# Functional ecology

Functional ecology is a field of ecology that has grown considerably in the past two decades. Functional ecology can offer a more mechanistic and predictive understanding of how traits are related with demographic tree patterns such as survival, growth and mortality rates. This approach is based on the study of functional traits. A functional trait is defined as any morphological, phenological, or physiological features of an individual that affect the growth, survival, reproduction, and fitness of that individual (Violle et al. 2007).

#### Functional traits in trees and forest

There is growing consensus that wood density (WD), the maximum height of adult trees, the specific leaf area (a measure of the surface produced to intercept the solar radiation per unit of dry weight invested in the construction of the leaves) and seed volume are the functional traits that best explain demographic attributes such as tree growth and mortality rates (Chao et al. 2008, King et al. 2006, Poorter et al. 2008). WD is the most functional trait analyzed because it is easy to measure and is strongly correlated with growth rates (lower density and, therefore, lower construction cost per unit xylem volume was associated with high growth rates) and mortality rates (Poorter et al. 2008).

#### Wood density as a functional trait

WD also has a positive relationship with some mechanical properties of wood (van Gelder et al. 2006), such as modulus of elasticity (MOE), modulus of rupture (MOR), toughness and hardness. High MOE values indicate that a tree can maintain its structural integrity with a low risk of mechanical failures. Also, MOE affects self-supporting capacity as it influences the critical height of trees (Fournier et al. 2013). Woods with high MOR and MOE values have a high breaking strength, which reduces the risk of buckling. This is critical for the mechanical integrity of the stems under variable loads, such as falling branches and trees, liana colonization of tree crowns and wind speed (King et al. 2006). Toughness is important because dynamic loads can change substantially and unpredictably over short periods of time and induce tree fall (Niklas 2016). On the other hand, susceptibility to coleopteran insects’ attack relates to wood and bark tree resistance to penetration, which can be estimated by wood and bark hardness (Paine et al. 2010). Species with low WD have low MOR and are more susceptible to breaking than species with high WD and high MOR (Putz et al. 1983).
WD can also be related to the hydraulic architecture of trees (Bucci et al. 2004, Campanello et al. 2008, Chave et al. 2009). Water transport efficiency (hydraulic conductivity per xylem area) correlates positively with vessel size and negatively with WD (Bucci et al., 2004); although in some studies no relationship was found between hydraulic conductivity and WD (Poorter et al. 2010). The area of the vessels, in general, decreases with the increase in WD (Preston et al. 2006). Resistance to cavitation, a measure of water stress resistance, could be associated in some cases with a higher WD (Hacke & Sperry 2001). However, other studies do not agree with this hypothesis (Cochard et al. 1999, Pratt et al. 2007).

#### References

Bucci, S. J., Goldstein, G., Meinzer, F. C., Scholz, F. G., Franco, a C., & Bustamante, M. (2004). Functional convergence in hydraulic architecture and water relations of tropical savanna trees : from leaf to whole plant. Tree Physiology, 24(Meinzer 2003), 891–899. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/15172839
Campanello, P. I., Gatti, M. G., & Goldstein, G. (2008). Coordination between water-transport efficiency and photosynthetic capacity in canopy tree species at different growth irradiances. Tree Physiology, 28(1), 85–94. http://doi.org/10.1093/treephys/28.1.85
Chao, K. J., Phillips, O. L., Monteagudo, A., Torres-Lezama, A., and Vásquez Martínez, R. (2009). How do trees die? Mode of death in northern Amazonia. J. Veg. Sci., 20(2): 260–268. http://+doi.org/10.1111/j.1654-1103.2009.05755.x.
Chave, J., Coomes, D., Jansen, S., Lewis, S. L., Swenson, N. G., & Zanne, A. E. (2009). Towards a worldwide wood economics spectrum. Ecology Letters, 12(4), 351–66. http://doi.org/10.1111/j.1461-0248.2009.01285.x
Cochard, H., D. Lemoine & E. Dreyer. (1999). The effects of acclimation to sunlight on the xylem vulnerability to embolism in Fagus sylvatica L. Plant, Cell and Environment 22: 101-108.
Fournier, M., Dlouhá, J., Jaouen, G., and Almeras, T. (2013). Integrative biomechanics for tree ecology: Beyond wood density and strength. J. Exp. Bot., 64(15): 4793–4815. http://doi.org/10.1093/jxb/ert279.
Hacke, U. G., & Sperry, J. S. (2001). Functional and ecological xylem anatomy. Perspectives in Plant Ecology, Evolution and Systematics, 4, 97–115. http://doi.org/10.1078/1433-8319-00017
King, D. A., Davies, S. J., Tan, S., and Noor, N. S. M. (2006). The role of wood density and stem support costs in the growth and mortality of tropical trees. J. Ecol., 94(3): 670–680. http://doi.org/10.1111/j.1365-2745.2006.01112.x.
Niklas, K. J. (2016). Tree Biomechanics with Special Reference to Tropical Trees. In Tropical Tree Physiology, Adaptations and Responses in a Changing Environment. Editors: Guillermo Goldstein and Louis S. Santiago. Series: Tree Physiology. Volume 6. Springer. Switzerland.
Paine, C. E. T., Amissah, L., Auge, H., Baraloto, C., Baruffol, M., Bourland, N., … Hector, A. (2015). Globally, functional traits are weak predictors of juvenile tree growth, and we do not know why. J. Ecol., 103(4), 978–989. http://doi.org/10.1111/1365-2745.12401
Poorter, L., McDonald, I., Alarcón, A., Fichtler, E., Licona, J.-C., Peña-Claros, M., … Sass-Klaassen, U. (2010). The importance of wood traits and hydraulic conductance for the performance and life history strategies of 42 rainforest tree species. The New Phytologist, 185(2), 481–92. http://doi.org/10.1111/j.1469-8137.2009.03092.x
Poorter, L., Wright, S. J., Paz, H., Ackerly, D. D., Condit, R., Ibarra-Manríquez, G., … Wright, I. J. (2008). Are functional traits good predictors of demographic rates? Evidence from five neotropical forests. Ecology, 89(7): 1908–1920. https://doi.org/10.1890/07-0207.1
Pratt, R.B., Jacobsen, A.L., Ewers, F.W. & Davis, S.D. (2007). Relationships among xylem transport, biomechanics and storage in stems and roots of nine Rhamnaceae species of the California chaparral. New Phytol., 174, 787–798.
Preston, K. a., Cornwell, W. K., & DeNoyer, J. L. (2006). Wood density and vessel traits as distinct correlates of ecological strategy in 51 California coast range angiosperms. New Phytologist, 170, 807–818. http://doi.org/10.1111/j.1469-8137.2006.01712.x
Putz, F.E., Coley, P.D., Lu, K., Montalvo, A. and Alello, A. (1983). Snapping and uprooting of trees: structural determinants and ecological characteristics. Can. J. For. Res. 13: 1011–1020. DOI: 10.1139/x83-133
van Gelder, H. A., Poorter, L., and Sterck, F. J. (2006). Wood mechanics, allometry, and life-history variation in a tropical rain forest tree community. New Phytol. 171(2), 367–378. https://doi.org/10.1111/j.1469-8137.2006.01757.x.
Violle, C., Navas, M. L., Vile, D., Kazakou, E., Fortunel, C., Hummel, I., & Garnier, E. (2007). Let the concept of trait be functional!. Oikos, 116(5), 882-892. https://doi.org/10.1111/j.0030-1299.2007.15559.x