The seasonal growth advantage hypothesis (SGAH) posits that plant species abundance can be explained by their ability to grow during seasonally stressful periods (e.g., seasonal droughts). Greater growth during seasonally stressful periods results in more months of extra growth compared to co-occurring species than lack this ability. If so, the abundance of stress-adapted species relative to stress-tolerating or stress-avoiding species is expected to be higher in areas of greater seasonal stress. We tested the SGAH with lianas and co-occurring trees in a tropical forest in Panama. Pan-tropically, liana density increases with the severity of seasonal drought compared to co-occurring trees, and the SGAH may explain this pattern. However, whether lianas consistently grow more than trees during droughts is poorly understood. Previous studies were limited in their ability to compare liana and tree seasonal growth because they measured only juvenile plants or short-term seasonal physiological responses of larger plants, and no study has examined how co-occurring liana and tree growth is affected by strong el Niño drought. We measured the dry season and wet season diameter growth of > 1800 large trees and > 800 large lianas from 2011-2016, which included the third strongest el Niño drought ever recorded (2015-2016).
Results/Conclusions
We found that lianas grew considerably more during the dry season months than the wet season months in each of the five years, and that lianas achieved a full half of their annual growth during the 4-month dry season. By contrast, trees grew far more during the wet season, and they achieved only a quarter of their annual growth during the dry season. During the strong 2015-2016 el Niño drought, tree stopped growing during the dry season, whereas lianas grew unimpeded and as well as during any of the previous four years. Our findings support the hypothesis that dry season growth gives lianas a decided advantage over trees in seasonal forests, and may explain the unique pan-tropical distribution of lianas, which peak in abundance in highly seasonal tropical forests. Furthermore, the ability of lianas to grow during strong el Niño drought indicates that lianas will benefit from predicted increasing drought severity compared to trees. By contrast, our data indicate that trees will suffer with increasing drought severity, and thus are predicted to decrease in abundance relative to lianas, especially in highly seasonal forests.