Since trees, like corals in the ocean, form the physical structure of many terrestrial biomes, their response to climate change will be vital in understanding effects on ecosystems. Along with their more conventional ecological role as primary producers and physical habitats, trees provide global services such as Carbon, Nitrogen, and water storage. These factors which connect the biotic realm to the global climate system, show how trees and forests both affect and are impacted by changes to climate. As the global climate continues to change with increasing temperature, atmospheric carbon levels, and altered precipitation, how trees adapt or are affected will have both a local and global feedback. This short set of current research looks into how some forests have responded and may change in response to climate change.
How Trees Respond to Higher CO2 Concentration
As anthropogenic activities have increased atmospheric carbon levels, this might seem like a boon to trees, who as any secondary school student will tell you, gain their chemical energy through photosynthesis of carbon dioxide from the air and water from soil. That same secondary school student will also tell you that trees produce water as a waste product from respiration; so trees that photosynthesize more are also at risk of water loss through leaf pores known as stomata that transpire into the air (evapotranspiration).
The ratio of carbon intake for photosynthesis over conductance through stomata is known as intrinsic water use efficiency, iWUE*. This metric can be used to examine tree response to higher CO2 levels and consequent increases in temperature.
A joint-research team from China, Australia, the UK, and Ireland found that evergreen trees increased iWUE more rapidly than deciduous trees in response to rising CO2 levels across a 25 year period. This response suggests a competitive advantage for evergreens in future drier climates. The authors conclude that past climate warming, such as in the Eocene (55 mya) may have led to evergreen expansion at that time period.
Challenges to applying this simplified tree response system to understanding future forest growth systems include other climatic variables such as warmer temperatures and resulting increased water access.
*this response can be seen at the cellular level as higher CO2 levels and warmer temperatures can influence guard cells to contract the opening of stomata in leaves / needles.
While Some Decline, Others Thrive
While many forests around the world have declined in productivity and biomass due to global changes, some forested regions have grown. In the Tibetan Plateau, a joint Chinese – USA research team found that trees have grown steadily there since the 1900s and are at their highest growth levels since the 18th century. So why the growth?
Scientists examining the trees used Carbon, Oxygen, and Nitrogen isotopic analysis from dendrochronology (historical tree ring data) to infer that higher carbon levels in the past had led to increased water & nutrient availability. The scientists postulate that the warmer temperatures from increased atmospheric CO2 has led to higher precipitation levels and melting of permafrost. The melted permafrost has not only allowed access to soil moisture but also nutrients trapped within the permafrost layer.
So, trees could respond to higher CO2 levels by increasing water efficiency or by thriving due to new water resources made available by a warmer climate. But these responses are limited to certain climates and tree types. Higher CO2 and temperature levels can also lead to lower precipitation rates, increasing severity of drought and tree mortality.
Drought Resistance & Ecosystem stability
As studies are showing, anthropogenic climate warming is likely behind the “mega-drought” in the Western U.S. – the long term severe drought from 2000 – present. How trees respond to continuous drought stress is important for conservation, fire management, and understanding ecosystem stability as droughts become longer, more frequent, and more intense.
Broderick, et al. used remote sensing to analyze drought resistance in California forests by examining canopy water content (CWC) or the total amount of water held in the tree canopy. By looking at changes to CWC over time using satellite and ground-based data, tree vulnerability to drought can be examined. A tree’s drought resistance, identified by the authors, would be measured as the amount of drought withstood by the tree prior to CWC levels falling below a long term level. The authors found that California’s forest showed heterogeneous drought resistance with certain forest types, such as the Redwoods, showing greater resistance than other forests such as the lower elevations of the Sierra Nevada ranges.
Whether the Redwoods took advantage of additional water sources, improved iWUE like their evergreen cousins from the 1st mentioned studies, or another strategy is unclear. The authors caution though that drought resistance does not confer drought resilience, or the capacity to recover fully from the abiotic stress. If drought induced stress does lead to extensive tree mortality, this may have largescale impacts on whole ecosystems and could induce succession of other tree species able to colonize these stressed regions.
