Snow and frost in a warming world

shrub snow poke

During the snowmelt period, upper shrub branches poke through the snowpack and are therefore exposed to frosts

A warming world will cause the snow in Australia’s alpine regions to melt much earlier than in previous decades; a potentially devastating situation for many alpine plants that rely on the protection and insulation that snow offers for their survival. Without snow in early spring, alpine plants may be subjected to severe frosts at a time when they are most physiologically active; rapidly growing and preparing for the short growing season ahead.

But how early would the snow need to melt for it to be notably earlier than what plants might experience anyway due to inter-annual variability in snowmelt date, and subsequently cause frost damage and affect plant growth? In recent months, I’ve been testing these questions in two ways: 1) by examining the freezing resistance of alpine shrub foliage that is exposed to frost early, compared with foliage that is protected under snow for almost two weeks longer; and 2) by contributing to an global effort to understand the interacting effects of exposure to frost and drought on plant growth.

So what’s going on?

close snow poke

Shrub foliage is often the first to be exposed during the period of snowmelt. Despite few opportunities for frost-hardening, Australian alpine shrubs are very freezing resistant.

1) Overall, shrub foliage that is exposed early, i.e., the upper branches that poke through the snow in spring, are no more or less resistant to freezing than the foliage which is lower down on the shrub, buried in snow and exposed about 14 days later. This was an unexpected finding, as plants generally ‘frost-harden’ over time, but obviously not markedly in these species at the start of the growing season. As the growing season progressed, the freezing resistance of all four shrub species in this study did improve over time; with some of the species tested being able to withstand -10 to -21°C. Since air temperatures never reached below -9 °C during the period of study (and temperatures never really get much colder than that in the Snowy Mountains anyway), it appears that these shrub species may not be affected by frosts as the snow melts.

The full story is here:
Venn SE and Green K (2018) Evergreen alpine shrubs have high freezing resistance in spring, irrespective of snowmelt timing and exposure to frost: an investigation from the Snowy Mountains, Australia. Plant Ecology 219:209-216. DOI 10.1007/s11258-017-0789-8

Frost graph

The minimum air temperatures at the study site over the course of early spring recorded from within the lower protected branches of the target shrubs (closed circles) and at the exposed canopy (shaded) branches of the target shrubs (open circles), and the LT50 values (inferred freezing resistance) of the target shrubs measured at three times over early spring (9 September, 18 October and 15 November 2010), for lower protected foliage (closed shapes) and upper exposed foliage (open shapes) in Grevillea australis (black triangles), Nematolepis ovatifolia (black squares), Prostanthera cuneata (grey diamonds) and Leucopogon mon- tanus (black crosses).

What else is going on?

snow pit drought net

Snow is removed in order to expose plants to frost events, before the drought treatment is imposed the following summer.

rainout shetlers

Rainout shelters provide a 40% reduction in precipitation in the summer growing season, although in winter snow the following season is not impeded

2) In a global study across 13 sites, co-ordinated by Hugh Henry at Western University, Canada and within the framework of the International Drought Experiment and DroughtNet, we investigated the interaction between and imposed drought treatment (by using rainout shelters), and snow removal (by digging). Among sites, we observed a negative correlation between the snow removal effect on minimum soil temperature and plant growth (measured the following growing season by means of biomass production). Only three sites exhibited a significant rainout shelter effect on plant productivity, and there was actually no significant interaction between snow removal and imposed drought on plant growth. However, these two factors did exhibit significant effects simultaneously for a single site. The local Bogong High Plains site showed no real trends. So overall, our results reveal that reduced snowfall, when it decreases minimum soil temperatures, can be an important component of the total effect of reduced precipitation on plant productivity.

The full story is here:
Henry HA, Abedi M, Alados CL, Beard KH, Fraser LH, Jentsch A, Kreyling J, Kulmatiski A, Lamb EG, Sun W, Lreyling J, Kulmatiski A, Lamb E, Sun W, Vankoughnett MR, Venn SE, Werner C, Beil I, Blindow I, Dahlke S, Dubbert M, Effinger A, Garris HW, Gartzia M, Gebauer T, Arfin Khan MAS, Malyshev AV, Morgan JW, Nock C, Paulson JP, Pueyo Y, Stover HJ, Yang X (2018) Increased Soil Frost Versus Summer Drought as Drivers of Plant Biomass Responses to Reduced Precipitation: Results from a Globally Coordinated Field Experiment. Ecosystems Accepted 4 February 2018. https://doi.org/10.1007/s10021-018-0231-7 in press

Henry et al pic

Mean (+/- standard error) A aboveground biomass and B total percent cover (that is, total of all species cover values) for the ambient snow and snow removal subplots. Data pooled over the rain-out shelter treatments. Sites are ordered on the x-axis (left to right) from coldest to warmest mean January air temperature. Asterisks indicate significant differences (P < 0.05) within sites.

