Edge Effects on Foliar Stable Isotope Values in a Madagascan Tropical Dry Forest

Abstract

Edge effects represent an inevitable and important consequence of habitat loss and fragmentation. These effects include changes in microclimate, solar radiation, or temperature. Such abiotic effects can, in turn, impact biotic factors. They can have a substantial impact on species, communities, and ecosystems. Here we examine clinal variations in stable carbon and nitrogen isotope values for trees along an edge-interior gradient in the dry deciduous forest at Ankarafantsika National Park. We predicted that soil respiration and differences in solar irradiance would result in stratified δ13C values where leaves collected close to the forest floor would have lower δ13C values than those growing higher up in the canopy. We also anticipated that plants growing at the savannah-forest boundary would have higher δ13C and δ15N values than plants growing in the forest interior. As expected, we detected a small but significant canopy effect. Leaves growing below 2 m from the forest floor exhibit δ13C values that are, on average, 1.1‰ lower than those growing above this threshold. We did not, however, find any relationship between foliar δ13C and distance from the edge. Unpredictably, we detected a striking positive relationship between foliar δ15N values and increasing distance into the forest interior. Variability in physiology among species, anthropogenic influence, organic input, and rooting depth cannot adequately explain this trend. Instead, this unexpected relationship most likely reflects decreasing nutrient or water availability, or a shift in N-sources with increasing distance from the savannah. Unlike most forest communities, the trees at Ampijoroa are growing in nutrient-limited sands. In addition to being nutrient poor, these well-drained soils likely decrease the amount of soil water available to forest vegetation. Continued research on plant responses to edge effects will improve our understanding of the conservation biology of forest ecosystems in Madagascar.

Connecting the Green and Brown Worlds

“…Further, C:N changes with soil depth (Fig. Crowley et al, 2012) and significant increase in d 13 C values with soil depth under grasslands (e.g. Crowley et al, 2012) and significant increase in d 13 C values with soil depth under grasslands (e.g.…”

Edge Effects on Morphometrics and Body Mass in Two Sympatric Species of Mouse Lemurs in Madagascar

“…Although some researchers have studied edge effects in Madagascar, most have focused on biotic effects in humid forests [Ramanamanjato, 2001;Lehman et al, 2006a;Irwin, 2008]. Only recently have researchers studied how plants and animals respond to edges in the rarer tropical dry forests [Lehtinen et al, 2003;Watson et al, 2004;Crowley et al, 2012]. For example, Crowley et al [2012] found a negative relationship between foliar δ 15 N values and edge proximity in western dry forests, which likely reflected spatial variations in nutrient and water availability, or a shift in N sources with increasing distance from the matrix (savannah).…”

“…Only recently have researchers studied how plants and animals respond to edges in the rarer tropical dry forests [Lehtinen et al, 2003;Watson et al, 2004;Crowley et al, 2012]. For example, Crowley et al [2012] found a negative relationship between foliar δ 15 N values and edge proximity in western dry forests, which likely reflected spatial variations in nutrient and water availability, or a shift in N sources with increasing distance from the matrix (savannah). Moreover, some biotic edge effects, such as tree dendrometrics, can penetrate up to 500 m into the interior of dry forest [McGoogan, 2011].…”

Variation in hair δ13C and δ15N values in long-tailed macaques (Macaca fascicularis) from Singapore

“…Using comparative research on δ 13 C in nonhuman primate hair presented by Schoeninger (2010, Fig. 15.3), we established a threshold value of ≤−24 ‰ to identify a closed canopy environment (but see Crowley et al 2012). With the exception of one individual from Rifle Range Road, all hair samples with δ 13 C ≤−24 ‰ were from monkeys from Bukit Timah.…”

“…The relative proportion of carbon isotopes (δ 13 C) varies from −35 to −21 ‰ in C 3 plants, −20 to −10 ‰ in C 4 plants, and −17 to −13.2 ‰ in CAM plants (see O'Leary 1981, 1988; Kohn 2010). In the tropics, variation in δ 13 C values in C 3 plants can sometimes be attributed to higher concentrations of atmospheric carbon dioxide associated with more closed forest canopies (Medina and Minchin 1980; van der Merwe and Medina 1989; Schoeninger 2010; Crowley et al 2012). Because the degree of forest canopy and the distribution of C 3 and C 4 plants can vary spatially, variation in δ 13 C values within and among animal groupings may reflect differences in feeding ecology, differences in geographic location, or both.…”