Germination sensitivity to water stress in four shrubby species across the Mediterranean Basin

Dry/wet cycling reduces spore germination and viability in six peatland bryophytes

Dry/wet cycling driven by water level fluctuation in wetlands may strongly influence the destiny of seeds. However, how dry/wet cycling affects spore survival and germinability in peatland bryophytes is poorly understood. Six peatland bryophytes, three hummock- and three hollow-dwelling Sphagnum species, were chosen as study species. We tested the effects of dry (60% air RH)/wet (waterlogging) cycle frequency (once per 12, 8 or 4 days for low, medium or high, respectively) and ratio (3:1, 1:1 or 1:3 dry:wet time per cycle) on spore germinability, viability, dormancy percentage and protonema development. Dry/wet cycling significantly reduced spore germination percentage and viability and slowed protonema development in all Sphagnum species, being more pronounced with higher dry/wet cycling frequencies. The hummock species S. capillifolium and S. fuscum had higher spore germination percentage after the continuous dry treatment, while the hollow species S. angustifolium, S. squarrosum and S. subsecundum showed the opposite response, compared to the continuously wet treatment. Except for S. squarrosum, spore viability was higher after the dry than after the wet treatment. Spore viability and dormancy percentage were higher after a dry/wet ratio of 1:3 than after ratios of 3:1 and 1:1. Our study shows that both germinability and viability of bryophyte spores are reduced by dry/wet cycling (especially when frequent) in peatlands. This emphasizes the need to ensure constant water levels and low frequencies of water level fluctuation, which are relevant in connection with wetland restoration, to promote Sphagnum spore survival and establishment in peatlands after disturbances.

Within-species variation of seed traits of dune engineering species across a European climatic gradient

Within-species variation is a key component of biodiversity and linking it to climatic gradients may significantly improve our understanding of ecological processes. High variability can be expected in plant traits, but it is unclear to which extent it varies across populations under different climatic conditions. Here, we investigated seed trait variability and its environmental dependency across a latitudinal gradient of two widely distributed dune-engineering species (Thinopyrum junceum and Calamagrostis arenaria). Seed germination responses against temperature and seed mass were compared within and among six populations exposed to a gradient of temperature and precipitation regimes (Spiekeroog, DE; Bordeaux, FR; Valencia, ES; Cagliari, IT, Rome, IT; Venice, IT). Seed germination showed opposite trends in response to temperature experienced during emergence in both species: with some expectation, in populations exposed to severe winters, seed germination was warm-cued, whereas in populations from warm sites with dry summer, seed germination was cold-cued. In C. arenaria, variability in seed germination responses disappeared once the seed coat was incised. Seed mass from sites with low precipitation was smaller than that from sites with higher precipitation and was better explained by rainfall continentality than by aridity in summer. Within-population variability in seed germination accounted for 5 to 54%, while for seed mass it was lower than 40%. Seed trait variability can be considerable both within- and among-populations even at broad spatial scale. The variability may be hardly predictable since it only partially correlated with the analyzed climatic variables, and with expectation based on the climatic features of the seed site of origin. Considering seed traits variability in the analysis of ecological processes at both within- and among-population levels may help elucidate unclear patterns of species dynamics, thereby contributing to plan adequate measures to counteract biodiversity loss.

Longer summer seasons after fire induce permanent drought legacy effects on Mediterranean plant communities dominated by obligate seeders

The ecological stability of Mediterranean ecosystems is being threatened by climate change. One of the impacts that is expected to be aggravated is the effect of summer drought prolongation toward previous or subsequent seasons by becoming more frequent. This, along with wildfires, could trigger synergistic negative effects on ecosystem regeneration capacity. Here we assessed how extending summer drought in two different ways (to autumn, AutExcl treatment, or bringing it forward to the following spring, SprExcl treatment) would affect plant recovery after an experimental fire carried out in summer in a Mediterranean seeder community. By installing rainout shelters, we assessed differences in seedling emergence, survival and establishment in the main families (Cistaceae, Labiatae, Leguminosae), and the effect on species richness and community composition. We observed that these post-fire dry season extensions reduced the total number of established seedlings and species richness. The most impacting drought treatment was AutExcl. However, the regeneration response was variable depending on the studied family. SprExcl was also determinant for Labiate survival rates. These results suggest that drought events which prolong the usual summer season may have a permanent drought legacy effect on seeder communities as practically all the seeder species populations were established in the first post-fire year. This fact is relevant for Mediterranean ecosystems dominated by seeder species as severer and longer droughts are increasingly recorded and are expected to become more frequent in forthcoming decades.

