There are certain places to which I’m strongly drawn. Every so often, I feel the need to return to these places to observe the biological changes that occur throughout the year. I especially like places dominated by deciduous forests. Thus, not long ago, I went hiking up Cerro El Roble, a small mountain 80 kilometers north of Santiago, Chile. To a scientist interested in climate and biological interactions, it’s becoming my living laboratory.
Phenology is the study of the temporal aspects of recurrent natural phenomena and their relationship to weather and climate. I’ll use the cycle of foliage in Chile’s northern oak forests (from its emergence to when it falls) to look at changes to landscape and explore climate’s role.
In Chile and around the world, the climate is changing and with it the distribution of species, the introduction of invasive species, the dominance of generalist species, the change in ecosystemic interactions and endless consequences. These changes in climate are directing and possibly altering trophic interactions that result in community instability.
Seasons and foliage
In Chile’s deciduous Nothofagus macrocarpa forests, the emergence of foliage occurs in the spring. In September the buds begin to open and thin vegetative axes appear with bundles of leaves that are expanding rapidly with each passing day. First, the trees that are at the low altitude start to change and then, sequentially, they are followed by the rest to the top. The leaves’ color is bright green and the stands of forest are darker. Along with the emergence of this foliage insects appear, which start to eat during the leaf expansion process. For certain plant species, leaf expansion is a vulnerable time for the leaves as they are rich in nutrients and low in secondary compounds—like toxins—making them more palatable to the insect.
As the season progresses, the foliage becomes firmer and the color a little darker. Insect larvae are less frequent as the summer progresses. But perhaps one of the finest times for this type of forest is the fall. The leaves begin to lose their chlorophyll and gradually start turning a range of red, orange and yellow. The spectacle of colors is beautiful and can easily distinguish the geographical boundaries of the population of oaks. Slopes are observed along a South-East exposure, and disintegrating down into the ravines, as if they were roots that gradually ended in thin rows of trees.
In winter, it is no less striking of a landscape. Lead-colored and almost dry looking, the branches of the trees, like skeletons, have no leaves and hide nothing. At higher altitudes, the trees are smaller in size and have more branching. It becomes difficult to observe the main trunk, most are a bundle of stem , perhaps a permanent product of human intervention (signs of having been cut at some point. If visiting after some rain, one will see clear layers of snow and ice in some places, especially in the more shaded spots.
Climate and temperature especially direct these phenological changes; indeed, climate change predicts changes in current phenological patterns—either by extending or advancing the start of the growing season in plants and altering their relationship with insects. Scott Altmann and I have studied this and have found a pattern completely opposite to the forests at more southern latitudes. Vegetation is expected to be lower at higher elevations, as the lower temperatures affect the performance of insects. This is precisely what has been found in the forests of the south.
However, in our work along the northernmost limit, we found that vegetation was greater at the highest elevations.
One possibility is that the insect boom coincides with the late emergence of foliage, or that insects are gradually migrating to heights with temperatures favorable to their development.
The importance of snow
Snow is important in these systems, but I fear that every year it becomes scarcer or its duration is shorter. Information that has been corroborated by the inhabitants of the area–recalling the past 30 or 50 years–the depth of snow that reached 1-2 meters and is now reduced to a few centimeters and quickly disappearing. Oak seeds need a cold stimulus—for a certain period of time—to leave their dormant state and germinate. My very personal impression is that today’s short periods of snow are insufficient for optimal germination. Coupled with the fact that the seeds that do manage to germinate then undergo a dry summer experience high water stress and most die.
I’ve visited in the month of July (Chile’s winter) on a hot sunny day where at 2,200 meters above sea level, the noon temperature was 23.5°C with no sign of wind. And there were the seeds, on the surface of the ground, with no snow, no cold, no optimal conditions for their development.
The oak buds at the end of the branches look pretty vigorous at that point: big, strong and robust, waiting for the time and the right temperature to emerge with a set of leaves. This temporary change in the landscape is the product of deciduous forest in a Mediterranean climate, seasonal, with hot, dry summers and rainy winters.
In the past, how did climate change alter biological interactions?
Geological evidence shows that some past episodes of climate change altered biological interactions, causing some species to die out, others into speciation, and altering the distribution and abundance of others. The diversity of functional groups has varied greatly in geologic time, as shown by marine ecosystems (which have a fairly complete fossil record) that passed through periods of stability and other mass extinction events as a result of climate change and associated changes to acidification, eutrophication and anoxia of the oceans.
Mass extinctions show a complex nonlinear response between climate change and biological interactions indicating the likely change that could be expected in the future. Both for terrestrial and marine systems, the community tends to become homogenized, resulting in assemblages dominated by generalist species that can live in a broad environmental range. This is shown by the analyses of different groups in the fossil record, but also with episodes of dispersal and invasion of species.
Blois et al. (1) mention an example from the Paleocene-Eocene era with rapid global warming where the North American Bighorn Basin experienced changes in the composition and immigration of new species which led to an increase in the intensity and frequency of insects feeding on plants. This link between insect damage and temperature is consistent with what has been observed in modern southern gradient in damage from herbivorous insects, suggesting that insect damage could be a persistent effect of future climate change. (That is a taste of the importance of the work Scott Altmann and I are pursuing) (2).
The work continues to talk about the extinction of the megaherbivores, the reconstruction of food webs and the changes experienced post-extinction.
At present, how are biological interactions altered by climate change?
Recent observations and experiments show that on a time scale of years to decades, climate change may alter the distribution and abundance of species and alter biological interactions through local extinctions, changes in geographic ranges and relative abundance. Predation will intensify and competition will change. In general, climate change may favor species tolerant to heat and varying environmental conditions, resulting in an increase in their fitness and their ability to move into new locations.
Experiments in the Arctic show that high temperatures favor bushes. By 2100, the spatial extent of the bushes is expected to expand by 20% to 52% in areas north of latitude 60° , leading to an increase in regional temperature due to increased evapotranspiration from those bushes.
But how much change will we see?
The climate is changing and with it, the distribution of species, the introduction of invasive species, the dominance of generalist species, the change in biotic interactions and endless consequences. It is true that everything changes and has changed forever, but today it is happening with tremendous speed thanks to our human intervention in natural systems and our contribution to global warming. How long will this change be happening? I do not know.
Photos: Sandra Claros
(1) Blois JL, Zarnetske PL, Fitzpatrick MC, Finnegan S. (2013). Climate Change and the Past, Present, and Future of Biotic Interactions. Science Vol 341: 499-504
(2) Altmann, S. & Claros, S, 2013. Insect abundance and leaf damage, tree growth, and leaf chemical components across an altitudinal gradient of central Chile. V Reunión Binacional de Ecología. XX Reunión de la Sociedad de Ecología de Chile, Pto Varas, 3-5 November.