Temperature and tree growth

Tree growth helps US forests take up 12% of the fossil fuelsemitted in the USA (Woodbury et al. 2007), so predicting treegrowth for future climates matters. Predicting future climatesthemselves is uncertain, but climate scientists probably havethe most confidence in predictions for temperature. Tempera-tures are projected to rise by 0.2 °C in the next two decades,then by 1.5–3.5 °C at the end of the century, depending onmodel and emissions scenario (IPCC 2007). In this issue,Way and Oren (2010) provide a thorough, timely and impor-tant synthesis of the effects of temperature on tree growth. Iwill highlight some of theirfindings and think about someother ways to approach the problem.Way and Oren (2010) found that increased temperaturegenerally increases tree growth, except for tropical trees.They suggest that this probably occurs because temperateand boreal trees currently operate below their temperatureoptimum, while tropical trees are at theirs. The response ofgrowth to temperature was not simply accelerating thesame trajectory of ontogeny achieved at current tempera-tures. Remarkably, temperature shifted the trajectory.Warmer trees were taller and skinnier, with more foliageand fewer roots! These changes were more pronouncedin deciduous species than in evergreen species, as wasthe overall response of growth to temperature. Contraryto expectations in the literature (Ryan 1991), plant respira-tion responded less than photosynthesis to increased temper-ature, because respiration acclimated while photosynthesisdid not. Way and Oren (2010) also developed and testedgeneral equations for estimating temperature effects ontree growth that should be useful for adjusting models.Because the literature was dominated by pot studies donewith limitations to water and nutrients removed, they sus-pectthattheequationsmighttendtooverestimatetheresponseofgrowthtotemperatureinecosystems,especiallywheretheseare limiting. As a final comment, Way and Oren (2010) offeran excellent model of how to synthesize diverse studies,because the methods are clear and statistically rigorousand the limitations and potential confounding factors areidentified and addressed.How well can equations developed from a synthesis ofstudies across sites predict the response for an individual site?This is important to consider, because an individual site iswhere the equation will be applied. Problems with cross-siterelationshipsmightariseifthepopulationataspecificsitehadadifferent response than the combined populations acrosssites. As an example, imagine if a cross-site relationshipwas developed from the 10 populations depicted in Figure 1(Rehfeldt et al. 2002). Since each of the populations are cur-rently growing at their optimum temperatures, a cross-siterelationship would show a response connecting the peaks(dashed line), but the response of any individual populationwould be much different. Indices of dispersion or overallmodel fit statistics for a cross-site model can help assessthis. However, because the within-site data used for thecross-site relationship represent only a small fraction ofthe overall response, they sample only a small part of thepopulation response. Way and Oren (2010) did test theircross-site relationship for a single species (Douglas-fir)and found that the more specific the cross-site relationship(for example, warming only for evergreens), the better it fitthe individual site. That the projected increases in tempera-ture over the next century (IPCC 2007) are within the rangefor most of the experiments suggests that a cross-site rela-tionship is a good initial estimate for the next century.A mechanistic understanding of temperature effects on treegrowth might also come from an understanding of the effectsof temperature on cell division and expansion, which are gen-erally more sensitive to environmental variability than arephotosynthesis and respiration (Hsiao 1973, Korner 2003).In many trees in many situations, photosynthesis does notcontrol the tree’s carbon balance (Korner 2003). Rather, thecontrol over the sinks by growing cells does, and sink feed-back can also regulate photosynthesis and respiration (Can-nell and Thornley 2000, Wiemken and Ineichen 2000).Moving towards a better understanding of the environmentalcontrols over cell division, cell expansion and partitioningphotosynthesis into various sinks might help achieve a bettermechanistic understanding of how tree growth will respondin future environments.Will faster tree growth in a warmer climate act to help mi-tigate CO