John C. Volin

GENERALITY OF LEAF TRAIT RELATIONSHIPS: A TEST ACROSS SIX BIOMES

Convergence in interspecific leaf trait relationships across diverse taxonomic groups and biomes would have important evolutionary and ecological implications. Such convergence has been hypothesized to result from trade-offs that limit the combination of plant traits for any species. Here we address this issue by testing for biome differences in the slope and intercept of interspecific relationships among leaf traits: longevity, net pho- tosynthetic capacity (Amax), leaf diffusive conductance (Gs), specific leaf area (SLA), and nitrogen (N) status, for more than 100 species in six distinct biomes of the Americas. The six biomes were: alpine tundra-subalpine forest ecotone, cold temperate forest-prairie ecotone, montane cool temperate forest, desert shrubland, subtropical forest, and tropical rain forest. Despite large differences in climate and evolutionary history, in all biomes mass-based leaf N (Nmass), SLA, Gs, and Amax were positively related to one another and decreased with increasing leaf life span. The relationships between pairs of leaf traits exhibited similar slopes among biomes, suggesting a predictable set of scaling relationships among key leaf morphological, chemical, and metabolic traits that are replicated globally among terrestrial ecosystems regardless of biome or vegetation type. However, the intercept (i.e., the overall elevation of regression lines) of relationships between pairs of leaf traits usually differed among biomes. With increasing aridity across sites, species had greater Amax for a given level of Gs and lower SLA for any given leaf life span. Using principal components analysis, most variation among species was explained by an axis related to mass-based leaf traits (Amax, N, and SLA) while a second axis reflected climate, Gs, and other area-based leaf traits.

Relationships of leaf dark respiration to leaf nitrogen, specific leaf area and leaf life-span: a test across biomes and functional groups

Abstract Based on prior evidence of coordinated multiple leaf trait scaling, we hypothesized that variation among species in leaf dark respiration rate (Rd) should scale with variation in traits such as leaf nitrogen (N), leaf life-span, specific leaf area (SLA), and net photosynthetic capacity (Amax). However, it is not known whether such scaling, if it exists, is similar among disparate biomes and plant functional types. We tested this idea by examining the interspecific relationships between Rd measured at a standard temperature and leaf life-span, N, SLA and Amax for 69 species from four functional groups (forbs, broad-leafed trees and shrubs, and needle-leafed conifers) in six biomes traversing the Americas: alpine tundra/subalpine forest, Colorado; cold temperate forest/grassland, Wisconsin; cool temperate forest, North Carolina; desert/shrubland, New Mexico; subtropical forest, South Carolina; and tropical rain forest, Amazonas, Venezuela. Area-based Rd was positively related to area-based leaf N within functional groups and for all species pooled, but not when comparing among species within any site. At all sites, mass-based Rd (Rd-mass) decreased sharply with increasing leaf life-span and was positively related to SLA and mass-based Amax and leaf N (leaf Nmass). These intra-biome relationships were similar in shape and slope among sites, where in each case we compared species belonging to different plant functional groups. Significant Rd-mass−Nmass relationships were observed in all functional groups (pooled across sites), but the relationships differed, with higher Rd at any given leaf N in functional groups (such as forbs) with higher SLA and shorter leaf life-span. Regardless of biome or functional group, Rd-mass was well predicted by all combinations of leaf life-span, Nmass and/or SLA (r2≥ 0.79, P < 0.0001). At any given SLA, Rd-mass rises with increasing Nmass and/or decreasing leaf life-span; and at any level of Nmass, Rd-mass rises with increasing SLA and/or decreasing leaf life-span. The relationships between Rd and leaf traits observed in this study support the idea of a global set of predictable interrelationships between key leaf morphological, chemical and metabolic traits.

Reexamining the empirical relation between plant growth and leaf photosynthesis.

