Susanna E. Venn

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra

Soil warming increases plant species richness but decreases germination from the alpine soil seed bank

Global warming is occurring more rapidly above the treeline than at lower elevations and alpine areas are predicted to experience above average warming in the future. Temperature is a primary factor in stimulating seed germination and regulating changes in seed dormancy status. Thus, plant regeneration from seed will be crucial to the persistence, migration and post disturbance recruitment of alpine plants in future climates. Here, we present the first assessment of the impact of soil warming on germination from the persistent alpine soil seed bank. Contrary to expectations, soil warming lead to reduced overall germination from the soil seed bank. However, germination response to soil temperature was species specific such that total species richness actually increased by nine with soil warming. We further explored the system by assessing the prevalence of seed dormancy and germination response to soil disturbance, the frequency of which is predicted to increase under climate change. Seeds of a significant proportion of species demonstrated physiological dormancy mechanisms and germination of several species appeared to be intrinsically linked to soil disturbance. In addition, we found no evidence of subalpine species and little evidence of exotic weed species in the soil, suggesting that the soil seed bank will not facilitate their invasion of the alpine zone. In conclusion, changes in recruitment via the alpine soil seed bank can be expected under climate change, as a result of altered dormancy alleviation and germination cues. Furthermore, the alpine soil seed bank, and the species richness therein, has the potential to help maintain local species diversity, support species range shift and moderate species dominance. Implications for alpine management and areas for further study are also discussed.

Using plant functional traits to explain community composition across a strong environmental filter in Australian alpine snowpatches

Environmental filters act to limit the local community assemblage from the regional species pool by restricting the viable trait states that can occur there. In alpine snowpatches, the timing of snowmelt is a strong environmental filter. In coming decades, the strength of this filter is likely to relax with global climate change. We used three continuous plant functional traits (leaf area, plant height, seed mass) and their divergence (using the FDvar index) to document current patterns of community assembly and predict plant community responses to future environmental filters in alpine snowpatch vegetation. The community trait-weighted mean for leaf area and height, but not seed mass, was significantly higher in early snowmelt zones relative to mid and late melting zones across all snowpatches. Mean FDvar for height (but not leaf area or seed mass), by contrast, was substantially lower in early snowmelt zones, indicating that species growing in early melt zones are consistently taller than those growing in other zones. These results suggest that if climate change leads to earlier snowmelt and hence, a longer growing season, taller (more competitive) species with larger leaf areas (more productive) may replace short species in snowpatches as these plant communities re-assemble in response to changing environmental filters.

Patterns in alpine seedling emergence and establishment across a stress gradient of mountain summits in south-eastern Australia

Background: Past research and ecological theory supports the hypothesis that alpine plants should be predominantly clonal, long-lived and reproduce by seed infrequently. Aims: To challenge the entrenched view that alpine seedling establishment is uncommon and perhaps unsuccessful in maintaining alpine plant populations. Methods: We looked for patterns in seedling establishment across a stress gradient of alpine sites and tested the notion that natural seedling recruitment would be reduced at higher altitudes due to a combination of stressful environmental conditions such as frosts, wind and extended snow cover. Results: Seedlings were common at all study sites and there was a partial trend in mean seedling density with altitude. Seedling emergence was best predicted by a combination of altitude, plant litter and soil wilting point. Many seedlings survived to become established plants (37–61%) but there was no significant relationship between establishment and altitude. There was low similarity between the seedling flora and the standing vegetation, suggesting that patterns in seedling establishment are unlikely to be driving the patterns in the standing vegetation assemblage. Conclusions: Although Australian alpine species predominantly reproduce vegetatively, this study shows that many plants depend on recruitment from seed in order to become established.

Do Facilitative Interactions with Neighboring Plants Assist the Growth of Seedlings at High Altitudes in Alpine Australia

Abstract During the early life-history stages, plants are especially susceptible to the abiotic conditions present in high mountain environments. At high altitudes, facilitative interactions between close neighboring plants may buffer seedlings from these abiotic pressures by providing shelter from frosts and winds. At lower altitudes, seedlings may not be so limited by the abiotic environment, and may therefore compete for resources with close neighboring plants. Using four alpine sites at different elevations (representing an abiotic stress gradient), we investigated how the presence of close neighboring plants influences seedling growth in their first growing season. We experimentally cleared above-ground vegetation and transplanted seedlings of three species into cleared and control plots. We quantified the stress gradient of abiotic conditions across sites by measuring ambient and soil temperatures, soil moisture, and soil frost heave. We used the “Relative Neighbor Effect” index to show the direction of the interaction between transplanted seedlings and their close neighboring plants. Aciphylla glacialis seedlings showed neutral interactions across the gradient of alpine sites, with undetectable change across the growing season, compared with Brachyscome rigidula seedlings which showed positive interactions with neighbors across the growing season at most sites. Trisetum spicatum seedlings showed mostly neutral interactions with neighbors at the higher elevations, and often negative interactions at the lower elevations, particularly midway through the growing season. Our findings highlight the importance of spatial and temporal plant-plant interactions with regard to seedling performance across altitudinal stress gradients.

