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

Abstract

In temperate climates with cool-season (winter/spring) dominated rainfall, river regulation typically reduces natural flow peaks and increases warm-season flows for irrigation, often to the detriment of stream health. However, the mechanisms driving seasonal effects of elevated river flows on herbaceous plants are not well understood. Our study aimed to improve understanding of the role of water temperature in herbaceous plant responses to season of inundation to inform flow management in temperate regulated rivers. In a tank-based experiment in late spring, we tested the effect of inundation durations typical of environmental or managed flow pulses (0–36 days) in both cool (ambient, c. 18°C) and artificially warmed water (c. 23°C) on eight herbaceous plant species selected across a range of expected inundation tolerances, growth forms, and origins (locally exotic or native). All species showed negative effects of inundation and greater reductions in survival and/or growth with longer inundation durations, and inundation tolerance varied among species as expected. Plants inundated in warm water died faster and/or at higher proportions than those inundated in cool water, for inundation intolerant to moderately inundation-tolerant species, with no mortality effects in highly inundation-tolerant species. Plant growth was relatively unaffected by temperature, except for minor reductions in growth and biomass, primarily in response to warm compared to cool inundation. Our study clearly demonstrates that extended inundation was detrimental to all species tested and that warmer water exacerbated these negative effects, especially for inundation-intolerant and moderately inundation-tolerant plant species. This suggests that warm-season flows in temperate regulated rivers should be short in duration to reduce the risk of damage to vegetation in riparian zones. Our findings strengthen our mechanistic understanding of herbaceous plant responses to inundation in different seasons, providing further evidence that water temperature is a key factor affecting these responses.