Lianas explore the forest canopy more effectively than trees under drier conditions

Lianas rely on trees for support and access to high light positions in the
forest canopy, but the implications for how lianas explore the canopy
compared to trees remain understudied. We present an in situ forest canopy
study to test the hypotheses that: (1) lianas favour leaf display over
stem investment compared to trees and (2) lianas have greater potential to
colonize non-shaded, high-light areas effectively than trees. We compared
branches of 16 liana species with those of 16 sympatric tree species in
two lowland tropical forest canopies in Panama using 40-50 m tall canopy
cranes. One forest was relatively dry and seasonal in rainfall and in
associated light availability. The other forest was relatively wet and
evergreen. We observed that lianas were more efficient in leaf display
over stem investment than trees, particularly in the forest with lower
precipitation and stronger seasonality. Specifically, lianas had a lower
LMA (leaf mass per unit leaf area), stronger apical dominance, higher stem
slenderness and fewer leaf layers than trees. In the forest with higher
precipitation and weaker seasonality, lianas also had stronger apical
control and fewer leaf layers than trees, but both lianas and trees were
relatively similar in LMA and stem slenderness. Our study shows that
lianas more effectively explore the canopy than trees under drier
conditions, but much less so under wetter conditions. We argue that lianas
display a functional strategy that allow them to better intercept light
than the tree species in forests with low precipitation and strong
seasonality, while they are constrained to display such strategy at high
precipitation – light-limited – sites.