Street Trees

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Greenscape

Trees, shrubs, grasses, and other landscape plantings, or “greenscape,” play an important role in making streets comfortable, delightful, memorable, and sustainable. Used appropriately, they can help define the character of a street or plaza, provide shade and cooling, reduce energy consumption, and absorb and cleanse stormwater. They also absorb greenhouse gases and help filter airborne pollutants. In proximity to other green spaces, street trees can contribute to native wildlife systems.

In addition to providing environmental benefits, a healthy greenscape provides psychological and social benefits. People are attracted to places that have well-maintained plantings. Healthy greenscapes are good for city life and for business. The changing light and color along a tree-lined street reminds us of the changing seasons. By connecting us with nature in its beauty and complexity, plants help reduce stress and restore a sense of calm and focus.

Maintaining landscape plantings on Boston’s dense streets is challenging. Sidewalk space is at a premium and the hard surfaces required to support concentrated activity can be hostile to trees and other plantings. Soil compaction, lack of rooting space, poor soils, road salt, temperature fluctuations, physical damage, and even air pollution and litter all put stress on plants. These guidelines seek to balance the benefits of a healthy greenscape with the realities of limited space and the ongoing need for care and maintenance.

The guidelines in this section are intended to enable street trees and plantings to thrive, and to use stormwater as a resource to support plant life and replenish groundwater. The following sections provide a discussion of the benefits of street trees, plantings, and vegetated stormwater management along sidewalks, as well as the importance of soil selection and management in cultivating plant life.

Greenscape elements in the public right-of-way must be approved by the Boston Parks Department and PWD. Enhanced and pilot treatments will require special maintenance agreements.

Phytoremediation/Phytotechnologies

Phytoremediation or phytotechnologies—the use of plants to clean, remove, and stabilize contaminates—should be strongly considered in the design of greenscape elements. Many common organic contaminates, including petroleum hydrocarbons, can be easily processed and degraded by plants and associated soil biology. Contaminates are found in stormwater, air, existing site soils, and groundwater; it is encouraged that plantings not only be designed to treat stormwater, but other pollution sources as well. Greenscape can be designed to prevent the spread of contamination spills before they occur, or to remediate areas where a previous contamination is suspected. This is especially important on Industrial Street Types, in maintenance yards, brown fields, and other areas where high concentrations of pollutants may be of concern. For additional information on phytoremediation, please refer to the Environmental Protection Agency’s website page on Contaminated Site Clean-Up Information (CLU-IN) and phytotechnologies.

Benefits of Street Trees

Environmental

Reduced energy use and heat island effects: Trees reduce energy use by shading buildings and cooling the air through transpiration. A study on heat island effects in New York City concluded that trees and green roofs substantially reduce air temperatures, with street trees providing the most cooling per unit area.*

• Reduced greenhouse gases and airborne particulates: Trees reduce greenhouse gases by sequestering carbon dioxide and reducing the need for air conditioning. Trees also improve air quality by capturing gaseous pollutants and particulates in the tree canopy surface. Street trees have been shown to remove as much as 60% of the airborne particulates at street level.
• Improved water quality and groundwater recharge: Trees capture rainfall in their leaves and in the soil, trapping sediments, breaking down organic pollutants, and stabilizing non-organic pollutants such as metals. Trees also reduce the volume and temperature of stormwater runoff and help recharge groundwater.
• Support of natural diversity: Trees make beneficial use of rainfall and runoff to support the growth of the urban forest ecosystem.

Social

• Enhanced comfort, beauty, and attractiveness of streets and public spaces: Trees provide shade and scale; define and accentuate streets and spaces; and provide a soft, colorful counterpoint to the hard surfaces in the city.
• Reduced stress and improved concentration: Studies have shown that even brief encounters with nature at a small scale can reduce stress and mental fatigue, restoring the ability to focus and concentrate.†
• Reduced exposure to UV rays: Shade provided by street trees makes it possible to walk, bicycle, and linger in public spaces with reduced risk of sunburn, skin cancer, and other harmful effects of UV rays. 5
• Symbolic connection to the natural world: Trees in the urban environment are reminders that nature is ubiquitous and interconnected through the climate, seasons, and the larger ecosystem.

