Old-growth and regrowth forest structure
Compared with regrowth forests, old-growth forests in Napo and Dong Na Tard had higher structural heterogeneity because of higher canopy height variations (uneven canopy), vertical stratification, and tree sizes and their variations. It appears that it takes considerably longer time (more than 15 years) for regrowth forests in Napo and Dog Na Tard to attain the structure of an old-growth forest. Earlier studies have also demonstrated that it takes several decades for regrowth forests to attain the structure of old-growth forests (Guariguata and Ostertag 2001; Chua et al. 2013; Mukul et al. 2016).
Above-ground biomass (AGB) of the old-growth forests at both sites is similar to other well-stocked mixed deciduous forest in the region (Table 9), but higher than the values reported by Vicharnakorn et al. (2014) for mixed deciduous forest in the Savannakhet Province (143.95 Mg ha−1), which included heavily degraded forests. This reflects the semi-protected status of the old-growth forest areas in this study, where access rights by the surrounding populations for collecting specific products from the forest might influence forest biomass (signs of recent logging were few in both places, but signs of human activities were clear).
Comparable references for ABG in regrowth forests are difficult to obtain as biomass values are strongly influenced by among other factors, the time the area was under a land use different than forest, the activity in place before abandonment, any soil treatment, and whether remnant trees were maintained during the non-forest phase. Regrowth forests of similar ages from Myanmar report average biomass values close to the ones reported in this study (Table 10). However, these values are well below other less disturbed regrowth forests ranging from 148 to 224 Mg ha−1 in the Philippines as reported by Mukul et al. (2016).
Napo’s regrowth forest is dense and homogeneous with small diameters and few trees reaching above the closed canopy and these characteristics suggest an even aged forest sharing the same genesis. Dong Na Tard’s regrowth forest was structurally less homogeneous and remnant trees were easily spotted. The remnant trees in the canopy were dominated by Xylopia vielana, Cratoxylum formosum, and Anisoptera costata, which were all used locally; Xylopia and Cratoxylum are both medicinal plants while Anisoptera is an endangered valuable timber tree.
The size class structure of regrowth forests differs from that of old-growth forests, such that it is more skewed towards the younger trees (Fig. 4). The high number of seedlings and saplings in the regrowth forests show that there was a robust natural regeneration, which in turn indicates that there was adequate supply of seeds via seed rain and/or the soil seed bank contained sufficient amount of viable seeds, which may be the pre-disturbance legacy of the site. In addition to germination from seeds, resprouting may have also contributed substantially to the natural regeneration, particularly if the site was cultivated for a short period.
Floristic diversity and composition
In general, the regrowth forests tend to have similar species richness (except in Dong Na Tard), diversity indices and evenness values compared to the old-growth forests (Tables 2, 3). The findings of this study are in agreement with earlier studies which state that species richness of tropical regrowth forests can increase very fast and reach that of old-growth forest in a few decades (Guariguata and Ostertag 2001; van Breugel 2007; McNamara et al. 2012). The proximity of the regrowth forests to the old-growth forests (literally bordering each other) may be the main reason for the fast recovery of species richness in the regrowth forests. It was demonstrated in this study that in general species richness recovers more rapidly than forest structure and this is in line with earlier findings (Piotto et al. 2009; Mukul and Herbohn 2016).
On the other hand, the woody species abundance of regrowth forests was considerably higher than that of old-growth forests. For instance, the woody species abundance of regrowth forests in Napo was almost twice that of the old-growth forest. The finding of this study is in accordance with other studies which state that stem density in regrowth forests is generally higher than old-growth forests (Chazdon et al. 2007).
Although by and large similar in species richness, the regrowth forests were significantly different from the old-growth forests in terms of species composition as shown by Fig. 5a, b and Table 4. This difference in species composition is also illustrated by Table 5a, b, i.e. the indicator species for old-growth forests in Napo and Dong Na Tard were entirely different than that of the regrowth forests. It seems regrowth forests were dominated by some widespread species as indicated by the relatively lower standard deviation of the Bray–Curtis similarity index and lower range of similarity values (Fig. 6). This finding concurs with earlier studies which state that regrowth forests are dominated by early successional and widespread habitat generalist species (Cain and Shelton 2001; Peña-Claros 2003; van Breugel 2007; Edwards et al. 2017).
In the later phase, because of the persistence of long-lived pioneer species and the long turnover time of canopy trees, species composition of regrowth forests takes considerably longer time to approach that of old-growth forests compared with forest structure and species richness (Guariguata and Ostertag 2001; van Breugel 2007; Dent and Wright 2009; Chua et al. 2013). In general, it appears that regrowth forest woody species richness recovers first followed by forest structure and species composition. By and large, regrowth forest recovery in terms of structure, species richness, and composition depends on land-use history, distance to an old-growth forest, ongoing disturbance, etc. (Sovu et al. 2009; McNamara et al. 2012).
