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PubMed2005

Effect of canopy position on germination and seedling survival of epiphytic bromeliads in a Mexican humid montane forest.

BACKGROUND AND AIMS: Seeds of epiphytes must land on branches with suitable substrates and microclimates to germinate and for the resulting seedlings to survive. It is important to understand the fate of seeds and seedlings in order to model populations, but this is often neglected when only established plants are included in analyses. METHODS: The seeds of five bromeliad species were exposed to different canopy positions in a Mexican montane forest, and germination and early seedling survival were recorded. Additionally, the survival of naturally dispersed seedlings was monitored in a census over 2.5 years. Survival analysis, a procedure rarely used in plant ecology, was used to study the influence of branch characteristics and light on germination and seedling survival in natural and experimental populations. KEY RESULTS: Experimental germination percentages ranged from 7.2 % in Tillandsia deppeana to 33.7 % in T. juncea, but the seeds of T. multicaulis largely failed to germinate. Twenty months after exposure between 3.5 and 9.4 % of the seedlings were still alive. There was no evidence that canopy position affected the probability of germination, but time to germination was shorter in less exposed canopy positions indicating that higher humidity accelerates germination. More experimental seedlings survived when canopy openness was high, whereas survival in census-seedlings was influenced by moss cover. While mortality decreased steadily with age in juveniles of the atmospheric Tillandsia, in the more mesomorphic Catopsis sessiliflora mortality increased dramatically in the dry season. CONCLUSIONS: Seedling mortality, rather than the failure to germinate, accounts for the differential distribution of epiphytes within the canopy studied. With few safe sites to germinate and high seedling mortality, changes of local climate may affect epiphyte populations primarily through their seedling stage.

Environment↗
PubMed1997

Microhabitat-independent regional differences in survival of unfed Ixodes scapularis nymphs (Acari:Ixodidae) in Connecticut.

The effects of habitat and microclimate on survival of unfed nymphal black-legged ticks, Ixodes scapularis Say (approximately I. damnini Spielman, Clifford, Piesman & Corwin), were studied under natural conditions in southcentral and northwestern Connecticut. At both coastal and inland locations, survival of 3 groups of 20 wild-caught questing nymphs placed in nylon mesh bags was monitored in each of 3 different habitats (field, forest canopy, and forest/field edge) during summer 1995. Simultaneously, soil temperature, ground-level air temperature, and relative humidity were measured continuously within each habitat at both sites. The number of ticks surviving in each habitat was monitored weekly. Average daily survival rates of nymphs were related inversely to soil temperature but were not related to air temperature or humidity. Overall, nymphal ticks at the inland site survived significantly longer than those at the coastal site; however, no significant differences in mortality rates were found among habitats. These results suggest that inland environmental conditions are suitable for lengthy survival of unfed nymphal I. scapularis in regions where this tick is not yet abundant.

Animals↗
PubMed1998

Air temperature and relative humidity effects on behavioral activity of blacklegged tick (Acari: Ixodidae) nymphs in New Jersey.

Air-temperature and relative humidity data were used to explain variation in behavioral activity of Ixodes scapularis Say nymphs. We estimated behavioral activity as the residual variation in drag-sample data after seasonal changes in population density were removed by regression. The seasonal decline in drag samples between June and August 1995 on field plots at Morristown National Historical Park, NJ, can be described by a simple negative exponential function. Residuals around a fitted exponential were significantly correlated with temperature and with relative humidity measured at the leaf-litter surface, and explained 34 and 44% of the variance, respectively. Multiple regression on temperature and relative humidity explained 51% of the variance. These regressions estimated the explanatory power of microclimate, independent of seasonal correlations, and might provide a basis for day-to-day prediction of human exposure to Lyme disease.

Animals↗
PubMed1998

Dermacentor hunteri (Acari: Ixodidae): seasonal variation in questing adults and on-host juvenile stages, and host associations and feeding behavior of larvae and nymphs.

