The use of X-ray densitometry to evaluate the wood density profile of Tectona grandis trees growing in fast-growth plantations

Tectona grandis (teak) is an important commercial tree species that is widely used in tropical dendrochronology due to the formation of climate-sensitive annual growth rings. However, young trees growing in plantation conditions exhibit poor ring visibility during the first years of growth, limiting the dendrochronology application. In the present study, we use x-ray densitometry to determine the wood density profile between and within annual rings and at the sapwood-heartwood boundary in trees from fast-growth plantations. The resulting wood density profiles (WDP) can be categorized as uniform, stable growth, unstable growth, and false. The annual ring boundaries were indistinct in trees less than 8 years old. In mature trees, the annual ring boundaries are more defined. In relation to the sapwood-heartwood boundary, the WDP showed a decrease in the wood density; however, this decrease is influenced by the annual ring boundary when the two boundaries coincide. The identification of annual rings in trees growing in fast-growth plantations should be combined with X-ray densitometry and visual identification if wood density data are necessary for deriving other analysis, as climate change, from annual ring.     

Age and Long-term Growth of Trees in an Old-growth Tropical Rain Forest, Based on Analyses of Tree Rings and 14C1

In an old‐growth tropical wet forest at La Selva, Costa Rica, we combined radiocarbon (¹⁴C) dating and tree‐ring analysis to estimate the ages of large trees of canopy and emergent species spanning a broad range of wood densities and growth rates. We collected samples from the trunks of 29 fallen, dead individuals. We found that all eight sampled species formed visible growth rings, which varied considerably in distinctiveness. For five of the six species for which we combined wood anatomical studies with ¹⁴C‐dates (ring ages), the analyses demonstrated that growth rings were of annual formation. The oldest tree we found by direct ring counting was a Hymenolobium mesoamericanum Lima (Papilionaceae) specimen, with an age of ca. 530 years at the time of death. All other sampled individuals, including very large trees of slow‐growing species, had died at ages between 200 and 300 years. These results show that, even in an everwet tropical rain forest, tree growth of many species can be rhythmic, with an annual periodicity. This study thus raises the possibility of extending tree‐ring analyses throughout the tropical forest types lacking a strong dry season or annual flooding. Our findings and similar measurements from other tropical forests indicate that the maximum ages of tropical emergent trees are unlikely to be much greater than 600 years, and that these trees often die earlier from various natural causes.     

Nonannual tree rings in a climate‐sensitive Prioria copaifera chronology in the Atrato River,Colombia

In temperate climates, tree growth dormancy usually ensures the annual nature of tree rings, but in tropical environments, determination of annual periodicity can be more complex. The purposes of the work are as follows: (1) to generate a reliable tree‐ring width chronology for Prioria copaifera Griseb. (Leguminoceae), a tropical tree species dwelling in the Atrato River floodplains, Colombia; (2) to assess the climate signal recorded by the tree‐ring records; and (3) to validate the annual periodicity of the tree rings using independent methods. We used standard dendrochronological procedures to generate the P. copaifera tree‐ring chronology. We used Pearson correlations to evaluate the relationship of the chronology with the meteorological records, climate regional indices, and gridded precipitation/sea surface temperature products. We also evaluated 24 high‐precision ¹⁴C measurements spread over a range of preselected tree rings, with assigned calendar years by dendrochronological techniques, before and after the bomb spike in order to validate the annual nature of the tree rings. The tree‐ring width chronology was statistically reliable, and it correlated significantly with local records of annual and October–December (OND) streamflow and precipitation across the upper river watershed (positive), and OND temperature (negative). It was also significantly related to the Oceanic Niño Index, Pacific Decadal Oscillation, and the Southern Oscillation Index, as well as sea surface temperatures over the Caribbean and the Pacific region. However, ¹⁴C high‐precision measurements over the tree rings demonstrated offsets of up to 40 years that indicate that P. copaifera can produce more than one ring in certain years. Results derived from the strongest climate–growth relationship during the most recent years of the record suggest that the climatic signal reported may be due to the presence of annual rings in some of those trees in recent years. Our study alerts about the risk of applying dendrochronology in species with challenging anatomical features defining tree rings, commonly found in the tropics, without an independent validation of annual periodicity of tree rings. High‐precision ¹⁴C measurements in multiple trees are a useful method to validate the identification of annual tree rings.     