Economic

• Improved comfort and appeal of retail districts: In preference surveys, consumers indicate a willingness to travel further, stay longer, visit more frequently, and even pay more for parking in shaded, well-landscaped business districts.‡
• Perception of quality and care, which extends to adjacent businesses: Healthy trees signal that a place is well managed and maintained. This benefits the image of adjacent businesses, suggesting attention to detail and good customer service.
• Increased residential property values: Trees on streets and in front yards add value to home properties, with increases generally in the range of 7% for homes in areas with good tree cover.§ 6

Benefits of Vegetated Stormwater Management

Overview

Boston’s streets and sidewalks are one of the city’s most valuable resources, and they offer tremendous opportunities to improve stormwater management. New green strategies for managing runoff along streets and sidewalks can reduce flooding, increase groundwater recharge, and reduce pollution to our rivers and streams as well as to Boston Harbor. Capturing rainfall before it flows into the city’s drainage and sewer system can also help reduce sewer overflows and save the city money on upgrading and repairing infrastructure. Many of the best techniques for managing stormwater runoff use trees and other vegetation to capture rainwater as it falls, and to collect and filter runoff from streets, sidewalks, and other paved areas. Increasing vegetation also helps keep streets cooler, both by the shade from large trees, and by evaporation and plant transpiration, which cool the air just as perspiring cools the skin.

The City of Boston encompasses just over thirty-one thousand acres of land, over half of which is paved over with streets, buildings, and parking lots. Stormwater runoff from Boston flows into four major watersheds: the Charles River, the Mystic River, the Neponset River, or directly into Boston Harbor. Boston also has a major challenge maintaining groundwater levels, mainly in areas that are on filled land that was previously open water and marsh. In these areas, wood pilings that support many buildings may rot if groundwater levels drop. Recharging stormwater rather than directing runoff into pipes is one strategy for maintain groundwater levels.

The City of Boston owns and controls about one quarter of the land area of the city, and over half of city-owned property is streets and roads. The streetscape is one of the city’s best areas for controlling and managing stormwater runoff.

Environmental

• Reduced pollution to rivers and the harbor: Stormwater is the main source of pollution to Massachusetts’ waters. When rain falls, it washes pollutants from the roads, lawns, and built environment into local waterways. Stormwater can also cause overflows of “combined” sewers—sewer pipes that carry both sanitary sewage and stormwater in the same pipe. Reducing the amount of stormwater runoff from urban areas will reduce pollution from direct runoff and from combined sewer overflows. Phytoremediation, or the use of plants to filter pollutants, is another benefit of vegetated stormwater management techniques.
• Decreased flooding: By capturing more stormwater in trees and vegetation and by recharging more of it back into the ground, there will be less street flooding and lower peak flows, which often cause flooding of local streams and low lying areas.
• Increased groundwater recharge: Healthy vegetation and porous soils dramatically increase how much rainfall filters into the soil instead of running off into storm drains. Increasing recharge and decreasing runoff can help 2 maintain Boston’s groundwater levels.
• Reduced energy use: When stormwater flows into the combined sewer system, it is carried out to the Deer Island Wastewater Treatment Plant, where it is treated and discharged out into Massachusetts Bay as if it were sanitary sewage. Keeping stormwater out of the sewer system 4 reduces the use of energy to pump and treat this water. Increased urban vegetation can also reduce ambient air temperatures, reducing the demand for air conditioning

Social

• Enhanced understanding of water: When people see water flowing into planted areas in the urban environment, rather than disappearing into underground drains, they are more likely to 3 understand the importance—and the challenges—of managing water in urban areas.
• Increased support for stormwater management: Visible stormwater management in the public right-of-way can increase people’s awareness of water pollution and the importance of taking action to protect the environment. Individual activities like picking up pet waste, reducing litter, and improving lawn care practices can reduce pollution in runoff.
• Sense of connection to Boston’s water resources: In Boston, streets function like small streams, carrying stormwater to rivers and harbors. People can appreciate these connections even when they are far away from the water.

