Evaluation of Drought Tolerance in Five Native Caribbean Tree Species With Landscape Potential Michael Morgan and Thomas W. Zimmerman Agroforestry Research Specialist II, Agricultural Experiment Station, University of the Virgin Islands, Kingshill, St. Croix, U.S. Virgin Islands; Research Associate Professor of Biotechnology and Agroforestry, Agricultural Experiment Station, University of the Virgin Islands, Kingshill, St. Croix, U.S. Virgin Islands Abstract wind. Landscape planting around buildings, in parks, and along roads differs from reforestation or restoration plantings Seedlings of five tree species native to the U.S. Virgin Islands in scale (square yards and square meters versus acres and and Puerto Rico with potential for landscape plantings were hectares) and in the size of the trees planted. The American grown in a greenhouse and subjected to three different water- National Standards Institute (ANSI) recommends that the ing intensities. We wanted to determine how fast nursery minimum size for a nursery-grown tree planted for landscap- stock would reach an appropriate size for outplanting and how ing purposes is a 0.5-in (12.5-mm) diameter measured at plant biomass would be allocated. Tree heights were mea- 6 in (15 cm) above the ground, and 4.0 to 5.0 ft (1.2 to 1.5 m) sured weekly for 22 weeks, after which trees were harvested (ANLA/ANSI 2004). By comparison, bare root seedlings to determine root, stem, and leaf weights. All species survived planted for reforestation purposes are typically 1- to 2-ft under the different watering regimes but had different (30- to 60-cm) tall. responses in both height growth and biomass allocation. Only one species, Andira inermis, when subjected to abundant Plant nurseries use abundant water, particularly those watering reached outplanting height by the end of 22 weeks. specializing in showy tropical plants such as species in the Plumeria alba growth did not respond positively to increas- Heliconaceae, Musaceae, and Zingiberaceae families. These ing water and the soil’s lack of field capacity wasted excess plants, after being planted in their final location, continue water. In terms of biomass allocation A. inermis was plastic to need abundant water—which is a problem. Nurseries and in the allocation of biomass by dedicating more biomass to landscape plantings cannot depend on rainfall alone in the roots while under water stress and dedicating more biomass to USVI. During the dry season, access to well, municipal, or stem wood when watered at field capacity. Other species, in pond water is necessary to keep these plants alive. particular, Bucida bucera did not change biomass allocation On the islands, supplies of fresh water are limited. Rainfall is in response to watering levels. The results indicate that U.S. seasonal. No perennial streams or lakes exist to provide fresh Virgin Islands nursery managers can save water during grow- water. People collect rainwater from rooftop-fed cisterns, pump ing of these species by controlling watering levels and still water from wells, or buy desalinated seawater from the Virgin obtain marketable local trees. Islands Water and Power Authority. Yet, drought in the USVI is a more serious concern than ever before in recent history. Introduction Subterranean reservoirs have become depleted, although ac- cording to records, the islands receive about the same amount The U.S. Virgin Islands (USVI) consists of four small islands of rainfall as in past periods of time. Subterranean water in the Caribbean. When Europeans arrived in the Caribbean sources have also become contaminated with salt water intru- at the end of the 15th century, these islands were mostly sions, waste water, and petroleum (USDA NRCS 2000). covered in tropical dry forest and inhabited by natives. In the 17th and 18th centuries, African slaves cleared the islands To reduce water use by plant nurseries and property own- of forest so sugar cane could be cultivated for the benefit of ers, we proposed the use of native tree species adapted to European planters. The cultivation of sugar cane has ceased, landscaping uses. As mentioned previously, tropical dry forest and the forests have grown back, but the islands are quickly was, and still is, the predominant vegetation type in the USVI. urbanizing (Thomas and Devine 2005). Tropical dry forests are adapted to seasonal rainfall regimes and low levels of precipitation. Worldwide, tropical dry New urban and residential developments require bushes and forests and woodlands are characterized by annual precipita- trees to beautify the newly constructed areas and provide tion between 40 and 80 in (1,000 and 2,000 mm) and very environmental services such as shade and protection from 49 Volume 57, Number 1 (2014) dry tropical forests have annual precipitation between 20 and in the length different species were subjected to treatments 40 in (500 and 1,000 mm) (Holdridge 1978). Tropical