A fallow stand (FS) in northwestern Vietnam that was created by shifting cultivation 32 years earlier had 43% of the species number, 72% of the stem density, and 53% of the basal area when compared with nearby old-growth forest (OGF); however, the values for commercial species were lower at 35%, 67%, and 26%, respectively. In terms of species diversity, the Shannon index of OGF (3.4) was significantly higher than that of FS (2.6), while the differences were not significant in terms of Evenness and species-size class distribution. Both FS and OGF had similar patterns of stem diameter frequency distribution but the diameters were more diverse in OGF compared to FS according to the Shannon index. Fallow stand was characterized by only 2 canopy layers (lower than 10 m and 10–20 m) and was simpler in vertical structure than that of OGF which included an additional upper canopy layer higher than 20 m. Our results indicate that increasing stem density of commercial species is necessary and can be realized by artificial seeding, planting seedlings, and/or natural regeneration from remaining mother trees in the fields.
Agricultural encroachment by shifting cultivation (swidden or slash-and-burn agriculture) has been an important topic in the debate on tropical deforestation. Despite rapid economic development in many tropical countries, millions of people, particularly in the humid tropics, still practice shifting cultivation [
Regeneration of secondary forest is an essential part of shifting cultivation [
Studies of succession after shifting cultivation in tropical regions have also indicated that the diversity of woody species gradually increases with fallow age [
Since launching The Sedentary Farming and Resettlement Program by the Vietnamese Government in the 1970s for regions of northwestern Vietnam, shifting cultivation has declined, leading to increase in fallow areas, which have partly been converted to timber-producing forests. However, little attention has been given to the study of forest recovery after abandonment of shifting cultivation. Some of the important questions to be answered are recovery of species diversity, especially for commercial tree species, and forest structure. In this study, we evaluated species composition, diversity, and structure of a 32-year-old fallow stand (FS) recovered after shifting cultivation and compared it to old-growth forest (OGF) in northwestern Vietnam. The following hypotheses were tested.
(i) Species composition and richness of the 32-year-old FS are lower than that of the OGF; we plan to test this hypothesis with respect to the commercially valuable timber species in particular.
(ii) Forest structure in terms of diameter frequency distribution and canopy height is simpler in FS than in OGF.
The study was conducted in northwestern Vietnam at 21°15′N-103°24′E. The elevation of the area ranged from 600 to 1824 m above sea level. In this area, the vegetation is characterized by evergreen broad-leaved forest [
There are two main ethnic minorities living in the field site region: Thai and H’Mong. The Thai, consist of 15% of the total population, and live in lower elevations (<800 m) where they mainly cultivate paddy rice. The H’Mong (80% of the population) live in mountainous areas of middle and upper elevations (>800 m), and mainly practice pioneer shifting cultivation [
A small part of forest land in the research area is covered by old-growth forest (OGF), which is mostly located on mountain tops or high elevation zones or along streams. These areas are reserved by custom for water supply of the paddy rice grown at lower elevations and for daily water use. Disturbance of OGF is prohibited by law, but there has been some unauthorized logging of high value trees (e.g.,
A tree census was conducted 5 in a 32-year-old fallow stand (FS) recovered after pioneer shifting cultivation and in an old-growth forest (OGF). Age of the FS was identified by interviewing village patriarchs and was confirmed by using growth rings taken with tree increment borer. The eight largest trees, which varied from 20 cm to 54 cm in diameter at breast height (one of
Two main plots of one hectare (
Diversity indices [
In the present analysis, “species-size class” diversity is also used. Suppose that stand A contains
A one hectare survey plot for the 32-year-old fallow stand (FS) or that for the old-growth forest (OGF) may not represent all species present in that stand. Because a complete census is feasible only under a few special situations, it is necessary to estimate species richness by sampling the target species assemblage. There are a number of equations for species estimation; however, Chao [
The Shannon index [
The inequality of basal area-distributions was described by using the Gini coefficient [
The relationship between stem height and diameter was fitted following equation (
In total, we found 32 tree species belonging to 30 genera and 20 families in the 32-year-old fallow stand (FS), while 74 tree species belonging to 57 genera and 35 families were found in the old-growth forest (OGF). All species found in FS were also found in OGF. Individuals of all species in FS were present in the seedling stratum. This decreased to 29 species in the sapling stratum, and to 26 species in the tree stratum. Conversely, in the OGF sixty-one species were present in the seedling stratum, 65 in the sapling stratum, and 69 in the tree stratum. There was a total density of 5,550 stems
Total number of species and stem densities of tree, sapling and seedling strata in each one-hectare plot, as well as those (mean ±SD) in the subplots for the 32-year-old fallow stand and old-growth forest. Diversity indices (mean ± SD) are also given for both stands.
