Stand Stability of Pure and Mixed Eucalyptus Forests of Different Tree Species in a Typhoon-Prone Area

Stand Stability of Pure and Mixed Eucalyptus Forests of Different tree Species in a Typhoon-prone Area

Haiyan Deng

South China Agricultural University

Linlin Shen

Jiaqi Yang

Xiaoyong Mo (  [email protected] )

South China Agricultural University https://orcid.org/0000-0002-8004-1499

Research

Keywords: Mixed forest,Preservation rate,Stability,Stemform,Non-spatial structure Posted Date: February 3rd, 2021

DOI: https://doi.org/10.21203/rs.3.rs-160935/v1

License:   This work is licensed under a Creative Commons Attribution 4.0 International License.

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1 Stand stability of pure and mixed eucalyptus forests of different tree 2 species in a typhoon-prone area

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Haiyan Deng, Linlin Shen, Jiaqi Yang, Xiaoyong Mo^*

Full names and email addresses:

Haiyan Deng, [email protected], South China Agricultural University. Linlin Shen, [email protected], South China Agricultural University. Jiaqi Yang, [email protected], South China Agricultural University. Xiaoyong Mo (Corresponding author), [email protected], South China Agricultural University.

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Abstract

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Background: Stable stand structure of mixed plantations is the basis of giving full play to forest ecological function and benefit, and the pure Eucalyptus plantations with large-area and successive planting have presented to be unstable and vulnerable in typhoon-prone area. In this study, we investigated eight 30 m ×

30 m plots randomly in pure and mixed eucalyptus forests on growth status, characterized and compared

the distribution of non-spatial structure of mixtures with that of mono-species plantation, and evaluated the stand quality and stability from eight indexes including preservation rate, stand density, height, diameter, stem form, degree of slant, tree species composition and age structure, so as to find out the best mixed composition and pattern of eucalyptus and other tree species in typhoon-prone area.

Results: Eucalyptus surviving in the mixed forest of Eucalyptus and A. mangium (EA) and the mixed forest

of Eucalyptus and P . e lliottii × P . c aribaea (EP) were 5.0% and 7.6% greater than those in pure Eucalyptus

forest (E) respectively, while only the stand preservation rate of EA was greater (+2.9%) than that of pure Eucalyptus forest. The proportions of all mixtures on the height class that greater than 7 m were fewer

* Correspondence: [email protected] College of Forestry and Landscape Architecture, South China Agricultural University, Guangzhou 510642, China

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than that of monoculture. The proportion of EA and EN (mixed forest of Eucalyptus and N.cadamba) on the diameter class that greater than 7 m were 10.6% and 7.8% respectively more than that of monoculture. EN got a highest ratio of branching visibly (41%), EA got a highest ratio of slant stems (8.1%) and EP got a most straight and complete stem form (68.7%). The stand stability of the mixed forest of Eucalyptus and A. mangium presents to be optimal for its subordinate function value (0.76) and status value (ω = 0.607) of real stand were the largest.

Conclusion: A. mangium is a most superior tree specie to mix with Eucalyptus for a more stable stand structure in the early growth stage to approach an evident and immense stability and resistance, which is of great significance for the forest restoration of Eucalyptus in response to extreme climate and on the forest management.

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Keywords: Mixed forest, Preservation rate, Stability, Stem form, Non-spatial structure

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Background

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As a fast-growing tree species, Eucalyptus have been introducing and promoting genetic improvement in many regions to produce large amounts of wood for economic development (Zhou et al. 2020). But large-area planting and continuous-planting rotation of commercial Eucalyptus plantations have caused various problems such as reduced species diversity, loss of soil nutrients, which threatens ecological and

timber security regionally and worldwide (Wen et al. 2005; Ye et al. 2010; He 2019; Zhu et al. 2019; Zhou

et al. 2020). Constructing the mixed forests can form stratified stand structure, which helps to make full use of forest land space and environmental resources, increase the light energy utilization, regulate the climatic environment within and outside the forest, improve forest land environment, so as to improve forest productivity, increase species diversity, enhance the forests ability to resist the disaster, exert forest ecological protection benefits, promote the ecological balance (Le Maire et al. 2013; Santos et al. 2017; Liu et al. 2018). So it is necessary to choose suitable and superior tree species to build high quality Eucalyptus mixed plantation. Nevertheless, the growth of mixed plantation is mainly restricted by the choice of the site and tree species, the collocation of row spacing, the mixing proportion and pattern. In view of the

