Response of inland dune vegetation to increased nitrogen and phosphorus levels

Question
How does pioneer vegetation of acid inland dunes respond to addition of nitrogen (N) and phosphorus (P)?

Location
Two inland dune reserves in The Netherlands with low and high N deposition.

Methods
During 2.5 yr, N and P addition (control, N, P, NP) took place in three different vegetation types of inland dunes (Polytrichum piliferum mats, Campylopus introflexus mats and lichen-dominated vegetation). In each site, changes in the vegetation were recorded in three replicate quadrats of 1 m × 1 m per treatment. Differences in element content of grasses and lichens were recorded, together with vegetation parameters, including cover and height of grasses, bryophytes and lichens.

Results
In the site with high N deposition, grasses were taller, had higher N:P ratios and a generally lower lichen cover than in the low-deposition area. Experimental N application resulted in higher N and lower base metal concentrations in grasses and an increase in grass cover and size. In contrast, lichens showed a general decline in the N treatment and the lichen:grass ratio decreased. The effect of N addition was larger in the low-deposition area. The P treatment had an opposite effect: lichens increased in size and cover and overgrew grasses, especially when reindeer lichens were present. This suggests that the cryptogam layer was P-limited even in the low N deposition site.

Conclusions
Pioneer vegetation in inland dunes is susceptible to N and P addition. The results provide evidence for a decrease in lichen cover due to increased N deposition.