roots to whole plant functioning. We present three causal models,
each one with a different causal structure. Model 1 states that soil
compaction affects directly and independently different variables:
root morphology (SRL) and anatomy (xylem vessel proportion and
xylem vessel diameter); plant physiology (transpiration rate, the
latter affecting the photosynthesis rate) and plant area (this affecting
the plant biomass). This model was rejected, as the data did
not agree with the proposed causal relationships (P = 0.021, Fig. 5
and Table S3 in Supplementary Material). Model 2 states that soil
compaction affects directly root variables: root morphology (SRL)
and anatomy (xylem vessel proportion and xylem vessel diameter)
but not plant physiology variables, which are independent of
the changes in root characteristics caused by soil compaction. This
model was also rejected (P = 0.014, Fig. 5 and Table S3 in Supplementary
Material).
In contrast, model 3 was accepted (P = 0.777, Fig. 5 and
Table S3 in Supplementary Material). In this model we consider
that soil compaction produces changes in the root traits that have
consequences on plant physiology and growth. Soil compaction
has an effect on root morphological variables, such as a decrease
in SRL (specific root length). SRL positively affects the transpiration
rate and leaf water potential (less negative). Transpiration rate
affects positively the photosynthesis rate. Soil compaction also has
an effect on root anatomy variables, decreasing the xylem vessel
proportion. Xylem vessel proportion has a positive effect on xylem
vessel diameter, which influences positively the plant area and this
affects positively the plant biomass. Therefore, plant biomass can