Intensive Forest Management
Genetic Improvement
Given that the primary metric of a forest’s value has been its merchantable volume, plantation forestry that is based upon successful breeding of superior tree genotypes (fast-growth species) is becoming used more widely to maximize productivity [16••], [26], [41••]. Selection for fast-growing genotypes may generally increase C sequestration rates by approximately 10 to 20%, depending upon what species is being grown [46]. Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) is grown as a plantation species in Europe, South America, New Zealand and Australia, and throughout its extensive natural range in western North America. Chappell et al. [47] reported that Douglas-fir growing on a good site (site index = 32 m) would be expected to yield 174 m3 ha−1 at the end of a 50-year rotation if they were naturally regenerated. Through a similar site regeneration process using genetically improved stock, which was subsequently thinned and fertilized, Douglas-fir can produce 643 m3 ha−1 in 50 years [47]. In Canada, boreal forest hybrid poplar clones (e.g. Populus balsamifera L. × P. simonii Carrière, P. deltoides × P. × petrowskyana) are generally selected for C uptake because of their rapid rates of growth [48]. In addition, there is a difference between monoclonal and polyclonal cultures, in terms of growth and tree size, thereby affecting the C stock in the aboveground biomass [48], [49]. Elferjani et al. [49] showed that the stem volume of hybrid poplar in a monoclonal culture is 6.1 m3 ha−1, while it is 21% higher in a polyclonal culture (7.4 m3 ha−1). Fast-growing tree species typically sequester more C at a young age, whereas C sequestration rates for slow-growing trees may reach a peak after many years have elapsed, depending on growth rate patterns and rotation length [25].
Today, the main issues of concern around the application of genetic engineering in forestry are about native biodiversity and the provision of ecosystem services. Using such techniques may reveal several drawbacks, such as less genetic diversity, invasive behaviour and less favourable habitat than natural forests [50]. Maintaining natural patterns of heterogeneity, including species diversity, across spatial and temporal scales is consistent with current principles of ecosystem management. The risk assessment of using such techniques is considered important and should include aspects such as the evaluation of the probability of establishment, spread and related undesirable consequences of establishing species that have been improved genetically [51]. Given the complex nature of the risks, Mathews and Campbell [51] state that an interdisciplinary approach including ecologists and molecular biologists is required.
Fertilization
The most critical elements for forest NPP are macro-nutrients (N, P, K, Mg and Ca). Both N and P are particularly important nutrients for ecosystem structure, processes and function, since their absence limits the production of plant biomass and growth. From previous work, N has been found to limit forest NPP on relatively young soils, whereas P is limiting on old soils [[52]]. Economically optimal intensive fertilization increased timber supply by 20% [47], [53], [54]. Fertilization is usually associated with forest plantations, especially those composed of conifer species, such as Douglas-fir, spruce (Picea spp.) or maritime pine (Pinus pinaster Aiton); supplemental nutrient applications can dramatically increase forest C sequestration capacity [54]–[56]. For instance, Jassal et al. [56] found that fertilization of 58-year-old Douglas-fir stands with 200 kg ha−1 of urea–N-elevated NEP by 64%, i.e. NEP increased from 326 to 535 g m−2 of C. Generally, nutrient limitation can reduce leaf area index (LAI), photosynthetic capacity or both [[52]]. Higher soil nutrient contents could increase soil decomposition and microbial activity, thereby decreasing SOC in the short term [57]; SOC could increase in the long term, since fertilization increases vegetation growth. Yet, N fertilization has been demonstrated to be the only forest management practice that has had a positive effect on the soil C pool, mainly in the mineral soil (A horizon) [58]. Many recent studies also have demonstrated that fertilization of temperate and boreal forests has a high potential for reducing ecosystem respiration (Rh and Ra) [54]. Nevertheless, in boreal forests as the growing season is likely to be prolonged by ongoing climate change, this would lead to higher N demand. N can be supplied by atmospheric deposition, but deposition is unlikely to cover the N demand at the appropriate rate, particularly in boreal forests where the inputs are lower [59] and tend to be intercepted by the thick bryophyte groundcover [60], [61]. Beyond macro-nutrients, forest C sequestration capacity may be limited by deficiencies of some trace elements, such as Fe, Cu and Zn [55].
