Data from: Ectomycorrhizal fungi and root water uptake respond independently to water availability

Temperate forests on their warm and dry distribution limits are expected
to be most vulnerable to reductions in water availability. This prediction
is mostly based on studies assessing single forest functions, mainly
growth. Water and nutrient cycling are functions that rely on tree roots
and their symbiotic association with ectomycorrhizal (ECM) fungi. Trees
can compensate for seasonal reductions in water availability by shifting
root water-uptake (RWU) towards deeper soil layers, but ECM fungi mostly
dwell in the topsoil, thus suffering from desiccation and compromising
nutrient uptake. We hypothesised that drier sites should depict larger
seasonal shifts in RWU, but at the expense of lower ECM fungal diversity
and colonization of fine roots by ECM fungi. We selected three beech
(Fagus sylvatica) forests in their warm distribution limit with
contrasting geographic locations and mean annual precipitation: northern
Atlantic (2500mm), intermediate transitional (1150mm), and southern
Mediterranean (780mm). We collected soil, stem, and root samples in spring
(wet) and summer (dry) to: (1) quantify fine-root density and colonization
by ECM fungi, (2) infer RWU from isotopic composition of plant and soil
water, and (3) characterize ECM communities through DNA-metabarcoding.
Generalized and linear mixed models revealed that high topsoil moisture
benefited ECM diversity, but higher diversity and ECM colonization did not
imply larger contributions of the topsoil to RWU. The prevailing climate
and abiotic conditions determined how ECM communities were structured,
more than seasonal climatic variability. Across sites, communities
differed in their functional diversity: ECM fungi with long hyphae, more
vulnerable to water scarcity, dominated at the southernmost site, where
water availability was the highest. Our results suggest that, in a climate
change scenario, increasing drought might not compromise RWU, but it would
still be detrimental for ECM communities, compromising key ecosystem
services such as nutrient cycling and productivity.