*4.3.2.1 Active-restoration methods*

These are methods involving management techniques, such as planting of seeds or seedlings. They are needed to restore severely degraded lands and are particularly relevant for water-limited environments, where self-restoration processes of severely degraded lands may be limited. One such method is the Framework Species Approach developed in Queensland, Australia [42]. The method involves a single planting of both early and late successional species. Planted species must survive in the harsh conditions of an open site as well as fulfill the functions of (i) fast growth of a broad dense canopy to shade out weeds and reduce the chance of forest fire and (ii) early production of flowers or fleshy fruits to attract seed dispersers and kickstart animal-mediated seed distribution to the degraded site [43]. Framework trees are indigenous, non-domesticated, forest tree species, which, when planted on deforested land, help to reestablish the natural mechanisms of forest regeneration and accelerate biodiversity recovery. A rainforest-restoration experiment established on abandoned pasture in northeastern Queensland to examine the effectiveness of five different restoration-planting frameworks concluded that some restoration success measures increased with planting diversity, but overall the rate of recovery was similar in framework species and maximum diversity method [44].

Active restorations, using afforestation and reforestation methods, are effective biological approaches with the potential to help restore and rehabilitate degraded dryland ecosystems and halt desertification [45]. Among other benefits, rehabilitation improves the soil biological activities where high rates of soil organic matter, organic C and N, suitable soil acidity range, and abundance of forest litter are considered the predisposing factors promoting higher microbial populations in enrichment planting as compared to secondary forest [46].

Afforestation is the planting of forests on land that historically had no forests [47]. The main purpose of afforestation is to reduce soil erosion by planting trees, which increases soil stability and enables forest regrowth. Other purposes include improving the potential wood extraction in the future and improving the visual landscape. However, there have been concerns that conversion of "natural" drylands to dryland forestry may have adverse ecological and environmental impacts on the environment, thus risking a wide range of ecosystem functions and services. Attempts have been made to demonstrate the potentially adverse implications of dryland forestry and highlight the caution needed when planning and establishing such systems [45]. For instance, in order to negate suppression of understory vegetation and sustain plant species' richness and diversity, low-density savanization by non-allelopathic tree species is preferred over high-density forestry systems by allelopathic species. According to the author, and wherever possible, it is preferable to plant native tree species rather than introduced or exotic species, in order to prevent genetic pollution and species invasion. In addition, mixed-species forestry systems should be favored over single-species plantations, as they are less susceptible to infestation by pests and diseases. In addition, drought-tolerant, fireresistant, and less-flammable tree species should be preferred over drought-prone, fire-susceptible, and more flammable species.

#### *4.3.2.2 Passive-restoration methods*

These are methods in which no action is taken except to cease environmental stressors, such as agriculture or grazing, and are effective for restoring moderately degraded lands. In Eastern Kenya, for instance, the results of a study involving passive restoration show that woodlands have a high potential to recover if put under a suitable management regime since they have a high number of saplings [12]. The most commonly used of these approaches is the Assisted Natural Regeneration (ANR) approach, which acts on natural regenerates that are already present in deforested sites. The word "assist" in ANR refers to helping the naturally growing young trees to grow faster [48]. Assisted Natural Regeneration accelerates the natural succession process by protecting against disturbances, such as fire, stray domestic animals, and humans, and by reducing competition from grasses, bushes, and vines that would hinder the growth of naturally regenerated trees [48]. Forest restoration using ANR has advantages over conventional reforestation through planting by being cheaper to implement as costs associated with seedling production, site preparation, and planting are greatly reduced. The plant community that is established is well adapted to the site conditions, and the naturally regenerating plant community typically comprises a mixture of species that result in more diverse, multilayered vegetative cover [48].