Dermacentor hunteri Bishopp is the only completely desert adapted tick in the Nearctic realm, and chiefly parasitizes desert bighorn sheep (Ovis canadensis Shaw) as an adult. The remainder of its life history has been unknown. We conducted field investigations in the Sonoran desert of the temporal and spatial variation of adult host-seeking ticks and of the host associations of juvenile ticks. Additionally, the feeding success of juvenile ticks was assessed in the laboratory. Adult ticks were found in significant numbers only in plateau and rocky slopes habitats, chiefly during the period from January to June. Questing adults were not found in July and August, and they were present in small numbers from September through December. Juvenile stages were found only on desert woodrats, Neotoma lepida Thomas (larvae and nymphs), and cactus mice, Peromyscus eremicus Baird (larvae only), in March, May, and early June. In the laboratory; both larvae and nymphs fed on N. lepida, but only larvae fed on P. maniculatus bairdii (Wagner). We concluded that the life history of D. hunteri may be constrained by the co-distribution of desert bighorn, desert woodrats, and perhaps cactus mice; and that adults oversummer either on desert bighorn or sequestered in favorable microclimates off the host.

Animals↗
PubMed1999

Effects of temperature and humidity on oviposition, molting, and longevity of Dermanyssus gallinae (Acari: Dermanyssidae).

The juvenile development and survival of Dermanyssus gallinae (De Geer) kept in vitro at different temperatures and humidity were investigated to obtain biological baseline data for a Swedish population. Individual females, eggs, larvae, and protonymphs were observed with regard to egg production, duration of various stages, and longevity when kept at different temperatures and relative humidities. Female mites laid eggs at temperatures between 5 and 45 degrees C with the highest numbers laid at 20 degrees C and 70% RH, but development to larvae and protonymphs was only observed at temperatures ranging from 20 to 25 degrees C. The average duration of oviposition varied from 1.0 to 3.2 d within the temperature range 20-45 degrees C but was gradually increased to 28 d at 5 degrees C. Specimens survived for up to 9 mo without access to food when kept in the temperature range of 5-25 degrees C. Temperatures > 45 degrees C and at -20 degrees C were found to be lethal. Longevity was similar for females and protonymphs kept at 30 and 45% RH, but it was enhanced at 70 and 90% RH for protonymphs. This study showed that D. gallinae can survive for a long time without feeding if the microclimate is suitable, but it does not thrive at low relative humidities and at temperature extremes. This indicates that changing of the abiotic conditions in infested poultry houses could be a possible measure to reduce mite populations.

Animals↗
PubMed2002

Effect of local irradiance on CO(2) transfer conductance of mesophyll in walnut.

The acclimation responses of walnut leaf photosynthesis to the irradiance microclimate were investigated by characterizing the photosynthetic properties of the leaves sampled on young trees (Juglans nigraxregia) grown in simulated sun and shade environments, and within a mature walnut tree crown (Juglans regia) in the field. In the young trees, the CO(2) compensation point in the absence of mitochondrial respiration (Gamma*), which probes the CO(2) versus O(2) specificity of Rubisco, was not significantly different in sun and shade leaves. The maximal net assimilation rates and stomatal and mesophyll conductances to CO(2) transfer were markedly lower in shade than in sun leaves. Dark respiration rates were also lower in shade leaves. However, the percentage inhibition of respiration by light during photosynthesis was similar in both sun and shade leaves. The extent of the changes in photosynthetic capacity and mesophyll conductance between sun and shade leaves under simulated conditions was similar to that observed between sun and shade leaves collected within the mature tree crown. Moreover, mesophyll conductance was strongly correlated with maximal net assimilation and the relationships were not significantly different between the two experiments, despite marked differences in leaf anatomy. These results suggest that photosynthetic capacity is a valuable parameter for modelling within-canopies variations of mesophyll conductance due to leaf acclimation to light.

Carbon Dioxide↗
PubMed2004

Annual and seasonal variation of sap flow and conductance of pine trees grown in elevated carbon dioxide and temperature.

Measurements of sap flow, crown structure, and microclimate were used to estimate the transpiration of individual 30-year-old Pinus sylvestris L. trees grown in elevated temperature and CO2. The trees were enclosed in closed-top chambers and exposed either to current ambient conditions (CON), or elevated CO2 (+350 micromol mol(-1); EC), or elevated temperature (+2 to +6 degrees C; ET) or a combination of EC and ET (ECT) since 1996, and the measurements were made from 1999 to 2001. EC significantly increased annual sap flow per tree (Ft.m) by 14% in 1999, but reduced it by 13% in 2000 and 16% in 2001. The CO2-induced increase in Ft.m in 1999 was due to a large increase in foliage area of trees, which more than compensated for a small decrease in crown conductance (Gc). The CO2-induced decreases in Ft.m in 2000 and 2001 resulted from a pronounced decline in Gc, which was much greater than the increase in foliage area. The CO2-induced increase in sensitivity of Gc at high vapour pressure deficit (VPD) did not alter the general response of sap flow to CO2 enrichment, but it did affect the diurnal courses of sap flow on some days during the main growing season (days 150-240). ET increased Ft.m by 53%, 45%, and 57% in 1999, 2000, and 2001, respectively, attributable to the combined effects of greater foliage area and maximum crown conductance, lower stomatal sensitivity to high VPD, and higher transpiration demand relative to the control treatments. There was no significant interaction between CO2 and temperature on sap flow, because ECT entailed approximately similar patterns of sap flow to ET, suggesting that the temperature played a dominate role in the case of ECT under boreal climate conditions.