Perhaps the future interactions between snow cover, frost exposure and drought won’t be as devastating to alpine plant life as anticipated? Or, it might take several consecutive poor snow seasons for the conditions to be well outside of the background inter-annual variability for the plants to notice sufficiently and respond.

In the meantime, it appears Australian alpine plants are reasonably well equipped to take on the challenges of short and sporadic exposure and frosts and drought.

 

 

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Posted in Shrubs, Susanna's research updates

Alpine plant species dispersing into the future?

Seed dispersal will be essential for plants to track future climate space, but species are not equally capable of dispersing through the landscape, nor will the landscape be equally affected by climate change. The notional plight of alpine vegetation, with nowhere higher and cooler to migrate to, lies in the expectation that species have the ability to disperse to their current range-edge in the first place.

In a recent paper, John Morgan and I employed a quantitative seed dispersal model (developed by Tamme et al. 2014) to calculate the dispersal capacity of an entire alpine flora in the Snowy Mountains, Australia. In doing so, we were able to model species’ maximum dispersal distances on the basis of inherent plant traits such as dispersal syndrome, growth form, seed release height and seed mass.

Morgan JW and Venn SE (2017) Dispersal opportunities for alpine plants in the face of rapidly changing climates. Plant Ecology online DOI: 10.1007/s11258-017-0731-0

seed picture with lables

Seed dispersal syndromes vary in the Australian alpine flora, from species that rely on wind dispersal (seeds 2–6), animal dispersal (seeds 1, 7 and 8), or have no dispersal syndrome and rely on gravity for movement (seeds 9–13)

The flora of the Snowy Mountains is made up of mostly low growing (0.4 m) herbaceous graminiods and forbs and most species (63%) had no inherent dispersal syndrome (i.e. they rely on gravity for dispersal). It wasn’t surprising therefore to find that 75% of species were modeled to disperse 100 m. Of course, species with animal or wind dispersal syndromes were modelled to disperse much further (>600 and >140 m respectively).

Celmisia

The Silky Snow-daisy, Celmisia sericophylla, has very specialised habitat requirements, is listed under the Flora and Fauna Guarantee Act, and a conservation status in Victoria as endangered. While wind dispersed, migration options for this species are very limited.

But is this far enough?

Modelling dispersal distances is a critical first step for understanding where alpine plants might go as the climate changes. But for species with highly specialized habitat requirements, the ability to disperse away from the parent plant becomes a secondary concern if the habitat disappears. Also, simply being a wind-dispersed species won’t get you out of trouble if your seeds don’t land in the right location.

While alpine species around the world may be migrating to higher elevations, we must not forget that a migration could be occurring ‘around’ the mountain too; whereby conditions might become more favourable on a different aspect, or simply in the lee of another landscape feature.

The next research question surrounding the dispersal of alpine seed is obvious:
“How much seed is landing in a suitable substrate, both now and into the future?”

Reference:
Tamme R, Gotzenberger L, Zobel M, Bullock JM, Hooftman DA, Kaasik A, Partel M (2014) Predicting species’ maximum dispersal distances from simple plant traits. Ecology 95:505–513
Posted in Susanna's research updates

Alpine shrubs as ecosystem engineers

orites-snow-drift

Alpine shrub, Orites lancifolia, effectively creates a snowdrift in its lee, which can promote soil nutrient cycling feedbacks

Worldwide, shrub cover is increasing across alpine tundra. In Australia, alpine shrub increases match a trend spanning four decades of rising temperatures and declining snowpack. Repeat photography, long-term monitoring, field warming experiments and dendrochronology have revealed that alpine shrubs are responding by encroaching into otherwise non-shrubby communities, such as alpine herbfields and grasslands.

Alpine shrubs readily restrict the growth of other plants via shading and smothering with leaf litter, and they can alter wildlife habitats. Warmer conditions may also exacerbate a feedback between shrubs and fire, whereby increased fire activity due to highly flammable foliage and leaf litter, stimulate vigorous re-sprouting and seeding, resulting in further increases in shrub cover.

