No abstract

Non-structural carbohydrates (NSCs) form a fundamental yet poorly quantified carbon pool in trees. Studies of NSC seasonality in forest trees have seldom measured whole-tree NSC stocks and allocation among organs, and are not representative of all tree functional types. Non-structural carbohydrate research has primarily focussed on broadleaf deciduous and coniferous evergreen trees with distinct growing seasons, while broadleaf evergreen trees remain under-studied despite their different growth phenology. We measured whole-tree NSC allocation and temporal variation in Eucalyptus obliqua L'Hér., a broadleaf evergreen tree species typically occurring in mixed-age temperate forests, which has year-round growth and the capacity to resprout after fire. Our overarching objective was to improve the empirical basis for understanding the functional importance of NSC allocation and stock changes at the tree- and organ-level in this tree functional type. Starch was the principal storage carbohydrate and was primarily stored in the stem and roots of young (14-year-old) trees rather than the lignotuber, which did not appear to be a specialized starch storage organ. Whole-tree NSC stocks were depleted during spring and summer due to significant decreases in starch mass in the roots and stem, seemingly to support root and crown growth but potentially exacerbated by water stress in summer. Seasonality of stem NSCs differed between young and mature trees, and was not synchronized with stem basal area increments in mature trees. Our results suggest that the relative magnitude of seasonal NSC stock changes could vary with tree growth stage, and that the main drivers of NSC fluctuations in broadleaf evergreen trees in temperate biomes could be periodic disturbances such as summer drought and fire, rather than growth phenology. These results have implications for understanding post-fire tree recovery via resprouting, and for incorporating NSC pools into carbon models of mixed-age forests.

Non-structural carbohydrates (NSCs) are crucial to support tree resprouting after disturbances that damage the crown or stem. Epicormic resprouting (from stem) could demand more from NSC reserves than basal resprouting (following top-kill), since epicormically resprouting trees need to maintain a greater living biomass. Yet, little is known about NSC use during epicormic resprouting, particularly the relative importance of stem and below-ground NSC reserves. We compared the distribution and magnitude of NSC decreases during epicormic and basal resprouting by experimentally removing crowns or stems of 14-year-old Eucalyptus obliqua L'Hér. trees in native forest, then harvesting these trees over a 10-month period (start, sprouts emerged, sprouts expanded) to measure changes in NSC concentration and mass by organ (stem, lignotuber, roots). We hypothesized that (i) NSC depletion during resprouting is primarily due to decreases in starch rather than soluble sugars concentrations; (ii) during epicormic resprouting, stem NSC concentrations are decreased irrespective of any decreases in roots; and (iii) absolute decreases in NSC mass are greater for epicormic than basal resprouting during the leafless period due to the carbon demands associated with maintaining greater living biomass. Results confirmed our hypotheses; starch was the primary storage carbohydrate, stems were an important source of starch during epicormic resprouting and carbon demands of maintenance functions were greater for epicormic resprouting, leading to greater decreases in NSC reserves. Roots were a more important starch storage organ than the lignotuber for both epicormic and basal resprouting. The proportional decrease in starch was severe for both modes of resprouting due to a long leafless period, after which trees resprouting epicormically relied on starch reserves for longer than those resprouting basally. It remains to be seen how the timing of disturbance affects the timing and vigour of resprouting, and how long-term NSC recovery differs for epicormic and basal resprouting.

No abstract

<p>Forest growth is considered as an important global carbon sink but its responses to environmental changes remain uncertain. Tree stems are a predominant carbon pool in temperate eucalypt forests, representing a susbstantive component of their net productivity and carbon dynamics. Despite their importance, our understanding of factors controlling stem growth in these evergreen forests remains limited partly because the dominant eucalypts lack distinct growth rings. Unravelling eucalypt species' growth responses to climate from other factors, such as competition and disturbances like fire, is challenging due to the lack of long-term growth data. To address this gap, we present six years of monthly measurements of stem-diamter changes (as basal area increment, BAI) of two co-occurring dominant eucalypts from different sub-genera (<em>Eucalyptus obliqua</em> and <em>E. rubida</em>) across seven sites in a natural temperate forest of south-eastern Australia. We used linear mixed-effect models to examine the relative importance to monthly BAI of species, monthly climate indices and their potential lag effects (temperature and rainfall), inter-tree competition, and recent fire history (long-unburnt, prescribed fires, wildfire). Monthly BAI peaked in spring and autumn and was lowest in summer with signficant differences between species during spring and summer. Overall BAI variation was most clearly associated with maximum mean temperature, having a hyperbolic relationship with increases in BAI up to species-specific temperature optima and decreases thereafter. Rainfall, particularly autumn rainfall, influenced seasonal patterns in BAI, while inter-tree competition and recent fire history were of comparatively minor importance. BAI also varied strongly between years reflecting the opportunistic growth behaviour of eucalypts including higher annual growth rates during and after periods of high rainfall and transient decreases in BAI during extended drier periods. Our study provides field-based evidence of different growth niches for co-existing eucalypts in natural temperate mixed forests and highlights the importance of intra-annual climate variation to better understand overall productivity in temperate evergreen forests.</p>