Gerhard Zotz

The epiphyte vegetation of the palm Socratea exorrhiza - correlations with tree size, tree age and bryophyte cover

We conducted a survey of the epiphyte flora growing on the stilt palm Socratea exorrhiza in a primary lowland rain forest in Panama by means of a canopy crane. For each palm in a 0.9-ha plot, we determined diameter at breast height, tree height, per cent bryophyte cover and the number, identity and attachment site of all vascular epiphytes. The 118 palm trees hosted a total of 701 epiphytes and hemi-epiphytes, belonging to 66 species. Trees were estimated to be c. 20 y old before colonization with vascular epiphytes began. Epiphyte species were highly clumped and segregated along the vertical axis of the trunk. Sequential colonization led to an increased number of species and individuals as the tree grows. Epiphytes were associated with bryophyte patches much more than expected by chance, but no species seemed to depend upon them for establishment. The influence of tree size, age and bryophyte cover on the composition of the epiphyte community are discussed.

Heteroblasty—A Review

Virtually all plants show a certain degree of variation among individual metamers during ontogeny. In some cases, however, there are abrupt and substantial changes in form and function (e.g. in leaf form, leaf size, phyllotaxy, internode length, anthocyanin pigmentation, rooting ability, or wood structure). These plants were called “heteroblastic” by Karl Goebel more than a century ago, but the functional significance of this type of ontogenetic change, the evolutionary trajectories in different plant groups, even their frequency in the plant kingdom are still unresolved issues. We argue that slow progress is partly due to an on-going terminological confusion and the lack of distinction between other developmental processes such as ontogenetic drift. This review develops a conceptual framework for future scientific work, proposes a quantitative index of heteroblasty, and discusses the evidence for developmental regulation, functional significance, and evolutionary implications of heteroblasty to provide a stimulating basis for further research with this fascinating group of plants.ZusammenfassungFast alle Pflanzen zeigen während der Individualentwicklung ein gewisses Maß an Variabilität einzelner Metamere. Bei manchen Arten kommt es jedoch zu einer ausgeprägten und sprunghaften Änderung in Form und Funktion (z.B. in Blattform oder -stellung, Internodienlänge, Pigmentierung, Holzstruktur, oder hinsichtlich der Fähigkeit zur Bildung von Adventivwurzeln). Obwohl diese Arten von Karl Goebel schon vor mehr als einem Jahrhundert als “heteroblastisch” beschrieben wurden, sind die funktionelle Bedeutung des Phänomens, dessen Evolution innerhalb einzelner Pflanzengruppen, wie auch die Häufigkeit im Pflanzenreich immer noch weitgehend ungeklärt. Dieser schleppende Fortschritt mag mit einem weit verbreiteten terminologischen Durcheinander und dem Fehlen einer klaren Abgrenzung von anderen Entwicklungsprozessen wie der “ontogenetischen Drift” zusammenhängen. Unser Übersichtsartikel entwickelt einen klaren konzeptionellen Rahmen, um eine Basis zu schaffen für zukünftige Forschungsarbeiten dieses faszinierenden Entwicklungsphänomens. Dazu schlagen wir einen quantitativen Index der Heteroblastie vor, skizzieren den gegenwärtigen Wissensstand der Regulierung von Entwicklungsprozessen bei Pflanzen, die bisherigen Untersuchungen zur funktionelle Bedeutung heteroblastischer Veränderungen, beleuchten aber auch die evolutionären Implikationen der Heteroblastie.

Demography of the epiphytic orchid, Dimerandra emarginata

Vegetative growth, reproductive effort, seedling establishment, and mortality of the bark epiphyte, Dimerandra emarginata (Orchidaceae), were studied over a 3-y period in the moist lowland tropical forest on Barro Colorado Island, Panama. The study included more than 350 individuals growing on 12 branches of five different host tree species. Mortality was highest among small individuals. Most deaths occurred during the dry season and were probably related to drought. Mortality among older individuals, on the other hand, was always related to the instability of the substrate, i.e. flaking bark, breaking branches or falling trees. Vegetative growth was slow. The average increase in size (= height of the most recent, fully developed shoot) was 2.7 cm over 3 y (not considering ‘stemless’ seedlings). This increase was negatively correlated with initial plant size. Growth was highly seasonal, with little variation between years. The reproductive effort increased strongly with plant size. Larger individuals produced fruits more frequently, in larger numbers and of larger size. After reproduction, plants showed reduced vegetative growth in the following year (in 1994). There was no negative effect on future reproduction. Compared to seed production, annual recruitment was very low and showed large year-to-year variation. Fewer than 50% of the seedlings survived the first dry season after germination. After 3 y, their average size was 0.5 cm. The results suggest that slow-growing epiphytes such as D. emarginata can attain considerable age and that their longevity – after an initial vulnerable juvenile stage – is almost exclusively limited by substrate durability.

