Light dynamics: production ecology, mixing effects and modelling

Greater absorption of photosynthetically active radiation (APAR) is often proposed as a reason for greater productivity in mixed-species forests than in monocultures, however, mixing effects on APAR had rarely been quantified. It was therefore difficult to determine which canopy structure or crown architectural characteristics most strongly influenced light-related mixing effects in forests and how these effects might differ between sites, species compositions and stand ages. Without a good understanding of light absorption, it is also difficult to develop reliable light absorption models, which are critical components of process-based forest growth models. Therefore, this work examined the light-related interactions in many different forests, and developed stand-level light models for mixed-species forests.

Main findings:

* Stand characteristics or treatments that lead to faster growth are often associated with greater APAR and/or greater light-use efficiency, both at the tree and stand levels (12,4,3,1). Similarly, large trees within a stand often absorb more light and use it more efficiently to produce biomass than smaller trees. This was examined using a production ecology approach, which is described here.

* Mixed-species forests can have greater APAR than monocultures and also use that APAR more efficiently, but not always (12,7,1). This was examined in Eucalyptus-Acacia mixtures in Australia (1), Picea-Abies mixtures in Germany (7), Pinus-Fagus mixtures distributed across Europe (12), and high diversity mixtures in Brazil (14).

This figure shows the stand structures of many different Pinus sylvestris (green trees) and Fagus sylvatica (red trees) in mixtures and monocultures used for study 12. These figures were created using the measured tree positions, tree heights, crown lengths and crown widths.

* The greater APAR in mixtures can be caused by different types of interactions and the relative importance of these are likely to change from site to site, with age, and depending on the species in the mixture (13,12,10,7,1). These interactions can include:

- Canopy stratification where fast growing (light-use efficient) species occupy the upper canopy, while shade tolerant species occupy the lower canopy to increase total stand APAR (12,1).

- Complementary crown shapes and architectures. Inter-specific differences in the vertical distributions of leaf area (12,9) and in crown shapes can enable species to occupy a greater canopy volume compared with their monocultures, potentially increasing light absorption (12).

- Intraspecific variability in crown architecture and size. Interactions between species can influence the crown architecture and allometry of a given species and hence its ability to absorb light (12) (also see this page).

- Physiological differences. Combinations of species with contrasting physiology (photosynthetic rates, light-use efficiency, and shade tolerance) and appropriate positioning within the canopy (e.g., shade-intolerant species overtopping more tolerant species) (12,1). These inter-specific differences can be exacerbated when species interactions influence the physiology of a given species (1).

- Phenological differences. Competition for light will be reduced when an evergreen species is growing next to a deciduous species. Even deciduous species may benefit when they are next to other deciduous species that produce their leaves later or lose them earlier (12).

* A greater APAR of mixtures than monocultures can lead to greater productivity in mixtures (12,10,7,1), however, even if mixtures have greater APAR, this does not mean that the increase in APAR caused any mixing effect on growth (see this study for examples relating to water- or nutrient-related interactions; correlations ≠ causality).

* Patterns of light absorption in relation to leaf area differ fundamentally for trees and stands (2).

- At the tree level, the relationship between leaf area and APAR is often linear or even exponential.

- The absorption of light through the crown of an individual crown typically follows a logarithmic trend where each successive layer of leaves absorbs a consistent proportion of incident light. This pattern is often related to Beer’s Law.

- For a given species, across different stands with different leaf area index, the relationship (leaf area index vs. APAR) is often not logarithmic and therefore not consistent with Beer`s law.

* APAR by individual species within a mixture can be predicted using relatively simple stand level light models (6,5). However, the approaches that are currently used in many stand-level forest growth models have not been validated against measurements of light absorption and/or are already known not to be appropriate for mixed-species forests. This was examined by reviewing many forest growth models (8) and by testing different approaches using data sets containing a wide range of species and stand structures, including data from Australia, Vietnam, Brazil, China and Germany (6). The relatively simple stand level light models that were produced were incorporated into the 3-PG_mix model (see this page).

* Some misconceptions that were tested:

- Higher “canopy packing” or canopy density (e.g. leaf area per canopy volume) is not necessarily correlated with higher stand APAR. It may be positively correlated with the APAR of one species within a mixture, but can be simultaneously negatively correlated with another species within the same mixture. That is, any increase in APAR due to a higher LAI may be offset by an increase in self-shading (12). Similarly, in response to changes in growing conditions, trees appear more likely to increase their APAR by changing crown sizes and positions within the canopy, rather than by changing the leaf area density of their crowns (9,6,3).

- More distinct canopy stratification, quantified in terms of a lower proportion of vertical overlap in the leaf area of different species within a mixture, was not correlated with an increase in stand APAR or with the mixing effect on growth in Pinus-Fagus mixtures (12).

