Although black locust produces annually a very large number of seeds, because of their thick tegument, successful natural stand regeneration is missing almost completely ^[4][13]. Exceptionally isolated trees can be recognized as being from seeds in very particular sites, such as upper sides of river banks ^[6][14] or at pile burning sites where forest residues are burned ^[7][15]. Therefore, vegetative regeneration of harvested stands by mechanical stimulation of root sprouting and artificial regeneration by plantation of nursery seedlings in reforestation and afforestation of non-forest lands are preferred and largely practiced across the country ^[8][2].

2.2. Vegetative Regeneration

While the harvesting technique is always clear cutting, stand regeneration occurs via sprouts on the aboveground part of the stump or suckers by mechanical stimulation of roots by: (a) ploughing of the soil among the stumps at 20 cm depth; or (b) digging out the stump together with the roots around it in a radius of 50 cm combined with additional ploughing. When regeneration targets root-suckers, the sprouts must be removed through early tending operations, so as to not overwhelm the slow growing root suckers ^[4][13]. After two cycles of vegetative regeneration from sprouts, the stumps weaken and lose their sprouting capacity (due to fungal impact and soil nutrient depletion); and by the third generation, the stand’s productivity drops to 40% of that of plantations ^[9][16]. That is why, after a second generation of sprouts, regeneration must be ensured from root-suckers ^[10][17].

2.3. Generative Regeneration

The recommended technique for seedling production includes harvesting the seeds directly from trees (late in the winter or early January) or separated from litter and mechanical or chemical ‘forcing’ before sowing next spring. Seedlings are almost without exception ready for transplantation after one season of growth (e.g., root collar diameter of 4 mm). After a second year in the nursery, they generally surpass the planting dimension standards which are associated with substantial risk of not surviving summer drought. Although, for afforestation in such areas, specific standards apply to ensure more robust plants are used—e.g., root collar diameter of 8 mm ^[10][17]. The seeds for nursery production of seedlings are often collected from seed orchards and have to meet standard requirements for purity, technical germination, and mass ^[11][18].

2.4. Afforestation Techniques

According to technical norms in force in Romania ^[12][19], afforestation with black locust is exclusively carried out by planting bare-root seedlings in hand-dug holes. This method has proved so successful in practice that there has been no more research on other methods, although associated costs may be prohibitive in an expensive labor market. The literature states that sowing the previously treated seeds can be used to create black locust stands ^[4][13]. Planting can be done both in spring as in autumn. Spring planting ensures adequate soil humidity (common in upper altitude regions), while planting in autumn (usually in the southern ad low-altitude regions) ensures root growth over winter and avoids spring flooding and early droughts ^[13][20]. The technical norms state that, in the case of sandy soils or degraded lands, optimum planting density for black locust is 5000 seedlings per hectare—with 2.0 m of distance between rows and 1.0 m between seedlings—and the planting pit is 40 cm wide and 40 cm deep. In the case of non-degraded lands, the planting density for black locust is 4000 seedlings per hectare—with 2.0 m of distance between rows and 1.25 m between seedlings—and the planting pit width, length, and depth is 40 cm. In order to ensure the highest survival rate, a key technical intervention after planting seedlings is trimming of their stems 1 cm above the soil before growing season starts ^[13][20].

2.5. Tending Operations

Being a fast-growing species, canopy closure takes place early (1–3 years following planting), therefore tending operations (especially cleaning and thinning) need to be made at shorter periods of time (3–5 years) depending on the tree origin (seedlings or root-suckers) and density of the stand. Periodicity of tendings varies in function of stand development (size of trees) and influences its productivity (stand density influences the rate of growth) as well as its protective function, in which case higher stand densities are preferable only on degraded lands due to the slenderness coefficient ^[10][17].

2.6. Stand Management

Forest management planning sets either productive or protective objectives for black locust forests, following strict management indicators: land use, silvicultural regime, stand structure, rotation, and harvesting technique. When dealing with such technical requirements, the forest management plan has to follow ecological, social, and economic local needs. While some stands’ fate is wood production (mainly timber and construction wood), many stands have additional strong protective functions (e.g., degraded land reclamation, protection of soils and infrastructure, etc.).

2.7. Silvicultural Regime

In Romanian forestry law and official technical norms ^[12][19], the recommended form of management system for black locust stands is the coppice. Two forms can be differentiated: the simple one applied extensively and the selection coppice applied locally and occasionally. The simple coppice consists of clear-cutting of the stand followed by vegetative regeneration or seedling plantations. In the coppice selection system, a part of the sprouts/root-suckers from each stump can be preserved, eliminating only the crooked ones and those that have reached the targeted diameter. It can be applied with experimental purpose in the case of plantations on severely degraded lands or with small privately owned forests.

2.8. Stand Composition and Structure

Black locust generally achieves large areas of continuous pure stands. Being a typical exclusivist species, stands are generally pure and even aged, their intraspecific competition being poor ^[14][24]. Nevertheless, on degraded lands, it is planted together with other exotic species. Black locust can tolerate other species, most commonly honey locust (Gleditsia triacanthos L.) on sandy soils in the south and east of the country, or with black pine (Pinus nigra L.) on eroded slopes in the east of Romania ^[14][15][24,25]. It behaves as an exclusivist species for ground vegetation, black locust stands have very poor herbaceous diversity (e.g., frequent are Urtica sp., Sambucus ebulus).

