OOS 63-2 - Impacts of shrub encroachment on soil seed bank dynamics in the Northern Chihuahuan Desert, U.S.A.

Soil seed banks – living seeds in the soil profile and on the soil surface – represent primary sources of regenerative potential and buffering capacity against disturbance in dryland ecosystems. Soil seed bank and aboveground plant community compositions are often dissimilar when dryland ecosystems are in a stable state, but are theorized to become more similar as vegetation communities near state transitions. This aboveground-belowground similarity may be an early warning sign for impending alternate state transitions and help determine when to implement management actions and active restoration interventions.
Woody plant encroachment into drylands historically dominated by perennial grasses is a global phenomenon. Encroachment is caused by numerous ecosystem drivers and has resulted in landscape-level state transitions from grasslands to shrublands. We want to understand the soil seed bank’s spatial distribution across a shrub encroachment gradient and test if aboveground-belowground community similarities increase as the plant community nears state transition. To examine how shrub encroachment impacts soil seed bank community dynamics, we established 120 pairs of shrub-island and interspace soil seed bank sampling locations (240 total sample locations) across twenty 5-ha plots on sandy and shallow-sandy ecological sites on the Jornada Basin Long Term Ecological Research site and Experimental Range in southern New Mexico, U.S.A. These plots are distributed across a gradient of shrub encroachment including low encroached black grama (Bouteloua eriopoda (Torr.) Torr.) grasslands to alternate state honey mesquite (Prosopis glandulosa Torr.) shrublands. Soil seed bank samples were collected in fall 2020 and 2021 and underwent greenhouse emergence trials to quantify their composition during 2021 and 2022, respectively. Aboveground vegetation data were collected concurrently with the seed bank samples, characterizing the aboveground vegetation community. Here we compare two years of soil seed bank emergence and aboveground vegetation data (2020 & 2021) to understand their spatio-temporal dynamics and community similarity.
As shrub encroachment increased, seeds became concentrated under shrub-island canopies and were depleted from interspaces. Seed bank species richness was low but stable between years in low encroachment plots, highest and stable between years in medium encroachment plots, and unstable between years in high encroachment plots. Additional work will be completed to characterize and compare the aboveground-belowground plant community similarity, but our initial results support the idea that community similarity may be a warning sign of state transition. Additionally, they indicate there is a dense propagule bank in medium encroachment sites that may respond well to shrub management at that stage.