Some shrub growth forms interact with winter processes; they can accumulate snow in their lee, thereby insulating soils from extreme winter temperatures. These effects may also promote a second feedback whereby deeper snowpack, warmer soils and higher soil moisture, coupled with leaf litter under shrub canopies, increases microbial activity. These effects in turn, can enhance soil nutrient cycling and ultimately promote shrub growth. Deeper snowpack around shrubs also contributes to winter and spring water yields in mountain catchments.

Given that alpine shrub range-expansion has the potential to significantly modify existing landscape flammability, winter processes and ecosystem function, alpine shrubs effectively act as ecosystem engineers. There is an urgent need for land managers to monitor changes in shrub abundance, and for stakeholders to understand these processes in order to determine whether increases in shrub cover and shrub encroachment will result in alternate stable states in alpine vegetation, local plant and/or animal extinctions, and whether an overall declining snowpack will mitigate or exacerbate these processes.

Isla Myers-Smith, James Camac and Adrienne Nicotra contributed to this piece.

The full story and references

Posted in Shrubs, Susanna's research updates, Uncategorized

It’ll be a cup of tea…

This past summer season, the Australian Alps were used in the International Tundra Teabag Experiment – inspired by the ongoing research by the Tea Bag Index.

What do we do? We bury two types of tea (Green and Rooibos) at sites across the tundra biome – the cold environments found in the Arctic or the tops of mountains. After a few months we dig up the tea and look at how much it has decomposed. This helps us understand how rapidly it has broken down – and how fast the carbon and nutrients it contains move into the soils and air.

Normally when we look at decomposition we have to consider both what it is (how fast does it break down) and where it is (where does it break down fastest). But because all the tea we use is the same, we can be more confident that any differences are due to site conditions. This means we can look at how things like temperature, moisture and vegetation cover affect decomposition – which helps us make predictions about the future.

Why?  The tundra biome covers a huge area of the earth’s surface, but it’s changing very quickly. This is mostly due to global warming – temperatures in the Arctic have increase by about 2℃ since the 1960s. As things heat up in the Northern Hemisphere, all the plant matter stored in cold and frozen soils will start to rot, releasing carbon to the atmosphere and speeding up global warming. This could cause a runaway positive feedback affecting the earth as a whole. The tundra tea bag experiment helps us understand if this will happen, and if so, how fast.

tundra tea bag sites

The International Tundra Teabag Experiment takes in six countries across the tundra biome

Haydn Thomas at the University of Edinburgh is compiling Tundra tea decomposition results from around the world. Similar experiments are also taking place elsewhere, spearheaded by the dECOlab in Urtrecht in the Netherlands (Tea Bag Index). Researchers aligned with the GLORIA summit monitoring protocol are also getting involved in burying tea bags on summits – with tea also decomposing on the Australian GLORIA summits in Kosciuszko National Park, to be recovered in December 2016.

photo 5

Get down, get all dirty, you just gotta bend your knees…

In the Victorian Alps, we buried tea bags into a snowpatch plant community on the Bogong High Plains, specifically in early, mid and late snowmelt zones. With this approach, we will also be able to determine the role that differential snow-lie has on decomposition, and be able to determine how important snow is for such underground processes. Stay tuned for the post-winter results, when we will recover the remaining tea bags that will have been buried for 9 and 12 months.

photo 4

The 2016 Aussie teabag team, including the Team Shrub member, Haydn Thomas (second from left), all the way from the UK

Posted in Susanna's research updates, Uncategorized | Tagged , , ,

Snowpatches or just patches of snow?

Snowpatches across the Main Range, Kosciuszko National Park

Snowpatches across the Main Range, Kosciuszko NP

A snowpatch (or snowbank) plant community is simply a re-occurring assemblage of plants that exist where snow lies late into the summer. In Australia, this occurs on the south-east side of high elevation ridges and mountain sides, in the lee of the prevailing north-westerly winds. The highest elevation snowpatches can develop impressive cornices of snow that may be up to 10 m or so deep. Snowpatches are one of the rarest communities in the landscape due to topographical constraints and the restricted nature of the alpine zone in general; Australian alpine and subalpine ecosystems occupy less than 1% of the area of the continent, and snow patch herbfields occupy less than 1% of that.

Areas where snow lasts well into summer are obvious in the landscape and create distinct  vegetation patterns. But are they true snowpatches?

Areas where snow lasts well into summer are obvious in the landscape and create distinct vegetation patterns. But are they true snowpatches?

