(15)N in tree rings as a bio-indicator of changing nitrogen cycling in tropical forests: an evaluation at three sites using two sampling methods.
Anthropogenic nitrogen deposition is currently causing a more than twofold increase of reactive nitrogen input over large areas in the tropics. Elevated (15)N abundance (δ(15)N) in the growth rings of some tropical trees has been hypothesized to reflect an increased leaching of (15)N-depleted nitrate from the soil, following anthropogenic nitrogen deposition over the last decades. To find further evidence for altered nitrogen cycling in tropical forests, we measured long-term δ(15)N values in trees from Bolivia, Cameroon, and Thailand. We used two different sampling methods. In the first, wood samples were taken in a conventional way: from the pith to the bark across the stem of 28 large trees (the "radial" method). In the second, δ(15)N values were compared across a fixed diameter (the "fixed-diameter" method). We sampled 400 trees that differed widely in size, but measured δ(15)N in the stem around the same diameter (20 cm dbh) in all trees. As a result, the growth rings formed around this diameter differed in age and allowed a comparison of δ(15)N values over time with an explicit control for potential size-effects on δ(15)N values. We found a significant increase of tree-ring δ(15)N across the stem radius of large trees from Bolivia and Cameroon, but no change in tree-ring δ(15)N values over time was found in any of the study sites when controlling for tree size. This suggests that radial trends of δ(15)N values within trees reflect tree ontogeny (size development). However, for the trees from Cameroon and Thailand, a low statistical power in the fixed-diameter method prevents to conclude this with high certainty. For the trees from Bolivia, statistical power in the fixed-diameter method was high, showing that the temporal trend in tree-ring δ(15)N values in the radial method is primarily caused by tree ontogeny and unlikely by a change in nitrogen cycling. We therefore stress to account for tree size before tree-ring δ(15)N values can be properly interpreted.
Forest Ecology and Management Group, University of Wageningen Wageningen, Netherlands ; Instituto Boliviano de Investigación Forestal Santa Cruz de la Sierra, Bolivia.
Peter
P
van der Sleen
Forest Ecology and Management Group, University of Wageningen Wageningen, Netherlands.
Mart
M
Vlam
Forest Ecology and Management Group, University of Wageningen Wageningen, Netherlands.
Peter
P
Groenendijk
Centre for Crop Systems Analysis, University of Wageningen Wageningen, Netherlands.
Niels P R
NP
Anten
Forest Ecology and Management Group, University of Wageningen Wageningen, Netherlands.
Frans
F
Bongers
Department of National Parks, Wildlife and Plant Conservation Bangkok, Thailand.
Sarayudh
S
Bunyavejchewin
Institut für Botanik, University of Natural Resources and Life Sciences Vienna, Austria.
Peter
P
Hietz
Plant Ecophysiology, Institute of Environmental Biology, Utrecht University Utrecht, Netherlands.
Thijs L
TL
Pons
Forest Ecology and Management Group, University of Wageningen Wageningen, Netherlands.
Pieter A
PA
Zuidema
anthropogenic nitrogen deposition; nitrogen cycling; stable nitrogen isotopes; tree ontogeny; tree rings; tropical forest; tropical tree