2.9. Harvesting and Rotation/Cycle Length

Black locust stands have the shortest rotation length in Romanian forestry. Rotation length differs as a function of the forest primary purpose (wood production or protection). If the stand primarily has a production purpose (e.g., timber or construction wood), harvesting age varies between 15 years for coppices classified under the lowest production class to 35 years for the planted stands classified as the highest production class. If classified as having protection function (e.g., steep terrains prone to soil erosion), the harvesting age can reach 40 years in stands ^[12][19]. Wood harvesting occurs through clear-cuts in plots with a maximum area of 5 ha.

3. Genetics, Selection, and Tree Breeding

An outstanding natural variety—Robinia pseudoacacia var. oltenica—was identified in the stands located in the south-west part of Romania (Oltenia region) by Bârlănescu, Costea, and Stoiculescu in 1966 ^[16][26]. Due to its valuable auxological and morphological characteristics ^[17][18][27,28], this variety was the subject of tree selection and breeding by grafting/cutting ^[19][20][29,30] and in vitro micropropagation. Later on, eco-physiological research by Bolea at al. ^[21][31] showed that this variety has both higher intensity of photosynthesis and tolerance to droughts, due to larger leaf area index (LAI), compared to common stands. Conservation and expansion of the Oltenica variety was continued in recent years by producing seedlings from cuttings, as maternal lineages of identified plus-trees ^[4][13]. Genetics research prior to 1990 focused on tree selection and breeding for enhancement of productivity and wood standing volume ^[19][22][23][29,32,33], as well as selection of valuable forest and beekeeping genotypes by hybridization. Trials consisting in testing different provenances in comparative cultures, identified “plus” trees and enhanced the number of clones and establishing vegetative propagation methods (e.g., grafting) for the valuable forms ^[24][34].

4. Vulnerability of Black Locust

Systematic records of biotic and non-biotic factors and damage data in Romanian forests have been collected through the Forest National Survey and Forecast System operations since 1958. Based on such data, the main cryptogamic diseases affecting Robinia pseudoacacia L. were listed as follows ^[11][18]: virosis, Fusarium sp. and Cuscuta sp. attacks on seedlings, mildew (Oidium sp.), sooty mold (Coniothyrium sp., Alternaria sp., Cladosporium sp.), leaf spots (Phleospora robiniae) and tar spots on leaves (Ectostroma sp.), twig blight (Pseudovalsa sp.) and Chorostate sp. on young shoots, black spots (Cucurbitaria sp.) on old shoots, and wood-destroying fungi (Hironela sp, Trametes sp., Phellinus sp.).

5. Wood Products and Other Uses of Black Locust

5.1. Wood Production

As a result of high percentage of coppice stands ^[25][54], the size of the black locust round wood usually does not exceed 20 cm in diameter at thin end, and quality of timber is relatively poor compared to key native species. In most of the Romanian regions, short longevity because of the tree dieback is the main cause for rather small tree dimensions comparative to local broadleaved tree species. Therefore, majority of wood generated by black locust stands is mostly used as firewood or in rural construction (e.g., fences, sheds, poles) or less for props for gardening and vineyards ^[26][55]. The good quality wood can also be used for interior and garden furniture, parquet and floors or woodchip boards while more common uses are for fenceposts, poles, railroad crossties, stakes and fuel wood ^[27][28][29][56,57,58].

5.2. Non-Wood Products

Black locust is so appreciated by foresters and farmers alike due to a range of non-wood products that are available already at an early age of stands ^[30][60]. The most valued non-wood byproduct is honey, considered of the highest quality ^[31][61], which makes it the most expensive on the market. Compared to the traditional melliferous species (linden/lime, black locust, sunflower, rapeseed), black locust is the first to bloom (in May); therefore, beekeepers start the pastoral beekeeping in the black locust forests. Black locust stands have a high melliferous potential in Romania with up to 697,000 tonnes of honey per year. In order to ensure that the full benefit is reached, an application to support the planning of the pastoral beekeeping was developed based on forest maps, as a tool for decision makers at a national level in the planning of pastoral activity of the beekeepers. Black locust flowers show medicinal use in teas or infusions for digestive and pulmonary effects, and they have a calming effect on the nervous system ^[32][33][34][62,63,64]. A mix of flowers and leaves can be used as a tea drink to treat stomach pains and migraine.

6. Landscape Improvement Contribution

In Romania, black locust was very successful in reclaiming degraded lands ^[35][67] by exercising its anti-erosion role together with phytoremediation ^[36][68], and biomass accumulation in site conditions strongly prohibitive to other species ^[37][69]. On industrial or mining dumps, black locust plantations reached volumes of up to 73 m³/ha among 8-year-old stands ^[38][70]. It was also successfully used in the ecological remediation of historically heavily polluted lands ^[39][71]. In a Kyoto Protocol project of afforestation implemented in Romania, about 2500 ha were afforested with black locust in the south of the country to reclaim degraded and marginal agricultural lands ^[40][72]. Ten years after their afforestation, black locust plantations have reached heights of 14 m and basal diameters of 16 cm (Figure 2).