Crown Index: a canopy balance indicator to assess growth and regeneration in uneven-aged forest stands of the Coastal Range of Chile

A balanced diameter structure, i.e. similar crown area allocatedto the different cohorts in uneven-aged forest stands, has beentraditionally regarded as a requirement for achieving bettergrowth rates and regeneration. Long-term experiments have demonstratedthe sustainability of these structures, but quantitative datainforming on how growth and regeneration behave under a balanceddiameter structure compared with other structures have beenscarce. Similarly, vertical structure has not been properlyaccounted for as a variable that can affect growth and regenerationin uneven-aged stands. In this study a Crown Index (CI) wasused as a surrogate of a balanced diameter structure and anindicator of vertical structure. This new index is obtainedby dividing the canopy area of trees in the main and upper canopypositions in a stand by the canopy area in lower strata. A CIof 1.0 indicates, on average, an even allocation of canopy areato each of four strata, namely, a balanced structure; values>1.0 or <1.0 reflect unbalanced structures, with excesscanopy cover of large trees or medium and small trees, respectively.From data obtained from stands at relatively poorer and bettersites in uncut and partially cut old-growth stands in the CoastalCordillera of southern Chile, the effects of CI, proportionof large and small trees, canopy area, and basal area on diametergrowth of Laurelia philippiana Looser (Monimiaceae) –the most important species in these forests – and regenerationof all canopy tree species were tested. Results indicated thefollowing: the CI was most strongly correlated to the proportionof large and of small trees (i.e. structural indicator), andto a lesser extent to basal area (a density indicator); theCI was by far the best explanatory variable of growth at bettersites and of regeneration at poor sites; and, in general, greatergrowth and more seedlings occurred as CI values decreased towards1, which reflects a balanced diameter structure. Results supportthe value of balanced diameter structures in managed uneven-agedstands, but also suggest that alternative structures could besought.