Rising temperatures, carbon-dioxide levels, and falling rainfall levels are commonly associated with climate change. However, different parts of the world, and the different plants and animals that inhabit them, will not necessarily show a consistent response. While certain plants that need more moisture may bear the brunt of a warming climate, those that are – in general – more drought tolerant, may end up thriving and colonising newer warm areas that open up to them. Writing in Nature this month, () a team of ecologists led by Peter Reich from the University of Minnesota examine how tree species in boreal and temperate forests are expected to respond to climate change in the twenty-first century.
Boreal forests, occurring in 50-60 degree N latitudes, are spread over most of the Russian and North American sectors. Unlike temperate forests, which can well receive rainfall of over 100 cms annually, boreal biomes settle for annual rainfall of 25-100 cms. Since the temperatures in boreal forests are sub-zero for most part of the year, most of the precipitation occurs as snow and the soil freezes during winters. When spring comes around, temperatures rise just enough to allow for the frozen surface snow to thaw into the water, but not enough for the water to evaporate, especially as the spring/summer season is also quite short and cold. The result is a landscape pockmarked with muddy bogs and lakes.
In an environment where most of the moisture exists on the ground – either as snow or cold water – and the atmosphere is fairly dry, trees like conifers – which have needle-like leaves that prevent water loss – have come to dominate the region. A similar logic also holds true for cacti in deserts. Besides, these trees have evolved to be conical in shape so as to let the heavy snow glide off to the ground. Temperate forests, spreading from anywhere around 25-degrees N to around 50-degrees N, on the other hand, are dominated by trees like oak, maple, hickory, and alder.
That biomes are not watertight compartments is something often overlooked, and the community composition characteristic of a temperate biome merges gradually into that of the boreal as one moves North. In fact, there are tree species, such as birch and spruce, that often co-occur in both biomes.
Over a five-year period from 2012-16, Reich and colleagues planted saplings of these nine tree species at two sites in North Eastern Minnesota. The nine species were a mixed bag of boreal trees like firs, spruces, birches, and pines as well as temperate ones like oaks and maple. In order to replicate future climate projections, these nine species were treated with different levels of temperature (normal, +1.6 °C, +3.1 °C) and rainfall (normal and reduced), thereby constituting six different combinations. This was done to isolate the effect of the two parameters – rainfall and temperature – on each other. The methodology was careful to also replicate the native communities of these tree species and introduced appropriate herbs, shrubs and ferns as well.
The study found that species – quite expectedly – respond differently to these changes. The growth of most boreal species was severely affected even at the 1.6 degrees C increase, and was almost one-third lower than normal conditions, while that of temperate oak and maple species witnessed an overall increase in growth. Rainfall was a key driver, for even those temperate tree species that witnessed an increase in growth with a +1.6 degrees C increase in temperature, did so under normal rainfall levels.
In other words, their increase in growth was not very significant at +1.6 degrees C when rainfall was reduced. Similarly, the paper birch – a boreal tree – for instance, grew more than twice as large as normal at a +3.1 degrees C temperature increase under normal rainfall but was half its normal size at normal temperature and reduced rainfall.
What could explain these divergent responses? The photosynthetic response could be one reason behind divergent responses. Higher temperatures have an effect on the amount of moisture held by the soil. This upsets the hormonal balance of the plant, leading to a build-up of toxins and, thus, less photosynthetic ability. Although tree species in general witness a reduction in photosynthetic ability when soils become drier, the extent differs from species to species.
The experiment also brought about changes in the relative abundance of tree species. In the progress towards a warmer or drier climate, boreal conifers (firs and pines) became less and less abundant, while the maples and birch trees became more abundant. However, under extreme conditions i.e. +3.1 degrees C temperature increase and reduced rainfall, all species seem to perform badly (for example, the paper birch example above). These results echo results from studies done in 2006 and 2015 that compared forest land survey records from the late 1800s and 1990s found that temperate trees like aspen, maple and spruce had become more abundant over time.
A 2013 study too reported that ‘the growth in most needleleaf species stalled and declined during the second half of the 20th century.’
The picture these currently boreal landscapes will paint a few decades from now is not entirely clear, for other factors that could certainly throw a spanner in the works. These include rising CO₂ levels, insects, diseases and fires. Reich et al (2022) have some answers to what these landscapes might look like at the close of the twenty-first century. Temperate maple, pine and oak tree species that are at the temperate-boreal transition zone will tend to proliferate with a warming climate. So will boreal tree species that currently exist in locations that are even colder than what these trees are capable of withstanding. Which of these tree species will be able to adapt and establish themselves quicker with respect to a changing climate? The evolutionary chase is on.
The author is a research fellow at the Indian Institute of Science (IISc), Bengaluru, and a freelance science communicator. He tweets at @critvik