Assessing the utility of direct and indirect methods for estimating tropical tree age in the Western Ghats, India

The knowledge of tree age is important for understanding tree growth and forest dynamics. It may be estimated by ‘direct’ methods involving growth ring counts, or by ‘indirect’ methods involving field measurements of growth rates. Direct methods are considered more accurate, but it is not clear if they are appropriate for all species, notably from the humid tropics. In this paper we assess the occurrence of annual growth rings and their utility for age estimation in three tropical tree species, Acrocarpus fraxinifolius, Dalbergia latifolia (Fabaceae) and Syzygium cumini (Myrtaceae), growing in traditional shade coffee plantations of the southern Western Ghats, India. These species previously were described as having “indistinct or absent” growth rings. We used anatomical studies, field measurements and computational methods to characterise growth rings and assess similarities between directly and indirectly estimated tree ages. Our study revealed that annual growth rings were characterised by different sets of anatomical features per species and were most distinct in the fast-growing deciduous A. fraxinifolius. Growth rates measured in the field showed annual periodicity in all three species, and reflected annual rainfall-drought cycles in D. latifolia and S. cumini. Direct age estimates were most similar to indirect estimates in D. latifolia, and least so in S. cumini. The results of direct age estimation by counting rings are consistent with them being annual in nature in tropical species with distinct and reliable annual growth ring formation. However, for species with poorly defined growth rings, indirect age estimation methods might be more useful.     

Ancient juniper trees growing on cliffs: toward a long Mediterranean tree-ring chronology

Juniperus phoenicea is a tree that can grow on vertical cliff faces in dry and warm Mediterranean climate conditions. These trees are adapted to extreme growing conditions where the main constraints are verticality, compact hard limestone, and low water supply. They respond to these constraints via various specific features and high longevity. The objective of this study is to confirm whether or not their tree-rings are annual in order to specify growth strategies and accurately date these trees. Trunk morphology, anatomical wood anomalies and radial growth were analyzed on 53 trees in the Ardèche canyon. Crossdating of the ring widths using traditional dendrochronological techniques was unsuccessful, so radiocarbon dating of tree pith was used to assess tree age, and wiggle-match dating was used to test for differences between number of rings counted and radiocarbon dates. Radiocarbon dates span the period 2520–685 BP. Minimal difference between radiocarbon dates and ring counts was apparently small—missing rings occur, but not in large numbers. Tree-ring formation is annual and radial growth is low, which creates stunted old trees. Such old living trees are uncommon in the Mediterranean basin, especially at low elevation. They can provide long tree-ring chronologies back to 792–524 cal BC. Results from the radiocarbon dating indicate that accurate annual dating of these rings may be possible by crossdating. J. phoenicea growing on cliffs offer a valuable model to better understand cliff population ecology and the functional responses of trees that can live in harsh environmental conditions.     

Climatic signals in tree‐rings of Araucaria angustifolia in the southern Brazilian highlands

Araucaria angustifolia (Bertol.) O. Kuntze (Araucariaceae) is a Neotropical tree, widely distributed in subtropical mountain rain forests and nearby natural grasslands of Southern Brazil. This species produces annual growth rings, but its dendroclimatic potential is barely known. In the present paper, the long‐term growth patterns of A. angustifolia were investigated using annual growth ring time series and association to climate over the last century. Wood cores of A. angustifolia trees growing in forest and grassland habitats were obtained with an increment borer. The cores were surfaced, measured and cross‐dated. The dated ring‐width time series were standardized and submitted to correlation and principal component analysis to verify growth trends among sites and trees. Growth‐climate relationships were investigated using correlation and regression analyses, comparing the ordination axes scores to regional time series of precipitation and temperature. Due to anatomical irregularities, mainly partial rings, only 35 out of 60 trees were cross‐dated. The correlation and ordination analyses showed common tree‐growth trends within and between sites, indicative of a regional environmental force determining inter‐annual cambial activity variation. Despite growing in distinct habitats and disturbance regimes, A. angustifolia trees share a common long‐term growth pattern, which is significantly related to thermal conditions during the current and previous growing seasons. Moreover, site‐specific characteristics may have influenced opposite growth responses and association to climate conditions between forest and grassland trees.     

