Interactions of landscape position, stream flow, and litter quality on litter decomposition in intermittent to ephemeral streams in the American Southwest

Ephemeral streams are an important but little studied resource in the American Southwest. Many streams in arid and semi-arid regions are ephemeral (e.g. 89% of western US), only flowing in response to rainfall, and the distribution of these streams is likely to increase in a changing climate. Given the hydrologic variability and additional water present in ephemeral stream systems, decomposition rates are likely to vary as a function of stream flow presence and soil-water presence.

Here we examine how different types of litter decompose across a climate gradient in Arizona with a range of ephemeral, intermittent to perennial streams. We used a space for time substitution and deployed common gamble oak and sycamore litterbags at three replicate reach transects at 13 sites in Arizona over an 18-month period. Within each of these sites, we deployed little bags in channel, riparian, and channel landscape positions for (1100 total litter bags). We measured soil-water presence and stream flow days with electrical resistivity sensors.

Cumulative rainfall ranged from 9 to 38 cm across the sites whereas cumulative stream flow presence varied dramatically across the sites, from 11 to 433 days over the 18-month deployment. As expected, rates of decomposition were higher in channels than riparian and upland positions at sites with higher soil-water and stream flow presence and slower for oak compared to sycamore leaves. Eighty-six of the oak compared to seventy four percent of the sycamore mass was remaining after 180 days. Cumulative days of soil –water presence and stream flow associated with each site were positively related to rates of decomposition of litter located in channels suggesting that changes in streamflow and soil-water presence associated climate change will have large effects of rates of nutrient release via the process of decomposition.