Are snowpatches really that special?
Snowpatch plant communities, or more correctly defined as ‘snow patch herbfields’, are not explicitly listed under national legislation (The Environment Protection and Biodiversity Conservation Act, 1999; EPBC), however they are within National Heritage areas and therefore considered as ‘matters of National Environmental Significance’ under the EPBC Act, and in Victoria, they are explicitly listed under the Flora and Fauna Guarantee Act 1988 (Government Gazette G 27, p. 1494, published on 5 July 2012). Despite formal recognition, Australian snowpatch vegetation includes many structural and floristic types including ‘tall-alpine herbfield’, ‘short alpine herbfield’, ‘short turf’ and ‘feldmark’. No vascular plant species appear to be totally restricted to snowpatch habitats, adding to the confusion about which patches of snow actually accommodate a snowpatch community. La Trobe University students grapple with this question every summer at Mt Hotham and invariably there is hot debate among the researchers, and bewilderment among the students, trying to discern a snowpatch community from the surrounding the landscape using multi-dimensional scaling and ordination.

Snowpatch dynamics
Snow is the primary environmental filter in a snowpatch community, creating a repeating pattern of melt every year and thus predictable zones of growing season length across the snowpatch. Many species time their growth and phenology on these patterns, as I demonstrated a while back during my honours research:

Venn SE and Morgan JW (2007) Phytomass and phenology of three alpine snowpatch species across a natural snowmelt gradient Australian Journal of Botany 55:450–456

Not all snowpatch species’ flowering is determined by snowmelt date, rather photoperiod also plays a significant role here. Snow also determines the type and function of species that grow in different snowmelt zones; those which are relatively taller and larger-leaved (competitive and productive) tend to be around the perimeter of a snowpatch where snow melts earlier, compared to the shorter and smaller leaved species which grow in the centre of the snowpatch where snow melts last, as we demonstrated for seven snowpatches in Kosciuszko National Park:

Venn SE, Green K, Pickering CM and Morgan JW (2011) Using plant functional traits to explain community composition across a strong environmental filter in Australian alpine snowpatches. Plant Ecology 212:1491-1499

Ranunculus perched high up in a snowpatch community above Blue Lake in KNP

Ranunculus perched high up in a snowpatch community above Blue Lake in KNP

Those competitive and productive species include shrubs and tall grasses, and are the most likely candidates to encroach into snowpatches in the future as global warming essentially weakens the environmental filter currently restraining them from entering these habitats. However, after a re-survey of the Kosciuszko snowpatches, my colleagues and I have revealed that this process is already underway:

Pickering CM,  Barros AA, Green K, Venn SE (2014) A resurvey of late-lying snowpatches reveals changes in both species and functional composition across snowmelt zones Alpine Botany DOI:10.1007/s00035-014-0140-0

Notably, between 2007 and 2013 there was an increase in species richness in the late snowmelt zone and an increase in the cover of the tall tussock grass Poa costiniana across all snowmelt zones. This research highlights that snowpatch vegetation can change within relatively short time periods and that snowpatch plant communities may not remain as discrete units in the near future due to the encroachment of more competitive and productive species from the surrounding landscape.

Future patches of snow?
Recently, Australian mainland snowpatches were assessed using the IUCN Red List Criteria for ecosystems, given the present threats from climate change, land use (ski resort development, summer tourism) and invasions by exotic plant and animal species. Evidence is mounting that climate change may also substantially alter vegetation state by increasing rate of encroachment of shrubs as well as tall grasses into snowpatches that are otherwise dominated by short herbs and graminoids. The assessment (citation below) recommends that the ecosystem (IUCN terminology) is ‘Endangered’ due to the restricted geographical distribution, the substantial and highly likely decline in the abundance of snow (the principal abiotic driver of the ecosystem), and the prospect of invasion of many snowpatches by taller-growing native shrubs and grasses thus causing ecosystem collapse.

Williams RJ, Wahren C-H, Stott KAJ, Camac JS, White M, Burns E, Harris S, Nash M, Morgan JW, Venn SE, Papst WA, Hoffmann AA. (2015) An IUCN Red List Ecosystems Risk Assessment for Alpine Snow Patch Herbfields, South-Eastern Australia. Austral Ecology in press

One of the oldest manipulative experiments in the world, the snowfence on the Old Man Range, New Zealand, creates a regular snowdrift accompanied by snowpatch vegetation

One of the oldest manipulative experiments in the world, the snowfence on the Old Man Range, New Zealand, creates a regular snowdrift since colonised by snowpatch vegetation

It seems that vegetation dynamics are one of the key culprits here. On the flip-side, vegetation dynamics could also produce snowpatches… Really??

