Is resprouting a persistence trait in tropical forests?

Author James Dalling discusses recent paper “Nutrient availability predicts multiple stem frequency, an indicator of species resprouting capacity in tropical forests” by Heineman, Turner and Dalling. Read on to find out more about variations in the resprouting ability of tropical trees.

The life of a tree is seldom the unobstructed path of a single stem from seedling to adult. Stuff happens. Limbs crash from the canopy, herbivores trample and browse, and stem-boring insects and fungal diseases kill off developing shoots. Most plants can stage a recovery from this damage via resprouting, thanks to energy and nutrient reserves that are stored in hard-to-access woody stem and root tissues. However, the frequency and mode of resprouting varies among species, which has implications for tree community assembly and forest recovery from disturbance.

*A multi-stemmed individual of Billia rosea (Sapindaceae), abundant in lower montane forest, Fortuna Forest Reserve, Panama. (Photo: Arturo Morris)*

Resprouting has mostly been viewed as a persistence strategy. After all, the resources that support resprouting are stored, rather than immediately used to power growth and reproduction. Species that exhibit high resprouting frequency are therefore expected to share other persistence traits, such as high wood density and shade tolerance. A hypothesized relationship between persistence and resprouting capacity also leads to the prediction that frequent resprouters should be more common where resources are more limited. In tropical forests, this includes sites where key soil resources such as phosphorus, are in short supply. An opposing argument, however, can also be made. The ability to sustain growth and store nutrients for potential future use requires ample soil resources. Plants that grow in low fertility soils may simply forgo storage, particularly if competition prioritises investment in growth.

Resprouting individual of Alseis blackiana (Rubiaceae) in lowland forest on Barro Colorado Island, Panama. (Photo: James Dalling)

We explored how resprouting capacity in tropical trees varies with life history and demographic traits, and with nutrient availability, at sites in the Panama Canal watershed and in lower montane forests in western Panama. These sites are underlain by a complex geology resulting in steep gradients in soil phosphorus availability and large variation in tree species wood and leaf phosphorus concentrations. The first challenge in this study was to develop a measure of resprouting capacity. Resprouting events are relatively rare and hard to capture over short periods. We used the 30-year census data from the Barro Colorado Island (BCI) 50 ha plot to show that resprouting frequency over the long term is strongly associated with the occurrence of multiple stemmed individuals in a population.

*Lower montane forest, Palo Seco Forest Reserve, Panama. (Photo: James Dalling)*

Our study showed that multiple stem frequency is highly variable among tropical tree species, with no evidence that it aligns with classical persistence traits of slow growth, low mortality, and high wood density. At the montane sites, variation in multiple stem frequencies among species were significantly positively correlated with foliar and wood phosphorus. At the community level, we also found that multiple stem frequency was strongly positively correlated with soil nutrient availability, even when including secondary and old-growth forest. Our results point to potentially distinct relationships between resprouting and resource availability for dry versus wet and infertile habitats, and highlights the importance of not just considering carbohydrate storage, but also nutrient storage in woody tissues in supporting regrowth and reproduction. Finally, allocation of hard to acquire resources, like phosphorus, into storage rather than immediate growth may help explain variation in stand-level forest growth rates across soil fertility gradients.