Wuyts K, De Schrijver A, Verheyen K - The importance of forest type when incorporating forest edge deposition in the evaluation of critical load exceedance

Introduction As the extent of forest fragmentation in­ creases worldwide, forest edges are becom­ ing an increasingly important feature in the landscape matrix (Harper et al. 2005). At forest edges, acidifying (N+S) and nitrogen (N) throughfall deposition are increased up to four times compared to the forest interior, and this edge effect decreases exponentially with increasing distance from the edge (Bei­ er & Gundersen 1989, Draaijers et al. 1994, Spangenberg & Kölling 2004, see De Schrijver et al. 2007 for an overview). Des­ pite their importance, edges are rarely taken into account in the assessment of critical load (CL) exceedance in forest ecosystems, but see Lövblad et al. (1995). De Schrijver et al. (2007) quantified edge impact on critical load exceedance in Flanders, but only provided a rough insight in the error on cur­ rent calculations, partly because a fixed edge effect was assumed. Yet, the magnitude (i.e., the level of deposition enhancement) and depth of edge influence depend on pollutant type, meteorological conditions, edge orient­ ation, and edge structure (like forest type, edge shape, and leaf area index Draaijers et al. 1994, Wuyts et al. 2008a, Wuyts et al. 2008b). The study’s aims were to asses: (i) the ef­ fect of incorporating edge deposition in the evaluation of CL exceedance in forests, tak­ ing into account pollutant type, meteorolo­ gical conditions, edge orientation, and forest type; and (ii) the importance of forest type in this effect. Therefore, we calculated CL ex­ ceedance in five Flemish regions differing in forest fragmentation extent and/or share of coniferous forest. Summary of methods In our calculations, we used data on SO4, NO3, and NH4 throughfall deposition along transects perpendicular to the abrupt edges of different forest types, from two field stud­ ies performed by Wuyts et al. (Wuyts et al. 2008b, Wuyts et al. 2009) in 2005-2006 and 2006-2007 (Tab. 1 see Wuyts et al. 2008b for a full description of methods). Integrated Forest Edge Enhancement (IFEE) factors (Tab. 1), which account for both the depth and magnitude of edge effects (DEI and MEI), were computed as the ratio of the throughfall flux that actually reaches the forest floor in the first 64 m of the edge to the throughfall flux that would reach the same area in the absence of edge effects (Wuyts et al. 2008b). Next, by means of met­ eorological data from the nearest weather stations of the Royal Meteorological Institute of Belgium, the forest edge exposure was de­ termined, which we defined as the propor­ tion of time a forest edge is exposed to wind oriented perpendicular (± 45°) to the edge (in %). Based on year-round wind direction data and the linear relationship between the IFEE factor and the edge exposure (Wuyts et al. 2008a), we derived for each of the edges hy­ pothetical IFEE factors for SO4, NO3, and NH4 for the four principal wind directions. We assumed that no considerable edge ef­ fects occur at the lee side of a forest (Pahl 2000). Subsequently, tree species specific, year-round forest interior throughfall depos­ ition was derived from (i) the forest interior plots of the stands and (ii) five Level II plots of the UNEP/UN-ECE Program for the year 2003 (Genouw et al. 2004). Using ARCVIEW 3.1 (ESRI 2004) and the digital forest cover map “Bosreferentielaag” (Aminal Afdeling Bos en Groen 2001), the total forest area and forest edge area (forest located within 64 m of an open area-to-forest