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Highlights • Different types of stress gradient may favour different types of biotic interactions (e.g., facilitation or competition); elevational gradients are typically related to changes in temperature. • We investigate an elevational gradient on La Palma, Canary Islands, where harshness inversely correlates with elevation because of aridity, using a novel approach combining a co-occurrence and network analysis. • We find a decrease of positive co-occurrences with increasing harshness, and different patterns for species richness and abundance. • Co-occurrence patterns and their drivers appear to differ depending on whether temperature or aridity determine the underlying harshness gradient. • Addressing the effects of harshness on co-occurrences helps to improve general hypotheses about biotic interactions.

<p>Plant water use during drought depends on atmospheric demand and soil water supply. Typically, transpiration response to drought is evaluated in two types of experiments: either exposure to a stepwise increase in vapor pressure deficit (VPD), or exposure to soil drying over the course of weeks. Surprisingly however, the extent of similarities and differences of the underlying mechanisms remains poorly documented. This hampers progress towards breeding for well-adapted crops targeting environments with high VPD, high risk of soil moisture deficit, or both. We present an extensive review of the two experimental approaches and use a soil-plant hydraulic model to simulate transpiration responses to both environmental drivers. Existing experimental results lead to contradicting results regarding the role of the plant hydraulic conductance for the transpiration response to atmospheric drying vs. to soil drying: a high plant hydraulic conductance triggers an earlier transpiration decline (i.e. in wetter soil conditions) during soil drying; but enables plants to sustain transpiration at high VPD. A hydraulic framework hypothesizing that transpiration responds to a decline in soil-plant conductance helps to explain the contradiction. At high VPD, water potential gradients mainly develop within the plant, and thus it is the plant hydraulic conductance that limits the water flow during atmospheric drying. During soil drying, the gradients develop in the soil, and thus the soil hydraulic conductivity controls the flow. The plant hydraulic conductance is expected to impact the plant’s sensitivity to the development of water potential gradients around the roots that occurs during soil drying. Thus, stomatal closure and hence transpiration response is related to a drop in hydraulic conductivities in both scenarios but the relevant hydraulic traits differ between the two environmental changes in a predictable way. Such a finding could better guide breeding efforts targeting adaptation to specific drought regimes.</p>

<p>The spatial arrangement of the soil surrounding the root can improve plant resource acquisition under drought and is closely related to the fate of soil organic carbon (SOC). Thus, the formation of soil structure and the establishment of a stable rhizosheath can potentially improve plant drought resistance and contribute to maintained crop yields during drought events. Yet, soil structure formation is a complex process determined by the interaction between various functional plant and soil properties, such as the soil (micro)biome, root exudation, or root morphological characteristics. To date, it is not understood how water scarcity affects soil aggregation in the vicinity of roots, by which functional traits these drought effects can be modified, and how this feedbacks on the cycling of SOC. </p> <p>Thus, we investigated drought effects on rhizosheath properties and their link with functional plant traits. We conducted a greenhouse experiment with 38 maize varieties where half of the plants were grown under optimum moisture, while the second half of replicates were subjected to drought stress after an initial establishment phase. For each plant, the rhizosheath soil was sampled and its aggregate size distribution, carbon (C) and nitrogen (N) content, and the proportion of newly maize-derived C were analysed via natural abundance <sup>13</sup>C. In addition, we recorded functional plant and rhizosphere traits, such as morphological and chemical root properties, microbial enzyme activities, and plant biomass.</p> <p>Drought-stressed plants formed lower amounts of rhizosheath, with a decreased physical aggregate stability and increased concentrations of SOC, N, and newly maize-derived C. Furthermore, under drought larger proportions of the elements were allocated into the microaggregate fractions. In particular, maize-derived C, along with N, accumulated under drought stress in the smaller aggregate size classes of the rhizosheath. Maize varieties forming larger amounts of roots under drought stress tended to maintain higher macroaggregate stability in the rhizosheath. In contrast, cultivars that invested little in root growth but promoted higher microbial enzyme activities in the rhizosheath and maintained root N contents under drought were associated with a strong accumulation of maize-C and N in the smaller aggregate size classes. </p> <p>Trait-based experimental approaches, such as the one presented here, are deepening our mechanistic understanding of drought effects in the crop rhizosheath and can thus help to guide future crop selection for improved drought resistance.</p>