Phytomass and phenology of three alpine snowpatch species across a natural snowmelt gradient

Alpine snowpatch vegetation in Australia is restricted to high mountain areas and occurs in locations where winter snow persists longest into the summer. The timing of annual snowmelt is considered an important determinant of vegetation patterns in alpine areas because it affects the length of the growing season for plant species at landscape scales. There are few studies in Australia that have examined the effects of the date of snowmelt on the performance of plant species at small spatial scales. The phytomass and phenology of three common snowpatch species (Celmisia pugioniformis, Luzula acutifolia, Poa fawcettiae) was examined during one growing season across a natural snowmelt gradient to examine their response to time of snow release. Peak phytomass was significantly higher in early than late-melting zones for L. acutifolia and marginally higher there for C. pugioniformis. P. fawcettiae, however, produced higher mean peak phytomass in late- melting zones where soil was initially wetter in the growing season. Flower buds of L. acutifolia were evident as the snow melted, and flowering occurred at the same time in all areas of the snowpatch. The number of days from the date of snowmelt to the date of the first observed flower bud in C. pugioniformis and P. fawcettiae was 22-25 days shorter in late-melting areas than in early melting areas. For both of these species, flowering and subsequent seed set occurred simultaneously across the snowpatch regardless of the date of the initial snowmelt, suggesting that photoperiod controls flowering in these species. Our study suggests that the predicted declines in snow cover in Australia in coming decades may affect the phytomass of species that are currently constrained by late-lying snow. This, in turn, may affect their long-term patterns of distribution. If plants respond to photoperiod for flowering, as seems to be important here for C. pugioniformis and P. fawcettiae, it is unlikely that the periods following earlier than usual snowmelt will be fully utilised by these species. Any attempts at predicting or modelling future alpine plant distribution on the basis of warming scenarios may therefore need to account for photoperiod constraints on flowering as well changes in phytomass production.

Spatial and temporal functional changes in alpine summit vegetation are driven by increases in shrubs and graminoids

In order to determine the mechanisms that drive changes in plant community composition across spatial and temporal scales, plant functional traits were used to interpret the results of a repeat species survey across a gradient of five alpine summits in south-east Australia. Vegetation changes were strongly affected by the high and increasing proportion of tall shrubs and graminoids, especially at the lower elevation summits. Several significant relationships between the community trait-weighted mean of different traits and elevation may suggest processes such as competition are influencing vegetation preferentially across the elevation gradient, with shrubs and graminoids driving these patterns.

Soil seedbank composition and dynamics across alpine summits in south-eastern Australia

Alpine soil seedbanks are generally regarded as small and unimportant to regeneration. Here, we investigate for the first time the composition of the readily germinable soil seedbank across alpine summits in south-eastern Australia. We aimed to compare the species in the seedbank with the standing vegetation, show seasonal variations in seedbank composition and identify regeneration strategies of alpine seedbank species. By using standard glasshouse and cold-stratification germination techniques, the germinable soil seedbank across the study region was found to comprise 39 species from 25 families, with species from the Asteraceae the most common. Persistent seedbanks were found across all eight alpine summits (1668–1970 m), comparable in seed density (150 ± 27 to 1330 ± 294 per m2) with those of other alpine areas in the northern and southern hemispheres. The density of germinable seeds varied widely among sites and between collection times (autumn, spring) and there were no trends in seed density with altitude. The qualitative and quantitative similarity between the seedbank species and the standing vegetation was low. Correlations between the proportions of species in regeneration categories (from obligate seeders, through to vegetative regenerators) in the standing vegetation and the seedbank were also poor. Our results indicate a divergence between the species in the current standing vegetation and those present in the readily germinable soil seed bank. The current patterns and predominance of seed-regenerating species in the seedbank indicate that these species may have an important role to play in regulating and contributing to future changes in the vegetation assemblage.

Tree-Limit Ribbons in the Snowy Mountains, Australia: Characterization and Recent Seedling Establishment

Abstract Tree-limit ribbons, isolated ribbons of trees growing above, but close to the alpine tree limit, have been described previously only for North America. Here, we describe such ribbons from the Snowy Mountains, Australia. Spread of trees above the treeline on lee slopes is generally as ribbons perpendicular to the prevailing wind, with snowdrifts accumulating downwind suppressing seedling establishment. The ribbons exhibit long-term stability, with estimated stem ages of snowgum (Eucalyptus pauciflora subsp. niphophila) up to 500 years, and the lignotubers considerably older. Windblown branches containing viable seed may allow initial establishment of trees above treeline leading to the formation of ribbons. Seedling establishment uphill of the highest ribbons is rare because snowgum has no inherent seed dispersal mechanism, depending on gravity for dispersal. However, seedling establishment immediately downslope of the highest ribbons, normally suppressed by snowdrifts, is more common and appears to have occurred mainly post-1970. Whether seedlings that established under snowdrifts post-1970 will remain as krummholz or proceed to full tree status will depend on the future snow regime and the persistence of regular snowdrifts. However, there are trees that have established below ribbons but outside the influence of snowdrifts, that exist now as younger ribbons, in clumps, or as individuals, in areas that previously did not support trees.