Diversity and structure of the arthropod fauna within three canopy epiphyte species in central Panamá

The arthropod fauna inhabiting 90 individuals of three different spe- cies of epiphyte was investigated in the moist lowland forest of the Barro Colorado National Monument in Panama. In total, 3694 arthropods belonging to 89 morpho- species and 19 orders were collected. While arthropod abundance was primarily a function of host plant biomass irrespective of epiphyte species, there were pro- nounced differences in species richness, species composition and guild structure of the arthropod faunas of the three epiphyte species. Although all study plants were growing in close proximity on the same host tree species, there was remarkably little overlap in the species assemblages across epiphyte taxa. The inhabitant species also differed dramatically in their ecological functions, as feeding guild and hunting guild analyses indicated. The influence of plant size, structure and impounded leaf litter on arthropod diversity is discussed. We conclude that epiphytes are microhabitats for a diverse and numerous fauna, and that different species of epiphytes foster both taxonomically and ecologically very distinct arthropod assemblages.

Diel patterns of CO2 exchange in rainforest canopy plants.

Over the course of a day, plants in their natural environments are exposed to fluctuations in temperature, light, relative humidity, precipitation, and, to a lesser extent, atmospheric CO2 concentration. These factors, combined with seasonal changes in soil water content, nutrient availability, and leaf developmental status, profoundly affect the plant’s gas exchange with the surrounding atmosphere. Diel (24 h) measurements of CO2 and water vapor exchange in situ provide information on how these environmental and leaf ontogenetic factors affect photosynthesis and respiration, leaf primary productivity, and the water and carbon economy of plants. Diel gas exchange measurements, in combination with other analytical tools (e.g., fluorescence analysis, enzyme assays, pressure bomb measurements, stem flow measurements, lysimetry) allow integrative studies from the leaf level to the entire plant.

Water relations of two co-occurring epiphytic bromeliads

Summary The water relations of the co-occurring epiphytic bromeliads, Guzmania monostachya and Tillandsia fascicular , were studied in situ in the lowland tropical forest of Barro Colorado Island, Panama. Particular attention was paid to differences in the water relations of smaller and larger plants. Both species showed moderate, but significant seasonal changes in leaf water status and CAM-activity, measured as nocturnal acidification. When water was withheld from plants, 24-h whole plant water loss decreased to a minimum after 4–5 days. There were no significant interspecific or size-related differences in area-based transpiration rates. Due to a lower ratio of plant leaf area to plant water content (LA/PWC), similar area-based transpiration rates resulted in lower relative water losses in T. fasciculata . Smaller plants suffered significandy higher relative water losses in G. monostachya , but not in T. fasciculata . The observed differences in the water relations of smaller individuals may be responsible for the differences in the spatial distribution in host trees.

High rates of photosynthesis in the tropical pioneer tree, Ficus insipida Willd.

Summary Diurnal courses of CO 2 and water-vapour exchange of canopy leaves of mature trees of Ficus insipida W illd . (Moraceae) were studied under natural conditions in the tropical dry forest of Parque Metropolitano near Panama City, Republic of Panama. Measurements were conducted from early January 1993 (dry season) until July 1993 (wet season). Rates of net CO 2 uptake were high throughout the study period, with up to 33.1 μmol m − 2 S −1 in the early dry season. During the dry season, gas exchange was characterized by strong reductions (“midday-depressions”) of net CO 2 uptake and stomatal conductance at noon. However, daily carbon gain and diurnal water-use efficiency of CO 2 uptake did not differ between seasons. East-exposed leaves that received direct sunlight during early morning hours showed higher CO 2 uptake rates and lost less water per CO 2 fixed than west-exposed leaves. As previously demonstrated for several canopy species from a tropical moist forest, the maximum rate of net CO 2 uptake was a good predictor of daily carbon gain.

