Influence of crown class and shade tolerance on individual tree development during deciduous forest succession in Connecticut, USA

Sixty years of individual tree crown class records were used to elucidate the influence of crown class (dominant, codominant, intermediate, or suppressed), shade tolerance (intolerant, midtolerant, or tolerant), and their interactions on the probability of individual tree movement among crown classes. Trees were measured at 10 year intervals between 1927 and 1987, excluding 1947, on 364 nominal 0.01 ha plots. A total of 14 154 individual tree records were used in this analysis. The transition rates among crown classes for 30 year intervals (1927–1957 and 1957–1987) were examined for all combinations of antecedent crown classes and tolerance rankings. The distribution of ingrowth among tolerance rankings was also examined. Mortality rates increased with decreasing crown class for all tolerance rankings, and mortality rates increased with decreasing tolerance. Compared with tolerant trees, midtolerant and intolerant trees had higher rates of ascension into dominant and codominant crown classes and exhibited higher persistence rates in the dominant crown class. These factors suggest that midtolerant and intolerant trees have an advantage over tolerant trees in the higher crown classes, In contrast, tolerant trees had the advantage in suppressed and intermediate crown classes, with lower mortality rates, higher persistence, and higher rates of crown class ascension than for midtolerant or intolerant trees. Crown class stratification was driven by the change in relative advantage of each tolerance ranking among the crown class. Results of this study suggest that the canopy stratum (upper canopy, lower canopy, or mixed) affected by disturbance is as important as disturbance scale, intensity, and frequency in influencing the composition of the suppressed crown class. Because the suppressed crown class is in a constant state of high flux, with fewer than 30% of the suppressed trees remaining in the suppressed crown class for any 30 year period, a small change in the relative persistence or ascension rates among tolerance rankings, whether by a different disturbance or climate regime, could alter the proportion of tolerance rankings in the suppressed crown class and ultimately result in an alternative succession.