Tree cavities are important structural elements of forest ecosystem that host numerous birds, mammals and other cavity-dependent organisms. Pattern of cavity distribution in temperate and boreal forests are relatively well studied, yet little is known about cavities in tropical and subtropical forests. We compared cavity availability in relation to tree condition (living tree and snag), tree species and DBH class between two different sites in a subtropical deciduous sal forest in Nepal: the Chitwan National Park Forest (the park site) and the Khorsor Buffer Zone Forest (the buffer site). Surveys for tree cavities were conducted in 2013 on 50 circular sample plots of size 0.1 ha. We recorded 40 cavity trees in the park site and 31 cavity trees in the buffer site. Density of cavities was on average 22.4 ha-1 in the park site and 19.2 ha-1 in the buffer site. Cavities occurred mostly in living trees (85.9% cavity trees) and were formed mostly by damage and decay (natural cavities: 74%) or by woodpecker activity (excavated cavities: 26%). Most were observed on three tree species:
Cavities in trees are an important ecological resource as nesting sites or shelters for many birds, small mammals, reptiles and other organisms (
Cavities are predominantly found in old living or dead trees, however, bird species often occupy cavities located in middle-age living trees of medium size (
It has been demonstrated that woodpeckers play a keystone role in some forests by providing cavities for other cavity-nesting species (
Although tree cavities are critical component of forest ecosystems worldwide, they have been mostly investigated in Europe and North America (
In this study we examined the density and origin of tree cavities in the sal forests (
The study was conducted in the subtropical lowlands of the inner Terai region, namely in the Chitwan National Park (CNP), and in the adjoining buffer zone forest at Khorsor, in a valley between Siwalik and Mahabharat Mountain ranges, south-central Nepal (
The sal forests covering substantial part of the CNP are regularly burned in early spring with low intensity fires, mostly burning the dry litter. Removal of dead wood is illegal though occurring (
Fieldwork was done during the dry season in March 2013. The study area included two sites with different forest conditions: (1) a site within the CNP, with relatively low human pressure (the park site); and (2) a managed forest corridor of buffer zone at Khorsor (the buffer site), where human pressure was higher (
For all trees within each sample plots, the species, the diameter at breast height (DBH) and the overall condition (live or dead) were recorded. Only trees with DBH ≥ 10 cm were recorded. Each tree was carefully inspected by at least two persons with binoculars. Detected cavities were checked using a ladder or/and a telescopic pole (up to 13 m long) with a low light diode and a webcam connected to a laptop. Using this device and a ladder we could investigate cavities up to 22 m. Therefore, the number of tree cavities should be considered as the minimum number of cavities present on the plots, as some trees with cavities were out of reach. We defined a cavity as a hole in the tree with a minimum entrance diameter of 2 cm, with the interior and bottom sufficient for small birds to nest in (a minimum bottom diameter of 5 cm). Very large cavities (entrance diameters > 15 cm or depth > 70 cm) were excluded from the analysis, since the study was focused on small and middle-size cavity-dependent birds.
Regarding the origin, cavities were divided into two categories: (i) non-excavated (natural cavities); (ii) excavated by woodpeckers (woodpecker-made cavities).
All sampled trees were divided into eight DBH classes: (I) 10-19 cm; (II) 20-29 cm; (III) 30-39 cm; (IV) 40-49 cm; (V) 50-59 cm; (VI) 60-69 cm; (VII) 70-79 cm; (VIII) 80+ cm. Six continuous density variables were analyzed: (a) of cavity trees; (b) all cavities; (c) natural cavities; (d) woodpecker-made cavities; (e) live trees; and (f) dead trees. First, differences in the above-mentioned variables between the two study sites were examined by applying the Mann-Whitney’s and the Chi-square tests, using the software package STATISTICA® version 10.0 (
where the electivity (
Second, to analyze the relationship between several predictor variables (tree characteristics) and the binary response variable “incidence of tree cavities” (0, 1), generalized linear models (GLMs) with binomial error distribution were computed using the statistical software R version 3.0.2 (
In total, we recorded 1041 live trees and 38 snags. The density of live trees was very similar at both sites (228.0 ha-1 in the park site, N = 25; and 228.4 ha-1 in the buffer site, N = 25). The density of snags was twice as high in the park site compared to the buffer site, and this difference was significant (on average 10.0 and 5.2 ha-1, respectively,
The DBH of all live trees in this study ranged from 10 to 111 cm. Overall, trees belonging to DBH classes I and II corresponded to 54.0% in the park site (N = 25) and 51.3% of all live trees in the buffer site (N = 25). Trees with DBH ≥ 50 cm were 23.9% of the total trees in the park site and 22.4% in the buffer site (
Density of trees with at least one cavity was slightly higher in the park site (on average: 16.0 ha-1, N = 25) compared to the buffer site (12.4 ha-1, N = 25), but such difference was not significant (p > 0.05 -
Considering all 50 plots from both sites together, only one cavity was found in 50 out of 71 cavity trees, while 21 trees had from 2 to 5 cavities. In 16 trees two cavities were found, and only five trees had more than two cavities. In live cavity trees an average of 1.41 ± 0.88 cavities (n = 61) were found, and in snags 1.80 ± 1.23 cavities (n = 10), but this difference was not significant (z = 1.17, p = 0.2406).
