The role of plant species diversity in maintaining ecological balance in oil palm plantation

The level of plant species diversity in oil palm plantations is thought to be decreasing in number. The existence of other plant species is considered to be a competitor for oil palm plants in obtaining water and plant nutrients. Thus, the diversity of plant species gets less attention because they are considered to have no role. We address this problem by analyzing the level of plant species diversity and the role of plant species diversity in maintaining the ecological balance in the plantation environment. This research was conducted on 4 types of landcover in TPR oil palm plantations (young growth, medium growth, old growth, HCV) and 2 smallholder oil palm plantations, by vegetation analysis with the single plot method. The results showed that the number of species on the research site was 49 species. The highest value of species richness and diversity was obtained in smallholder oil palm plantations (Dmg=3.11 and H′=2.18), while the highest evenness value was found in old growth (E=0.79). Based on the literature study, there were 23 species that have a direct ecological role in oil palm plantations. The ecological role of plants in oil palm plantations are as a ground cover for regulating soil moisture (4 species), producing litter for nutrient formation (5 species), fertilizing the soil and absorbing toxins (3 species), regulating water management and preventing erosion (3 species), increase soil carbon stocks and groundwater availability (2 species), animal feed (5 species), feed for wildlife (6 species), control pests and diseases (4 species), etc. The ecological role of this plant species will have a positive impact on oil palm plantations, especially in maintaining soil fertility, availability of groundwater, providing feed for animals, and also as biopesticides for oilpalm plants to avoid pests and diseases.


Introduction
Palm oil is a plantation commodity that has a large area of land which is estimated to reach 15.08 million hectares in 2021, has a high production was 49.7 million tons by 2021 [1], and improves the economy in Indonesia [2].However, the existence of oil palm plantations is controversial because it is suspected to threaten the existence of biodiversity [3,4], especially plant species diversity.The level of plant species diversity is expected to decrease in number.Therefore, sustainable management of oil palm plantationsis needed to protect biodiversity and the important ecosystems within it.The adoption of this sustainable practice can provide economic benefits [5], as well as meet market demand for palm oil certified by ISPO and RSPO [6].One of the principles in fulfilling RSPO certification is environmental responsibility and biodiversity conservation.The implementation of these principles can be started by analyzing the level of plant species diversity and the role of plant species diversity in maintaining the ecological balance in the plantation environment.
Currently, the diversity of plant species in oil palm plantations is getting less attention.This is because monocultures from oil palm plantations only focus on one type of oil palm.In addition, this type of plant species diversity is considered to be a competitor for oil palm in obtaining water and nutrients [7].[8] 1243 (2023) 012015 IOP Publishing doi:10.1088/1755-1315/1243/1/012015 2 revealed that undergrowth in oil palm plantations is often seen as weeds that are considered to competewith oil palm for nutrients, so plantation management overcomes with weeding and herbicides.Meanwhile, according to [9] wild plants in oil palm plantations can provide benefits in the agroecosystem, and weeds in oil palm plantations can be categorized as detrimental and beneficial.Some of the beneficial weeds such as Asystasia gangetica, Ageratum conyzoides, Paspalum conjugatum, Nephrolephi biserrata can be tolerated in oil palm plantations and contribute to preventing soil erosion [10], and some can be made as bioherbicides [11], with the population having to be kept under control [12].Data regarding plant species diversity and its role in ecology on oil palm plantations in Indonesia is limited.This study aimed to examine the diversity of plant species and determine the role of the plant species in maintaining ecological balance on an oil palm plantation.The information will contribute to plant conservation so that the sustainable management of oil palm plantations can be carried out correctly and has a good impact on the ecology and economy of oil palm plantations.

Study area
This research was conducted in 1 large-scale oil palm plantation namely PT TPR, and 2 smallholder oilpalm plantations (SOP) in the regency of Ogan Komering Ilir, Province of South Sumatra.The study in PT TPR was conducted on 4 kinds of land cover, namely: young growth oil palm plantation (YG) established in 2007; medium growth oil palm plantation (MG) established in 2008; old growth oil palmplantation (OG) established in 2010, and High Conservation Area (HCV).