Carbon Dioxide↗
PubMed2006

Intra-annual radial growth and water relations of trees: implications towards a growth mechanism.

There is a missing link between tree physiological and wood-anatomical knowledge which makes it impossible mechanistically to explain and predict the radial growth of individual trees from climate data. Empirical data of microclimatic factors, intra-annual growth rates, and tree-specific ratios between actual and potential transpiration (T PET(-1)) of trees of three species (Quercus pubescens, Pinus sylvestris, and Picea abies) at two dry sites in the central Wallis, Switzerland, were recorded from 2002 to 2004 at a 10 min resolution. This included the exceptionally hot and dry summer of 2003. These data were analysed in terms of direct (current conditions) and indirect impacts (predispositions of the past year) on growth. Rain was found to be the only factor which, to a large extent, consistently explained the radial increment for all three tree species at both sites and in the short term as well. Other factors had some explanatory power on the seasonal time-scale only. Quercus pubescens built up much of its tree ring before bud break. Pinus sylvestris and Picea abies started radial growth 1-2 weeks after Quercus pubescens and this was despite the fact that they had a high T PET(-1) before budburst and radial growth started. A high T PET(-1) was assumed to be related to open stomata, a very high net CO2 assimilation rate, and thus a potential carbon (C)-income for the tree. The main period of radial growth covered about 30-70% of the productive days of a year. In terms of C-allocation, these results mean that Quercus pubescens depended entirely on internal C-stores in the early phase of radial growth and that for all three species there was a long time period of C-assimilation which was not used for radial growth in above-ground wood. The results further suggest a strong dependence of radial growth on the current tree water relations and only secondarily on the C-balance. A concept is discussed which links radial growth over a feedback loop to actual tree water-relations and long-term affected C-storage to microclimate.

Carbon↗
PubMed2003

Occupational and patient doses in the therapeutic cave, Tapolca (Hungary).

The radon concentration has been measured for three years in a hospital cave used for medical treatment of respiratory diseases. A mean value of the actual equilibrium factor measured in the cave in different seasons was used, different from the commonly used 0.4. The dose contribution to the patients and the staff was calculated using these data. The results of the dose assessment show that the staff in the hospital cave can receive doses up to the dose limit for occupational exposure (20 mSv y(-1)) when working 4 h per day in the cave. Patients receive 0.18-4.22 mSv committed effective dose during the treatment period depending on the exposure periods. The only solution to reduce the dose to the staff seems to be decreasing the time they spend underground, because intensive ventilation would disturb the special microclimate of the cave.

Air Pollutants, Radioactive↗
PubMed1999

Comparison of egg hatchability and in vitro survival of goose embryos of various origins.

Hatchability of 69,324 fertilized eggs of White Italian geese from the WD1 maternal strain, selected for egg production, and of 25,226 eggs from the WD3 paternal strain, selected for weight gain, were compared. A significant difference was noted, but its magnitude varied during the reproduction season. Mean results for the WD1 strain (80.9%) were always higher (P < 0.01) than those of WD3 strain (75.8%). To determine the causes of this difference, goose eggs from WD1, WD3, and the Kuban (K) strain that demonstrate high reproductive performance (egg number, fertility, and hatchability) were incubated in a laboratory scale incubator. Embryonic mortality and egg weight loss during incubation were determined. Best hatchability (78.5%) was noted in eggs from K goose strain and the weight loss attained 10.9% of the initial egg weight, until Day 25 of incubation. With the higher egg weight loss in Strains WD1 (11.8%) and WD3 (13.2%), hatchability was diminished to 67.3 and 65.5%, respectively. Better hatchability resulted from lower mortality up to Day 6 of incubation and, especially, between Days 7 and 25 of incubation. The differences found in the egg weight losses and in embryonic mortality suggest a need for adjusting the incubation technique to the particular goose genotype. Embryo cultivation in vitro in plastic containers made the comparison of embryo survival possible, because the interaction of incubator microclimate by eggshell quality was eliminated. The results noted due to strain were ranked in the following order: K, WD1, and WD3. The experimental findings suggest that, apart from that of shell quality effect on various egg weight losses during incubation, there appear to be other factors that cause differences in hatchability of the goose strains studied.

