INTRODUCTION
Tropical rainforests represent a disproportionate and dynamic component
of the world’s carbon budget (Mitchard 2018, Friedlingstein et
al. 2022), contain a majority of Earth’s terrestrial biodiversity
(Barlow et al. 2007, Gibson et al. 2011), and provide a
diverse array of ecosystem services (Watson et al. 2018). Despite
their significance, these ecosystems are under severe anthropogenic
pressure, with an estimated 17% reduction in moist tropical forest
extent from 1990 to 2019 (Vancutsem et al. 2021). At the same
time, active restoration of terrestrial ecosystems is essential for
addressing global challenges such as climate change, biodiversity
decline, and land degradation (Girardin et al. 2021). The latest
IPCC report (Riahi et al. 2022), highlights afforestation,
reforestation, and revegetation perhaps in addition to improved forest
management, agroforestry, and soil carbon sequestration as the primary
CO2 removal methods currently in use. The United Nations
(UN) declared 2021-2030 the Decade of Ecosystem Restoration (United
Nations General Assembly 2019). However, the increasing frequency of
hot, dry periods is exacerbating global tree mortality (Hammond et
al. 2022) and is predicted to increase the frequency of lethal canopy
temperatures being reached in tropical forests (Doughty et al. 2023), posing a significant threat to the success of restoration
efforts.
In response to climate change, plants can migrate, adapt, or become
extinct (Corlett & Westcott 2013, Merila & Hendry 2014). However, the
rapid rate of anthropogenic climate change and the shallow latitudinal
temperature gradient in the tropics mean that migration is largely
restricted to upward shifts in elevation (Colwell & Feeley 2024).
Additionally, the long generation times of forest tree species limit
their ability to adapt quickly, making intraspecific phenotypic
variation – both genetic variation accumulated over generations and
phenotypic plasticity – crucial for adaptation (Merila & Hendry 2014).
When variation exists for divergent selection to act on, local
adaptation can result in local populations with a fitness advantage over
nonlocal populations in their home site (Brancalion et al. 2018,
Muehleisen et al. 2020). Understanding and leveraging
intraspecific variation is therefore essential for assessing and
enhancing the success of restoration plantings in the face of rapid
climate change.
One strategy to ensure that restoration plantings are successful in
establishing resilient forests is the appropriate choice of seed
sources. Restoration guidelines provide advice on how to maintain the
genetic diversity and adaptive capacity of restored forest populations
through the consideration of the geographic origin of seed, i.e.
provenance (Offord & Meagher 2009, McDonald et al. 2016,
Malavasi et al. 2018). Although a strategy of selecting local
provenances has traditionally been preferred by practitioners (Cooperet al. 2018), this can reduce fitness in cases when forest
fragmentation results in inbreeding (Schlaepfer et al. 2018) or
when rapid climate change causes a mismatch between the current or
future climate with the historic conditions a population is adapted
(Aitken & Whitlock 2013, Gellie et al. 2016). These issues have
led to the recommendation of alternative provenancing strategies
incorporating non-local seed sources (Breed et al. 2013, Jordanet al. 2024). Given the strong role climate can have in driving
selection (Steane et al. 2014, Cordero et al. 2021, Ravnet al. 2024), the matching of seed sources to future climates
(known as predictive or climate-adjusted provenancing) aims to introduce
to an area genetic material that is better adapted and therefore also
more resilient to climate change (Breed et al. 2013).
Despite the known advantages that strategies such as climate-matching
provenance can have for restoration outcomes, the current industry
default in rainforest restoration plantings is to source seed locally
(Cooper et al. 2018). A key challenge in adopting alternative
provenance selection in tropical rainforests is the extensive genetic
information required to recommend a suitable strategy. Evidence for
local adaptation requires direct evidence of local populations having
fitness advantages over non-local populations, with full life-cycle
analysis under reciprocal common garden conditions being considered the
gold standard for determining true local adaptation (Kawecki & Ebert
2004, Meek et al. 2022, Schwinning et al. 2022). Synthesis
studies to date are heavily biased toward temperate species of
commercial importance with very few, if any, tropical trees included
(Hereford 2009, Matesanz & Ramirez-Valiente 2019). Their findings also
reveal how the presence and strength of local adaptation are species and
context-dependent. This limits our understanding of the prevalence and
strength of local adaptation in tropical rainforest trees, which
typically have lower population densities than temperate species, but
high reproductive efficiency over large distances maintaining similar
levels of genetic diversity (Degen & Sebbenn 2015). However, provenance
trials to test for local adaptation are highly resource-intensive (Sorket al. 2013) and typically focus on a single species. This has
limited the broad applicability of climate-matching provenance
strategies in tropical systems where forests are extremely speciose and
understanding the genetic background of each species is not feasible.
While a ‘one-size fits all’ approach to provenancing is generally not
recommended or ideal, the combination of extremely high species
diversity in tropical rainforests, and the limited resources available
for restoration activities necessitates a simplified, easy-to-implement
provenancing strategy. One possibility could be grouping the strategy
according to species plant functional traits, such as wood density, that
describe the slow-fast life-history continuum (Reich 2014).
Faster-growing species such as pioneers have shorter lifespans and
earlier reproductive capacity than slower-growing species such as
mature-phase species. As such, early successional stage species may show
higher rates of molecular evolution and therefore be more likely to have
developed local adaptation than slower-growing species (Smith &
Donoghue 2008, Smith & Beaulieu 2009, Müller & Albach 2010). However,
with studies of local adaptation severely lacking for tropical
rainforest tree species, there is currently insufficient experimental
evidence to support this. Additionally, while meta-analyses are useful
to discern the patterns and drivers of local adaptation in studies
across multiple species, these studies often vary in methodology, site
maintenance, and conditions that are not easily accounted for. There is
therefore a need for multi-species studies of local adaptation in
tropical trees that are key to rainforest restoration.
We investigated the prevalence of local adaptation in tropical
rainforest tree species commonly used in restoration plantings in the
Australian Wet Tropics. To do so we established three field-based
provenance trials, including an upland and lowland site that differ in
climate but have similar high-nutrient soils, and an additional lowland
site on a low-nutrient soil. Saplings of 16 species were sourced from
lowland and upland populations and reciprocally planted at each site,
where initial growth and survival were monitored over 1.5 years.
Specifically, we tested the following questions:
- How do provenance, site, and provenance × site interactions affect
sapling growth and survival?
- If provenance × site interactions exist, do local provenances
outperform non-local provenances at their local site?
- If there is variation across species in their provenance
differentiation, is this more pronounced in faster-growing species?