thorn resulted from delays in obtaining seeds of the various species scrub forests 10 to 20 in (250 to 500 mm) rain yearly. Based and differences in seed germination. Because St. Croix is a on annual precipitation and temperatures, the USVI has both tropical island, freezing temperatures and low levels of winter dry tropical forest and very dry tropical forest. light were not a problem. Mean daily temperature is 79 ˚F (26 ˚C), and St. Croix has roughly 12 hours of sunlight per Most tropical forests experience seasonal droughts that last day, year round (NOAA 2013). for weeks or months, even in forests classified as moist or humid (Mulkey and Wright 1996). Forest plants can also Each pot was filled with a potting mix of two partsSphagnum suffer water limitation daily, during the heat of the midday or peat, one part coarse sand, and one part top soil. Top soil was even from competition with other plants in the shade of the obtained from agricultural fields at the UVI-STX campus. understory (Kainer and others 1998). Fresh water is a limited A soil survey map indicates that the soil is a Sion Clay resource and plants have various physiological responses derived from alkaline marine deposits. It is considered prime and strategies in response to water availability. Some of agricul tural soil if irrigation is available; elsewhere, it is used the adaptations of tropical dry forest plants to drought are as range land (USDA NRCS 2013, 2000). The soil is moderately leathery leaves, leaves that are deciduous in times of drought, alkaline and sometimes causes problems for crops because of photosynthesis through the trunk instead of the leaves during iron deficiencies. Soil tests contracted out by UVI to Waters times of drought, dedication of more growth to roots instead Agricultural Laboratories indicate that the pH is 8. of leaves, and storage of water within the roots and tree trunk One month after transplanting, plants were assigned a weekly itself (Slayter 1967, Farquhar and Sharkey 1982, Sharkey and watering regime: 100 percent field capacity (FC), 66 percent Badger 1982, Gardner and others 1985, Shulze 1986, An- field capacity, and 33 percent field capacity (designated here- derson and others 1995, Brodribb 1996, Sanford and Cuevas after as 100 FC, 66 FC, and 33 FC, respectively). Six plants 1996, Manter and Kerrigan, 2004). of each species were in each watering regime. The pots were To remain profitable and in business, plant nurseries need to color-coded blue, green, and yellow to avoid confusion while produce a sufficient supply of plants at a price people are will- watering and to symbolize a gradient from abundant watering ing to pay. Two ways to reduce costs are to closely monitor through drought. In a previous experiment using 3-gal pots water use and to grow native plants that are adapted to the dry (data unpublished), the watering treatments were field capac- environment of the USVI. The objective of our study was to ity, 50 percent field capacity, and 33 percent field capacity, determine biomass production and allocation to leaves, stems, but we found no increase in plant growth when watering was and roots for five different species subjected to three watering increased from 33 to 50 percent field capacity. By increasing wa- regimes. This information can be an asset to nursery growers tering to 50 and 66 percent of field capacity, we were able to test and landscapers to better understand a plant species’ water- water-conserving treatments while still promoting plant growth. saving strategy. To determine the amount of water that each plant would re- ceive, we needed to determine the field capacity of the plant- Methodology ing substrate. Field capacity refers to the amount of water held in the soil after excess water has drained away. Gravity Five native tree species were included in this study: Andira causes the excess water to drain from the macropores; water inermis (W. Wright) Kunth ex DC, Bucera bucida L., Jac- for plant use is held within the micropores via capillary action quinia arborea Vahl, Pimenta racemosa (Mill.) J.W. Moore, (Brady and Weil 2002). We determined field capacity two and Plumeria alba L. Each species was grown from seed. different ways; the first is theoretical and the second empirical After two adult leaves emerged, the plants were transplanted with the expectation that both methods would produce similar into 61 in3 (1,000 cm3) pots and grown until they reached a results. A theoretically ideal growing medium consists of size of 6 in (15 cm) at which point 18 individuals of each 50 percent mineral or organic particles and 50 percent pore species were transplanted again into 3-gal (11,400-cm3) black space (Brady and Weil 2002). To simplify calculations, we esti- plastic pots (Custom ™Containers) and allowed to recover mated pore space to be evenly divided between micropores and from transplant for 5 weeks before the experiment started.