Pigment dynamics and autumn leaf senescence in a New England deciduous forest, eastern USA

The leaves of woody plants at Harvard Forest in Central Massachusetts, USA, changed color during senescence; 70% (62/89) of the woody species examined anatomically contained anthocyanins during senescence. Anthocyanins were not present in summer green leaves, and appeared primarily in the vacuoles of palisade parenchyma cells. Yellow coloration was a result of the unmasking of xanthophyll pigments in senescing chloroplasts. In nine red-senescing species, anthocyanins were not detectable in mature leaves, and were synthesized de novo in senescence, with less than 20 m g cm - 2 of chlorophyll remaining. Xanthophyll concentrations declined in relation to chlorophyll to the same extent in both yellow- and red-leaved taxa. Declines in the maximum photosystem II quantum yield of leaves collected prior to dawn were only slightly less in the red-senescing species, indicating no long-term protective activity. Red-leaved species had significantly greater mass/area and lower chlorophyll a / b ratios during senescence. Nitrogen tissue concentrations in mature and senescent leaves negatively correlated to anthocyanin concentrations in senescent leaves, weak evidence for more efficient nitrogen resorption in anthocyanic species. Shading retarded both chlorophyll loss and anthocyanin production in Cornus alternifolia , Acer rubrum , Acer saccharum , Quercus rubra and Viburnum alnifolium . It promoted chlorophyll loss in yellow-senescing Fagus grandifolia . A reduced red : far-red ratio did not affect this process. Anthocyanins did not increase leaf temperatures in Q. rubra and Vaccinium corymbosum on cold and sunny days. The timing of leaf-fall was remarkably constant from year to year, and the order of senescence of individual species was consistent.

Anthocyanins in Autumn Leaf Senescence

Anthocyanins are synthesized during leaf senescence in certain plants across virtually all biomes, but are most spectacular in the autumn foliage of temperate deciduous forests. The patterns of color production in senescing foliage depend at least partly upon species composition and their phenology. Both ecological and physiological explanations have been raised to explain why plants produce this pigment just before leaf fall. Physiological explanations, as photoprotection, predict that cyanic leaves would be better able to resorb nitrogen during the process of chlorophyll degradation. Ecological explanations predict better dispersal of propagules advertised by association with the brilliantly colored leaves (plausible for only a minority of species), or warning against egg-laying activity of herbivorous insects, as aphids. These hypotheses make predictions that we now can test, to help us understand this old mystery - and majestic phenomenon.

Leaf morphology scales multi-annual trends in nutrient cycling and leaf, flower, and fruiting phenology among species in the sub-tropical hardwood forests of the northern Florida Keys

The subtropical hardwood forests of southern Florida are formed by 120 frost-sensitive, broadleaved angiosperm species that range throughout the Caribbean. Previous work on a series of small sized forest component patches of a 20 km2, forest preserve in northern Key Largo indicate that a shift in species composition was associated with a 100 year forest developmental sequence, and this shift was associated with an increasingly evergreen canopy. This document investigates the underlying differences of the biology of trees that live in this habitat, and is specifically focused on the impact of leaf morphology on changing nutrient cycling patterns. Measurements of the area, thickness, dry mass, nutrient content and longevity of several leaves from 3-4 individuals of ten species were conducted in combination with a two-year leaf litter collection and nutrient analysis to determine that species with thicker, denser leaves cycled scarce nutrients up to 2-3 times more efficiently than thin leaved tree species, and the leaf thickness/density index predicts role in forest development in a parallel direction as the index predicts nutrient cycling efficiency. A three year set of observations on the relative abundance of new leaves, flowers and fruits of the same tree species provides an opportunity to evaluate the consequences the leaf morphology/nutrient cycling/forest development relationship to forest habitat quality. Results of the three documents support a mechanistic link between forest development and nutrient cycling, and suggests that older forests are likely to be better habitats based on the availability of valuable forest products like new leaves, flowers, and fruits throughout the year.

Importance of water source in controlling leaf leaching losses in a dwarf red mangrove (Rhizophora mangle L.) wetland

