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An evaluation of Israeli forestry trees and shrubs as potential forage plants for bees
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Tamar Keasar(1) and Avi Shmida(2)
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(1) Department of Science Education - Biology, University of Haifa – Oranim, Tivon
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36006, and Dept. Life Sciences, Achva College, Mobile Post Shikmim 79800,
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Israel.
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(2) Department of Evolution, Systematics and Ecology and Center for the Study of
Rationality, Hebrew University, Jerusalem 91904, Israel.
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Corresponding author: Dr. Tamar Keasar, phone: 972-52-871-8860, fax: 972-4-953-
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9608; e-mail: tkeasar@research.haifa.ac.il
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Key words: forestry; honeybee; nectariferous plant; nectar production.
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ABSTRACT
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Loss and fragmentation of foraging habitats limit honeybee populations in Israel.
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This problem can be alleviated by the planting of bee forage plants in forests, parks, and
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along roadsides. To provide recommendations for such planting, we combined a
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literature survey and qualitative evaluations of experts to compile a list of 266 local wild
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plant species that have high food potential for bees. We also quantitatively evaluated the
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food potential of 32 species of trees and shrubs planted by the Israeli Forestry Service.
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We recorded the following parameters of each species: main flowering season; flower
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morphology; type of food reward; number of flowers per plant; nectar standing crop;
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hourly nectar production rate; type of insect visitors; and frequency of insect visits. We
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ranked the surveyed species according to their potential importance as food plants,
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assigning high ranks to species that (a) bloom between July and February (the period of
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dearth in flowering natural vegetation), (b) produce large amounts of nectar, (c) are
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highly attractive to honeybees. Of the species surveyed, Amygdalus communis,
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Eucalyptus camaldulensis, Ceratonia siliqua, and Ziziphus spina-christi best combined
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these benefits. A regression model indicated that nectar production rates, but not the other
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plant parameters measured, significantly explain differences among species in insect
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visitation rates. Our study highlights the importance of diversified forestry planting to
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address agricultural, conservation, and recreational needs.
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INTRODUCTION
Populations of honeybees and other pollinating insects have been decreasing
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worldwide in recent years, limiting the reproduction of food crops and wild plants (Allen-
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Wardell et al., 1998). A major reason for these population declines is loss and
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fragmentation of natural and agricultural foraging habitats for bees, along with their flora
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(Kremen et al., 2002; Goulson, 2003). Food limitation for honeybees may be particularly
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severe in Israel, because colonies are kept at high densities (95,000 colonies at 6,100
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forage points on less than 7,000 km2, Israel Honey Board, 2008).
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The planting of forage plants for bees in non-agricultural and open areas can help
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sustain honeybee populations, and thus improve honey production and pollination
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services. Such planting is already in practice in field margins and small gardens in some
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countries (Carvell et al., 2006; Comba et al., 1999; Dag et al., 1998). Recent work has
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aimed at identifying the plant species that are most attractive to pollinators for such
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planting (Pontin et al., 2006). Designing forest areas for apiculture has been suggested in
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the US, especially for small privately owned plots (Hill and Webster, 1995).
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The Israeli Forestry Service (Keren Kayemet Leisrael, KKL) is in charge of
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public planting of trees and shrubs in forests, parks, and along roadsides in Israel.
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Planting is designed to answer a combination of needs, such as prevention of soil erosion
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and desertification, stabilization of sand dunes, and formation of recreation sites
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(http://www.kkl.org.il/kkl/english/main_subject/forest_and_places/fgeneral/kkl%20affore
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station%20work.htm). Enhancement of foraging areas for honeybees may also be
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achieved in planted forests, and requires the use of pollinator-friendly plant species. The
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present work aims to form recommendations for trees and shrub species that are suitable
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as food plants for bees, and that also meet additional forestry needs in Israel. This aim is
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compatible with current KKL policy to diversify the species used in forestry (Ginsberg,
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2006).
