Changes in Long-Term Light Properties of a Mixed Conifer—Broadleaf Forest in Southwestern Europe

Abstract

Natural and anthropogenic factors affect forest structure worldwide, primarily affecting forest canopy and its light properties. However, not only stand-replacing events modify canopy structure, but disturbances of lower intensity can also have important ecological implications. To study such effects, we analyzed long-term changes in light properties of a conifer–broadleaf mixed forest in the Southwestern Pyrenees, placed in the fringe between the Mediterranean and Eurosiberian biogeographical regions. At this site, a thinning trial with different intensities (0%, 20%, and 30–40% basal area removed) took place in 1999 and 2009, windstorms affected some plots in 2009 and droughts were recurrent during the sampling period (2003, 2005, 2011). We monitored light properties during 14 years (2005–2019) with hemispherical photographs. We applied partial autocorrelation functions to determine if changes between years could be attributed to internal canopy changes or to external disturbances. In addition, we mapped the broadleaf canopy in 2003, 2008, and 2016 to calculate broadleaf canopy cover and richness at the sampling points with different buffer areas of increasing surface. We applied generalized linear mixed models to evaluate the effects of light variables on canopy richness and cover. We found that light variables had the most important changes during the period 2008 to 2010, reacting to the changes caused that year by the combined effects of wind and forest management. In addition, we found that an area of 4.0 m radius around the sampling points was the best to explain the relationship between light properties and species richness, whereas a radius of 1.0 m was enough to estimate the relationship between light and canopy cover. In addition, light-related variables such as diffuse light and leaf area index were related to species richness, whereas structural variables such as canopy openness were related to canopy cover. In summary, our study demonstrates that non stand-replacing disturbances such as windstorms, thinning, or droughts can have an important role in modifying structural and light-related canopy properties, which in turn may influence natural processes of stand development and ecological succession.

The influence of stand composition and season on canopy structure and understory light environment in different subtropical montane Pinus massoniana forests

“…1 and 2 ), with Tdir exhibiting the highest spatial distribution heterogeneity. P. massoniana , with its small leaf area, clustered leaves, and high tree height, exhibits low sunlight interception efficiency throughout the year ( Ruiz et al, 2021 ). Moreover, it often forms gaps between adjacent trees ( Messier, Parent & Bergeron, 1998 ).…”

“…2 and 4 , Table S1 ). In addition, lower P. massoniana light interception efficiency increases the survival possibilities of other broadleaf tree species in the understory ( Ruiz et al, 2021 ), leading to trees having canopies of varying heights in different seasons, which can effectively absorb and reflect light. As a result, the Tdif fluctuations in the ULE across different seasons were minimal, and its distribution was relatively uniform ( Promis et al, 2009 ).…”

Environment and density-dependency explain the fine-scale aggregation of tree recruits before and after thinning in a mixed forest of Southern Europe

“…In addition we found that the local distribution of trees and recruits also depended on the patch dynamics, as we found strong differences between plots. In particular, in plots 2, 3, 5 and 9 trees and recruits were aggregated whereas in plot 4 they were repulsed from 1.5 to 6.0 m; the latter spatial patterns was probably due to the local dynamics, such as a strong tree fall as a consequence of a windstorm before the thinning event that created large openings in the local proximity of trees ( Ruiz de la Cuesta et al., 2021 ). We additionally found that recruits in control plots had proportionally higher survival beyond 2.0 m from trees, probably as a consequence of higher tree-recruit competence for resources; for instance, rainfall was proportionally low during the summer season in 2008 and that potentially affected the tree-recruit competence in that year.…”