Study area

This study was carried out in the tropical montane cloud forest region of central Veracruz, Mexico. The climate is mild and humid year-round. Total annual precipitation is ca. 1600 mm and mean temperature is 14-18 °C. In this area, we selected two locations. Xolostla (XO, 19º 32’ 13’’N, 96° 58' 06’’W, 1430 m a.s.l.), which was converted to pasture in 1980, and abandoned after 10 years of use, at which time it had a few Acacia pennatula (Schltdl. & Cham.) Benth. trees. Mesa Yerba (MY, 19º33’ 42” N, 97º 01' 21”W, 1875 m a.s.l.) was deforested for wood and carbon extraction, and abandoned in 1990 at which time it was dominated by Pteridium arachnoideum (Kaulf.) Maxon. A detailed disturbance history for each location is given in Pedraza & Williams-Linera (2003).

In 1998, ecological restoration plantations were established at each site. In six 30 × 36 m blocks, 720 seedlings of four native tree species were planted, 10 seedlings of each species along three lines with 3 m spacing between lines. The species used were Carpinus tropicalis Walt., Juglans pyriformis Liebm., Liquidambar styraciflua L. and Podocarpus matudae Lundell. The vegetation around each tree planted was cleared with a machete during the first 18 months, then once in 2004 and again in 2014 (Pedraza & Williams-Linera, 2003; Williams-Linera et al., 2016).

 

Experimental design

In 2016-2017, we monitored the trees planted in both plantations. Each tagged tree was located or declared a non-survivor. All planted trees were surveyed, their diameter was measured at 1.3 m above ground level, and height was estimated using a clinometer. We selected a forest fragment 3 Restoration plantations and recovery of tropical cloud forest Results near each plantation (< 1 km away) as its reference forest. Our original databases consisted of ten 10 m x 10 m plots in each reference forest and were used to analyze tree species composition and vegetation structure. In each plantation, ten 10 m x 10 m plots were distributed throughout the planted area randomly. In each plot, tree species ≥ 5 cm diameter were measured, counted and identified. Unknown species were collected for later identification in the XAL herbarium of the Institute of Ecology, A.C. (INECOL). Scientific names were checked using the Tropicos database (www.tropicos.org) of the Missouri Botanical Garden. Richness (S), the Shannon diversity index (H’) and the Shannon equitability index (EH = H’/ln S) were estimated per site.

The successional stages used to classify tree species were pioneer (shade-intolerant) and non-pioneer or forest tree species based on previously published research, unpublished shade-tolerance databases and field observations (Muñiz-Castro et al., 2006; López-Gómez et al., 2008). For the tree species, seed dispersal mode follows van der Pijl (1972): anemochorous (seeds dispersed by wind), barochorous-synzoochorous (large seeds dispersed by gravity or animals that do not ingest the seeds) and endozoochorous (small seeds dispersed by birds and small mammals) (Muñiz-Castro et al., 2006; López-Gómez et al., 2008).

 

Statistical analyses

Basal area, density and mean and maximum height were calculated for all plots, and differences between active restoration (planted trees and woody plant regeneration) and reference forests were tested with Student’s t-test, using a significance level of p < 0.05. Prior to running the t-test, data normality (Shapiro-Wilk’s test, p > 0.05) was assessed and the data log-transformed when necessary (basal area, mean and maximum height). Statistical analyses were run in JMP v. 10.0.0 (SAS Institute, Cary, NC, USA). Values reported are mean ± SE unless otherwise indicated.

The number of species and individuals classified by shade tolerance and seed dispersal mode were compared between active plantations (natural regeneration only) and reference forests using the G statistic for the log-likelihood ratio goodness-of-fit test (G-test). To describe similarities in both the species composition and abundance of tree species in the active restoration (without planted trees) and forests, we used nonmetric multidimensional scaling (NMDS). The presence of groups of sites was determined using a matrix of 40 plots and 36 species with two or more individuals. This analysis was carried out in autopilot mode, slow and thorough, Sørensen distance measurements, and 500 iterations. Ordination was run in PC-ORD software (McCune & Grace, 2002).

 

ResultsTop

Planted tree species performed differently between the two 19-year-old active restoration plantations; Liquidambar styraciflua had a higher basal area and better survival in MY than in XO, whereas Juglans pyriformis and Podocarpus matudae grew and survived better in XO than in MY, and Carpinus tropicalis had relatively high values of both variables in MY and XO (Table S1 [suppl.]). Survival in planted trees was 59% and 44% in XO and MY, respectively. The basal area of planted trees was 7.2 and 7.9 m2/ha, while density was 658 and 493 trees/ha in XO and MY, respectively. Planted tree species reached a maximum average height of 5 to 16 m (Table S1 [suppl.]).

