Soil climate underpins year effects driving divergent outcomes in semi-arid cropland-to-grassland restoration

Converting croplands to grasslands can restore ecosystem functions and services, but there is uncertainty about why some restoration treatments succeed and others do not. One likely explanation for variation in restoration outcomes is that interannual variation in the drivers of community assembly, or “year effects,” is often overlooked in restoration planning. Existing restoration strategies tailor species compositions of seed mixes according to long-term climate means and hardiness zones. However, individual years typically deviate from average climate norms such that restoration activities may be better informed by recent conditions than by climate averages. We monitored a 109-ha field in northeastern Colorado that was converted from a winter wheat-fallow rotation to native perennial grassland via seeding. The same seed mix was used to seed 6 of 12,120-m strips in 2013 (drier) and 6 in 2014 (wetter). In the strips seeded in 2013, only one native grass and one shrub species from the seed mix established widely, whereas in 2014, all native grasses established. Higher soil moisture preceding seed application was positively associated with perennial grasses, while rhizomatous grasses, shrubs, and introduced annuals were associated with other variables. After seeding, high summer soil moisture was positively associated with a rhizomatous C₃ grass, while the planted C₄ bunchgrasses were negatively associated with high summer soil moisture and positively associated with high fall soil temperatures. We found evidence of facilitatory interactions between grasses and forbs, as well as antagonistic interactions between native perennial grasses and non-native annuals. Our results suggest that the conditions immediately before and after planting govern community assembly and leave a lasting legacy and should be considered in planning treatments. We suggest composing seed mixes that are tailored to commonly encountered extremes of temperature and moisture availability. Land managers can also use split-seeding or repeated seeding approaches, within or between years as bet-hedging strategies. The development of more flexible funding mechanisms could allow for regional go/no-go climate thresholds to avoid wasting resources.