Post-fire summer rainfall differentially affects reseeder and resprouter population recovery in fire-prone shrublands of South Africa

Summer rainfall can have strong effects on post-fire mediterranean-type shrubland recovery patterns, with potentially long-lasting implications on communities. Our three-year field rainfall manipulation experiment tested post-fire survival and physiological responses of reseeders and resprouters to contrasting summer rainfall patterns in Fynbos and Renosterveld shrublands in South Africa. Climate projections are uncertain for this region but indicate that increased convective summer rainfall events could occur. We irrigated treatment plots during the hottest summer months (i.e. Jan, Feb, March) to contrast the naturally dry summer conditions. This allowed for assessments of the potential limiting effects of summer drought on post-fire vegetation recovery and the responsiveness of vegetation to moisture inputs during this time. Natural summer droughts led to leaf dehydration, reduced photosynthesis and reduced photosynthetic capacity. This had a particularly severe effect on reseeders during the first summer after fire leading to high mortality rates. Summer irrigations strongly reduced levels of reseeder stress and mortality. Resprouters in both vegetation types were physiologically less sensitive to rainfall patterns and showed little drought-related mortality. Comparisons of final population sizes with emergence and survival patterns showed that summer rainfall during the first summer after fire had the potential to strongly alter reseeder population sizes. The physiological sensitivity of plants to summer rainfall patterns was higher in shrubland communities occurring on fine-textured, moderately fertile soils (e.g. Renosterveld). Shrublands occurring on sandy, nutrient-poor soils (e.g. Fynbos) were remarkably insensitive to summer drought after the first summer with lower irrigation responses. Our study demonstrated the potential for variation in post-fire summer rainfall to strongly affect reseeder and resprouter population recovery patterns.

Germination response to water availability in populations of Festuca pallescens along a Patagonian rainfall gradient based on hydrotime model parameters

Sensitivity to water availability is a key physiological trait for grassland species located in arid and semiarid environments, where successful germination is closely related to rainfall dynamics. Festuca pallescens inhabits diverse environments along a steep precipitation gradient in North Patagonia, thus offering a suitable model for the study of germination behavior in response to water availability. By analyzing germination in nine populations using a hydrotime model approach, we aimed to find within-species variation. Seed population behavior was analyzed under different hydric conditions using hydrotime model parameters (hydrotime, mean base water potential and its standard deviation). We estimated the mean base water potential for F. pallescens (ψb₍₅₀₎ = − 2.79 ± 0.45 MPa), which did not differ significantly between populations. However, the hydrotime parameter (θ_H) varied markedly, suggesting physiological adaptation to local environments. Higher values of θ_H were found in populations located at the extremes of the distribution gradient, indicating that germination may be prevented or delayed in conditions that are suboptimal for the species. Since the variation in hydrotime model parameters did not follow a cline, micro-environmental cues may have the greatest influence on the physiological behavior of the species, rather than the macroscale rainfall gradient.

