Linking environmental heterogeneity and plant diversity at intermediate scales - Our new paper in Science of the Total Environment

Our new paper about the link between environmental heterogeneity and plant diversity has recently been accepted in Science of the Total Environment.

Deák, B., Kovács, B., Rádai, Z., Apostolova, I., Kelemen, A., Kiss, R., Lukács, K., Palpurina, S., Sopotlieva, D., Báthori, F., Valkó, O. (2021): Linking environmental heterogeneity and plant diversity: the ecological role of small natural features in homogeneous landscapes. Science of the Total Environment 763: 144199 [IF2019: 6.551]

The paper is open access and can be downloaded by clicking here. 

Environmental heterogeneity (EH), encompasses spatial and temporal heterogeneity in biotic (land cover, vegetation) and abiotic (climate, soil and topography) components. EH is considered one of the most important drivers of species richness patterns from local to continental scales. The conservation importance of EH is being recognised globally, due to the generally positive relationships between EH and biodiversity. However, there are several open questions regarding EH-biodiversity relationships. Although the effects of EH components are often interrelated and all of them affect biodiversity patterns, most studies consider only one, or a few, of them. Moreover, studies generally focus either on a continental/landscape-scale or a fine-scale of a few centimetres/metres, but there are only a few studies concentrating on intermediate spatial scales.

The relationship between environmental heterogeneity and plant diversity is well known in mountain environments.

For establishing links between EH components and biodiversity at intermediate scales of a couple of metres, small natural features (SNFs) provide an outstanding opportunity. SNFs include field margins, road verges, midfield islets, stone walls, karst dolines, ancient earthworks and burial mounds, all of these can be regarded as distinct entities differing from their surroundings in their abiotic and biotic conditions, and they are often characterised by a high level of EH. These SNFs are characteristic elements in many landscapes, but little is known about the relationship between EH components and the associated biodiversity patterns found on them. For our study, we chose several millennia-old ancient burial mounds (built by nomadic steppe tribes), which are one of our favourite natural features. If you are particularly interested in these iconic landscape elements, please read our former blog post here.

Ancient burial mounds in Bulgaria - the most impressive ones can be as high as ten metres and they are covered by dry grasslands.

In this study, we focused on all components of EH that are potential drivers of vegetation patterns, and can contribute to the maintenance of grassland biodiversity on topographically heterogeneous SNFs. We surveyed ancient burial mounds covered by dry grasslands in the Pannonian (Hungary) and Continental (Bulgaria) biogeographical regions of Europe, in order to test whether the effects of EH on vegetation composition depends on the biogeographical region. In order to understand the interactions between EH components and reveal their effects on plant diversity and species composition, we studied the five main EH components on mounds using nearby plain grasslands as a reference. On each mound, we investigated five mound microsites (slopes with different main aspects and top).

Figure showing our sample design.

We found very interesting patterns in the microclimate parameters on the slopes of the mounds. The figure below shows that mound microsites were characterised by considerable fluctuations between extreme positive and negative relative values of air temperature (T), relative humidity (RH) and vapour pressure deficit (VPD). In the morning, the air of the east-facing slopes warms up faster than the plain grassland or the top, and becomes dry, while west-facing slopes remain cool and humid. In the afternoon, this pattern reverses, but west-facing slopes dry and warm up to a lesser degree. Diurnal variations are due to the temporal changes in solar radiation received by the different microsites. East-facing slopes warms up and consequently become drier earlier than plain grasslands and other mound microsites. It is very impressive that these patterns can be detected in landscape elements that are maximum 10 metres high!

Diurnal variations of air temperature (ΔT), relative humidity (ΔRH) and vapour pressure deficit (ΔVPD) in the studied mound microsites, relative to the values detected in plain grasslands (dotted line).

As shown by the diurnal variations, the difference between north- and south-facing slopes was greatest when the Sun is at the zenith (at about 13:00). At that time north-facing slopes were 5.1°C cooler and south-facing slopes 7.6°C warmer than plain grasslands. In the case of mounds occurring in regions with a generally harsh continental macro-climate, small-scale micro-topographical differences are equivalent to large latitudinal (several hundreds of kilometres) and elevation differences (the observed 12.7°C range in temperature within the small area of a mound is roughly equivalent to a shift of couple of hundred metres in altitude) over very short horizontal distances. The microclimatic differentiation detected on the mounds in our study highlights the importance of small landforms in plain areas that can introduce a high level of EH to the otherwise homogeneous landscape.

