Invasion often leads to vegetation community type conversion, such as conversion from native shrubland to invasive grassland. Invasive species may have life history traits with flexible resource acquisition strategies. Many invasive grasses are drought avoiders with a short lifespan and fine roots for rapid water uptake. Alternatively, drought tolerant shrubs may possess long-lived, coarser roots. Flexible responses to precipitation could make invasive species stronger competitors than native shrubs in a changing climate.
We hypothesized that an invasive grass (Ehrharta calycina) would produce fine roots and mycorrhizal hyphae more rapidly, resulting in a greater depletion of soil moisture relative to a native chaparral shrub (Adenostoma fasciculatum). We installed sensor arrays in adjacent native and invasive stands consisting of thermal dissipation sap flux probes (n = 7 shrubs), soil moisture sensors, soil temperature sensors, and an atmospheric humidity and temperature probe. Soil sensors were installed at 30 cm in three replicate manually-weeded and intact grass sub-plots of 1 x 1 m each. Seasonality of root production was followed using a manual minirhizotron. Imagery was taken bi-weekly from December 2015 until May 2016. Root and hyphal abundances were quantified visually using these images.
Ehrharta calycina rapidly depleted soil moisture starting in early April to lower levels than the native plots and plots without vegetation. Transpiration of A. fasciculatum increased following rain events in January before noticeable increases in root production. Transpiration of A. fasciculatum peaked in mid-March alongside peak root density. E. calycina had greater root density than A. fasciculatum later in the growing season. Earlier soil dry-down under E. calycina may be linked to earlier upregulation of root production. Root and soil moisture (at 30cm) was coupled for A. fasciculatum, but soil moisture decreased as root density increased for E. calycina. Hyphal abundance tracked root abundance for A. fasciculatum, but hyphal abundance for E. calycina peaked in mid-March whereas root abundance did not peak until May. Both species depleted soil moisture at 30 cm as they grow down to produce most of their roots between 70-80cm, possibly leading to greater competition in a mixed stand. Greater soil moisture extraction by the grass coupled with higher rooting density are likely to reduce the ability of the native shrub to recover from drought.