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Our immediate goal was to provide information about the foraging value of a
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number of specific trees and shrubs for honeybees. We addressed this goal both
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qualitatively and quantitatively. The qualitative approach consisted of compiling a list of
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local plant species that are attractive to insects, based on a literature survey, personal
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information, and consultations with experts. For the quantitative evaluation, we sampled
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32 species, in use or under testing by the Israeli Forestry Service, for food value and
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insect visit rates. We selected only trees and bushes for the quantitative evaluation, while
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the qualitative list additionally covers herbaceous species. We considered both local and
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introduced species in the quantitative evaluation, while the qualitative list includes local
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species only.
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More generally, we sought to identify which plant/flower traits most affect
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pollinator attraction in an interspecific comparison. These traits should be measured in
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the evaluation of further potential plant species. For this purpose we recorded several
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floral and plant parameters, which were previously shown to influence pollinator
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attraction to plants within species: The number of flowers per plant (Brody and Mitchell,
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1997; Goulson et al., 1998), nectar standing crops (Hodges, 1995; Pappers et al., 1999),
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nectar production rates (Delph and Lively, 1992; Mitchell, 1993), and parameters of
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floral morphology (Conner and Rush, 1996; Galen and Stanton, 1989). We tested the
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contribution of these parameters to the variability in visit rates among the plant species of
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our survey.
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METHODS
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Qualitative evaluation
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A list of local bee forage plants was compiled based on data by Dag et al. (1993, 1998),
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Fahn (1948), Gindel (1951), Lupo & Eisikowitch (1987), Reves (2004), and Zohary
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(1947). It was updated based on consultations with researchers from the Israeli
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Agricultural Research Organization (Arnon Dag, Haim Efrat, Sima Kagan, Yossi
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Slabezki), Tel Aviv University (Dan Eisikowitch) and Haifa University (Ofrit Shavit).
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We also consulted with the following beekeepers: Tomer Erez (Klil), Shalom Israeli
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(Ayelet Hashachar), Yehuda Kendel (Kfar Pines), Pini Nahmani (Yokneam), Shmuel Nir
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(Eilon), Noga Reuven (Manot), Hagai Schitzer (Gamla), Ya'akov Stam (Kfar Kish), Uri
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Surkin (Ein Harod Meuchad), Dan Weil (Yad Mordechai Pollination Services) and Eitan
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Zion (Yad Mordechai Apiary). The updating process allowed us to reduce an initial list of
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562 potential honeybee forage plants to 266 species. Data on the plants’ distribution and
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abundance in Israel and on their phenology were obtained from our Ecological Data-Base
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of the Israeli Flora (Shmida and Ritman, 1985).
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Quantitative evaluation
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Selection of plant species
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The surveyed plant species are a subset of the assemblage of trees and shrubs that
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are grown in KKL’s nurseries for wide-scale planting in forests, open areas, roadsides
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and parks, or for experimental planting and evaluation. We surveyed species that were
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considered attractive for bees, based on qualitative preliminary observations by our team
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and by the staff of KKL, the Shaham agricultural extension service, and the Volcani
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Center Dept. of Horticulture. We biased the survey toward plants that bloom in summer,
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autumn, and winter (July – February), since this is the period of dearth in floral food
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resources in Israel. The food potential of several Eucalyptus species for bees was
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assessed by a different research team (Eisikowitch and Dag, 2003). We therefore
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restricted our survey of Eucalyptus sp. to four common species that were sampled in
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autumn. On each day of the survey, 2-3 individuals of one species were observed. Most
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species were studied for two days (totaling 4-6 individuals), but some of the species were
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observed more intensively (see Table 2). Most observations were carried out in KKL
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nurseries, forests, and parks in Israel’s coastal plain and Negev desert. Repeated
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observations were conducted at different locations and flowering seasons to reduce local
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effects. The survey species, numbers, and months of observation are detailed in Table 2.
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Plant and flower morphology
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We counted the total number of flowers of surveyed individuals whenever
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possible, and estimated the number of flowers by counting a sector of the plant in
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individuals that were too large for complete counting. We recorded the following
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parameters of floral morphology in ten flowers from each of three individuals per
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species: total length of the flower (from the base of the tube to the tip of the corolla);
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length of the tube; and maximal diameter of the corolla.
