Tree Growth Dynamic and Thinning response in Mediterranean Pine-Oak Forest Stands

Abstract

Climate change forecasts are particularly severe for the western Mediterranean Basin, where rising temperatures and decreased precipitation could increase the frequency of drought events. Mixed forest stands via species niche complementarity may lead to greater functional services, over-yielding or increased temporal stability compared to monospecific stands. Controlling competition through thinning may also provide a drought adaptation tool to minimize potentially adverse ecological and socio-economic impacts of climate change. In this study, we sought to better understand temporal complementarity among intra-annual radial increment and thinning effect in Mediterranean pine-oak coexisting species. Besides, we required to uncover differences in the weather drivers of radial variation for the two co-existing species. The forest stands under study were originated by pine forestation, which were diversified over time due to the recovery and sprout of initially oak coppice stands. Nowadays, they present a pine-oak mixed stands structure where pine species dominates the oak coppice trees. We studied species differences in stem daily radial variation cycles, intra-annual cumulative radial increment pattern and thinning response of Pinus pinaster-Quercus pyrenaica mixed stands over climatically contrasted years (2010-2014) at two sites with dissimilar drought conditions. Moreover, intra-annual cumulative radial increment pattern and thinning response of Pinus sylvestris-Quercus pyrenaica mixed stands were evaluated for two contrasted year (2016-2017). A severe drought event occurred during 2012 and 2017 years in both sites respectively. Nine rectangular plots were established in a Latin square design at the three study sites. The experimental design consisted of two thinning treatments with different intensity and unthinned control with three replicates each. Thinning treatment comprised moderately and heavy thinning removing 25% and 40-50 % of initial basal area. Thinning treatment was focused only on pine in maritime pine-oak mixed stands, while both species were harvested in Scots pine-oak mixed stand. Band and high-resolution point dendrometers were installed in oak and pine trees sample, so that stem radial variation was analyzed. Site weather variables were continuously monitored using a variety of sensors, a specifically weather station and via the Spanish State Meteorological Agency automatic network stations. Non-linear mixed models were fitted to show species differences in intra-annual cumulative radial increment pattern: bi-logistic, a Richards curve for spring and autumn period and a double-Richards curve. Linear mixed models were fitted to analyze species-specific response to weather. Besides, linear and non-linear statistical metrics were also used to evaluate radial variation synchrony. Radial increment commenced on 30 March for maritime pine, while deciduous oak stem increment began 1–3 weeks later coinciding in time with leaf budburst. Air temperature leads to radial stem-size changes in daily variation cycles with different species responses, while precipitation increased daily amplitude and cycle duration in the same way for both species. Daily radial increment and number of cycles with increment phase during spring were mostly higher for maritime pine than oak, being water availability the most important control factor. Accordingly, intra-annual cumulative patterns showed greater radial increment for maritime pine than oak regardless year and site. Species differences in intra-annual cumulative radial increment patterns also revealed a possible species-specific complementarity growth strategy in certain periods, despite synchronization of radial variation. Heavy thinning resulted in the greatest annual radial increment for maritime pine regardless drought episode. Besides, heavy thinning positively affected the intra-annual pattern for pine increasing spring and autumn asymptotes. Thinning treatment, independently of intensity, had also a slight positive effect on inter- and intra-annual radial increment for oak species, but not under drought conditions. Species-specific differences on intra-annual cumulative radial increment patterns showed an obvious asynchrony of radial growth for Scots pine-oak mixed stand. Only heavy thinning increased, but not prolonged, spring radial increment for both species in regardless climatic conditions. These results suggest a species niche complementarity, involving an efficient use of resources, which could release abiotic stress compared to monospecific stands. Besides, heavy thinning enhances the species-specific radial increment response to drought in Mediterranean pine-oak stands, so may be recommended to mitigate climate change impacts.