Lessons from Tropical Frontline Regions

Today’s fast-paced world requires an understanding of not only the relationship between green assets, biodiversity and mental/social health, but also the need to implement dynamic strategies in a changing climate. The utility, urban forestry resources and management programs are of growing importance to the communities they serve. The readiness of the arbor-resiliency programs is tested during more extreme and frequent weather events.

In the United States, the strategies are evolving as utilities gather data and employ newer technologies, ranging from advanced plant health-care detection tools to mapping and the use of satellite data. Companies are using this data to better inform and guide work planning and deliver arboricultural, ecological and economic return on investment. Globally, the urban tree canopy cover and tree resources are now of special concern due to the interconnectedness of climate effects.

In this light, the nation is now paying attention to and becoming appreciative of the lessons learned from frontline regions of the world. These areas tend to have longer growing seasons and sometimes more heightened and apparent climate effects. In addition, they have experienced, or are experiencing, even more challenges and changes in vegetation and a higher frequency of erratic weather systems.

Plants respond to change and evolve with sites as conditions change, resulting in more rapidly occurring plant dynamics and/or plant migration. As climate effects are not always apparent, research narratives in these frontline areas, such as island nations and the global south, can help Americans understand the relatable challenges that may come their way.

Exploring International Regions

For several years, researchers have looked at changing plant populations and plant dynamics in utility and urban areas through several international frontline regions. Sea level rise is a global phenomenon. Apart from water levels rising, the secondary effects of salinity intrusion and loss of biodiversity to more salt-tolerant plant communities are very telling. A drought’s primary effects of stunting, defoliation and branch dieback may cause loss of structural integrity. Secondary effects may include the rise of tree pests and pathogens. Conversely, in many cases, heavy rainfall and flooding lead to whole tree failure, broken branches and pathogens associated with earlier tree mortality.

Yohann Govia of Trinidad and Tobago’s Ministry of Agriculture Land and Fisheries, points to an endemic double (Warszewiczia Coccinea) Chaconia tree, the national tree of Trinidad and Tobago, that is biomechanically suited for urban environments.

Under these dynamic conditions, the management of urban trees regularly pruned for sight, safety and reliability will rely on real-time data and analytics. In past decades, cycle- or time-based programs not only sufficed but were effective. To continue building upon this foundation, more effective and dynamic programs and integrated approaches are needed. With site conditions changing rapidly, programs must be effective at the tree species level with system-wide efficiency.

Considering Tree Resources as a System

In agriculture, it’s easier to consider a systems approach. As in many cases, it may be a monoculture of plants or a limited mixed crop situation. These agro-ecosystems can be monitored and managed. Although cropping in climate resiliency efforts has considerable challenges, its core objective is to raise plants from seeds to seedlings to the whole plant, then yield. This process may be relatively short-term in nature, but still wholly-related to the arboricultural process.

Outplants of nursery-raised trees (Serianthes nelsonii) with officials of the Guam Plant Extinction Prevention Program (GPEPP).

The similar, longer-term objective in the exploration of tree systems is to realize the near-full or full potential of trees. The current estimates in the urban and utility space for tree survivorship to maximum age potential are much lower than ideal (45 to 60 percent longevity or yield of tree ecosystem benefits on average by some estimates). To improve these statistics, we can start by upping our acceptability levels as agriculture demands a higher success level.

In terms of limitations, apart from site, care of seedlings, and plant establishment, seed stock has often been touted as a limiting factor. To build tree resources globally, this must be addressed. In several tropical frontline areas, seed stock is derived from naturally occurring and other local plantings and re-introduced to the landscapes via area nurseries as seedlings grown in bags. These smaller seedlings take off with relative ease and can adapt to sites with sometimes higher survivorship compared to planting larger trees. Notably, trees in the tropics are often planted in areas where similar species are already doing well in adjacent locations (Insert Figures 1 a, b, and c).

Pioneer species, such as ironwood (Casuarina equitsifolia) in Micronesia, are strong trees individually and are stable in stands with the ability to tolerate high wind loads.