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complexity of these factors, it is still rough to mix different tree species with Eucalyptus in line with expectations and to ensure the ecological and economic benefit and realize sustainable development of

Eucalyptus plantation.

The research on Eucalyptus mixed plantations have got under way for nearly 43 years since J.J. Burdon and G.A. Chilvers did preliminary studies on a native Australia Eucalyptus forest invaded by exotic Pinus radiata in 1977 (Burdon and Chilvers 1977), and few years later rigorous research had been done on mixed-species plantations of Eucalyptus and other tree species with scientific experimental design at one site (Chu et al. 1981; He et al. 1988; Kelty 2006). Later, other scholars also did a lot of researches on mixed Eucalyptus plantation,but most studies on Eucalyptus mixed plantation focused on water use (Forrester et al.

2010), light use (Le Maire et al. 2013), carbon allocation (Forrester et al. 2006; Nouvellon et al. 2012; Huang 2013; Wen et al. 2020; Zhang 2020), productivity (Forrester et al. 2007; Nouvellon et al. 2012; He 2019), nutrient cycle (Santos et al. 2017; Voigtlaender et al. 2019; Yao et al. 2019; Zhang et al. 2021), microbial communities (Huang et al. 2017; Pereira et al. 2019) or succession dynamics (Forrester et al. 2004;

Santos et al. 2016) and less research had been done on the structural features of mixed Eucalyptus plantation.
Forest structure determines the service function of forest ecosystem by influencing forest environment and biological factors (Gong et al. 2012). A reasonable stand structure is not only the basis of giving full play to forest function and benefit (Kong 2013) but also the main driving force of forest ecosystem succession. The study on stand structure is the theoretical basis of forest management and analysis and the comprehensive reflection of stand development process such as tree species competition, natural succession and disturbance activities (Jiang 2015).
The stand structure includes spatial structure and non-spatial structure. Spatial structure, generally using mingling, neighborhood comparison, uniform angle index, open degree, competition index and forest layer index for evaluation, mainly refers to the point pattern of individuals and the spatial distribution of their attributes. Non-spatial structure, evaluated by factors reflecting stand characteristics, including the tree species composition, the stand density, the tree height distribution, the diameter distribution, the canopy structure, the tree species diversity, the tree vigor, the tree stability, etc (Gadow et al. 2012; Hui 2013; Li et al. 2016; Zhu 2016; Wei et al. 2019), generally describes the average state of stand structural characteristics, which is independent of the spatial properties of single trees. The distribution structure of these

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characteristic factors can reflect the overall stability and resistance of the forest and the quality of the stand, to some extent. Peng Shaolin analyzed the age structure of the forest community and the species diversity of each age level, and then concluded that the age structure of the forest community can represent the stability and succession dynamics of the community (Peng 1987). The research results of O 'Connor et al. showed that the population structure of tree species had a significant influence on stability, which could be used as a conclusion for future stability (O 'Connor et al. 2017). Through the study on the structure of stand, we can understand their distribution rules and the interaction relationship between tree species (Kong 2013),so as to provide theoretical guidance for forest management. The spatial structure of pure Eucalyptus forests is particularly monotonous. We hypothesized that the structure of Eucalyptus plantations mixed with other tree species is improved and change greatly, the stand stability and stress resistance on resisting external adverse disturbance is enhanced tremendously, indicating the correct selection of mixed species and the success of mixed pattern. In windy areas, the forest with a successful mixed pattern can slow down wind speed and reduce the economic loss caused by wind damage, which is also the full embodiment of ecological function (Yang et al. 2009). Therefore it is of great significance to study the stand structure of Eucalyptus plantation mixed with different trees species, especially the Eucalyptus plantation in windy areas.

