Adam R. Martin

A Reassessment of Carbon Content in Tropical Trees

Accurate knowledge of carbon (C) content in live wood is essential for quantifying tropical forest C stocks, yet generic assumptions (such as biomass consisting of 50% carbon on a weight/weight basis) remain widely used despite being supported by little chemical analysis. Empirical data from stem cores of 59 Panamanian rainforest tree species demonstrate that wood C content is highly variable among co-occurring species, with an average (47.4±2.51% S.D.) significantly lower than widely assumed values. Prior published values have neglected to account for volatile C content of tropical woods. By comparing freeze- and oven-dried wood samples, we show that volatile C is non-negligible, and excluding the volatile fraction underestimates wood C content by 2.48±1.28% (S.D.) on average. Wood C content varied substantially among species (from 41.9–51.6%), but was neither strongly phylogenetically conserved, nor correlated to ecological (i.e. wood density, maximum tree height) or demographic traits (i.e. relative growth rate, mortality rate). Overall, assuming generic C fractions in tropical wood overestimates forest C stocks by ∼3.3–5.3%, a non-trivial margin of error leading to overestimates of 4.1–6.8 Mg C ha−1 in a 50-ha forest dynamics plot on Barro Colorado Island, Panama. In addition to addressing other sources of error in tropical forest C accounting, such as uncertainties in allometric models and belowground biomass, compilation and use of species-specific C fractions for tropical tree species would substantially improve both local and global estimates of terrestrial C stocks and fluxes.

Size-dependent changes in leaf and wood chemical traits in two Caribbean rainforest trees

Tree functional traits and their link to patterns of growth and demography are central to informing trait-based analyses of forest communities, and mechanistic models of forest dynamics. However, few data are available on how functional traits in trees vary through ontogeny, particularly in tropical species; and less is known about how patterns of size-dependent changes in traits may differ across species of contrasting life-history strategies. Here we describe size-dependent variation in seven leaf functional traits and four wood chemical traits, in two Dominican rainforest tree species (Dacryodes excelsa Vahl. and Miconia mirabilis (Aubl.) L.O. Williams), ranging from small saplings to the largest canopy trees. With one exception, all traits showed pronounced variation with tree size (diameter at breast height, DBH). Leaf mass per area (LMA), thickness and tissue density increased monotonically with DBH in both species. Leaf area, leaf nitrogen (N) and carbon (C) : nitrogen (N) ratios also varied significantly with DBH; however, these patterns were unimodal, with peak trait values preceding the DBH at reproductive onset in both species. Size-dependent changes in leaf structural traits (LMA and leaf thickness) were generally similar in both species, while traits associated with leaf-level investment in C gain (leaf area, leaf C : N ratio) showed contrasting ontogenetic trends between species. Wood starch concentration varied with DBH in both species, also showing unimodal patterns with peaks preceding size at reproductive onset. Wood C concentration increased linearly with DBH in both species, though significantly only in M. mirabilis. Size-dependent patterns in wood chemical traits were similar between both species. Our data demonstrate pronounced variation in functional traits through tree ontogeny, probably due to a combination of environmental factors and shifts in resource allocation. Such ontogenetic variation is comparable in magnitude with interspecific variation, and so should be accounted for in trait-based studies of forest dynamics, structure and function.

Size-dependent changes in wood chemical traits: a comparison of neotropical saplings and large trees

We have a fundamental and applied understanding of how differences in the wood chemistry of trees affects the durability of wood products. By comparison, relatively little is known about the ecological causes and consequences of species differences in wood chemistry; even less is known about how or why wood chemistry differs within species, across trees of different sizes. In this study we find strong and consistent differences in wood chemistry of saplings and canopy trees, in several tropical hardwood species. These differences point to the importance of pathogens and tree biomechanics as evolutionary causes of size-dependent changes in wood chemistry.

Island Invasion by a Threatened Tree Species: Evidence for Natural Enemy Release of Mahogany (Swietenia macrophylla) on Dominica, Lesser Antilles