Thirty-two-year-old fallow stand (FS) | Old-growth forest (OGF) | |||||||||
One-ha plot | Mean of 16 sub-plots ( | One-ha plot | Mean of 16 sub-plots ( | Percent recovery | ||||||
Numberof species | Density (stems | Number of species | Density | Number of species | Density (stems | Number of species | Density | Number of species | Density | |
Total | 32 | 5,550 | ***11.5 ± 1.4 | **5,554 ± 2,472 | 74 | 7,670 | ***18.6 ± 3.6 | **7,570 ± 1,410 | 43.2 | 72.4 |
Tree stratum | 26 | 540 | ***9.2 ± 1.9 | 541 ± 167 | 69 | 550 | ***15.7 ± 2.7 | 553 ± 128 | 37.7 | 98.2 |
Sapling stratum | 29 | 1,190 | 10.3 ± 1.6 | *1,193 ± 432 | 65 | 940 | 11.1 ± 1.9 | *906 ±182 | 44.6 | 126.6 |
Seedling stratum | 32 | 3,820 | ***11.2 ± 1.0 | **3,819 ± 2,691 | 61 | 6,180 | ***8.8 ± 1.1 | **6,111 ± 1,458 | 52.5 | 61.8 |
2.66 | **2.15 ± 0.13 | 3.44 | **2.60 ± 0.21 | 77.3 | ||||||
0.77 | 0.89 ± 0.03 | 0.80 | 0.90 ± 0.03 | 96.3 | ||||||
2.99 | 1.88 ± 0.42 | 3.37 | 1.96 ± 0.39 | 88.7 | ||||||
0.65 | 0.54 ± 0.11 | 0.63 | 0.49 ± 0.08 | 103.2 | ||||||
Basal area | 14.2 | 26.7 | 53.2 |
Seedling stratum (
A mean of 11.5 (±1.4) species per
Species diversity index (
Fifty-six and 30 species were used by the local people for traditional uses (see the appendix). The traditional uses include collection of plant organs as food (fruit, leaf, etc.), medicine (headache, stomachache, etc.), construction material (housing, etc.), and other uses (resin, tannin, ornamental, etc.). The species that can be used for multiple purposes were numbered 18 in OGF and 12 in FS (see the appendix).
There were large differences between the observed species number and estimated species number in both FS and OGF. The number of species estimated for OGF was 96.3 (±12.8), which was 130% of the observed species number (74 species). The estimated number of species for FS was 40.0 (±6), 125% of the observed species number (32 species) (Figure
Chao’s species number estimation by number of surveyed plots. Percentage of overestimate = (number of estimated species − number of observed species)/number of observed species * 100. This was calculated for corresponding number of samples (sub-plots). OGF is old-growth forest and FS is a 32-year-old fallow stand.
Growth rings of
The shapes of the Lorenz curves were similar between 32-year-old fallow stand (FS) and old-growth forest (OGF). Both showed very slow basal area accumulation at the first 90% of population accumulation. In the 32-year-old fallow stand, accumulation of basal area was 0.14, 1.26, 3.27, 6.45, and 18.6% for 10, 30, 50, 70, and 90% of population accumulation in ascending order, respectively. On the other hand, accumulation of basal area was 0.04, 0.22, 0.60, 1.49, and 11.9% for 10, 30, 50, 70, and 90% of population accumulation in OGF. The value of the Gini coefficient was also similar between FS (0.71) and OGF (0.78) (Figure
The Lorenz curve as applied to size inequalities in plant populations. The horizontal axis is the cumulative percentage of individuals in a stand, summed from the smallest to the largest. The area between the curve and line of absolute equality expressed as a proportion of the area under the diagonal is called the Gini coefficient (G, [
Table
Commercial species presence in 32-year-old fallow stand and in old-growth forest.