The fertilization timing and frequency generally depend upon the site nutrient content and the planted species (broadleaves vs. coniferous) [53]. In contrast, the application of fertilizers releases nutrients (including phosphate P) to the freshwater bodies that contribute to their eutrophication (accumulation of minerals and nutrients), particularly nearby rivers or lakes [62]. However, in order to both improve forest C sequestration capacity and reduce environmental pollution, appropriately balanced fertilization is required.
Mechanical Soil Preparation
Forest C sequestration capacity can be significantly enhanced by careful mechanical soil preparation (MSP) prior to afforestation or following forest harvesting. Scarification, mounding, sub-soiling/ripping and deep soil cultivation (ploughing to 50 cm depth) are common MSP treatments that are used in forestry. Superficial cultivation of soil layers could be included among those treatments, since this process aims to remove competing vegetation and logging slash, to facilitate planting or direct seeding [63], [64•]. MSP is carried out for many reasons: to promote natural regeneration, to reduce competition between tree seedlings and understory vegetation, to create micro-sites that favour seedling survival and to enhance forest growth by increasing nutrient content, and element mobilization in soils [55], [61], [64•].
MSP techniques could also temporarily reduce soil C stocks for the short or medium term, since they can increase decomposition rates [65••]. Nevertheless, the real effect of MSP on forest soil C depends upon the methods that are used and their intensities [63], [64•]. For instance, Jiménez Esquilín et al. [66] noted that soil C (O horizon and uppermost mineral soil) had decreased by 50%, as measured 24 years after scarification. Nordborg et al. [63] asserted that no differences emerged among MSP treatments when the whole soil profile was considered; however, losses were higher in the O horizon after patch scarification (5 cm deep) compared to deep-soil cultivation. Also, Mjöfors et al. [[67]••] demonstrated that site preparation increases forest ecosystem C stocks over the long term. They further recommended mounding or disc trenching to promote C sequestration, since these treatments did not affect soil C stocks down to a depth of 30 cm. Consequently, low- or moderate-intensity MSP techniques had no appreciable effect on soil C, but increased the volume of soil that roots can readily exploit by decreasing bulk density. Therefore, these treatments enhanced forest productivity, leading to greater litter production, thereby increasing soil C content over the long term [63], [64•], [68], [69]. In previous studies, roots have been shown to concentrate in micro-sites with mixed soil organic matter and mineral soil in a deep cultivated soil profile [69]. If the organic material is ploughed into the mineral soil, water relations are often improved by higher infiltration capacity so that moisture is preserved, thereby enhancing forest NPP [68].
By contrast, intensive MSP treatments are recommended in wetland soils, since these enhance tree establishment, lower the water table and increase oxygenation of the soils [20••], [42•]. All MSP techniques definitely reduce soil C sequestration and turn peatlands into significant C sources, because of the high rates of decomposition. Moreover, MSP reduces the water content while improving soil and root aeration, thereby enhancing tree productivity and increasing ecosystem Rh [42•], [70]. Yet, Lavoie et al. [42•] have stressed that in peatlands where wildfires are severe and frequent, MSP could become problematic for future C emissions. Nevertheless, peatlands hold vast stocks of C in their soils; their conservation is potentially a natural solution for mitigating climate change.
Afforestation and Reforestation
Planted forest areas have increased from 168 to 278 Mha since 1990 [14]. These plantations account for 7% of the world’s forest area due to massive afforestation programs to achieve CO2 emissions reduction targets (Kyoto Protocol, REDD+) [13]. The Kyoto Protocol allows countries to claim as credit any C that is sequestered as a result of reforestation and afforestation. C sequestration in both soil and aboveground biomass is one of the essential benefits of afforestation and reforestation, as is increasing HWP C stocks [25], [71]. However, fast-growing species are the most frequently used species in afforestation, given that they have a high potential for C sequestration [25], [46], [47]. Species composition, genetics (fast-growth species), spacing and initial land use are the main factors affecting CSS in forested land [46], [72•]. Also, the C sink capacity of forest plantations can be maximized by prolonging the rotation length and adopting suitable practices for each species (e.g. spacing, fertilization, MSP and composition) [50], [54], [55].
