THE CATASTROPHIC LOSS OF TORREYA-TAXIFOLIA - ASSESSING ENVIRONMENTAL INDUCTION OF DISEASE HYPOTHESES

The Florida torreya (Torreya taxifolia Arn.) has decreased to near extinction during the past 30 yr, allegedly as the result of a fungal disease induced by environmental stress. Increasing regional temperatures, micro-climatic warming, drought, and soil nutrient stress are among the environmental changes proposed as triggers for the decline of T. taxifolia. These putative disease inciting mechanisms were evaluated through historical observations of the magnitude of environmental change, as well as physiological and growth experiments used to assess the relative sensitivity of T. taxifolia to environmental change. Historical evidence of climatic warming, regionally or locally, is lacking. Likewise, physiological experiments failed to support the temperature change hypothesis. A drought, concurrent with the timing of the decline, was not notably severe and T. taxifolia was observed to be relatively drought tolerant. Greenhouse growth experiments demonstrated that low soil nutrition is not likely to limit growth in T. taxifolia. Another hypothesis states that fire suppression in the surrounding uplands triggered a disease epidemic. This hypothesis states that smoke deposition in ravines may limit foliar pathogen populations and that a chronic increase of foliar pathogens, associated with fire suppression, triggered the decline of T. taxifolia. Our experiments suggest that foliar pathogens may impose health risks to T. taxifolia and that one very common foliar fungal associate is susceptible to smoke. Decreased light levels, as a result of increased woody biomass in the absence of fire, may also increase plant stress, leading to increased disease incidence. While the low light levels characteristic of field situations limited T. taxifolia growth in the greenhouse, light level was not correlated with growth, or vigor, in the field. These results suggest that the fire suppression hypothesis is, at least, plausible and requires further examination.