Study methods
The inventory of plant species was carried out by analysis of vegetation using a single plot in every type of land cover.The plot used was 113.14 x113.14 m 2 which is divided into smaller plots The observation plot has measuring 113.14 x 113.14 m 2 for data collection of vegetation trees growth rate (diameter ≥ 20 cm) and liana, the 60 x 60 m 2 plot was used for the pole growth rate (diameter arange 10-20 cm), the 40 x 40 m 2 plot was used for sapling growth rate (diameter < 10 cm, height 1.5 m), and the 10 x 10 m 2 plot was used for seedling growth (height < 1.5 m, diameter < 3 cm) and undergrowth.The data collected for all growth include the species name, and the number of individuals of each species, while for the pole and trees is added the circumference or diameter of the stem at chest height.Data on the role of plant species were collected from study literature.

Data analysis
Data were analyzed using quantitative and qualitative methods.Analysis of the data used to determine the level of species diversity were Shanon wiener index, margalef index, and evenness index.Data on the role of plant species were analyzed by qualitative methods.The formula used in analyzing the data,namely: 2.3.1 Species diversity, the density of plant species was obtained using the formula: Information : H' = Shanon-Wiener diversity index ; n = number of individuals of a particular species ; ln = natural logarithm ; N = number of individuals of all species 2.3.2Species richness.To determine the value of the species richness index using the formula of Margalef's index: (2) Information : Dmg = Margalef index ; S = total species ; N = number of individuals of all species Information : E = evennes index ; H' = shanon-wiener diversity index ; S = total species

Number of species and species diversity
The results of the vegetation analysis showed that the number of plant species in the study area was 49 species.The highest amount of land cover was found in the smallholder oil palm 2 which was 22 species.While the number of species of medium growth land cover found at least 9 species.Previous research on plant diversity in oil palm plantations in North Sumatra found 7 species [13], 27 species in Bengkulu [14], and 39 species in Aceh [15].The most common plant growth rates were at the seedling and undergrowth levels.This finding is not different from research which states that in oil palm plantationsthere is a lot of diversity of undergrowth species that can be divided into three categories, namely grasses, leguminosae, and weeds [16].Meanwhile, plants at the growth stage of saplings, poles, and trees were only found in HCV areas that have a forest form.Data on the number of species in each typeof land cover and growth rate are presented in Table 1.The difference in the number of species in each land cover is influenced by several things such as environmental conditions, as well as human activities.The number of plant species affects the value of species diversity.The diversity value of plant species in each land cover is presented in Figure 1 using the Margalef Species Richness Index (2), Shannon Wienner Species Diversity Index (1) and Species Evenness Index (3).SOP 1 has the highest value of species richness and species diversity, namely Dmg = 3.11 and H' = 2.18.Meanwhile, the highest evenness value was found in medium growth land cover (E=0.79).The lowest index value was found in HCV land cover (Dmg=1.24; H'=1.17 ; E=0.6).The value of the plant species diversity index (H') in the oil palm plantation area ranged from 1.17 to 2.18.It can be seen that the level of plant diversity is in the medium category, where index values ranging from 1-3 are included in the medium category, values below 1 are low, and above 3 are high.This value is the same as that found by previous research, namely the value of plant diversity in oil palm plantations is in the medium category [13,15].The value of plant species richness in smallholder oil palm plantations is higher because oil palm plants are still small (aged 2 years old) so the area is more open and causes a lot of undergrowth species to grow and develop.While the highest evenness value isfound in the medium growth oil palm, this shows that the community in that area is more balanced than other communities.The evenness value can describe the dominance of each species in the community, if the evenness value is high, then the species found in the area have relatively equal.The lowest evenness value in HCV indicates that there is a dominant species compared to other species.It can be seen from the results of the study that the Piper aductum species dominated with 61 individuals.The value of diversity and species richness in HCV has a lower value than the others.It is because the land cover before the area was converted into plantations was shrubs, so in the HCV area in oil palm plantation, there were only a few trees.This result is no different from the study on HCV of oil palm plantations in South Kalimantan which found that the number of plants species was relatively small, that was only 8 species [17], on the other hands, reference [18] reported that the number of plants species in HCV of oil palm plantation Riau was relatively higher, that were 129 species in HCV forest, 58 species and 27 species in non-forest HCV.Another study stated that the number of plant species in Kalimantan's oil palm plantation HCV was 106 species [19].
The results showed that the highest density was found in the smallholder oil palm plantation, whichwas 47,833.The dominant plant species are Zoysia japonica and Piper aduncum.This is different fromprevious research, which shows the dominant plant species in oil palm plantations are Asystasia gangetica dan Nephrolepis biserrata [20], Axonopus compressus [21].Each land cover has a different dominant plant species.The dominant plant species in each land cover are presented in Table 2. Stenoclaena palustris dominates in several types of land cover, namely young growth, HCV, and Smallholder oil palm plantation (SOP 1).Stenoclaena palustris known as Climbing Fern belongs to the Blechnaceae family which has habitats in the lowlands, open places and secondary forests, and swamp areas [22].This plant is a type that is easy to grow in open and humid areas, so this species can thrive in oil palm plantation areas.