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Taken the preservation rate, stand density, height, diameter, the stem form, the degree of slant, tree species composition and age structure as evaluation indexes of the non-spatial structure, this paper aims to analyze the structure difference on Eucalyptus plantation of monoculture and mixtures that mixed

respectively with three different tree species (Neolamarckia cadamba, Acacia mangium and Pinus elliottii ×

P. c aribaea), embarking from the actual growth status of each plantation, to explore the stand stability and resistance of the monoculture and mixtures of eucalyptus forests and find out the best composition and pattern of eucalyptus mixed with tree species in typhoon-prone area.

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Material and methods

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Site characteristics

The experimental field was located in Leizhou Peninsula in Jijia Town, Zhanjiang City, Guangdong Province (20º 26' - 21º 11' N, 109º 42' - 110º 23' E) of , with an elevation of 35 m and a gentle terrain of nearly 0º slope. The region is dominated by humid monsoon climate of tropics with a mean annual temperature of 23.5 ºC, a mean annual precipitation of 1855 mm, a mean annual evaporation capacity of

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1762.9 mm, and a relative humidity of 82-84%, presenting an obvious dry and wet season (May - September - rainy season, with south wind blowing mainly, and November - next April - dry season, with north wind blowing mainly). Frequent tropical storms or typhoons lands in Leizhou Peninsula for 1 - 3 times every year, which is the primary natural calamities in Leizhou Peninsula.

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Characteristics of the experimental plantations

These experimental plantations were constructed in February 2014, including one pure plantation of

Eucalyptus urophylla × E. grandis ‘DH32-29’ (E) as control and three mixed Eucalyptus (DH32-29)

plantations that mixed with three kinds of tree species respectively: (1) Mixed plantation of Eucalyptus and

Neolamarckia cadamba (EN) was mixed by strips for four rows of Eucalyptus and four rows of N. cadamba;

(2) Mixed plantation of Eucalyptus and Acacia mangium (EA) was mixed by strips for four rows of

Eucalyptus and four rows of A. mangium; (3) Mixed plantation of Eucalyptus and Pinus elliottii × P . caribaea (EP) was mixed by rows for one row of Eucalyptus and one row of P . e lliottii × P . c aribaea. Since

the biological characteristics of different tree species are different intrinsically, Eucalyptus ‘DH32-29’ and P .

elliottii × P . c aribaea were planted at spacing of 1.3 m ×3.0 m, N. cadamba was planted at spacing of 3.9 m

×3.0 m, and A. mangium was planted at spacing of 2.6 m ×3.0 m. They are all fast growing excellent tree species with strong adaptability to grow in Leizhou Peninsula, However, They were damaged by severe wind damage every year after planted (See Table 1 for details). Due to the catastrophic damage of typhoon "Mujigae"(Super Typhoon, Wind force ≥ 51.0m /s, Beaufort scale ≥ 16) in October 2015, all Eucalyptus trees were blown down with stems broken, hence all Eucalyptus were cut off from basal stem to facilitate new branches germinated and renew the whole stands. Recover measures that erecting leaning or fallen trees were taken for mixed species to restore stands. In June 2016, one strong branch of each individual of Eucalyptus was preserved and the others were cut off. Other tending measures were consistent across the four stands.