Some frequently planted species are more vulnerable than others to disturbances. For example, Eucalyptus is a major component of fire-prone or fire-driven ecosystems of its native Australia. Yet, species in this genus are vulnerable to wildfires in the Mediterranean basin and other regions of the world. Moreover, regions where it has been introduced have undergone massive land use change, especially when they are planted as industrial monoculture forest stands [73]. Likewise, balsam fir (Abies balsamea [L.] Miller), white spruce (Picea glauca [Moench] Voss) and black spruce (Picea mariana [Miller] BSP) are vulnerable to a co-evolved insect defoliator, i.e. the spruce budworm (Choristoneura fumiferana [Clemens]) in the boreal forest of eastern North America [74], [75]. For these tree species, their C sequestration potential and stability of C stocks during a disturbance event is still a matter for research and debate. In the interim, tree species that are less vulnerable to catastrophic fire, wind damage, insects and drought can be planted or maintained on the landscape [76]. Yet, with genetic control, planted species vulnerability could be reduced. For instance, Dale et al. [76] have noted that tree invasion by phloem-feeding insects, such as bark beetles (e.g. Dendroctonus spp., Scolytus spp.), is controlled in part by the ability of trees to produce oleoresins; indeed, the primary defence of pine trees (Pinus spp.) is oleoresin, which is a mixture of monoterpenes and resin acids that provides a chemical and physical barrier against biotic intrusions [77], [78]. Wildfires are not only followed by bark beetle outbreaks in coniferous forests (e.g. Pinus radiata D. Don, P. pinaster), but insect irruptions can result in their genesis by weakening and killing susceptible individuals, thereby creating a ready source of fuel [79]. Therefore, planting selected tree species and genotypes with relatively high oleoresin contents could limit insect outbreaks. Furthermore, the use of mixed stands may reduce the risk of pest and pathogen outbreaks, compared to monocultures, since there is a negative correlation between tree species diversity and the level of damage [80].
The effects of afforestation and reforestation on SOC are not fully understood. Some researchers have pointed out that afforestation could decrease SOC, whereas others have reported positive effects. For instance, afforestation of pastures with pine plantations leads to decreased SOC [81], but other studies have presented contradictory results [82]. For broadleaf plantations, the results are quite variable [81], [83]. A review of the literature suggests that the effect of afforestation on SOC is generally related to initial land use (e.g. forest land, grassland, cropland) [20••], [84]. From a meta-analysis on data that were obtained from 74 publications, Guo and Gifford [81] determined that soil C declines after land use conversions from pasture to plantation (− 10%) and native forest to plantation (− 13%), but it increases after land use changes from cropland to plantation (+ 18%) and cropland to secondary forest (+ 53%).
Applied Silvicultural Treatments in Extensive and Intensive Forest Management
This section presents common silvicultural treatments and practices that are applied in EFM and IFM (or both), and which are used to improve forest growth and stand harvested volume after stand establishment. Generally, practices with high operating and investment costs such as pre-commercial thinning (PCT) and pruning (including coniferous brushing and formative shaping of broadleaves) are used in IFM (afforestation) [85]–[88]. PCT also could be used in EFM to improve the uneven-aged stand volume over a long rotation period. Clear-cutting (CC) is globally the most widely used silvicultural treatment that is applied in IFM [89]–[91]. About 50% and 93% of all timberlands in temperate and boreal forests respectively involve CC methods [90], [93•], while both CC (mainly in the tropical rainforest) and selective logging have been widely employed to harvest tropical forests [91]. In EFM, PC and selective cutting treatments are frequently used to promote natural regeneration [93•]. PC ranges from retention silviculture to more selection-oriented treatments that have been adapted to the boreal forest. It generally has been more recently applied to this biome (< 20 years) [89], despite being used in other biomes for at least a century [91].
Pruning
Pruning is applied to specific high-value tree species to improve stem quality (reduce knots size and presence) and vigour, together with overall tree growth [85], [86]. Pruning can also increase forest C sequestration rates. For example, a study by Medhurst et al. [85] on native Australian blackwood (Acacia melanoxylon R.Brown) demonstrated increased photosynthetic capacity between 2 and 6 weeks after pruning, to levels that were as much as 50% higher than the no-pruning controls, depending upon pruning location (i.e. upper-, middle- or lower-crown). C sequestration was found to be 33% and 62% higher in the upper- and middle-crown treatments, respectively, compared to the unpruned treatment [85]. Yet, Pinkard and Beadle [86] have claimed that lower pruning of 50% of the crown depth had no effect on height or diameter increment, while removal of 70% of the lower crown depth resulted in severe reductions in both parameters, thereby reducing forest C sequestration rates. Regarding SOC, pruning has an effect similar to stem-only harvesting [94]. Indeed, when the branches are removed and left in situ, their C and nutrient contents are transferred to the deadwood pool, increasing SOC in the long term as a direct result of the decomposition process.