The southern Everglades mangrove ecotone is characterized by extensive dwarf Rhizophora mangle L. shrub forests with a seasonally variable water source (Everglades – NE Florida Bay) and residence times ranging from short to long. We conducted a leaf leaching experiment to understand the influence that water source and its corresponding water quality have on (1) the early decay of R. mangle leaves and (2) the early exchange of total organic carbon (TOC) and total phosphorus (TP) between leaves and the water column. Newly senesced leaves collected from lower Taylor River (FL) were incubated in bottles containing water from one of three sources (Everglades, ambient mangrove, and Florida Bay) that spanned a range of salinity from 0 to 32‰, [TOC] from 710 to 1400 μM, and [TP] from 0.17 to 0.33 μM. We poisoned half the bottles in order to quantify abiotic processes (i.e., leaching) and assumed that non-poisoned bottles represented both biotic (i.e., microbial) and abiotic processes. We sacrificed bottles after 1,2, 5, 10, and 21 days of incubation and quantified changes in leaf mass and changes in water column [TOC] and [TP]. We saw 10–20% loss of leaf mass after 24 h—independent of water treatment—that leveled off by Day 21. After 3 weeks, non-poisoned leaves lost more mass than poisoned leaves, and there was only an effect of salinity on mass loss in poisoned incubations—with greatest leaching-associated losses in Everglades freshwater. Normalized concentrations of TOC in the water column increased by more than two orders of magnitude after 21 days with no effect of salinity and no difference between poisoned and non-poisoned treatments. However, normalized [TP] was lower in non-poisoned incubations as a result of immobilization by epiphytic microbes. This immobilization was greatest in Everglades freshwater and reflects the high P demand in this ecosystem. Immobilization of leached P in mangrove water and Florida Bay water was delayed by several days and may indicate an initial microbial limitation by labile C during the dry season.

Temporally dependent C, N, and P dynamics associated with the decay of Rhizophora mangle L. leaf litter in oligotrophic mangrove wetlands of the Southern Everglades

We performed two litter decomposition experiments using nearly-senesced red mangrove (Rhizophora mangle L.) leaves collected from an Everglades dwarf mangrove wetland to understand the short-term (3 weeks) and long-term (1 year) changes in mass, as well as C-, N-, and P-content of decomposing leaf litter. We expected that leaves decomposing in this oligotrophic environment would be short-term sources of C, N, and P, but potential long-term sinks for N and P. In May 1998, we conducted a 3-week leaching experiment, incubating fresh, individual leaves in seawater for up to 21 days. From May 1997 to May 1998, leaf litter in mesh bags decomposed on the forest floor at two dwarf mangrove sites. Leaching accounted for about 33% loss of dry mass from R. mangle leaves after 3 weeks. Leaching losses were rapid, peaking by day 2, and large, with leachate concentrations of total organic carbon (TOC) and total phosphorus (TP) increasing by more than an order of magnitude after 3 weeks. Mean leaf C:N increased from 105 to 115 and N:P increased from a mean of 74 to 95 after 21 days, reflecting the relatively large leaching losses of N and P. Loss of mass in the litterbags leveled off after 4 months, with roughly 60%dry mass remaining (DMR) after nearly 1 year of decomposition. The mass of carbon in each litterbag declined significantly after 361 days, but the mass of nitrogen and phosphorus doubled, indicating long-term accumulation of these constituents into the detritus. Subsequently, the leaf C:N ratio dropped significantly from 90 to 34 after 361 days. Following an initial 44-day increase, leaf N:P decreased from 222 to 144, reflecting high accumulation of P relative to N. A review of several estuarine macrophyte decomposition studies reveals a trend in nitrogen accumulation through time regardless of site, but suggests no clear pattern for C and P. We believe that the increase in litter P observed in this study was indicative of the P-limited status of the greater Everglades ecosystem and that decomposing mangrove litter may represent a substantial phosphorus pool in the system.

Leaf gas exchange characteristics of three neotropical mangrove species in response to varying hydroperiod

We determined how different hydroperiods affected leaf gas exchange characteristics of greenhouse-grown seedlings (2002) and saplings (2003) of the mangrove species Avicennia germinans (L.) Stearn., Laguncularia racemosa (L.) Gaertn. f., and Rhizophora mangle L. Hydroperiod treatments included no flooding (unflooded), intermittent flooding (intermittent), and permanent flooding (flooded). Plants in the intermittent treatment were measured under both flooded and drained states and compared separately. In the greenhouse study, plants of all species maintained different leaf areas in the contrasting hydroperiods during both years. Assimilation-light response curves indicated that the different hydroperiods had little effect on leaf gas exchange characteristics in either seedlings or saplings. However, short-term intermittent flooding for between 6 and 22 days caused a 20% reduction in maximum leaf-level carbon assimilation rate, a 51% lower light requirement to attain 50% of maximum assimilation, and a 38% higher demand from dark respiration. Although interspecific differences were evident for nearly all measured parameters in both years, there was little consistency in ranking of the interspecific responses. Species by hydroperiod interactions were significant only for sapling leaf area. In a field study, R. mangle saplings along the Shark River in the Everglades National Park either demonstrated no significant effect or slight enhancement of carbon assimilation and water-use efficiency while flooded. We obtained little evidence that contrasting hydroperiods affect leaf gas exchange characteristics of mangrove seedlings or saplings over long time intervals; however, intermittent flooding may cause short-term depressions in leaf gas exchange. The resilience of mangrove systems to flooding, as demonstrated in the permanently flooded treatments, will likely promote photosynthetic and morphological adjustment to slight hydroperiod shifts in many settings.