Economic

• Reduced costs for wastewater treatment: When less water enters the combined sewer system, 4 wastewater treatment costs can be lowered.
• Potential capital project savings: In many cities, stormwater management systems designed to mimic natural processes, also called “green infrastructure,” have been found to be less expensive than conventional pipe and gutter systems or “gray infrastructure.”
• Potential to create new green jobs: The installation and maintenance of vegetated stormwater treatment systems requires a combination of engineering, construction and operational labor skills. There is significant potential for job creation and growth in these fields as stormwater management requirements become more demanding.
• Enhanced property values: Numerous economic studies have shown that property values are higher in areas where there are water features, open space, and vegetation in the public right-of-way. Designing stormwater management systems to provide public amenities such as open streams, ponds, and street trees will 5 increase overall economic benefit.

Soils Selection and Management

Overview

Proper soil selection and management is one of the best ways to support healthy vegetation and to improve stormwater management in urban areas. Healthy soils—soils that have a high organic content and plenty of pore space—support healthier trees and plants and promote more groundwater recharge and better filtration of stormwater. Heavily compacted soils act almost like pavement, absorbing little water, and supporting less biological activity than well aerated soils.

Existing trees and planted areas that have become compacted and degraded can be significantly improved with aeration to restore porosity and/or the addition of soil amendments, such as weed-free compost, to help retain soil moisture. Soil improvements can make a significant difference in the health and longevity of trees and other vegetation. They can also improve stormwater management. Soil maintenance should be part of an operation and maintenance plan for urban vegetation.

New street trees and plantings present an opportunity to use engineered soils to grow a much larger and healthier greenscape and to clean and recharge significant volumes of stormwater runoff. Design details for planting street trees and implementing vegetated stormwater management techniques are found in the following sections. In all of these applications, careful selection of soil type and providing maximum soil volume should be priorities.

In constrained situations where existing street trees cause sidewalk heaving or where space is limited, consider using structural soils. Structural soils are a type of engineered soil that is designed to meet the load bearing requirements of urban streets while still maintaining adequate porosity and organic content to support healthy vegetation. Some structural soils also contain materials that specifically retain moisture. In urban contexts, structural soils allow the placement of ample, healthy soil beds beneath sidewalks and parking areas. Trees and plantings can be grown in dense urban settings with paved surfaces above the root systems, provided there is a way for water to enter the structural soil mixture.

Structural soils require irrigation (passive or active) to support a variety of plant types. Overflow drains may be necessary depending on the characteristics of the surrounding soils. Structural soil applications can both provide a healthier environment for plants and better capture, filter, and recharge of stormwater.

As an alternative to structural soils, soil cell systems can be used to provide appropriate soil volumes. See Covered Tree Trenches later in this chapter for more information about structural soils.

Street Trees

Street trees help define many of Boston’s best-loved streets and are a critical component of Boston’s urban forest ecosystem. This section describes how and where to plant street trees to achieve both environmental and urban design benefits.

Any resident of Boston can request to have a street tree planted in front of their home or business, provided the sidewalk is wide enough, by calling the Park Line at 617-635-PARK (7275). An arborist must inspect the site to determine if a tree can be planted.

The Boston Parks Department oversees maintenance and planting of trees in the public right-of-way. The maintenance program includes pruning, disease control, removal, and storm damage repairs. The Department’s oversight includes review and approval of trees to be planted by others and the planting of new trees throughout Boston’s neighborhoods. Tree selection and planting design in the public right-of-way must be approved by the Boston Parks Department and PWD.

Street Trees and Urban Design

Street trees can be used to serve a variety of urban design functions. Based on their location, arrangement, and spacing trees can:

• Frame, define, and accentuate spaces
• Emphasize linearity and long views
• Create a ceiling and sense of enclosure
• Provide needed shade and filtered light
• Reinforce the rhythm of a streetwall
• Add texture, delight, and human scale

Iconic plantings of street trees associate neighborhoods with seasons, and contribute to a unique sense of place. Red oaks in autumn on the Jamaicaway embody the essence of New England. Magnolias in bloom on Commonwealth Avenue mark the arrival of spring.

Trees are an ideal form of shade, providing protection on hot summer days while allowing heat and light to penetrate during cold winter months. They can also calm traffic by narrowing the apparent width of the roadway.

Street trees should be used in thoughtful compositions that respect the overall street context, local environment, and adjacent land uses.

Street Trees and Street Types

Street trees should be considered in every street design project; however, on some Street Types, trees are essential. For example, Boulevards and Parkways are defined in large part by the presence of trees. Below are guidelines for using trees on Boston’s Street Types.