Thirty-two-year-old fallow stand (FS) | Old-growth forest (OGF) | ||||
Commercial species | Proportion to all species (%) | Commercial species | Proportion to all species (%) | aPercent recovery in FS | |
Species number | 12 | 37.5 | 34 | 45.9 | 35.3 |
Tree stratum | 217 | 40.2 | 347 | 63.1 | 62.5 |
Sapling stratum (stems | 270 | 23.2 | 320 | 34.9 | 84.4 |
Seedling stratum (stems | 1,420 | 37.3 | 2,170 | 35.1 | 65.4 |
Total density (stems | 1,907 | 36.6 | 2,837 | 37.6 | 67.2 |
Basal area ( | 5.6 | 39.2 | 21.4 | 80.1 | 26.2 |
aPercent recovery is defined as the number of commercial species found in FS stand, divided by that in OGF forest.
List of species, families, commercially valuable species, and their traditional uses in 32-year-old fallow stand and in old-growth forest.
Species | Family | Commercially valuable species | Traditional uses | |||
Food | Medicine | Construction | Others | |||
Styracaceae | * | * | ||||
Betulaceae | * | Antiseptic | * | |||
Burseraceae | * | F | Antiseptic | * | ||
Burseraceae | * | Antiseptic, antithermal | * | |||
Fagaceae | * | * | ||||
Fagaceae | * | Fu | ||||
Anacardiaceae | * | Antiseptic | * | |||
Juglandaceae | * | Antiseptic | * | Po/fishing | ||
Fagaceae | * | * | ||||
Fagaceae | * | * | ||||
Theaceae | * | * | Fib/rope | |||
Myrtaceae | * | Antiseptic, antithermal | * | Po/fishing and hunting | ||
Euphorbiaceae | Antiseptic, antithermal | |||||
Mimosaceae | Antiseptic | |||||
Euphorbiaceae | F | Antiseptic, antithermal | ||||
Theaceae | Fu | |||||
Malvaceae | Fu | |||||
Rutaceae | Antiseptic | Po/fishing | ||||
Theaceae | Fu | |||||
Moraceae | ||||||
Clusiaceae | F, L | Antioxidant | ||||
Lauraceae | Fu | |||||
Lauraceae | Antiseptic, antithermal | |||||
Euphorbiaceae | Antithermal | Fib/rope | ||||
Rosaceae | F | Antiseptic | ||||
Verbenaceae | Antiseptic | |||||
Rubiaceae | Antiseptic | |||||
Anacardiaceae | Antithermal | |||||
Actinidiaceae | F | Antiseptic, antithermal | ||||
Styracaceae | * | |||||
Rubiaceae | ||||||
Annonaceae | ||||||
Aceraceae | * | O | ||||
Meliaceae | * | |||||
Alangiaceae | * | F | ||||
Lauraceae | * | * | ||||
Sapindaceae | * | * | ||||
Fagaceae | * | S | * | |||
Fagaceae | * | |||||
Fagaceae | * | * | ||||
Fagaceae | * | S | ||||
Meliaceae | * | |||||
Lauraceae | * | Oi/indoor fragrance | ||||
Tiliaceae | * | Fib/rope | ||||
Lauraceae | * | |||||
Fabaceae | * | * | Oi/indoor fragrance | |||
Cupressaceae | * | * | Ha | |||
Clusiaceae | * | * | ||||
Flacourtiaceae | * | |||||
Fagaceae | * | * | ||||
Lauraceae | * | Oi/ indoor fragrance | ||||
Annonaceae | * | |||||
Combrecaceae | * | * | ||||
Xylopia vielana | Annonaceae | * | * | |||
Aceraceae | ||||||
Rutaceae | ||||||
Meliaceae | ||||||
Theaceae | O | |||||
Sapotaceae | F | |||||
Elaeocarpaceae | * | Ta/dyeing | ||||
Rutaceae | ||||||
Clusiaceae | F, L | |||||
Ulmaceae | ||||||
Myristicaceae | ||||||
Lauraceae | ||||||
Sapotaceae | * | Oi/indoor fragrance | ||||
Lauraceae | ||||||
Rutaceae | Antiseptic, antithermal | |||||
Annonaceae | F | |||||
Rosaceae | * | |||||
Fagaceae | * | |||||
Sterculiaceae | F | |||||
Hamamelidaceae | ||||||
Symplocaceae | ||||||
Total in fallow stand | 12 | 5 | 18 | 12 | 10 | |
Total in old-growth forest | 34 | 12 | 19 | 26 | 19 |
Total number of species used were 30 and 56, while number of species used for multipurpose were 12 and 18 in fallow stand and in old-growth forest, respectively.
