Increased soil temperature and decreased precipitation during early life stages constrain grass seedling recruitment in cold desert restoration

Environmental Effects during Early Life-History Stages and Seed Development on Seed Functional Traits of an Australian Native Legume Species

Understanding how seed functional traits interact with environmental factors to determine seedling recruitment is critical to assess the impact of climate change on ecosystem restoration. This study focused on the effects of environmental factors on the mother plant during early plant life history stages and during seed development. Desmodium brachypodum A. Gray (large tick trefoil, Fabaceae) was used as a model species. Firstly, this study analyzed seed germination traits in response to temperature and moisture stress. Secondly, it investigated how seed burial depth interacts with temperature and soil moisture to influence seedling emergence traits. Finally, it determined if contrasting levels of post-anthesis soil moisture could result in changes in D. brachypodum reproductive biology and seed and seedling functional traits. The results showed that elevated temperature and moisture stress interacted to significantly reduce the seed germination and seedling emergence (each by >50%), while the seed burial improved the seedling emergence. Post-anthesis soil moisture stress negatively impacted the plant traits, reducing the duration of the reproductive phenology stage (by 9 days) and seed production (by almost 50%). Unexpectedly, soil moisture stress did not affect most seed or seedling traits. In conclusion, elevated temperatures combined with low soil moisture caused significant declines in seed germination and seedling emergence. On the other hand, the reproductive output of D. brachypodum had low seed variability under soil moisture stress, which might be useful when sourcing seeds from climates with high variability. Even so, a reduction in seed quantity under maternal moisture stress can impact the long-term survival of restored plant populations.

Effects of warming on seed germination of woody species in temperate secondary forests

Seed germination, a critical stage of the plant life cycle providing a link between seeds and seedlings, is commonly temperature-dependent. The global average surface temperature is expected to rise, but little is known about the responses of seed germination of woody plants in temperate forests to warming. In the present study, dried seeds of 23 common woody species in temperate secondary forests were incubated at three temperature sequences without cold stratification and after experiencing cold stratification. We calculated five seed germination indices and the comprehensive membership function value that summarized the above indicators. Compared to the control, +2 and +4 °C treatments without cold stratification shortened germination time by 14% and 16% and increased the germination index by 17% and 26%, respectively. For stratified seeds, +4 °C treatment increased germination percentage by 49%, and +4 and +2 °C treatments increased duration of germination and the germination index, and shortened mean germination time by 69%, 458%, 29% and 68%, 110%, 12%, respectively. The germination of Fraxinus rhynchophylla and Larix kaempferi were most sensitive to warming without and with cold stratification, respectively. Seed germination of shrubs was the least sensitive to warming among functional types. These findings indicate warming (especially extreme warming) will enhance the seedling recruitment of temperate woody species, primarily via shortening the germination time, particularly for seeds that have undergone cold stratification. In addition, shrubs might narrow their distribution range.

Soil surface treatments and precipitation timing determine seedling development across southwestern US restoration sites

Restoration in dryland ecosystems often has poor success due to low and variable water availability, degraded soil conditions, and slow plant community recovery rates. Restoration treatments can mitigate these constraints but, because treatments and subsequent monitoring are typically limited in space and time, our understanding of their applicability across broader environmental gradients remains limited. To address this limitation, we implemented and monitored a standardized set of seeding and soil surface treatments (pits, mulch, and ConMod artificial nurse plants) designed to enhance soil moisture and seedling establishment across RestoreNet, a growing network of 21 diverse dryland restoration sites in the southwestern USA over 3 years. Generally, we found that the timing of precipitation relative to seeding and the use of soil surface treatments were more important in determining seeded species emergence, survival, and growth than site-specific characteristics. Using soil surface treatments in tandem with seeding promoted up to 3× greater seedling emergence densities compared with seeding alone. The positive effect of soil surface treatments became more prominent with increased cumulative precipitation since seeding. The seed mix type with species currently found within or near a site and adapted to the historical climate promoted greater seedling emergence densities compared with the seed mix type with species from warmer, drier conditions expected to perform well under climate change. Seed mix and soil surface treatments had a diminishing effect as plants developed beyond the first season of establishment. However, we found strong effects of the initial period seeded and of the precipitation leading up to each monitoring date on seedling survival over time, especially for annual and perennial forbs. The presence of exotic species exerted a negative influence on seedling survival and growth, but not initial emergence. Our findings suggest that seeded species recruitment across drylands can generally be promoted, regardless of location, by (1) incorporation of soil surface treatments, (2) employment of near-term seasonal climate forecasts, (3) suppression of exotic species, and (4) seeding at multiple times. Taken together, these results point to a multifaceted approach to ameliorate harsh environmental conditions for improved seeding success in drylands, both now and under expected aridification.