Parkways are lined with continuous green spaces for trees, either on the sides or in the median. If sufficiently wide, green spaces provide an excellent rooting environment for largestature shade trees. Trees should be planted no more than 40’ apart to help create a continuous canopy. Species of a similar size, scale, and form should be planted along the length of the road for consistency and to maximize visual impact. Avoid monocultures, as disease and insects may destroy street trees along an entire street.

Trees on Boulevards are planted at regular intervals in a formal pattern with street lights, emphasizing linearity and long perspective views. The pattern draws the eye to the horizon or to an important terminus, such as the State House on Beacon Street, the Public Garden on Commonwealth Avenue, or the Blue Hills on Blue Hill Avenue. Trees are planted in the Greenscape/Furnishing Zone and are usually surrounded by pavement. Modern planting techniques such as covered tree trenches should be used to provide sufficient soil volume. Large-stature shade trees of similar size, scale, and form are typically planted 30’ apart to create a continuous canopy.

Neighborhood Connector Streets are similar to Boulevards but are less formal. Trees should be planted where they can best survive, such as in open or covered tree trenches.

Neighborhood Main Streets benefit enormously from trees, as visual preference studies have found commercial districts with shade trees are consistently preferred over districts without trees. Shade trees create the sense of an outdoor room and make streets more comfortable for sitting, café dining, window browsing, and socializing. Trees should complement and not interfere with first floor uses, entryways, cafés, or other activities in the Frontage Zone. Trees should not be planted in loading zones. Limbs should be pruned to maintain sight lines and maximize visibility of the street wall. Different species can be used in clusters to highlight special areas and create a sense of place.

Large canopy shade trees are attractive and add value to homes on Neighborhood Residential Streets. They help keep homes cool in the summer while allowing light and heat to penetrate in colder months. The branches also have the benefit of tempering winter winds. Street trees should be spaced far enough apart to allow light to reach front lawns and gardens. Open tree trenches or front yards (with permission from owners) should be used where possible to maximize rooting space.

Downtown Commercial and Downtown Mixed-Use Street Types require trees that can adapt to low light depending on building heights, street width, and street orientation. Where there is insufficient rooting depth due to underground utilities, raised tree beds can be considered.

Trees in Industrial settings must to be able to withstand drought and harsh conditions resulting from heavy traffic, green-house gas (GHG) emissions, and heat island effects from surrounding lots. Where possible, trees should be set back from the street and planted in continuous filter strips between the paved lots and the sidewalks. Tree species that can uptake and remove urban contaminates and air pollutants should be considered wherever possible.

Choosing the Right Tree

Tree selection needs to address the ability of the tree to mature in a given microclimate, as well as its ability to meet design objectives. Scale and form are key design considerations.

Large canopy shade trees play a critical role in the urban forest ecosystem, and offer a unique presence on city streets. Providing sufficient rooting space is a challenge, however this does not limit plantings to smaller trees; even small trees will suffer in a limited rooting environment. Given all the uncontrollable variables in a street it is worth taking a chance that a shade tree will survive in less than ideal conditions. Appropriate details should be used to enable trees to grow without roots rising to the surface and deforming sidewalks.

Choosing a tree for the right habitat can help minimize conflicts with adjacent infrastructure. For example:

• Shallow rooted species should be considered near sewer or drain pipes
• Open-form trees should be considered near overhead wires
• Trees with deeper roots and small trunk flares should be used adjacent to pavements

Other considerations for selecting the right tree include: the scale and form; sight line requirements; the type of microclimate; tolerance to drought and insects; inundation; resistance to vehicular emissions and salt; the ability to remediate pollutants; and the amount of maintenance. From an aesthetic perspective, spring flowers, fall color, the quality of light and shade, and the abundance of fruit, nuts, and leaf litter should also be considered.

Examples of Parks Department Approved Street Trees

Tree Siting and Spacing

Trees should be planted in locations that provide the best conditions for growth within a given design framework. This could mean planting in private yards in residential areas (with permission from owners), or clustering trees in open planting areas on wide sidewalks or in plazas. Large, contiguous planting areas should be employed where feasible to enable large canopy shade trees to reach maturity.

Street tree plantings should strive for continuity along a street while respecting adjacent uses. Each tree should complement and not interfere with first floor uses, entryways, cafés, or other activities in the Frontage Zone. Trees should not be planted in loading zones or within 10’ of bus stop landing zones. Trees limbs should be pruned to maintain sight lines and maximize visibility of the street wall.

