About 2 million years ago, the Earth’s climate entered a new and restless phase. The onset of the Pleistocene glacial periods or ‘ice ages’ rapidly reorganized landscapes and biota right around the globe. During each glacial period, solar radiation weakened, mountains heaved with glaciers, and sea levels fell as ice caps blanketed parts of Europe and North America. The climate of temperate regions grew colder, drier or more variable. Accustomed to the relative warmth and stability of the preceding Tertiary period (65–2 Ma), some plants and animals responded to their new environment in a most decisive way – they became extinct.
Fossil records show that at least 20 tree genera disappeared from Europe and North Africa through the Pleistocene. They include Carya, Halesina, Liquidambar, Magnolia, Nyssa and Sassafras, genera that are still alive in the forests of other continents. Europe’s Pleistocene extinctions illustrate how climatic changes of global extent can lead to starkly different outcomes in different regions.
Why was Europe’s flora disproportionately affected by the glaciations? Part of the answer is to do with Europe’s geography: Europe is warmed by the Gulf Stream, the ocean current that keeps that continent’s climate much milder than it otherwise would be. The Gulf Stream weakens during each glacial period, making Europe’s climate cold and continental. For plants intolerant of low temperatures, the mountain chains of the Alps, Balkans, Pyrenees, Carpathians and Caucasus, as well as the Mediterranean Sea itself, all present geographic barriers to migration (Svenning, 2003).
There is, however, a group of plants that remained in Europe and North Africa, surviving the long glacial winters and flourishing briefly during interglacials. Tertiary relict plants are the last survivors of forests that prevailed in Europe and other parts of Eurasia from the late Cretaceous until the late Pliocene. Two categories of European Tertiary relicts are recognized: ‘Palaeotropical’ elements descended from evergreen tropical rain forests, and ‘Arctotertiary’ elements from warm-temperate deciduous forests (Mai, 1989).
Tertiary relict plants have fascinated biogeographers for decades. They are, if you like, living dinosaurs of the plant world. The questions they raise have similar resonance: What triggered the extinctions? Why have some species been able to endure 2 million years of climatic change, while others could not? How do these plants cope with glacial conditions? Do they hide in sheltered locations (‘refugia’), or does their physiology allow them to adapt? How do they react to warmer interglacial climates? Why is their current distribution so limited? Can we predict how they will respond to future environmental change?
A recent study has shed valuable light on these questions by tracing the extinction of two Tertiary relict plants in an island ecosystem. Lea de Nascimento et al. (2008) analysed pollen fossilized in lake sediments on Tenerife, one of the Canary Islands off the north-west coast of Africa, to understand how the vegetation of the island has changed in recent millennia. Tenerife is home to several Tertiary relict plants that survive there because of the mild, oceanic climate. The pollen reveals the extinction of two Arctotertiary species from Tenerife’s forests: oak (Quercus) and hornbeam (Carpinus). The results are surprising because no one had found evidence for their previous existence. Hornbeam is particularly unexpected: if the pollen represents Carpinus betulus, its nearest living population is 2000 km from Tenerife.
Oak and hornbeam went into decline around 2000 years ago, coinciding with human settlement of the island. In the absence of any other ‘smoking gun’, the study’s authors argue that humans – through burning, grazing, selective felling and agricultural development – were ultimately responsible for the extinctions.
Island ecosystems are, of course, more vulnerable to extinctions than their mainland counterparts. Or are they? The past, present and future of a Palaeotropical Tertiary relict, Portuguese laurel (Prunus lusitanica), is the subject of a detailed analysis by Juan Calleja et al. (2008). Along with the Canary Islands, this evergreen tree occurs on the Azores and Madiera, in Morocco and south-western Europe. The authors counted every Portuguese laurel on the Iberian Peninsula: 31,076 in total. Such hard-won results highlight laurel’s fragmented distribution: some populations have only two trees and the majority comprise fewer than 50 individuals, often separated by hundreds of kilometres. In effect, these populations are islands in the Iberian landscape.
