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Plant Ecology

Plant Ecology

Prominent tundra forb, alpine avens (Geum rossii), photo credit: Bill Bowman

The productivity of biomass by vegetation is a crucial component of the energy, carbon and nutrient fluxes that occur in ecosystems. The spatial distribution, abundance, and productivity of plants in the subalpine forest and alpine tundra are controlled by physical forces, like wind, snow, and topography, as well as biotic factors, like other plants and soil microbial communities. Historic research at NWT has given much attention to the influence of physical forces on plant communities in the tundra; more recently, NWT scientists have begun to investigate the role of biotic factors. Changes in environmental conditions due to climate change (temperature, precipitation) and dust and nitrogen deposition (nutrient availability) are also affecting plant communities in high mountain ecosystems, and are the focus of a number of research experiments at NWT.

Interactive effects of climate and nitrogen on alpine plant communities

Indirect and interactive effects of changes in nutrient availability, temperature, and precipitation (snowpack) patterns on alpine tundra ecosystems are being tested in an on-going global change experiment (Farrer et al. 2014). Over time, enhanced nitrogen (N) availability and winter snowpack were found to increase the abundance of a grass species, Deschampsia caespitosa, and decreasedthe abundance of a forb, Geum rossii. By changing the abundances of these two prominent tundra plant species, N and snow additions had a strong indirect effect on overall plant community diversity, with the strength of this effect increasing over time (Fig. 1). Increased N, summer temperature, and winter snowpack also directly affected ecosystem functions like plant productivity, N mineralization, and winter N availability. However, over time, the indirect effects are stronger than the direct effects of these environmental drivers. For example, most of the negative effect on plant community diversity caused by N addition was due to nitrogen’s effect of increasing Deschampsia abundance. Overall, these results suggest that explicitly accounting for changes in dominant plant species abundances may be necessary for forecasting plant community response to environmental change, but predicting ecosystem function without knowledge of plant responses to global change may also be possible.

Investigators: Katie Suding, Emily Farrer

Figure 1. Development of direct and indirect effects of increased (a) nitrogen,(b) snow and (c) temperature (warming) on plant community diversity over time.

Figure 1. Development of direct and indirect effects of increased (a) nitrogen,(b) snow and (c) temperature (warming) on plant community diversity over time.

Shrub expansion in the alpine tundra

The expansion of woody plant cover in historically herbaceous-dominated plant communities has been observed in a range of ecosystems around the world, including arctic and alpine tundra. These changes are being driven by various factors including climate and other global factors, like N deposition, as well as land management strategies. Depending on the rate and extent of woody encroachment in these systems, such shifts in vegetation cover could have major implications for important ecosystem processes like nutrient and carbon cycling and storage.

Formica et al. (2014) quantified the rate of shrub expansion from 1946 to 2008 in an 18 ha area of alpine tundra on Niwot Ridge by analyzing aerial photographs and used a global change manipulation experiment to assess the effects of three important global change factors – N deposition, temperature, and precipitation (snowpack) – on willow shrub seedling survival and growth. They found that over the 62-year period willow cover increased by 441% and at an exponential rate (Fig. 2), translating to a 137 kg ha-1 increase in carbon storage. Increased snowpack was shown to increase willow seedling survival and increased N deposition and warmer summer temperatures to facilitate willow growth (Fig. 3), supporting the conclusion that, in addition to release from grazing pressure, global change factors may be driving shrub expansion in the alpine tundra.

Investigators: Katie Suding, Emily Farrer

Figure 2. The increase in willow cover on the Niwot Ridge Saddle fits with an exponential growth curve (R2 = 0.989, P < 0.001) (Formica et al. 2014).

Figure 2. The increase in willow cover on the Niwot Ridge Saddle fits with an exponential growth curve (R2 = 0.989, P < 0.001) (Formica et al. 2014).

Figure 3. Willow (Salix) seedling (a) survival and (b) height five years after transplant into the field site. Significant effects of all possible treatment factor combinations are presented in each figure (Formica et al. 2014).

Figure 3. Willow (Salix) seedling (a) survival and (b) height five years after transplant into the field site. Significant effects of all possible treatment factor combinations are presented in each figure (Formica et al. 2014).

Significant inertial impacts of nitrogen deposition

**Preliminary results – DO NOT CITE**

In a long-term fertilization experiment the Bowman lab has added several levels of nitrogen (N) as NH4NO3 (0, 2, 4, or 6 kgN ha-1 yr-1) for 17 years, and discontinued fertilization of half of each plot after year 11. Plots were sampled in 2014 to assess recovery six years after stopping the fertilization treatment. Soils and vegetation (NMDS ordination) in N fertilized plots show few signs of recovery, except for a slight increase in soil pH.  Oddly, the recovery plots did not exhibit increases in base cations, and actually had higher extractable soluble aluminum concentrations. Soil solution nitrate sampled from plots at the beginning of the growing season, and prior to any addition of fertilizer for the current year, indicate elevated levels in both treatment and recovery plots, with no significant difference in the amount of increase (Fig. 4). The continued elevation of soil solution nitrate is probably due to previously noted higher mineralization and nitrification rates from litter and soil organic matter with lower C:N ratios.

