On the possible links between tree growth and galactic cosmic rays


(*Author for correspondence: tel +358 (0)9 19150756; email markku.kulmala@helsinki.fi)

The relationship between galactic cosmic radiation and climate has been under discussion for at least several decades (see, for example, Carslaw et al., 2002). Now, in this issue of New Phytologist, Dengel et al. (pp. 545–551) have investigated a new aspect related to this phenomenon, namely the relationship between galactic cosmic radiation and tree diameter growth.

Because the radiation field is affected substantially by clouds, it is possible that the observed correlation of cosmic radiation flux with tree growth might be related to cosmic ray-induced changes in cloud properties.

Dengel et al. investigated the interannual variation of growth rings formed by Sitka spruce (Picea sitchensis) trees in northern Britain (55°N, 3°W) over a 35-yr period (1961–2005). A statistically significant correlation between the growth of trees and galactic cosmic radiation was found. The authors present the following hypothesis to explain this interesting finding: the tendency of galactic cosmic radiation to produce new aerosol particles enhances the scattering of solar radiation in the atmosphere, which in turn increases diffuse solar radiation intensity and thereby the photosynthesis of the forest canopy.

The study by Dengel et al. raises interesting questions from a biological and physical point of view.

  • 1Is the observed relationship characteristic for the studied stand in northern Britain or is it a more general phenomenon, for example, for all coniferous stands in Britain or even for boreal forests?
  • 2Is the enhanced photosynthesis caused by increased diffuse radiation the mechanism that provides the link between radial growth and cosmic radiation, and what is the role of atmospheric aerosols in this context?
  • 3Is it possible that cosmic rays have a direct effect on the radial growth of trees?

In principle, the first two questions can be approached in a straightforward manner. The geographical extent of the effect can be easily studied from the existing tree-ring series. The effect of diffuse radiation is also quite simple to study, even though diffuse radiation measurements covering several decades are very scarce. However, the third question, the direct effect of cosmic rays on tree growth, is rather problematic to explain. If the observation by Dengel et al. turns out to be characteristic for large areas, including boreal forests, then this finding is very important for our understanding of tree growth and even for the relationship between vegetation and its environment.

In this commentary we present some relevant results that we hope will shed light on questions (1) and (2). First, in order to obtain some insight into the geographical coverage of the phenomenon, we investigated the relationship between cosmic ray flux and growth anomaly by using the same methods as Dengel et al. The period was the same 35 yr, and the investigated trees were Norway spruce (Picea abies) trees from Finnish Lapland. While the correlation obtained was clearly weaker (0.23; Fig. 1) than that obtained by Dengel et al. (0.39), our analysis demonstrated that the association between the cosmic ray flux and tree growth might be a common phenomenon and not just a specific feature of a single location.

Figure 1.

 Annual growth anomaly of tree rings as a function of cosmic ray flux (counts h−1). The growth anomalies are from Norway spruce (Picea abies) trees growing in Finnish Lapland (Lännenpääet al., 2008), and the cosmic ray flux is from the same Kiel databank used by Dengel et al. (2009).

For a given solar input, the radiation field experienced by trees, including the intensity of diffuse radiation, is affected by the amount of optically active aerosol particles in the atmosphere, and the properties and type of clouds. It is well known that cosmic rays produce ions that help atmospheric aerosol formation (e.g. Carslaw et al., 2002); however, to perturb the solar radiation field, these aerosols need to be formed in large quantities and they need to grow to optically active sizes (larger than approx. 100 nm in diameter) in the atmosphere. The SMEARII station (Hari & Kulmala, 2005), located in southern Finland, has the longest record of measured aerosol-formation events (e.g. Kulmala et al., 2001; Dal Maso et al., 2005). These measurements, spanning a full solar cycle, show no connection between the cosmic ray-induced ionization rate (CRII) and the frequency of aerosol-formation events on a monthly basis (Fig. 2). The same is true between the concentration of optically active aerosol particles and cosmic radiation flux. Furthermore, the contribution of ions to atmospheric new-particle formation has been shown to be relatively small (< 10%), at least in the atmospheric boundary layer (Kulmala et al., 2007). Additionally, Pierce & Adams (2009) showed recently, using a global model, that aerosol formation related to cosmic rays has a negligible influence on the concentrations of cloud/optically active atmospheric aerosol particles, even when conditions are assumed to be most favorable to cosmic ray-induced effects. Taking these pieces of information together, it seems very unlikely that aerosol particles formed by cosmic rays would be able to affect tree growth via altering the atmospheric radiation field.

Figure 2.

 Particle formation events and cosmic ray-induced ionization intensity (CRII) at the SMEAR II station in Hyytiälä, Finland, during 1996–2008 (Kulmala et al., in press). The monthly number of particle-formation events is expressed as a function of the corresponding median CRII. Particle-formation events are determined as described by Dal Maso et al. (2005). CRII at the SMEAR II station was determined using the model of Usoskin & Kovaltsov (2006).

A number of studies have looked for possible connections between cosmic rays and cloud properties (e.g. Palle, 2005;Kristjansson et al., 2008; Sloan & Wolfendale, 2008; Svensmark et al., 2009), and in some (but not all) of these studies statistically significant associations have been observed. Because the radiation field is affected substantially by clouds, it is possible that the observed correlation of cosmic radiation flux with tree growth might be related to cosmic ray-induced changes in cloud properties. The exact mechanism responsible for such cloud effects remains to be elucidated.

To investigate the third option, the direct effect of cosmic rays on tree growth, to explain the new findings by Dengel et al., we need to ask several questions, such as: is it possible that the ionization affects the electrical properties of the trees by, for instance, enhancing their water uptake? Are there potential electrical forces inside the trees that could result in effects like this? However, answering these questions is likely to be a tedious task and needs more rigorous investigations.

To determine reliably the reason for the findings of Dengel et al., further studies need to be carried out to establish how common (geographically) the observed association between the cosmic ray flux and tree growth really is and to pinpoint the actual mechanism responsible for this association. Concerning the latter point, we probably need to search for something unrelated to cosmic ray-induced aerosol formation, such as the direct influence of cosmic rays in the forest canopy, or indirect effects of cosmic rays on cloud properties.