Facilitation interactions in dryland ecosystems of Cyprus, based on Ziziphus lotus

Abstract

Drylands cover ca. 40% of the Earth’s land surface. Positive interactions between nurse plants and their facilitated species are highly present in these habitats, where water availability is the key limiting factor for plant productivity. Plant communities characterized by a network-like structure have been observed in various ecosystems. However, it is unknown if facilitation governs the structure of phryganic plant communities, which adult species are effective as facilitators, and which species are more dependent on facilitation. To answer this, we assessed the percentage of young plants under the canopy of adult species compared to open ground and the presence of a nested pattern, in a 0.136 km2 experimental site. Phryganic plant communities with Ziziphus lotus, in Cyprus, are characterized by a non-random, nested pattern exhibiting high NODF values (p<0.001). Ziziphus lotus, Thymus capitatus, and Noaea mucronata are the main facilitators in the community. Asparagus stipularis, Phagnalon rupestre, Noaea mucronata, and Sarcopoterium spinosum proved to be highly dependent on nurses. For such ecosystems to retain productivity and biodiversity, would be valuable to identify/promote keystone plant species that (i) have developed strategies to more efficiently utilize moisture resources not easily accessible and (ii) improve moisture conditions for neighboring plants. The very deep-rooted Ziziphus lotus, considered an ecosystem engineer, is one such example. However, it is not known which biotic traits: (a) canopy interception of moisture/rainfall, (b) hydraulic redistribution of deep ground moisture by roots, or non-biotic factors: (c) soil’s volume, and (d) organic matter content, Z. lotus activates/modulates to play such a role. We, thus, selected dryland ecosystems where the plant dominates and measured for potential effects on the less deep-rooted Thymbra capitata. For assessing impacts on ecosystem productivity, we measured the spatial aggregation of ca. 3600 T. capitata plants. As a proxy for soil moisture availability (SMA) and its spatial variability, we conducted a seven-year-long study using thymes’ nighttime rehydration. Sampling extended up to 15m away from Z. lotus. The density of T. capitata plants growing up to 5m around Z. lotus vs. thymes growing 10–15m away was found significantly increased (2.5 to 4.5 times), while their stem/leaf moisture was ca. 10% higher at predawn compared to nightfall during the dry season. This suggests that ecosystem productivity is driven by a greater SMA around Z. lotus permitting more thyme daytime transpiration, in contrast to thymes growing further away. The phenomenon appeared only under dry topsoil (during the dry season; becoming stronger during dry climatic years). Nor did morning dew/rainfall interception from the canopy or soil depth/organic matter show significant effects, leaving only the hydraulic lift (HL) properties of Z. lotus as the most likely driver for SMA. Density and stem moisture for thymes growing near Z. lotus do not seem to be significantly affected by topography. Thus, the deep-rooting properties and HL potential of Z. lotus may be the key to enabling it to boost ecosystem productivity. Knowing species interactions and the traits that form them will allow us to better understand how biodiversity in the phryganic communities is shaped. This way we will be able to apply this knowledge to restore species and ecosystem functions.