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Table 1 Appearance situation of tropical cyclones in Leizhou Peninsula from 2014 to 2017

Suffering time
Maximum wind speed at Beaufort

Landing date

Name

Number code

Intensity rank

landfall (m/s)

scale

Page 6 of 29 Super Typhoon

(SuperTY)

7:30 p.m. 9:40 a.m. 2:10 p.m. 3:40 p.m.
Rammasun Kalmaegi
1409 1415 1522 1608
60/72
42
17 13
July 18, 2014
Severe Typhoon
(STY)
September 16, 2014

SuperTyphoon

(SuperTY)
October 4, 2015 August 18, 2016
Mujigae Dianmu
50/52 20/28
15/16
8
Tropical storm
(TS)

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Data measurements

By the end of April 2017, two 30 m × 30 m plots were set randomly in each mixed Eucalyptus forest (including three mixed stands and one pure stand) adopting the random sampling method to measure the

height, the diameter, the straight-fullness and brunching status of stem form, the slant of stem of each tree

individual and record the number of preserved and missing trees. The height (H, unit: m) was measured with a laser altimeter (Nikon Rangefinder Rieho 1000AS) (precision: 0.1 m). When the diameter at breast height (DBH) of an individual tree was less than 4 cm, the groud diameter (unit: cm) was measured with a vernier caliper, otherwise the diameter at breast height (DBH, unit: cm) of each individual tree was measured with a tape (precision: 0.1 cm). Both the stem form and the degree of slant of each individual were evaluated by the classification methods as Table 2 for statistics.

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Table 2 Classification criteria of tree stem form

Stem form description
Degree of slant

description

Classification

The tree has one single stem that is complete and straight (well-formed).
The tree stem grow vertically without lean to any side.

Ⅰ

The tree has one single stem that is complete and curved slightly.

Ⅱ

The tree stem has 0° to 30° oblique.
The tree has more than two stems that are branched below a third of its height.

ⅢⅣ
ⅢⅣ

The tree stem has 30° to 60° oblique.

The tree has more than two stems that are

Page 7 of 29 branched higher than one third and lower than two thirds of its height. The tree has more than two stems that are branched higher than two thirds of its height.

ⅤⅥ
ⅤⅥ

The treetop was broken off.

The tree has one single stem that is bent badly.

The tree stem was broken off.

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Data Processing and Statistical Analysis

Eight indicators of non-spacial structure were assigned as preservation rate, stand density, height, diameter, stem form, degree of slant, tree species composition and age structure.

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Preservation rate = Number of retained plants/number of planted plants ×100%. The tree height distribution was calculated by using the tree height class integration method: every two m was integrated as one tree height class, and the median value of the group represented the tree height class and was involved in the calculation. The diameter distribution was calculated by using the diameter class integration method: every two cm was integrated as one diameter class, and the median value of the group represented the diameter class and was involved in the calculation. The stem form and slant status of stem were analyzed through their classification.
Two tree species and two ages exist in the mixed forests, one tree species and one age exist in the pure forests, hence the tree structure and age structure of the mixed forest were assigned a value of while that of the pure forest was assigned a value of 1. Other evaluation indexes were assigned as their average values when evaluated. The values of stability indexes were standardized firstly adopting the method of subordinate function value of fuzzy mathematics (Guo et al. 2009; Yu et al. 2015; Yang et al. 2020) and forward to make it dimensionless between [0,1]. The formula is as follows:

(x_ij x_{i min}

)

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U_ij

(1)

(x_{i max} x_{i min}

)

Where U_ijis the subordinate function value of i indexes of j stand, and U_ik∈[0,1]. x_ijis the measured

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value of i indicator of j stand, x_iminis the minimum value of i indexes of j stand and x_imaxis the maximum value of i indicator of j stand.
The stability of four Eucalyptus forests were assessed by the method of π value rule of optimal stand state
(Hui et al. 2016; Wan et al. 2020). The methods are as follows: Draw a circle with a radius of 1 and divide

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the circle of 360° into 8 sector areas which representing eight non-spatial structure indexes of the stand respectively; Starting from the center of the circle of the eight sector areas, draw corresponding index lines in the form of radiation and mark the index names; Mark the corresponding index value of each stand on the radiation with points, connect adjacent points in turn to form a closed curve, which represents the stability status of the real stand.
When all indicators equal to one, the maximum circular area of π can be regarded as optimal stand state.
The ratio of the state value of the real stand to the optimal stand is used to judge the stable degree of the state value of the real stand, and the formula is:

nm1

 (m 1)



s

2i



s₁ s₂



n

i1

181 182

 