Short-Term Canopy and Long-Term Effects Soil of a Native Shrub Under Arid Bioclimate: A Case Study From Tunisia

Interactions between plants (competition and facilitation) in terrestrial ecosystems include: (1) short-term effects primarily quantied with experimental removals; and (2) long-term effects primarily quantied with observational methods. This study, conducted in the National Park of Bou Hedma, examined (1) the relative contributions of short-term canopy and long-term soil effects of a shrub species in explaining differences in biomass, species diversity (richness) and species density of understory plants (i) between shrubs and open areas, (ii) between shrubs and removed shrubs; and (2) the role of grazing in driving changes in direction of short-term and long-term effects in shrub/understory species interactions. Differences in species richness, density and biomass of understorey communities between shrub removed and open areas were mostly due to long-term soil effects, whereas differences beneath shrubs and shrub removed were due to short-term canopy effects, in particular on soil water content. Our study provides the positive effect of savannas shrubs on the understorey vegetation biomass, species density and diversity in arid ecosystems. Additionally, grazing by large herbivores negatively inuenced the dynamics of vegetation under an arid bioclimate.


Introduction
Interactions between plants plays a key role in de ning community structure and dynamics, and regulating and determining the composition, functioning and productivity of plant communities and ecosystems (Brooker 2006;Craine and Dybzinski 2013). Interactions between neighbours in plant communities range from competitive, where interacting species limit each other's performance, to facilitative, where at least one of the interacting species bene ts (Callaway and Walker 1997).
Interactions are due to the habitat-modifying capacity of nurse plants, which can alter its environment both above ('canopy effect') and below-ground ('soil effect') (Gomez-Aparicio et al. 2005). Co-existing plants may interact and compete for light, nutrients, water, and space, but, at the same time, protect each other from stress such as herbivore impact, potential competitors or extreme environmental uctuations, and/or provide additional resources through canopy leaching, microbial enhancement, decomposition, and mycorrhizal networks. Neighbours have short-term effects (STE) on other species, acting at a timescale from less than a day to a season, and mostly due to resource consumption (light, water, nutrient) or to the alteration of direct non-resource factors (microclimate, high irradiance, salinity, disturbances) by a living neighbour. Organisms are also known to have long-term effects (LTE) on other species through ecosystem-engineer processes, acting on a scale of up to several hundreds of years, and including sediment trapping in marine (and dune) systems or soil weathering in terrestrial ecosystems (Michalet 2006; He and Bertness 2014).
Most arid ecosystems have been grazed or are currently grazed by large introduced herbivores, suggesting that any indirect plant interaction mediated by herbivores should have a strong and obvious impact on a large scale (Illius and O'Connor 1999). Therefore, although abiotic grazing refuges (e.g. boulders, dead branches) provide better plant protections than biotic refuges (Milchunas and Noy-Meir 2002), notably because the outcome of plant-plant interactions is highly variable ).
Under the arid bioclimate of Tunisia, shrubs and trees have a strong capacity to modify soil properties with increasing soil organic matter, improving the soil structure, sequestering C and assisting in nutrient cycling (Noumi et al. 2012;Noumi 2015;Abdallah et al. 2016). In addition to the uptake of soil resources such as water and nutrients, shrubs may decrease the evaporative demand by shading (Maestre et al. 2003 Muhamed et al. 2013) and increases soil moisture through reduced evapotranspiration (Holmgren et al. 1997). Negative STE concern competition for the main resources (light, nutrient and water) and occur under a variety of climatic conditions depending on the resources and the species functional strategies The main aim of our study was: (1) to assess the relative contribution of short-term and long-term canopy vs. soil effects in shrub/understory species interactions in an arid savanna ecosystem of central-south Tunisia; (2) to assess the role of grazing in driving changes in direction of short-term and long-term effects in shrub/understory species interactions.

Study area and vegetation
The study site is located in the Bou Hedma National Park (348°390 N, 9°480 E, southern Tunisia). The park covers an area of 5115 ha. Climate is Mediterranean lower arid in the nomenclature of Emberger (Le

Experimental design
The experiments were conducted from October 2017 to October 2019. In order to study plant interactions occurring during the recruitment of A. tortilis, we chose a pioneer shrub, Lycium shawii, as the nurse in this study. L. shawii is a species of thorny shrub adapted to desert environments, and can be found throughout the Arabian peninsula and in Africa. The thin leaved, rigid bush grows to 3 meters high, with many branches and alternating spines along the branches and on their tips that vary in size. The leaves narrow towards their base. It produces small whitish-pink or purple owers from September until April, and red pea-sized seedy berries that are edible by large introduced herbivores of the Park plants often growing nearby include Acacia tortilis and Prosopis cineraria.
In order to assess the relative importance of the canopy and soil effects of L. shawii on herbaceous understorey species, we randomly selected 8 experimental plots (50 m x 50 m). Half of the plots was randomly chosen and fenced to exclude large herbivores using 2 m high fences (grazing treatment) with a mesh size of 50 × 50 mm. To assess biotic interactions with both the removal and observational methods, we created a patch treatment within each plot by selecting ten individuals of Lycium shrub and ve naturally open areas. For ve shrub plants the above-ground parts were cut at ground level. The basic design consisted of planting A. tortilis in three conditions of the patch treatment (Open, Lycium and Lycium removed).

