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?

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.

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.

Glorious vegetation change in the mountains

Long-term vegetation monitoring, by definition, takes a long time. But observing changes over long time periods isn’t just a matter of sitting back and waiting for something to happen. Actually making sure you, or someone else, can return to the same site and repeatedly measure the same things is the most accurate way to build a vegetation monitoring database, thereby providing the empirical evidence to support the predicted trends or challenge perceived patterns in species composition.

The Global Observation Research Initiative in Alpine Environments (GLORIA) is a long-term monitoring strategy aimed at gathering empirical evidence for species migration in mountain areas as a response to climate change. In Australia, we have five such GLORIA summits, all within the Main Range region of Kosciuszko National Park, in the Snowy Mountains. The sites are re-surveyed every 5-7 years and the data is directly comparable with other sites around the world; vegetation is monitored and the data is collected according to strict protocols.

The layout of sampling methodology of the GLORIA summits showing A the positioning of the upper and lower summit area sections (SAS) and clusters of 1 m2 quadrats, and B as viewed from above on a hypothetical summit. Source: Pauli et al. (2011).

Our first re-survey of these summits in January 2011 revealed some interesting results regarding changes in species richness, recently published in Biodiversity and Conservation:

Venn SE, Pickering CM and Green K (2012) Short-term variation in species richness across an altitudinal gradient of alpine summits. Biodiversity and Conservation 21:3157-3186  DOI:10.1007/s10531-012-0359-2

Long-term vegetation monitoring – best done using permanent plots and simple equipment.

On the study summits, there was an increase in species richness from 2004 to 2011 across all spatial scales investigated, at an overall rate of almost one new species per year, which is consistent with reported trends from European summits. There was moderate turnover of species moving in or out of the plots over the years, and shrub and graminoid species showed the greatest increases in species richness. These results suggest shrub migration onto the lower areas of the summits is underway, whereas a dramatic change to the uppermost summit vegetation at these sites seems unlikely in the short term. Given that the local lower elevation species pool is dominated by expanding shrubby vegetation, over longer time periods shrubs are expected to continue to increase in abundance at the lower summit sites, potentially causing decreases in overall species richness at those sites. There was no evidence of new species from outside the local species pool entering any of the study plots. Additionally, the strength and direction of change was not related to site altitude nor the variation in climate between years, but is more likely to reflect the longer-term trends in climatic variation of the region.

Although this study is in its infancy, the continuation of long-term monitoring in alpine areas is essential for detecting and predicting Australian alpine species’ responses to local climate change. The climate predictions for the Australian alpine areas include increases in temperature, particularly minima, and further reductions in snowpack; potentially down from the present mean of 183 days with least 1cm of snow cover at the highest summit in the region, Mt Kosciuszko 2228 m, to 87 days by 2050. Reflecting this, minimum and maximum temperatures in high mountain areas are already rising and snow cover has declined over the past few decades.

The value of long-term vegetation monitoring cannot be overstated. An excellent post by Ian Lunt explores this topic in more detail: http://ianluntresearch.wordpress.com/2011/10/21/stake-your-future/

In the meantime, our GLORIA vegetation database holds many more clues as to how the vegetation is changing in response to various environmental factors. Stay tuned for the next glorious instalment that will tell of how functional diversity and plant functional traits may lead to a greater understanding of the processes driving alpine vegetation community change.

Is climate change forcing alpine plants out into the cold?

Nematolepis ovatifolia emerging from snow in spring, Kosciuszko National Park

It’s snowing in the mountains now and most of the highest elevation areas in Australia should be snow covered until at least September/October… Or will they? Recent trends and predictions all point towards more variable snowfalls and increased variability in the timing of snowmelt. So, how will changed snowmelt regimes affect plant tolerances of very cold conditions? In Australia, it’s usually a comfortable 0-1 degrees C under the snow and the soil rarely freezes. But after snowmelt, plants (and animals and soil) can experience temperatures much lower. There’s growing concern that more variable snow falls and earlier snowmelt may also leave plants ‘out in the cold’ at a time when spring frosts are common and plants are busy making buds, flowers and seeds, making them more vulnerable to freezing damage. My recent paper with Janice Lord (Otago University) and John Morgan (La Trobe University) (in Early View with Austral Ecology) focuses on this topic.

Venn SE, Morgan JW, Lord JM (2012) Foliar freezing resistance of Australian alpine species over the growing season. Austral Ecology in press.

In this paper, we used chlorophyll flurometry to measure ‘leaf health’ after leaves of various alpine species were frozen at different temperatures. We then related subsequent measures of freezing resistance to habitat affiliation (snowpatch, herbfield, heathland); and to the timing of snowmelt using samples collected from different locations. We predicted that those species ‘expecting’ snow to melt early in the growing season (heathland species up on the ridge) would tolerate lower temperatures than those used to being protected from freezing conditions (sheltered snowpatch species), and that species should show patterns of acclimatisation over the growing season.

The results showed mixed patterns in freezing resistance among community affiliation/exposure and only one species showed any improvement in freezing resistance over the growing season (Craspedia aurantia). Freezing resistance was significantly related to site altitude, and overall most species showed very high foliar freezing resistance and were well adapted to very cold conditions; many shrubs being able to withstand temperatures down to -12 and -13 C. Pimelea axiflora could withstand -18.7 C. Although, most of the forb and graminoid species were in trouble below -10 C. Temperatures this low in Australian alpine areas are particularly infrequent and the most freezing resistant species in Australia are probably those growing in marginal sub-alpine cold-air drainage valleys at lower elevations, in which woody plants are notably absent and only very hardy grasses and forbs persist.

So for now, it appears alpine species don’t mind being left out in the cold so much, whereas untimely, freaky cold snaps over summer or unexpected mid-winter snowmelt might pose more of an unexpected threat to alpine plant species.

The Australian alpine treeline: marching up the mountain?

Ken Green and I have just had our research on alpine tree ribbons published in Arctic, Antarctic and Alpine Research:

Green K and Venn SE (2012) Alpine tree ribbons in the Snowy Mountains, Australia: characterization and potential mechanisms for treeline advance. Arctic, Antarctic and Alpine Research 44:180-187

Tree ribbons as seen from the air during winter near Mt Twynam, Kosciuszko National Park

Snowgum (Eucalyptus pauciflora sub. sp. niphophila), Australia’s alpine treeline species, is unlikely to track a changing climate and encroach into the alpine zone any time soon. It lacks any useful uphill dispersal mechanisms, has poor seedling recruitment and recent fires did not produce a significant pulse of recruitment above, or promote any advance on the current treeline. One potential mechanism for uphill treeline movement lies in ‘ribbons’ of trees that appear to promote in-filling between the ribbons and the extant treeline.

A very old snowgum in a ribbon above the alpine treeline near Mt Twynam, Koscisuzko National Park

Such ribbons, aged >300 years, have been discovered to aid in the creation of a downwind and downslope snowdrift from the ribbons. Here, late-lying snow appears to have historically prevented tree seedlings and saplings from becoming adult trees. Recent declines in snow, however, appear to be facilitating the growth of saplings downslope of the historic ribbons. It appears treeline movement, in the Snowy Mountains at least, is a case of ‘two steps forward, one step back’ as ribbon formation appears random and episodic infilling between ribbons will take many decades if not hundreds of years.