Protecting biodiversity is key to maintaining the health and resilience of ecosystems in times of global change. Although soil organisms form the most diverse community on land and are the driving force behind almost all ecosystem processes, are rarely taken into account in nature conservation plans. The new study sheds light on a critical difference between aboveground and belowground biodiversity, specifically between vascular plants and mycorrhizal fungi, which has fundamental implications for conservation strategies.

Mycorrhizal symbioses are of great importance, occurring in over 90 % vascular plant species and having a significant impact on ecosystem productivity and function. However, until now, there has been no comprehensive global overview of the relationships between plant and fungal diversity and how they vary across scales.

Correlation Relationships Across Scales

The researchers examined the strength and direction of the relationships between plant diversity and arbuscular mycorrhizal (AM) fungi, as well as ectomycorrhizal (ECM) fungi, across different spatial scales—global, biome, and ecoregional. They found that correlations between plant and fungal diversity were weak at the global level. For AM fungi, the global correlation was r = 0.4, and for ECM fungi it was even weaker, r = 0.04.

However, the situation changes at smaller spatial scales:

• Regional correlations are stronger and can be both positive and negative.
• Correlations between plants and AM fungi are generally negative in forest biomes (such as tropical moist forests and temperate deciduous forests) and positive in grassland biomes (such as tropical grasslands, temperate grasslands, montane grasslands, boreal forests, tundra, and deserts).
• Correlations between plants and ECM fungi are mostly positive or neutral, being significantly positive in all biomes except tropical coniferous forests.

These findings suggest that plant diversity may be an adequate indicator of mycorrhizal fungal diversity only in specific situations and locations.

Guiding Mechanisms of Diversity Correlations

The study examined three main hypotheses about what influences correlation patterns of diversity:

1. Effects of symbiosis: It is hypothesized that interrelationships can lead to a "coupling" of diversity, where an increase in the diversity of one group directly leads to an increase in the diversity of the other. The study found that relationships between AM fungi and plants were stronger and more positive as between ECM fungi and plants. This is likely because most vascular plants (72 %) host AM fungi, while only 2 % are hosts to ECM fungi. However, the hypothesis that higher abundance of host plants would lead to stronger diversity coupling was not confirmed.
2. Environmental impacts: If plants and microorganisms respond differently to the environment, negative correlations can arise. It was found that correlations were more often positive when plant and fungal diversity responded to temperature in the same direction, and negative when they responded differently. For example, in tropical moist forests and temperate deciduous forests, AM fungal diversity was positively related to soil pH (higher diversity in more alkaline soils), while plant diversity was negatively related, which could lead to negative correlations.
3. Legacy effects: Long-term evolutionary patterns or recent disturbances can influence diversity relationships. The study confirmed that correlations between plants and AM fungi were more positive in regions with historically more stable climatesHowever, no significant relationship was demonstrated between the correlations and human activity.

Diversity Hotspots Overlay

One of the key findings of the study is significant discrepancy in the location of plant and fungal diversity hotspotsHotspots are defined as areas with diversity values in the upper 95th percentile.

• Only 8.8 % hot spots AM mushrooms a 1.5 % Hotspot ECM Mushrooms overlaps with plant hot spots.
• Global plant diversity hotspots are concentrated in the tropical and subtropical forests of Central and South America, Central Africa, Madagascar, and Southeast Asia.
• AM fungal hotspots are also predominantly in tropical regions such as the Brazilian Cerrado, the Guinean forests of West Africa, and Southeast Asia.
• However, major ECM fungal hotspots are distributed predominantly in temperate and boreal forests in Canada, the northwestern USA, central Europe, Russia, and eastern Asia.
• The main areas of hotspot overlap occurred in Central America and Southeast Asia for both types of fungi.

Implications for Nature Conservation

The limited extent of overlap of plant and fungal diversity hotspots clearly suggests that Plant hotspots are inadequate indicators for mycorrhizal fungal hotspots in most situationsFocusing conservation decisions exclusively on above-ground plant communities may overlook the most diverse areas of mycorrhizal fungi, which are critical for ecosystem functioning.

This strongly supports recent calls for policymakers to better incorporate fungi and other microorganisms when designing conservation and management strategies. Given that relationships between plant and fungal diversity are highly context-dependent and influenced by a range of interactive ecological mechanisms, Effective protection of belowground biodiversity is likely to require specific management considerationsFuture work should focus on investigating relationships between other types of plant and fungal diversity measures, such as beta diversity, gamma diversity, phylogenetic diversity, rarity and endemicity, to gain a more comprehensive picture of the links between aboveground and belowground biodiversity. JRi