Leaf-out in the Bio­sphere

The Victoria Standard - - Environment - ANNAMARIE HATCHER

In the Mi’kmaw cal­en­dar, June is Nip­niku’s which trans­lates to ‘Trees Fully Leafed Time’. If you travel around the Bras d’or Lake Bio­sphere in early June, you will no­tice that there is a sub­stan­tial dif­fer­ence in the de­gree of ma­tu­rity of leaves among dif­fer­ent trees, and in the same tree species in dif­fer­ent places. So, what causes the leaves to ma­ture and what ef­fect does the en­vi­ron­ment have on the tim­ing? Even though the leaves are un­fold­ing in the spring, the whole process ac­tu­ally started last au­tumn. As the days get shorter and the tem­per­a­ture drops in the au­tumn, trees start to pre­pare them­selves for win­ter by be­com­ing dor­mant. Sci­en­tists have de­ter­mined that the dom­i­nant cues are de­creas­ing tem­per­a­ture and day length. Af­ter a cer­tain pe­riod of shorter days and cooler tem­per­a­tures, hard buds form over the de­vel­op­ing leaves to pro­tect them from the win­ter weather. But what con­trols the time that that buds open in the spring, ex­pos­ing the del­i­cate new leaf tis­sue? If they open too early, the young leaves will freeze solid. Most species of trees are adapted to the lev­els of win­ter cold fol­lowed by spring warmth com­mon in the en­vi­ron­ment where they are na­tive. In the Bras d’or Lake Bio­sphere, cer­tain trees tend to leaf out early (birches, alders, many poplars) and oth­ers late (oaks and ashes). So why is there a dif­fer­ence? It would make sense to push the new leaves out as early as pos­si­ble be­cause they carry out pho­to­syn­the­sis and pro­vide sug­ars for the tree. A tree that leafs out in mid-may has four ad­di­tional weeks to pho­to­syn­the­size than a tree that leafs out in mid-june. How­ever, the early-leaf­ing tree faces the dan­ger of a late frost that will kill its leaves and dam­age its ves­sels, the main tis­sues that con­duct wa­ter from the roots to the rest of the tree. So, the tree faces a trade-off be­tween early food pro­duc­tion and po­ten­tial dam­age. The type of ves­sel struc­ture is a good clue to ex­plain why some trees leaf out early and some later. Early leaf­ing species tend to have smaller ves­sels that are less prone to frost dam­age than the larger ves­sels of other species that leaf out later in the spring. Also im­por­tant is the evo­lu­tion­ary his­tory of a plant group. Just be­cause a tree is na­tive to the Bio­sphere now does not mean that it evolved here. If it orig­i­nated in a warmer cli­mate (for ex­am­ple, red oak) , it may not have fully adapted mech­a­nisms for deal­ing with ex­treme cold and there­fore may have dif­fer­ent fac­tors reg­u­lat­ing leaf-out than a plant group orig­i­nat­ing in a colder cli­mate (for ex­am­ple, white birch).

So, as I was ex­plain­ing the in­flu­ence of tem­per­a­ture on leaf-out times, I’ll bet that you were won­der­ing about the im­pacts of global warm­ing on our spring for­est-scape. The rule of thumb has been that spring leaves come out five days ear­lier in the year for ev­ery de­gree of warm­ing. Mon­i­tored na­tive trees in New Hamp­shire (beech, sugar maple, and yel­low birch) are leaf­ing out 5 to 10 days ear­lier than they were 50 years ago. That may pro­vide longer pe­ri­ods of pho­to­syn­te­sis for the trees but it has many reper­cus­sions for the rest of the ecosys­tem. Many birds and in­sects are cued into the tim­ing of leaf-out in par­tic­u­lar habi­tats. Newly emerged leaves pro­vide crit­i­cal habi­tat for some in­sects that have gone through a long win­ter with lit­tle avail­able food, and th­ese in­sects may be a crit­i­cal food source for bird species com­plet­ing an en­ergy-de­mand­ing mi­gra­tion north. Tim­ing is ev­ery­thing! Ev­ery­thing in the nat­u­ral world is con­nected and in Mi’kmaq this prin­ci­ple is beau­ti­fully put as ‘MSIT No’kmaq’, or ‘all my re­la­tions’.

Dr. Annamarie Hatcher is an ad­junct pro­fes­sor at Unama’ki Col­lege, Cape Bre­ton Univer­sity and a board mem­ber of the Bras d’or Lake Bio­sphere Re­serve As­so­ci­a­tion. For more in­for­ma­tion about the Bras d’or Lake Bio­sphere Re­serve, check out blbra.ca.

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