Add to this diagram – “Respiration (when overheated).”
We’ve talked about how older forests may sequester less carbon and dead forests release carbon (for example, here). New research indicates that forests also sequester less carbon and start to release carbon (while they are still alive) if the temperature gets too high. As reported here:
‘We’re in Bigger Trouble Than We Thought’
The data show a clear temperature limit, above which trees start to exhale more CO2 than they can take in through photosynthesis, said co-author Christopher Schwalm, an ecologist and earth system modeler at the Woodwell Climate Research Center. The findings mark a tipping point, of sorts, at which “the land system will act to accelerate climate change rather than slow it down,” Schwalm said.
“Seeing such a strong temperature signal globally did not surprise me,” he said. “What I was surprised by is that it would happen so soon, maybe in 15 to 25 years, and not at the end of the century.”
Other researchers commented on management implications of drought-stressed dying trees:”
It may come down to looking at options for saving valuable, individual stands of trees, and protecting genetically distinct and more resilient species. It could also be important to conserve corridors and patches of woodland to reduce the distance seeds must travel to enable forests of the future to spread or reconnect under more favorable climate conditions, he explained.
“We think a lot of these areas are going to go down, so where can we save some of it?” he asked.
There are obviously implications here for national forest planning. It seems like it should be the role of the national headquarters to review and interpret the implications of new research for forest management, and to advise national forests regarding its implications for their plans and whether they should consider making changes.
7 thoughts on “Climate tipping point for forests”
This is another paper that relies on modeling based on Representative Concentration Pathway 8.5 (RCP8.5), what some call the worst-case scenario.
A 2020 Nature article states that RCP8.5 “paints a dystopian future that is fossil-fuel intensive and excludes any climate mitigation policies, leading to nearly 5 °C of warming by the end of the century2,3. That one is named RCP8.5.” And adds that “RCP8.5 was intended to explore an unlikely high-risk future.”
The authors of the “We’re in Bigger Trouble Than We Thought” paper note that, “In contrast to Representative Concentration Pathway 8.5 (RCP8.5), warming associated with scenario RCP2.6 could allow for near-current levels of biosphere productivity, preserving the majority land carbon uptake (~10 to 30% loss).”
Jon, I hope you don’t take this study very seriously. As I’ve said numerous times (1) we don’t know how climate change will affect microclimates as trees experience them, (2) we don’t know how much of a change, in what parameters (moisture, heat, time of year etc.) will affect trees of any species. (3) we don’t know if their age and condition affects responses (4) we don’t know how any changes will affect diseases, insects, beneficial mycorrhizae and so on and (5) we don’t know how well the offspring of current trees can adapt to the changes.
None of the analyses the authors did is even related to that.. whether it’s 2.6, 8.5 or whatever, the fundamental question is that “it is too complicated for us to be able to predict how trees can react” or “really, no one has a clue.”
Here is what the authors did:
“To address these questions, we used measurements from the largest continuous carbon monitoring network, FLUXNET (9), as an observational constraint to determine the temperature dependence of global rates of photosynthesis and respiration. Across ~1500 site years of daily data from all major biomes and plant functional types, we applied a 30-day moving window partial correlation analysis at each flux tower site to extract the temperature signal (a change in photosynthesis or respiration solely attributable to changes in temperature, i.e., the signal excludes other climatic effects such as water availability and sunlight) from daytime partitioned gross primary productivity [photosynthesis (P)] and total ecosystem respiration (R). We then normalized each site-level temperature dependence curve and applied macromolecular rate theory (MMRT) (10) in conjunction with Monte Carlo resampling to avoid length-of-record bias. The curves were subsequently aggregated to the biome level and then area-weighted to arrive at a global constraint of temperature dependence (see Materials and Methods). MMRT is a framework rooted in the principles of thermodynamics, which provides a mechanistic basis to extract the temperature dependence of rates across scales from individual enzyme kinetics to organismal and ecosystem metabolism (see Materials and Methods) (11). This framework is based on classical transition state theory from physical chemistry (12) and describes temperature rate dependence using three parameters, with emphasis on a maximum or optimal temperature value, Tmax, above which rates decline exponentially. The Arrhenius function is a special case of MMRT where the heat capacity term is zero and the temperature-rate relationship is exponential without a maximum (see Materials and Methods) (10). MMRT is applicable across a range of processes and levels of biological organization and has been successfully used to model the temperature dependence of enzyme kinetics (13), microbial growth (14), soil respiration (15), and leaf respiration (16). Here, we extend this analysis to include global land photosynthesis and net carbon fluxes, producing the first observationally derived curves for the temperature dependence of global carbon metabolism, using a single function grounded in thermodynamics.”
IMHO, we can’t model our way out of our fundamental ignorance of the complexity of the real world.
Sharon, we don’t know what will happen tomorrow. Yet we plan for it based on the best information we can find. I can’t give much weight to arguments that what I know isn’t good enough. I’m more interested in flaws in the reasoning or contradictory evidence (more like what Anon contributed), and suggested that it should be the job of the Forest Service Washington Office to be in charge of figuring that out. (‘All models are wrong, but some may be useful.’)
Anonymous was me. I thought my name/email was there — sorry. Here’s another take on RCP8.5 from BBC News last year:
Climate change: Worst emissions scenario ‘exceedingly unlikely’
“Referred to as ‘business as usual’, the scenario assumes a 500% increase in the use of coal, which is now considered unlikely.”
So why do researchers keep basing models on RCP8.5? There are good reasons to look at worst-case scenarios, of course, but why not the more realistic, middle-of-the road RCP4.5 or RCP6.0? One reason may be that “We’re in Bigger Trouble Than We Thought” gets attention.
So using this to predict when trees will die based on measurements from Fluxnet is not a “flaw in the reasoning”? It’s not a logical “bridge too far”? We actually don’t need information from disciplines, like, say, tree physiology to predict what will happen to trees under higher temperatures?
“FLUXNET is a global network of micrometeorological tower sites that use eddy covariance methods to measure the exchanges of carbon dioxide, water vapor, and energy between terrestrial ecosystems and the atmosphere. More than 500 tower sites around the world are operating on a long-term basis. The overarching goal of the FLUXNET data collection at ORNL DAAC is to provide information for validating remote sensing products for net primary productivity (npp), evaporation, and energy absorption.”
I didn’t really intend for this to be a debate about climate science, but here is one of the authors’ defense of using the “business as usual” scenario: “Historical and anticipated future total CO2 emissions to 2050 show more agreement with Representative Concentration Pathway 8.5 (RCP8.5) than other Coupled Model Intercomparison Project 5 (CMIP5)-era RCPs (1). “https://www.pnas.org/content/117/45/27793
And there’s some common sense to the idea that in this short-term (30 years), “business as usual” is pretty much baked into the results.
(At least one of the authors has a forestry degree.)
The western US has already warmed 2 degrees. Winters are warmer and droughts have increased. Rainforest species like western redcedar and western hemlock are dying back and declining in some areas of the Pacific Northwest. Bigleaf maple is also being affected by some sort of decline. Southern Oregon experienced dieback and mortality of Douglas-fir on drier sites several years ago. Is this a short-term event or a harbinger of larger things to come? Extremes are likely to have more impacts on forests (IMHO) than more gradual changes.