Differential Interpretation of Mountain Temperatures by Endospermic Seeds of Three Endemic Species Impacts the Timing of In Situ Germination

Predicting seed germination in the field is a critical part of anticipating the impact of climate change on the timing of wild species regeneration. We combined thermal time and soil heat sum models of seed germination for three endemic Mediterranean mountain species with endospermic seeds and morphophysiological dormancy: Aquilegia barbaricina, Paeonia corsica, and Ribes sandalioticum. Seeds were buried in the soil within the respective collection sites, both underneath and outside the tree canopy, and their growth was assessed regularly and related to soil temperatures and estimates of the thermal characteristics of the seeds. The thermal thresholds for embryo growth and seed germination of A. barbaricina assessed in previous studies under controlled conditions were used to calculate soil heat sum accumulation of this species in the field. Thermal thresholds of seed germination for P. corsica and R. sandalioticum were not previously known and were estimated for the first time in this field study, based on findings of previous works carried out under controlled conditions. Critical embryo length and maximum germination for A. barbaricina were reached in April, and in December for R. sandalioticum. Seeds of P. corsica stay dormant in the ground until the following summer, and the critical embryo length and highest germination were detected from September to December. Soil heat sum models predicted earlier germination by one month for all three species under two Intergovernmental Panel on Climate Change (IPCC) scenarios, based on the assumption that the estimated thermal thresholds will remain constant through climate changes. This phenological shift may increase the risk of mortality for young seedlings. The models developed provide important means of connecting the micro-environmental niche for in situ seed germination and the macro-environmental parameters under a global warming scenario.

No genetic adaptation of the Mediterranean keystone shrub Cistus ladanifer in response to experimental fire and extreme drought

In Mediterranean ecosystems, climate change is projected to increase fire danger and summer drought, thus reducing post-fire recruitment of obligate seeder species, and possibly affecting the population genetic structure. We performed a genome-wide genetic marker study, using AFLP markers, on individuals from one Central Spain population of the obligate post-fire seeder Cistus ladanifer L. that established after experimental fire and survived during four subsequent years under simulated drought implemented with a rainout shelter system. We explored the effects of the treatments on marker diversity, spatial genetic structure and presence of outlier loci suggestive of selection. We found no effect of fire or drought on any of the genetic diversity metrics. Analysis of Molecular Variance showed very low genetic differentiation among treatments. Neither fire nor drought altered the small-scale spatial genetic structure of the population. Only one locus was significantly associated with the fire treatment, but inconsistently across outlier detection methods. Neither fire nor drought are likely to affect the genetic makeup of emerging C. ladanifer, despite reduced recruitment caused by drought. The lack of genetic change suggests that reduced recruitment is a random, non-selective process with no genome-wide consequences on this keystone, drought- and fire tolerant Mediterranean species.

Drought differentially affects the post-fire dynamics of seeders and resprouters in a Mediterranean shrubland

In fire-prone ecosystems, changes in rainfall after fire could differentially affect seeders and resprouters, thus leading to long-lasting impacts on the vegetation. Climate change in the Mediterranean region is projected to reduce precipitation, expand the summer drought and increase fire danger. Understanding the sensitivity to changes in rainfall during the post-fire regeneration stage is critical to anticipate the impacts of climate change on Mediterranean-type areas of the world. Here, we investigated how species differing in post-fire regeneration strategy (seeders vs resprouters) responded to rainfall changes in a Cistus-Erica shrubland of central Spain. Drought treatments were implemented using a system of automatic rainout shelters with an irrigation facility before (one season) and after (four years) burning a set of experimental plots. Treatments applied were: environmental control (natural rainfall), historical control (mimicking the long-term rainfall), moderate drought (-25% rainfall), and severe drought (-45% rainfall). Plant demography and vigour (main woody shrubs), as well as abundance (shrubs and herbs) were monitored during the first four years after fire. The first post-fire year was the key period for the recovery of seeders (Cistus ladanifer and Rosmarinus officinalis), and their recruitment, cover and size significantly decreased with drought. However, density four years after fire was larger than unburned and it was significantly correlated with emergence during the first year, indicating that population controls were more on emergence than on establishment. In contrast, resprouters (Erica arborea, Erica scoparia and Phillyrea angustifolia) were hardly affected by drought. Plant community dynamics in the burned control plots progressively converged with the unburned ones, while that in the drought-treated plots lagged behind them, maintaining a higher cover, richness and diversity of herbs. This post-fire "herbalization" due to drought might facilitate an untimely fire, before seeders would reach sexual maturity, which could have major implications for the maintenance of the community.