Animals↗
PubMed1997

Water use by Eucalyptus tereticornis stands of differing density in southern India.

We studied water use by Eucalyptus tereticornis Sm. in two plantations, differing in tree density (1800 stems ha(-1) at Site I and 1090 stems ha(-1) at Site II), in different years. At both sites, stomatal conductance, predawn and midday water potentials and microclimate were measured and used to estimate hourly transpiration by the Penman-Monteith equation. Growth in girth was also measured. Stomatal conductance was closely correlated with atmospheric vapor pressure deficit (D); however, stomata did not close completely even at high D ( approximately 5.0 kPa). Midday leaf water potentials did not fall below -2.0 MPa during any part of the year at either site. Predawn leaf water potentials were greater than -0.25 MPa during the postmonsoon period, but declined to -0.7 MPa at Site I during the premonsoon period. Transpiration estimates ranged from 0.6 to 1.2 mm h(-1) at Site I and from 0.2 to 0.6 mm h(-1) at Site II. The extrapolated transpiration values for the rain-free days of the year were 1563 mm and 853 mm for Sites I and II, respectively. Growth in girth was negligible during the premonsoon period. Photosynthesis was not affected by the minor water stress that developed during the premonsoon period.

Journal Article↗
PubMed1999

Thinning, fertilization, and crown position interact to control physiological responses of loblolly pine.

To examine physiological responses to thinning, fertilization, and crown position, we measured net photosynthesis (P(n)), transpiration (E), vapor pressure difference (VPD), stomatal conductance (g(s)), and xylem pressure potential (Psi(1)) between 0930 and 1130 h under ambient conditions in the upper and lower crowns of a 13-year-old loblolly pine (Pinus taeda L.) plantation six years (1994) after the treatments were applied. Photosynthetic photon flux density (PPFD) and air temperature (T(a)) within the canopy were also recorded. Needle P(n) of thinned trees was significantly enhanced by 22-54% in the lower crown, because canopy PPFD increased by 28-52%. Lower crown foliage of thinned plots also had higher E and g(s) than foliage of unthinned plots, but thinning had no effect on needle Psi(1) and predawn xylem pressure potential (0430-0530 h; Psi(pd)). Tree water status did not limit P(n), E and g(s) during the late-morning measurements. Fertilization significantly decreased within-canopy PPFD and T(a). Needle Psi(1) was increased in fertilized stands, whereas P(n), E and g(s) were not significantly altered. Upper crown foliage had significantly greater PPFD, P(n), VPD, g(s), E, and more negative Psi(1) than lower crown foliage. In both crown positions, needle P(n) was closely related to g(s), PPFD and T(a) (R(2) = 0.77 for the upper crown and 0.82 for the lower crown). We conclude that (1) silvicultural manipulation causes microclimate changes within the crowns of large trees, and (2) needle physiology adjusts to the within-crown environmental conditions.

Journal Article↗
PubMed1999

Fine root respiration in mature eastern white pine (Pinus strobus) in situ: the importance of CO(2) in controlled environments.

We measured seasonal fine root respiration rate in situ while controlling chamber temperature and [CO(2)]. Atmospheric [CO(2)] ([CO(2)](a)) and measured soil [CO(2)] ([CO(2)](s)) were alternately delivered to a cuvette containing intact fine roots of eastern white pine (Pinus strobus L.). Respiration rates were consistently higher in [CO(2)](a) than in [CO(2)](s) and were almost three times higher during midsummer. Respiration rates were immediately reversed after returning to the alternate [CO(2)] (i.e., [CO(2)](a) --> [CO(2)](s) --> [CO(2)](a), and vice versa) suggesting a direct effect of elevated [CO(2)] on apparent respiration. Soil-[CO(2)]-based respiration rates decreased with increasing [CO(2)] on a dry mass and tissue [N] basis. We conclude that estimates of soil CO(2) flux and soil carbon budgets may be improved by more completely accounting for the rhizosphere microclimate (i.e., soil temperature and [CO(2)](s)) during measurement of fine root respiration.

Journal Article↗
PubMed2000

Transpiration and forest structure in relation to soil waterlogging in a Hawaiian montane cloud forest.

