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1. Middle Rio Grande riparian plant cover sensitivity to variability in groundwater depth collected by the Bosque Ecosystem Monitoring Program

   https://doi.org/10.6073/pasta/b0bb59fd39e327b2452be059cc6a8440  

   Eichhorst, Kim D ( January 2020 , Environmental Data Initiative)

   Determining the ecological consequences of interactions between slow changes in long-term climate means and amplified variability in climate is an important research frontier in plant ecology. We combined the recent approach of climate sensitivity functions with a revised hydrological ‘bucket model’ to improve predictions on how plant species will respond to future changes in both the mean and variance of groundwater resources. We leveraged spatiotemporal variation in a long-term dataset of riparian vegetation cover to build the first groundwater sensitivity functions (GSFs) for common plant species of dryland riparian corridors. Our results demonstrate the value of this approach to identifying which plant species will thrive (or fail) in an increasingly variable climate layered on top of declining groundwater stores. Riparian plant species differed in sensitivity to both the mean and variance in groundwater levels. Rio Grande cottonwood (Populus deltoides ssp. wislizenii) cover was predicted to decline with greater interannual groundwater variance, while coyote willow (Salix exigua) and other native wetland species were predicted to benefit from greater year-to-year variance. No non-native species were sensitive to groundwater variance, but patterns for Russian olive (Elaeagnus angustifolia) predict declines under deeper mean groundwater tables. Warm air temperatures modulated groundwater sensitivity for cottonwood, which was more sensitive to variability in groundwater in years/sites with warmer maximum temperatures than in cool sites/periods. Cottonwood cover declined most with greater intra-annual coefficients of variation (CV) in groundwater, but was not significantly correlated with inter-annual CV, perhaps due to the relatively short time series (16 y) relative to cottonwood lifespan. In contrast, non-native tamarisk (Tamarix chinensis) cover increased with both intra- and inter-annual CV in groundwater. Altogether, our results predict that changes in groundwater variability and mean will affect riparian plant communities through the differential sensitivities of individual plant species to mean versus variance in groundwater stores. 

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2. Flood regime alters the abiotic correlates of riparian vegetation

   https://doi.org/10.1111/avsc.12572  

   Steinberg, Kelly A. ; Eichhorst, Kim D. ; Rudgers, Jennifer A. ; Hölzel, ed., Norbert ( April 2021 , Applied Vegetation Science)

   Predicting the influence of climate change on riparian plant communities improves management strategies. The sensitivity of riparian vegetation to climate and other abiotic factors depends on interactions between properties of the ecosystem, like flood regime, and plant characteristics. To explore these interactions, we addressed three questions: (a) does the composition and diversity of riparian vegetation vary with the flood regime; (b) do abiotic correlates of vegetation, including climate and groundwater, differ between sites that flood compared to locations that did not experience floods; and (c) which plant functional groups account for differential plant community sensitivity to abiotic factors between flood regimes?

   Middle Rio Grande Valley, New Mexico.

   We used long‐term observations of plant community composition, groundwater depth, precipitation and interpolated temperature from 24 sites spanning 210 km of the Rio Grande riparian cottonwood–willow forest to explore the relative importance of climate and hydrologic correlates of riparian vegetation diversity and composition.

   Riparian plant diversity was higher at sites flooding compared to non‐flooding sites. Plant diversity positively tracked shallower groundwater depth at flooding sites, but was best predicted by intra‐annual groundwater variability at non‐flooding sites. Plant community composition correlated with groundwater depth and air temperature at all sites, but at non‐flooding sites, also with intra‐annual groundwater variability and precipitation. Relationships between native plant cover and potential environmental drivers diverged strongly between the two flood regimes; non‐native plant cover had only weak relationships with most environmental predictors.

   The current flood regime of a site determined the climate and hydrologic factors that best predicted riparian plant community composition and diversity. Relationships between plant diversity or total cover and groundwater, temperature, precipitation, or groundwater variability can change in strength or direction depending on a site's flood history, highlighting the importance of flood regime to predicting the sensitivity of riparian woodlands to future environmental change.

    

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3. Local groundwater decline exacerbates response of dryland riparian woodlands to climatic drought

   https://doi.org/10.1111/gcb.16376  

   Williams, Jared ; Stella, John C. ; Voelker, Steven L. ; Lambert, Adam M. ; Pelletier, Lissa M. ; Drake, John E. ; Friedman, Jonathan M. ; Roberts, Dar A. ; Singer, Michael Bliss ( January 2022 , Global Change Biology)

