Tree differences in primary and secondary growth drive convergent scaling in leaf area to sapwood area across Europe

Trees scale leaf (A(L)) and xylem (A(X)) areas to couple leaf transpiration and carbon gain with xylem water transport. Some species are known to acclimate in A(L) : A(X) balance in response to climate conditions, but whether trees of different species acclimate in A(L) : A(X) in similar ways over their entire (continental) distributions is unknown. We analyzed the species and climate effects on the scaling of A(L) vs A(X) in branches of conifers (Pinus sylvestris, Picea abies) and broadleaved (Betula pendula, Populus tremula) sampled across a continental wide transect in Europe. Along the branch axis, A(L) and A(X) change in equal proportion (isometric scaling: b similar to 1) as for trees. Branches of similar length converged in the scaling of A(L) vs A(X) with an exponent of b = 0.58 across European climates irrespective of species. Branches of slow- growing trees from Northern and Southern regions preferentially allocated into new leaf rather than xylem area, with older xylem rings contributing to maintaining total xylem conductivity. In conclusion, trees in contrasting climates adjust their functional balance between water transport and leaf transpiration by maintaining biomass allocation to leaves, and adjusting their growth rate and xylem production to maintain xylem conductance.