Empirical determination of N critical loads for alpine vegetation

1. Empirical determination of N critical loads for alpine vegetation William D. Bowman, Julia L. Gartner, Keri Holland, and Magdalena Wiedermann Department of Ecology and Evolutionary Biology and Mountain Research Station, University of Colorado, Boulder

2. N Critical Loads: Does one size fit all?

4. Indicators of Ecosystem Response to Elevated N Inputs: • episodic acidification= loss of acid neutralizing capacity and elevated [NO3-] in upper Green Lakes Valley (Nel Caine & Mark Williams) • changes in diatom composition (lake cores) (Jasmine Saros, Alex Wolfe and Jill Baron) • needle and forest floor chemistry in old-growth subalpine forests (East-West slope comparison) (Heather Rueth and Jill Baron) • changes in alpine plant species composition in long-term monitoring plots

5. Paradox of simultaneous N limitation & N excess Experimental N additions in alpine result in greater plant growth, yet growing season export of NO3- is occurring (?) • Adaptation to low soil nutrient supply- some species don’t respond to increased N availability

6. Addressed experimentally in alpine (species rich dry meadow), using additions of 2, 4, 6 g N/m2/yr • response variables: • species composition • soil solution chemistry • N leaching (resin bags) • biomass production • soil N transformation rates • soil cation chemistry Paradox provides an opportunity: changes in species composition indicative of N inputs Alternative view: how much N input does it take to produce a change in species composition? (= N critical load using biotic response)

7. Carex rupestris species composition response: treatment x year P < 0.01 similar response for Trisetum spicatum

8. Community response: ordination score treatment x year P < 0.05

9. Establishing a critical load from response data: • assume a dose response i.e. magnitude of change is related to treatment level • 2) assume no other forcing factor is altering response variable (e.g. climate change) • 3) set “0” level to ambient deposition rate (8 kg/ha/yr)

10. Empirical estimation of N critical load for plant species responses in alpine dry meadows N Criticalload: 4-12 Kg N/ ha/ yr

11. Estimates of N critical loads in the alpine: Amount: source: basis: (kg ha-1 yr-1) 4-12 this study vegetation change 4 * Williams & Tonnessen surface water chemistry (2000) 1.5 Baron (2006) hindcasting analysis 3-4 Baron et al. (1994) CENTURY model (N leaching) 10-15 Bobbink et al. (2002) vegetation change *wet only

12. Indications of ongoing vegetation response to N deposition on Niwot Ridge • Recensus of long-term plots (Marr plots- Korb & Ranker) • Analysis of LTER monitoring plots (Suding & Bowman):

13. Ecosystem (soil) responses: inorganic N loss to resin bags (15 cm depth) during the growing season

14. Soil solution NO3-- N (early season-prior to fertilization) note apparent higher critical load for N leaching relative to vegetation response

15. (from Aber et al. 1998) N cycling rates: net N mineralization and nitrification b b b ab ab ab a a

16. Exchangeable Aluminum

17. Summary: Take-Home Messages • N Critical load estimation possible using community/ population level approach (most probable in chronically N limited vegetation: alpine, arctic, grassland, herbaceous understory); coupled experimental – monitoring approach • Sampling intensity and disturbance lower using plant species monitoring • Responses by vegetation may precede more serious soil changes that may lead to greater environmental degredation (acidification) • Changes in plant species composition may have a positive feedback on inorganic N leaching

18. Research needed to establish N critical loads in sensitive sites e.g. governed as class 1 areas of Clean Air Acts e.g. similar empirical approach will be used to establish N critical loads for alpine vegetation in Rocky Mountain and Glacier National Parks Chapin Pass Appistoki Valley