Despite its appeal to explain plant invasions, the enemy release hypothesis (ERH) remains largely unexplored for tropical forest trees. Even scarcer are ERH studies conducted on the same host species at both the community and biogeographical scale, irrespective of the system or plant life form. In Cabrits National Park, Dominica, we observed patterns consistent with enemy release of two introduced, congeneric mahogany species, Swietenia macrophylla and S. mahagoni, planted almost 50 years ago. Swietenia populations at Cabrits have reproduced, with S. macrophylla juveniles established in and out of plantation areas at densities much higher than observed in its native range. Swietenia macrophylla juveniles also experienced significantly lower leaf-level herbivory (∼3.0%) than nine co-occurring species native to Dominica (8.4–21.8%), and far lower than conspecific herbivory observed in its native range (11%–43%, on average). These complimentary findings at multiple scales support ERH, and confirm that Swietenia has naturalized at Cabrits. However, Swietenia abundance was positively correlated with native plant diversity at the seedling stage, and only marginally negatively correlated with native plant abundance for stems ≥1-cm dbh. Taken together, these descriptive patterns point to relaxed enemy pressure from specialized enemies, specifically the defoliator Steniscadia poliophaea and the shoot-borer Hypsipyla grandella, as a leading explanation for the enhanced recruitment of Swietenia trees documented at Cabrits.

Farmer perception and utilization of leaf functional traits in managing agroecosystems

Using knowledge of leaf functional traits, such as those forming the leaf economics spectrum (LES), to understand plant responses to environmental change is well-established and now being more widely applied to agroecosystems. Yet, little is known about how farm managers invoke leaf functional traits to inform management decisions. The objectives of this research were to (1) evaluate whether farmers use knowledge of intraspecific trait variation (ITV) in LES traits (or trait proxies) of target crops as response indicators of management conditions; (2) determine whether LES trait values are ranked consistently among multiple farmers along a “Farmer Leaf Economics Spectrum” (FES); (3) evaluate how a FES corresponds to the LES; and (4) identify the farmer and farm attributes that best predict the agreement between the FES and the LES. We collaborated with coffee (Coffea arabica) farmers in the Turrialba Valley, Costa Rica. We used a visual elicitation tool of fresh leaves along an intraspecific spectrum of leaf size, leaf thickness and leaf colour (as a proxy for leaf nutrients); respondents were asked to rank leaves in response to shade and nutrient scenarios as well as yield potential. On-farm biophysical data, management practices and socio-economic attributes were also collected. The majority of farmers demonstrated a developed system of utilizing coffee leaf and whole-plant ITV as indicators of management practices. Farmers managing smaller farms tended to more commonly acknowledge ITV in LES chemical–morphological traits, as compared to those managing large farms. The agreement between a respondent-identified ranking of leaf thickness ITV as a function of light environment and an empirically defined thickness-to-light ranking was partially explained by farmers’ physical engagement with plants. Synthesis and applications. In scientific literature, analyses of crop intraspecific trait variation have provided important insights into the mechanistic bases of multiple key agroecological processes. We demonstrate that farmers use crop leaf trait variation as an indicator to both evaluate management prescriptions and to initiate management actions including shade-tree species selection and abundance, crop- and shade-tree pruning regimes and fertilization treatments. These findings signify that functional traits represent a key nexus between scientific and local knowledge.

Resistance and resilience of root fungal communities to water limitation in a temperate agroecosystem

Abstract Understanding crop resilience to environmental stress is critical in predicting the consequences of global climate change for agricultural systems worldwide, but to date studies addressing crop resiliency have focused primarily on plant physiological and molecular responses. Arbuscular mycorrhizal fungi (AMF) form mutualisms with many crop species, and these relationships are key in mitigating the effects of abiotic stress in many agricultural systems. However, to date there is little research examining whether (1) fungal community structure in agroecosystems is resistant to changing environmental conditions, specifically water limitation and (2) resilience of fungal community structure is moderated by agricultural management systems, namely the integration of trees into cropping systems. Here, we address these uncertainties through a rainfall reduction field experiment that manipulated short‐term water availability in a soybean‐based (Glycine max L. Merr.) agroforest in Southern Ontario, Canada. We employed terminal restriction fragment length polymorphism analysis to determine the molecular diversity of both general fungal and AMF communities in soybean roots under no stress, stress (rainfall shelters added), and poststress (rainfall shelters removed). We found that general fungal and AMF communities sampled from soybean roots were resistant to rainfall reduction in a monoculture, but not in an agroforest. While AMF communities were unchanged after stress removal, general fungal communities were significantly different poststress in the agroforest, indicating a capacity for resiliency. Our study indicates that generalist fungi and AMF are responsive to changes in environmental conditions and that agroecosystem management plays a key role in the resistance and resilience of fungal communities to water limitation.

Inter- and intraspecific variation in leaf economic traits in wheat and maize

Wheat and maize are among the world’s most important crops, covering nearly 400 million ha of Earth’s land surface (an area roughly twice the size of Mexico) and playing a major role in the diets and livelihoods of millions of people. Leaf characteristics (or leaf traits) of these crops—including photosynthetic rates—are widely used in models that predict how wheat and maize will respond to environmental change. Using a large global database, we evaluate how leaf traits of wheat and maize differ across cultivars, and link this variability to climate conditions. In doing so, we assess how leaf traits can be expected to vary under climate change, and in turn, influence predictions of future food security.

