The Acorn Effect

The Acorn effect

In the autumn of every year, you might hear acorns cascading off an oak tree and landing on your roof, clattering and rustling as they fall. For some, this is an indication of a bumper crop and a time to gather the acorns. For others, it is a nuisance. Acorns can be a serious nuisance for homeowners, as they can cause property damage and blockage of driveways, streets, and sidewalks, as well as attract pests such as squirrels. Acorns can also be a hazard for children, as they often fall from high in the canopy and may be hard to reach. Thankfully, there are some ways to minimize the nuisance of acorns, including mulching, picking them up regularly, and pruning the tree so that it can support less weight from the acorns.

Oaks are often a dominant plant species in natural forests and make up a significant share of urban and suburban landscapes. Their abundant acorns are an important source of food for many species, especially birds and mammals. Acorns are also a popular food for humans and provide a valuable source of dietary fiber and essential fatty acids such as a-linolenic acid (which promotes heart health), omega-3 fatty acids, and phytochemicals such as lignans.

Acorn size varies considerably among and within oak species (Rice et al. 1993; Ke and Werger 1999; Navarro et al. 2006; Quero et al. 2007; Gonz-Rodriguez et al. 2011a; Pesendorfer 2014). Acorn size is also a major factor in seedling emergence and growth, with bigger acorns producing larger seedlings (Quero et al. 2007; Sage et al. 2010).

We investigated the legacy effects of severe droughts on BAI, acorn index, and male inflorescence production in a mature Q. ilex stand. Legacy effects were quantified as the difference between observed and predicted values of the response variable (BAI, acorn index, or male inflorescence production) based on the corresponding GAMMs. BAI and acorn index showed moderate inter-individual synchrony, but male inflorescence production was correlated with climatic factors at the population level.

At the individual tree level, acorn index showed a positive correlation with a seasonal temperature index and a negative correlation with a daily maximum air temperature. Acorn index was also positively correlated with the previous season’s acorn production and negatively correlated with radial growth during the following year. Our results suggest that acorn production and radial growth are coupled at the individual tree level, but uncoupled in response to severe drought events. This is in agreement with other studies that indicate a decoupling of leaf production and acorn production in Q. ilex after severe droughts. The legacy effects of the 2005 and 2012 droughts on acorn index and radial growth were similar, with lower-than-expected values two and three years after the drought. However, the differences were not statistically significant. This may be due to the strong spatial heterogeneity of drought legacies in Q. ilex, which makes it difficult to find consistent patterns. A lower-than-expected acorn index during the first year after the drought could be due to delayed acorn maturation, or to a lack of sufficient reserves during this period.