Preferred Tree Spacing and Offsets

Root Environment for Street Trees

The ability of a tree to grow beyond a certain size is directly related to the volume of soil available for roots. Providing sufficient rooting soil in a dense, urban environment can be costly, but is worthwhile given the unique benefits that mature shade trees provide.

Tree roots do not survive well in highly compacted soil because it lacks the void spaces needed for air and water to circulate. Roots in compacted soil will migrate toward the surface for air and water, causing sidewalks to crack and heave.

When the rooting space is severely constrained, the tree roots will grow to capacity, and then the tree will decline and die.

Trees in the Northeast U.S. need approximately 2 cubic feet of soil per square foot of canopy area. † For example, a tree growing in a constrained 3’ by 8’ by 4’ pit would be expected to reach about an 8’ diameter canopy before becoming stressed and showing signs of decline. If the tree has access to soil outside the pit, the canopy can grow much larger.

Landscaped areas in the Frontage Zone or on the edge of adjacent properties (with permission from owners) can be excellent places to plant trees, as they may offer open areas for roots to spread. Examples include the residential edges on Commonwealth Avenue, where most of the iconic Magnolias are planted. When open landscape areas are not available, more intensive strategies are required.

The last decade has brought several innovations in engineered soils and sidewalk designs to support root growth. Below are four strategies for planting trees in constrained sidewalk settings. These strategies are intended to increase the volume of rooting soil while maintaining accessible sidewalks, and are discussed in detail on the following pages. Methods include:

• Open Tree Trenches
• Covered Tree Trenches
• Tree Pits
• Raised Tree Beds

Open Tree Trenches

Overview

An open tree trench is an area of soil connecting a row of trees that is covered with mulch, groundcover, grass (or “tree lawn”), or other greenscape. Tree trenches are generally located in the Greenscape/Furnishing Zone, though they can also be located in the Frontage Zone. For stormwater benefits, the sidewalk should be pitched toward the open tree trench. Non-linear open tree areas can also be used for planting trees in clusters.

Trees planted in open tree trenches and areas with a sufficient amount of uncompacted soil have the greatest chance of surviving and thriving in an urban environment.

Use

• Curbside open tree trenches or “tree lawns” are commonly used on Neighborhood Residential Street Types.
• Provide as large of a trench as needed for sufficient rooting volume while maintaining appropriate sidewalk clearances. The typical size of a tree trench is 4’ wide by 3’ deep. If sidewalk constraints prohibit wider tree trenches, smaller trenches, as narrow as 2’-6" wide, may be approved.
• Existing trenches that are smaller may be replanted at the discretion of Boston Parks Department.
• Plant the tree so that the top of the root ball is flush or nearly flush with the surrounding soil.
• The surface of the tree trench should be level with the sidewalk to avoid creating a tripping hazard.

Considerations

• Areas with heavily-used, high-turnover curbside parking are not compatible with open tree trenches, as the soils become compacted over time. Consider including pavement breaks to provide intermittent access to parking.
• Consider planting bare-root trees (trees with no soil around the roots). Confirm with Boston Parks Department or a tree nursery specialist if bare-root planting is appropriate for the given species and timing.

Covered Tree Trenches

Overview

A covered tree trench is a linear trench covered by pavement designed to support root growth while providing structural support for sidewalks. A covered tree trench makes it possible to have large canopy shade trees in even the most constrained urban environments.

Support for the sidewalk is typically provided by using engineered structural soils. Structural soil is designed to be loose enough to allow air exchange, water movement, and root growth, yet compactable enough to support pavement||. Soil cells and structural soils can be used in clusters around trees as well as in a linear trench if needed to avoid underground obstructions.

Tree trenches should be covered with pavement and permit passive irrigation to allow water to reach the soil. Provisions may include the use of pervious pavement or flexible, perforated pipes beneath the pavement. Covered tree trenches are considered an enhanced treatment and require a special maintenance agreement.

