Richard K. Kobe

LIGHT GRADIENT PARTITIONING AMONG TROPICAL TREE SPECIES THROUGH DIFFERENTIAL SEEDLING MORTALITY AND GROWTH

To characterize species differences in light-dependent mortality and growth, I transplanted seedlings of four related tree species into a range of light environments (<1% to 85% full sun) on alluvial soils at La Selva Biological Station, Costa Rica. With maximum likelihood techniques, I fit species-specific, nonlinear regression models of seedling probability of mortality and radial growth as functions of light availability for Trophis racemosa, Castilla elastica, Pourouma aspera, and Cecropia obtusifolia (Family Moraceae, latter two also classified as Cecropiaceae). Models characterizing mortality in the first year of seedling life showed significant differences among the species, both in functional forms and in model parameter estimates. All species decreased in mortality with increases in light to 20% full sun. Above 20% full sun, the mortality of T. racemosa and Castilla elastica continued to decrease, while that of Cecropia obtusifolia remained constant and that of P. aspera increased. At <10% full sun, there was a fivefold difference in mortality probability; T. racemosa had the highest survivorship, followed by C. elastica, P. aspera, and C. obtusifolia. Mortality models for the initial 4 mo of the experiment revealed significant size effects for all species except P. aspera and increased mortality under high light for P. aspera and T. racemosa. In contrast, radial growth of all species increased with light availability. The radial growth models, based upon only variation in light availability and tree size, accounted for 60–86% of the variation in growth. Cecropia obtusifolia occupied the lowest rank in radial growth at 75% full sun. When considered together, the mortality and growth models revealed that differences in species performance could lead to light gradient partitioning. At distinct light levels, each species showed the highest growth relative to other surviving species. Neither growth nor mortality considered alone revealed this specialization. Light gradient partitioning provides an explanation for successional dynamics and suggests that tropical tree species coexistence and diversity is at least partly maintained through niche differentiation.

Carbohydrate allocation to storage as a basis of interspecific variation in sapling survivorship and growth

Total nonstructural carbohydrates (TNC) of roots were sampled in early autumn from saplings of four species that differ in light-dependent growth and survivorship (shade tolerance). The two deciduous species (Acer saccharum and Fraxinus americana) had higher TNC concentrations than the evergreens (Tsuga canadensis and Pinus strobus), presumably because of autumn build-up of reserves for spring refoliation. In separate comparisons of deciduous and coniferous pairs, A. saccharum and T. canadensis had higher low-light TNC concentrations and survivorship than F. americana and P. strobus, respectively. In high light, TNC levels were not significantly different between A. saccharum and F. americana and both species had >98% survivorship. An analytical model of carbohydrate allocation demonstrates that variation in storage allocation can influence survivorship and growth and that the opportunity cost of storage is lower under low light. The model and empirical data are consistent with an observed correlation among species between growth determinancy and shade tolerance and a negative correlation between high-light growth rates and low-light survivorship. Allocation to storage may be an effective strategy of shade tolerance because it is relatively inexpensive under low light and provides a buffer against stresses.

Interspecific and intraspecific variation in tree seedling survival: effects of allocation to roots versus carbohydrate reserves

Abstract We examined interspecific and intraspecific variation in tree seedling survival as a function of allocation to carbohydrate reserves and structural root biomass. We predicted that allocation to carbohydrate reserves would vary as a function of the phenology of shoot growth, because of a hypothesized tradeoff between aboveground growth and carbohydrate storage. Intraspecific variation in levels of carbohydrate reserves was induced through experimental defoliation of naturally occurring, 2-year-old seedlings of four northeastern tree species –Acer rubrum, A. saccharum, Quercus rubra, and Prunus serotina– with shoot growth strategies that ranged from highly determinate to indeterminate. Allocation to root structural biomass varied among species and as a function of light, but did not respond to the defoliation treatments. Allocation to carbohydrate reserves varied among species, and the two species with the most determinate shoot growth patterns had the highest total mass of carbohydrate reserves, but not the highest concentrations. Both the total mass and concentrations of carbohydrate reserves were significantly reduced by defoliation. Seedling survival during the year following the defoliation treatments did not vary among species, but did vary dramatically in response to defoliation. In general, there was an approximately linear relationship between carbohydrate reserves and subsequent survival, but no clear relationship between allocation to root structural biomass and subsequent survival. Because of the disproportionate amounts of reserves stored in roots, we would have erroneously concluded that allocation to roots was significantly and positively related to seedling survival if we had failed to distinguish between reserves and structural biomass in roots.

