Description

According to the last report on climate change, global temperatures are planned to increased of + 2.4-6.4 °C by 2100 (IPCC 2007), leading to an increase of +0.4 °C every 10 years. This global temperature rise will affect unique and threatened systems. There is new and strong evidence of the observed impacts of climate change on unique and vulnerable systems (such as polar and high mountain communities and ecosystems), but there is much uncertainty (few studies) of the effect of global warming on tropical biotas.

Tropical forests are among the biologically richest ecosystems on Earth, but are being threatened by habitat degradation and conversion. These forests may also be vulnerable to climate change but much uncertainty exists as to the magnitude and nature of this anthropogenic impact on tropical organisms (Laurance et al. 2011). Besides the striking diversity, they host a particular and restricted endemic fauna and flora, thermally specialized to these humid conditions. They serve in watershed protection and as biodiversity reservoirs. Furthermore, they are of socio economic importance, being recreational areas for the population and represent a cultural heritage.

Among the effects of global warming in the Tropics, rising temperature would alter the height of the cloud base, moisture inputs from cloud–stripping and the diversity (Pounds 1999) and virulence of pathogens (Laurance, 2008; Pounds, 2001). 

Photo: Upper cloud layer on the Piton des Neiges volcano (Réunion).

Changing the precipitation regime could strongly influence persistence of tropical forests and affect their vulnerability to fire (Cochrane 2003). The limited data available for the tropics suggests that species are increasingly shifting towards higher elevations (Chen et al. 2009, Feeley et al. 2010). The high elevation specialists in the tropics could be among the most endangered species on Earth. If temperatures increase, species may move or adapt or become locally extinct. But for high elevation specialists, moving up mountaintops will not be an option. Lowland species could also be vulnerable to global warming. In lowland tropics, the lack of a source pool of species adapted to higher temperatures to replace those driven upslope by warming, raises the possibility of substantial attrition in species richness (Colwell et al. 2008).

According to Thomas et al. (2004) 18 - 35 % of all plant and animal species will go extinct by 2050. The following bio-ecological effects of global warming have been reported to affect the fauna and flora: change in growing season length, earlier flowering of plants, earlier emergence of insects, earlier migration and egg-laying in birds, breakdown in symbiotic relationships, changes in abundance and local extinctions, altitudinal and latitudinal shifts in species range. On the other hand, it has been recently proposed that many high elevation plant species can locally survive higher temperatures in cool habitats (Scherrer & Körner, 2010). Clearly there is enormous uncertainty involved in all these predictions.

Tropical and subtropical island forests are particularly vulnerable to the future impacts of climate change: rising sea level might cause the regression of littoral plant communities and coastal wetlands; the leeward sides of islands will experienced more droughts and fires causing the decline of dry and mesic forests, and the incidence and intensity of cyclones related to sea temperature increase will alter the structure, composition and dynamics of montane rainforests and favour invasions by alien species (Loope and Giambelluca 1998). Island biota, because of their small populations, with often-restricted distribution range and specialized habitats, their inherent fragility (genetically impoverished, poor dispersers, the so-called “island syndrome” (Carlquist 1974)) as a consequence, they will be the first to be affected.

Among the highest priorities to prevent these catastrophic scenarios:

1) to document the rates of ongoing elevational shifts in montane bioindicator species and functional groups,

2) to identify the upper thermal limits and acclimatising capacity of a representative suite of tropical species, and the physiological, genetic and behavioural traits that influence thermal tolerance,

3) to understand the evolutionary histories that shape species diversity, distribution and community assembly, which will help in future to identify the most sensitive species i.e.  species specialising on a narrow range of  temperature, microhabitat and altitude,

4) to monitor long term vegetation changes in permanent plots for assessing temporal variability of biodiversity in space and time, providing tools for the sustainable management of natural resources.

