Abstract
Woody encroachment into herbaceous ecosystems is emerging as an important ecological response to global change. A primary concern is alterations in C and N cycling and associated variations across a variety of ecosystems. We quantified seasonal variation in litterfall and litter N concentration in Morella cerifera shrub thickets to assess changes in litterfall and associated N input after shrub expansion on an Atlantic coast barrier island. We also used the natural abundance of 15N to estimate the proportion of litterfall N originating from symbiotic N fixation. Litterfall for shrub thickets ranged from 8,991 ± 247 to 3,810 ± 399 kg ha−1 year−1 and generally declined with increasing thicket age. Litterfall in three of the four thickets exceeded previous estimates of aboveground annual net primary production in adjacent grasslands by 300–400%. Leaf N concentration was also higher after shrub expansion and, coupled with low N resorption efficiency and high litterfall, resulted in a return of as much as 169 kg N ha−1 year−1 to the soil. We estimated that ∼70% of N returned to the soil was from symbiotic N fixation resulting in an ecosystem input of between 37 and 118 kg ha−1 year−1 of atmospheric N depending on site. Considering the extensive cover of shrub thickets on Virginia barrier islands, N fixation by shrubs is likely the largest single source of N to the system. The shift from grassland to shrub thicket on barrier islands results in a substantial increase in litterfall and foliar N concentration that will likely have a major impact on the size and cycling of ecosystem C and N pools. Increasing C and N availability in these nutrient-poor soils is likely to permanently reduce cover of native grasses and alter community structure by favoring species with greater N requirements.
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Acknowledgements
This study was supported in part by NSF grant DEB-008031 to the University of Virginia for LTER-related work at the Virginia Coast Reserve. Paul Bukaveckas assisted with N concentration analyses. Colorado Plateau Stable Isotope Laboratory provided stable isotope analyses. The Virginia Coast Reserve LTER staff, especially Arthur Schwarzschild, assisted with island logistics. Data collection and experiments in this study comply with all laws of the United States of America.
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Communicated by Tim Seastedt.
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Brantley, S.T., Young, D.R. Shifts in litterfall and dominant nitrogen sources after expansion of shrub thickets. Oecologia 155, 337–345 (2008). https://doi.org/10.1007/s00442-007-0916-7
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DOI: https://doi.org/10.1007/s00442-007-0916-7