Nitrogen-15 natural abundance in a montane cloud forest canopy as an indicator of nitrogen cycling and epiphyte nutrition

Abstract

Nutrients obtained by epiphytes may either be of atmospheric origin or from organic matter in the canopy, which decomposes to form canopy soil on large branches. We hypothesised that the N supply for epiphytes on small branches was lower, and a larger proportion provided by rainwater, than for epiphytes rooting in canopy soil. We tested this by measuring the N concentration and isotopic composition in terrestrial and canopy soil and in various canopy compartments of a Costa Rican cloud forest. In general, epiphytes on small branches without canopy soil had lower N foliar concentrations and δN signals than plants rooted in canopy soil, suggesting that the former receive a higher proportion of N directly from the rain. Epiphytes on small branches also had less negative δC values, indicating more frequent water stress. Epiphytes had lower δN values (-3.9±2.3‰) than ground-rooted trees (-1.1±1.6‰), and canopy soil had lower values (0.7±1.2‰) than terrestrial soil (3.8±0.7‰). Assuming that the isotopic effect of terrestrial and canopy soil organic matter formation is similar, our findings support earlier results showing that canopy soil is derived mainly from epiphytes, with only minor inputs from host tree matter. Thus, the epiphyte N cycle appears to be largely detached from the tree-soil cycle. Epiphylls on leaves of understorey shrubs had higher δN signals than cryptogams in the upper canopy, as a result of either N accumulation in throughfall or increased N fixation. The correlation between epiphyll and understorey host leaf δN suggests some exchange of N between epiphylls and host leaves. Differences between epiphyte groups also appear to be related to uptake of N through mycorrhizas or N fixation. Thus, the source and quantity of N supply is highly variable, depending on the systematic group and canopy position.

Stable N isotope composition of nitrate reflects N transformations during the passage of water through a montane rain forest in Ecuador

“…In the tropics, stable isotope studies on N transformations have been either limited to lowland rain forests or focused only on single processes of the N cycle or selected ecosystem compartments (Marrs et al 1988;Vitousek and Matson 1988;Hall and Matson 2003;Soethe et al 2006). The same holds true for the few stud-ies conducted in tropical montane forests (Hietz et al 2002;Houlton et al 2006;Kiese et al 2008).…”

Fractionnement isotopique 13C et 15N dans les arbres, le sol et les champignons pour un peuplement de forêt naturelle et une plantation d’épicéas

“…Stable isotope techniques are efficient tools for ecophysiology and ecosystem research [25,35,37]. 13 C and 15 N natural abundance and 14 C measurements have been used to study fungal sources of carbon and nitrogen [28].…”

The effect of small-scale land use on vegetation in the Valdivian Coastal Range (Chile)

“…It was present in 24% of the SDE forest plots and may indicate less disturbed conditions. Since vascular epiphytes are slow growing (Zotz 1995, Hietz et al 2002 they are susceptible to anthropogenic disturbances that alter their habitat (tree crowns) and changes in microclimatic conditions on which epiphytes depend (Freiberg 1997). In contrast, the tall fern Lophosoria quadripinnata is associated with the typical hyperhumid conditions of Valdivian woodlands occurring on steep slopes (Oberdorfer 1960).…”