Scientists say climate change could cause a ‘massive’ tree die-off in the U.S. Southwest

Desdemona Despair

By Chris Mooney
21 December 2015

(Washington Post) – In a troubling new study just out in Nature Climate Change, a group of researchers says that a warming climate could trigger a “massive” dieoff of coniferous trees, such as junipers and piñon pines, in the U.S. southwest sometime this century.

The study is based on both global and regional simulations — which show “consistent predictions of widespread mortality,” the paper says — and also an experiment on three large tree plots in New Mexico. The work was led by Nate McDowell of the Los Alamos National Laboratory who conducted the research along with 18 other authors from a diverse group of universities and federal agencies, including the U.S. Geological Survey.

“We have fairly consistent predictions of widespread loss of piñon pine and juniper in the southwest, sometime around 2050,” said McDowell. The paper concludes that the consequences could be vast, citing “profound impacts on carbon storage, climate forcing, and ecosystem services.”

The study examined both an extreme warming scenario — which recent climate policies suggest we may be able to avert — and also a more modest scenario that would likely bring temperatures above 2 degrees Celsius above pre-industrial levels by the year 2100, but not necessarily by that much. The more extreme scenario was certainly worse for these trees, but even under the moderate scenario, the negative results were merely “delayed by approximately one decade,” the study found.

The problem is that climate change is expected to not only increase the risk of drought, but will also drive heat up in general. And this could injure trees in two ways — simply drying them out, but also leading to “carbon starvation.” This could occur if, faced with dry conditions, tree leaves or needles close their stomata to keep water in, but therefore cannot bring in more carbon dioxide and thus suffer from reduced or even fully halted photosynthesis.

In the field experiment, conducted over five years, the researchers found that depriving trees of 48 percent of usual rainfall led to 80 percent mortality of piñon pines more than 100 years old – in other words, fully grown trees – and a 25 percent loss for junipers. […]

Most striking, of course, is the study’s suggesting that even a lesser warming scenario, closer to what the world is now shooting for under the Paris agreement — limiting warming well below 2 degrees Celsius — but still likely beyond it, would still trigger these devastating consequences.

“2 C is huge for trees,” says McDowell. [more]

Scientists say climate change could cause a ‘massive’ tree die-off in the U.S. Southwest

ABSTRACT: Global temperature rise and extremes accompanying drought threaten forests^{1, 2} and their associated climatic feedbacks^{3, 4}. Our ability to accurately simulate drought-induced forest impacts remains highly uncertain^{5, 6} in part owing to our failure to integrate physiological measurements, regional-scale models, and dynamic global vegetation models (DGVMs). Here we show consistent predictions of widespread mortality of needleleaf evergreen trees (NET) within Southwest USA by 2100 using state-of-the-art models evaluated against empirical data sets. Experimentally, dominant Southwest USA NET species died when they fell below predawn water potential (Ψ_pd) thresholds (April–August mean) beyond which photosynthesis, hydraulic and stomatal conductance, and carbohydrate availability approached zero. The evaluated regional models accurately predicted NET Ψ_pd, and 91% of predictions (10 out of 11) exceeded mortality thresholds within the twenty-first century due to temperature rise. The independent DGVMs predicted ≥50% loss of Northern Hemisphere NET by 2100, consistent with the NET findings for Southwest USA. Notably, the global models underestimated future mortality within Southwest USA, highlighting that predictions of future mortality within global models may be underestimates. Taken together, the validated regional predictions and the global simulations predict widespread conifer loss in coming decades under projected global warming.

Multi-scale predictions of massive conifer mortality due to chronic temperature rise