Post-burning regeneration of the Chaco seasonally dry forest: Germination response of dominant species to experimental heat shock

Background/Question/Methods

Some seasonally dry forests of South America, such as the Chaco of central Argentina, do not have a fire history as long, frequent and/or intense as Mediterranean-climate ecosystems. However, some of these forests are experiencing more frequent fires as a consequence of climatic and land-use changes. The capacity of the dominant species in this ecosystem to regenerate from seeds after fire is poorly known. Presumably, plant species in this ecosystem have been under a lower evolutionary pressure than Mediterranean-climate species to developed specialized fire-response traits, such as heat-stimulated germination. Nevertheless, adaptations to other factors with long history in the system, such as herbivory, could have led some species to develop seeds with the capacity to tolerate heat shock, for example as a consequence of endozoochorous dispersion. Therefore, heat-tolerant germination rather than heat-stimulated or heat-sensitive germination should be more common. To test this hypothesis, the germination responses of 30 dominant species were classified as (a) heat-stimulated; (b) heat-tolerant; or (c) heat-sensitive. Seeds were exposed to heat treatments at 70ºC (1h), 100ºC, 120ºC, 180ºC (5min), and a control treatment (25ºC). Seeds were then incubated for 30 days. Influence of growth form (trees, shrubs, graminoids and forbs) and endozoochory on germination type was also tested.

Results/Conclusions

Seventeen of the studied species showed heat-tolerant germination, including all the native graminoids studied. Seven species showed heat-stimulated germination, although in general it was under the less intense heat treatment (70ºC). Only two species (Castela coccinea and Larrea divaricata) showed heat-sensitive germination. Four species did not germinate, either in the control or any of the heat treatments. Only four species were able to tolerate the 180ºC treatment, of which only the native tree Prosopis flexuosa and the exotic graminoid Cenchrus ciliaris exceeded 10% germination. Furthermore, growth form and endozoochory had no influence on the germination response of the studied species. These results suggest that most of the studied species can regenerate well from seeds after low intensity fires, but germination response can be severely reduced in the presence of higher-intensity fires. This supports the hypothesis of fire being a weaker evolutionary force in Chaco woodlands when compared to Mediterranean-climate ecosystems. This study is a contribution to the understanding of the role of fire in the evolution of Chaco plant species and of their dynamics in the face of increasingly frequent fires.