Notwithstanding, regrowth forests being useful in conservation of woody species, they do not, however, harbour the full suite of species found in old-growth-forests, particularly the dispersal limited late-successional species. Furthermore, old-growth forests tend to contain relatively more rare species as found in this and previous studies (Brunialti et al. 2010; Ngo and Hölscher 2014; Shima et al. 2018). The structural heterogeneity of old-growth forests, which is manifested by high tree height and diameter class variations and higher number of strata, provides niches for various species (McElhinny et al. 2005; Gao et al. 2014). Hence, the conservation of old-growth-forests nearby regrowth forests, which serve as source of propagules and a habitat for numerous species are crucial for the conservation of the woody species diversity within a landscape (Dent and Wright 2009; Gibson et al. 2011).
Woody climbers (lianas) and vines were quite abundant in the old-growth and regrowth forests at Napo and Dong Na Tard. This observation concurs with earlier findings which state that woody climbers are common and they are one of the main growth forms in tropical forests—they constitute ca. 25% of the woody stem density (abundance) and species diversity (Schnitzer and Bongers 2002). The most frequent and dominant woody climber species found in old-growth forests were by and large different than the ones found in the regrowth forests (Table 7). In other words, the woody climbers found in the regrowth and old-growth forests may represent the early-successional and late-successional species, respectively.
The herbaceous ground cover percentage of the old growth-forests were found to be significantly higher than that of the regrowth forests at both sites. This is most probably due to the low amount of light reaching the forest floor as a result of the high stem density and relatively uniform canopy (in terms of height) of the regrowth forests.
Implications for conservation and rehabilitation
Regrowth forests are continuing to proliferate throughout the tropics. It is reported that as much as 60% of the world’s remaining tropical forests are degraded and regrowth forests (Dent and Wright 2009). Hence, in addition to the declining pristine forests, regrowth forests are of paramount importance in conservation and restoration of tropical biodiversity and they can also serve as a conduit for the restoration of degraded lands and forests.
This study showed that after about 10 to 15 years, regrowth forests can have a similar species richness as that of an old-growth forest and they have a potential to serve as biodiversity repositories. One of the main factors affecting the recovery of regrowth forests is the proximity to old-growth forests and hence the protection of particularly older regrowth forests, which are close to old-growth forests should be given a priority. In addition, regrowth forests of different age groups may have species assemblages belonging to different successional groups and need to be protected (Chazdon et al. 2009). In general, as Crouzeilles et al. (2016) suggest, areas with an intermediate degree of disturbance such as regrowth forests, should be considered a priority for landscape restoration activities as they have the potential for larger gains associated to biodiversity and vegetation structure, thus increasing the conservation value of any given investment. However, at present regrowth forests are underrated and the deforestation of regrowth forests is about three times higher than other forest categories in the lower Mekong Basin (Heinimann et al. 2017).
As there is significant difference in species composition between regrowth and old-growth forests, the regrowth forests may not conserve all the species found in old-growth forests. Hence, enrichment planting of rare/threatened and dispersal-limited species with seedlings grown from germplasms that are better adapted to future climate change (Dawson et al. 2011) may be necessary to speed up the recovery process depending on the objectives of the rehabilitation endeavour. On the other hand, regrowth forests are dynamic and transient in nature and may develop to a forest comparable to an old-growth forest through natural succession, if they are well-connected to an old-growth forest and the past land use was not intensive. In such cases, minimal or no human intervention is required.
Pockets of regrowth forests embedded within a landscape can also serve as a source of propagules for the autochthonous re-colonization of the surrounding degraded or bare forest lands. Degraded lands in the vicinity of regrowth forests can be rehabilitated by converting them to regrowth forests through natural seed dispersal and regeneration, which is relatively inexpensive compared to afforestation or reforestation.
Regrowth forests have also the potential to serve as buffer zones around the fragmented remnant old-growth forests and may ameliorate edge-effects, reduce anthropogenic disturbances, and enhance landscape connectivity (Pardini et al. 2005; Van Breugel et al. 2013). Buffer zones composed of regrowth forests provide additional habitats for the various organisms residing in the core habitat (Chazdon et al. 2009). In addition, regrowth forests can serve as ecological corridors by connecting isolated remnant forest and agroforestry patches, which improves the dispersal and movement of the flora and fauna as well as the viability of small populations in fragmented landscapes (Chazdon et al. 2009; Morse et al. 2009; Schroeder et al. 2010). In particular, regrowth forests located along the riparian zone function as stream bank stabilizer and ecological corridor connecting the up and down stream areas (Heartsill-Scalley and Aide 2003). In general, the strategic location of regrowth forests in the human-modified landscapes is important in terms of in situ biodiversity conservation and rehabilitation of degraded forest lands.
In Laos, about a quarter of the rural population is involved in shifting agriculture and around 34.6% of the forests are affected by shifting cultivation, which is a considerably large forest area (Sovu et al. 2009; Higashi 2015). By protection of the extensive area of regrowth forests, which are established on fallow lands as a result of shifting cultivation, Laos can rehabilitate a substantial part of its degraded forests and increase the forest cover of the country. Laos aims to increase its forest cover to 70% by 2020 (Sovu et al. 2009). This type of mainly passive restoration is a cheap and acceptably quick method. In conclusion, regrowth forests established on fallow lands in Laos can play a key role in biodiversity conservation and restoration of degraded forests and lands; hence, regrowth forests should be given the due considerations in the national forest conservation and restoration policy of Laos.