We recorded cavities in eight tree species. In both sites most cavities were found on trees of three species,
Cavity trees were on average thinner in the park site (DBH 38.7 ± 19.97 cm, n = 40) compared to the buffer site (47.7 ± 22.53 cm, n = 31), but these differences were not significant (z = -1.95, p = 0.0508). DBH of cavity trees ranged from 12 to 111 cm, and differed depending on the species: the thickest were
The generalized linear models indicated that tree species, tree condition and DBH were important variables for predicting the presence of cavities (
The density of cavities was on average 22.4 ± 4.04 ha-1 in the park site (N = 25) and 19.2 ± 3.31 ha-1 in the buffer site (N = 25), and these differences were not significant (p >0.05 -
Despite the different management regimes, we found that the two study sites (the park site and the buffer site) were structurally similar. This may be explained by the fact that both sites are included in a conservation area, with only some differences in the human impact (see below). Indeed, the two study sites represent an intermediate part of a gradient from natural to heavily-managed sal forests, whose extremes are likely to present more pronounced differences. Nonetheless, our study sites differed in several structural parameters like density of snags, density of several tree species and the distribution of size classes of trees. Moreover, we found a positive effect of tree size and condition on the presence of cavities,
The lower density of snags observed at the buffer site may be easily explained by a more intense removal of dead wood by villagers. The Chitwan National Park authorities does not allow removal of dead trees within the park, though some snags are illegally removed regardless (
As compared to temperate forests, snags are generally less abundant in the tropical forest due to their higher decomposition rates (
The thinnest DBH classes (I and II) of living trees represented half of all trees found in each studied site, indicating a well established regeneration at both sites. The most common species
An interesting finding is the reverse pattern of occurrence of
The average density of cavities in our study was higher than the global average estimated by
Although most cavities were found in
Cavity trees were found mostly to be relatively thick (average DBH: 43 cm). Most cavities were found on living trees rather than on snags in both forest types. Similar result were found in a tropical forest of Costa Rica (
Our study revealed that non-excavated cavities in both forest types (77%) were more common than excavated (woodpecker-made) cavities. In a mature forest in Mongolia three-fourth of secondary cavity birds nested in non-excavated cavities (
Although we did not find any evidence that human impact may reduce the density of tree cavities, a lower density of snags at the buffer site compared to the park site was found. Dead trees are relatively important for the occurrence of tree cavities. Furthermore, based on our results we cannot exclude that several differences could exist in the usage by dependent organisms of snag cavities as compared with those found on living trees. According to the operational plan of forest management in the Khorsor buffer zone, selected dead, dying, diseased, depressed wood is allowed to be removed from the forest. Based on the above considerations, we recommend to ensure the presence of some snags in the managed forests, as they are the potential substrate for cavities of specialized species, as well as the habitat for beetles and other insects which represent the preferential food for woodpeckers. Moreover, we found that non-excavated cavities on living trees were more abundant than woodpecker-made cavities at both study sites. Based on our findings,
Our study is based on a limited dataset not covering the whole spectrum of environmental conditions of the sal forests in Nepal. Indeed, the two study sites represent a small part of the broader gradient from natural to heavily-managed sal forests. Therefore, our results should be considered as an initial assessment of cavity resources in this broadly-distributed forest ecosystem. Further studies are needed to throw light on the use of cavity resources by the community of cavity-dependent organisms, as well as on
This research was funded by the Swedish University of Agricultural Sciences, Sweden, and Siedlce University of Natural Sciences and Humanities, Poland (19/91/S). Anil Gurung, Suresh Chaudhari, Shiva Raj Thanet, Prakash Chapaghai and Bindu Adhikari helped us in the field work. The Institute of Forestry, Pokhara Campus, Department of National Park and Wildlife Conservation, and the Chitwan National Park and National Trust for Nature Conservation authority made our work possible by providing permission to conduct fieldwork. We thank all of the contributors. We also heartily thank Kerry L. Nicholson for valuable comments on an earlier version of manuscript and for language improvement.