Technological advances during the past several decades have greatly enhanced our ability to measure leaf photosynthesis virtually anywhere and under any condition. Associated with the resulting proliferation of gas-exchange data is a lingering uncertainty regarding the importance of such measurements when it comes to explaining intrinsic causes of plant growth variation. Accordingly, in this paper we rely on a compilation of data to address the following questions: from both statistical and mechanistic standpoints, how closely does plant growth correlate with measures of leaf photosynthesis? Moreover, in this context, does the importance of leaf photosynthesis as an explanatory variable differ among growth light environments? Across a wide array of species and environments, relative growth rate (RGR) was positively correlated with daily integrals of photosynthesis expressed per unit leaf area (Aarea), leaf mass (Amass), and plant mass (Aplant). The amount of RGR variation explained by these relationships increased from 36% for the former to 93% for the latter. Notably, there was close agreement between observed RGR and that estimated from Aplant after adjustment for theoretical costs of tissue construction. Overall, based on an analysis of growth response coefficients (GRCs), gross assimilation rate (GAR), a photosynthesis-based estimate of biomass gain per unit leaf area, explained about as much growth variation as did leaf mass ratio (LMR) and specific leaf area (SLA). Further analysis of GRCs indicated that the importance of GAR in explaining growth variation increased with increasing light intensity. Clearly, when considered in combination with other key determinants, appropriate measures of leaf gas exchange effectively capture the fundamental role of leaf photosynthesis in plant growth variation.

The reproductive biology of the invasive ferns Lygodium microphyllum and L. japonicum (Schizaeaceae): implications for invasive potential.

The effect of culture system and population source on sexual expression and sporophyte production was examined for two invasive fern species in Florida, USA, Lygodium microphyllum and L. japonicum (Schizaeaceae). Both species are currently spreading through Florida. Long-distance dispersal of ferns is thought to rely on successful intragametophytic selfing. Given the rate of spread observed in both Lygodium species, we hypothesized that both species are capable of intragametophytic selfing. To test this hypothesis, gametophytes of both species were grown in vitro as isolates, pairs, and groups. Both species were capable of intragametophytic selfing; 78% of L. microphyllum isolates produced sporophytes and over 90% of the L. japonicum isolates produced sporophytes. Lygodium microphyllum also displayed the ability to reproduce via intergametophytic crossing, facilitated by an antheridiogen pheromone. Sporophyte production was rapid across mating systems for both species, an advantage in Floridas wet and dry seasonal cycles. The high intragametophytic selfing rate achieved by both species has likely facilitated their ability to colonize and spread through Florida. The mixed mating system observed in L. microphyllum appears to give this species the ability to invade distant habitats and then adapt to local conditions.

Recent and historic drivers of landscape change in the everglades ridge, Slough, and Tree Island Mosaic

More than half of the original Everglades extent formed a patterned peat mosaic of elevated ridges, lower and more open sloughs, and tree islands aligned parallel to the dominant flow direction. This ecologically important landscape structure remained in a dynamic equilibrium for millennia prior to rapid degradation over the past century in response to human manipulation of the hydrologic system. Restoration of the patterned landscape structure is one of the primary objectives of the Everglades restoration effort. Recent research has revealed that three main drivers regulated feedbacks that initiated and maintained landscape structure: the spatial and temporal distribution of surface water depths, surface and subsurface flow, and phosphorus supply. Causes of recent degradation include but are not limited to perturbations to these historically important controls; shifts in mineral and sulfate supply may have also contributed to degradation. Restoring predrainage hydrologic conditions will likely preserve remaining landscape pattern structure, provided a sufficient supply of surface water with low nutrient and low total dissolved solids content exists to maintain a rainfall-driven water chemistry. However, because of hysteresis in landscape evolution trajectories, restoration of areas with a fully degraded landscape could require additional human intervention.

