The use of local seed is widely advocated for habitat restoration and is based on the premise that locally sourced seed will be the best adapted for the local conditions at restoration sites.
However, a ‘local is best’ seed sourcing practise (where seed for planting establishment is only sourced from native habitat within a few km of the restoration site) misses two important points, which may be seriously impacting on restoration outcomes, particularly resilience in the face of future environmental and climate change.
The first potential problem is that there is a serious risk of establishing populations that will not harbour sufficient evolutionary potential for future environmental change (or ‘genetic ghettos‘ as they have been termed. In addition, strict adherence to ‘local is best’ protocols may encourage the selection of poor seed sources, when genetically healthier sources further a field may produce a more efficacious restoration result. This may serve to perpetuate the number of small inbred populations across highly degraded landscapes that are unlikely to persist in the long term.
The second issue is that environmental conditions that drive local adaptation can change very rapidly, and sourcing only local seed may fix adaptations to past environments. The environment is continually changing at different rates and scales, from diurnal to decadal to Milankovitch (100,000 year) cycles. The most notable recent environmental change (at least in geological time periods) has been the emergence from the last ice age, over the last 10,000 years ago or so. During the ice age the atmosphere was significantly cooler and dryer than it is today, but since that time plants (and their genes) have redistributed across the landscape, some over thousands of kilometres and some at exceptionally rapid rates. In addition, recent anthropogenically forced environmental change (e.g. climate change, habitat fragmentation, increased salinity, irrigation, and heavy metal deposition) will have dramatically changed selection pressures.
In the face of rapidly changing environments it is pertinent to ask how “local environments” should be defined in contemporary landscapes, especially for long-lived species such as trees. In many regions of the world the conditions under which a 200-year-old tree was established are now very different to those existing today, and it could be legitimately argued that source material from more distant (geographically and ecologically) populations may harbour adaptations that more closely match the environment of the focal restoration site today.
So can we improve the selection of seed provenances to maximize evolutionary potential in restoration plantings? And can we facilitate long-term adaptive response to contemporary and future selection pressures? In answering this question it is informative to note two main processes, the migration of genetic adaptations between populations (through gene flow) and the evolution of new adaptive variants, which have allowed species to keep pace with environmental change naturally. The answer to provenance selection for future adaptive potential surely then lies in mimicking these natural gene flow and evolutionary dynamics.
For some species gene flow via pollen and seed has been documented to occur over tens, and in some rare cases over hundreds of kilometres. However many species are now limited in their capacity to disperse propagules (both pollen and seed), following habitat alteration and fragmentation. Gene flow in many species is leptokurtic – a term which means that most seed falls close to he mother plant, but that a significant proportion moves over much longer distances (see fig below)
To simulate the natural mixing of genes during a restoration programme, it would be necessary to restore populations using a mixture of material sampled at different distances from the focal site, a practise defined as composite provenancing. This ‘composite provenance’ would be predominantly composed of locally sourced material, taken from genetically healthy stock, but would also incorporate local and ecogeographically matched sources. In addition, a smaller proportion of material, depending on the natural gene flow dynamics of the focal species (but usually somewhere between 10 and 30% ), should be comprised of material from much further a field.
Whilst a composite provenancing approach may risk introducing some maladapted genes (a phenomena termed outbreeding depression), it crucially provides an opportunity for the migration of adapted genes and the evolution of new adaptive gene combinations through mixture of multiply sourced stocks, a key driver of evolution.
For restoration plantings, we need to be initiating plantings that will allow natural selection to act to produce a suitable and adaptively fit restored stand.
Since we first proposed these ideas (2008-2010), a number of restoration organisations have changed their seed sourcing strategy and starting taking into consideration genetic health issues and and introducing provenances that should allow plantings to be more resilient to climate changes in the future – which is a great outcome
Staff, students and volunteers planting experimental designs into restoration projects on the Yorke Peninsula, SA
This post is an updated version of an article that was first published as:
Lowe AJ (2010) Composite provenancing of seed for restoration: progressing the ‘local is best’ paradigm for seed sourcing. In The State of Australia’s Birds 2009: Restoring Woodland Habitats for Birds. (Eds David Paton and James O’Conner). Supplement to Wingspan20(1) March. pp 16-17.
and incorporates ideas published in
Breed MF, Ottewell KM, Gardner MG, Marklund MHK, Dormontt EE, Lowe AJ (2015) Mating patterns and pollinator mobility are critical traits in forest fragmentation genetics. Heredity. Published online doi:10.1038/hdy.2013.48.
Breed MF, Stead M, Ottewell K, Gardner MG, Lowe AJ (2013) Which provenance and where? Seed sourcing strategies for revegetation in a changing environment. Conservation Genetics 14: 1–10.
Breed MF, Marklund MHK, Ottewell KM, Gardner MG, Harris JBC, Lowe AJ (2012) Pollen diversity matters: revealing the neglected effect of pollen diversity on fitness in fragmented landscapes. Molecular Ecology 21(24): 5955-5968. doi: 10.1111/mec.12056
Broadhurst LM, Lowe A, Coates DJ, Cunningham SA, McDonald M, Vesk PA, Yates C (2008) Seed supply for broadscale restoration: maximising evolutionary potential. Evolutionary Applications 1: 587-597.
Sgrò CM, Lowe AJ, Hoffmann AA (2011) Building evolutionary resilience for conserving biodiversity under climate change. Evolutionary Applications 4: 326–337. doi: 10.1111/j.1752-4571.2010.00157.x.
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Ward, M., Dick, C.W., Gribel, R., Lemes, M., Caron, H. & Lowe, A.J. (2005) To inbreed, or not to inbreed: A review of mating systems and pollen dispersal variance in neotropical trees. Heredity, 95, 246-54.