Pecan trees are really attuned to the continental climate we experience in the middle of the U.S. They are almost always the last tree to break bud in the spring. Why is this? Dr. Darrell Sparks from the University of Georgia published a study back in 1993 that suggested both chilling and heat accumulation were responsible for budbreak timing in pecan (The link to the paper is here). Below is a short description of pecan budbreak that I wrote for a conference back in 2007:
“Pecan trees, like most temperate fruit species, exist under a physiologically mandated rest period. This rest period, also called dormancy, helps to regulate the timing of budbreak. The start of dormancy generally begins in late summer when shoot growth stops and apical dominance ceases. Amling and Amling (1980) stated that rest is a growth-inhibiting physiological condition that can develop internally in buds. The rest period can be satisfied by the exposure of buds to periods of cold temperatures. Temperature, as well as hormones such as abscissic acid largely control the activity of the buds along with light intensity and day length (Nesbitt, 2002). The hormone levels that induce dormancy dissipate during the process of chilling accumulation which generally occurs when temperatures are below 45 F, but above 32 F. After the required number of chilling hours have been met (and this varies among cultivars and genotypes), an accumulation of heat over time will activate the buds and growth will begin again. The in-between period when chilling has been satisfied, but heat accumulation has not been met, is known as quiescence.
Budbreak during the spring is closely associated with the chilling requirement. Trees with a long chilling requirement will normally begin growth later than trees with a short chilling requirement. Budbreak regulation by heating and chilling is an evolutionary survival mechanism derived through adaptation resulting in pecan being native throughout a large area of the United States. In cold winter regions the high chilling received in the winter enables buds to break with minimum heating in the springtime. Growth commences within a short period of time, thus increasing the probability that the fruiting cycle will be completed within the abbreviated growing season associated with cold areas.
Conversely, pecans are one of the most adapted plants to the southern U.S. because they have a relatively low chill hour requirement, but a high heat unit requirement. However, there are some cultivars that break bud very early which increases the danger of bud damage to spring frost (Nesbitt 2002). The lack of a mandated chilling requirement contributes to pecan’s survival in regions with little or no chilling (Sparks, 2003). In these cases, the dormant period is prolonged in the absence of chilling temperatures; however, a deficiency of chilling temperatures can delay foliation, increase fruit drop, and reduce yield when pecans are grown in warm climates that lack sufficient chilling hours (Smith, 1994). The need for greater heat unit accumulating temperatures delays budbreak and minimizes the chance of damage from late spring freezes (Smith et al. 1992). The mechanism of increased heat unit accumulation is evident in the southern U.S., where pecan is one of the last deciduous tree species to breakbud in the spring (Sparks 2005).
Budbreak in pecans is described as being under the interaction of chilling and heat accumulation. Problems begin when sufficient heat is accumulated for the re-initiation of growth, leading to budbreak, in early spring when the chance of cold weather and damaging frost conditions has not yet passed. The typical continental climate that exists in the Southwest, with wildly fluctuating winter temperatures, can pose a threat to those pecan trees that awaken from their quiescent phase and initiate budbreak because the heat requirement has been satisfied.
Nesbitt, M. 2002. The pecan tree in winter. Pecan South 34(12):4-5.
Smith, M.W. 1994. Freeze injury to pecans. Proc. 28th Western Pecan Conf. pp. 155-157.
One aspect of this I do not hit upon is tree response to day length (photoperiod). Although timing of budbreak is highly heritable, other factors can influence it, such as shorter photoperiods. Pecan trees are great survivors in our highly variable climate because they are native to the region. In some ways they are very conservative in the spring, with late budbreak, but dangerously reckless in the fall (with late fruit ripening). It is an interesting crop to observe and work with. Right now (at the end of March) they are just beginning to show some green whereas other native and non-native trees have long since broke bud. I wonder if such late budbreak is in some way a contributing factor to alternate bearing.