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

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Abstract
A balanced diameter structure, i.e. similar crown area allocated to the different cohorts in uneven-aged forest stands, has been traditionally regarded as a requirement for achieving better growth rates and regeneration. Long-term experiments have demonstrated the sustainability of these structures, but quantitative data informing on how growth and regeneration behave under a balanced diameter structure compared with other structures have been scarce. Similarly, vertical structure has not been properly accounted for as a variable that can affect growth and regeneration in uneven-aged stands. In this study a Crown Index (CI) was used as a surrogate of a balanced diameter structure and an indicator of vertical structure. This new index is obtained by dividing the canopy area of trees in the main and upper canopy positions in a stand by the canopy area in lower strata. A CI of 1.0 indicates, on average, an even allocation of canopy area to each of four strata, namely, a balanced structure; values >1.0 or Laurelia philippiana Looser (Monimiaceae) – the most important species in these forests – and regeneration of all canopy tree species were tested. Results indicated the following: the CI was most strongly correlated to the proportion of large and of small trees (i.e. structural indicator), and to a lesser extent to basal area (a density indicator); the CI was by far the best explanatory variable of growth at better sites and of regeneration at poor sites; and, in general, greater growth and more seedlings occurred as CI values decreased towards 1, which reflects a balanced diameter structure. Results support the value of balanced diameter structures in managed uneven-aged stands, but also suggest that alternative structures could be sought.