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.
Highlights
Forests can be altered in multiple ways by a wide array of natural and anthropogenic factors, causing physical, structural, and functional changes
We found nonstationary trends of the values of light variables, with two major disturbance events occurring in 2010 and 2015 (Figure 2), and followed by recovery periods of the initial values recorded at the beginning of our time series
When accounting for light variables explained by forest canopy variables, we found that an area of 4.0 m radius around each sampling point had the highest performance for Canopy openness (CanOpen), DirectBelowYr, N.Sunflecks, and Max.Sunflecks, whereas an area of 2.0 m radius had the highest performance for Leaf area index (LAI)
Summary
Forests can be altered in multiple ways by a wide array of natural and anthropogenic factors, causing physical, structural, and functional changes. Factoring out the relative importance of the drivers and alterations on forest systems is one of the priorities in forest research and management [1]. Abiotic factors, such as strong winds, drought, and other climatic events affect forest dynamics and structure [2]. Middle-intensity strong winds can cause uprooting, stem and branch breakage [3], and gap generation [4], leading to modifications in ecosystem dynamics [5], species composition [6], tree growth [7], and litter inputs to the soil [8]. Biotic factors such as pests or pathogens can modify canopy structure and properties [11]
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