Assisted Natural Regeneration is a flexible and adaptable approach that can be applied in a variety of contexts. It can, for instance, be combined with enrichment planting for various reasons including to fill in patches that may not have enough wildlings to establish tree canopy cover within the desired time frame, enhance the

#### *Combating Desertification through Enhancement of Woody Floral Diversity in the Drylands… DOI: http://dx.doi.org/10.5772/intechopen.100399*

success and quality of forest restoration, and restore ecologically and/or economically valuable species to meet specific restoration objectives [48]. Enrichment planting may be defined as the introduction of valuable species to degraded forests without the elimination of valuable individuals who already existed at that particular site [49]. In this technique, trees are planted in gaps, lines, or open sites as plantations of mixed species or under canopies of young dryland forests. In a study to identify the optimal enrichment planting method vis-à-vis gap and line planting, and to evaluate the performance of two dipterocarps and three legumes planted in logged-over mixed deciduous forest of Laos, the diameter and height growth were favored more in gaps than in planting lines [50]. Furthermore, the use of logged-over gaps for enrichment planting was recommended given the difficulty to maintain constant line width and even light condition, the cost of annual clean operation, and the rigid geometric patterns of planting lines [50]. In Indonesia, gap planting with *Anthocephalus macrophyllus* to rehabilitate degraded natural forests increased soil density, although its value was categorized as a very loose soil class [51]. In another study from Malaysia, the total mean microbial enzymatic activity, as well as biomass carbon (C) and nitrogen (N) content, was significantly higher under enrichment planting than under secondary forest [46].

There is little consensus on whether active or passive restoration strategies are more successful for recovering biodiversity because few studies make adequate comparisons [41]. In some studies, recovery of species' richness and composition is similar in active- and passive-restoration sites, while in others, recovery of forest specialists is enhanced through active restoration [40, 41]. While both restoration strategies may lead to different vegetation structures, they may support similar biomass of foliage-dwelling arthropods and be similarly used by foraging insectivorous birds [40]. Passive restoration is generally less costly than active restoration and, if local and landscape characteristics do not impede recovery, may be a viable alternative. Where active restoration is adopted, it should be implemented using mixed plantations of native tree species and, whenever possible, select sites close to mature forest to accelerate the recovery of tropical forest biodiversity [41]. Because active restoration is more expensive than passive restoration, both strategies should be used in complementarity at the landscape level for cost-effectiveness and optimization of the different land management objectives for the wider landscape [40, 41].

#### *4.3.3 Challenges of tree growing in drylands*

Tree planting in the drylands poses challenges to land users, which are brought about by a combination of edaphic, ecological, and socioeconomic factors in these areas. These include, among others, moisture stress, termite infestation, animal damage, and competition from weeds [8, 52]. Although the farmers and tree growers have developed interest in tree planting as an investment activity, they are discouraged by the continuous low tree survival rate and thus are not able to reap the maximum benefits from their tree crop. In a guideline intended for farmers and tree growers living in the drylands of Kenya, the common factors contributing to tree mortality at all stages of tree growing have been presented [8]. They also provide interventions that can be applied during species' selection, raising seedlings in the nursery, out-planting, and tree management to enhance tree growing in the drylands. The Kenya Forestry Research Institute (KEFRI), through the dryland forest research program, has identified major factors contributing to low tree survival and developed mitigation measures, which include the selection of appropriate tree species and development of suitable methods for propagating, establishing, and managing trees [8]. KEFRI has demonstrated better ways of re-afforestation especially in areas under limited water availability [52], proposed

#### *Deserts and Desertification*

species-site matching as a key consideration, especially with drought tolerance, and found mycorrhizal inoculation to greatly enhance the survival of trees planted in degraded areas, which have low mycorrhizal inoculum potential [53]. Inoculated trees have been used to restore the soil inoculum and to enhance the growth of interplanted agricultural crops. There are also opportunities to exploit tree/crop symbiotic associations in agroforestry systems, using trees selected both for their own attributes and for soil-improving qualities [53]. Farmers and tree growers need to adhere to these measures to improve tree survival and thus realize maximum profits from tree-planting activities.