Seedling defoliation may enhance survival of dominant wheatgrasses but not Poa secunda seeded for restoration in the sagebrush steppe of the Northern Great Basin

Restoration of dryland ecosystems is often limited by low seedling establishment and survival. Defoliation caused by insects and small mammals could be an overlooked cause of seedling mortality. In the sagebrush steppe, we examined the effect of seedling defoliation on the survival of perennial grasses commonly used as restoration materials. Under field conditions, seedlings of three perennial bunchgrass species (non-native Agropyron cristatum, and native grasses Poa secunda and Pseudoroegneria spicata) were defoliated at two intensities (30 % and 70 % leaf length removal) and frequencies (one or two clippings) and compared to a non-defoliated control. Following emergence the first year, clippings occurred at the two-leaf stage; a second clipping occurred 1 month later for repeated defoliation treatments. We monitored seedling survival and tillering for 2 years. We expected higher defoliation intensity and frequency to reduce survival for all species, but only a few treatments reduced Po. secunda survival. Conversely, larger-statured Triticeae (wheatgrasses) benefited from some defoliation treatments. In both years, A. cristatum survival increased with repeated defoliation at both intensities. Defoliation did not affect Ps. spicata survival in the first year, but a single defoliation in the second year resulted in increased survival. In both A. cristatum and Ps. spicata, higher-intensity defoliation reduced the boost to survival resulting from defoliation frequency. Seedlings with more tillers had greater survival probabilities, but tiller number was unaffected by defoliation. Further research may elucidate mechanisms seedlings use to compensate for or benefit from defoliation. In the meantime, managers should aim to select defoliation-tolerant species if they anticipate herbivory will be problematic for restoration sites.

Seed encrusting with salicylic acid: A novel approach to improve establishment of grass species in ecological restoration

To achieve global ambitions in large scale ecological restoration, there is a need for approaches that improve the efficiency of seed-based interventions, particularly in overcoming the bottleneck in the transition from germination to seedling establishment. In this study, we tested a novel seed-based application of the plant stress modulator compound salicylic acid as a means to reduce seedling losses in the seed-to-seedling phase. Seed coating technology (encrusting) was developed as a precursor for optimising field sowing for three grass species commonly used in restoration programs, Austrostipa scabra, Microlaena stipoides, and Rytidosperma geniculatum. Salicylic acid (SA, 0.1mM) was delivered to seeds via imbibition and seed encrusting. The effects of SA on seed germination were examined under controlled water-limited conditions (drought resilience) in laboratory setting and on seed germination, seedling emergence, seedling growth and plant survival in field conditions. Salicylic acid did not impact germination under water stress in controlled laboratory conditions and did not affect seedling emergence in the field. However, seedling survival and growth were improved in plants grown from SA treated seeds (imbibed and encrusted) under field conditions. When SA delivery methods of imbibing and coating were compared, there was no significant difference in survival and growth, showing that seed coating has potential to deliver SA. Effect of intraspecific competition as a result of seedling density was also considered. Seedling survival over the dry summer season was more than double at low seedling density (40 plants/m²) compared to high seedling density (380 plants/m²). Overall, adjustment of seeding rate according to expected emergence combined with the use of salicylic acid via coating could improve seed use efficiency in seed-based restoration.