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

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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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Seeds, dispersal and coping with climate change in the Australian Alps

An alpine meadow at Mt Bogong with Celmisia costiniana (Asteraceae) flowering

An alpine meadow at Mt Bogong with Celmisia costiniana (Asteraceae) flowering in the foreground

Climatic changes will undoubtedly affect alpine plants considerably. Long-term resilience strategies of species may involve: persistence, whereby species remain on site and rely on inherent characteristics and plastic trait shifts to maintain populations; adaptation, whereby species remain on site and rely on genotypic trait shifts to match changing environmental filters; or dispersal, whereby species re-locate to suitable sites to avoid unfavourable conditions.

Pentachondra pumila (Ericaceae) berries among the mat-forming, prostrate plant

Pentachondra pumila (Ericaceae) berries among the mat-forming, prostrate plant. Photo: Friends of the ANBG

Interestingly, most of the seeds (including diaspores of all descriptions) from the higher vascular plants in the Kosciuszko alpine region do not have a particular dispersal syndrome to aid in spatial dispersal; most rely simply on gravity to disperse their seeds, which for many small herbaceous species is not very far. A few possess brightly coloured  fleshy fruits, however there is a distinct lack of suitable animal dispersers; birds are rare except for the occasional little raven, Richard’s pipit or flame robin. So many of these fruits remain on individuals to rot or are eaten by ants. However, there is no hard evidence that ants disperse these seeds very far from their source.

Microseris sp. with bristle pappus - perfect of wind dispersal. Photo: Victorian flora.org

Microseris lanceolata with bristle pappus seeds – perfect for wind dispersal. Photo: Victorian Flora.org

The largest group of alpine species with any special wind-dispersal mechanisms are the Asteraceae daisies, with a bristle pappus, perfect for being carried high up into the air and dropped down metres or tens of metres from the mother plant. Just how far they can go in the mountains remains to be seen.

In a new study with John Morgan, I’m exploring the dispersal strategy among plants restricted to the Kosciuszko alpine region, where climate change predictions all point towards weakening environmental filters; notably warmer temperatures and less snow. There’s a real need to integrate seed character and dispersal syndrome research with community ecological theory, in order to understand how well and how far alpine plant species can really get around the landscape, and how they will respond to the new environments in the alpine zone. This can be achieved by predicting species’ maximum dispersal distances from simple plant traits such as growth form, dispersal syndrome and seed release height. We’re using some nifty R code produced by Riin Tamme in Estonia, who has done exactly this with a large data set of 576 species in Europe.

Check out:

Tamme R, et al. (2013) Predicting species’ maximum dispersal distances from simple plant traits. Ecology in press

Xerochrysum subundulatum (Asteraceae) at Mt Hotham, another bristle pappus seeded species

Xerochrysum subundulatum (Asteraceae) at Mt Hotham, another bristle pappus seeded species

Our predictions for the Kosciuszko alpine flora include a high proportion of very short dispersal distances, with a small proportion of wind-dispersed species being able to get several metres away from their starting point. An understanding of dispersal syndromes and unassisted dispersal capacity is the first step in unravelling whether this strategy might be useful for species migration and/or persistence in alpine landscapes in future decades. Stay tuned for our the results – coming soon.

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Germination of alpine seeds in a warmer world

Summer flowering in an alpine herbfield, Kosciuszko National Park. But what is the fate of any viable seed produced?

Summer flowering in an alpine herbfield, Kosciuszko National Park. But what is the fate of any viable seed produced?

The climate in Australian alpine areas is changing more rapidly than in lowland areas.  Increased temperatures, particularly minima, and significant reductions in snowpack, snow duration and extent are some of the anticipated impacts. The expected climate warming will also present several unprecendented scenarios for plant reproduction; given that temperature is the primary factor for stimulating seed germination and regulating changes in seed dormancy. Successful regeneration via seed will be crucial to species persistence, migration and ongoing recruitment in the future.

But where is the seed? And will it germinate?

After seed matures on the flower; seeds may germinate immediately as suitable conditions arise, disperse to other areas, die, or be incorporated into the soil seed bank. Persistent seedbanks enable species to synchronise their germination with favourable years, while maintaining some seeds in the soil during periods of poor seed production. In contrast, seeds from a transient seedbank remain within the top-soil layers only for a short time and germinate quickly rather than accumulating and working their way deeper into the soil. Which begs the question; how will warmer alpine soils influence current germination rates from the soil seed bank?

Together with my colleagues at the Australian National University and the Australian National Botanic Gardens, we are beginning to understand the role of the alpine soil seed bank under climate change scenarios in Australian alpine areas:

Hoyle GL, Venn SE, Steadman KJ, Good RB, McAuliffe EJ, Williams ER and Nicotra AB. (2013) Soil warming increases plant species richness but decreases germination from the alpine soil seed bank. Global Change Biology in press (doi: 10.1111/gcb.12135)

By understanding the mechanisms driving germination from the alpine soil seed bank with an experimental warming treatment, we can begin to understand the potential effects of climate warming on alpine soils and population recruitment processes.

By understanding the mechanisms driving germination from the alpine soil seed bank with an experimental warming treatment, we can begin to understand the potential effects of climate warming on alpine soils and population recruitment processes.

We ran a simple germination experiment using soil collected from Kosciuszko National Park under two contrasting glasshouse temperature regimes; 1) designed to mimic present day soil temperatures during an optimal growing season; and 2) designed to mimic warmer alpine soil temperatures which reflects a growing season under the climate predictions beyond 2050. Overall, soil temperatures in the warm glasshouse were approximately 4°C warmer than those in the cool glasshouse, both day and night.

We expected warmed soil to increase germination rates and the total number of germinants, which is a common response in laboratory germination trials. Indeed, we recorded 12 more species in the warmed soil treatments; however, we actually saw an overall reduction in germination in the warmed soil treatment. When we examined the data more carefully, germination response to soil temperature was species specific, about 64% of the germinable soil seed bank was made up of Poaceae seeds which did not germinate well in warmed soil; reflecting the dominance of grasses in the system. However, there were some Cyperaceae, Juncaceae and Campanulaceae species which showed higher germination in the warmed soils. Only seeds of local alpine species germinated, and only 2% were identified as weeds.

What does this mean for alpine germination in a warmer world?

There were still 17 species that needed a dose of growth hormone (giberrellic acid, GA3) to overcome physiological dormancy to germinate, indicating that the alpine environment, characterized by cold, snowy winters, may be critical for germination among some species, especially those in the Apiaceae, Rosaceae, Crassulaceae and Boraginaceae. But overall, warmed soil appears to hinder, rather than help alpine seed germination. Should warmer soils in the alpine zone manifest along the lines of our experiment, we might still expect alpine species to maintain populations through transient and/or persistent seeds banks. However, species might begin to rely more heavily on germination immediately post flowering and seed maturation to maintain local species diversity, support species range shifts and maintain species populations under warmer and more variable climates.

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