An historic snowfence built in 1959 by Emeritus Professor Sir Alan Mark (Otago University, Dunedin) on the Old Man Range in Central Otago, New Zealand, creates a lasting patch of snow in its lee every spring, while snow in the surrounding landscape melts at the usual time. As a consequence, many local specialist snowpatch species have colonised the sheltered area behind the fence, thus creating a functioning snowpatch community, with similar internal dynamics, environmental filtering and vegetation patterns to those in Australia, within a matrix wind-blasted dwarf shrubs.

Mark AF, Korsten AC , Guevara DU, Dickinson KJM, Humar-Maegli T, Pascale M, Halloy SRP, Lord J, Venn SE, Morgan JW, Whigham PA and Nielsen JA. (2015) Ecological responses after 52 years to an experimental snow fence in high-alpine cushionfield, Old Man Range, south-central New Zealand. Arctic, Antarctic and Alpine Research in press

Perhaps patches of snow elsewhere could also become snowpatch communities in the future? As long as there is snow, there will always be places where snow lasts longest in the landscape, and therefore there will be plant species in that location that must either cope with shorter growing seasons or enjoy the protection that lasting snow can offer. If we are willing to continually re-define what constitutes a snowpatch habitat and the species that make a true snowpatch community (or ecosystem), then recognised snowpatches per se are only as threatened as the amount of snow that exists in the alpine landscape.

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Australian alpine shrubs: turning global patterns upside down.

Across an altitudinal gradient, you might expect more plants with low specific leaf area (SLA; the ratio of one-sided area of a fresh leaf to its oven-dry mass) to be in higher abundance at the highest altitude areas; where plants are subjected to strong winds, freezing temperatures and a short growing season. Up there, plants may invest in tough, long-lived leaves with structural and chemical defences to protect them from the harsh environmental conditions and herbivore damage. Given the extra leaf investments, lasting a long time makes sense, several years or more, as leaves need to pay back their own construction costs, as well as make a positive contribution to the overall carbon balance of the plant over it’s lifetime.

Small summits along the ridge line of Mt Clarke, Snowy Mountains, Kosciuszko National Park, have revealed some counter-intuiative patterns in leaf traits; high proportions of species with low SLA at the lower altitude summits.

Small summits along the ridge line of Mt Clarke, Snowy Mountains, Kosciuszko National Park, have revealed some counter-intuiative patterns in leaf traits; high proportions of species with low SLA at the lower altitude summits.

However, the dominance of shrubs and graminoids in the Australian alpine regions are producing some counter-intuitive patterns in terms of leaf traits, in particular SLA, and the ecological processes that they represent. In a recent paper with my colleagues Ken Green and Catherine Pickering we show just how dramatic the patterns in the Snowy Mountains are:

Venn SE, Pickering CM and Green K (2014) Spatial and temporal functional change in alpine summit vegetation is driven by increases in shrubs and graminoids. AoB PLANTS 6: plu008; DOI:10.1093/aobpla/plu008

In the Snowy Mountains, Kosciuszko National Park, we used an altitudinal gradient of summits, part of the international GLORIA project (see my earlier post about Glorious vegetation change in the mountains), to investigate the patterns in plant functional traits. We measured simple morphological traits like plant height (which infers competitive ability), leaf area and leaf dry matter content (which infers how productive a species may be) and SLA (which infers leaf life span, toughness and durability). As expected at high elevations, the plants are relatively shorter, but when we measured the abundance of the leaf traits – not species abundance, but instead the community trait-weighted mean, we found high proportions of species with low SLA at the lower altitude summits; a surprising and counter-intuitive pattern.

So what’s going on here?

One of the dominant shrub species at the lowest altitude summit; Nematolepis ovatifolia (Rutaceae).

One of the dominant shrub species at the lowest altitude summit; Nematolepis ovatifolia (Rutaceae).

It turns out the dominance of some graminoids, namely Poa hiemata (Poaceae) and Carex breviculmis (Cyperaceae) and several shrubs such as Kunzea muelleri (Myrtaceae), Epacris microphylla (Ericaceae) and Nematolepis ovatifolium (Rutaceae) at the lower altitudes are really driving the overall patterns across the gradient. These are the key, low SLA species from typical, sclerophylous, Australian plant families. And so relative to these species, the forbs at high elevations do have high SLA, thus creating this upside-down pattern. The dominant shrubs are also much taller than the other life forms present; probably making them superior competitors with their neighbours. In addition, the climate change predictions for the mountains all point towards more favourable conditions for shrubs; less snow, longer growing seasons and warmer temperatures. And so we expect the shrubs to maintain their dominance, increase in abundance, out-compete other life forms and make some big advances into areas currently not dominated by shrubs in the medium to long term.

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