We have reported here on intra-annual growth patterns in Pinus radiata, a plantation species of important in several countries (Lavery and Mead 1998; Ivković et al. 2016). We focused on responses as observed at two case study sites at which different thinning treatments had been applied. The silvicultural histories at the two sites were very different, and also quite unusual. At Flynn, a heavy thinning was undertaken only once, and the patterns of growth seen in the remaining trees contrasted with the extreme case of no thinning at all. At Mt. Gambier, the contrast was different: in the study reported here we were studying trees which had “arrived” at the same stand density (556 stems∙ha− 1) via three very different silvicultural “pathways”. While the level of these effects is quite coarse (a stand level effect, which occurred in all cases some time prior to our fine-scale monitoring), our interest was in the fine-scale effects which could be seen on average in plots which had experienced/were currently experiencing different stand-level competition. It was not our goal to explore tree-level effects.
Unsurprisingly, we found at the Flynn site that trees in the previously heavily thinned stand were significantly larger than those which had not been thinned. On the other hand, at Mt. Gambier, where trees had all been thinned to a similar stand density, only those trees that had been thinned from a very high stand density (2222 stems∙ha− 1) were still significantly different (smaller) in size from trees which had either been planted at 1111 stems∙ha− 1 and thinned to 556 stems∙ha− 1, or those which had always been at 556 stems∙ha− 1. At Flynn, the advantage from thinning and subsequent marked reductions in stand density was maintained by trees in the thinned plots during the period of our study, with the thinned plots exhibiting markedly higher growth rates. At Mt. Gambier, however, this was not the case, and despite the previously unthinned (but more sparsely planted) trees being larger than trees in the other two treatments (i.e. having grown faster up until the time of our study), they were the slower growing trees while monitored during our study. Other research has shown, however (e.g. Valinger et al. 2000 in scots pine), that increased rates of growth in thinned stands could still be detected as long as 12 years after thinning, and differences in growth have been observed as long as 6 years following certain types of thinning in P. radiata (Cremer and Meredith 1976). It is possible, therefore, that trees in the thinned plots at Mt. Gambier were still exhibiting something of a “thinning response” (Snowdon 2002; Burkhart and Tomé 2012) during our study, even years after thinning occurred. We speculate that potentially trees in these previously thinned plots were still able to further explore the site in a way those in the never--thinned plots could not do.
Unfortunately, however, our data did not allow us to determine in more detail what was behind these treatment-level effects, e.g. differences in canopy development. However, we were able to explore concomitant short-term dynamics. The use of dendrometers proved to be a useful approach to tackling this question. While in some studies, difficulties in assessing onset of growth due to ambiguity resulting from effects of non-growth-related hydration events (Korpela et al. 2010), a simple method of using a proportional point to ascertain timing of growth onset and cessation worked well in our study. More complex methods can be used (Korpela et al. 2010; van der Maaten 2013), but this was not deemed necessary here where growth increments were large, relative to the “noise” often present in dendrometer data, and generally continuous.
Effects of thinning and stand density on growth dynamics
We postulated that the differences in cumulative growth seen between trees in the different thinning treatments could, in principle, be explained by differences in one or more of three main dynamics (Linderholm 2006). We conjectured that some combination of all three of these possibilities led to the observed differences between the two treatments at Flynn, and between the two thinned vs unthinned (but now at constant SPH) treatments at Mt. Gambier.
Some studies have found that larger, dominant trees retained a competitive advantage by starting to grow earlier than smaller and suppressed trees (Rathgeber et al. 2011; Vieira et al. 2015). Other studies (e.g. van der Maaten 2013 in European beech) have observed that thinning can stimulate earlier onset and a longer season duration. In our study, while we did not find evidence that the timing of onset of seasonal growth was a clear factor in determining differences in overall growth between thinned/unthinned plots, it was clear that thinning affected the timing of the cessation of growth in the major growing season: growth in trees in thinned plots ceased later in the season at both sites. It is particularly notable that even in the case of Mt. Gambier, where all stands were (for our study) at the same stand density, those trees which had previously been thinned ceased growing later in the season than those never thinned.
In addition, we found at both the Mt. Gambier and Flynn sites that the slower growing, never-thinned treatments had fewer days of detectable increment (within the season) than in previously thinned plots. Interpreting the occurrence of a detectable “over-bark” increment as measured by the dendrometers is, of course, complicated by water movement into expansible tissues, and other effects, and is not purely a function of the occurrence of cell production and irreversible expansion (Mäkinen et al. 2003; Zweifel et al. 2006; Drew et al. 2014) and ultimately “growth”. However, it is almost certain that cambial activity would be substantially reduced, if not stopped, when no growth was detectable (Abe et al. 2003; Gruber et al. 2010). Certainly, trees in unthinned plots maintained an overall smaller cambial zone than thinned plots. It is interesting, however, and consistent with previous findings in P. radiata by researchers like Skene (1969), that the sizes of these zones remained fairly constant throughout the season (although possibly a small decline was evident very late in the season in the unthinned plots). Potentially, the relatively more erratic growth during the season in the unthinned plots may have been more a function of variable meristematic activity in a constant-sized cambial zone than of constant expansion/reduction in the size of the meristematic population. The higher ratio between cambial and enlarging cells suggests that the unthinned plots also had a lower duration of enlargement than the thinned plots (cf. Drew and Pammenter 2007).
