Responses of grasses to experimental submergence in summer: implications for the management of unseasonal flows in regulated rivers

Abstract

River regulation has altered the seasonal timing of flows in many rivers worldwide, impacting the survival and growth of riparian plants. In south-eastern Australia, demand for irrigation water in summer often results in high river flows during a season that would naturally experience low flows. Although unseasonal high summer flows are thought to significantly impact waterways, their effects on vegetation are poorly quantified. We investigated the responses of five grass species commonly occurring in riparian zones to different durations of submergence in summer. We experimentally tested the response of three exotic and two native grasses to four submergence treatments (4 weeks, 8 weeks, 2-week pulses and no submergence), and two levels of shading (no shading and 80% light reduction), over 8 weeks in summer and early autumn. All submergence treatments, including the 2-week pulse, resulted in the death of all plants of three species (Bromus catharticus, Dactylis glomerata and Rytidosperma caespitosum). Lolium perenne exhibited moderate survival rates in the shorter-duration unshaded submergence treatments, while Poa labillardierei largely survived all treatments. Similar responses across species were observed for plant height and biomass, although height generally increased while biomass growth was reduced by shading. These results show that even 2-week periods of summer submergence can reduce growth and cause the death of some riparian grasses. Although some species may survive longer submergence durations, impacts on other aspects of fitness, and ongoing effects of repeated unseasonal submergence, remain uncertain. Our study highlights that the impacts of unseasonal flows require further investigation and careful management.

Warmer water temperatures exacerbate the negative impacts of inundation on herbaceous riparian plants

“…5°C difference, suggesting a relatively high sensitivity to temperature differences. The accelerated mortality and reduced growth we observed in response to our warm water treatments adds to evidence suggesting that warm‐season inundation has greater negative effects on survival and growth of a range of species compared to cool‐season inundation (Kitanovic, 2019; Van Eck et al., 2006; Vivian et al., 2020).…”

“…The terrestrial grass species included in this study ( B. catharticus , R. caespitosum , P. labillardierei ) represent a gradient of increasing inundation tolerance from intolerant to moderately tolerant, respectively (Vivian et al., 2020). Oxalis pes‐caprae , a terrestrial species and a geophyte, is known to be intolerant of inundation from field observations in Victoria (C. Jones, personal observation, August 2019).…”

“…After 2–3 weeks, bulbs, seedlings and established stolons were planted out into 1.4‐L round black plastic pots with approximately 3 cm of pine‐bark in the base to prevent soil leaking, and filled to approximately 2 cm from the top with a 7:1 sand:topsoil mix as per previous studies (Kitanovic, 2019; Vivian et al., 2020). Pots were placed in an unheated polytunnel with automatic irrigation for 4 min once per day (increased to twice per day 10 days before the start of the experiment).…”

“…There is evidence that this seasonal flow flattening and extended warm‐season flows negatively impact vegetation in riparian zones by decreasing plant vigour (growth and survival) and recruitment (Greet et al., 2013b; Vivian et al., 2020). Cool‐season flows that correspond with natural flow regimes appear to have fewer negative impacts on plant health (Kitanovic, 2019; Van Eck et al., 2006).…”

“…However, even river‐associated biota, including riparian plants, have evolved in the context of natural water temperature patterns and the bioenergetic function of plants can be negatively impacted by extreme warm temperatures (Olden & Naiman, 2010). Further experimental research on the role of water temperature is needed to help understand the mechanisms behind negative effects of warm‐season inundation on herbaceous plants (Vivian et al., 2020).…”