The role of plant in maintaining ecological balance
Plant diversity has an important ecological function for life, including in the oil palm plantation environment.High plant diversity will produce a stable ecosystem, while the extinction or decrease in plant diversity can cause damage to the food chain and disrupt ecosystem stability.In addition, plant diversity also plays a role in providing key ecosystem functions and services, such as nutrient recycling, pest and disease control, and mediating greenhouse gas fluxes [23].Plant diversity plays a role in maintaining nutrients in the soil so it can affect biogeochemical cycles in nature.In addition to plants, the diversity of animals can also help the pollination process needed for plant regeneration.Various types of fungi and decomposing bacteria are needed to degrade and decompose organic waste and organisms to fertilize the soil.So the diversity of plants greatly affects the stability and sustainability of nature.Based on the literature study, it was found that there are 25 plant species (51%) that have the potential to play a special role in the ecological system in the plantation environment.In Table 3, it can be seen plant species that have an ecological role in oil palm plantations.There are 10 types of ecological roles that can occur with the diversity of plants in oil palm plantations.
Table 3 Notes : YG = Young growth oil palm plantation, MG = Medium growth oil palm plantation, OG = Old growth oilpalm plantation, HCV = High Conservation Value, SOP = Smallholder oil palm plantation Several species of plants that have an ecological role are ferns such as Nephrolephis biserrata, Stenoclaena palustris, Lygodium japonicum, Dicranopteris linearis, Adiantum sp, Diplazium esculentum.Ferns have ecological and economic benefits.The ecological functions of ferns include ground cover vegetation, producing litter for the formation of soil nutrients, and food chain producers [24], as environmental bioindicators because they are cosmopolitan [25], besides that they also have aneconomic function, namely as ornamental plants, food sources, and medicines [26].In addition to ferns, Eleocharis dulcis also has many ecological roles in the ecosystem of oil palm plantations.Eleocharis dulcis is known as pest control that contains organic nutrients with high Fe content which can increase soil pH, Mg, and Ca to provide soil fertility.Eleocharis dulcis can also be used as an organic fertilizer and biofilter to improve water quality and absorb toxic compounds such as iron, sulfur, lead, mercury, and cadmium [27].[28] stated that this plant was able to absorb lead from the liquid waste of the palm oil industry by 0.32-0.54ppm at roots and 0.24-0.27ppm in stems.Not only Eleocharis dulcis, Asystasia gangetica can also be used as cover and increase the availability of nutrients N, P, K as much as 18.49%,9.20%,38.36%. A. Gangetica can increase soil carbon stock by 100% compared to soil without groundcover [29,30].Other studies have shown that besides A. Gangetica, N. Biserrata also has functioned as a cover crop and reduces the water deficit by 51.53%, and contributes nutrients by 15.7 tons dry weight/ha/year or 7.7 tons C/ha/year [31].This shows that several plant species have a very important role in increasing the availability of soil nutrients, so that the soil in the oil palm plantation environment is more fertile, avoids erosion, has better air and water availability, and is protected from attacks by toxic compounds.It is will have an impact on the success of oil palm production.
In addition to having a role in soil fertility, the diversity of undergrowth species in oil palm plantations can also be used as animal feed [14,32].Cattle grazing in oil palm plantations can provide ecological and economic benefits.Oil palm plantations can reduce weed control costs by more than 16% [33], and with controlled system grazing up to 30-60% [32].Besides, cattle grazing can help increase biodiversity and maintain soil ecosystem functioning [23].Research shows that 70.37% of undergrowth in oil palm plantations can be used as animal feed [14].The plant species found at the research site thatcan be used as animal feed are Asystasia gangetica favored by cattle, Digitaria ciliaris favored by goats, Eleocharis dulcis favored by buffalo, Imperata cylindrica, and Ottochloa nodusa.Generally, plant species as animal feed belong to the Poaceae family.Besides as animal feed, the Poaceae family can beused as ground covers that can regulate moisture in the soil [34] and erosion control [35] Axonopus compesus and Imperata cylindrica are able to withstand the rate of erosion that occurs on a different slope compared to the land without planting grass [35].
Not only animal feed, but several types of plants found can also potentially be used as feed for wildlife.There are 6 species of plants that have the potential to be used as food for wildlife, namely Syzigium sp, Stenoclaena palustris favored by mammals [22], Ficus grossularioides, Melastomamalabathricum favored by bats and birds, Macaranga sp favored by bee, Ploaiarium alternifolioum andAlstonia scholaris.Eleusine indica, Eleocharis dulcis, Imperata cylindrical are the type of plant that can attract insects as bird food, so the diversity of plants in oil palm plantations can become a food web by providing food for wild animals.This is an advantage for insectivorous birds because the plant will provide food for them.Research by [36] found that the most common insectivorous bird species were found in PT TPR.In addition, 4 species of fruit-eating birds used Melastoma malabathricum as their food, and 2 species of nectar-eating birds used nectar from Melaleuca leucadendra.
Pest and disease control can be done by managing botanical pesticides in the form of herbicides, larvicides, and environmentally friendly insecticides.The use of chemical pesticides needs to be reduced to increase soil biodiversity, and also reduce costs incurred by companies [37].So that the management of botanical pesticides can be developed using the types of plants found in oil palm plantations.Piper aduncum is one of the dominant plant species found in HCV which can be useful as larvicides because it has bioactive substances such as phenylpropanoids, lignoids, alkaloids, chromones, dihydrochalcones, flavonoids, etc. which are insecticidal [38].In addition, Chromolaena odorata is an allelopathic plant that can be used as a natural herbicide [39].Eleocharis dulcis can be a pest control for rats in fields [27].Based on the literature study, it can be seen that the diversity of plant species in oil palm plantations has an important ecological role in the environment, ecosystem functions, and will have an impact on economic improvement, as well as supporting sustainable oil palm plantation management activities.