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Nectar measurements
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When sampling three individuals of a species, we determined nectar standing
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crops for ten flowers per plant on each sampling day. When sampling two individuals, we
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measured nectar standing crops from 16 flowers per plant. Thus, nectar measurements are
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based on 30-32 flowers per sampling day. We used 1-µl or 5-µl micropipettes to measure
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nectar volumes, and Bellingham-Stanley hand-held refractometers to measure sugar (w/w
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%) concentrations. Sugar concentrations could be measured only for nectar standing
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crops that exceeded 1/3 µl. We bagged ten sampled, depleted flowers (3-5 per plant) with
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bridal-veil netting (Wyatt et al., 1992), and harvested them again after 24 h. The nectar
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that accumulated in the sampled flowers represents the plant’s 24 h nectar production.
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We divided the produced nectar volume by the covering time to obtain hourly per-flower
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nectar production rates. We calculated mean per-plant nectar production rates for each
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species by multiplying the mean per-flower production rate by the mean number of
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flowers per plant. The logarithm of this rate was defined as the species’ nectar score.
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Insect visits
We observed each plant for insect visits for 10 minutes on each observation day.
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The observation unit was a patch of flowers on the plant that allowed convenient
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recording of visits. A visit was defined as a touch of the corolla, the stigma, or the stamen
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by an insect. We did not record numbers of arrivals to the patch, which generally
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correlate with the number of visits (Bar-Shai, 1995). Observations were conducted during
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peak pollinator activity (typically between 10-12 am). We classified the pollinators into
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the following functional groups: honeybees, large bees (larger than honeybees), small
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bees (smaller than honeybees), flies, butterflies, and beetles. We noted nectar-extraction
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behavior and loading of pollen sacs. Accordingly, we classified the visitors’ food reward
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as pollen, nectar, or both. We recorded the number of flowers observed per individual
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(usually 100), and calculated the 60-minute visit rate per flower.
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Insect visit rates may vary widely between dates and locations because of
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differences in weather conditions, proximity to honeybee colonies, or composition of the
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surrounding plant community. To partially correct for these confounds, we used
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Rosmarinus officinalis as a reference species. Immediately after recording insect visits to
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a survey species, we counted visits to nearby R. officinalis shrubs using the same
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protocol. R. officinalis was selected as a reference because it is abundant, has a long
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flowering season (August – April) and is frequently visited by honeybees. R. officinalis
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did not grow in all the survey sites, and some of the survey species were observed outside
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its flowering season. The reference observations could thus only be obtained for some of
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the survey plants (see Table 5).
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Regression model
We tested which floral and plant parameters best predicted insect visitation rate
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on a between-species basis, and thus should be measured in future evaluations of
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additional plant species. We defined the mean per-species visit rates as the dependent
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variable in a regression model. Mean per-species nectar standing crops, nectar production
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rates per flower, number of flowers, nectar score, flower length, tube length, and corolla
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diameter were treated as independent variables.
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RESULTS
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Qualitative survey
Table 1 lists 266 species from the native floral that were proposed as bee forage
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plants in the qualitative survey, out of an initial list of 562 potential plants. The table also
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reports these species’ main food reward, and scores for their distribution and abundance
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in Israel. The seasonal distribution of blooming peaks of the potential forage species is
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shown in Fig. 1. The seasonal frequency of species at peak bloom is also plotted for the
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whole flora of Israel, for the sake of comparison. The plots show that the period between
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July and February is characterized by a low number of flowering local species in general,
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and bee forage plants in particular. This period comprises a dry season between July and
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November, and a rainy, cold one between December and February. These seasons of
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floral dearth are therefore difficult for honeybee survival, and forage plants that flower
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during the dearth period should be particularly useful. We therefore biased our
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quantitative survey towards the subset of forage plants in Table 1 that bloom outside
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spring. With this consideration in mind, we also included non-native species in the
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quantitative evaluation. We relied on the information of Table 1 to focus on species that
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are widespread in Israel, and attractive to insects. These considerations further aided in
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the selection of species for the quantitative survey.