Pre-commercial Thinning
PCT is conducted in the interval between canopy closure and final harvest, removing the smaller-, weaker- and poorer-quality stems, and could be applied several times during the rotation period [95], [96]. It is largely applied in even-aged, post-fire stands of black spruce and jack pine (Pinus banksiana Lambert), and to second-growth stands of balsam fir. PCT generally precedes the final cut by 15–20 years [89], [97]. PCT from below initially redistributes C among pools, i.e. from the biomass to deadwood, and increases forest C sequestration capacity as a long-term effect, which mainly is expressed in the biomass [87], [88]. In fact, the application of this method reduces competition for light, water and nutrients, thereby improving tree growth; this positive effect depends upon the intensity and frequency of thinning [95], [98]. Moreover, in a 25-year study by Hoover and Stout [96], thinned-from-below plots (about 35% basal area removal) had greater volume production and C sequestration rates than plots where thinning from the middle or above was applied (Supplementary Information). Thinning-from-below as a pre-commercial treatment could increase CSS, because all harvested trees remain on the ground (increasing soil C stocks), while residual trees accumulated C at a faster rate, depending upon the species [87], [94], [99••], [100]. For instance, Ruano et al. [100] report that 5 years after thinning, the height and the basal area of Aleppo pine (Pinus halepensis Miller) had increased by 20% compared to control stands. PCT treatments from above or middle have been shown to display negative C sequestration rates, storing significantly less C than thinning-from-below or control treatments (no thinning) [96]. Thinning-from-above could increase the decomposition process and ecosystem respiration (C loss), since this treatment exposes deadwood and litter to light and stimulates microbial activity [101]. For instance, Concilio et al. [102] and Campbell et al. [101] report a 43% increase in soil respiration 2 years following thinning-from-above of a mixed-conifer forest. On the other hand, thinning could stimulate the shrub layer, which can also result in net C loss and a lower C sequestration rate of the stand [101]. In some circumstances in boreal forests, root grafting could affect the growth response to thinning. For instance, in jack pine boreal forest, root grafting could considerably increase the radial growth of trees after thinning [103].
Harvesting Treatments: Clear-Cuts Versus Partial Cuts
Like severe wildfires, CC may switch forests from being a sink to a source of C by increasing respiration and reducing leaf biomass and, therefore, photosynthesis in the period following disturbance (Fig. 3) [30]. Using more than 180 site-years of eddy covariance measurements of C fluxes that were made on forest chronosequences in North America, Amiro et al. [104] found that NEP exhibited C losses from all ecosystems following a stand-replacing disturbance. Post-disturbance C balances after CC depend upon many ecosystem processes, including regeneration and vegetation succession, photosynthesis and respiration [18]. Canopy removal that is associated with CC can increase the amounts of solar radiation and precipitation that reach the soil surface, thereby increasing soil temperature and soil moisture, and consequently increasing Rh and reducing NEP [105], [106]. Over time, NPP and biomass accumulation increase in the youngest stands, and inputs of new litter cause the detrital pools to build up, resulting in an extended period of C accumulation and a positive NEP. In the oldest stands, mortality and decomposition losses accelerate, which causes NEP to approach zero (Fig. 3) [18], [30]. Overall, the ecosystem attains net C gain after about 20 years following CC, due to forest regrowth [107]. CC exports close to 100% of the aboveground biomass pool as HWP and transfers the rest in the deadwood pool as harvest residues, thereby reducing NEP and altering the ecosystem NPP of the replacement stand [18], [30].
PC are thus gaining interest as alternatives to CC in the face of climate change, since they could enhance forest C sequestration, with possibly lower impacts on SOC [23], [58]. Bose et al. [89] defined PC as “a generic term, which refers to a whole range of treatments from clear-cutting with sparse, dispersed retention in which a few merchantable stems are left on site, to single-tree selection systems where the very evidence of a harvesting treatment might be too subtle to be noticed by an untrained eye”. PC includes a broad range of treatments, such as shelterwood cutting, selection (distant or close), retention systems and seed-tree systems [89], [108•]. Based upon a simulation model in the Canadian boreal forest focusing on red spruce stands (Picea rubens Sargent), Taylor et al. [109] found that total ecosystem C increased in PC stands throughout the 240-year simulation from 308.9 to 327.3 Mg ha−1, while it decreased in CC stands to 305.8 Mg ha−1. Based upon an ecosystem process model (CENTURY) that ran for 5000 years, Peng et al. [110] determined that PC could increase C sequestration, by about 36–40% in the boreal forest region.
