Thinning effects on litterfall remaining after 8 years and improved stand resilience in Aleppo pine afforestation (SE Spain)

Chlorophyll fluorescence variation in two Mediterranean forest species over a 21-year drought treatment period

The Mediterranean is among the regions predicted to be most affected by climate change due to rising temperatures and increasing frequency as well as intensity of extreme weather events, such as drought. These shifts in climatic conditions might lead to changes in species community composition by enabling the increase of drought-tolerant species at the expense of less tolerant ones. This hypothesis was tested in the current study using chlorophyll fluorescence data from a 21-year precipitation exclusion experiment in a Mediterranean forest for two co-dominant Quercus ilex L. and Phillyrea latifolia L. species with contrasting low and high levels of drought tolerance, respectively. The maximum potential quantum efficiency of photosystem II (PSII) (Fv/Fm), photochemical efficiency of PSII (yield) and non-photochemical quenching (NPQ) varied seasonally. The Fv/Fm and NPQ levels were positively correlated with air temperature and standardized precipitation-evapotranspiration index (SPEI), whereas yield, which was greater under drought treatment, was negatively associated with vapor pressure deficit and SPEI. The Fv/Fm values showed similar increase in the two species over the 21-year study period regardless of treatment and in parallel to progressive warming. By contrast, yield values were higher in Q. ilex than in P. latifolia, while NPQ values were higher in P. latifolia than in Q. ilex. Notably, high yield values were also observed in the drought-treated plots. Throughout the study, plants in the drought-treated plots exhibited decreased basal area, leaf biomass and aerial cover due to high stem mortality. In addition, a continuous increase in temperature was detected in summer and autumn, which might explain the observed increase in Fv/Fm values over the study period. Higher yield and lower NPQ detected in Q. ilex could be attributed to less competition for resources in the drought-treated plots and acclimation of Q. ilex plants over the study period. Our results indicate that reduction in stem density could improve forest resilience to climate change-induced drought conditions.

Effects of Forest Thinning on Soil Litter Input Nutrients in Relation to Soil CO2, CH4, and N2O Fluxes in Greece

The contribution of litterfall (dead leaves, twigs, etc., fallen to the ground) and forest floor (organic residues such as leaves, twigs, etc., in various stages of decomposition, on the top of the mineral soil) is fundamental in both forest ecosystem sustainability and soil greenhouse gases (GHG) exchange system with the atmosphere. The effect of different thinning treatments (control-no thinning, traditional-low thinning, selective-intense thinning) on litterfall and forest floor nutrients, in relation to soil GHG fluxes, is analyzed. After one year of operations, thinning had a significant seasonal effect on both litterfall and forest floor, and on their nutrient concentrations. The intense (selective) thinning significantly affected the total litterfall production and conifer fractions, reducing them by 46% and 48%, respectively, compared with the control (no thinning) sites. In the forest floor, thinning was able to significantly increase the Fe concentration in traditional thinning by 59%, and Zn concentration in the intense thinning by 55% (compared with control). Overall, litterfall acted as a bio-filter of the gasses emitting from the forest floor, acting as a GHG regulator.

Thinning and plantation of resprouting species redirect overstocked pine stands towards more functional communities in the Mediterranean basin