   Dryland riparian woodlands are considered to be locally buffered from droughts by shallow and stable groundwater levels. However, climate change is causing more frequent and severe drought events, accompanied by warmer temperatures, collectively threatening the persistence of these groundwater dependent ecosystems through a combination of increasing evaporative demand and decreasing groundwater supply. We conducted a dendro-isotopic analysis of radial growth and seasonal (semi-annual) carbon isotope discrimination (Δ13C) to investigate the response of riparian cottonwood stands to the unprecedented California-wide drought from 2012 to 2019, along the largest remaining free-flowing river in Southern California. Our goals were to identify principal drivers and indicators of drought stress for dryland riparian woodlands, determine their thresholds of tolerance to hydroclimatic stressors, and ultimately assess their vulnerability to climate change. Riparian trees were highly responsive to drought conditions along the river, exhibiting suppressed growth and strong stomatal closure (inferred from reduced Δ13C) during peak drought years. However, patterns of radial growth and Δ13C were quite variable among sites that differed in climatic conditions and rate of groundwater decline. We show that the rate of groundwater decline, as opposed to climate factors, was the primary driver of site differences in drought stress, and trees showed greater sensitivity to temperature at sites subjected to faster groundwater decline. Across sites, higher correlation between radial growth and Δ13C for individual trees, and higher inter-correlation of Δ13C among trees were indicative of greater drought stress. Trees showed a threshold of tolerance to groundwater decline at 0.5 m year−1 beyond which drought stress became increasingly evident and severe. For sites that exceeded this threshold, peak physiological stress occurred when total groundwater recession exceeded 3 m. These findings indicate that drought-induced groundwater decline associated with more extreme droughts is a primary threat to dryland riparian woodlands and increases their susceptibility to projected warmer temperatures. 

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4. The Roles of Climate Forcing and Its Variability on Streamflow at Daily, Monthly, Annual, and Long‐Term Scales

   https://doi.org/10.1029/2020WR027111  

   Yao, Lili ; Libera, Dominic A. ; Kheimi, Marwan ; Sankarasubramanian, A. ; Wang, Dingbao ( July 2020 , Water Resources Research)

   The temporal variability of precipitation and potential evapotranspiration affects streamflow from daily to long‐term scales, but the relative roles of different climate variabilities on streamflow at daily, monthly, annual, and mean annual scales have not been systematically investigated in the literature. This paper developed a new daily water balance model, which provides a unified framework for water balance across timescales. The daily water balance model is driven by four climate forcing scenarios (observed daily climate and observed daily climate with its intra‐monthly, intra‐annual, and inter‐annual variability removed) and applied to 78 catchments. Daily streamflow from the water balance model is aggregated to coarser timescales. The relative roles of intra‐monthly, intra‐annual, and inter‐annual climate variability are evaluated by comparing the modeled streamflow forced with the climate forcings at two consecutive timescales. It is found that daily, monthly, and annual streamflow is primarily controlled by the climate variability at the same timescale. Intra‐monthly climate variability plays a small role in monthly and annual streamflow variability. Intra‐annual climate variability has significant effects on streamflow at all the timescales, and the relative roles of inter‐annual climate variability are also significant to the monthly and mean annual streamflow, which is often disregarded. The quantitative evaluation of the roles of climate variability reveals how climate controls streamflow across timescales.

    

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5. Beavers, Bugs and Chemistry: A Mammalian Herbivore Changes Chemistry Composition and Arthropod Communities in Foundation Tree Species

   https://doi.org/10.3390/f12070877  

   Durben, Rachel M. ; Walker, Faith M. ; Holeski, Liza ; Keith, Arthur R. ; Kovacs, Zsuzsi ; Hurteau, Sarah R. ; Lindroth, Richard L. ; Shuster, Stephen M. ; Whitham, Thomas G. ( July 2021 , Forests)

   null (Ed.)

   The North American beaver (Castor canadensis Kuhl) and cottonwoods (Populus spp.) are foundation species, the interactions of which define a much larger community and affect a threatened riparian habitat type. Few studies have tested the effect of these interactions on plant chemistry and a diverse arthropod community. We experimentally examined the impact of beaver foraging on riparian communities by first investigating beaver food preferences for one cottonwood species, Fremont cottonwood (P. fremontii S. Watson), compared to other locally available woody species. We next examined the impact of beaver foraging on twig chemistry and arthropod communities in paired samples of felled and unfelled cottonwood species in northern Arizona (P. fremontii) and southwestern Colorado (narrowleaf cottonwood, P. angustifolia James, and Eastern cottonwood, P. deltoides W. Bartram ex Marshall). Four major patterns emerged: (1) In a cafeteria experiment, beavers chose P. fremontii six times more often than other woody native and exotic species. (2) With two cottonwood species, we found that the nitrogen and salicortin concentrations were up to 45% greater and lignin concentration 14% lower in the juvenile resprout growth of felled trees than the juvenile growth on unfelled trees (six of seven analyses were significant for P. fremontii and four of six were significant for P. angustifolia). (3) With two cottonwood species, arthropod community composition on juvenile branches differed significantly between felled and unfelled trees, with up to 38% greater species richness, 114% greater relative abundance and 1282% greater species diversity on felled trees (six of seven analyses with P. fremontii and four of six analyses with P. angustifolia were significant). The above findings indicate that the highest arthropod diversity is achieved in the heterogenous stands of mixed felled and unfelled trees than in stands of cottonwoods, where beavers are not present. These results also indicate that beaver herbivory changes the chemical composition in 10 out of 13 chemical traits in the juvenile growth of two of the three cottonwood species to potentially allow better defense against future beaver herbivory. (4) With P. deltoides, only one of five analyses in chemistry was significant, and none of the four arthropod community analyses were significant, suggesting that this species and its arthropod community responds differently to beaver. Potential reasons for these differences are unknown. Overall, our findings suggest that in addition to their impact on riparian vegetation, other mammals, birds, and aquatic organisms, beavers also may define the arthropod communities of two of three foundation tree species in these riparian ecosystems. 

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