How did laurel populations become so isolated, fragmented and small? To answer this question, the authors used a sophisticated modelling technique based on the climate of laurel habitats. It shows that laurel trees today cover an area of 301 km2, a mere 1% of their potential range. Human activities, now and in the past, have almost certainly contributed to habitat fragmentation. Long-term dispersals are also important. During the height of the last glaciation (about 21,000 years ago), laurel probably survived in refugia scattered along the Atlantic coast. Relying on birds to disperse its seeds, laurel migrated out into the mountains to arrive at where it is today.
Calleja et al. (2008) predict that the species’ future distribution, under climatic conditions simulated for the year 2080, will differ substantially from today’s. Laurel’s main population in Portugal is expected to decline, leaving behind a sprinkling of suitable habitats in the mountains of central and northern Spain. Already laurel’s southernmost populations show signs of the drought-stress predicted to accelerate in coming decades. The species’ long-term viability will depend on its ability to migrate or adapt, an ability that may well be hampered by human activities.
Human impact is a common theme in these new studies. So will we humans put the last nail in the coffin of Tertiary relict plants? Before making any bold predictions, we must examine the role that humankind has played in long-term environmental change. To return to the example of the Portuguese laurel: if it were so adversely affected by humans in Iberia, one might expect a similar outcome on the Canary Islands. Instead, it seems that laurel increased on Tenerife after the arrival of humans. Iberian laurel populations were perhaps vulnerable to disturbance because of drought stress. Likewise, Tenerife’s oak–hornbeam forests may have been at risk because they existed outside their ecological optimum. However, in the absence of a firm understanding of what ‘human impact’ constitutes, such conclusions are difficult to assess rigorously. Human impact remains a vague concept, encompassing many different processes, rates, scales and ecological outcomes (Head, 2008).
The danger is that a concept as fuzzy as human impact could be used uncritically – as a kind of ecological gap-filler. Of course this does not mean that we can ignore human agency altogether. There is now abundant evidence that humans have been pervasive in global environmental change for millennia (Roberts, 1998). Calleja et al. (2008) and de Nascimento et al. (2008) must be applauded for recognizing the importance of non-climatic factors in the history and distribution of plants. Our problem now is measuring the anthropogenic effect in the long term. This is no simple task when we consider the variety of human interactions with ecosystems today, but to fail to integrate humans into our understanding of Nature is to fail to understand Nature herself.
The way forward in illuminating past human–ecosystem interactions could be considered to have two fronts: data calibration and ecosystem modelling. The first concern is to improve reconstructions of past environments. Much has already been done. Charcoal pieces fossilized in sediments reveal the occurrence of fires in the past. Certain fossil spores indicate the presence of grazing herbivores. Some types of fossil pollen are associated with human activities. The challenge is to calibrate these data to produce estimates of palaeoenvironmental conditions. Can we match charcoal quantities to a fire’s area and intensity? Can we use spores to measure grazing pressure? Can we determine the nature and extent of past human activities using pollen? We must find ways to answer ‘yes’ to these questions.
Ecosystem modelling can approach the same problem from another direction. It is now possible to create computer-simulated ecosystems that develop in response to fire, disease, temperature, rainfall, drought, soil fertility and other variables. It should be possible to model past human activities as well, informed by historical and archaeological data. Fossil records can be used to verify the simulations so that they can be used as robust predictive tools. Of course, such simulations will only be as good as the ecology that underpins them, so we must continue to increase our knowledge of individual species.
What does this all mean for Tertiary relict plants? It means we should be careful about pointing the finger at ourselves when it comes to explaining their rarity. Drought seems to be an important factor for some species (Hampe & Arroyo, 2002; Pulido et al., 2008), but the long-term effects of Mediterranean climate on their distributions have only recently begun to be explored (Rodríguez-Sánchez & Arroyo, 2008). Ecological strategies of Tertiary relict plants are certain to be critical in predicting their adaptability to change (Svenning, 2003). ‘Opportunistic’ and ‘thermophilous’ species may fare better than those with a ‘conservative stenoecious’ strategy (sensuDenk et al., 2001).
Indeed, as Calleja et al. (2008) suggest, we need a new, comprehensive framework for understanding Tertiary relict plants. Integrating some measure of past human activity into this framework should ensure that our efforts to conserve these ancients will be positive rather than negative.