Journal articles related to this project:

1. Forrester, D. I., Lancaster, K., Collopy, J. J., Warren, C. R., Tausz, M. (2012). Photosynthetic capacity of Eucalyptus globulus is higher when grown in mixture with Acacia mearnsii. Trees-Structure and Function 26, 1203-1213. doi:10.1007/s00468-012-0696-5

2. Binkley, D., Campoe, O. C., Gspaltl, M., Forrester, D.I. (2013). Light absorption and use efficiency in forests: Why patterns differ for trees and forests. Forest Ecology and Management. 288, 5-13.doi:10.1016/j.foreco.2011.11.002

3. Forrester, D.I., Collopy, J.J., Beadle, C.L., Baker, T.G. (2013). Effect of thinning, pruning and nitrogen fertiliser application on light interception and light-use efficiency in a young Eucalyptus nitens plantation. Forest Ecology and Management 288, 21-30. doi:10.1016/j.foreco.2011.11.024

4. Forrester, D.I. (2013). Growth responses to thinning, pruning and fertiliser application in Eucalyptus plantations: A review of their production ecology and interactions. Forest Ecology and Management 310, 336-347. doi:10.1016/j.foreco.2013.08.047

5. Forrester, D.I. (2014). A stand-level light interception model for horizontally and vertically heterogeneous canopies. Ecological Modelling. 276, 14-22. doi:10.1016/j.ecolmodel.2013.12.021

6. Forrester, D.I., Guisasola, R., Tang, X., Albrecht, A.T., Dong, T.L., le Maire, G. (2014). Using a stand-level model to predict light absorption in stands with vertically and horizontally heterogeneous canopies. Forest Ecosystems. 1, 17 doi:10.1186/s40663-014-0017-0

7. Forrester, D. I., Albrecht, A. T. (2014). Light absorption and light-use efficiency in mixtures of Abies alba and Picea abies along a productivity gradient. Forest Ecology and Management 328, 94-102. doi: 10.1016/j.foreco.2014.05.026

8. Pretzsch, H., Forrester, D.I., Rötzer, T. (2015). Representation of species mixing in forest growth models. A review and perspective. Ecological Modelling 313, 276-292. doi:10.1016/j.ecolmodel.2015.06.044

9. Guisasola, R., Tang, X., Bauhus, J., Forrester, D.I. (2015). Intra- and inter-specific differences in crown architecture in Chinese subtropical mixed-species forests. Forest Ecology and Management 353, 164-172. doi:10.1016/j.foreco.2015.05.029

10. Forrester, D.I., Bauhus, J. (2016). A review of processes behind diversity - productivity relationships in forests. Current Forestry Reports. 2, 45-61 doi:10.1007/s40725-016-0031-2

11. Dong, T.L., Forrester, D.I., Beadle, C., Doyle, R., Hoang, N.H., Giap, N.X., Worledge, D. (2016). Effects of light availability on crown structure, biomass production, light absorption and light-use efficiency of Hopea odorata planted within gaps in Acacia hybrid plantations. Plant Ecology and Diversity 9, 535-548. doi:10.1080/17550874.2016.1262471

12. Forrester, D.I., Ammer, C., Annighöfer, P.J., Barbeito, I., Bielak, K., Bravo-Oviedo, A., Coll, L., Río, M.d., Drössler, L., Heym, M., Hurt, V., Löf, M., Ouden, J.d., Pach, M., Pereira, M.G., Plaga, B., Ponette, Q., Skrzyszewski, J., Sterba, H., Svoboda, M., Zlatanov, T., Pretzsch, H. (in press). Effects of crown architecture and stand structure on light absorption in mixed and monospecific Fagus sylvatica and Pinus sylvestris forests along a productivity and climate gradient through Europe. Journal of Ecology. doi:10.1111/1365-2745.12803

13. Forrester, D.I., Rodenfels, P., Haase, J., Härdtle, W., Leppert, K.N., Niklaus, P.A., Oheimb, G.v., Scherer-Lorenzen, M., Bauhus, J., (2019). Tree species interactions increase light absorption and growth in Chinese subtropical mixed-species plantations. Oecologia, 191, 421-432. doi:10.1007/s00442-019-04495-w

14. Amazonas, N.T., Forrester, D.I., Silva, C.C., Almeida, D.R.A.d., Oliveira, R.S., Rodrigues, R.R., Brancalion, P.H.S., 2021. Light- and nutrient-related relationships in mixed plantations of Eucalyptus and a high diversity of native tree species. New Forests 52, 807-828. doi:10.1007/s11056-020-09826-x