Effect of climate on the growth of annual rings in the main roots of perennial forbs in an Inner Mongolian semi‐arid grassland,China

Question: Is there a pattern in growth of annual rings in roots of perennial forbs in relation to climate and climate extremes in grassland ecosystems? Location: Semi‐arid grassland in Duolun (42°27′N, 116°41′E, 1380 m a.s.l.), central Inner Mongolia, China. Methods: Main roots of three perennial species, Potentilla anserina L., Cymbaria dahurica L. and Lespedeza daurica Schindl., were sampled. Cross‐sections (10–15‐μm thick) were produced from the proximal end of sampled roots using a sledge microtome. Annual growth rings in the main roots were identified and measured by differentiating between earlywood and latewood in the secondary xylem. Relationships between annual growth rings and monthly mean temperature and total monthly precipitation were identified using correlation analysis. Differences in an annual ring width to the previous and following years were examined by calculating a distinctness score. Results: The three perennial forbs showed clearly demarcated annual growth rings in all individuals and the same fluctuation patterns. Their ring widths were generally positively correlated with precipitation from April to October (except for August) and with temperature from February to June (except June for L. daurica), September to October, and the annual mean. Strong deviations of annual ring widths from their neighbour rings were observed in 1998 and 2000. The trend of absolute distinctness scores (D_m) increased significantly from 1988 to 2003, indicating an increase in the frequency of annual ring width variation. Conclusions: Annual growth rings in the main roots of three perennial forb species can be used as an indicator of the influence of climate on below‐ground grassland growth. The change in below‐ground conditions and effects on the functioning of grassland should receive more attention in future studies.     

Beyond CO2-fixation by Rubisco – an interpretation of 13C/12C variations in tree rings from novel intra-seasonal studies on broad-leaf trees

Evidence is presented for a very specific, seasonally recurring tri‐phase carbon isotope pattern in tree rings of broad‐leaf deciduous tree species. It is derived from highly resolved intra‐annual measurements of ¹³C/¹²C ratios of wood and cellulose from tree rings of Fagus sylvatica, Populus nigra, Quercus petraea and Morus alba. Investigations on δ¹³C from buds and leaves of Fagus sylvatica revealed a similar tri‐phase δ¹³C pattern. At the very beginning of a growing season, the δ¹³C trend of tree rings and foliage shows a marked increase of up to 5‰. The maximum δ¹³C‐value of each vegetation period always occurs in young heterotrophic leaves shortly after bud burst and persistently in the early wood of each tree ring, when growth depends on carbon reserves. Thereafter, δ¹³C profiles represent the autotrophic stage of the leaves, which show different patterns of variation, by and large characterized by a decline. The minimum δ¹³C‐value always shows up in the late wood of each tree ring. At the very end of each tree ring δ¹³C‐values start rising again. This increase in δ¹³C marks the gradual switch‐over to storage‐dependent growth and can also be observed in senescent leaves. Seasonal changes of more than 4‰ were measured, whereas contiguous δ¹³C values rarely differed from each other by more than 0.3‰. This tri‐phase pattern cannot be explained by the common model of carbon isotope fractionation during photosynthesis. It appears to be primarily an indication of seasonal changes in down‐stream processes of the carbohydrate metabolism. Environmental influences on the carbon isotope fractionation during photosynthesis are presumably of secondary importance and expressed by certain peculiarities showing up during the autotrophic phase, i.e. the mid‐section of the seasonal δ¹³C pattern.     

Mangrove growth rings: fact or fiction?

The analysis of tree rings in the tropics is less straightforward than in temperate areas with a demarcated unfavourable winter season. But especially in mangroves, the highly dynamic intertidal environment and the overriding ecological drivers therein have been a reason for questioning the existence of growth rings. This study aimed at casting light on growth rings in mangroves. In six mangrove species growing in Gazi Bay, Kenya (Sonneratia alba, Heritiera littoralis, Ceriops tagal, Bruguiera gymnorrhiza, Xylocarpus granatum and Lumnitzera racemosa), the occurrence of growth rings was examined. Growth rate of each species was determined based on a 1-year period using the cambial marking technique. The effect of climate was furthermore considered by comparing the results with a number of wood samples originating from contrasting climatic regions. We can conclude that for growth rings to appear in mangroves more than one condition has to be fulfilled, making general statements impossible and explaining the prevalent uncertainty. Climatic conditions that result in a range of soil water salinity experienced over the year are a prerequisite for the formation of growth rings. For species with an anatomy characterized by indistinct ring boundaries, this should be combined with a growth rate of at least 0.3 mm/year. The use of growth rings for age or growth rate determinations should thus be evaluated on a case by case basis.     