Holocene dynamics of the Florida Everglades with respect to climate, dustfall, and tropical storms

Significance Wind-blown dust is seldom considered an important source for nutrients in large peatlands, such as the Everglades. However, a sedimentary record suggests that high loadings of dust-borne nutrients once prevailed in the central Everglades during a period of moister climate with intense tropical storms that ended 2,800 y ago. Afterwards, a drier climatic regime with a steep decline in dustfall may have been the impetus for the striking surface patterning of the Everglades. This study provides additional support for the importance of aeolian dust in ecosystem development. Aeolian dust is rarely considered an important source for nutrients in large peatlands, which generally develop in moist regions far from the major centers of dust production. As a result, past studies assumed that the Everglades provides a classic example of an originally oligotrophic, P-limited wetland that was subsequently degraded by anthropogenic activities. However, a multiproxy sedimentary record indicates that changes in atmospheric circulation patterns produced an abrupt shift in the hydrology and dust deposition in the Everglades over the past 4,600 y. A wet climatic period with high loadings of aeolian dust prevailed before 2800 cal BP (calibrated years before present) when vegetation typical of a deep slough dominated the principal drainage outlet of the Everglades. This dust was apparently transported from distant source areas, such as the Sahara Desert, by tropical storms according to its elemental chemistry and mineralogy. A drier climatic regime with a steep decline in dustfall persisted after 2800 cal BP maintaining sawgrass vegetation at the coring site as tree islands developed nearby (and pine forests covered adjacent uplands). The marked decline in dustfall was related to corresponding declines in sedimentary phosphorus, organic nitrogen, and organic carbon, suggesting that a close relationship existed between dustfall, primary production, and possibly, vegetation patterning before the 20th century. The climatic change after 2800 cal BP was probably produced by a shift in the Bermuda High to the southeast, shunting tropical storms to the south of Florida into the Gulf of Mexico.

Does release from natural belowground enemies help explain the invasiveness of Lygodium microphyllum? A cross-continental comparison

Lygodium microphyllum (Cav.) R. Br., a climbing fern native to the Pantropics of the Old World, is aggressively colonizing natural ecosystems in the Florida Peninsula. Here, we examined soil factors that might affect the fern’s invasiveness, specifically addressing the hypothesis that a release from natural belowground enemies contributes to its vigorous growth in Florida. We also investigated phenotypic differences of sporophytes raised from spores collected in Florida and the fern’s native range in Australia, hypothesizing that the Florida population would possess traits resulting in faster growth and superior competitive ability than the two Australian populations. We tested our hypotheses in parallel greenhouse experiments—one in Australia using soil from the fern’s native habitat, and another in Florida, USA, with soil from a recently colonized ecosystem. Fern growth rate and its principal determinants were expressed relative to the optimal growth with a common sand culture in each experiment and compared among treatments in which soil was altered through either sterilization or nutrient amendment, or both. Contrary to the expectation, the optimal growth rates in the sand culture were higher for Australian populations than the Florida population, while the comparatively poor growth of all populations in unaltered soil was stimulated by nutrient amendment and sterilization. The overall effect of sterilization, however, was muted under high-nutrient conditions, suggesting that the effect of soil sterilization may be due to greater nutrient availability in sterilized soils. The only exception was the local population from the site where the soil was collected for the experiment in Australia, which grew significantly faster in sterilized than in non-sterilized soil, and also more rapidly in response to soil insecticide application. Our results indicate that the invasiveness of L. microphyllum in Florida is not a simple phenotypic difference in inherent growth rate as predicted by the evolution of increased competitive ability hypothesis, but it may be mediated in part by release from soil-borne enemies that vary in their effectiveness even within the native geographical range of the fern.

