Testing two measures of shade tolerance in a mesic forest in southeast Texas

Abstract

I used sapling demographic data to investigate the relationship between shade tolerance and parameter estimates of a mortality-growth model and a height-diameter model. The study site is at Wier Woods, a mesic forest in southeast Texas. The results show that species order for probability of mortality at zero growth corresponds closely to the standard shade tolerance classification: the probability of mortality at zero growth decreases as shade tolerance rank increases. Also, the probability of mortality decreases rapidly as growth increases for shade-intolerant species, while showing little variation for shade-tolerant species. Therefore, this study provides strong support for the assertion that the mortality-growth relationship is a key life-history characteristic that determines shade tolerance. The results of a linear regression of height against DBH show that shade-intolerant species have steeper slopes than shade-tolerant species. This implies that a trade-off of photosynthate allocation between height growth and diameter growth may be an additional mechanism that influences shade tolerance.