest Tamarix cover sites. These guilds can be used to anticipate plant community response to restoration efforts and in selecting appropriate species for revegetation.

Environmental changes have resulted in significant declines in native riparian forests that are comprised largely of dioecious tree taxa, including boxelder and iconic cottonwood/willow gallery forests. Dioecious species may be especially vulnerable to the effects of climate change given that they often exhibit skewed sex ratios that are reinforced by physiological and morphological specialization of each sex to specific microhabitats. A comprehensive data synthesis suggests that male individuals of boxelder and cottonwood taxa have a higher representation on dry microhabitats than females and are less physiologically sensitive to increased aridity than co-occurring females. Consequently, extreme male-biased sex ratios are possible under future climate conditions that could reduce population fitness below a sustainable threshold. Riparian willows, on the other hand, generally do not express obvious sexual dimorphism in habitat preference or physiological sensitivity to aridity. Thus, it is unclear whether climate change will impact population structure of willows in ways that parallel other dioecious riparian tree taxa. Future riparian tree restoration programs should aim to maintain future sex ratio balance that maximizes population fitness under projected hydro-climatological conditions. Recent advances in genomics will likely provide the critical tools for early sex determination in pre-reproductive trees across riparian tree species such that sex ratio balance could be targeted during initial stages of restoration, along with adaptations for drought tolerance and other key traits that are essential for survival under future conditions.

The isotopic composition of tree ring cellulose was obtained over a 2-year period from small-diameter riparian-zone trees at field sites that differed in source water isotopic composition and humidity. The sites were located in Utah (cool and low humidity), Oregon (cool and high humidity), and Arizona (warm and low humidity) with source water isotope ratio values of –125/–15‰ (δD/δ18O), –48/–6‰, and –67/–7‰, respectively. Monthly environmental measurements included temperature and humidity along with measurements of the isotope ratios in atmospheric water vapor, stream, stem, and leaf water. Small riparian trees used only stream water (both δD and δ18O of stem and stream water did not differ), but δ values of both atmospheric water vapor and leaf water varied substantially between months. Differences in ambient temperature and humidity conditions between sites contributed to substantial differences in leaf water evaporative enrichment. These leaf water differences resulted in differences in the δD and δ18O values of tree ring cellulose, indicating that humidity information was recorded in the annual rings of trees. These environmental and isotopic measurements were used to test a mechanistic model of the factors contributing to δD and δ18O values in tree ring cellulose. The model was tested in two parts: (a) a leaf water model using environmental information to predict leaf water evaporative enrichment and (b) a model describing biochemical fractionation events and isotopic exchange with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulose, indicating that the model parameterization from controlled experiments was robust even under uncontrolled and variable field conditions.

Invasive species are a major cause of biodiversity loss worldwide, but their impact on communities and the mechanisms driving those impacts are varied and not well understood. This study employs functional diversity metrics and guilds-suites of species with similar traits-to assess the influence of an invasive tree (Tamarix spp.) on riparian plant communities in the southwestern United States. We asked: (1) What traits define riparian plant guilds in this system? (2) How do the abundances of guilds vary along gradients of Tamarix cover and abiotic conditions? (3) How does the functional diversity of the plant community respond to the gradients of Tamarix cover and abiotic conditions? We found nine distinct guilds primarily defined by reproductive strategy, as well as growth form, height, seed weight, specific leaf area, drought and anaerobic tolerance. Guild abundance varied along a covarying gradient of local and regional environmental factors and Tamarix cover. Guilds relying on sexual reproduction, in particular, those producing many light seeds over a long period of time were more strongly associated with drier sites and higher Tamarix cover. Tamarix itself appeared to facilitate more shade-tolerant species with higher specific leaf areas than would be expected in resource-poor environments. Additionally, we found a high degree of specialization (low functional diversity) in the wettest, most flood-prone, lowest Tamarix cover sites as well as in the driest, most stable, highest Tamarix cover sites. These guilds can be used to anticipate plant community response to restoration efforts and in selecting appropriate species for revegetation.

Riparian habitats are characterized by substantial flows of emergent aquatic insects that cross the stream-forest interface and provide an important source of prey for insectivorous birds. The increased availability of prey arising from aquatic subsidies attracts high densities of Neotropical migratory songbirds that are thought to exploit emergent aquatic insects as a nestling food resource; however, the prey preferences and diets of birds in these communities are only broadly understood. In this study, we utilized DNA metabarcoding to investigate the extent to which three syntopic species of migratory songbirds—Acadian Flycatcher, Louisiana Waterthrush, and Wood Thrush—breeding in Appalachian riparian habitats (Pennsylvania, USA) exploit and partition aquatic prey subsidies as a nestling food resource. Despite substantial differences in adult foraging strategies, nearly every nestling in this study consumed aquatic taxa, suggesting that aquatic subsidies are an important prey resource for Neotropical migrants nesting in riparian habitats. While our results revealed significant interspecific dietary niche divergence, the diets of Acadian Flycatcher and Wood Thrush nestlings were strikingly similar and exhibited significantly more overlap than expected. These results suggest that the dietary niches of Neotropical migrants with divergent foraging strategies may converge due to the opportunistic provisioning of non-limiting prey resources in riparian habitats. In addition to providing the first application of DNA metabarcoding to investigate diet in a community of Neotropical migrants, this study emphasizes the importance of aquatic subsidies in supporting breeding songbirds and improves our understanding of how anthropogenic disturbances to riparian habitats may negatively impact long-term avian conservation.

Spatial genetic patterns can be influenced by a broad range of factors across a landscape. The hypothesis that heterogeneous vegetation and habitat fragmentation rather than water flow influence genetic patterns in two riparian plant species with different niches was tested. Genotyping by sequencing was used to assess the genetic diversity and structure of each species at 12 sites across a river catchment. Generalized dissimilarity modelling examined the relative influence that vegetation type and habitat fragmentation had on patterns of genetic differentiation across the landscape. Restricted gene flow in the widespread species, Callistachys lanceolata, resulted in lower genetic differentiation than that exhibited by Astartea leptophylla, a restricted riparian species with high gene flow. Geographic distance and vegetation type explained the patterns of genetic differentiation in the widespread species, whereas habitat fragmentation and, to a lesser extent, vegetation type explained patterns in the restricted species. Water flow was not found to have significant impacts on patterns of genetic diversity in riparian plant species with restricted and widespread distribution. Impacts of vegetation type on genetic differentiation were most likely due to change in canopy density and associated pollinator communities in the vegetation types across the catchment. Reduced connectivity caused by habitat fragmentation was evident in the restricted riparian species, while reduced connectivity in the widespread species was related to the change of vegetation type between sites. Natural causes of reduced connectivity as well as anthropogenic causes need to be considered in future work to predict persistence and resilience under a changing climate.