Transpiration, leaf characteristics and forest structure in Metrosideros polymorpha Gaud. stands growing in East Maui, Hawaii were investigated to assess physiological limitations associated with flooding as a mechanism of reduced canopy leaf area in waterlogged sites. Whole-tree sap flow, stomatal conductance, microclimate, soil oxidation-reduction potential, stand basal area and leaf area index (LAI) were measured on moderately sloped, drained sites with closed canopies (90%) and on level, waterlogged sites with open canopies (50-60%). The LAI was measured with a new technique based on enlarged photographs of individual tree crowns and allometric relationships. Sap flow was scaled to the stand level by multiplying basal area-normalized sap flow by stand basal area. Level sites had lower soil redox potentials, lower mean stand basal area, lower LAI, and a higher degree of soil avoidance by roots than sloped sites. Foliar nutrients and leaf mass per area (LMA) in M. polymorpha were similar between level and sloped sites. Stomatal conductance was similar for M. polymorpha saplings on both sites, but decreased with increasing tree height (r(2) = 0.72; P < 0.001). Stand transpiration estimates ranged from 79 to 89% of potential evapotranspiration (PET) for sloped sites and from 28 to 51% of PET for level sites. Stand transpiration estimates were strongly correlated with LAI (r(2) = 0.96; P < 0.001). Whole-tree transpiration was lower at level sites with waterlogged soils, but was similar or higher for trees on level sites when normalized by leaf area. Trees on level sites had a smaller leaf area per stem diameter than trees on sloped sites, suggesting that soil oxygen deficiency may reduce leaf area. However, transpiration per unit leaf area did not vary substantially, so leaf-level physiological behavior was conserved, regardless of differences in tree leaf area.

Journal Article↗
PubMed2001

Ecosystem respiration in a young ponderosa pine plantation in the Sierra Nevada Mountains, California.

We estimated total ecosystem respiration from a ponderosa pine (Pinus ponderosa Dougl. ex Laws.) plantation in the Sierra Nevada Mountains near Georgetown, California, from June to October, 1998. We apportioned ecosystem respiration among heterotrophic, root, stem and foliage based on relationships for each component that considered microclimate and vegetation characteristics. We measured each respiration component at selected sampling points, and scaled the measurements up to the ecosystem based on modeled relationships. Over the study period, total mean ecosystem respiration was 5.7 +/- 1.3 mumol m-2 s-1 (based on daily mean), comprising about 67% from soil-surface CO2 efflux, 10% from stem and branch respiration and 23% from foliage respiration. Shrub leaves contributed about 24% to total foliage respiration, and current-year needles (1998 age class) accounted for 40% of total tree needle respiration. Root respiration accounted for 47% of soil-surface CO2 efflux. We conclude that ecosystem respiration can be estimated based on daily mean air and soil temperatures through exponential relationships with r2 values of 0.85 and 0.87, respectively. When based on both air and soil temperatures, about 91% of the variation in total ecosystem respiration could be explained by a linear regression.

Biomass↗
PubMed2002

Age-related effects on leaf area/sapwood area relationships, canopy transpiration and carbon gain of Norway spruce stands (Picea abies) in the Fichtelgebirge, Germany.

Stand age is an important structural determinant of canopy transpiration (E(c)) and carbon gain. Another more functional parameter of forest structure is the leaf area/sapwood area relationship, A(L)/A(S), which changes with site conditions and has been used to estimate leaf area index of forest canopies. The interpretation of age-related changes in A(L)/A(S) and the question of how A(L)/A(S) is related to forest functions are of current interest because they may help to explain forest canopy fluxes and growth. We conducted studies in mature stands of Picea abies (L.) Karst. varying in age from 40 to 140 years, in tree density from 1680 to 320 trees ha(-1), and in tree height from 15 to 30 m. Structural parameters were measured by biomass harvests of individual trees and stand biometry. We estimated E(c) from scaled-up xylem sap flux of trees, and canopy-level fluxes were predicted by a three-dimensional microclimate and gas exchange model (STANDFLUX). In contrast to pine species, A(L)/A(S) of P. abies increased with stand age from 0.26 to 0.48 m(2) cm(-2). Agreement between E(c) derived from scaled-up sap flux and modeled canopy transpiration was obtained with the same parameterization of needle physiology independent of stand age. Reduced light interception per leaf area and, as a consequence, reductions in net canopy photosynthesis (A(c)), canopy conductance (g(c)) and E(c) were predicted by the model in the older stands. Seasonal water-use efficiency (WUE = A(c)/E(c)), derived from scaled-up sap flux and stem growth as well as from model simulation, declined with increasing A(L)/A(S) and stand age. Based on the different behavior of age-related A(L)/A(S) in Norway spruce stands compared with other tree species, we conclude that WUE rather than A(L)/A(S) could represent a common age-related property of all species. We also conclude that, in addition to hydraulic limitations reducing carbon gain in old stands, a functional change in A(L)/A(S) that is related to reduced light interception per leaf area provides another potential explanation for reduced carbon gain in old stands of P. abies, even when hydraulic constraints increase in response to changes in canopy architecture and aging.