The use of bryophytes and ferns as model organisms:

Bryophytes (mosses, liverworts, hornworts) are a particularly diverse group in tropical systems. They are poikilohydric (state of hydration closely resembles local environment), possessing no real vascular tissues, and are consequently very dependent on their local environment. They are particularly sensitive to climatic variation and are therefore appropriate candidates to detect the biological effects of climate change. They are also ideal model systems for testing predictions from population genetic and metapopulation theory due to their relatively high colonization-extinction rates, high substrate specificity, dominant haploid condition and small size (Pharo and Zartman 2007). They are ideal candidates for macroecological questions because many families, genera and even species are characterized by distributions across more than one continent, a feature that allows the opportunity of examining their distribution at a regional scale whilst minimizing the confounding effects of phylogenetic differences among study groups (Shaw 2001).

Photo: Herbertus sp. (a leafy liverwort) in the cloud forest of Bélouve (Réunion)

Photo: Ulota sp. (a moss) corticolous species, on shrubs above 2100 m in Réunion

Ferns and lycophytes differ from bryophytes in having a prominent sporophytic generation and a generally better control of their water relations, usually having vessels, multicellular leaves and stomata (exceptions are moss-like filmy ferns as well as poikilohydric species). However, recent evidence suggests that ferns have much less control over their water balance than angiosperms (Brodribb & Field, 2010; Brodribb & McAdam, 2011), which may explain their well-known preference for moist habitats and renders them particularly suitable organims for studying effects of climate change. At the same time, this implies that ferns use more water per unit of carbohydrate assimilated, which increases the nutrient to assimilate ratio in the plants and hence reduces nutrient limitation to growth (L. Salazar, M. Kessler, J. Kluge & J. Homeier, unpubl. data). 

Photo: Cyathea sp. in Bélouve forest (Réunion).

On the other hand, ferns share with the bryophytes their dispersal by spores, which allow for efficient long-distance dispersal and render them independent from biotic vectors to pollination and dispersal. This is especially relevant on islands where the establishment of animal-pollinated plants is commonly inhibited by the lack of suitable pollinators. Accordingly, ferns are much better represented on remote oceanic islands than angiosperms (Kreft et al. 2010). In combination, all these traits imply that ferns are particularly suitable organisms to study climate-distribution relationships on islands.

	Guadeloupe	La Palma, Canaries	Pico, Azores	La Réunion, Mascarenes	Tahiti, French Polynesia
Area (km2)	1700	729	445	2512	1045
Highest summit (m)	1467	2400	2350	3069	2241
Bryophytes	611 (Lavocat Bernard, com.pers)	339 (González-Mancebo et al., 2010)	285 (Gabriel et al. 2010)	807 (Ah-Peng et al. 2010)	c.a. 200 (Whittier 1976)
Ferns and allies	308 (Bernard 2010)	45 (Izquierdo et al.2004)	58 (Borges et al. 2010)	249 (Grangaud, 2010)	c.a.200 (Murdock & Smith 2003)
Angiosperms	1535 (Fournet 2002)	761 (Izquierdo et al.2004)	551 (Borges et al. 2010)	848 (CBNM, 2010)	315 (Florence, 1993)

Table: Characteristics of the targeted study islands and their native plant species richness.

In this project, we propose a trans-national research project between the small islands of La Réunion (Mascarenes), Guadeloupe, Pico (Azores), La Palma (Canaries) and Tahiti (French Polynesia) by using bryophytes and ferns as bioindicators of global change.

The  project aims to: 

(1) Characterize the biodiversity of poorly known but rich groups of plants (bryophytes and ferns), 

(2) Elucidate the processes which govern species richness and distribution along altitudinal transects (from the gene to community structuring), and relate them to life history and functional traits of species, 

(3) Link richness patterns to environmental and spatial predictors along elevational gradients between the islands, 

(4) Model the shift of species range with temperature and precipitation

(5) Establish permanent plots for long term monitoring, managing responses for vegetation and raising new conservation directions for decision making.