The positive effect of PC on forest C sequestration depends upon harvesting intensity. In fact, Lee et al. [92] affirm that annual C assimilation rates in the post-harvest period (after 5 years) in boreal mixedwood stand were substantially higher in control (uncut forest: 3.1 Mg ha−1 year−1) than PC with two-thirds of the volume being removed (1.8 Mg ha−1 year−1), or when clear-cut (0.3 Mg ha−1 year−1). Most empirical studies, however, have been conducted at the plot scale (e.g. [23], [92], [111], [112]). Furthermore, experimental results from individual studies are highly variable, mainly due to differences in harvest intensity (removed volume) and the number of recovery years after cutting. For example, some studies have documented substantial increases in forest NPP and soil C storage following PC [92], [111], whereas others have shown non-significant effects or decreases in forest CSS [101], [113]. Consequently, the intensity at which PC is conducted is a very important consideration, to maintain this positive effect on residual trees. We must be able to predict long-term changes in forest C dynamics at regional and global scales under different intensities. For instance, Øyen and Nilsen [114] reported that PC intensity should not exceed 65% in southeast Norway to maintain forest biomass sustainability and preserve its contributions to C sequestration.
The success of the PC approach requires consideration of not only its intensity, but also its form. In fact, some forms of PC could increase post-cutting mortality that is incurred by disturbance, particularly windthrow [108•], thereby reducing NEP. It was determined from several studies that post-cutting mortality following PC ranges from 15 to 74%, which is attributable to windthrow [108•], [115]. Previous research has shown that mortality is proportional to harvest intensity (r2 = 0.3988 [109]); it was highest for the treatment that had the highest intensity compared to the control and other treatments [108•], [116•]. Mortality rates that are associated with PC depend upon a range of factors: spatial patterns (edge, residual strips and trails), forest fragmentation and harvesting intensity. Montoro Girona et al. [108•] reported that 60% of residual trees were dead in seed-tree treatments, compared to 30% for shelterwood cuts. These results showed that experimental uniform shelterwood cuttings with 50% harvest intensity successfully reduced the proportion of tree loss compared to the seed-tree system that is employed in Québec’s boreal forest. Shelterwood and seed-tree harvesting followed by scarification promotes regeneration through the creation of uniform openings in the canopy, while limiting the growth of competing vegetation [93•].
With respect to SOC, CC might reduce total SOC stocks relative to the effects of PC [65••]. Despite this potential difference, harvesting (PC and CC) exerts a more limited effect on SOC than it does on biomass [23], [65••]. After CC, soil C stocks appeared in many cases to remain relatively constant, with their variation being linked to C transfer from the litter (Fig. 3) (e.g. leaves, lifted branches after the cut) and to the site conditions (mainly temperature) [18]. The meta-analysis that was conducted by Nave et al. [117] for temperate forests worldwide shows that harvesting generally results in a small reduction (− 8%) in total soil C stocks, whereas the mineral horizons showed no significant change overall. In other cases though, forest harvesting had a critical effect on SOC. Covington’s [107] results described SOC dynamics following forest harvesting in northern hardwood forests (New Hampshire, USA), showing a decline of 50% within 20 years following harvest. This decrease was attributed to accelerated decomposition and changes in litter inputs after harvest [118] and to changes in depth distribution of plant roots, altered soil water regimes and temperature regimes that accelerate decomposition [30], [87], [107]. SOC responds to the harvesting methods that have been applied: cut-to-length (short-wood), tree length and full tree (Supplementary Information). Cut-to-length and tree length contribute to the build-up of the SOC pool [94], while the full tree method increases ecosystem respiration and decreases total ecosystem and SOC stocks [[110]]. Soil respiration rates increase following harvesting but are substantially higher after full-tree harvest [119]. Johnson and Curtis [58] reported that harvest residues left on site after cut-to-length and tree length treatments increased SOC by + 18%, while full tree harvest caused a decrease (− 6%) over the long term. Post-harvest SOC dynamics are thus highly variable and depend not only upon silvicultural treatments, but also on a myriad of abiotic and biotic factors (e.g. soil texture and moisture, temperature, precipitation, species, stands). For instance, in three contrasting climatic zones (e.g. cool temperate, warm temperate and subtropical), Sun et al. [120] report that the total SOC of the 0–20-cm soil layer decreased with increasing mean annual temperature. Climate affects forest SOC by shaping both SOC inputs (changing plant productivity) and outputs (affecting soil fauna metabolism) [121].