Intraspecific trait variation and coordination: Root and leaf economics spectra in coffee across environmental gradients

Hypotheses on the existence of a universal “Root Economics Spectrum” (RES) have received arguably the least attention of all trait spectra, despite the key role root trait variation plays in resource acquisition potential. There is growing interest in quantifying intraspecific trait variation (ITV) in plants, but there are few studies evaluating (i) the existence of an intraspecific RES within a plant species, or (ii) how a RES may be coordinated with other trait spectra within species, such as a leaf economics spectrum (LES). Using Coffea arabica (Rubiaceae) as a model species, we measured seven morphological and chemical traits of intact lateral roots, which were paired with information on four key LES traits. Field collections were completed across four nested levels of biological organization. The intraspecific trait coefficient of variation (cv) ranged from 25 to 87% with root diameter and specific root tip density showing the lowest and highest cv, respectively. Between 27 and 68% of root ITV was explained by site identity alone for five of the seven traits measured. A single principal component explained 56.2% of root trait covariation, with plants falling along a RES from resource acquiring to conserving traits. Multiple factor analysis revealed significant orthogonal relationships between root and leaf spectra. RES traits were strongly orthogonal with respect to LES traits, suggesting these traits vary independently from one another in response to environmental cues. This study provides among the first evidence that plants from the same species differentiate from one another along an intraspecific RES. We find that in one of the world’s most widely cultivated crops, an intraspecific RES is orthogonal to an intraspecific LES, indicating that above and belowground responses of plants to managed (or natural) environmental gradients are likely to occur independently from one another.

Global patterns in wood carbon concentration across the world’s trees and forests

Wood carbon concentrations play a central role in forest carbon accounting, and are fundamentally linked to the growth strategies of woody plants. Yet there are no comprehensive assessments of wood carbon among trees globally, and coarse approximations of wood carbon (for example, 50%) are employed in virtually all benchmark models and assessments of forest carbon. We consolidated the largest database for any wood chemical trait—2,228 wood carbon observations from 636 species across all forested biomes—to derive robust wood carbon fractions for forest carbon accounting. Carbon fractions show substantial variation among forest biomes, and indicate errors in the existing forest carbon estimates of 4.8%, on average, and most extreme errors of 8.9% in tropical forests. The data also demonstrate that wood carbon concentrations show a phylogenetic signal and are co-evolved with, and negatively related to, wood density, thus representing a key plant trait that links plant functional biology to ecosystem processes worldwide.Large variability of wood carbon fractions in different trees can lead to an error of up to 8.9% in carbon estimates for forests, according to an analysis of wood carbon data across global forested biomes.

Functional traits of coffee along a shade and fertility gradient in coffee agroforestry systems

Intraspecific variation in leaf functional traits has gained widespread attention as a means to evaluate, predict, and manage plant responses to environmental conditions, however there are considerable uncertainties regarding the extent and drivers of intraspecific trait variation (ITV) in domesticated plants. In a coffee (Coffea arabica) agroforestry system, we quantified ITV in seven leaf traits [i.e. area (LA), mass per area (LMA), dry matter content (LDMC), thickness (Lth), nitrogen concentrations (LNC), maximum photosynthetic rate on area and mass bases (Asat, Amass, respectively)] across managed gradients of soil fertility and light availability. Leaf physiological traits (Asat, Amass), as well as LA, showed the greatest extent of variation within coffee, while morphological traits (LMA, LDMC, Lth) and leaf N were less variable. All traits differed significantly as a function of light and fertilization treatment, however light was more influential in driving ITV in coffee leaves. Low light availability resulted in greater ITV for physiological leaf traits (Asat and Amass), while high light constrained ITV in most morphological-(LA, LMA, LDMC), physiological-(Asat, Amass) and chemical-(LNC) traits. Fertilization treatments did not induce systematic shifts in the extent of ITV. In addition, shade management treatments explained 9.2% of the variation in multivariate trait syndromes, while nutrient management regimes explained only 2.9%. Our results indicate that highly heterogeneous aboveground resource environments such those created by agroforestry, results in greater ITV for key crop physiological parameters. Based on ecological theory, such patterns indicate that management systems promoting resource heterogeneity should promote higher rates of resource partitioning, and greater resource-use efficiency in agroecosystems.