Species-Specific Growth Responses to Climate Variations in Understory Trees of a Central African Rain Forest

Basic knowledge of the relationships between tree growth and environmental variables is crucial for understanding forest dynamics and predicting vegetation responses to climate variations. Trees growing in tropical areas with a clear seasonality in rainfall often form annual growth rings. In the understory, however, tree growth is supposed to be mainly affected by interference for access to light and other resources. In the semi-deciduous Mayombe forest of the Democratic Republic of Congo, the evergreen species Aidia ochroleuca, Corynanthe paniculata and Xylopia wilwerthii dominate the understory. We studied their wood to determine whether they form annual growth rings in response to changing climate conditions. Distinct growth rings were proved to be annual and triggered by a common external factor for the three species. Species-specific site chronologies were thus constructed from the cross-dated individual growth-ring series. Correlation analysis with climatic variables revealed that annual radial stem growth is positively related to precipitation during the rainy season but at different months. The growth was found to associate with precipitation during the early rainy season for Aidia but at the end of the rainy season for Corynanthe and Xylopia. Our results suggest that a dendrochronological approach allows the understanding of climate–growth relationships in tropical forests, not only for canopy trees but also for evergreen understory species and thus arguably for the whole tree community. Global climate change influences climatic seasonality in tropical forest areas, which is likely to result in differential responses across species with a possible effect on forest composition over time.     

Arabia's tallest trees: ecology,distribution and conservation status of the regionally endangered tree species Mimusops laurifolia

Mimusops laurifolia (Forssk.) Friis is the largest tree species in the Arabian Peninsula, and a characteristic species of the threatened southwest Arabian valley forest habitat. In order to understand the distribution and conservation status of M. laurifolia, this paper collates the historical records of M. laurifolia and compares them with recent survey data. This comparison highlights that the abundance of M. laurifolia has dramatically declined at sites that were previously considered important for this species. In Arabia, approximately 100 individuals of M. laurifolia remain at eleven localities. From their size, it is likely that many of these are extremely old trees, but there is currently little regeneration. The main threats to this species are the loss of valley forest habitat through overgrazing and road construction, overexploitation of M. laurifolia for wood as well as climate change. Based on these findings, we recommend that M. laurifolia should be classified as Endangered (EN) in the region on the IUCN red list. We recommend some basic conservation measures and identifies an urgent need for further research on the distribution, propagation and restoration of this species.     

On the reliability of the growth rings in two species of the family Characidae in the Sudan

The scales, opercula and vertebrae from 69 Hydrocyon forskali and 63 Alestes dentex were investigated for their reliability as aging structures. In both species the scales were unreliable due to their regenerative property, in spite of the clarity of their growth rings.Back-calculated lengths from the first growth ring laid on opercula and vertebrae resulted in values that coincided with the annual growth of the two species. In H. forskali, the mean back-calculated length from the two bones were 16.2 ± 5.9 and 16.8 ± 4.4 respectively, and for A. dentex, 11.1 ± 1.7 and 12.8 ± 1.8, respectively.Predictive equations for bone radius from fish length were written for both species. In this study, reliability was maximal between the observed and back-calculated lengths for both species (r > 0.95).     

An assessment of growth ring identification in subtropical forests from northwestern Argentina

Most subtropical forests in South America are located in regions with a marked seasonality in precipitation, which may induce the formation of annual bands in woody species. Due to the lack of precise information on tree-ring visibility, we evaluated the wood characteristics of 37 tree species in the subtropical Yungas and Chaco forests from northwestern (NW) Argentina. Anatomical features associated with the delimitation of growth bands were examined to establish the presence of tree rings. Different forest types reflect the precipitation gradients and wood anatomical features vary accordingly. Characteristics of wood structure are closely related to the dominant climatic patterns of each forest, revealing a common pattern of anatomical arrangements in terms of water transport and safety. In the Chaco and transitional forests, ring boundaries are related to marginal parenchyma whereas in montane forests growth ring boundary is mostly associated with the presence of thicker fibers at the end of the ring. The largest proportion of species with clearly marked growth rings occurs in the montane forest type of NW Argentina. Clear growth rings is a requisite for dendrochronological applications, hence the present work represents the first regional attempt to address the potential of subtropical species in South America to be used in dendrochronological studies.     

