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Australasian Plant Conservation

Originally published in Australasian Plant Conservation 20(4) March - May 2012, p 16-17

Monitoring for climate driven floristic shift in Australian subtropical rainforest

Melinda Laidlaw
Queensland Herbarium, Toowong, Qld. Email: Melinda.Laidlaw@derm.qld.gov.au

Araucarian notophyll vine forest surveyed at 500 m altitude in Lamington National Park, south east Queensland. Photo: Melinda Laidlaw

Araucarian notophyll vine forest surveyed at 500 m altitude in Lamington National Park, south east Queensland.
Photo: Melinda Laidlaw

Background

The potential impacts of climate change on subtropical rainforest vegetation communities are being studied in south east Queensland by monitoring along an altitudinal gradient. This transect forms part of a larger international survey project known as IBISCA: Investigating the biodiversity of soil and canopy arthropods. IBISCA originated in Panama in 2003 as a collaborative project between a group of 40 scientists from more than 20 countries, including Australia. Following the success of the Panama study, IBISCA-style collaborative surveys have also been conducted in Vanuatu (2006) and France (2008-2010).

In 2006, the IBISCA survey protocol was applied to the subtropical rainforests of Lamington National Park in south east Queensland. This study revolved around the establishment of an altitudinal transect ranging from 300 m to 1100 m above sea level, whereby adjacent altitudes could serve as surrogates for different climates. Such a study design has provided a unique research environment in which to study the potential impacts of a changing climate at a single point in time, as well as establishing a permanent baseline transect which can be tracked over time. Soil and canopy arthropods were intensively studied over several years and although I will focus on the vegetation results here, the results from all IBISCA Qld studies, including the vegetation studies, have recently been published in a special volume of the Memoirs of the Queensland Museum (Burwell et al. 2011).

Study location and methods

The IBISCA Queensland transect was established in the West Canungra Creek catchment of Lamington National Park in 2006. The transect consisted of vegetation plots established at each of five altitudes: 300 m, 500 m, 700 m, 900 m and 1100 m above sea level. As such, the transect traversed a steep moisture and temperature gradient where the low altitude plots experience generally hotter and drier conditions than the cooler and moister high altitude plots. Each adjacent altitude surveyed also represented approximately 1ºC change in mean annual temperature. All plots were established on basalt derived soils with a similar aspect, a minimum of 50 m from a major water course and with no recent disturbance, in order to remove as much non-climate related variation as  possible.

At each altitude, four permanently marked 20 x 20 m plots were established with star pickets and all trees ≥5 cm diameter at breast height (dbh) tagged. Diameter was measured at 1.3 m from the ground on the uphill side of the trunk and tree tags nailed in 10 cm above this. Where vines or epiphytes obstructed measurement, these were gently lifted and the tape passed underneath. All other vascular plants on the site, including tree seedlings, were identified and given a score for cover.

The study and what we found

In establishing the altitudinal transect at Lamington National Park, we were looking for early evidence of climate related floristic shift, or turnover in the vegetation community, as has been identified elsewhere in the world in similar studies. In particular, we were looking for a stepwise, upslope movement of tree species where seedlings were found growing upslope from their parent trees. Such a pattern may be an indication of the upslope movement of cool, moist environmental envelopes as is predicted to occur under warming and drying climate change scenarios. In order to do this, we compared the established and juvenile tree communities recorded at different altitudes. An ‘established’ tree was defined as one with a stem of ≥5 cm (dbh), and a ‘juvenile’ tree was one with a stem below this dbh cut-off, but capable of reaching this size class. The presence or absence of species in each of these groups was compared at each altitude by using the Bray-Curtis dissimilarity metric.

Our results did not, as it turns out, reveal a stepwise upslope movement of seedlings, but instead a pattern which was quite unexpected (Laidlaw et al. 2011). At the two highest altitudes, 900 m and 1100 m, the established and juvenile tree communities were quite similar, suggesting that the tree communities are largely reproducing themselves in situ at these sites. At altitudes of 700 m and below, however, a strong division was found between the established and juvenile tree communities, regardless of what altitude they were growing at. This result suggests that at mid to low altitudes at Lamington National Park, the recruiting tree community was not reflecting the composition of the canopy, and a floristic shift may be occurring.

Of course, such one-off surveys highlight the lack of long-term information known about forests in their ‘normal’ state. It is possible we had simply stumbled upon a normal pattern in these forests which has just not been recorded before. The timing of the 2006 survey coincided with an extended period of drought where the death of some seedlings could be expected, but why was this pattern not found along the length of the transect? We suggest a possible reason is that sites at higher altitudes are regularly exposed to fog and cloud, even during droughts, which may help to mitigate the worst effects of dry weather on the understorey. We know that cloud and fog can constitute significant moisture inputs in montane forests (Hutley et al. 1997), and that one of the many predicted impacts of climate warming is that the elevation of cloud formation may rise. In the case of Lamington National Park, it is possible that in the future, cloud may no longer make regular contact with the vegetation at the highest altitudes, dramatically reducing the amount of moisture available during drought.

Conclusions and further work

Vitally, the results of the 2006 survey have allowed us to make predictions about what we may find the next time this site is resurveyed. For example, for the sites which sit at the base of the current cloud cap (approximately 900 m above sea level) we can predict an increase in the Bray‑Curtis dissimilarity between established and juvenile tree communities should the base of the cloud cap rise and the mitigating effects of cloud during drought be reduced. Only ongoing monitoring of this and other similar transects established along environmental gradients will reveal if the floristic shifts identified are intermittent, or the start of a long term compositional change in Australia’s subtropical rainforests. The first of many resurveys of the vegetation of the IBISCA Queensland transect is currently underway and we look forward to seeing how the forest has changed following two La Niña events.

References

Burwell, C.J., Nakamura, A. and Kitching, R.L. (2011)  Biodiversity, altitude and climate change in an Australian subtropical rainforest: Results from the IBISCA-Queensland project 2006-2010, Memoirs of the Queensland Museum, 55(2).

Hutley, L.B., Doley, D., Yates, D.J. and Boonsaner, A. (1997) Water balance of an Australian subtropical rainforest at altitude: the ecological and physiological significance of intercepted cloud and fog, Australian Journal of Botany, 45, 311-329.

Laidlaw, M.J., McDonald, W.J.F., Hunter, R.J., Putland, D.A. and Kitching, R.L. (2011) The potential impacts of climate change on Australian subtropical rainforest, Australian Journal of Botany, 59, 440-449.

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