Leaf defenses have been the focus of ecological, evolutionary, and biogeographical studies for decades. For example, several studies have found higher levels of leaf defenses at low latitudes, perhaps due to a combination of increased resource availability and increased herbivore pressure. However, other forms of defense, such as floral defenses, have only gained interest more recently, despite the fact that floral defenses can be just as common as leaf defenses and frequently occur at much higher levels. While several studies have started to look at the presence of floral defenses, their effects on potential floral antagonists, and their ability to be induced, the field of floral defense ecology is still fairly new. In this study, we investigated patterns in floral defenses across geographical clines and compared them to leaf defenses to test for tradeoffs in various plant defense strategies across latitude. We collected leaves and flowers from Chamaecrista fasciculata plants at nine sites across latitudes from Minnesota to Texas and extracted their condensed tannin defenses.
Results/Conclusions
We found that plants had higher levels of leaf condensed tannins at lower latitudes, which agrees with previous studies on the topic. Additionally, we found that floral condensed tannins were also higher at low latitudes, suggesting that there is no tradeoff between chemical defenses in flowers and leaves. Frequently, both defenses were also correlated with some measure of plant size, suggesting condensed tannins may be more common in plants with higher resource availability, such as in productive lower latitudes. However, extra floral nectaries (EFNs), which are used to attract ant defenders that protect both flowers and leaves, were larger at higher latitudes, suggesting there may be a tradeoff between chemical defenses and indirect defenses in Chamaecrista fasciculata. This may mean that EFNs are cheaper to maintain in relatively resource poor northern climes, or that plants have responded to local variation in ant presence or other environmental characters when adapting their defense strategies. To our knowledge, this is the first study looking at floral defenses or EFNs across latitudinal clines. Such studies may become increasingly important to understand how plants interact with their antagonists across geographic space as climate change alters plant and animal phenologies and ranges.