Overall, the vegetation structure values for the active restoration (including both planted trees and natural regeneration) were close to those of the reference forests (Fig. 1). In XO, the forest had a higher basal area than active restoration (t = 4.10, p = 0.0007, Fig. 1a). In MY, basal area did not differ between forest and active restoration (t = 0.60, p = 0.56; Fig. 1b). Tree density in XO did not differ between sites (t = 1.29, p = 0.21; Fig. 1c), but in the MY tree density was higher in active restoration than in the forest (t = 5.29, p < 0.0001; Fig. 1d). Forests had a higher mean (XO, t = 4.16, p = 0.0017; MY, t = 4.95, p = 0.0004; Fig. 1e, f) and maximum height (XO, t = 2.32, p = 0.039; MY, t = 7.88, p < 0.0001; Fig. 1h, g) than the restoration plantations.

In total, 49 tree species > 5 cm diameter were recorded (Table S2 [suppl.]). In the reference forests, richness was 14 tree species in XO and 21 in MY. In plantations, woody regeneration included 16 tree species in XO and 15 in MY (Table S2 [suppl.]). Shannon’s equitability index tended to be similar in both XO and MY reference forests (0.85 and 0.83, respectively), and also in XO active restoration (0.86), but was lower in MY active restoration (0.75) (Table S2 [suppl.]).

The proportion of tree species classified as pioneer and non-pioneer was different in forest and active restoration in XO and was similar in MY (G-test = 6.02, p < 0.05, G-test = 1.78, p > 0.05, respectively, Fig. 2a). However, the proportion of individuals differed between treatments (XO G-test = 75.91, MY G-test = 68.6, p < 0.01, Fig. 2b). Non-pioneers represented > 75% of the tree species, and > 90% of the individuals in the reference forests (Fig. 2a, b). In active restoration, non-pioneers represented ca. 50% of the species and individuals recorded (Fig. 2).

The proportion of tree species with different seed dispersal modes was similar in forests and active restoration plantations (XO G-test = 1.63, MY G-test = 0.41, p > 0.05, Fig. 3a), however, the proportion of individuals was different between treatments (XO G-test = 28.85, MY G-test = 66.0, p < 0.01, Fig. 3b). Anemochorous, barochorous-synzoochorous and endozoochorous seed dispersal modes were represented in both active restoration and reference forests (Fig. 3). In XO, the proportion of wind-dispersed species and individuals was similar between the forest and active restoration. In MY, the proportion of endozoochorous species and individuals was similar in forest and active restoration, however the proportion of anemochorous was greater and the proportion of barochorous-synzoochorous individuals was smaller in active restoration than in forest (Fig. 3a, b).

The NMDS ordination achieved the greatest reduction in stress with a three-dimensional solution (final stress = 19.1, final instability = 0.00031, 500 iterations). The proportion of variance represented by Axis 1 and Axis 2 was 0.24 and 0.16, respectively. The Monte Carlo test indicated that the extracted axes were significantly different from those expected by chance (p = 0.004). NMDS analysis showed that species composition clearly differs between the XO and MY locations. In XO there is a clear 5 Restoration plantations and recovery of tropical cloud forest trend of active restoration to merge with forest plots, whereas in MY, there is a trend separating active restoration and forest plots (Fig. 4).

 

Figure 1.  Vegetation structure in the Xolostla (XO) and Mesa Yerba (MY) cloud forest areas of central Veracruz, Mexico. Sites are mature forest and 19-year-old active restoration (planted trees and natural regeneration). Basal area (a, b), density (c, d), mean height (e, f), maximum mean height (g, h). Values are the mean and standard error are shown. * indicates significant differences between means (p < 0.05) and NS indicates non-significant differences.

 

Figure 2.  a) Proportion of species and b) Proportion of individuals. Tree species are classified by successional stage (pioneer and non-pioneer trees) in mature forest and 19-year-old active restoration (natural regeneration without planted trees) in XO and MY cloud forest areas of central Veracruz, Mexico.

 

Figure 3.  a) Proportion of species and b) Proportion of individuals. Tree species are classified by seed dispersal mode (anemochorous, barochorous-synzoochorous, endozoochorous) in the reference forest and 19-year-old active restoration (natural regeneration without planted trees) in XO and MY cloud forest areas of central Veracruz, Mexico.