North-facing slope of a mound in Bulgaria covered by Elymus elongatus and Ranunculus illyricus.

We found that the different abiotic environmental conditions represented by microsites were reflected in the vegetation patterns; thus, the microsites studied function as microhabitats for plant species. On the site level we detected clear patterns in both regions; the vegetation composition of mild and harsh mound microhabitats and the grassland were well-separated in all the sixteen sites.

Ordination diagrams from the non-metric multidimensional scaling, displaying the species compositions of the studied microsites (HU – Hungary; BG – Bulgaria). Large panels show the patterns on the regional level (points with the same colour denote the average of five plots per microsite per site), whilst small panels show the species composition of different microsites on the site level (one point represents one plot).

In general, the mild north- and west-facing slopes had similar species composition, dominated by specialists such as Elymus hispidus, Stipa capillata, Inula germanica, Helianthemum salicifolium, and Inula oculus-christi, and only a few species indicating degradation. The harsh south-facing slopes and the top shared similar species composition, harbouring stress- and disturbance-tolerant generalists and weeds (e.g. Elymus repens, Bromus spp., Senecio vernalis). The varying but harsh east-facing slopes showed a scattered pattern. Plain grasslands were characterised by a rather distinct species pool in both regions.

Our findings highlight that SNFs have a crucial conservation importance in plain landscapes, which cover one third of the global land area. We found that even landforms, which are only a few metres high, can introduce a large variation of environmental heterogeneity into these areas. Besides the more than half a million Eurasian mounds, there are also many analogous and globally widespread landforms (inselbergs, dolines, mounds built by ecosystem engineering rodents) that can have a similar function in introducing EH to environmentally homogeneous landscapes, and therefore driving local and landscape-scale diversity patterns within these landscapes.

Graphical abstract summarising the key findings of the paper.

Abstract

Small natural features (SNFs), such as road verges, midfield islets, rocky outcrops and ancient burial mounds, provide safe havens for species of natural habitats in human-modified landscapes; therefore, their great ecological importance is in contrast to their small size. SNFs often have a high topographical heterogeneity and abiotic conditions, which differ from their surroundings; therefore, they provide a unique opportunity for establishing links between environmental heterogeneity (EH) and biodiversity. However, no study has so far investigated the EH components of topographically heterogeneous SNFs in a comprehensive framework, by linking environmental and biotic parameters. To fill this knowledge gap, we evaluated the EH components and their effect on biodiversity on ancient mounds covered by semi-natural grasslands in the Pannonian (Hungary) and Continental (Bulgaria) biogeographical regions. We designated 16 study sites, each containing a few-metre-high mounds with five microsites (top, north-, east-, south- and west-facing slopes) and a nearby plain grassland. At each microsite, we measured soil moisture, soil chemical properties, solar radiation and microclimate; and recorded the cover of vascular plants in a total of 480 plots. On the mounds, topographical heterogeneity was associated with sharp differences in microclimate and soil properties. Besides the contrast between mild north-facing and harsh south-facing slopes, east- and west-facing slopes also sustained unique microsites characterised by dynamic diurnal changes in air temperature and vapour pressure deficit. Various combinations of the EH components resulted in unique plant species compositions within the microsites, and supported the co-occurrence of species typical of contrasting habitat types, even within a couple of metres. By combining high-resolution measurements of abiotic factors with fine-scale vegetation sampling, our study provides evidence that widespread SNFs with complex topography harbour several grassland-specialist plant species and introduce a high level of EH to otherwise homogeneous plain landscapes, which cover one third of the global land area.

Group photo in our expedition in Bulgaria; from left to right: Salza Palpurina, Iva Apostolova, Desislava Sopotlieva, Réka Kiss, Katalin Lukács, András Kelemen, Orsolya Valkó, Balázs Deák.