Genetic Diversity and Plant Preservation

Article by Neil Diboll
Republished from the Summer 2022 Wild Ones Journal

The divergence between the goals of horticulture and ecology create a conundrum for the modern gardener, especially those committed to the preservation of plant species and the organisms that depend upon them for survival. 

Horticulture is directly dependent upon the maintenance of a broad genetic storehouse within each plant species to serve as stock for future plant selections. Thus, it is incumbent upon us, as stewards of the planet, to preserve and propagate open-pollinated, genetically diverse native plants within our respective regions. In this way we can ensure the survival of these plants, as well as their use in sustainable, ecologically sound and economically feasible landscapes of the future. 

Plant genetic diversity plays an important role in our gardens, landscapes and natural areas, with latitude, longitude, elevation and the general climate being key factors. Isolated populations can experience in-breeding depression due to a narrowing of the gene pool in a single location with no opportunity to outcross with other populations to maintain genetic diversity. Outcrossing with other distant populations of the same species can reinvigorate a local population through “hybrid vigor.” 

Hybrids between different populations are often more vigorous than non-hybrids, and masking of homozygous recessive genes can cause harmful mutations, retard the plant’s vigor and other problems. For example, hybrid corn consistently out-yields corn from standard non-hybrid corn. 

Research has shown that plants with multiple ploidy, or the degree of repetition of the basic number of chromosomes, are often more capable of adapting to difficult or changing growing conditions due to their extra genetic information. However, there is a cost to the plant for maintaining multiple ploidy in the materials and energy required to carry this extra “genetic baggage.” 

Polyploidy is generally more prevalent in long-lived perennial plants. It also tends to increase with higher latitudes and elevations. It is also common in wetland plants, particularly in salt marshes and estuaries where the water fluctuates between salty ocean, brackish and fresh waters.  

How important are local ecotype plants? 

There is much debate regarding the importance of local ecotype. The importance of planting local ecotypes within a plant’s native range depends upon the individual plant species, the distance it is being moved and the climatic differences between the two sites. 

Studies have shown that plants can be successfully moved longitudinally east to west hundreds of miles, provided that the climatic conditions are not appreciably different. However, a generally accepted rule of thumb is that plant material can only be moved two to three degrees latitudinally, north or south of its original location. This is due to differences in temperature regimes, day length and length of growing season.   

In addition, studies have shown conclusively that moving the same species up or down a mountain slope results in decreased plant vigor and survival at the new elevation.  

Learn more about ecoregions, native ranges, and hardiness zones from this previous Journal article. 

Apomictic plants – natural “seed clones” 

Some plant species exhibit a condition called apomixis, in which each flower produces “cloned seeds” that do not undergo fertilization by another flower. This phenomenon has been documented in members of the rose family (Rosaceae), aster family (Asteraceae/Compositae) and grass family (Poaceae), among others.  

It is believed that practically all the black chokeberry plants in North America are identical clones from apomictic reproduction. This species occurs in both a diploid form (two sets of chromosomes like people) and tetraploid (four sets of chromosomes). The tetraploid is most common, and there is almost no genetic variation between the individual plants and no sexual reproduction except for isolated diploid populations in New England.   

Interestingly, many apomictic plants originate from natural hybrids, and many are polyploid, such as tetraploid (four sets of chromosomes), hexaploid (six sets) and octoploid (eight sets). Perhaps the presence of the wider diversity of genetic options afforded by polyploidy helps overcome any limitations in adaptability incurred by the lack of variation through sexual reproduction. 

Perils of inbreeding depression 

Some plant populations or local ecotypes have been documented to suffer from loss of vigor due to inbreeding depression. 

For example, lakeside daisy (Tetraneuris herbacea, formerly Hymenoxis acaule) is a rare plant that grows on sand dunes in the Great Lakes region. Populations on Lake Michigan were found to not be reproducing and were crossed with a population on Lake Erie hundreds of miles away. The offspring were fully capable of setting viable seed. Thus, the introduction of “fresh genes” into a small, local population that had become inbred led to more vigorous plants and prevented the local population from being extirpated.  

There is an inherent conflict between horticulture and ecology. Horticulture seeks to select and breed genetically “superior” plants to better serve the needs of humankind, while ecological gardening emphasizes biodiversity and the preservation of distinct local gene pools of each species. Many ecologists criticize gardeners’ focus on showier cultivars as a self-serving indulgence that potentially threatens the genetic integrity of the species, especially with regard to native plants.   

‘Nativars’ – improved varieties or not? 

There is much controversy surrounding the introduction of native cultivars, or nativars, that result from selection, breeding and hybridization of herbaceous native plants. The contention by ecologists is that selecting plant strains for purely aesthetic characteristics may rob them of the genetic flexibility they need to adapt to an ever-changing world through loss of important genetic traits. The priorities of many gardeners include bigger, longer-blooming flowers, bolder foliage coloration, improved disease resistance and enhanced plant appearance. But the priorities of most ecologists include preservation of diverse gene pools so plants can adapt to future climatic perturbations and provide essential ecological services to pollinators and other fauna. 