Species sampling and environmental variables
During October 2018, 12 months after shrub removal, all plants growing in each plot were collected and identi ed to the species level. Plant material was dried at 70°C for 48 h and weighed. Aboveground biomass was recorded for each species in each quadrat and total aboveground biomass and species richness were calculated per quadrat (50 × 50 cm). Three soil variables were analyzed: oxidizable soil organic matter, which was determined by the Walkley-Black procedure (Nelson and Sommers 1982); extractable phosphate and total nitrogen, which were determined by Olsen's bicarbonate extraction (Olsen and Sommers 1982 -STE; relative difference in target performance between shrub-control and shrub-removed plots.
-LTE; relative difference in target performance between shrub-removed and open plots.

Statistical analyses
The effects of our factors on survival of transplants, biomass, richness, density and environmental variables (soil water content and soil nutrient OM, TN and Extractable P) were assessed with a two-way ANOVA model. All univariate analyses were done using JMP software 10.0 (SAS Institute, Cary, N.C.).
Tukey's HSD tests were used to determine the signi cant differences between treatment means.

Results
There were signi cant effects of patch and grazing treatments on survival of Acacia transplants (Table 1; Fig. 1). Grazing had a signi cant negative effect on survival of Acacia transplants. One year after transplantation, the survival rate of Acacia transplants in ungrazed areas was twice higher that in grazed areas (59% and 25%, respectively).  Fig. 1). After one year, survival rate was 48% in the Lycium patch, 55% in the Lycium removed and 75% in the open patch.
In order to distinguish the effect of Lycium shrubs in the survival rate of A. tortilis we calculated the RII using two methods (observational vs. removal). The RII values were always negative, which again highlights that Acacia was always negatively affected by neighbours (see Tukey test in Fig. 2). Overall, grazing reduced the intensity of competition. Moreover, there was a highly signi cant method effect wither higher intensity of competition with the observational method than with the removal method (see Tukey test in Fig. 2).
In this study, we compared the relative LTE and STE soil vs. canopy effects of nurse plants on species diversity (richness), density and biomass. In ungrazed plots, statistical analyses of vegetation parameters showed a highly signi cant effect of patch (see Tukey test in Fig. 3). A similar trend was detected in grazed plots. Overall, our results showed lower vegetation parameters in open patches compared to Lycium and Lycium removed patches. Even if the effect of Lycium shrubs on species diversity and density was relatively low, our results showed a signi cant effect of shrubs on biomass. These differences can be attributed to differences in soil moisture and fertility.
The overall ANOVA of soil attributes showed signi cantly higher concentrations of all soil nutrients in the Lycium removed patch compared with open areas and the Lycium patch (see Tukey test in Fig. 4).There were strong differences in soil characteristics among grazing treatment (see Tukey test in Fig. 3). The soil organic matter content (OM), the soil nitrogen content (N) and the extractable phosphorus (P) was signi cantly higher in the ungrazed soils compared to grazed one.
One day after the rain, we did not observe any difference between different treatment (Fig. 5). Seven days after a fall of 40mm of rain, soil water was signi cantly higher in the Lycium patch, less in the Lycium removed patch, with lowest values in the open patch (see Tukey test in Fig. 5, P < 0.05). This tendency remained constant for 10 days in grazed and ungrazed plots. Finally, 15 days after the rainfall event, neither patch nor grazing had any effect on soil moisture values.

Discussion
Our main objective was to assess rstly the relative contribution of short-term and long-term canopy vs. soil effects in shrub/understory species interactions in an arid savanna ecosystem of central-south Tunisia, and secondly the role of grazing in driving changes in direction of short-term and long-term effects in shrub/understory species interactions.
The LTE were stronger than STE for the Lycium shrub. The LTE on species density and richness were more strongly positive (facilitative) than STE. STE on biomass were strongly positive effect. Facilitation evidenced by the positive LTE was associated with habitats with higher soil organic matter and nutrient content, but the positive STE was associated with soil water content. The most important differences between open areas and shrubs were in soil moisture, but the removal treatment displayed the highest values of soil nutrient content (total N and P, organic matter  Michalet et al. 2015). However, few have separated LTE from STE, to assess their relative contribution to understory performances, as done here. Contrasting results have been observed, but these mainly depend on climatic conditions. In arid southern Australia Weedon and Facelli (2008) found that positive LTE of a chenopod shrub were overwhelmed by negative STE. In contrast, Anthelme and Michalet (2009)

Conclusions
Our results con rmed the positive effect of L. shawii on the understorey vegetation biomass, species density and diversity in arid ecosystems. Shrubs have been used and studied as nurse plants in degraded rangeland in Kenya, in Mediterranean open woodlands, in the semi-arid subtropical Andes, in Californian woodlands (Callaway, 1992) and in the Sonoran Desert (Carrillo-Garcia et al. 1999). The separation of nurse-shrub species LTE from their STE demonstrates that positive LTE (i.e. facilitation) are due to increased soil nutrients in the shrub patches and the positive STE are due to increase in soil water content.
Declarations Figure 1 Survival rate (means ± SE) of the target species in the three patches (Lycium, Lycium removed and open) of the ungrazed and grazed plots. Capitals letters represent results of Tukey's HSD tests for the patch treatment (P < 0.05) and lowercase letters between bars are results of the grazing treatment Variations of the RII of the nurse-target species interaction along the grazing treatment using the two methods (observational versus removal) after the rst measurements (October 2018)