Responses to canopy openings in architectural development of saplings in eight deciduous broad-leaved tree species

Crown-architectural responses to small canopy openings, in relation to relative height growth rate (RHGR), were examined in saplings (0.5-2.3 m tall) of eight deciduous broad-leaved tree species in northern Japan. Five species were lateral-growth type with high leaf-support cost (branch mass per leaf area) to display many small leaves over a wide area, while three species were vertical-growth type with low leaf-support cost due to a small number of large leaves along a main trunk. In closed-canopy conditions, the lateral-growth species showed a greater increase in leaf area with increasing sapling size than the vertical-growth species. Net production rate (NP) per leaf area was not clearly different between the two groups. As a result, NP per sapling also increased to a greater extent with increasing sapling size for the lateral-growth species. The NP per leaf area was increased in canopy openings for the two groups, but this increase was not clearly distinguished between the two groups. The vertical-growth species supported greater leaf area per sapling in canopy openings because of the low cost of leaf support, which resulted in a greater increase in NP per sapling compared with the lateral-growth species. The lateral-growth species allocated more to trunk in canopy openings, keeping leaf area constant and decreasing the cost of leaf support. The RHGR of the verticalgrowth species tended to be greater than that of the lateral-growth species in closedcanopy conditions. RHGR of all species was increased in canopy openings to a similar degree in both groups. Multiple regression analysis showed that RHGR of the lateralgrowth species depended on both NP per sapling and allocation to trunk, while that of the vertical-growth species depended strongly on NP per sapling alone. Thus, saplings of each group responded to canopy openings in contrasting ways to increase RHGR.