On days when increment did occur, rates did not differ at all at Flynn, and at Mt. Gambier, by a far smaller margin, between the thinned and unthinned treatments. That is, growth rates were relatively high, temporarily, on “growth days” in the unthinned treatments, but not maintained. These temporary, pulse-like (Zeppel et al. 2008; Drew et al. 2009; De Swaef et al. 2015) responses will be a combination of both a short-term hydration of flexible tissues, and a substantial, but temporary, enhancement of rates of cell division and irreversible tracheid radial/longitudinal expansion (Downes et al. 2009; Zweifel et al. 2014).
It is reasonable to conclude, from our data, that during and at the end of the season limitations from lack of resources became acute in cases where trees were competing more heavily. By contrast, the onset of growth was not affected by these resource limitations.
Seasonal conditions and the timing of growth
Early season temperatures have been shown by a number of authors, often in boreal conifers, to have an important effect in controlling the onset and ending of xylogenesis (Kramer et al. 2000; Rossi and Deslauriers 2007; Rossi et al. 2008; Moser et al. 2009; Bryukhanova and Fonti 2012). In our study, however, the onset of seasonal growth was not apparently affected by whether or not winter conditions were warmer or colder between years. In fact, it is notable that in onset of growth occurred right in the middle of winter, when temperatures were at (or close to) their lowest values, and day length was also not much longer than the winter solstice (about 21 June). Similar early (winter) onset of growth has been seen in radiata pine in places like New Zealand (Tennent 1986) where the conditions on the North Island are comparable to what we had in our study. These findings suggest very strongly that temperature in the very moderate and mild conditions at which the trees were growing in our study, in southern mainland Australia, is of no consequence as a determinant of the timing of growth in Pinus radiata. Certainly, this has been found in Australian P. radiata, in terms of general physiological activity, including bud elongation and internode extension in P. radiata, which occurred throughout the year (Cremer 1973). Similarly, Barnett (1971) found that P. radiata the cambium never became truly dormant. These attributes of P. radiata, growing in this relatively mild environment, may also be a reason why growth was not maximized later in the season. We did not find, as has been found in studies on northern conifers in colder regions (e.g. Rossi et al. 2006b) that maximum growth rates occurred around the time of the longest days. Rather, it would seem that growth rates are as high as ever, if not maximized (this was not always distinct), soon after growth onset, or (at the latest) towards the end of the Spring flush.
By contrast, it was evident that the cessation of growth did differ between years. Growth ceased, at both sites, earlier in the drier (based on both rainfall and evaporation) years. This was not surprising, as other studies have shown that trees growing under drier conditions have been shown to cease growth earlier than trees in less xeric conditions (Gruber et al. 2010). Through the effect of water availability on tree water relations, these conditions can be expected to cause a reduction in cambial activity, tracheid expansion and general phenology and metabolism (Hinckley et al. 1979; Kramer et al. 2000; Drew et al. 2014).
Other phenological changes, in concert with these differing conditions will likely also play a role. Timing of flowering has been shown to play in a role in P. radiata, for example (Fernández and Cornejo 2016) and cessation of growth may be in large part due to phenomena at the tree-level which are themselves linked to the drier conditions. Some work suggests that bud activity and shoot extension in P. radiata is also reduced or temporarily ceases in late summer/early autumn, probably also for drought-related reasons (Bollmann and Sweet 1979). Overall, however, our data would suggest that growing season in P. radiata growing under conditions such as in our study, is more determined by conditions in the late summer months, not temperature related per sé (except insofar as high temperatures may lead to drought-stressed conditions).
Data supporting fine-scale models of growth
Detailed fine-scale data on growth dynamics provide an opportunity to validate and calibrate models on new metrics, in addition to only yield. That is, process-based or hybrid models should predict not only that overall growth rates are higher in thinned stands, but also the changes in duration of season and number of growth days. This is particularly the case with fine temporal scale models such as CABALA (Battaglia et al. 2015) which are applied in predicting P. radiata growth and yield in Australia and internationally (Drew and Downes 2015; Drew et al. 2017). It makes particular sense to link fine-scale dynamics in stem radial growth and wood formation to morphological and phenological variables, in appropriately developed functional–structural models, such as the framework proposed by Fernández et al. (2011).