Phytogeographic origin determines Tropical Montane Cloud Forest hydraulic trait composition

Tropical montane cloud forests (TMCF) have unique climatic conditions,
which allow the coexistence of plant lineages with different
phytogeographic origins from tropical versus temperate climates. Future
climate projections suggest TMCFs will be subjected to increasing drought
stress due to fog uplift and higher temperatures, possibly leading to tree
mortality and local extinctions, and consequently changes in forest
composition and functioning. Characterising community functional
composition, trade-offs among traits and the drivers of community assembly
is of utmost importance to improve our capacity to predict the response of
montane plant communities to forecast climate change. Here, we aimed to
test if species from different phytogeographic origins (i.e. tropical -
evergreen x deciduous - and temperate) differ in drought vulnerability and
how the co-existence of these groups change the hydraulic composition of
TMCF`s. We used a framework based on measurements of key hydraulic traits
(i.e. xylem embolism resistance, hydraulic safety margin, stomata control,
turgor loss point, minimum water potential) of 16 dominant species
(> 70% of the forest basal area) within a TMCF in the Atlantic Rain
Forest Domain in southeast Brazil. We used community-weighted means to
model whether removing each species group would change the community
hydraulic functional composition. Temperate, tropical deciduous and
tropical evergreen groups differ in their hydraulic functioning and these
differences explain forest functional composition and taxa dominance.
Temperate and tropical deciduous taxa were consistently more vulnerable
hydraulically (i.e. lower safety margins and embolism resistance). The
coexistence of different phytogeographic lineages is a key determinant of
TMCF hydraulic composition. We also used models including phylogeny to
evaluate the variation of hydraulic traits across Phytogeographic groups,
and the results suggest some niche conservatism associated with plant
hydraulic functioning. Our results provide evidence of the importance of
species phytogeographic origin on TMCF functioning, and niche conservatism
in the evolution of hydraulic traits. The higher drought vulnerability
observed in temperate group might be a mechanistic explanation for the
expansion of temperate taxa distribution to wetter places during past
colder and drier climate. Thus, we suggest hydraulic functional traits may
be useful to predict future dynamics of TMCFs under changing climatic
conditions.