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Quantitative evaluation
The species that were included in the quantitative survey are listed in Table 2.
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Species that were studied during the period of floral dearth are shaded in the Table.
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Table 3 lists the plants’ growth form, cultivation status, height (measured for only part of
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the species), number of flowers per plant, and the parameters of floral morphology.
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Column 2 of Table 4 provides a classification of the survey species according to their
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main food reward to pollinators (nectar (N), pollen (P), or both (N+P)). This
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classification is based on qualitative observations of pollinator behavior, and does not
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express the amount of nectar and pollen produced per plant. A quantitative assessment of
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a plant’s food potential should take into account its average size (i.e., number of flowers,
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Table 3) and the quantity of food reward produced per flower (Table 4). In the present
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study, we quantified per-flower nectar production, but not per-flower pollen yields. The
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product of the mean number of flowers per plant and the mean per-flower nectar
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production rate provides an estimate of per-plant nectar production rates (Table 4,
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column 6). We observed large variations in per-plant production rates both within and
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among species. To facilitate among-species comparisons, we defined a plant’s “nectar
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score” as the base-10 logarithm of its mean per-plant production rate (Table 4, column 7).
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Nectar scores for our survey species ranged between 1.5 and 4.5, i.e., across three orders
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of magnitude of per-plant nectar production rates. The species with the highest nectar
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scores (above 4.0) appear shaded in Table 4. These species have large numbers of flowers
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per plant, high nectar production rates per flower, or both. Table 5 lists the main
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pollinator groups for each of the studied species, mean visit rates, and, when available,
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insect visitation rates relative to visits to R. officinalis.
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Mean per-flower nectar production rates significantly explained the variation in
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insect visit rates among the survey species (P=0.001). The remaining parameters included
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in the regression model (nectar standing crop, number of flowers, nectar score, flower
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length, tube length, and corolla diameter) had no significant effects. The complete
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regression model was statistically significant (R2= 0.52, P=0.027).
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DISCUSSION
Our qualitative and quantitative surveys focused mainly on several criteria for the
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selection of bee forage plants in forestry: blooming season, type of food reward, amount
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of food reward, and insect attraction. None of the 32 species that were studied
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quantitatively ranked highest according to all criteria (Tables 2, 4, 5). Nevertheless, the
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trees Amygdalus communis, Ziziphus spina-christi, and Eucalyptus camaldulensis shared
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the following desirable traits: blooming during the period of floral dearth, production of
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both nectar and pollen, high nectar scores, and high rates of visits by honeybees
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(compared with R. officinalis). These species therefore seem the most promising for
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pollinator-friendly forestry. Ceratonia siliqua should also be considered, due to its
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favorable flowering season, low water requirements, and large number of flowers. Plants
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that bloom in spring (e.g., Styrax officinalis, Cercis siliquastrum) should receive lower
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priority in planting as bee forage plants.
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Two of the surveyed shrubs, Myrtus communis and Leucophyllum frutescens,
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produce pollen as their only (M. communis) or main (L. frutescens) food reward, and
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therefore received low nectar scores. We recommend their planting as pollen sources, in
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combination with other plants that can serve as sources for nectar. We further recommend
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the subshrub Rosmarinus officinale var. Blue Lagoon, which is native to the western
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Mediterranean but is well adapted to the local ecosystem. This chamaephyte blooms
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during the summer, produces both nectar and pollen, and is frequently visited by
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honeybees.
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Our regression analysis indicates that nectar production rates significantly explain
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differences among species in insect attraction. We therefore suggest that nectar
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production rates be determined for additional potential forage plant species in the future.
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Other plant traits account for the variability in pollinator attraction among individuals
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within a species. These include plant size (Robertson and McNair, 1995; Goulson et al.,
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1998), variability in nectar standing crops (Keasar et al., 2008), and flower size (Conner
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and Rush, 1996). However, these traits did not significantly contribute to the variability
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in insect visits among species in our study.