Climate Induced Range Expansion
In southern Canada near the Great Lakes region, Goldblum & Rigg from the University of Wisconsin-Whitewater and N. Illinois University found that 3 primary tree species residing along the boundary between boreal and temperate forests will likely see different ranges under a warming climate by 2080. They examined tree core records (dendrochronology) of sugar maple, white spruce, and balsam fur and pollen accumulation to examine growth responses to temperature and precipitation. These data when applied to future modeled warming scenarios, showed a change in the location of the three tree populations. Under all future warming & precipitation scenarios, the sugar maple would show a northward growth trend while the balsam fir would likely show a significant decline.
These changes indicate that the temperate dwelling maple may thrive at more northerly latitudes in the warmer future and that the organismal composition of future forest biomes may be altered. But forests around the world are not homogeneous and their responses to climate change will vary as many different factors play into their growth.
Many, Many Variables to Consider
Foster, et al., examined boreal forests in Alaska and the many interactions between trees and the soil, fire events, and climate to make predictions about warmer climates using a highly complex model. Their model showed an overall decrease in biomass in the Tanana Valley River Basin in Alaska with significant declines in spruce forests (the dominant tree species) but smaller gains in deciduous trees such as Quaking Aspen.
Their model and the assumptions of tree response touch on many of the earlier studies mentioned above including thresholds to consider for trees under climatic stress and the complex interplay of many variables. While White Spruce is a drought-tolerant species and can apply strategies such as iWUE, there is a limit to this tactic. Under long-term moisture stress and increased temperature as shown in the researchers model, spruce forests declined after initial growth from higher CO2 and photosynthetic capacity.
Many other variables added to the model showed differentiation between individual tree site, soil type, water availability due to moss cover and organic soil layer depth, rate of drought and fire, and other factors. The researchers stress that despite the improvement of their model of including many variables to determine future tree growth under warming climate scenarios, their model is missing some key information such as bark beetle infestation which has had significant impact on forests in southern latitudes.
While the small sample of research above is by no means a sufficient review of the literature, it does show the complexity and challenge of predicting forest response to climate change in specific locales. While this post examines some specific responses to climate change, other literature provides a more comprehensive outlook of forests under warming scenarios.
As forests attempt to adjust to increased atmospheric carbon and warmer climates, researchers are able to see tree responses and ecological impacts in real time. This work can help to predict what future forests may look like, ecosystem impacts to the communities that depend on forests for shelter or fodder, and global budgets of carbon and water storage that impact and feedback into the climate system.
What we have seen so far of rapid warming and forest response is not promising despite the many arrows that trees have in their quiver. This somber reminder is needed in order to accelerate research and change global goals for carbon emissions.
Sources and Further Reads:
Brodrick, P. G., et al. “Forest Drought Resistance at Large Geographic Scales.” AGU Journals, John Wiley & Sons, Ltd, 1 Mar. 2019, agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018GL081108.
Foster, Adrianna C., et al. “Importance of Tree- and Species-Level Interactions with Wildfire, Climate, and Soils in Interior Alaska: Implications for Forest Change under a Warming Climate.” Ecological Modelling, Elsevier, 3 Aug. 2019, http://www.sciencedirect.com/science/article/pii/S030438001930273X.
Silva, Lucas C. R., et al. “Tree Growth Acceleration and Expansion of Alpine Forests: The Synergistic Effect of Atmospheric and Edaphic Change.” Science Advances, American Association for the Advancement of Science, 1 Aug. 2016, advances.sciencemag.org/content/2/8/e1501302?intcmp=trendmd-adv&_ga=2.99413790.1400226102.1587489344-1536952118.1586289325.Soh, Wuu Kuang, et al. “Rising CO2 Drives Divergence in Water Use Efficiency of Evergreen and Deciduous Plants.” Science Advances, American Association for the Advancement of Science, 1 Dec. 2019, advances.sciencemag.org/content/5/12/eaax7906.