Guillermo Goldstein

Questions: What are the temporal and spatial variations of groundwater depth along topographic gradients in Neotropical savannas? Are patterns of tree density and species diversity along topographic gradients in Neotropical savannas controlled by water table depth? Do soil and groundwater nutrient concentrations differ along topographic gradients in Neotropical savannas?Location: Fire‐protected savannas of central Brazil.Methods: Over 16 mo, we monitored temporal and spatial variations in groundwater levels using automated submersible pressure transducers installed in ten wells located along two topographic gradients (five wells per gradient) of 950 m and 1703 m in length, representing elevations of 47 and 37 m a.s.l., respectively. We located the wells according to changes in vegetation physiognomies from woody savannas at high elevations, to open shrubby grasslands at low elevations. Along each topographic gradient we determined soil and groundwater nutrient concentrations as well as richness, density, basal diameter and height of trees within two plots of 14 × 14 m (392 m²) adjacent to each well.Results: Along the two gradients, groundwater levels exhibited larger fluctuations at lower than at higher elevations where the water table was deeper. Richness, density and diversity of trees decreased significantly at lower elevations where soils were waterlogged during the wet season. Soil pH and soil concentrations of carbon, nitrogen and manganese decreased significantly as elevation increased along the topographic gradients, but soil nutrient concentrations of phosphorus, aluminium and iron did not change with elevation. Groundwater samples contained only trace amounts of nutrients and were poorly correlated with elevation along the topographic gradients.Conclusions: In Neotropical savannas, the minimum distance between the soil surface and water table depth (reached during the wet season) and the relatively large fluctuations in groundwater limit tree density and diversity at low elevations as savanna trees cannot cope with extended waterlogging during the wet season and with low soil water availability during the dry season. Thus, variations of tree density and diversity along topographic gradients are more related to spatial and temporal variations in water table depth than to soil and groundwater nutrient variations in Neotropical savannas.

The relative advantages of being deciduous or evergreen in subtropical forests and the relationship between leaf phenology and nutrient resorption efficiency are not well understood. The most successful deciduous species (Lyonia ovalifolia) in an evergreen-dominated subtropical montane cloud forest in southwest (SW) China maintains red senescing leaves throughout much of the winter. The aim of this study was to investigate whether red senescing leaves of this species were able to assimilate carbon in winter, to infer the importance of maintaining a positive winter carbon balance in subtropical forests, and to test whether an extended leaf life span is associated with enhanced nutrient resorption and yearly carbon gain. The red senescing leaves of L. ovalifolia assimilated considerable carbon during part of the winter, resulting in a higher yearly carbon gain than co-occurring deciduous species. Its leaf N and P resorption efficiency was higher than for co-occurring non-anthocyanic deciduous species that dropped leaves in autumn, supporting the hypothesis that anthocyanin accumulation and/or extended leaf senescence help in nutrient resorption. Substantial winter carbon gain and efficient nutrient resorption may partially explain the success of L. ovalifolia versus that of the other deciduous species in this subtropical forest. The importance of maintaining a positive carbon balance for ecological success in this forest also provides indirect evidence for the dominance of evergreen species in the subtropical forests of SW China.

Studies on tree water relations, hydraulic architecture and effects of nutrients on growth have been carried out for many years, but not often concurrently. Only recently have the functional relations of these groups of plant traits been investigated with a more integrative approach (e.g. Harvey and van den Driessche 1997, Clearwater and Meinzer 2001, Ewers et al. 2000, 2001, Bucci et al. 2006, Hacke et al. 2010, Faustino et al. 2013). We attempt to answer the question of why the water relations and hydraulic architecture of trees are functionally related to their nutrient availability. We will focus on the results of studies where nutrient limitations have been removed or alleviated by the addition of the two most important macronutrients: nitrogen (N) and phosphorus (P). Plants can grow in soils where nutrient availability is high or in soils where either nitrogen, phosphorus or both chronically limit their growth and development. The response of tree species to the removal of nutrient limitation depends on which nutrient has been limiting. The typical method for assessing nutrient constraints is to fertilize the plants with each nutrient (e.g. N or P), which is expected to limit the growth and/or other plant processes. However, this approach has some drawbacks because it may overlook other factors or resources which are less available and essential for plant growth but which may control the distribution of a species in a given environment. From this point of view, adding N, P or N + P may not result in any significant response if some other resource is the bottleneck for plant growth. We assume that nutrients are the main limiting resources and that factors such as light, water availability, salinity and temperature do not interfere with the response to removal of nutrient limitations.

Rhizophora mangle L. trees of Biscayne National Park (Florida, USA) have two distinct growth forms: tall trees (5–10 m) growing along the coast and dwarf trees (1 m or less) growing in the adjacent inland zone. Sharp decreases in salinity and thus increases in soil water potential from surface soil to about a depth of 1 m were found at the dwarf mangrove site but not at the tall mangrove site. Consistent with our prediction, hydraulic redistribution detected by reverse sap flow in shallow prop roots was observed during nighttime, early morning and late afternoon in dwarf trees, but not in tall trees. In addition, hydraulic redistribution was observed throughout the 24-h period during a low temperature spell. Dwarf trees had significantly lower sapwood-specific hydraulic conductivity, smaller stem vessel diameter, lower leaf area to sapwood area ratio (LA/SA), smaller leaf size and higher leaf mass per area. Leaves of dwarf trees had lower CO₂ assimilation rate and lower stomatal conductance compared to tall trees. Leaf water potentials at midday were more negative in tall trees that are consistent with their substantially higher stomatal conductance and LA/SA. The substantially lower water transport efficiency and the more conservative water use of dwarf trees may be due to a combination of factors such as high salinity in the surface soil, particularly during dry periods, and substantial reverse sap flow in shallow roots that make upper soil layers with high salinity a competing sink of water to the transpiring leaves. There may also be a benefit for the dwarf trees in having hydraulic redistribution because the reverse flow and the release of water to upper soil layers should lead to dilution of the high salinity in the rhizosphere and thus relieve its potential harm to dwarf R. mangle trees.

Plasticity in hydraulic architecture of five dominant Atlantic forest species differing in light requirements and growth rates was evaluated in saplings grown at different irradiances to determine if hydraulic architecture changes in coordination with photosynthetic capacity. Saplings were grown in shade-houses at 10, 30, 45 and 65% of full solar irradiance for 4 months. In four of the five species, maximum relative growth rates were observed at intermediate irradiances (30 and 40% of full sun). Slow-growing species had lower maximum electron transport rates (ETR_max) than fast-growing species. A positive correlation between ETR_max and maximum leaf hydraulic conductivity (K_L) was found across species, suggesting that species-specific stem hydraulic capacity and photosynthetic capacity were linked. Species with relatively high growth rates, such as Cedrela fissilis Vell., Patagonula americana L. and Cordia trichotoma (Vell.) Arrab. Ex Stend, exhibited increased K_L and specific hydraulic conductivity (K_S) with increased growth irradiance. In contrast, K_S and K_L did not vary with irradiance in the slower-growing and more shade-tolerant species Balfourodendron riedelianum (Engl.) Engl. and Lonchocarpus leucanthus Burkart, despite a relatively large irradiance-induced variation in ETR_max. A correlation between K_S and ETR_max was observed in fast-growing species in different light regimes, suggesting that they are capable of plastic changes in hydraulic architecture and increased water-transport efficiency in response to changes in light availability resulting from the creation of canopy gaps, which makes them more competitive in gaps and open habitats.