Ecology and responses to climate change of biocrust-forming mosses in drylands

Abstract

Interest in understanding the role of biocrusts as ecosystem engineers in drylands has substantially increased during the past two decades. Mosses are a major component of biocrusts and dominate their late successional stages. In general, their impacts on most ecosystem functions are greater than those of early-stage biocrust constituents. However, it is common to find contradictory results regarding how moss interactions with different biotic and abiotic factors affect ecosystem processes. This review aims to (i) describe the adaptations and environmental constraints of biocrust-forming mosses in drylands, (ii) identify their primary ecological roles in these ecosystems, and (iii) synthesize their responses to climate change. We emphasize the importance of interactions between specific functional traits of mosses (e.g. height, radiation reflectance, morphology, and shoot densities) and both the environment (e.g. climate, topography, and soil properties) and other organisms to understand their ecological roles and responses to climate change. We also highlight key areas that should be researched in the future to fill essential gaps in our understanding of the ecology and the responses to ongoing climate change of biocrust-forming mosses. These include a better understanding of intra- and interspecific interactions and mechanisms driving mosses' carbon balance during desiccation-rehydration cycles.

Keywords: Abiotic interactions; biological soil crusts; biotic interactions; bryophytes; global change; hydrology; microbial community; nutrient cycles; plant interactions; soil properties.

Fig. 1.

Example of distribution and biotic interactions of moss-dominated biocrusts in drylands. Cabo de Gata-Níjar Natural Park (A) is a Mediterranean coastal steppe ecosystem in southeast Spain and one of the driest sites in Europe. The long-term average rainfall is around 200 mm, but an important source of water for vegetation comes from fog and dew. This allows a well-developed grassland vegetation dominated by sparse tussocks of Macrochloa tenacissima with open spaces where mosses are abundant (B). A preferential microhabitat for biocrust-forming mosses is located under the canopy of these tussocks (C). In Mediterranean drylands is very common to find biocrusts where mosses coexist with lichens such as Fulgensia spp. (D) or Squamarina lentigera (E).

Fig. 2.

Effects of mosses on soil properties. Soil variables and processes increased and decreased by the presence of mosses are indicated by + and – signs, respectively. Created with BioRender.com.

Fig. 3.

The effects of mosses on the soil water balance are determined by water infiltration into the soil (A) and evaporative processes (B). Positive signs on biotic and abiotic variables indicate their positive correlations with the presence of biocrust-forming mosses in (A). The increase of the variables located in the soil profile on the left positively affects the infiltration rate, whereas those located on the right have a negative effect on this rate. The representation of mosses in (B) varies in colour to indicate the effect of radiation reflectance on evaporation. Created with BioRender.com.

Fig. 4.

Effects of mosses on nutrient cycles. An interaction worth investigating in moss-dominated biocrusts is highlighted in red. Created with BioRender.com.

Fig. 5.

Different moss–vascular plant interactions. In the germination rates section, biotic factors with positive (+) signs and an arrow pointing to the soil profile on the left indicate positive correlations with germination rate, whereas those pointing to the right indicate negative correlations with this rate. An interaction worth investigating in moss-dominated biocrusts is highlighted in red. Created with BioRender.com.

Fig. 6.

Loss of moss-dominated biocrusts driven by negative carbon balances in two climate change field experiments (Reed et al., 2012; X. Li, Hui et al., 2021) and in a mesocosm experiment (Tucker et al., 2019) in cool drylands; and increasing (but not significant) trends of moss cover in two field experiments in temperate–hot drylands (Escolar et al., 2012; Maestre et al., 2015; Ladrón de Guevara et al., 2018). Effects related to temperature are highlighted in red. Biotic and abiotic effects found or suggested in these experiments are shown. The effects of mosses on soil properties are still inconclusive in temperate–hot drylands as lichens are the dominant biocrust component of these studies. In addition, several physiological effects derived from the desiccation–rehydration cycles caused by the treatments are presented as complementary mechanisms that could explain the observed changes in moss cover. Created with BioRender.com.