Rotation Length
Another factor to consider in FMS at the stand level is rotation length, which is an effective way of managing the forest C budget [122]. Shorter rotation lengths are used in IFM, while long rotations are favoured in EFM. Rotation length is commonly used to manage timber yield, but it may also affect forest CSS depending upon tree species and management goals [123]. For instance, Scots pine (Pinus sylvestris L.) stands stored the largest total amount of C when applying the longest rotation length, which has a duration that can extend beyond 300 years [123]. Boisvenue et al. [122] have stated that C stocks were lowest in even-aged stands with lower rotation length (100 years), while it was higher in high rotation lengths with 400-year fire return intervals. Pérez-Cruzado et al. [124] asserted that increasing rotation length from 10 to 20 years for southern blue gum (Eucalyptus globulus Labillardière) in a humid temperate region (Galicia, Spain) could increase the C sequestration rate from 7.73 to 10.93 Mg ha−1 year−1 and mineral soil C from − 0.24 Mg ha−1 year−1 (lost) to 0.18 Mg ha−1 year−1 (gained). On the one hand, shortening the rotation length generally decreased the C stock in living biomass, but increased the C stock of soil, because the production of litter and harvest residues increased [125]. On the other hand, lengthening rotation not only retains more C by increasing average stem size, but also delays emissions that occur during harvesting [126].
Even-Aged Versus Uneven-Aged System
Considering industrial purposes and ease of operations, forests are managed conventionally as even-aged systems under IFM [99••]. However, silviculture systems affect forest CSS differently. Even-aged forests sequester and store more C in biomass than do uneven-aged systems that are used mainly in EFM, but these store more C in soils [126], [127]. For instance, Nilsen and Strand [127] found that long-term timber production in the uneven-aged stand was estimated to be 95% of the even-aged stand, and the difference in net C sequestration was 37 Mg ha−1 in 81 years in favour of the even-aged stand. Over 81 years, there was an increase in mineral soil C (21 Mg ha−1) in an uneven-aged stand compared to an even-aged stand, which was linked mainly to differences in the O horizon. In fact, the C content in the humus layer in the uneven-aged stand was 20% higher compared to that in the even-aged stand [127]. Moreover, based on model simulations, several studies have shown that uneven-aged systems may be the better alternative strategy. This should be taken into consideration when increasing C storage and sequestration rates over the long term. Indeed, Seidl et al. [128] concluded that a transition to uneven-aged forestry has considerable potential with regard to increasing C storage in forest ecosystems and achieving multiple management objectives. Taylor et al. [109], through a simulation that compared management effects of CP and CC in the Canadian boreal forest, found that total ecosystem C increased in uneven-aged stands. Similarly, in temperate forests, Nunery and Keeton [129] reported that C sequestration capacity was greater in uneven-aged systems.
Ruiz-Peinado et al. [99••] noted that uneven-aged stands have continuous litter inputs, thereby ensuring permanent soil and watershed protection, whereas in even-aged stands, there will be periods with no soil cover or only partial soil cover, which could lead to C losses. Laiho et al. [130] reported a 48% higher diameter increment in uneven-sized Norway spruce forests compared to that in even-sized forests over a 15-year period where the uneven-sized forest was treated with selection cuttings and the even-sized forest with thinning from below. Also, in Finnish forest stands, Pukkala et al. [131] found that uneven-aged systems were more profitable than even-aged plantations when other management objectives (timber and C) were considered. Nevertheless, designing silvicultural approaches that aim to establish an uneven-aged stand structure requires a deep understanding of the competition process. Harvesting intensity should also be considered. In fact, high harvesting intensities could decrease NEP and increase C emissions; in contrast, lower intensities could increase CSS in the long term, regardless of stand age structure [132].