Conclusion
The number of plant species found at the research site was 49 species.The diversity value based on the Shanon-Wiener index is in the medium category.Around 51% of plant diversity in oil palm plantations has an important ecological role that has a positive impact on oil palm plantations.The ecological rolesof plant species diversity include regulating soil moisture, producing litter for the formation of nutrients, fertilizing soil and absorbing toxins, regulating water management and preventing erosion, increasing soil carbon stock and availability of groundwater, animal feed, wildlife feed, and tree cover.wildlife, organic fertilizers, and pest and disease control.The existence of plant species diversity will provide benefits and positive impacts on oil palm plantations in maintaining ecosystems, improving the economy, and supporting the sustainable management of oil palm plantations.

3
Notes : YG = Young growth oil palm plantation, MG = Medium growth oil palm plantation, OG = Old growth oilpalm plantation, HCV = High Conservation Value, SOP = Smallholder oil palm plantation, S = number of species,Dmg = species richness, H' = species diversity, E = evenness index

Figure 1 .
Figure 1.Comparison between the number of species (S), species richness (Dmg), species diversity (H'), and evenness index (E) in each land cover

Table 1 .
Number of species and structures of vegetation in each land coverNo.
Notes : YG = Young growth oil palm plantation, MG = Medium growth oil palm plantation, OG = Old growth oil palm plantation, HCV = High Conservation Value, SOP = Smallholder oil palm plantation

Table 2 .
Dominant of plant species in oil palm plantations Notes : YG = Young growth oil palm plantation, MG = Medium growth oil palm plantation, OG = Old growth oilpalm plantation, HCV = High Conservation Value, SOP = Smallholder oil palm plantation

.
Plant species that have a role in the ecology of oil palm plantations