FSSpecies appeared in fallow stand; F: edible fruits; L: edible leaves; S: edible seeds; O: ornamental; Fu: fuel; Fib: fiber; Oi: oil; Ta: tannin; Po: poison; Ha: handicraft.
Both FS and OGF had a skewed bell-shaped distribution of stem diameters with greater representation of the small diameter classes; however, the positions of the peaks were different between the stands (Figure
Stem diameter distribution and diameter diversity index (Ddi) of 32-year-old fallow stand (FS; a, b) and old-growth forest (OGF; c, d). Ddi was calculated based on the Shannon index, in which relative frequency of stems in diameter classes and number of diameter classes were used. The asterisk (*) indicates significant difference of Ddi at 5% level by the
Seventy six percent of stems were smaller than the 10 cm diameter in OGF; the proportion increased to 88.6% for stems <20 cm dbh. Those proportions were 85.6% and 99.3%, respectively, in FS. There were only 0.7% of stems in the diameter classes greater than 20 cm in FS; this proportion was higher (11.4%) in OGF (Figures
Relationships between stem height and stem diameter of FS stand (a) and OGF forest (b). Only the individuals in tree and sapling strata were included for calculation. The curves were fitted by using a
By using chronosequence sampling of fallow stands, Tran et al. [
Recovery rate of the species number in the tree stratum (37.7%) was lower than that of the sapling stratum (44.6%) and seedling stratum (52.5%) (Table
The number of species increased from tree to sapling and seedling strata in FS while this pattern was reversed in OGF. This may be caused by the fact that seedlings and saplings of the pioneer species tended to be absent in OGF because of light deficiency on the forest floor. This pattern was not observed in FS, since recruitment into the sapling or tree strata always requires the seedlings to go through the seedling stage. In western Thailand, the number of seedling species in both OGF and 6-year-old fallow stands was only 30% of all tree species found. This may result from the limitation of seed rain and/or light deficiency in FS, since rotational shifting cultivation applied in that area resulted in high growth rate of vegetation in the first years after land abandonment [
Old-growth forest showed greater species diversity than FS both in terms of species number and the Shannon index. However, if variation of stem size class was also considered, there was no difference between OGF and FS in terms of species size-class diversity (Table
Traditional uses of plants for medicinal purposes have been reported by many studies [
The real species number present in OGF and FS may be much higher than the number of species observed (Figure
Since a portion of fallow land in the present study area is being converted to timber producing forests, presence of commercially valuable species is an important indicator for evaluation of forest restoration. Forty-six percent of the species found in the old-growth forest (OGF) were commercially valuable, while it was 37.5% in the 32-year-old fallow stand (FS). Proportion of the stems of commercial species was quite high (63.1%) in OGF and much higher than that of FS (40.2%) (Table
The patterns in frequency distribution of stem diameters were similar between OGF and FS for the pool of all species (Figures
Even if stem numbers of saplings and trees are high, the values of stand basal area are mostly determined by the contribution of a small number of large individuals in both FS and OGF (Figure
Vertical structure (canopy height) of OGF was different from that of FS. Canopy of OGF included an upper layer of trees taller than 20 m (Figure
The complexity of forest recovery after shifting cultivation that has been observed in various forest types, disturbance regimes, climate conditions, and so forth challenge restoration ecologists. Appropriate restoration strategy should be sought by considering this complexity. Requirements on the rate of recovery and the similarity in species composition to the surrounding old-growth forest are most important considerations. If relying only on natural regeneration, the fallow stand in the present study may require 45 years for its stem density to return to the condition of OGF, 60 years for its basal area, and 70 years for its species diversity. It may require an even longer time for a group of commercially valuable species to reach abundance similar to that of OGF, especially in terms of basal area. Adding stems of commercial species by artificial seeding and/or planting may be an effective way to shorten the recovery process. Low ratio of seedlings (37.3%) and saplings (23.2%) of commercial species in FS may have resulted from their competition with seedlings of non-commercial species. Therefore, silvicultural treatments such as thinning and forest floor clearing may be necessary. However, the approach of stem addition and/or other silvicultural operations may not be applicable to a large area since it is costly to apply labor intensive techniques, compared to the approach of natural regeneration. Therefore, remnant trees of commercial species may be used as the seed source for natural regeneration in the fallow stand in the present study area. Then, it is also necessary to consider the trade-off of low crop production due to shading by crowns of other adult trees.
For more details see Table
The authors thank researchers of Chi