Tree Species Compositions: Mixed Versus Pure Forest Stands
Tree species richness promotes productivity and affects nutrient and light availability [133], [134]. According to the facilitation and niche complementarity theories (Supplementary Information), mixed stands have higher C sequestration potential than do pure stands [72•]. Several studies have shown that ecosystems with high tree species diversity are more productive and sequester more C than low-diversity ecosystems [133], [134]. Erskine et al. [135] found that throughout the tropics, plantations with mixtures were more productive than monocultures, leading to a 55% increase (on average) in mean tree basal area. Litter production, nutrient return and leaf litter decomposition are higher in mixed plantations than in monocultures [136], which can increase the mobilization of nutrients in the soil and increase the productivity of the mixture considerably [137]. In Scots pine (Pinus sylvestris L.) and European beech (Fagus sylvatica L.) plantations, tree volumes (+ 12%), stand density (+ 20%), basal area growth (+ 12%) and stand volume growth (+ 8%) of the species were higher in mixture compared to the weighted mean of neighbouring pure stands [72•]. Tree species richness also could increase soil C stocks, through alterations to litter quality, nitrogen fixation and rooting patterns [99••]. For instance, Dawud et al. [138] found that mixed forests (combinations of P. sylvestris, Betula pendula Roth, Carpinus betulus L., Quercus robur L. and Picea abies [L.] H. Karst) had higher SOC stocks in samples that were taken from deeper layers of the soil profile (20 cm and 40 cm), compared to topsoil (0–10 cm). Generally, the ratio C/N in deeper layers is significantly and positively related to species diversity [138].
Several avenues of research have shown that pure and mixed stands do not differ in terms of the aboveground standing volume that they attain. Examples include Norway spruce (Picea abies) and European beech mixtures in Northern Europe or mixtures of black spruce and trembling aspen (Populus tremuloides Michaux) in Canada [139•], [140]. In the Canadian boreal forest, Légaré et al. [141] reported that aspen exerted a positive effect on black spruce productivity, but only when the aspen constituted < 40% of stand basal area. It seems that increasing NPP in mixedwoods requires a balance between species that are used and their densities, and should consider the spacing and spatial arrangement of the stands [141], [142]. Therefore, it is important to emphasize that the effects of mixtures on productivity vary with stand development stage, stand density and site conditions [72•].
Old-Growth Forest Conservation
Old-growth and intact forests are critical in stabilizing terrestrial C storage, maintaining biodiversity and providing other ecosystem functions [17••], [22]. These forests represent 22% of the world’s forested land surface (12 M km2), of which three countries (Russia, Brazil and Canada) account for nearly two-thirds of that area. Furthermore, Canada represents 25% of the world’s remaining primary forest, while tropical South America (including Amazon rain forest) represents 35%, but with a high rate of reduction or loss (7.1% during 2000–2013), compared to the boreal (0.3% in North America) and temperate (0.9% in South America temperate forest) forests [17••]. However, the extent of the world’s intact forests has been reduced by 7.2% (a reduction of 919,000 km2) since 2000; the primary global cause of this loss is industrial timber extraction that has resulted in forest landscape alteration and fragmentation [17••], [143].
Old-growth forests alone sequester about 1.36 Pg year−1 of C [144••]. The older trees (and stands) may exhibit reduced uptake rates, but the C that is stored in soils and biomass within them can greatly exceed that of younger stands [18], [144••], [145]. McGarvey et al. [146•] found that total C density is 30% higher (154 Mg ha−1), and deadwood C density is 1800% higher (120 Mg ha−1) in old-growth forests than in surrounding younger stands (5 Mg ha−1). Old-growth forests contribute considerably more to increased soil C from accumulated deadwood than do IFM and EFM [146•]. On the one hand, stand age correlates positively with tree biomass and C accumulation until an advanced age where net C uptake is thought to be balanced by respiration and increased mortality [147]. Therefore, older tree cohorts store C less efficiently in live woody tissues, although they can continue accumulating C [144••]. On the other hand, numerous studies expect that much of this C will move back to the atmosphere if these forests are disturbed or replaced with younger forests [148]. Current deforestation emissions are about 1.2 ± 0.6 Pg year−1 of C, but only 12% of the global intact forest area is protected [17••], [143]. During the period 2000–2013, the tropical regions are responsible for 60% of the total reduction of the intact forest; tropical South America lost 322,000 km2, whereas Africa lost 101,000 km2. The temperate and southern boreal regions contributed 21% to the global area loss [17••]. Consequently, the old-growth conservation strategy must be promoted by reducing deforestation and degradation. The latter problems need to be addressed in the short term, especially in the tropics.