Location of the study area.
Distribution of all live trees according their DBH classes in the sal forest. Colors represent the two study sites: green - the park site (lower human pressure, N = 25) and yellow - the buffer site (higher human pressure, N = 25). Mean and SE are shown.
Distribution of
Distribution of
Distribution of
Distribution of dead trees according their DBH classes in the sal forest. Colors represent the two study sites: green - the park site (low human pressure, N = 25) and yellow - the buffer site (higher human pressure, N = 25). Mean and SE are shown.
Percentage of cavity trees in relation to tree species availability in: (a) the park site (low human pressure); and (b) the buffer site (higher human pressure). All live and dead trees combined. (SR):
Average DBH of the most common tree species with cavities (live and dead combined) in all study plots in the sal forest. Boxes represent the mean ± SD, whiskers represent ± SE.
Percentage of cavity trees in relation to DBH classes availability in: (a) the park site (low human pressure); and (b) the buffer site (higher human pressure). All live and dead trees combined. Ivlev’s electivity index in parentheses (positive values: overrepresented DBH classes; negative values: underrepresented DBH classes). Colors represent trees: grey - resources, orange - cavity trees.
A comparison of the stand characteristics between Chitwan National Park (the park site) and Khorsor buffer zone forest (the buffer site).
Variables | Park site(N = 25) | Buffer site(N = 25) | Mann-Whitneytest | |||
---|---|---|---|---|---|---|
Mean ± SE | Range | Mean ± SE | Range | U | P | |
Live trees ha-1 | 228.0 ± 14.15 | 40-330 | 228.4 ± 19.81 | 40-470 | 299.5 | 0.808 |
Snags ha-1 | 10.0 ± 2.30 | 0-50 | 5.2 ± 2.24 | 0-50 | 206.5 | 0.041 |
Cavity trees ha-1 | 16.0 ± 2.44 | 0-40 | 12.4 ± 1.66 | 0-30 | 267.5 | 0.388 |
Cavities (all) ha-1 | 22.4 ± 4.04 | 0-50 | 19.2 ± 3.31 | 0-60 | 274.0 | 0.461 |
Natural cavities ha-1 | 16.0 ± 2.82 | 0-40 | 14.8 ± 3.0 | 0-50 | 291.5 | 0.691 |
Excavated cavities ha-1 | 6.4 ± 1.72 | 0-30 | 4.4 ± 1.30 | 0-20 | 279.5 | 0.528 |
Density of tree species (trees ha-1) in the park site and the buffer site.
Tree species | Park site (N = 25) | Buffer site (N = 25) | ||
---|---|---|---|---|
Mean ± SE | Percentage | Mean ± SE | Percentage | |
|
112.8 ± 12.27 | 49.5 | 104.4 ± 13.68 | 45.7 |
|
44.4 ± 7.03 | 19.5 | 49.2 ± 11.42 | 21.5 |
|
43.6 ± 7.85 | 19.1 | 10.0 ± 3.78 | 4.4 |
|
6.8 ± 3.35 | 3.0 | 1.6 ± 1.24 | 0.7 |
|
4.8 ± 2.71 | 2.1 | 0 | 0 |
|
4.4 ± 2.45 | 1.9 | 1.2 ± 0.87 | 0.5 |
|
4.0 ± 2.16 | 1.8 | 0.4 ± 0.40 | 0.2 |
|
2.4 ± 1.44 | 1.1 | 4.0 ± 1.6 | 1.8 |
|
2.0 ± 1.63 | 0.9 | 0 | 0 |
|
0.8 ± 0.55 | 0.4 | 0.4 ± 0.40 | 0.2 |
|
0.8 ± 0.80 | 0.2 | 0 | 0 |
|
0.4 ± 0.40 | 0.2 | 2.4 ± 1.44 | 1.1 |
|
0.4 ± 0.40 | 0.2 | 0 | 0 |
|
0.4 ± 0.40 | 0.2 | 0 | 0 |
|
0 | 0 | 51.6 ± 9.18 | 22.6 |
|
0 | 0 | 2.4 ± 1.32 | 1.1 |
|
0 | 0 | 0.4 ± 0.40 | 0.2 |
|
0 | 0 | 0.4 ± 0.40 | 0.2 |
Total | 228.0 | 100 | 228.4 | 100 |
Number of cavity bearing trees and number of cavities according to tree species in the park site (totally 2.5 ha) and in the buffer site (2.5 ha).