Effect of soil pH on growth, nutrient uptake, and mycorrhizal colonization in exotic invasive Lygodium microphyllum

Lygodium microphyllum is an invasive exotic plant species taking over many sites in freshwater and moist habitats in Florida. Managing it has been a significant challenge for land resource managers and researchers due to its extensive rapid invasion. To assess the effects of soil pH on growth, nutrient uptake, and mycorrhizal colonization in the roots of L. microphyllum, we conducted a 60-day greenhouse experiment by growing it in pots filled with pH-adjusted soils to a range from 4.5 to 8.0. L. microphyllum was able to survive and grow at all soil pH levels; however, final biomass, relative growth rate, photosynthesis, and specific leaf area were all greater in soil pH 5.5–6.5 compared to the other treatments. Correspondingly, nitrogen concentration was also related to these four plant parameters. Root colonization by mycorrhizal fungi was higher in soil pH 5.5–7.5 and lowest for plants growing in 4.5 or 8.0 and was correlated with plant growth parameters as well as elemental concentration in the leaves. Soil pH 8.0 was not strong enough for a pronounced growth decline, thus further increasing soil pH could provide a desired outcome and merit further investigation, although its potential negative impact on native flora (both plants and microorganisms) would need to be assessed.

Mycorrhizal symbiosis and Lygodium microphyllum invasion in south Florida - a biogeographic comparison.

Lygodium microphyllum (Old World Climbing Fern) is one of the most problematic weeds in south Florida, invading numerous habitats from mangroves to pine flatwoods natural ecosystems. Much of the research efforts on L. microphyllum has been focused on reproductive potential, spore release, growth under different environmental conditions, belowground rhizome dormancy and survival strategies that describes its invasiveness. However, the role of an important mutualistic association with arbuscular mycorrhizal fungi (AMF) in the competitive ability and successful invasion of L. microphyllum by enhancing nutrient uptake has not been previously considered. Analysis of field root and soil samples from the ferns introduced and native range as well as a 7-week growth chamber experiment were done to determine the level of mycorrhizal colonization in the roots of L. microphyllum and the dependency on mycorrhizal fungi for growth and phosphorus (P) uptake. The field root samples showed that L. microphyllum was heavily colonized by AMF in relatively drier conditions, which are commonly found on some Florida sites compared to more common wetter sites where the fern is found in its native Australia. The results from the growth chamber experiment showed that the mycorrhizal treatment plants had significantly higher relative growth rate and biomass compared to the non-mycorrhizal plants. Similarly, L. microphyllum was highly dependent on the mycorrhizal fungi for growth and P uptake. Our results suggest that AMF play a significant role in vegetative reproduction and likely enhance the invasiveness of L. microphyllum in south Florida natural areas.

Assessing species-level biases in tree heights estimated from terrain-optimized leaf-off airborne laser scanner ALS data

Canopy height is an important metric in forest research and management with uses that include estimating stand volume, scheduling silvicultural treatments, and inferring site quality. In recent years, airborne laser scanner (ALS) data have been frequently used to model canopy height continuously and remotely across large areas. A number of studies have demonstrated that ALS is effective in this regard when collected during leaf-on conditions; however, relatively few studies have investigated the accuracy of leaf-off ALS in modelling canopy height. In this article, we assessed species-level biases in heights estimated from terrain-optimized leaf-off ALS data (1.5 points m–2). We focused on several deciduous and coniferous species common to the forests of the northeastern USA. Our study area included 13 sites located in the temperate deciduous forests of eastern Connecticut. Tree heights were measured in the field for 1192 trees which included 17 deciduous and 2 coniferous species. For one site, terrestrial laser scanner (TLS) data were collected and used to estimate tree heights. The ALS data were used to create a 1 m resolution canopy height model (CHM)ALS in which cell values corresponded to the heights of the highest returns. The (CHM)ALS underestimated tree heights with a median difference of approximately 1.3 m when compared to field-based measurements. Height biases ranged from approximately 0.1 to 2.1 m with the smallest bias for black cherry, red maple, shagbark hickory, and black oak and the largest for white ash, red oak, and white oak. We found no significant differences in bias corresponding to species’ leaf-types (i.e. simple, compound, needle). Biases in tree height estimates increased substantially as the (CHM)ALS cell size increased above 1 m. Our study suggests that leaf-off ALS data with a density > 1 point/m2 can be used to estimate tree heights with relatively small bias regardless of the species type.