DOI: 10.5061/dryad.h70rxwdk6
title: Interplay of above- and belowground resource limitations: a competition-facilitation shift maintains species coexistence

description: Forest ecosystems are commonly characterised by a hierarchy of resources.
 During a disturbance of a forest community, increased light availability
 in the understorey can support competitive interactions at the expense of
 facilitation. This may overwhelm the role of belowground resource
 heterogeneity in maintaining species coexistence and so result in biotic
 homogenisation of a site. We re-surveyed species composition while
 estimating interspecific interactions along a microtopographic (moisture)
 gradient of a temperate swamp forest after increasing the availability of
 light into the undergrowth. We measured temporal changes in species
 diversity, compositional dissimilarity measures and functional traits,
 while belowground resource heterogeneity remained unchanged over time. To
 explain observed changes in terms of prevailing interspecific
 interactions, we evaluated temporal changes in the
 competition-facilitation balance. The productivity of understorey
 vegetation increased strongly over the five years of the study. This was
 accompanied by an increase in species diversity and the maintenance of
 compositional dissimilarity. The relative importance of competitive and
 facilitative interactions along the microtopographic gradient did not
 change over time. Rather than an overall biotic homogenisation of a site,
 we observed a spread of resource acquisitive species within the
 resource-rich sites and of stress-tolerant species within the
 resource-limited sites. We have shown that the temporary increase in
 productivity did not mitigate the effects of belowground resource
 heterogeneity on plant species turnover, which is likely maintained by
 contrasting interspecific interactions prevalent in different parts of the
 resource gradient. This suggests contrasting community assembly processes
 maintain diversity and productivity even in a local community.
description: In July 2013, we
 established 212 microtopographic gradients in all tree hummocks in three
 stands of the <i>Alnus</i> swamp forest. There were 71, 71 and
 70 hummocks in the three stands and the stands were at least 150 m apart.
 The coordinates of the geographical centre of each hummock were taken
 using high-accuracy measurements using a Leica GPS System 1200 (Leica
 Geosystems, Sankt Gallen, Switzerland). Within each microtopographic
 gradient, we divided a hummock into 2–6 vertical zones (Fig. 1a). The
 lowest zone of the microtopographic gradient (resource rich) captured the
 inundated hollow below the common shoreline (&lt; 0 cm) and was
 defined as a 100-cm-diameter horizontal area surrounding the dry part of
 the hummock. The other zones, each of 20 cm height, started at the common
 shoreline of the dry part of the hummock such that resource availability
 decreased with increasing height above the common shoreline. Hummock
 height was defined as the sum of the vertical distance between the highest
 point of each hummock and the shoreline and the mean depth of the
 inundated area measured on four opposing sides. The hummock area was
 defined as a horizontal area within the boundaries of an inundated zone
 estimated using georeferenced aerial photographs taken for each hummock
 2.5 m above the ground. Within each hummock we established 2–6 permanent
 plots each covering the whole surface area of each zone (i.e., zone area).
 In total we established 728 plots. In July 2013 and 2018, the percentage
 cover of all vascular plant species in each plot and whole
 microtopographic gradient was estimated visually by the same senior
 researchers.
description: DATAset_DoudaOikos.csv contains table with the following
 columns: ID – identification number of each vegetation
 sample<br> HummockID - identification number of hummock
 (microtopographical gradient)<br> LevelID - identification number of
 hummock zones <br> X_mean – longitude of hummocks in SJTSK
 projection<br> Y_mean – latitude of hummocks in SJTSK
 projection<br> HummockArea_m2 – vertical area of hummocks
 (m2)<br> HummockHeight_cm – the height of the terrestrial part of
 hummocks (cm)<br> WaterDepth_mean_cm – depth of water in the bottom
 zone of hummocks (cm) <br> Year – sampling year<br> Other
 columns (AgrCan and the others) represent species and their percentage
 coverage in the hummock zone