Post-fire regeneration in Pinus halepensis' forests, one of the most abundant vegetation types in the Mediterranean basin, often generates overstocked and vulnerable stands. They accumulate a high fuel load, increasing the risk of further fires, and present high levels of vulnerability due to their reduced seed production. In addition, these dense stands substantially reduce the availability of light and nutrients, which may hinder the recruitment of other species, often generating mono-specific and homogeneous stands, which potentially supply fewer ecosystem services than mixed forests with more heterogeneous structures. In these dense pine stands, management is of high priority to reduce fire hazards and promote their functionality. In overstocked pine stands (>75,000 trees·ha^-1), we assessed the long-term effects (10 years) of two thinning levels (600 and 1200 trees·ha^-1), in combination with the plantation of Quercus faginea (a resprouter species typical of advanced successional stages in our study area) on 28 above and belowground ecosystem attributes, including fire hazard. After ten years, thinning and plantation interacted to enhance ecosystem attributes associated with disturbance regulation and biodiversity conservation (up to 200%) and food production (up to 90%), while no effects were observed on those attributes related to carbon sequestration and supporting services. These effects were mainly driven by aboveground attributes, as they responded more strongly to our treatments than those belowground. Our results are relevant for the restoration of Mediterranean degraded ecosystems, and show that tree thinning in overstocked pine stands, combined with the plantation of resprouter species, may not only reduce fire risks and accelerate post-fire succession but also enhance the supply of multiple ecosystem services in the long run.

Contrasting effects of thinning on soil CO2 emission and above- and belowground carbon regime under a subtropical Chinese fir plantation

Thinning plays a major role in forest soil carbon cycling. However, the mechanisms governing soil C fluxes, i.e., C input through litterfall and fine root (FR) production and C output through soil heterotrophic respiration (R_h), remain unclear. To fill this gap, we quantified the C fluxes in the topsoil layer (0-20 cm) by measuring litterfall, FR production and total soil respiration (R_s) (R_a (autotrophic respiration) and R_h) at three thinning intensities (control; low-intensity thinning: extraction of 30% of individual trees; high-intensity thinning (HIT): extraction of 70% of individual trees) in a 26-year-old Chinese fir plantation in southern China. In the control plots, the total C input (110 g C m^-2 year^-1) via litterfall (59 g C m^-2 year^-1) and FR production (51 g C m^-2 year^-1) was much lower than the C output via R_h (518 g C m^-2 year^-1). This finding demonstrated that the soil is a C source (407 g C m^-2 year^-1). Furthermore, the C source increased with increasing thinning intensity, particularly under HIT, due to the decreased litterfall return and increased soil CO₂ emissions through R_h; this increase occurred despite the increased C input from FR production. In addition, the C output via R_s significantly increased by 42% under HIT due to the stimulation of R_a and R_h. Consequently, thinning reduced the topsoil C pool by 7-8%. Redundancy analysis indicated that the soil C fluxes following thinning were driven by increased FR mortality, understory plant biomass and diversity, and microbial biomass carbon (MBC). Overall, our results indicate that heavy thinning increases soil C loss by increasing soil CO₂ emissions and decreasing litterfall return, even under substantially increased FR production. This finding suggests that thinning practices should consider the trade-off between soil C inputs and outputs to reduce the impact of thinning on forest soil carbon sequestration.

Airborne Laser Scanning Cartography of On-Site Carbon Stocks as a Basis for the Silviculture of Pinus Halepensis Plantations

Forest managers are interested in forest-monitoring strategies using low density Airborne Laser Scanning (ALS). However, little research has used ALS to estimate soil organic carbon (SOC) as a criterion for operational thinning. Our objective was to compare three different thinning intensities in terms of the on-site C stock after 13 years (2004–2017) and to develop models of biomass (Wt, Mg ha−1) and SOC (Mg ha−1) in Pinus halepensis forest, based on low density ALS in southern Spain. ALS was performed for the area and stand metrics were measured within 83 plots. Non-parametric kNN models were developed to estimate Wt and SOC. The overall C stock was significantly higher in plots subjected to heavy or moderate thinning (101.17 Mg ha−1 and 100.94 Mg ha−1, respectively) than in the control plots (91.83 Mg ha−1). The best Wt and SOC models provided R2 values of 0.82 (Wt, MSNPP) and 0.82 (SOC-S10, RAW). The study area will be able to stock 134,850 Mg of C under a non-intervention scenario and 157,958 Mg of C under the heavy thinning scenario. High-resolution cartography of the predicted C stock is useful for silvicultural planning and may be used for proper management to increase C sequestration in dry P. halepensis forests.