In the United States, seed stocks, planting systems and care for the establishment are very important and need to be continuously addressed while building capacity for future tree canopies in cities. Nurseries can propagate superior plants and while plant quality can be improved, attention to plant species and microsite conditions is now of additional concern and should accompany the “right tree right place” philosophy to give better survivorship and maximize overall tree lifespans.

Tree Structural Assets as a Factor of Stability

Tree-inherent properties, including growth rates, branch aspect ratios and percentage branches with included bark are all indicators of tree potential for strength or failure. By assembling a database of recently planted trees, attaching inherent properties and comparing them to more established trees, it’s possible to begin to understand biodiversity and its relationship to tree species and structural tree integrity assets as trees grow in a particular geography. Trees planted in ecosystems or in stands generally fare better in high wind-loading environments (Insert Figure 2).

To capture this data globally, Tree Biomechanics International (TBI) was formed and is a growing series of sites that come together for tropical tree structural research. TBI sites are found in Belize, the Caribbean, Micronesia, Mexico, Columbia and other countries including the global south (Insert Figure 3).

Pruning cuts with percentage eclosion three years after pruning treatments. In most species, larger branches had less ability to heal. The exceptions were sweetgum, boxelder and tree of heaven species.

TBI’s aim is to gather data on tree structural characteristics in these tropical species and better understand the survivability of trees based on structure. With that data, researchers knit together the successes and challenges of trees in these areas that are historically subjected to heavy loading, high moisture, drought and other climate impacts.

Are there lessons that can be applied here in the United States? As the country experiences warmer winters and longer, hotter summers, for many species, the impact is already occurring. The fall season in temperate areas allows for trees to regroup energy and heal wounds such as accidental branch breaks, stem wounds and pruning cuts.

With reduced fall seasonal times, will this be a factor for tree survival and future tree structural integrity? Wound eclosion (healing in pruning cuts) is being measured across several sites in the United States, and several species of trees are beginning to demonstrate higher eclosion rates compared to others. This may be a more cohesive parameter to figure out what species are becoming more successful at surviving climate change (Insert Figures 4a and b.

Zoning in on Tree Biomechanics

Conventional edge-hardening efforts often rely on visual tools and management cycles based on system analytics over time. However, dynamic climate readiness and programs dictate a need for adaptability, additional metrics and overall system-level solutions and can borrow from national and international case studies on tree biomechanics.

Attaching a biomechanical stability score to tropical trees, coupled with structural integrity and tree inherent assets, can pinpoint the species that may impart more tolerance to loading.

This tool can assist in mapping efforts on the stability of edge and corridor trees and its implementation can be an additional layer to better understanding trees in rights of way (ROWs). These data also capture stability in relation to flooding, wind, rain, snow and ice loading, or the relationship of the tree structure to drought effects and insect and disease impacts.

The inclusion of tree biomechanics considerations in ROW forestry programs means that system foresters can better engage climate readiness at a level that provides greater emphasis on tree structure and stability. The lessons learned from frontline regions can inform processes in the United States. For example, the biomechanical scoring systems in tropical areas based on survivorship, coupled with plant inherent characteristics, can be adapted for temperate trees.

At a tree biomechanics workshop in Guadalajara, Mexico in August 2022, the group observes a fractometer, a mechanical measurement device determining characteristic values of bending and compression strength of wood evaluating samples taken from tropical ash (Fraxinus uhdei) trees.

Collaborating with government agencies both within the United States and internationally allows the Research, Science, and Innovation (RSI) department through ACRT Services to draw inspiration. Participating in global collaborative initiatives, as opposed to limiting research domestically, allows the team to produce better products, be better informed analytically and ultimately, better understand climate readiness. Climate is a global issue, which must be kept in mind with the continued work toward climate resilience.

Dr. Anand Persad ([email protected]) is the director of the Research, Science and Innovation (RSI) team at ACRT Services. He is the research committee chair for the Utility Arborist Association, chair of the International Society of Arboriculture Science and Research Committee and actively works with city, state and federal organizations. He earned his Ph.D. in invertebrate ecology/entomology from the University of the West Indies.