Read Wild Ones’ position statement on nativars here. 

Open pollinated plants vs. plant selections, hybrids, etc. 

In order to assure consistency in appearance and plant behavior, almost all cultivars are propagated asexually by division, cuttings and tissue culture. This potentially leads to a narrowing of the gene pool compared to “open pollinated” plants that are commonly propagated via pollination. The theory is that this preserves the natural diversity of a species’ genetic legacy, as well as its future adaptive capabilities. 

In a garden setting, the preservation of a species’ genetic diversity may seem irrelevant to the survival of any given species. However, with increasing fragmentation of native plant communities due to development and loss of habitat, gardens are becoming increasingly important repositories of native plant genetics. This becomes more important when taking into account the loss of habitat for insects and other invertebrates that depend upon native plants. The rapid decline in pollinator populations is a warning flag that the ongoing loss of natural areas is having a significant impact on these populations. One third of the foods we rely upon are dependent upon pollination, mostly by native insect species, via fertilization and fruit and nut production. 

Why native plants are essential to tomorrow’s gardens 

The garden of the future will be challenged by numerous ongoing changes, both locally and globally. The meteorological shifting sands caused by climate change is leading to highly variable weather patterns. Extreme heat, cold, drought and the increased frequency of high rainfall events will make gardening more challenging. As with any change, it may also present opportunities to grow new plants that were previously not tolerant of cold winters or other factors. 

Long-term ecological history indicates that during periods of warmer, drier conditions, woody plants tend to decline, while herbaceous flowers and grasses tend to survive. This makes prairie plants excellent candidates for landscaping in regions that experience higher temperatures and lower annual precipitation in the future. As competition for a finite supply of fresh water becomes more intense, the cost of irrigation will increase. At some point, the demand for water in agriculture may limit the availability of water for maintaining residential landscapes. Fortunately, many native plants are accustomed to growing under low rainfall conditions.   

Native plants are finding increased acceptance in our landscapes, not only because they are attractive in their own right, but because they reduce maintenance and total costs. A combination of wilder, open-pollinated native plant landscapes, along with more formal native gardens, appears to be the direction in which modern horticulture is heading. 

Climate change and horticulture 

There is much debate regarding how humans should respond to climate change with regards to preserving plants whose habitats may be impacted negatively by warming temperatures. Ecologists generally agree that preservation of diverse gene pools is essential to ensuring that plants will be able to adapt to a warmer climate. Some favor the active transportation of southern ecotypes northward, especially for species whose southern ranges may become inhospitable to their survival as temperatures increase. Others favor moving southern species northward into habitats where they are not known to occur, at least not in recent times; a process referred to as assisted migration. 

Some native plants have already made the move northward on their own over the past two decades. Seeds are constantly being transported to new locales by birds, wind and other natural vectors. Some southern species that did not survive in colder northern zones are now capable of doing so.  

Slower-growing, more conservative plant species, especially those that do not have the benefit of transport by birds and wind, are generally less able to migrate rapidly into new areas. This includes trees that produce large nuts such as oaks and hickories that cannot be carried over long distances by birds or other animals. Trees and shrubs that produce berries are more easily transported by animals due to their lighter weight and tendency to be ingested without harming the seed itself. However, almost all fruits are ripe in autumn when birds are migrating southward, so the opportunity for these species to move northward is limited. 

Many now ask if we have a responsibility to assist in the northward migration of species that may lose some or all of their natural plant ranges due to warming temperatures. Is there an ecologically ethical way to accomplish this without potentially creating other problems? If so, what guidelines should be used in this process? 

Ecological purists argue that plants should remain within the natural ranges they occupied at the time of European settlement. But botanical history shows this is neither possible nor consistent with the planet’s history of evolutionary change and continuous migration of species, be they plant or animal.  

Many herbaceous and woody plants have been found to be naturally cold hardy in locations far north of their existing ranges. Some have been bred for cold hardiness simply by selecting plants that survive cold winters and propagating them over multiple generations to develop reliably cold hardy strains. As with any selection process, there may be other important adaptive traits that are lost in this process, such as disease resistance, drought resistance, nectar quality, etc. Once again, the importance of retaining diversified genetic stock for every species provides the opportunity to select for other important characteristics in the future. 

To counter the above argument, there are woody trees and shrubs that are cold hardy north of their range as adults, but sensitive to cold as juvenile seedlings and saplings. These species are generally hardy in USDA Zones 4-5 when transplanted as young trees or shrubs. However, their seedlings are killed with the first hard frosts and self-sustaining populations cannot be established. 

Diboll, N. (Summer 2022). Genetic diversity and plant preservation. Wild Ones Journal, 35(2) p. 19-21.

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To learn more about genetic diversity and plant preservation, watch Neil’s 2022 webinar on the Wild Ones YouTube Channel.