Short-term photosynthesis measurements predict leaf carbon balance in tropical rain-forest canopy plants

Diel (24 h) courses of net CO2 exchange of leaves were determined in eight species of tropical rainforest plants on Barro Colorado Island, Panama, during 1990 and 1991. The species included three canopy trees, one liana, two epiphytes and one hemiepiphyte. One of the species studied was growing in a rain-forest gap. Daily carbon gain varied considerably across species, leaf age, and season. The analysis of data for all plants from 64 complete day/night cycles revealed a linear relationship between the diurnal carbon gain and the maximum rate of net CO2 uptake, Amax. Nocturnal net carbon loss was about 10% of diurnal carbon gain and was positively related to Amax. We conclude that short-term measurements of light-saturated photosynthesis, performed at periodic intervals throughout the season, allow the annual leaf carbon balance in these rain-forest plants to be predicted.

Short-Term Regulation of Crassulacean Acid Metabolism Activity in a Tropical Hemiepiphyte, Clusia uvitana.

Diel courses of net CO2 exchange of leaves were studied in Clusia uvitana (Clusiaceae), a tropical Crassulacean acid metabolism (CAM) hemiepiphyte, growing in the crown of a 47-m tall kapok tree on Barro Colorado Island, Panama. Measurements on days without precipitation showed that net uptake of atmospheric CO2 occurred at night, a feature of CAM, as well as in the early morning and late afternoon. During 36 h of almost continuous rainfall, nocturnal net CO2 uptake was abolished and the diel pattern of net CO2 exchange became similar to that of a C3 plant. Exposing well-watered, potted plants of Clusia in the laboratory to temperatures and photosynthetic photon flux densities similar to those during the tropical rainstorm also abolished nocturnal net CO2 uptake. In contrast, Kalanchoe pinnata (Crassulaceae), an obligate CAM plant, still showed net CO2 dark fixation following the same low-light and moderate-temperature conditions, albeit at decreased rates. During these 12-h photoperiods, titratable acidity in Clusia increased slightly above its high level measured at the end of the previous dark period, whereas in Kalanchoe, the acid content decreased by about 40%. A survey among outer canopy leaves of Clusia on Barro Colorado Island showed that leaves that exhibited little or no nocturnal acidification maintained high levels of H+ at dawn and dusk. Progressively lower levels of H+ at dusk were accompanied by progressively higher nocturnal increases in H+. The data suggest that in C. uvitana the rapid switching between CAM- and C3-type carbon fixation that may occur within 24 h in response to environmental changes is controlled by the acidity status of the leaves in the light. Nocturnal CO2 fixation is enhanced by conditions that decrease the organic acid content during the light period.

Light and dark CO2 fixation in Clusia uvitana and the effects of plant water status and CO2 availability

SummaryIn well-watered plants of Clusia uvitana, a species capable of carbon fixation by crassulacean acid metabolism (CAM), recently expanded leaves gained 5 to 13-fold more carbon during 12 h light than during 12 h dark periods. When water was withheld from the plants, daytime net CO2 uptake strongly decreased over a period of several days, whereas there was a marked increase in nocturnal carbon gain. Photosynthetic rates in the chloroplasts were hardly affected by the water stress treatment, as demonstrated by measurements of chlorophyll a fluorescence of intact leaves, indicating efficient decarboxylation of organic acids and refixation of carbon in the light. Within a few days after rewatering, plants reverted to the original gas exchange pattern with net CO2 uptake predominantly occurring during daytime. The reversible increase in dark CO2 fixation was paralleled by a reversible increase in the content of phosphoenolpyruvate (PEP) carboxylase protein. In wellwatered plants, short-term changes in the degree of dark CO2 fixation were induced by alterations in CO2 partial pressure during light periods: a decrease from 350 to 170 μbar CO2 caused nocturnal carbon gain, measured in normal air (350 μbar), to increase, whereas an increase to 700 μbar CO2, during the day, caused net dark CO2 fixation to cease. The increased CAM activity in response to water shortage may, at least to some extent, be directly related to the reduced carbon gain during daytime.