Seasonal cambium activity in the subtropical rain forest tree Araucaria angustifolia

Information on the timing and dynamics of tree ring formation is essential to assess the seasonal behavior of secondary wood growth and its associated environmental influences. Araucaria angustifolia is a dominant species in highland pluvial ecosystems of southeastern South America. Previous investigations indicated that their growth rings are formed annually, but no information exists about the timing of growth ring formation and the environmental triggers influencing cambium activity. In this paper we examine inter- and intra-annual cambial activity in A. angustifolia, through anatomical and dendrochronological evidence at two study sites, and model the relationships between regional climate variation and intra-annual tree ring formation. The results confirm the annual growth ring formation in A. angustifolia and indicate that its growth season extends from October to April. Day length and temperature were the main environmental factors influencing the seasonal cambium activity. Our results evidence the dendrochronological potential of A. angustifolia for ecological and climatological studies in southeastern South America.     

SPACING OF PARENCHYMA BANDS IN WOOD OF CARYA GLABRA (MILL.) SWEET,PIGNUT HICKORY,AS AN INDICATOR OF GROWTH RATE AND CLIMATIC FACTORS

Variations in the spacing of concentric, tangential bands of apotracheal axial wood parenchyma cells in the annual growth rings of Carya glabra (Mill.) Sweet constitute a record of changes in radial growth rate and of fluctuations in climatic factors. This relationship was determined from trees that were growing under moisture stress and consequently had highly consistent ring-width patterns from tree to tree. Band spacing decreased across a growth ring and was generally greater in wide than in narrow rings. Thus, the proportion of xylem cross-sectional area occupied by axial parenchyma tissue was inversely related to ring width. Average numbers of bands in each of 67 years were analyzed relative to ring widths. Years in which bands were unusually numerous relative to ring width were years of low April-May precipitation, leading to slow spring growth and closely spaced bands early in the growing season. Years in which band numbers were unusually low relative to ring width were years of high April-May precipitation, leading to rapid growth and widely spaced bands early in the season; low July precipitation in these years apparently curtailed growth before much of the typical late-season wood, with its closely spaced bands, could form. Parenchyma bands were aggregated within some rings, suggesting that a slowing of growth had been followed by a growth spurt. Evidence for a late-season spurt was that rings with aggregations were significantly wider than rings formed the same year and not having aggregations of bands. The number of vessels per unit of cross-sectional area was inversely related to ring width.     

Forest tree growth response to hydroclimate variability in the southern Appalachians

Climate change will affect tree species growth and distribution; however, under the same climatic conditions species may differ in their response according to site conditions. We evaluated the climate‐driven patterns of growth for six dominant deciduous tree species in the southern Appalachians. We categorized species into two functional groups based on their stomatal regulation and xylem architecture: isohydric, diffuse porous and anisohydric, ring porous. We hypothesized that within the same climatic regime: (i) species‐specific differences in growth will be conditional on topographically mediated soil moisture availability; (ii) in extreme drought years, functional groups will have markedly different growth responses; and (iii) multiple hydroclimate variables will have direct and indirect effects on growth for each functional group. We used standardized tree‐ring chronologies to examine growth of diffuse‐porous (Acer, Liriodendron, and Betula) and ring‐porous (Quercus) species vs. on‐site climatic data from 1935 to 2003. Quercus species growing on upslope sites had higher basal area increment (BAI) than Quercus species growing on mesic, cove sites; whereas, Acer and Liriodendron had lower BAI on upslope compared to cove sites. Diffuse‐porous species were more sensitive to climate than ring porous, especially during extreme drought years. Across functional groups, radial growth was more sensitive to precipitation distribution, such as small storms and dry spell length (DSL), rather than the total amount of precipitation. Based on structural equation modeling, diffuse‐porous species on upslope sites were the most sensitive to multiple hydroclimate variables (r² = 0.46), while ring‐porous species on upslope sites were the least sensitive (r² = 0.32). Spring precipitation, vapor pressure deficit, and summer storms had direct effects on summer AET/P, and summer AET/P, growing season small storms and DSL partially explained growth. Decreasing numbers of small storms and extending the days between rainfall events will result in significant growth reduction, even in regions with relatively high total annual rainfall.     