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Our measurements include several confounding effects. The most important ones
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are climactic differences between seasons and sampling sites, which may have affected
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both the plants’ nectar production patterns and the insects’ foraging activity (Wyatt et al.,
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1992). In addition, the composition of the plant community surrounding the survey plants
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varied among sampling days, and likely affected the pollinators’ attraction to the focal
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plants (e.g., Moeller, 2004). These confounds cannot be eliminated completely, but can
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be reduced by increasing sample sizes in future evaluations.
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The Society for the Protection of Nature in Israel (SPNI), co-founded by Azaria
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Alon, has been involved in campaigns to increase the use of local tree species in public
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forestry. KKL’s forestry policy has changed significantly in recent years, from focusing
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on planting of conifer stands to diversified forestry that emphasizes local tree species
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(Ginsberg, 2006,
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http://www.kkl.org.il/kkl/english/main_subject/forest_and_places/fprofession/kkl-
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tree%20data%20(website).mht). Diversification of forest composition in Israel promotes
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diversity of forest arthropods (Levanoni, 2005) and birds (Shochat and Tsurim, 2004).
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Additional potential advantages of diversified planting may include reduced impacts of
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plant diseases and pests, and a wider range of possibilities for recreation. We conclude
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that multiple organisms in forest ecosystems, including bees, are likely to benefit from
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diversified planting of trees and shrubs.
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ACKNOWLEDGMENTS
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Adi Sadeh assisted with field work. Yossi Slabezki commented on the manuscript. The
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study was funded by KKL. Data analysis and writing were supported by the research
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group on Evolution and Game Theory at the Institute of Advanced Studies, The Hebrew
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University.
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TABLES
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Table 1: A list of potential local bee forage plants, based on published literature and
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qualitative evaluations by experts. Pollination system: Z – zoophilous, W – wind;
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Attractivity score: + mildly attractive to bees, ++ - moderately attractive, +++ - highly
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attractive; Food reward: P: pollen, N: nectar, B: both pollen and nectar, NP: more nectar
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than pollen, PN: more pollen than nectar, L: very little nectar and pollen; Area score:
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estimate area occupied by the plants, scaled from 1 (least abundant) to 5 (most abundant);
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Peak blooming month: the main blooming period of the species; Total blooming months:
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range of blooming period throughout the plant’s area of distribution; Climactic region: D:
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Desert (70-250 mm rain annually), H: Hermon subalpine, M: Mediterranean, O:
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orographic (montane) belt, on Mt. Hermon above 1300 m, T: transition between
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Mediterranean and desert, X: Extreme desert (1-70 mm rain annually); Cultivated / Feral
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(in addition to occurrence as a wild plant): C: cultivated, F: feral, O: left over after
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cultivation but not feral.
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Table 2: A List of surveyed species, and the months in which they were surveyed, during
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2000-2002. Numbers denote survey days within a month. 2-3 individuals per species
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were studied on each day of the survey. Species that were surveyed between October and
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February, the period of floral dearth, are shaded.
375
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Table 3: Data on plant and flower morphology of the survey species. Growth form: C:
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chamaephyte (up to 0.5 m), S: shrub (0.5-2 m), T2: tree of 2-4 m height, T4: tree of 4-8
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m height, T8: tree above 8 m, V: vine; Cultivated / Feral: C: cultivated, F: feral, O: left
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379
over after cultivation but not feral, W: wild; NR: flower length is not relevant when the
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flower tube is rather wide and does not restrict insect foraging.
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Table 4: Reward parameters of survey species. Main food reward collected by visitors: N:
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nectar, P: pollen, N+P: nectar and pollen; NSC: nectar standing crop; NP: nectar
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production; NP / plant: mean NP per flower × mean number of flowers; Nectar score: log
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(NP/plant).
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Table 5: Insect visits to survey species. Main visitors: HB: honeybee, LB: large bees, SB:
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small bees, F: flies;
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FIGURE LEGENDS
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Fig. 1: The number of species at peak bloom throughout the year. Dashed: local potential
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bee forage plants, listed in Table 1; Solid: all species of the Israeli flora.
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