Abstract: Water availability in the soil profile varies with depth and time throughout the season following series of precipitation events and dry periods. Capacity of accessing shallow or deep soil-water depends on rooting patterns. This study aims to address the following questions: 1) At what depths do dryland shrubs uptake water, and how does this compare to neighboring grasses? 2) How does shrub uptake depth change with size? We implemented a tracer experiment in a semi-arid shrub-encroached grassland located in the northern Chihuahuan Desert to investigate the water uptake strategies of Bouteloua eriopoda (black grama), a perennial C4 grass, and Prosopis glandulosa (honey mesquite), a perennial C3 shrub. We injected a LiCl solution at depths between 10 and 120 cm beneath shrubs grouped into 4 categories of aboveground size (m3). 21 days after injection, we harvested photosynthetic tissue from the selected shrubs (N=64) and neighboring grasses (N=64) and measured tissue Li content using inductively coupled plasma optical emission spectroscopy to assess water uptake.
To compare grass and shrub absorption, we adopted an existing model of plant root distribution developed by Gale and Grigal (1987), Y = 1 - βd, to produce a numerical index of the vertical root absorption profile (β) based on the cumulative fraction of the total Li taken up across all samples of a given species (Y) from depths (d) of 10 cm to 120 cm. We found that 80% of Li uptake in grasses occurred within the top 10 cm of the soil profile while approximately 50% of shrub Li uptake occurred at depths below 50 cm. In addition, we found that larger shrubs had a deeper absorption profile than smaller shrubs, and grasses neighboring large shrubs absorbed greater proportions of Li from shallow soils compared to those neighboring small shrubs. In absolute terms, a larger amount of injected Li was taken up by all shrub size groups at every depth than by their neighboring grasses except in one case. Grasses neighboring the largest shrubs took up 44% more of the Li tracer injected at 10 cm depth. This research supports the occurrence of vertical spatial resource partitioning between grasses and shrubs but suggests that partitioning is contingent upon shrub size. Climate change through alterations in amount of precipitation and size distribution will modify the soil-water availability profile. Our results assist in understanding consequences of those soil-water changes on plant-species composition and ecosystem water dynamics.
