Montane Ecosystem Responses to Experimental Warming

A broader conceptual framework soon followed, laying out how warming experiments should be designed and interpreted differently from earlier carbon-dioxide enrichment studies (Shaver et al., 2000). Within a few years, researchers at the Warming Meadow had documented the first phenological responses: experimental warming advanced flowering by roughly two weeks across eleven subalpine species, and snowmelt date emerged as the single most important driver of flowering timing (Dunne et al., 2003). Parallel work showed that warming reshuffled the carbon balance of the meadow, altering when and how much carbon plants took up and released (Saleska et al., 1999).

Key findings

The most consistent result across decades of work is that warming dries soils and shifts plant communities away from productive forbs toward woody shrubs. A ten-year synthesis found that heated plots lost about 200 grams of carbon per square meter from soil organic matter, and the culprit was not faster microbial decomposition but a shift in plant community composition from high-productivity forbs to low-productivity shrubs, which reduced the amount and quality of plant litter entering the soil (Saleska et al., 2002). A later analysis spanning 23 years of warming confirmed that snowmelt advanced by an average of 7.7 days and that shrub encroachment continued to drive long-term soil carbon feedbacks (convergent ecosystem responses study, 2015). A subsequent global synthesis of 27 warming experiments reinforced the local pattern: warming reliably dries soils, but the temperature sensitivity of soil respiration itself changes little with experimental warming (Carey et al., 2016).

Individual species and trophic groups respond idiosyncratically. Warming reduced flower production by nearly 29 percent in glacier lily (Erythronium grandiflorum) and 49 percent in larkspur (Delphinium nuttallianum) while leaving other species unaffected (reproductive responses study, 2007). Plants in earlier-melting plots accumulated more damage from a wider range of pathogens and herbivores, though individual attacker species varied in their responses (Roy et al., 2004). Nitrogen cycling responded only in the drier sagebrush zone, where gross mineralization rates doubled in the first two years before returning to baseline (Shaw & Harte, 2001). Aboveground, sagebrush biomass increased in the dry zone while the dominant grass tufted hairgrass (Deschampsia cespitosa) declined in the wet zone (Rodriquez, 2022), and one alpine forb, Androsace septentrionalis, went locally extinct in a heated plot (climate warming extinction study, 2018).

A parallel theoretical program at RMBL developed the maximum entropy theory of ecology, which uses information entropy to predict patterns of biodiversity, abundance, and energy use from a few community-level numbers (Harte & Newman, 2014). The theory successfully predicts a universal species-area curve across diverse ecosystems (Harte et al., 2009) and, more recently, an equation of state linking species richness, biomass, abundance, and energy flow that captured the patterns in 42 datasets (Harte et al., 2022).

Current frontier

Early work in the 1990s and 2000s established that warming dries soils, advances snowmelt, and shifts community composition. Recent studies since 2020 have shifted focus to two new questions: how macroecological theory itself behaves under disturbance, and how soil carbon is redistributed rather than simply lost. A six-year study of a stressed alpine plant community showed that species-area and species-abundance patterns increasingly deviated from static maximum entropy predictions as the community lost roughly 10 percent of its species, with mortality and declining recruitment driving the breakdown (Franzman et al., 2021). To address this, researchers developed DynaMETE, a dynamic version of the theory that incorporates explicit mechanisms of disturbance such as shifts in birth, growth, and migration rates (Harte et al., 2021).

New soil carbon work suggests the picture is more nuanced than early experiments implied. A recent thesis found that warming and removal of dominant species shifted soil organic carbon from deeper to shallower layers without changing the total amount stored, and that early snowmelt had no detectable effect on total soil carbon (Waldron, 2023). Along an elevation gradient, soil moisture emerged as the best single predictor of soil carbon, explaining about 23 percent of the variation (Issa, 2022). Microbial activity declined sharply with soil depth, complicating predictions about deep-soil carbon vulnerability (long-term warming microbial study, 2019).

Open questions

Several major questions remain. How will compounded stresses — warming, drought, altered snowpack, and species loss — interact over the coming decades, and can dynamic theories like DynaMETE predict community trajectories accurately enough to guide management? Will the transient soil carbon losses observed in early decades of warming reverse, stabilize, or accelerate as shrub-dominated communities mature, and how do redistributions of carbon across soil depths affect long-term storage? How will idiosyncratic species responses — local extinctions in some species, compensation by others — reshape pollination, herbivory, and decomposition networks? Answering these questions will require continued long-term experimentation at sites like the Warming Meadow, paired with elevation-gradient studies and the next generation of macroecological theory.

References

Carey, J. C. et al. (2016). Temperature response of soil respiration largely unaltered with experimental warming. PNAS. →

Climate Warming Drives Local Extinction (2018). →

Convergent ecosystem responses to 23-year ambient and manipulated warming (2015). →

Dunne, J. A., Harte, J., Taylor, K. J. (2003). Subalpine meadow flowering phenology responses to climate change: integrating experimental and gradient methods. Ecological Monographs. →

Franzman, J. et al. (2021). Shifting macroecological patterns and static theory failure in a stressed alpine plant community. Ecosphere. →

Harte, J. et al. (2022). An equation of state unifies diversity, productivity, abundance and biomass. Communications Biology. →

Harte, J., Newman, E. A. (2014). Maximum information entropy: a foundation for ecological theory. Trends in Ecology and Evolution. →

Harte, J., Smith, A. B., Storch, D. (2009). Biodiversity scales from plots to biomes with a universal species-area curve. Ecology Letters. →

Harte, J., Torn, M. S., Chang, F.-R., Feifarek, B., Kinzig, A. P., Shaw, R., Shen, K. (1995). Global warming and soil microclimate: results from a meadow-warming experiment. Ecological Applications. →

Harte, J., Umemura, K., Brush, M. (2021). DynaMETE: a hybrid MaxEnt-plus-mechanism theory of dynamic macroecology. Ecology Letters. →

Issa (2022). The Effect of Climate Change on Soil Organic Carbon over an Elevational Gradient. →

Reproductive and physiological responses to simulated climate warming for four subalpine species (2007). →

Rodriquez (2022). Abundance and Identification of Hymenopteran Parasitoids in Euphydryas gilletti. →

Roy, B. A., Gusewell, S., Harte, J. (2004). Response of plant pathogens and herbivores to a warming experiment. Ecology. →

Saleska, S. R. et al. (2002). Plant community composition mediates both large transient decline and predicted long-term recovery of soil carbon under climate warming. Global Biogeochemical Cycles. →

Saleska, S. R., Harte, J., Torn, M. S. (1999). The effect of experimental ecosystem warming on CO2 fluxes in a montane meadow. Global Change Biology. →

Shaver, G. R. et al. (2000). Global warming and terrestrial ecosystems: a conceptual framework for analysis. BioScience. →

Shaw, M. R., Harte, J. (2001). Response of nitrogen cycling to simulated climate change: differential responses along a subalpine ecotone. Global Change Biology. →

The impacts of long term warming on potential soil microbial activity across soil depth (2019). →

Waldron (2023). Exploring the impact of climate change on soil carbon storage in montane meadows. →

Achnatherum spp.

A. Kinzig