Climate Change & the Hydrological Cycle in the Colorado Rockies

Isotopic investigations in the East River valley:
Vegetation usage of summer versus winter precipitation


Precipitation in the East River valley, near the Rocky Mountain Biological Laboratory, is strongly bimodal in nature. Although the vast majority falls as snow in winter, ecologically important contributions come from summer rainfall. The overarching hypothesis of this research is that different vegetation functional groups (trees, shrubs, and herbs (grasses and forbs)) rely on summer precipitation to differing degrees during the growing season. This hypothesis is set within the context of observed climate change at the Rocky Mountain Biological Laboratory (RMBL). This change is likely to impact the timing and magnitude of precipitation and thus the strategies that plants employ to metabolize carbon, grow, flower, and set seed in the short growing season.

This research will test the following specific hypotheses:

  1. Summer precipitation is important for forbs and grasses that emerge and flower in the middle and later parts of the growing season.

  2. Gymnosperms rely almost exclusively on snowmelt water that has percolated into the soil and that they access through deep roots.

  3. Shrubs rely on both water sources, since they have long taproots to access deep soil water but also extensive surface root biomass (ex. Artemisia tridentata).

  4. The carbon isotope composition of bulk leaf biomass from these functional groups reflects variations in water-use efficiency that are related to differential use of summer versus winter water sources and different timing of emergence and flowering.

GothicWinter and summer precipitation occur in different forms (i.e., snow crystals versus rain drops), and they are also imprinted with distinct hydrogen and oxygen isotopic signatures. Water isotope data collected at the University of Colorado field station (Niwot Ridge) document a large seasonal cycle in the 18O and 2H content of summer rainfall and winter snowfall, with strong depletions in these heavier isotopes in winter compared to summer precipitation. This seasonal cycle is driven by the differing source regions of summer and winter storms and the different condensation histories and temperatures of winter versus summer storms. Based on the similarity of the RMBL environment to Niwot Ridge, I anticipate a large seasonal cycle in the isotopic composition of precipitation in the East River valley. Thus, we can exploit the distinct isotopic content of summer versus winter precipitation to trace these hydrological inputs. Water isotope tracers provide a unique opportunity to quantify the importance of seasonal water sources to plant growth throughout the year. It should be possible to estimate the amount of winter versus summer precipitation used by trees, shrubs, forbs, and grasses. Although most vegetation around RMBL undoubtedly depends mostly upon snowmelt water, the timing and magnitude of summer rainfall is crucial to some plants.

This tracer may also help understand the impacts of climate change on the hydrological cycle in the upper Gunnison basin. Climate change is likely to alter both the ratio of winter:summer precipitation (at present, about 4:1) and the totalPedicularis Casseteja amount of precipitation in the upper Gunnison basin (about 24"). Indeed, there is evidence of a positive trend in both winter precipitation and springtime mean minimum temperatures at RMBL (Inouye et al. 2000). Stable isotopes in water may provide a probe of these changes. For example, a significant shift to more summer precipitation or to more spring snowfall would be accompanied by enrichment of 2H and 18O content in streams, soil water, and ponds. Finally, water isotopic tracers have the potential to address basic questions about watershed hydrology in the East River valley, including the contribution of snowmelt to stream flow and spring flow on a seasonal basis. With these data, it should be possible to construct a temporally detailed picture of the water isotope budget for the upper East River valley.

In addition to water isotope data, plant carbon isotopic contents are useful measures of plant function. Spatial and temporal patterns of carbon isotope discrimination can be inferred from leaf carbon isotope signatures. Plant carbon isotope discrimination can in turn be related to plant water-use efficiency (WUE), or the ratio of photosynthesis to transpiration. WUE is a sensitive diagnostic of both plant water use-carbon metabolism, and potential climate change impacts on RMBL vegetation. For example, stable isotope analysis of leaf biomass samples I collected in the climate warming manipulation at RMBL (Harte et al.) suggests measurable changes in the carbon isotope signature of vegetation Treebeardin the warming plots compared to the control plots. Plants in the warming plots were depleted in 13C content by ~0.5‰ (a unit equivalent to parts per thousand) compared to the control plots. This offset likely reflects differences in the seasonal timing of carbon uptake. Since plants in the warming plots can begin photosynthesizing earlier in the year when humidity is higher, they will have a lower WUE and thus higher carbon isotope discrimination.

This research will be largely observational in nature, as I am interested in basic spatial and temporal patterns as a starting point. My sampling will be limited to collecting leaf, soil, water, and tree core samples. I will also extract water from the Treebeardplant and soil samples for oxygen and hydrogen isotopic analysis. Because soil water often has large gradients in 2H and 18O within the top 15-20 cm of soil, it is possible to use the isotopic composition of stem water samples to infer the depth of soil water at which plant roots are withdrawing their water. Combined with data from RMBL's weather station and archived water samples, data from these samples will allow me to construct a picture of precipitation patterns and water usage in this ecosystem.