Dendrochronological analyses and climatic signals of Alchornea triplinervia in subtropical forest of southern Brazil

The predominance of secondary forest‐species in Brazilian subtropical forests highlights the importance of understanding the ecology of these taxa, and dendrochronology provides valuable information about the growth and climate response of tree species. The wide distribution of Alchornea triplinervia (Spreng.) Mull. Arg. (Euphorbiaceae), and the presence of growth rings in its wood, leds to its selection for this study. Samples were collected from 34 trees growing in rainy dense forest fragments in the cities of Blumenau and Brusque, in southern Brazil, and subjected to standard dendrochronological techniques. The local chronology spanned from 1889 to 2013 and the age of the trees ranged from 32 to 125 years. The mean age was 73 years and annual increment was 1.13 mm year^?1; the diameter and age showed a weak correlation. The results suggest that A. triplinervia growth responded negatively to hot summers and increased rainfall in the previous spring and winter. The high perdiodicity trend characteristic of ENSO activity was reflected in wavelet power spectra of A. triplinervia chronology, affecting positively its growth after strong ENSO events (1970–2009). Negative influences occurred before 1970 during normal ENSO events. Also, there is evidence of the existence of a connection between Pacific and Atlantic sea surface temperatures influencing on tree growth.     

Detecting invisible growth rings of trees in seasonally dry forests in Thailand: isotopic and wood anatomical approaches

We measured radial variation of carbon isotope composition and vessel traits in tree species in seasonally dry forests of Northeast Thailand to explore a more reliable and amenable method of tropical dendrochronology for trees that lack visually detectable and consistent growth rings. Six Dipterocarpaceae species (3 Shorea, 2 Dipterocarpus, and 1 Hopea species) with indistinct or irregular growth rings and teak (Tectona grandis), a species which forms distinct growth rings, were examined. The δ¹³C value variations in all species showed annual cyclicity. Dipterocarpaceae species usually marked the lowest values of δ¹³C in the middle of the growing season, whereas teak had the lowest values at nearly the end of the growing season. Since the growing season of the species examined almost corresponds to the rainy season in the study area, the δ¹³C variation was likely caused by the change in moisture availability. The different variation pattern of teak was attributable to its stronger dependence on ¹³C-enriched reserved material early in the growing season. Changes in tree vessel traits for all species examined also showed annual cyclicity. Dipterocarpaceae species showed significant correlation between δ¹³C values and vessel measurements. Vessel lumen (mean area, tangential and radial diameter, and proportion of total area) had a negative correlation, whereas vessel frequency showed a positive correlation. The correlations indicated that changes in vessel traits were caused by the seasonal variation of moisture available to the trees. Thus, we concluded that methods using wood anatomy, as well as δ¹³C, have great potential for use as tools in tropical dendrochronology within the context of seasonal climate.     

RELATIONSHIP AMONG VESSEL DIAMETER,VESSEL FREQUENCY,AND SPACING OF PARENCHYMA BANDS IN WOOD OF CARYA TOMENTOSA NUTT., MOCKERNUT HICKORY

Vessel-lumen diameter, vessel frequency, and spacing of concentric, tangential bands of parenchyma cells follow well defined trends across annual growth rings of Carya tomentosa Nutt., mockernut hickory. These cross-ring cellular changes were correlated with each other to a considerable degree but not tightly linked. Vessel-lumen diameter decreased in a strongly linear fashion, as a function of distance from the inner ring boundary. Distance between tangential, concentric bands of axial wood parenchyma cells also decreased across rings, but followed a less strongly linear trend. Vessel frequency decreased abruptly after the large-pore zone and then increased to the outer ring boundary. Radial growth rate, as indicated by ring width, strongly influenced these trends. For all three cellular characteristics, the cross-ring trend was abrupt in narrow rings and more gradual in wide rings. An anomalous ring of Carya tomentosa, and three similar rings of Carya glabra (Mill.) Sweet, showed a decrease in interband distance and then an abrupt increase followed by another decrease to the outer ring boundary, whereas vessel diameter remained small and uniform after an initial decrease. The three cellular characteristics were concluded to be under separate physiological controls, although growth rate seems to exert a considerable, parallel influence on them.