RESORPTION EFFICIENCY DECREASES WITH INCREASING GREEN LEAF NUTRIENTS IN A GLOBAL DATA SET

To investigate effects of green-leaf nutrient status on senesced-leaf nutrient concentrations and resorption efficiency, we developed a database of nitrogen (N) and phosphorus (P) concentrations in green and senesced leaves from 92 published studies. We fit power functions (i.e., [nutrient]sen = A [nutrient]grB) separately for N and P. The database encompassed 297 perennial species of different life-forms. Across these divergent species and conditions, a major control on senesced-leaf nutrient concentration was green-leaf nutrient status; nutrient concentrations in senesced leaves were positively associated with green-leaf nutrient concentrations (r2 values from 51% to 84%). Within-species variation as well as species differences contributed to the overall variation in nutrient concentrations. Moreover, N and P resorption efficiency decreased, respectively, with increased N and P green-leaf status; fitted power functions indicated that a disproportionate amount of nutrient remained in senesced leaves of h...

Sapling growth as a function of light and landscape-level variation in soil water and foliar nitrogen in northern Michigan

Interspecific differences in sapling growth responses to soil resources could influence species distributions across soil resource gradients. I calibrated models of radial growth as a function of light intensity and landscape-level variation in soil water and foliar N for saplings of four canopy tree species, which differ in adult distributions across soil resource gradients. Model formulations, characterizing different resource effects and modes of influencing growth, were compared based on relative empirical support using Akaike’s Information Criterion. Contrary to expectation, the radial growth of species associated with lower fertility (Acer rubrum and Quercus rubra) was more sensitive to variation in soil resources than the high fertility species Acer saccharum. Moreover, there was no species tradeoff between growth under high foliar N versus growth under low foliar N, which would be expected if growth responses to foliar N mediated distributions. In general, there was functional consistency among species in growth responses to light, foliar N, and soil water availability, respectively. Foliar N influenced primarily high-light growth in F. grandifolia, A. rubrum, and Q. rubra (but was not significant for A. saccharum). In A. saccharum and A. rubrum, for which soil water availability was a significant predictor, soil water and light availability simultaneously limited growth (i.e., either higher light or water increased growth). Simple resource-based models explained 0.74–0.90 of growth variance, indicating a high degree of determinism. Results suggest that nitrogen effects on forest dynamics would be strongest in high-light early successional communities but that water availability influences growth in both early successional and understory environments.

Tropical tree growth is correlated with soil phosphorus, potassium, and calcium, though not for legumes

Tropical forest productivity is widely assumed to be limited by soil phosphorus (P), but biogeochemical processes that deplete P also could deplete base cations, suggesting multiple resource limitation. Limitation by several resources could arise from species and functional diversity and from variation among groups in resource requirements, including ecophysiological strategies that minimize P limitation. We hypothesized that tree growth is positively related to soil base cation and P availability and negatively related to local competition; Fabaceae growth is weakly correlated with soil resources if fixed N is used indirectly to acquire other resources; growth of species with low wood density is more strongly related to soil resource availability than that of species with high wood density. Diameter growth and soil resource availability were measured in five mapped stands situated across natural soil resource gradients in lowland wet tropical forest (La Selva Biological Station, Costa Rica). Soil resourc...

Conspecific density dependence in seedlings varies with species shade tolerance in a wet tropical forest

Density-dependent seedling mortality could increase with a species relative abundance, thereby promoting species coexistence. Differences among species in light-dependent mortality also could enhance coexistence via resource partitioning. These compatible ideas rarely have been considered simultaneously. We developed models of mortality as functions of irradiance and local conspecific density (LCD) for seedlings of 53 tropical woody species. Species varied in mortality responses to these factors, but mortality consistently increased with shading and LCD. Across species, density-dependent mortality on a per-neighbour basis was inversely related to species community abundance, but higher LCD in more common species resulted in a weak relationship between species abundance and density-dependent mortality scaled to species maximum LCD. Species mortality responses to shading and maximum LCD were strongly and positively correlated. Our results suggest that species differences in density-dependent mortality are more strongly related to physiologically based life-history traits than biotic feedbacks related to community abundance.