Species | Park site | Buffer site | Total | ||||||
---|---|---|---|---|---|---|---|---|---|
Cavity trees | No. ofCavities | Cavity trees | No. ofCavities | Cavity trees | No. ofCavities | ||||
Live | Dead | Live | Dead | Live | Dead | ||||
|
14 | 2 | 20 | 15 | 2 | 21 | 29 | 4 | 41 |
|
13 | 2 | 20 | 3 | 1 | 6 | 16 | 3 | 26 |
|
4 | 0 | 5 | 6 | 0 | 13 | 10 | 0 | 18 |
|
0 | 0 | 0 | 1 | 0 | 5 | 1 | 0 | 5 |
|
0 | 0 | 0 | 2 | 0 | 2 | 2 | 0 | 2 |
|
0 | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 1 |
|
1 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 1 |
|
1 | 0 | 1 | 0 | 0 | 0 | 1 | 0 | 1 |
Unknown species | 0 | 3 | 9 | 0 | 0 | 0 | 0 | 3 | 9 |
Total | 33 | 7 | 57 | 28 | 3 | 48 | 61 | 10 | 104 |
Generalized linear models (GLMs) predicting the probability of a cavity occurrence in trees (N= 1049, cavity present = 68, cavity absent = 981). The top-ranked model (first line), selected by step-wise deletion using the Akaike’s information criterion (AIC), was used to estimate the parameters reported in
Model | AIC | ΔAIC |
|
AUC |
---|---|---|---|---|
Species + Condition + DBH | 474.52 | 0 | 0.02 | 0.743 |
Species + Condition + DBH + Site | 474.99 | 0.47 | 0.08 | 0.749 |
Species + Condition + DBH + Site + Condition × DBH | 475.71 | 1.19 | 0.21 | 0.750 |
Species + Condition + DBH + Site + Condition × DBH + Site × DBH | 477.71 | 3.19 | 0.31 | 0.752 |
Species + Condition + DBH + Site + Condition × DBH + Site × DBH + Species × DBH | 480.88 | 6.36 | 0.39 | 0.786 |
Parameter estimates for the minimal adequate model (AIC = 474.52) for the prediction of the probability of cavity occurrence in trees (N = 1049, cavity present = 68, cavity absent = 981). The incidence of cavities in trees was the response variable (binomial GLM).
Coefficients | Estimate | Std. Error | z value | Pr(>|z|) |
---|---|---|---|---|
Intercept | -2.307 | 0.612 | -3.768 | <0.001 |
Other species | -0.685 | 1.086 | -0.631 | 0.528 |
Species |
-14.820 | 1088.0 | -0.014 | 0.989 |
Species |
-0.244 | 1.090 | -0.224 | 0.823 |
Species |
0.008 | 1.105 | 0.007 | 0.994 |
Species |
-1.464 | 0.502 | -2.915 | 0.004 |
Species |
0.670 | 0.427 | 1.570 | 0.116 |
Species |
-2.296 | 0.850 | -2.701 | 0.007 |
Species |
0.313 | 1.096 | 0.286 | 0.775 |
Species |
-14.750 | 1314.0 | -0.011 | 0.991 |
Tree condition Live | -1.376 | 0.499 | -2.756 | 0.006 |
DBH | 0.044 | 0.009 | 5.217 | <0.001 |