Use

• Use covered tree trenches in locations with heavy pedestrian traffic and high turnover parking.
• Provide as large a trench as needed for sufficient rooting volume. The trench should be at least 5’ wide by 3’ deep, and should provide at least 450 cubic feet of soil for a single tree, or 350 cubic feet of soil per tree if the space is shared among several trees in a cluster. Plant the root ball nearly flush with the surrounding pavement, allowing for the depth of any covering such as pavement or mulch.
• Provide an opening around the trunk of 2’ by 2’ covered with mulch during the initial years; however keep the mulch away from the base of the trunk. Over time the roots in this zone will expand and thicken with bark, eliminating the need for mulch.
• Provide subsurface drain lines in areas where the subgrade drains poorly. If in doubt, install drainage infrastructure.
• Covered tree trenches must meet required load bearings.

Considerations

• Consider covered tree trenches whenever sidewalks are being replaced along the length of a corridor.
• Engineered soils are required for both structural soil and soil cells. The soil used with soil cells is similar to planting soil.
• Structural soils require stringent quality control to ensure proper mixing and compliance with specifications.

Raised Tree Beds

Overview

Raised tree beds can be appropriate for planting trees in locations where utilities or subsurface conditions prohibit planting in the ground. However, tree growth is strictly limited by the size of the raised bed. In this constrained situation, smaller stature trees should be considered. Raised tree beds can also provide seating if the height is between 16" and 2’, with 20" being the preferred height. They can also be used to define spaces and provide a sense of enclosure in plazas and other open sidewalk areas.

Use

• Size raised tree beds as large as needed to provide sufficient rooting volume while maintaining appropriate sidewalk clearances.
• Clustering trees in large planters is a good strategy to provide greater soil volumes to individual trees.
• If there is subsurface space available for root growth, provide a shallow layer of structural soil below the adjacent pavement.
• Provide subsurface drain lines in areas where the subgrade drains poorly. If in doubt, install drainage infrastructure

Considerations

• Raised tree beds should not obstruct the Pedestrian Zone and should only be used in sidewalks of generous width to avoid creating a tripping hazard.
• Consider slightly smaller container grown tree stock for raised tree beds.
• Consider planting bare-root trees. Confirm with Boston Parks Department or a tree nursery specialist if bare-root planting is appropriate for the given species and timing.

Tree Pits

Overview

Tree pits are used where space or resources do not permit the use of open or covered tree trenches. The tree pit should be made as large as possible to provide maximum rooting volume while maintaining the appropriate clear width for the Pedestrian Zone. The sides of the pit below the sidewalk should be open to the surrounding subgrade to allow for root penetration beyond the pit.

Tree grates require maintenance to adjust for tree growth and to correct for any settlement that may cause a tripping hazard. Tree grates are considered an enhanced treatment and will require a maintenance agreements.

Use

• Provide as large a tree pit as feasible while maintaining appropriate sidewalk clearances. The preferred size for a tree pit is at least 4’ by 10’ by 3’ deep or 120 cubic feet. Smaller tree pits, as narrow as 2’-6" wide, may be approved if sidewalk constraints prohibit the construction of a full size tree pit.
• Existing tree pits that are smaller than the recommended minimum may be replanted at the discretion of Boston Parks Department.
• Plant the tree so that the root ball is nearly flush with the surrounding pavement while allowing for the depth of any mulch or covering.
• Provide an opening around the trunk of 2’ by 2’. The remainder of the pit should be covered with mulch, pervious pavers set in sand, or, if there is a maintenance agreement, a tree grate. If mulch is used, keep it away from the base of the trunk. Over time the roots in this zone will expand and thicken with bark, eliminating the need for mulch.
• Install a wrapped 4" perforated water/aeration tube in each tree pit per the most current approved Boston Parks Street Tree planting details.
• Pitch the sidewalk toward the tree pit to use stormwater for irrigation.
• Provide at least 50% new soil and scarify soils at the interface with adjacent soil to promote blending. Depending on the project site and soil conditions, the amount of new soil may vary.

Considerations

• Where sidewalk space is limited and minimum dimensions cannot be achieved with the installation of street trees, consider providing curb extensions.
• The surface of a tree grate is not counted toward the width required for an accessible pedestrian pathway.
• Tree grates must have break-out pieces around the trunk to allow for growth.
• Consider slightly smaller container grown tree stock for tree pits.
• Consider planting bare-root trees. Confirm with Boston Parks Department or a tree nursery specialist if bare-root planting is appropriate for the given species and timing.