Soil water content and emergence time control seedling establishment in three co-occurring Mediterranean oak species

Tree species can differ in their responses to resource availability during the critical phase of establishment, which could influence forest dynamics. In Mediterranean forests, most of the attention has focused on the effects of shade and summer drought on seedling survival, but little is known about the effect of autumn to spring rains on earlier stages of recruitment. A sowing experiment was set up along natural light and water gradients with three co-occurring oak spe- cies (Quercus suber L. (cork oak), Quercus canariensis Willd. (Algerian oak), and Quercus pyrenaica Willd. (Pyrenean oak)) that show limited natural regeneration in southern Spain. Recruitment stages were monitored for 1 year. Models of seed germination, seedling emergence, and seedling survival as well as of overall recruitment patterns were developed as functions of light, soil moisture, and soil compaction. The influence of intraspecific variation in seed mass and emergence time were also tested. Excess soil water levels during the winter reduced germination and emergence and lengthened time to emergence (in waterlogged open areas), which in turn decreased seedling survival during the dry season. Seedlings from larger seeds were more likely to germinate and emerge. The results suggest that temporal and spatial variability of soil water content, mediated by emergence time and seed size, play a crucial role in the regeneration dynamics of Mediter- ranean oak forests.

Optimal partitioning theory revisited: Nonstructural carbohydrates dominate root mass responses to nitrogen

Under optimal partitioning theory (OPT), plants preferentially allocate biomass to acquire the resource that most limits growth. Within this framework, higher root mass under low nutrients is often assumed to reflect an allocation response to build more absorptive surface. However, higher root mass also could result from increased storage of total nonstructural carbohydrates (TNC) without an increase in non-storage mass or root surface area. To test the relative contributions of TNC and non-storage mass as components of root mass responses to resources, we grew seedlings of seven northern hardwood tree species (black, red, and white oak, sugar and red maple, American beech, and black cherry) in a factorial light x nitrogen (N) greenhouse experiment. Because root mass is a coarse metric of absorptive surface, we also examined treatment effects on fine-root surface area (FRSA). Consistent with OPT, total root mass as a proportion of whole-plant mass generally was greater in low vs. high N. However, changes in root mass were influenced by TNC mass in all seven species and were especially strong in the three oak species. In contrast, non-storage mass contributed to increased total root mass under low N in three of the seven species. Root morphology also responded, with higher fine-root surface area (normalized to root mass) under low vs. high N in four species. Although biomass partitioning responses to resources were consistent with OPT, our results challenge the implicit assumption that increases in root mass under low nutrient levels primarily reflect allocation shifts to build more root surface area. Rather, root responses to low N included increases in: TNC, non-storage mass and fine-root surface area, with increases in TNC being the largest and most consistent of these responses. The greatest TNC accumulation occurred when C was abundant relative to N. Total nonstructural carbohydrates storage could provide seedlings a carbon buffer when respiratory or growth demands are not synchronized with photosynthesis, flexibility in responding to uncertain and fluctuating abiotic and biotic conditions, and increased access to soil resources by providing an energy source for mycorrhizae, decomposers in the rhizosphere, or root uptake of nutrients.

Evaluation of irradiance metrics with respect to predicting sapling growth

Accurate and efficient measurement of photosynthetically active radiation (PAR) is critical in ecological studies. We evaluated 25 metrics of PAR with respect to predicting growth of sugar maple (Acer saccharum Marsh.) and white ash (Fraxinus americana L.) saplings from understory to large gap conditions. PAR metrics were derived from gallium arsenide photodiodes, hemispherical canopy photographs (film and digital), and a LI-COR LAI-2000 plant canopy analyzer. In general, percent canopy openness, estimated with film photographs or LAI-2000, best predicted growth. Mean daily photosynthetic photon flux density (PPFD) from photodiodes ranked intermediate; direct beam radiation (from digital and film photographs) was among the poorest growth predictors. Metrics that integrate direct radiation may be relatively poor predictors of growth, because sunflecks were above PPFD levels at which photosynthesis saturates but fully contributed to the calculation of mean daily PPFD. Mean daily PPFD based on truncated sunf...