These results indicate significant inertial impacts of N deposition even after simulated N deposition ceases, with continued elevation of nitrate in soil solution, acidified soils, and higher extractable aluminum.  Ecosystem recovery from the effects of elevated N deposition depends on lowering nitrate levels and increasing base cations in the soil. The former is influenced over the long-term by changes in the chemistry of soil organic matter. Weathering rates, which are very slow relative to the rates of cation leaching, influence the latter.  Base cations are also influenced by inputs of dust, which have been shown to significantly influence acid neutralizing capacity of soils on Niwot Ridge, and which have been increasing over the past two decades.  The role of dust in buffering the impacts of N deposition is not well known, but ongoing work to investigate this is using strontium as a tracer for calcium inputs from dust.

Investigator: Bill Bowman

Figure 4. Soil solution nitrate sampled from plots at the beginning of the growing season, prior to any addition of fertilizer for the current year, indicate elevated levels in both treatment and recovery plots. There was no significant difference in the amount of increase between the plots, which have received NH4NO3 for the past 17 years and those for which N addition treatments stopped six years ago.

Differential resource controls on productivity across a snowpack gradient

**Preliminary results – DO NOT CITE**

Nitrogen (N) deposition on Niwot Ridge is almost 40 times higher than background levels (currently 8 kgN ha-1 yr-1).  Dust deposition, which carries calcium and phosphorus (P), is some of the highest in the US, with estimates of as much as 300 mg calcium/m2  (Brahney et al 2013). Both N and dust deposition often accumulate in snow, and we find the redistribution of snow by wind and the melt out patterns affect both the delivery of snow water and nutrients to different areas on the landscape.  Thus, one of our goals is to understand how these changes will affect resource limitations across snowpack gradients. 

We find differential resource limitations across a snowmelt gradient (Fig. 5). Dry meadows, which receive very little snowmelt and are largely blown free of snow during the spring, show a slight N and P co-limitation, but a decrease in biomass when N added, likely due to the prevalence of N-fixing Trifolium in these areas.  Wetter areas of the landscape (mesic and wet meadows) that receive snowmelt through a large part of the growing season show strong N limitation largely due to the responsiveness of graminoids with little dependence on P.  These results suggest that in areas on the landscape that receive high snowmelt inputs, and possibly years where snowpack is high, N inputs may be a strong secondary driver after soil moisture. However, in areas that are drier and do not receive as much snow water inputs, production may be more dependent on the combination of N and dust deposition. Because we find that dust inputs often occur in conjunction with spring storms, these drier areas may not receive enough P inputs to alleviate the P limitation, and may actually decline in production in the short term due to continued N inputs and loss of N-fixers. 

Investigator: Bill Bowman

Figure 5. Principal Coordinates Analysis showing plant community compositional dissimilarity (Bray-Curtis) among meadow types (left) and fertilization treatments (right). Left:  Red = Dry; Blue = Mesic; Orange = Wet. Right: Red = Control; Green = +P; Orange = +NP; Blue = +N. Resource effects on composition depend on community types, with strong effects in the wetter (higher snowpack) areas.  

Related Discipline Data:

Plant/vegetation ecologySoil scienceBiogeochemistryMicrobial ecology


Related Research Topic Pages:


Associated Niwot Researchers:

Related Photo Galleries:

Niwot Ridge Flora

Browse The Complete set Here >>

  • Alpine avens (Geum rossii)
    Alpine avens (Geum rossii)
  • Columbine (Aquilegia coerulea)
    Columbine (Aquilegia coerulea)
  • Shootingstar (Dodecatheon pulchellum)
    Shootingstar (Dodecatheon pulchellum)
  • Alpine forget-me-not (Eritrichium nanum)
    Alpine forget-me-not (Eritrichium nanum)
  • Old man of the mountain (Tetraneuris grandiflora)
    Old man of the mountain (Tetraneuris grandiflora)
  • Alpine clover (Trifolium dasyphyllum)
    Alpine clover (Trifolium dasyphyllum)
  • Saffron groundsel (Packera crocata)
    Saffron groundsel (Packera crocata)
  • Moss campion (Silene acaulis)
    Moss campion (Silene acaulis)
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This material is based upon work supported by the National Science Foundation under Cooperative Agreement #DEB-1027341. Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necesarily reflect the views of the National Science Foundation.

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