@article {KNZ001351,
	title = {Cross-site comparison of herbivore impact on nitrogen availability in grasslands: the role of plant nitrogen concentration},
	journal = {Oikos},
	volume = {118},
	year = {2009},
	pages = {1613 -1622},
	abstract = {<p>Herbivores may influence the nitrogen (N) recycling rates and consequently increase or decrease the productivity of grasslands. Plant N concentration emerged as a critical parameter to explain herbivore effects from several conceptual models, which predict that herbivores decrease soil N availability when plant N concentration is low whereas they increase it when plant N concentration is high (Hobbs 1996, Ritchie et al. 1998, Pastor et al. 2006). However, a broader cross-site comparison among published studies to test these predictions is hampered by the different methodologies used to measure soil N availability or a proxy thereof, and a lack of measurements of plant N concentration. Therefore it remains unclear whether these model predictions are generally valid across a range of grasslands. We tested whether there is a relationship between plant N concentration and herbivore impact on soil N availability (measured with resin bags) with a study of replicate 6\&ndash;8 year old exclosures (with an unfenced control) of vertebrate herbivores (\&gt;1 kg) established at each of seven grassland sites in North America and Europe. Contrary to model predictions, we found a negative relationship between the effect of herbivores on resin bag soil N availability and plant N concentration. Our study confirms the importance of plant N concentration as a predictor of herbivore effect on soil N availability across grasslands, but contradicts the models. A possible explanation may be that the results represent a transient situation as the exclosures were relatively young whereas the models may refer to an equilibrium state. Simultaneous measurements of both plant N concentration and herbivore effect on soil N availability from more grassland sites, preferably with contrasting plant N concentrations and including exclosures of different ages, should resolve the contrast between model predictions and our field measurements.</p>},
	keywords = {LTER-KNZ},
	doi = {10.1111/j.1600-0706.2009.17199.x},
	url = {https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1600-0706.2009.17199.x},
	author = {E.S. Bakker and Knops, J.M.H. and Milchunas, D.G. and Ritchie, M.E. and Olff, H.}
}
@article {KNZ001352,
	title = {Herbivore impact on grassland plant diversity depends on habitat productivity and herbivore size},
	journal = {Ecology Letters},
	volume = {9},
	year = {2006},
	pages = {780 -788},
	abstract = {Mammalian herbivores can have pronounced effects on plant diversity but are currently declining in many productive ecosystems through direct extirpation, habitat loss and fragmentation, while being simultaneously introduced as livestock in other, often unproductive, ecosystems that lacked such species during recent evolutionary times. The biodiversity consequences of these changes are still poorly understood. We experimentally separated the effects of primary productivity and herbivores of different body size on plant species richness across a 10-fold productivity gradient using a 7-year field experiment at seven grassland sites in North America and Europe. We show that assemblages including large herbivores increased plant diversity at higher productivity but decreased diversity at low productivity, while small herbivores did not have consistent effects along the productivity gradient. The recognition of these large-scale, cross-site patterns in herbivore effects is important for the development of appropriate biodiversity conservation strategies.},
	keywords = {LTER-KNZ},
	doi = {10.1111/j.1461-0248.2006.00925.x},
	author = {E.S. Bakker and Ritchie, M.E. and Olff, H. and Milchunas, D.G. and Knops, J.M.H.}
}