Germany↗
PubMed2003

Midday depression of net photosynthesis in the tropical rainforest tree Eperua grandiflora: contributions of stomatal and internal conductances, respiration and Rubisco functioning.

High midday temperatures can depress net photosynthesis. We investigated possible mechanisms underlying this phenomenon in leaves of Eperua grandiflora (Aubl.) Benth. saplings. This tropical tree establishes in small gaps in the rainforest canopy where direct sunlight can raise midday temperatures markedly. We simulated this microclimate in a growth chamber by varying air temperature between 28 and 38 degrees C at constant vapor pressure. A decrease in stomatal conductance in response to an increase in leaf-to-air vapor pressure difference (deltaW) caused by an increase in leaf temperature (Tleaf) was the principal reason for the decrease in net photosynthesis between 28 and 33 degrees C. Net photosynthesis decreased further between 33 and 38 degrees C. Direct effects on mesophyll functioning and indirect effects through deltaW were of similar magnitude in this temperature range. Mitochondrial respiration during photosynthesis was insensitive to Tleaf over the investigated temperature range; it thus did not contribute to midday depression of net photosynthesis. Internal conductance for CO2 diffusion in the leaf, estimated by combined gas exchange and chlorophyll fluorescence measurements, decreased slightly with increasing Tleaf. However, the decrease in photosynthetic rate with increasing Tleaf was larger and thus the difference in CO2 partial pressure between the substomatal cavity and chloroplast was smaller, leading to the conclusion that this factor was not causally involved in midday depression. Carboxylation capacity inferred from the CO2 response of photosynthesis increased between 28 and 33 degrees C, but remained unchanged between 33 and 38 degrees C. Increased oxygenation of ribulose-1,5-bisphosphate relative to its carboxylation and the concomitant increase in photorespiration with increasing Tleaf were thus not compensated by an increase in carboxylation capacity over the higher temperature range. This was the principal reason for the negative effect of high midday temperatures on mesophyll functioning.

Carbon Dioxide↗
PubMed2006

Temperature regulation of bud-burst phenology within and among years in a young Douglas-fir (Pseudotsuga menziesii) plantation in western Washington, USA.

Past research has established that terminal buds of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings from many seed sources have a chilling requirement of about 1200 h at 0-5 degrees C; once chilled, temperatures > 5 degrees C force bud burst via accumulation of heat units. We tested this sequential bud-burst model in the field to determine whether terminal buds of trees in cooler microsites, which receive less heat forcing, develop more slowly than those in warmer microsites. For three years we monitored terminal bud development in young saplings as well as soil and air temperatures on large, replicated plots in a harvest unit; plots differed in microclimate based on amount of harvest residue and shade from neighboring stands. In two of three years, trees on cooler microsites broke bud 2 to 4 days earlier than those on warmer microsites, despite receiving less heat forcing from March to May each year. A simple sequential model did not predict cooler sites having earlier bud burst nor did it correctly predict the order of bud burst across the three years. We modified the basic heat-forcing model to initialize, or reset to zero, the accumulation of heat units whenever significant freezing temperature events (> or = 3 degree-hours day(-1) < 0 degrees C) occurred; this modified model correctly predicted the sequence of bud burst across years. Soil temperature alone or in combination with air temperature did not improve our predictions of bud burst. Past models of bud burst have relied heavily on data from controlled experiments with simple temperature patterns; analysis of more variable temperature patterns from our 3-year field trial, however, indicated that simple models of bud burst are inaccurate. More complex models that incorporate chilling hours, heat forcing, photoperiod and the occurrence of freeze events in the spring may be needed to predict effects of future silvicultural treatments as well to interpret the implications of climate-change scenarios. Developing and testing new models will require data from both field and controlled-environment experiments.

Ecosystem↗